PHOTOIZNOLIZATION AW TYPE II. PHUTOREACTIOII 0F ORTHO-nALKYL PRENI’I. KETOIIES A Dissorfaflon for the chrec of Mn. D. MICHIGAN STATE UNIVERSITY Andy Chi-Pang Chen I977 Date 0-7639 This is to certify that the thesis entitled Wfil‘mlfhtfox 45ml type IL Pliohraacflm off m 41114 P’W'vl Katine; presented by Ami; Chi-I’M} Chem has been accepted towards fulfillment of the requirements for P)“ 0' degree in Chmmfi/ %jor profisor LIE aux; J. (““13 IT {I ' hilLi11C)C-J.L U2 o‘w ‘- ““4 . . L 63.34:.)7 ABSTRACT PHOTOENOLIZATION AND TYPE II PHOTOREACTION OF ORTHO-ALKYL PHENYL KETONES BY Andy Chi-Peng Chen Photochemical studies of a series of orthg—alkyl sub; stituted phenyl ketones were conducted primarily to deter— mine the mechanism and kinetics of photoenolization reaction, particularly conformational effects on photoreactivities. The other major objective was to determine the substituent effect of an orthg-methyl group on type II reactivity. The low type II quantum yields for 9-methylbutyrophenone, g- methylvalerophenone, and o—methyl y-methylvalerophenone re- sults from a combination of low yield of long—lived triplet, low probability for product formation from the biradical in- termediate, and competing enolization. An orthg-methyl sub- stituent decreases the type II rate constant by a factor of 3.3, which is comparable to the decrease produced by a meta- methyl stustitution. The ability of ortho-methyl phenyl ketones to photo- sensitize the cis-trans isomerization of l,3-pentadiene indicates that such ketones form two triplets, one short- lived (m0.3 nsec) and one long—lived (NBS nsec). Quantita- tive studies of the type II photoelimination of orthg-alkyl ketones indicate that the reaction proceeds from the long- lived triplet. The competing reaction of this triplet is Andy Chi-Peng Chen presumed to be photeonolization, but its rate (ke = 3 x 107 sec ) is the same for ortho-CH CD and C H Since ke 3’ 3 2 5' is independent of C—H bond strength, it must be concluded that the rate determining step for enolization of the long- lived triplet is not hydrogen abstraction. 8—Methyl—l- tetralone, a model for syn-g—methylacetOphenone, also sen- sitizes the cis—trans isormerization of l,3—pentadiene, but displays a linear reciprocal quantum yield plot, and there- fore has only one short-lived triplet. It is concluded that the photoenolization of ortho— alkylphenyl ketones is dominated by conformational factors, in particular the ground state syn/anti ratio and the rate for anti+syn rotation in the excited state. The quantum yield for formation of long—lived triplet in fact measures the percentage of anti ground states in what is presumably a rapid conformational equilibrium. The rate-determining step for enolization of the anti triplet is rotation to a syn conformation (ke = 3 x 107 sec —1) in which enolization is very rapid (k > 109 3-1). Although the 2,6-disubstituted phenyl ketones probably have only one principal ground state conformation, two kine- tically distinct triplets have been observed in sensitiza— tion studies. The long-lived triplet decays with a rate of 5 x 107 sec—1 but does not produce benzocyclobutenol; the short-lived one forms benzocyclobutenol with a rate of m 109 sec’l. PHOTOENOLIZATION AND TYPE II PHOTOREACTION OF ORTHO-ALKYL PHENYL KETONES BY Andy Chi—Peng Chen A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1977 a f." 4‘.“ m1 dfz-"v’ . {v To My Wife and My Son ii ACKNOWLEDGMENTS First and formost, this author gratefully adknowledges Professor Peter J. Wagner for his guidance, support and en- couragement throughout the course of this research. The author thanks the Department of Chemistry for its excellent faculty and for the use of its fine research facil- ities. Thanks is due my fellow graduate students, and espec— ially those of the Wagner Group, for many enjoyable associa- tions during my stay at Michigan State. The author takes pleasure to thank Eastman Kodak Com- pany, I.E. du Pont de Nemours & Co. Inc. and National Science foundation for a three-year Quill Fellowship admin— istered by the Department of Chemistry at Michigan State University. TABLE OF CONTENTS INTRODUCTION. . . . . . . . . . . . . . . . . I. Photophysical Processes. . . . . II. Photochemical Processes. . . . . . . . III. IV. V. VI. 1. The Norrish Type I Photoprocesses. 2. The Norrish Type II Photoprocesses a. Definition . . . . . . . . . b. Multiplicity of Reactive Excited States c. The l,4-Biradical Intermediates in Triplet—State Reactions. . . . d. Effects of Ring Substituents in the Type II Photoreactions Photoenolization Reaction . . . . . 1. Early Observations. . . . . . . . 2. The Mechanism . . . . . . . . . . 3. Singlet versus Triplet Reactivity 4. n,n* versus fi,n* Reactivity . . . 5. Conformational Effects. . . . . Sensitization and Quenching Kinetics. Research Objectives . . . . . . . . . Practical ApplicatiOn . . 1. Stabilization of Polymers . . . . 2. Photochromism . . . . . . . . . . 3. Synthetic Applications. . . . . . iv Page 10 l6 18 20 23 27 28 28 29 30 TABLE OF CONTENTS (Continued) Page RESULTS. . . . . . . . . . . . . . . . . . . . . . . 33 I. Qrthg—Alkyl Phenyl Ketones. . . . . . . . . 33 1. Quantum Yield of PhotOproducts. . . . . 33 2. Quenching of Photoproducts. . . . . . . 36 3. Sensitization Studies . . . . . . . . . 40 4. Deuterium Incorporation Experiments . . 43 II. g-Methylated Ketones. . . . . . . . . . . . 49 1. Quantum Yields of Photoproducts . . . . 49 2. Radical Trapping Experiments. . . . . . 52 3. Quenching of Photoproducts. . . . . . . 52 4. Sensitization Studies . . . . . . . . . 52 III. 2,6—Dimethyl Ketones. . . . . . . . . . . . 52 IV. Methyl-Substituted l-Tetralones . . . . . . 64 V. Spectroscopic Studies . . . . . . . . . . . 67 VI. Sensitization Kinetic Analysis. . . . . . . 74 DISCUSSION I. Effects of g-Methyl Substitution on Type II Photoreactivity. . . . . . . . . . . . . 83 II. Comparisons between Type II Reaction and Photoenolization Reaction . . . . . . . . . 88 III. Comformational Considerations . . . . n . . 90 IV. Conformational Effect on Photoenolization . 95 V. 2,6-Disubstituted Ketones . . . . . . . . . 102 VI. Summary . . . . . . . . . . . . . . . . . . 107 TABLE OF CONTENTS (Continued) VII. Suggestions for Further Research. 1. Effects of g—Alkyl Substituents on the Lifetime of l,4-Biradical Intermediates 2. o—Methylphenyl d—Diketones. 3. Competition between Photoenolization and Type II Reaction in 2—Alky1-8-methyl—l- tetralone 4. Temperature—dependent Studies EXPERIMENTAL I. Chemical. 1. Solvents. a. Benzene b. t-Butyl Alcohol c. Pyridine. d. l—propanol. e. l-Pentanol. f. l-Heptanol. g. l,4-Dioxane h. Methyl Alcohol-d. i. n—Heptane 2. Ketones a. g-Methylbutyrophenone b. g-Methylvalerophenone c. g-Ethylvalerophenone. d. 8-Methyl—l-tetralone. e. 4,7-Dimethyl-l-indanone vi Page 109 109 109 110 111 113 113 113 113 113 113 114 114 114 114 114 114 115 116 116 116 116 117 TABLE OF CONTENTS (Continued) f. g—Methyl Y—methylvalerophenone. g. 2,3-Dimethy1 Y—methylvalerophenone. h. 2,4-Dimethyl—y—methylvalerophenone. i. 2,4—Dimethy1—d6—y—methylvaler0phenone j. 2,5-Dimethyl—y—methy1va1erophenone. k. 2,3,4,5-Tetramethyl y—methylvalero— phenone 1. 2,4,6—Trimethy1 y—methylvalerophenone m. 2,3,5,6-Tetramethylvalerophenone. n. 2,3,4,5,6—Pentamethylvalerophenone. o. 5,6,7,8—Tetramethy1—l—tetralone p. 2,4,6—Triisopropylacetophenone. q. 2,4,6-Triisopropyl y—methylvalero— phenone . . . r. g—Methyl g-methylvalerophenone. s. 9~Methy1 a,u-dimethylvalerophenone. t. g—Methylacetophenone. u. ngethylbenzophenone. v. 2,4,6—Trimethylacetophenone w. 3.3,6,8—Tetramethyl—l—tetralone Quencher. a. l,3—Pentadiene. . b. 2,5—Dimethy1—2,4—hexadiene. C. l-Methylnaphthalene Internal Standards. . . Page 117 118 118 119 119 119 119 119 120 120 120 120 121 122 122 122 122 122 122 122 123 123 TABLE OF CONTENTS (Continued) Page II. Methods. . . . . . . . . . . . . . . . . . . 123 l. PreparatiOn of Samples for Photolysis. . 123 2. Photolysis . . . . . . . . . . . . . . . 124 3. Photolysate Analysis . . . . . . . . . . 124 a. Gas Chromatography . . . . . . . . . 124 b. Identification of Photoproducts. . . 123 c. StandardizatiOn Factors for Internal Standards. . . . . . . . . . . . . . 127 4. Actinometry and Quantum Yields . . . . . 129 III. Photokinetic Data. . . . . . . . . . .y. . . 130 l. Stern-Volmer Quenching Studies . . . . . 131 2. Sensitization Studies. . . . . . . . . . 154 BIBLIOGRAPHY. . . . . . . . . . . . . . . . . . . . 166 viii TAELE 10 11 12 13 LIST OF TABLES Transients Observed with 2,4—Dimethylbenzo— phenone. . . . . . . . . . . . . . . . . . . . Activation Parameters of Phenyl Ketones. . Cycloadducts from 2—Methy1benzophenone . . . Photokinetics Parameters for Ortho Alkyl Ketones in Benzene . . . . . . . . . . . . . . . . . Photokinetics Parameters for Ortho Alkyl Ketones in t-BuOH. . . . . . . . . . . . . . . . . . Quenching Constants for g—Methyl y—Methylvalero~ phenone in Primary Alcohol Solvents of Varying Viscosity. . . . . . . . . . . . . . . . . Triplet Formation and Decay Rate Constant for orthg-Alkyl Ketones in Benzene and t—BuOH. . Photokinetic Parameters for g-Methylated Valero— phenones in Benzene. . . . . . . . . . . Photokinetic Parameters for 2,6-Disubstituted Ketones. . . . . . . . . . . . . . . . . . . Triplet Formation and Decay Rate Constant Ob— tained from Sensitization Plots for 2,6-Disubs— tituted KetoneS. . . . . . . . . . . . . . . . Triplet Yield and Decay Rate Constant for Cyclic Ketones . . . . . . . . . . . . . . . . UV Spectra of o-Substituted Ketones in n—Heptane 13C NMR Chemical Shifts Values of 9—Substituted Phenyl Ketones . . . . . . . . . . . . . . . . ix Page 11 21 32 34 39 44 50 62 63 67 68 7O TABLE 14 15 16 17 18 19 20 21 22 23 24 25 26 27 LIST OF TABLES (Continued) Triplet Lifetime and Rate Constants for g—Alkyl Phenyl Ketones. . . . . . . . . . . 13C NMR Chemical Shifts and Angle of Twist. Triplet Yield and Lifetimes for 2,6—Disubs— tituted Acetophenones and Tetralones. . . . Long—lived Triplet Lifetime and Rate Con- stants for 2,6—Disubstituted Ketones in Benzene . . . . . . . . . . . . . . . . . . Standardization Factors . . . . . . . . . Stern—Volmer Quenching Studies of ofMethylfi butyrophenone in Benzene at 313 nm. . . . . Stern—Volmer Quenching Studies of g—Methyl- valerophenone in Benzene at 313 nm. . . . . Stern-Volmer Quenching Studies of g-Methyl y-MethylvalerophenOne in Benzene. . . . . Stern-Volmer Quenching Studies of 2,3- Dimethyl y—Methylvalerophenone in Benzene Stern~Volmer Quenching Studies of 2,4— Dimethyl Y— Methylvalerophenone in Benzene. Stern—Volmer Quenching Studies of 2,5— Dimethyl y— Methylvalerophenone in Benzene. Stern—Volmer Quenching Studies of 2,3,4,5- Tetramethyl y-Methylvalerophenone in Ben— zene and E—BuOH . . . . . . . . . . . . Stern—Volmer Quenching Studies of g—Ethyl- valerophenone in Benzene and E—BuOH . . Stern—Volmer Quenching Studies of g—Methyl- butyrophenone in t—BuOH . . . . . . . . . . X Page 84 103 104 128 131 132 133 134 135 136 137 138 139 TABLE 28 29 30 31 32 33 34A 34B. 35 36 37 38 LIST OF TABLES (Continued) Stern-Volmer Quenching of g—Methylvalero— phenone in t—BuOH. Stern—Volmer Quenching Studies of g—Methyl y—MethylvalerophenOne in t-BuOH. Stern-Volmer Quenching Studies of o—Methyl y-Methylvalerophenone in 3.0 M Dioxane Stern-Volmer Quenching Studies of g—Methyl y—Methylvalerophenone in Primary Alcohols. Stern-Volmer Quenching Studies of 2,3- Dimethyl y—Methylvalerophenone in t—BuON Stern—Volmer Quenching Studies of 2,4- Dimethyl- and 2,5-Dimethyl y—Methylvalero— phenone in E—BuOH. . . . . Stern—Volmer Quenching Studies of Deuterium Incorporation in 0.05 M g—Methylacetophenone and ngethylbenzophenone Stern—Volmer Quenching Studies of Deuterium Incorporation in 0.05 M g—Methylacetophenone and g—Methylbenzophenone . . Stern-Volmer Quenching Studies of g—Methyl a—Methoxyacetophenone in Benzene with 1,3— Pentadiene as Quencher Stern—Volmer Quenching Studies of g—Methyl d-Methylvalerophenone in Benzene Stern-Volmer Quenching Studies of Photo— lysis of Ketone 40 in Benzene. Stern—Volmer Quenching Studies of 2,4,6— Trimethyl y—Methylvalerophenone in Benzene xi page 139 140 140 141 142 142 143 .144 145 146 147 148 TABLE 39 40 41 42 43 44 45 46 47 48 49 50 51 LIST OF TABLES (Continued) Stern—Volmer Quenching Studies of 2,3,5,6- Tetramethylvalerophenone in Benzene. . . . . Stern—Volmer Quenching Studies of 2,3,4,5,6- PentamethylvalerOphenone in Benzene. . . . . Stern—Volmer Quenching of Benzocyclobutenol Formation from 2,4,6-Trimethylacetophenone in Benzene Irradiated at 366 nm . . . . . . Stern—Volmer Quenching of Benzocyclobutenol Formation from 2,4,6-Triisopropylacetophenone in Benzene Irradiated at 366 nm . . . . . . . Stern-Volmer Quenching Studies of 2,4,6— Trimethylpivalophenone in Benzene . . . . . Stern—Volmer Quenching Studies of 2,4,6— Trimethyl a—MethylvalerOphenone . . . Sensitization Studies of Various Ketones with 0.2 M cis-l,3-pentadiene in Benzene. . . Sensitization Studies of Various Ketones with 0.15 M cis—1,3—pentadiene in E-BuOH. Sensitization Studies of 0.05 M 9—Methy1— acetophenone in Benzene . . . . . . . . . . Sensitization Studies of 0.05 M g-Methyl— benzophenone in Benzene . . . . . . . . . . . Sensitization Studies of 0.05 M g-Methyl- valerophenone in Benzene and t—BuOH . . . . . Sensitization Studies of 0.05 M o—Methyl y—Methylvalerophenone in Benzene. . . . . . Sensitization Studies of g-Ethylvalerophenone in Benzene. . . . . . . . . . . . . . . . . Page 149 151 152 153 154 155 156 157 158 160 TABLE 52 53 54 55 56 LIST OF TABLES (Continued) Sensitization Studies of g—Methyl d-Methyl— valerophenone (3%) and g—Methyl u,u—Dimethyl— valerophenone (40) in Benzene. Sensitization Studies of 2,4,6-Trimethy1 y-Methylvalerophenone (46) 2,3,4,5,6—Penta- methylphenone (48) in Benzene. Sensitization Studies of 8—Methy1—l-tetralone (51) and 5,6,7,8-Tetramethyl-1-tetralone (5%) in Benzene Sensitization Studies of 2,4,6-Triisopropy1 y—Methylvalerophenone in Benzene Reciprocal Quenching Study for the Unknown Product Formation from 2,4,6—Trimethylaceto— phenone in Benzene . . . . . . . . . . . . xiii Page 161 162 163 164 165 FIGURE 1 LIST OF FIGURES Page Photophysical transitions between electronic states in a typical organic molecule. . . . . 2 Stern-Volmer quenching plot for ofmethyl— butyrophenone (o), g—methylvalerophenone (A) and o—methyl y—methylvalerophenone (u). . . . 37 Stern-Volmer quenching plot for g-methyl y— methylvalerophenone in l-propanol (a), 1— pentanol (A), and l—heptanol (u). . . . . . . 38 Quenching constant for ketone 34 versus quenching constant for valerophenone in same solvent. . . . . . . . . . . . . . . . . 39 Quenching plot for g—methyl a-methoxyaceto— phenone in benzene with pentadiene as quen— cher. Solid curve is calculated from equa- tion 20 . . . . . . . . . . . . . . . . . . . 41 Concentration dependence of the gis—trans isomerization of cis-l,3-pentadiene (c—p) in benzene photosensitized by 0.05 M g— methylacetophenone. . . . , . . . . . . . . . 42 Sensitization studies for 9—methylvalero— phenone in benzene (a) and t—BuOH (A) . . . . 45 Sensitization studies for g—ethylvalero— phenone in benzene. . . . . . . . . . . . . . 46 Sensitization studies for 9—methy1benzo— phenone in benzene. . . . . . . . . . . . . . 47 FIGURE 10 11 12 13 14 15 16 17 18 19 20 21 LIST OF FIGURES (Continued) Page Sensitization studies for 9—methy1 Y" methylvalerophenone in benzene. 48 Quenching plot of deterium incorporation for 9-methylacetophenone (A) and 9—methyl— benzophenone (o) in CH3OD with pentadiene as quencher . . . . . . . . . . . 51 Stern—Volmer quenching plot for ketone 32 (o) and 40 (A) in benzene 53 Sensitization studies for ketone 39 (0) 54 and 40 (A) in benzene . . . . ’b’b Reciprocal quenching plot for unknown pro- duct formation from 2,4,6-trimethylacetophenone Stern-Volmer quenching of benzocyclobutenol formation from 2,4,6—trimethylacetophenone in benzene. . . . . . . . . . . Stern—Volmer quenching plots for ketone 4% (u), 47 (0), 48 (A) and 49 (o) in benzene ”\l’b ’\I’\J 'b'b Sensitization studies of 2,4,6—trimethyl y—methylvalerophenone (o) and 2,3,4,5,6— pentamethylvalerophenone (A) in benzene Sensitization plot for 2,4,6—triisopropyl y—methylvalerophenone . . Quenching plot of benzocyclobutenol formation from 0.1 M 2,4,6—triisopropylacetophenone in benzene. Quenching plot for 2,4,6—trimethylpivalophenone Sensitization studies of 8-methyl—1—tetralone (o), 5,6,7,8-tetramethyltetralone (A), 3,3,6,8— tetramethyltetralone (I) and 4,7—dimethy1-d- indanone (D). . . . . XV 56 57 59 6O 61 63 66 INTRODUCTION I. Photophysical processes The creation of an electronically excited molecule re- sults in the occurrence of photochemical as well as photo- physical processes. An understanding of the photophysical processes is essential to fully appreciate the photochem- ical processes, and vice versa. A modified Jablonski dia- gram1 is shown in Figure l to illustrate the photophysical processes for a typical organic molecule. Selection rules require that the spin angular momentum of the ground state be conserved in the light absorption process,2 so that trip- let states are generally populated by intersystem crossing from the directly excited singlet. Because of rapid vibra— tional relaxation in solution, only the lowest singlet and triplet excited states generally participate in chemical reactions. II. Photochemical Processesi 1. The Norrish Type I Photoprocesses The Norrish type I process of ketones involve the home- lytic scission of the bond between the carbonly carbon and the a—carbon to give acyl and alkyl radicals3 which go on to form various stable products. R - C — R' ————————> R — C’ + ~R' 7 3.- 3:: ””9 (IC) INCREASING ENERGY I ABSORPTION TO SECOND SINGLET I!!! I So ‘ - GROUND STATE 3 . Internal conver51on. blntersystem crossing. Figure l. Photophysical transitions between electronic states in a typical organic molecule. It is known that pivalophenone undergoes type I cleavage in solution through its triplet excited state.4 Many investi- gations of type I reactions of cycloalkanones have been con- 5 ducted. The photochemical reaction pathways of cyclic ke— . . . . . 6 tones in solution are summarized in the folloWing scheme. O [—‘V / (Grimm. (CI-12)J 19c (“H2)" 4%! Q n n / \ 3 / \ (CH ) ' ‘k '— /’ 2 H /0 /_ “O \n_-1 (CH 11:: CBHB (CH2)IT‘ \ / 2 n — ___:: :1 (CH2);— In the case of ketones with differing degrees of d—alkyl substitution, type I cleavage generally results in the formation of the more stable alkyl radical and the corres- 5 pending acyl radical. The introduction of either a-methyl 8’9 in cyclic alkanones in- substituents,7 or ring strain creases the rate constant for d—cleavage from the triplet state; it is likely that the rate constant for d—cleavage 10 from the singlet excited state also increases. Di—t- butyl ketone undergoes the type I process with a rate con- stant of 6 x 107 sec_1 from the singlet excited state and with a rate constant of 7 - 9 x 109 sec—1 from the triplet state.11 An analogous difference of at least two orders of magnitude is also observed in the reactivities of the singlet and triplet excited states of cyclic alkanones toward OL-cleavage.lO 2. The Norrish Type II Photoprocesses a. Definition Carbonyl compounds containing y C—H bonds undergo, upon electronic excitation, characteristic 1,5-hydrogen shifts to yield both cleavage and cyclization products. The cleavage reaction is commonly called Norrish type II photoelimination, after its discoverer.12 Cyclobutanol formation, which generally accompanies cleavage, was first reported by Yang in 1958.13 Together they are called the type II photoprocesses.l4 (Equation 1). I? . + = .\ O RCCIIRZ CR2 CFZ II hv RCCR2CR2CHR2 on R (1) R '1 .D. n b. Multiplicity of Reactive Excited States It is well established that aliphatic ketones undergo type II elimination from both singlet and triplet n,n* states.15_l7 The evidence is that only part of the reaction can be quenched by conjugated dienes, which are Very effi- cient triplet quenchers but inefficient singlet quenchers.l4 Cyclobutanol formation, however, occurs mostly from the tri- 15_l7 This fact is evidenced by a significant plet state. decrease in the ratio of cyclization to elimination products with increasing concentrations of triplet quencher. TheType II cyclization and elimination reactions of aromatic ketones apparently occur only from triplet states since intersystem crossing quantum yields are generally unity.l8 c. The l,4—biradical Intermediates in Triplet—State Reactions A l,4-biradical intermediate has been demonstrated, with little doubt, to intervene in triplet state type II reactions.19 The fact that this intermediate can undergo reversal of the hydrogen abstraction process, as well as product formation, was not taken into account by early studies which tried to compare quantum yields to triplet 20-22 reactivity. Wagner has shown that added Lewis bases maximize quantum yields by eliminating the reverse hydrogen transfer.23 Furthermore, the biradical of y—methoxy valer— ophenone was actually trapped using alkyl thiols as trap- ping agents. Using the photoprocesses of valerohenone, V, as an example, the following scheme reveals the mechanism 6 of the type II process in phenyl alkyl ketones.19 H . hv 1* wow. 3* «.1009. U vo —$ v ——9 v ——> ph . 0 Ph (”(30.99 H //j:\ + fl// OH . % Ph \ Ph ° 0 $56 OH 113 Ph’I::I‘H trans) i H ph/ ‘\ (cis) The partitioning of the biradical between the various processes varies according to the structure of each spe— cific ketone. d. Effects of Ring Substituents in the Type II Photoreactions The effects of ring substituents on the type II re— actions have been studied to ascertain structure—reactivity relationships. For mafia and paga substituents, there are two effects which determine quantum yields in type II reac— tions. The first is an inductive effect on the disproportionation of the biradical and the second is the lowering of the reactivity of the triplet through inversion 24 of the 3(n,Tr*) and 3(n,w*) triplets. The increase in re- activity for the strongly electron withdrawing groups seems to be an inductive effect on the already electrophilic 25,26 triplet. It is now well established that ketones with n,w* lowest triplet are appreciably less reactive than 27’28 Since those with n,n* triplets in type II reactions. the 3(n,n*) lies only a few Kcal above the 3(n,1T*) in phen- yl alkyl ketones, electron donating substituents or a high— ly polar solvent can invert the ordering of the states.29’31 32’33 of the two It has been suggested that vibronic mixing triplets might induce n,n*—like reactivity in the lowest triplet. Wagner has shown that when the two triplets are close enough in energy to equilibrate thermally before de- caying, hydrogen abstraction can occur from low concentra- tions of n,n* triplets even when then,n* triplets are 8,34 lower.2 The effects of ring methyl and polymethyl sub— stitution on photoreactivity of butyrophenones and valero— phenones has been determined by Wagner et al.35 The effect of a maga or paga methyl is to increase the energetic sep— aration between the reactive n,n* triplet and the lower n,n* triplet. As the energy difference E between n,n* T and n,w* triplets of the methylated ketones increased, the observed rate.constants for the type II reaction decreases and is proportional to (—AET/RT). These results seem to indicate that reactivity arise from equilibrium levels of the upper n,W* states, rather than from vibronic mixing of the n,W* and W,W* states. For ortho substituents, a steric effect will be oper- ative along with the electronic effect. The substitution of an g-alkyl group also introduce a competing enolization reaction. This reaction has been known for a long time and has been subjected to flash spectroscopy studies but not to a combination of quenching and sensitization tech— niques. Further evidence concerning the mechanism and kin— etics of the photoenolization reaction comprises a major part of the research presented herein. The photoenoliza— tion reaction will be reviewed and the kinetics involved will be derived in the next two sections. III. Photoenolization Reaction 1. Early Observations In 1904 Collie36 observed that crystals of pyrone 1, upon exposure to sunlight, turned brilliant yellow and that the color faded upon melting the crystals or disolving them in a solvent. This early report of photochromism was fol— lowed by Ullman's observations on some chromone derivatives 2.37 m Aryl ketones with ortho alkyl substituents are a o I l R O \ CIIZR 1 2a, R=C6H5 b, R=CH noteworthy for their photochemical stability. For example, the photoreduction of benzophenone to benzopinacol is mar- kedly suppressed when the ortho position is substituted by an alkyl group containing an g—hydrogen (-CHRZI. Thus, for a—methylbenzophenone, ¢reduction= 0.055 in 2—propan- 01.38 In contrast, both 3—methylbenzophenone and 4— methylbenzophenone show high quantum yields, Qreductionz ca. 0.5. In an important paper published in 1961,39 Yang and Rivas showed that this effect was due neither to adverse electronic interactions nor to steric hindrance, but rather to an internal hydrogen abstraction which yields an enol isomer of the ketone (Equation 2). Photoenoliza— CH R , 2 /CIIR OH \ h\) / / Czo ——~" —C (2) I ' \: C6H5 6H5 83' R=C6H5 4a, R=C6H5 bl RZH b, R2H tion of this type has been demonstrated by the following observations. When a solution of 3a in Ch30D was irrad- iated, the recovered 3a was found to contain 1.04-1.09 atoms of deuterium per molecule. By nmr spectrometry all the deuterium atoms were found to be located at the benzylic position. Also, the photoenol 4b or a-methyl— benzophenone reacts smoothly with dimethyl acetylenedi— carboxylate, a dienophile, to give an adduct 5 in I—" 0 excellent yield. The structure of 5 was established by its conversion to l—phenylnaphthalene—2,3-dicarboxylic acid 6, identical in all respects with an authentic sample. CH3OD //\\T/COC6H5 CHDC6H5 C6H5 OH COZCH3 H O+ __. g I + \\C02CH3 fi—CO2CH3 8 c—co2cn3 C6H5 we CO2H 8 2. The Mechanism Most early studies of photoenolization were centered around 2-a1ky1benzophenones. Detection of the triplet ke- tone species in a photoenolization reaction was first re— ported by Yang a3 a;.40 Thus, flash photolysis of 2— benzylbenzophenone produced two species of lifetimes ca. 10 S and 500 us. The longer—lived species was assigned as a dienol and the shorter-lived species as the n,n* triplet (Equation 3). CHC H CH2C6H5 k / ’6 3H . ' / (2&0 h) K*1 ISC K*3 '—C\ (3) \ K C6HS C6H5 11 The latter assignment, however, was subsequently dis— counted by Porter and Tchir41 on the basis of known lifetimes of triplet states for benzophenone in hydrogen donating sol- vents. In a more extensive investigation, Porter and Tchir4l'42 detected five transients in the conventional and laser flash photolysis of 2,4—dimethylbenzophenone. The transients observed and their lifetimes in ethanol and cyclo— hexane are listed in Table 1. Table l. Transients observed with 2,4-dimethylbenzophenone Transient Amax /nm (A) 535 (B) 420 (C) 390 (D) 430 (E) 390 Lifetimes Cyclohexane Cyclohexane—O2 EtOH 38 ns 38 ns 28 ns 67 ns 67 ns 1.7 us 250 s 20 ms 1.9 s 3.9 s 9.5 ms 1.7 5 Hours Hours The reaction pathway and structural assignments of the transients is given in the following scheme. The initial n,fl* singlet state is extremely short—lived (10 ps) and un— dergoes intersystem crossing with unit efficiency. The corresponding triplet state has a lifetime of 40 ns (A) and further decays to a transient of lifetime 67 ns (B). The nature of this species will be discussed later. The tran Sients C and D were assigned the two isomeric enol structures. The isomer C decays more slowly than D presumably because it cannot undergo internal tautomerization. The absorption 12 .H H2C ‘\U A l I triplet ///\\“/¢\\ _____. LC) 0 T = 40 ns H3C B singlet nn* (T< lOns) product spectrum of transient E is the same as that reported by Ullman43 for an intermediate to which he assigned a dihy— droanthrone structure. The formation of transient E, by a "photo-Elbs" reaction, would be avoided when light of 42’44 A low yield of longer wavelength was filtered out. anthrone was isolated on addition of oxygen to this inter— mediate and further photooxidation would eventually give anthraquinone. 13 In a study of the photoenolization of some photo- chromic chromones, Ullman a; al.37 suggested that a triplet state of enol intervenes between the triplet state of the ketone and the ground state of the enol (step 3). This KO >*1K 1K I 3K (step 3) 3K >‘3E 3E > Eo proposal was based on the fact that although the chromone 2a undergoes photoenolization, the corresponding 2-methyl analogue 2b does not. Ullman argured that since the chro- mophores of two compounds are Virtually identical, their reactvities must be controlled by the nature of the pro— ducts. However, the energies of the ground states of the two corresponding enols should be comparable since both have energy levels lying well below the level of their triplet state precursors. On the other hand if excited states of the enols were formed as intermediates, the more conjugated excited photoenol of the benzyl derivative 2a might be expected to be substantially less energetic than that of 2b. Porter and Tchir suggested later41 that this species, corresponding to transient B observed in flash photolysis, might be a "twisted" or "orthogonal" triplet state, a mole— cule which might be described as a l,4—biradical. However, the destabilization of this species by a phenyl group and its insensitivity to oxygen would argue against this 14 assignment. A further complication was that no single transient arising directly from the decay of this species was observed. Therefore, no definite assignment of this species was made. The most commonly accepted mechanism for photoenol- ization is outlined in the following scheme. R H ———> R R / OH ——-—) \ . -———> R The introduction of a second QEEaa—substituent into the ketones causes a drastic change in photobehaviour. For example, the transients observed in the flash photolysis of 2,6—dimethylbenzophenone41 are not at all similar to those observed with ortho-substituted benzophenones and no assignments were suggested. In an extensive study of 2,4,6- trialkylphenylketones, Matsuura and Kitaura45 noted that cyclobutenol formation was the preferred course of reac— tion. Even though no Diels—Alder adducts were observed, deuterium exchange was noted when the ketones were irra— diated in CH OD fo long periods. They deduced that di— 3 enols were first formed and then underwent ring closure to 15 give the cylobutenols. The following scheme describes some of the reactions of 2,4,6—trialkylphenylketones. CH R CH 3 - ' I jH3 If ‘ 3 R / II/§O hv \ -/V\on O L0H , > __J /’\\ /i\\ ' CH /\CH CH' \ \ CH \/ 3 3 3 3 7b m hv/O2 a: R=CH - = 7b R 3 8a. R CH3 . —C2H5 . f b: R=C2H5 C: R=l-C3H7 C: R=itC3H7 d: R=t—C4H9 d: R=t—C4H9 e: R=H e: R=H f: R=Ph .4 _ N Ph CH3 H i C H I 3 I 2\5/ on Lo) 0. lo ? H3 0 CHE/\\V// C 1 \ Since the photoinduced enols are hydroxy-a—quinodime- thanes Qa—xylylnes), it is worthwhile to examine the nature of the electronic structure of axylylene. Commonly asked questions about its electronic structure involve the relative importance of structuressnmfiiaslga—lgcixivarious electronic states; preferred geometry; and reaction modes of each state. 46,47 48,49 Both theoretical arguments and spectroscopic studies suggest that a-xylylene has a ground state singlet. Its Sl 16 A + .— I {—2. _ I1\ H i < > . \ § \/ . -\/ \ (——— . - + 1% 1%a . lab ago 42 state is planar (tight biradicaloid geometry, i.e., the two nonbonding orbitals in the same general region of space), while in T1 loose geometries are preferred, i.e., one CH2 group twisted out of the plane of the ring and the two orbi— tals in separate regions of Space. Recentcmlculationshased on Pariser—Parr—Pople (PPP) approximations suggest that the ground state of 12 deviates from the image of a "perfect" bi- radicaloid molecule.48 The ionic configurations 12b and 12c make a noticeable contribution (ml8%) to SO. In standard symbolism, formula 1% then is a fair representation of the ground state SO, although the contribution of structure 12a is higher than usual. 3. Singlet versus Triplet Reactivity The photoenolization reaction of a—alkylbenzophenones and 3-benzoylchromones seems to proceed exclusively via the 37’40 since addition of a triPlet quencher can triplet state, completely quench the reaction. Theljmmuofsignletreaction probably results from the rapid rate of intersystem crossing in these ketones. Exclusive triplet reaction is, however, not always true for alkyl phenyl ketones. In a study on the photoenolization 17 for 2-methylacetophenone”?O addition of 0.25 M cis-piperylene resulted in only a 20% depression of the enol formation ob- served by flash phocolysis. Lindquist argued that since pip- erylene is a very efficient acceptor for the energy of ketone triplets, while it does not quench their singlet excited state?0 it is apparent that the photoenolization of 2-methyl- acetophenone occurs to a large extent via the excited Singlet state of the ketone. Since only a single concentration of piperylene was employed and also since the unquenchable species could either be a short-lived triplet or singlet, a conclusion50 that 80% of the enolization occurs from the singlet state is not warranted. SammesSl further substantiated the participation of the singlet state by the following argument. Itifisknowntflmn:alkyl phenyl ketones containing -H atoms on the aliphatic chain undergo the type II process only from the triplet state}9 and the efficiency of the type II reaction is much reduced for 2- methyl substituted phenyl alkyl ketones because of competing photoenolization. Because the observed triplet decay rate of 2-methylacetophenone is slower than those of analogous ketones bearing a y-H atom it would be surprising if hydrogen abstrac- tion from the aggaa-methyl group in the triplet ketone could compete efficiently with that from a y-C atom in the aliphatic chain. The singlet state pathway was then suggested as the alternative route. However, Bergmark et al.52 demonstrated a 30:1 prefer- ence for a-benzylic:y-CH2 in the corresponding alkoxy radical 18 (Equation 4_. If the alkoxyl radical could serve as a model CH3 0 \JCHB hv 4 I . @ CH3 + () 3O : 1 for the n,n* triplet state of the corresponding ketone, the rate of a—methyl hydrogen abstraction should be at least 109 sec_1 instead of the ’blO7 sec.l found by flash photolysis. This result indicates that the unquenchable species observed by Lindqvist may well be a short—lived triplet rather than a singlet or a combination of the two. Further experiments are needed to ascertain the quantitative participation of the singlet state in photoenolization processes. 4. n,n* versus n,n* Reactivity The relative reactivities of the n,n*aumlfi,fi*states:hi . . . 27-34 the Norrish type II reaction have been well studied. Since the photoenzolization process strongly resembles the type II process in that Y—hydrogen migrates to the carbonyl oxyger, 38, 39, 41' 45 triplet was it is not suprising that the n,n* assumed to be the reactive species in the photoenolization reaction. However, hydrogen abstractions are known for systems where a n,n* excited state must be reacting. Etn:exampletju3 olefin l4 undergoes hydrogen scrambling53 and chemical trapping54 with maleic anhydride to give 16 (Equation 5). Similarly, l9 6-benzylbenzanthrone 17, which would be expected to posses a low-lying w,n* triplet, does form a photoenol, as determined by formation of yellow coloration at low temperature and deuterium exchange at the benzylic position (Equation 6). \\ 0 1 ellow color / 0 V y /’ 6 (6) / ON .900 O // CHD 7 // : : ’\ J l \\ 8H Similar n,n* state appears to be involved in the hydrogen abstraction reaction of the ketone 18, which produces the alcohols 19 and 20 by a 7-membered transition state55 (Equa— tion 7). The n,n* character of the excited state was 20 Ph Ph %Q demonstrated by the small bathochromic shift for the 0-0 band in the phosphorescence spectrum(of18xflmn1anon-polarsolvent 56 There has been, however, was replaced by a polar solvent. little work done to determine quantitatively the relative re— activities of n,n* and n,w* states in the photoenolization process. 5. Conformational Effects There is an increasing number of photochemical reactions for which the product composition apparently depends upon 57 ground—state molecular conformation. Thefirstexamplewhere such a relationship was postulated is the sensitized dimeriza— tion of butadiene57a (Equation 8). FormatiOn of vinylcyclo— hexene only upon excitation of the s—cis diene requires that the cis and trans triplets do not interconvert prior to addi— tion. Conformational control of product in the photoisomeri- Zation of l,3-cyclohexadienes to 1,3,5—hexatrienes has also 21 been investigated.58 Lewis has reported that y—hydrogen abstraction in poly- cyclic ketones 22 and 23 is more rapidthanijiacyclicketone 21.59 It was suggested that therate increases are 6.0 -— —— (IN 3 70 3.7 —4 22 In an elegant study of the photochemistry of 1-benzoyl— l—methylcyclohexane 24, Lewis g: a1.60 have provided an exam— ple of a system in which excited state reactions are faster than conformational changes. Excitation of 24 yields two Ph __0 O s— ”Q W P l 4 523a g’be I ihv ihv Ph 7 . . sec whicn discrete triplets: one with a lifetime of 10— undergoes only a—cleavage to radicals, and one with a much shorter lifetime which undergoes only y—hydrogen abstraction and cyclization. The rate of cyclohexane ring inversion (W105 sec-l) is too slow to equilibrate excited conformers with the benzoyl group equatorial and axial. In this case, the excited state reactions are determined by ground state conformational preferences. Cyclobutyl ketones provide another example of confor— mational effects on photochemistry. The quantum yield for the type II processes of benzoylcyclobutane is low61 presuma— bly because hydrogen abstraction can occur only when the benzoyl group is in a pseudoaxial conformatiOn. The rapid'y- . 62 hydrogenabstractioncafexo—5—benzoylbicyclo[2.l.l]hexane, 23 HO Ph A OH —./ \Ph a good model for a, certainly supports this interpretation. However, little attention has been given to conforma- tional effects on photoenolization processes. IV. Sensitization and Quenching Kinetics The quantum yield is a measure of the efficiency of a photoreaction. It is also the only kinetic parameter which can be measured under steady state cinditions since photoreac— tions generally follow zero order kinetics. For a triplet reaction, the quantum yield may be defined as follow. III = 6T - ®R° Pp (9) III = kiscfs - krTT - Pp (10) i: all the chemical and physical pathways for T. The intersystem crossing yield mm is the probability of J- triplet formation from its singlet precursor. It is defined as k. I , where k. is the rate constant of intersystem isc s isc Crossing and IS is the lifetime of the singlet. @R' the 24 probability that the triplet will react, is defined as kr IT, where kr is the rate constant for the reaction and TT is the lifetime of triplet. If an intermediate exists between the 63 should also triplet and the final products, revertibility be considered. Hence, the factor Pp is necessary to describe the probability that the intermediate will form product. If there are competing reactions from the triplet, the lifetime of the triplet will be determined by the rate of all reac- tions undergone by the triplet (Equation 11). The lifetime of a particular excited state can be de— rived from quenching studies analyzed by the Stern—Volmer expression (Equation 12). d) : ‘I ®O/. l + quQ’To (12) @O is the quantum yield in the absence of quenchen<§is the quantum yield in the presence of some quencher, and {Q} is the concentration of the quencher. Thereiszalinearrela— tiOn between ©0/® and the quencher concentration, with slope qu, kq being the bimolecular rate constantforquenchingand To being the lifetime of the particular excited state being quenched. The generalStern-Volmerquenchingequations flarsystems inwhiditwodifferentexcitedstatesbothreactandarequen— ched by a given quencher has been derived and analyzed by WaSJner.64 Similar kinetic analysis has been developed by 65 Dalton and Synder and by Shetlar.66 Intersystem crossing yields and lifetimes of triplets 25 can be obtained by measuring the efficiency with which the ketones sensitize the cis—to-trans isomerization of various concentrations of l,3-pentadiene. Equation 13 describes the ¢ “1 = a. ’la‘1 (1 + ——:~l—,——~.) <13) sens isc quTLQJ reciprocal dependence of sensitized quantum yield on quencher concentration, where ®isc is the intersystem crossing quantum yield for the sensitizer, a is the probability that the quen— cher triplet will yield the observed product, IT is the sen- sitizer triplet lifetime, and IQ] is the diene concentration. With acetophenone as sensitizer, Qcat equals 0.56,67 and this system can be used as an actinometer. In systems where two conformers give rise to different photoproducts, the kinetics will be more complicated (Equa- tion 14). hv kx A A* -————9- X II kaIIkb k. (14) B by 8* y Y There are three boundary conditions of interest: 1) excited state conformational changes are faster than triplet decay; 2) conformational changes are slower than triplet decay; 3) conformational changes are rate-determining. In case one the activation energy for conformational isomerization is lower than those for formation of X or Y (k; ,kg >> kX ,ky). In this case the ratio of products will depend upon the difference in energy for the transition states leadingIKDXaHle(Curtin—Hammettpminciple68). The observed 26 rate is the rate for each conformation times the equilibrium percentage of molecules in that conformation. obs. obs- - . k There is only one lifetime, which is defined as follOWS: _ -1 -1 l/T — XATA + XBTB In case two (k; ,kg << kX ’ky) the ratio of products de— pends upon the ground state populations of A and B and the A efficiencies of product formation from their excited states. The lifetimes of A* and 8* need not be the same. Iftflmalife- times of the two excited states differ greatly, the simple Stern- Volmer equation (Eq. 12) would be obtained for both excited states . In the last case where conformational change is rate de- termining (kg << kX ,ké % ky), the lifetime of A* would be different from that of B*, With l/TA* = k ,l/rB* = k + ka. Y The boundary conditions and product ratios are summarized in X the following scheme. Conformational Equilibrium Y ké kx k' ,k' >> k ,k —L— = b ‘ Y '. a x y kb ky Ground'State-Control ~ 'A‘k k' ,kg << k 1k '5“ = tAI j X TA* a x y Y tBrBJky TB* Rotational Control «r [l (1,1 , k5 << kX k; m k X _TA*IIA‘AJ+€B‘BkaTB*Ikx 27 V. Research Objectives The purpose of this research was to further investigate the photoenolization of a—substituted aryl ketones to provide more information about the relationship between molecular con- formation and photochemical reactivity. The ring substituent effect of an ammethyl group on type II reactivity was also of interest. The type II reaction was chosen as the monitoring system because certain problems inherent in other system can be 69 The C—H bond strength at the y—carbon can be varied avoided. without significantly changing the environment of the excited state of the ketone. In addition, the type II reaction is well understood and the products of this reaction can, in most case, be easily analyzed by VPC. The photochemistry of a series of a—methyl substituted butyrophenones, valerophenones, and Y-methylvalerophenones was studied. These compounds, upon irradiation, have two chemical options available: abstraction by oxygen of an ortho methyl hydrogen (photoenolization) or of a y-hydrogen from the alkyl ,chain (type II reaction) . The rate constant of enolization can be calculated if both the lifetime of the triplet and the rate constant of the type II reaction are known. In addition, 9- ethylvalerophenone and deuterated 2,4—dimethylvalerophenone were studied to determine the effect of C-H bond strength on the photoenolization processes. The lifetime and intersystem crossing yield of 8—methyl- tetralone, a good model for the syn conformer, was measured 28 to compare with that a—methyl phenyl ketones, which have both the aya and 3231 conformers. Polymethylsubstituted 8—methyl tetralones were also studied in order to estimate the reacti— vity of compOunds with a lowest n,n* triplet state. Sincel3CnmrspectracoldprovideEigenerallyapplicable method to study the conformational preferences of these com— pounds,7'0 the spectra oftheseketonesweretakentocnrrelate the photoreactivity with ground state conformations. The effect of solvent polarity on the photoenolization reaction was also studied by triplet decay measurement in various organic solvents. VI. Practical Application The study of photoenolization will not only extend our knowledge about the nature of excited states of ketones but also has several practical applications: 1. Stabilization of Polymers There are several ways ir1whichaapolymer*cankxaprotected from the action of ultraviolet light, apart from the obvious expedient of using an opaquescreencn:coatingtx>preventlight from reaching the polymer. One way is to mix with the polymer a compound which will absorb most of the light and use up the energy in some way which does not harm the polymer. Compounds which have been found particularly effective with polyethylene, for example, are the substituted 2—hydroxybenzophenones. These compounds undergo a reversible photoenolization which gives a low energy path for the photochemical energy to be degraded to heat (Equation 15). 29 H /\g/\OR_L,/\\C_ on 0 ll QC‘QORmeat Photochromism is defined as a reversible change of a (15) 2. Photochromism single chemical species between two states having distingui- shably different absorptiOn spectra. This definition can be represented by the following equation: A (i1) $ B (x2) (16) It is not surprising that a lot of photoenolizable ketones readily show photochromic characteristics. For example, 2- benzyl—3~benzoy1chromones show photochromism even at room temperature. The lifetime of these colored enols can be 0 OH I C\ h V C”3 (l7) \cmcefls Colorless Orange 30 controlled over a wide range by the choice of solvents, by the use of an acid or base catalyst, and by the use of metal ions to form coordination complexes. 3. Synthetic Applications The photoinduced dienols are potentially useful for new synthetic methods of annelation. A variety of dienophiles will add to the photodienol from 2—methylbenzophenone. Some of the example are shown in Table 3.71 An intramolecular addi— tion between the photoenol and olefinic bond of 25 afford a 72 major product 26 and a minor product 27. COIEt CHO (l8) 8% 8% £2, Kametani has used the opening of benzocyclobutene deriva— tives as a source of the reactive a—quinonedimethides in some elegant alkaloid synthesis.73 In particular the benzo— cyclobutenol 28 reacted with the imine 29 to give the adduct 30 which was then converted into the alkaloid xylopinine 31.74 The photochemical equivalent of this reaction has not so far been attemped. 31 OZO oSO OS 02 z\/ om: OOE aw . \fl ow: + E no \ ooz Table 3. Cycloadducts from 2—Methylbenzophenone 32 Dienophile Adduct Yield(%) Me02C°CEC-CO2Me 53 NC / CN C=C 22 \ NC / CN / O l o 50 I HO 0 Cone // 02Me l 35 \\ co Me COZMe HO/ ‘Ph 2 ///C02Me Icone J 58 MeOZC C0239 ] 27 C HO CIR) RESULTS I. anaafAlkyl Phenyl Ketones 1. Quantum Yield of Photoproducts Irradiation (313 nm) of degassed solutions of the 933E9- alkyl phenyl ketones listed in Table 4 resulted in the produc- tion of the type II cleavage product, the substituted aceto- phenone. No other products were observed by VPC analysis in any of the solvents used. The disappearance quantum yield for a—methyl y-methylvalerophenone in benzene is 0.036. This is somewhat larger than the 0.033 value of a-methylacetophenone formation, indicating that unidentifhaiproductsneylxaformed, but is no more than 8% chemical yield. These unidentified products are most likely the type II cyclization products, l—a-methylphenyl-2,2—dimethy1cyclobutanols. Solvent effects on type II quantum yields were determined for all the ketones. This was usually done by measuring the type II quantum yield of the ketone at various concentrations of added t—butyl alco- hol, pyridine, or l,4-dioxane. Additionscfiflargeconcentrations of EaEE—butyl alcohol caused type II yields to maximize, as 14 Irradiation of ketone solutions containing previously noted. pyridine produced a slight yellow color. No attempt was made to determine the origin of the color formation. A maximum quantum yield was usually obtained in 3.0 M l,4-dioxane. Quan- tum yields of acetophenones in benzene and maximum quantum yields in l,4-dioxane are listed in Table 4. Quantum yields in E-butyl alcohol are listed in Table 5. 33 34 .nozocosq mm econcmxon Iv.mla>ceofin©|m.m LuHB m#0nm noEno>Icnoum nmocnn mo momenmo.oconcmucoalm.Hlmoo E N.o mo coHumNonoEomn mennnpnmcom wn CmCHEnouoC pawn» nonmflne .ocmx0np E o.m an 0 COfimmEHOw mcocosmobooucnopm Memenn mo momenm mu .nozocosq mm econpmxoglv.NIHSLuoEnCIm.N gunB unend .oconcmncomlm.nlmno z mn.o mo cenumwnnoEOmn menmnunmcom >3 toenEneuoc Cnonm ponanneo .COHumEnOm ocococaouooquendhexcitedgrnethyly~nethylvalero~ phenone 34 in primary alcohols . As solvent viscosity increases k Ivalue do in fact decrease, as shown in Figure 3. Figure‘4 compares the k T values obtained from quenching excited 34 against those obtained from quenching valerophenone , with each point corresponding to measurements in a particular solvent. The irradiation of a-methyl—d-methoxyacetophenone:55in degassed benzene solution results in the production of a-methyl— acetophenoneamuil-(a-methylphenyl)-l-hydroxy-3—oxetane. The quantum yeild for the former product is 0. 171 t 0 . 009; that for oxetane formation is 0. 029 i O . 001. Values for :i—methoxyaceto- phenone itself are 0.57 and 0.42.72 The quenching of 45 by 37 7 I- // § / I’l/ / 6 .— 5- _ .A @O/c a 4 - / / . / / A 3* ’/// @t 5,] /’// ”/13. r _, / / I I L AL 0.02 0.03 0.04 0.05 IQuencher] , M Figure 2. Stern-Volmer quenching plot for a-methylbut- yrophenone (o), a-methylvalerophenone (A), and a-methyl y-methylvalerophenone (a). 38 / 2 5 0 / // / / / ’/x / // //I 2 . 0 ~— »' A //' /. , I/ i / / ,0 , A a /’I) . / /A//A I I / / / 1 F»— / / r/ // I .-”/ /I J”/ / x / ’ ‘K/ ’//I / / ,/ / i - ,. 1 . 0 . -"/ l l 1 l 0 0.01 0.02 0.03 0.04 0.05 [Quermflnar], I1 Figure 3. Stern-Volmer quenching plot for o-methyl ,—methyl— valerophenone in l—pi‘opanol (o), l-pentanol (A), and l-heptanol (I). 39 Table 6. Quenching Constants for a-Methyl y—methylvalerophe- none in Primary Alcohol Solvents of Varying Viscosity Solvent Viscosity (n, cP)a qu, M.l qub(VP) l—Propanol 1.9 37 i 2 52 1-Pentanol 3.1 25 r l 36 l-Heptanol 5.5 14 i l 23 E—BuOH 3.9 40 1 3 4O aRef. 75. bFrom quenching excited valerophenone, ref. 75. 40— o 30- _l o k I, M q 20— o 10* 0 l 1 I l l 0 10 20 30 40 50 60 k T, 51-1, vp Figure 4. Quenching constant for ketone 34 versus quenching constant for valerophenone in same solvent. 4O l,3—pentadiene in benzene deserves ammaattentionsinceiizdoes not give a straight Stern—Volmer plot, as shown in Figure 5. Two qu values of 4.9 M-1 and 1.4 M_1 are obtained based on equations 20 - 2377 for cases in whichtwm>excihaistatesreact o , - 0p = (1+iqulel)(l+qu2IQl> (20) 0 . k + o0 ' I~ 0 p 1 + qlelrz quLQ.Ll/¢l o 0 1+ @2/el no ®§ . . . '1 ' = __._____..__. + _______._ initial slope kqtl(q>o + ©0) qu2(¢° + $0) (21) l 2 l 2 , . o o . _ kqtl qu2(l+02/®l) final slope — ——~——*—*} “ 0" (22) a qu2 + @2k Tl/_1 (l+®O/®°)(k I +k I ) final intercept b2 = k ,2 lQquTl/®3 2 ‘ qL2 + 2 q 1 '1 (1+4 /© )2 k r k r 2 l U q l TIq 2_ (23) 0 - (qu2 + ’quTl/®1)2 and are both quenched. 3. Sensitization Studies The intersystem crossing quantum yields (eT) of ortho- alkyl phenyl ketones were determined from the quantum effi- ciencies with which these ketones sensitize the aia-to—trans isomerization (0 ) of various concentrations of gis-l,3- c+t pentadiene in benzene. Normally, such plots are linear. As shown in Figure 6, the plot for a-methylacetophenone is not Figure 5. 41 I I 1 I 0.5 1.0 1.5 2.0 2. [Quencher), M Quenching plot for a—methyl a—methoxyacetophe- none in benzene with pentadiene as quencher. Solid curve is calculated from Equation 20. 42 I I 3 . 0 ° . 5” ‘ o I . I I 4L , I I o I I o 3+- I a 0.55 . q.) I c+t ; o 25... l; I I I I 0! 1 1 I L Li. 0 2 4 6 8 10 IC-p]_l, M—l Figure 6. Concentration dependence of the cis-trans iso- merization of cis-1,3—pentadiene (c-p) in ben- zene photosensitized by 0.05 M a-methylaceto- phenone. IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII-I-I-I-I-I-I-I-I-I-r— 43 linear but has a steep slope at high (>1 M) diene concen- tration78 and a very low slope at low (<0.2 M) diene con- centration. Kinetic analysis of this plot, which is 1 derived in section VI, yileds ké'values of 150 M- for the l for a triplet formed triplet formed in 21% yield and 1.0 M— in 31% yield. Such nonlinear plots signify two kinetically independent triplets. Similar sensitization plots for a— methylvalerophenone, a—ethylvalerophenone and a—methylben- zophenone in benzene and t-butyl alchol are shown in Figure 7, 8 and 9 respectively. The finalslopeanmlintercept of these sensitization plots are not considered reliable be— cause the sensitized quantum yields are very close to each other at the lower concentrations of diene. As shown in Figdre 10, a sensitization plot for a-methyl y—methylvaler- ophenone at even lower concentration of diene (0.1 ~ 0.01 M) results in a straignt line with reasonably large varia- tion in quantum yields. The qu value thus obtained is 25 M-l, in good agreement with the slope of the Stern-Volmer quenching plot for that ketone. It is assumed hereafter that the triplet lifetime of any ketone determined by sen- sitization studies at low concentrations of diene would be the same as that obtained from a Stern-Volmer quenching plot. Table 7 contains triplet yileds and lifetimes of both triplets for several ketones. 4. Deuterium Incorporation Experiments Irradiation of a—methylacetophenone and a-methylben- zophenone in CH3OD results in deuterium incorporation in the recovered starting ketone. The quantum yield of 44 Table 7. Triplet Formation and Decay Rate Constant for Ortho Alkyl Ketones in Benzene and E—BuOH CHR / 2 1 <0 >“' ,CfR2 O R R Long-lived Short-lived 1 2 L :L/-c,'io7s‘l 1/1,10 atOtal T T Benzene H CH3 .219 2.9 .301 0.52 H C H .376 13.8 .624 1.0 6 5 CH3 (CH2)3CH3 .120 5.1 .20 0.32 _ . .2 . .4 H (CH2)3CH3 21 5 28 0 9 E-BuOH H CH .11 0.6 .15 0.26 H C6H5 .35 6.4 .65 1.0 1' CH3 (CH2)3CH3 .07 1.5 15 0.22 .11 2.0 .17 0.28 H (CH2)3CH3 45 1 —l —l [c—p} , M F‘ i 1 . . . . - . lClure /. oen5itization studies for a—methylvalerophenone in benzene (o) and t-BuOH (A). 46 , -l —l (c—p) , M F1*qure 8. Sensitization studies for o-ethylvalerophenone in benzene. _ n- ....__. 0......— ) 0 o o o 3-- o 0 o o 2.. 0.55 “13+t 0 o o o l._ 0 I l l l ' O 2 4 6 8 .10 -—l I<:-E)I , ll Ligure 9. SenSitization studies for O_m0thYlbvnzophenone in benzene. 48 28- 24- 20- W- _.—_~— I l 0 20 40 Ic-pl Figure 10. Sensitization studies phenone in Benzene. -1 - ,311 for g—methyl 80 100 ,-methylvalero- 49 deuterium incorporation, determined by mass spectral anal— ysis (m/e 91), is 0.21 i 0.06 for a—methylacetophenone and 0.38 i 0.06 for a—methylbenzophenone. Stern-Volmer quench- ing plots of this process for these ketones are shown in Figure 11. Small concentrations of pentadiene (O.5 M) continue to quench the reaction, but with less efficiency. Application of eq. 20-23 suggests that this process arises from two triplets, one long-lived (11 ns) and one short—lived (0.2 ns). a—Methylbenzophenone also produces two triplets, but with a total QT of unity. More~ over it undergoes photoinduced deuterium incorporation al- most twice as efficiently as does a—methylacetophenone. II. a-Methylated Ketones 1. Quantum Yields of Photoproducts Irradiation (313) nm) of a—methyl a—methylvalerophe- none 39 in benzene solution results in the type II cleav— age product a-methylpropiophenone and the type II cycliza— tion products, 1—9—methylphenyl—2,4—dimethylcyclobutanols. They are formed in comparable yields, just as reported for d-methylvalerophenone.79 On the other hand, photolysis of a—methyl a,d-dimethylvalerophenone 40 gives the type I cleavage product a-methylbenzaldehyde in addition to the type II cleavage and cyclization products. The quantum yields of product formation for both ketones in benzene and in 4.0 M l,4-diozane are listed in Table 8. .mpoHa cenumNopnmcom Eonmw .mmm 2 mo.o|ao.o we oocomonm CH0 .oC>£oCHmNco£H>:u®Eum .posoona H maxeo .mnmozueoHMQ an ooumna m0 Cnonw Esncmsq ESEnme ozu “cennwEnow HocmusnoHoxoo .ocmeAC z 00 en omenmbno .ocowox E mo.od Cnon> Esncosq ESEHRmE one no mnmonbconmm :n noQESC “cenumEHOw oCOCQCQOpoo<£ annmmnv onmoo.onmmoo.ovAnoo.onmmo.ov Amoco.000no.ov 88 on.a mnewn omm.o mnm.o neo.o mooo.on0moo.o wooo.onwoo.o mooo.on0mco.o mzo 0:0 o0 wnmnmnnv Amoo.onmmo.ov Amoo.onnmo.ov 83 ow.n mammn 0mm.o 00m.o Nnn.o .III mooo.onooo.o 0ooo.onmoo.o 0:0 2 mm nnz nu: v w . .e e e n 0o 00. 0 n econ .me x meoHe x Hmuone unonme omene be 09 39 m m tom N... 05:05:05:on I O _ nm 0 00o docmmcom CH monocosmonoam> Cmnoahzumzlo now mn®u®Emnmm Onuocnxouocm .w wanna 15 __ 0 O 0 L.— 0 O 10 t- 0 D/ A A A A O S**- A A A O “i 1 1 1 I Figure 11. Quenching plot of deuterium incorporation for g-methylacetophenone (A) and o-methylbenzophenone (9) in CH3OD with pentadiene as quencher. 52 2. Radical Trapping Experiments l-Dodecanethiol was employed at concentrations which would 80 to trap the free radicals not directly quench the triplet produced from type I cleavage of ketone $0. The virtually identical quantum yields of o-methylbenzaldehyde with both 0.01 M and 0.05 M thiol indicates complete trapping of all noncage free radicals. Thus the quantum yield for a—cleavage product formation is approximately 0.0065. Allowing for %SO% cage re- combination of benzoyl and alkyl radicals8O the quantum yields for d-cleavage can be estimated as 0.0l3. 3. Quenching of Photoproducts All the photoproducts of %% and £0 are quenchable by 2,5- dimethyl-2,4-hexadiene; Stern—Volmer plots were constructed using the data from the cyclobutanol products and are shown in Figure 12. Table 8 contains the values of qu obtained from Figure 12. 4. Sensitization Studies Sensitization of the gig—to-trans isomerization of Eli- l,3-pentadiene by these two w-substituted ketones also indi- cates the existence of two triplets with discrete lifetime. The curved double reciprocal plots are shown in Figure 13. Application of equations in section 6 indicates the @T and qu values listed in Table 8. III. 2,6—Dimethyl Ketones Introduction of a second ortho-methyl substituent on the phenyl ring of the ketones causes dramatic changes in molecu- lar conformations and photochemical behaviour. As indicated 53 03 O\ T / 5 _ / // / // ,5 lg) A x /r 0 - O / . ,. /l/ / 4* ./ ,/ / _/ 0/ // 3 —- / / / 4 / / , ,/ / ,/ / / * / /// /"«':// 1 :7 . l 1 1 1 L 0 0.01 0.02 0.03 0.04 0.05 ffl>u<2nt o O o 2" O o 1' 1 1 I I 1 0 2 4 6 8 10 {c—p}_1, M_1 Pigure 17. Sensitization studies of 2,4,6—trimethyl y—methyl- valerophenone (o) and 2,3,4,5,6-pentamethylvalero- phenone (A) in benzene. IIIIII=::______________________________________________4444444 61 ---__._‘. “awn“-.. 120.. y/ / ' 100- /// // 80 / / /'/. 60+— /// /'// 0 53 —3‘ / ‘Cvt // 40 — //{/ 20 _ ,// /II. / / l T 0 L 1 1 l . __J 0 1 2 3 4 5 1' a — 1— — iC-p; , H 1 Figure 18. Sensitization plot for 2,4,6-triis0pr0py1 E-methyl- valerophenone. 62 .coflchHOM mcocmcdoumom co cmmmmo .COeumEMOM HocmuseoHomcn .mflmocucmumm CH cmpmfla can mcmeec 2 o.m CH cmcHMpno we came» Esucmsq EsEflme “coflmeHOM ecocmcmoumo‘max E:max Xmax 8max >‘max 8max 32 2-CH 309 72 283 1300 237 11550 mm 3 33 2-CH 306 70 284 1350 238 11700 mm 3 34 2-CH 307 68 283 1320 236 11450 mm 3 35 2,3-(CH ) —— —— 282 1010 241 8610 mm 3 2 36 2,4-(CH ) 304 89 283 1010 243 11170 mm 3 2 37 2,5-(CH3) ~— —— 291 1430 241 9900 mm 2 38 2,3,4,5‘(CH ) —— .1_ 290 1010 251 8600 mm 3 4 39 2-CH , a-CH —— —— 281 1050 238 7500 mm 3 3 40 igfiHg, a,a- —— —— 276 870 236 3500 3 2 1 2-CH CH - .1 280 1030 242 8570 mm 2 3 45 2-CH , a-OCH 307 70 284 1340 238 11640 mm 3 3 46 2,4,6—(CH ) 308 110 273 386 240 2580 mm 3 3 47 2,3,5,6-(CH ) 306* 82 279 719 212 11500 mm 3 4 48 2,3,4,5,6- 306* 80 278 746 214 12050 “0’ (CH ) 3 5 49 2,4,6-(CH ) , ~— ~—- 271 398 215 10860 mm 3 3 u‘CI’I 3 5% 2,4,6- 305* 99 267 410 208 12380 CH (CH3) 2 3 51 2,4,6-(CH ) , -— —m - 210 11480 mm 3 3 0,0-(CH3)2 Table 12. Continued. n,0* Lb a Ketone R1 1 e A e A s max max max max max max 1-Tetralones 250 10100 22 8-CH3 301 2310 —— —- 246 10390 11450 _ 263 14900 54 5,6,7,8 (CH3)4 304 1822 —~ —~ 256 15340 * Shoulder, not a maximum - No maximum 70 Table 13. 13C NMR Chemical Shifts Values of o—Substituted Pheynl Ketones. _ R 1+3 218 Key— 0, ppm 71 .5 tones 1 2 3 4 5 6 7 %% 2~CH3 137.8 138.4 129.3 132.0 125.7 131.5 201.5 2% 273-(CH3)2 137.9 140.6 134.6 131.9 125.2 124.9 205. 2% 2,4-(CH3)2 135.5 138.4 132.8 141.5 126.3 128.9 204. 21 2,5-(CH3)2 138.5 134.6 128.9 131.8 135.0 131.6 204. gg 2,3,4,5-ICH3)4 136.5 137.8 133.3 138.4 132.0 126.0 207. 2% 2-CH3I a-CH3 137.6 139.0 127.6 131.7 125.6 130.7 208. 48 Z'CH3r 9:9- 134.2 141.1 124.8 130.9 124.4 128.6 213. (CH3)2 41 2-CH2CH3 138.8 143.7 130.2 130.8 125.6 128.0 205. 42 2:4:6-£uoEIm .owmeoHo>oH%prE CH mCOCmCmoemomH>CmeHHqu.v.m mo muuommm momouozmmonm *1. 73 I I I - _._._._.- - I I I I l _' ..___._.‘-.'_. -........- -_.--._. ._ 1 I l 1 I ‘ 1 L——— I 'L_.___.____ I l r 1 r » . , I—_..—_._._.__.___.___'_._-..- - _ . - l 1. -1 . i I . .- 1 _ _I _ I” _ W _ _ I ..I Ii“ . fur: I... . 1.! LII , . “.__.. _. .. “I... m n.4,.» 1:11 .1 1 S ‘ . 1.! I3 Ly} I .mm musmflm ‘ l —4 ' I I r 1. '.. ".l I L . - . J - " ‘I"‘f"‘?‘?:‘rr“T"I..:3.‘.rtl.3H'.;_J .. .l - -4 L ,.x q. . . 74 are shown in Figure 22 and 23 respectively. l3C nmr spectra of each ketone were measured in CDCl3 using tetramethylsilane (TMS) as an internal standard: the chemical shift values of aromatic and carbonyl carbons are listed in Table 13. It seems that the 13C nmr signal of the carbonyl carbon atom moves to lower fields with increasing steric hindrance by ortho-substituent. Thus, the carbonyl carbon of acetophenone has a chemical shift of 6 199.9 ppm, while the ortho—substituted ketones g—methylvalerophenone, g—ethylvalerophenone, 2,3-dimethylvalerophenone 2,4,6-tri- methylacetophenone and 2,4,6-triisopropylacetophenone show resonances at progressively lower fields: 6 201.54, 208.82, 205.98, 210.01, and 211.87, respectively. VI. Sensitization Kinetic Analysis In order to describe the behavior of sensitization plots where two triplet states can react with quencher Q, it is desired to derive a general expression relating the inverse of the total quantum yield for product formation, 1/0T, and l/[Q]. If species A and B are interconvertible both in the ground state and in the excited triplet state, product for— mation can occur from either species, say A, either directly or indirectly through interconversion from the other depending on the relative rate of interconversion and the rate of other processes available to these species. The processes of interest are summarized in the following scheme and the parameters involved are defined below: 75 A {—i ‘4 B l-x x lhv 111V 1A 1B A 1t2 t1 B :k. k Zk. 1 3 ____J._.. 3 1 Y 2....“— A 't k B-—————? x b A B k Iq. 3 3 Q Q a \ product X the fraction of species B in equilibrium t1 the intersystem crossing quantum yield of B t2 the intersystem crossing quantum yield of A ka the rate of conversion from 3A to 3B kb: the rate of conversion from 3B to 3A k: : the bimolecular quenching constant of A k2 the bimolecular quenching constant of B 2k? all the unimolecular reaction rates other t 2k? all the unimolecular reaction rates other t a the probability of product formation from 3 The total quantum yield for product formation, 0 equal the sum of the quantum yield for formation of T han k a han kb Q T' will from 3A and 3B: -A B 1) z + QT mT ®T Expressions for 0' and 4T, derived from A kA (Q1 kb kA {Q} 4 = (1 — x)t ———9—————— a + 4 t a T 2 1 A- . l 1 B 1 A ~ + k 1Q} __..._~_ + R [Q] -—-‘T + k [Q1 TA q LB q A q (25) r B 7 B kB LQ} ka k LQJ @ = x t g, d + (l-x)t q a T l 1 8. 2 1 Ar 1 8r . _?~ + quQ] —?— + quQ1 -?# + quQJ B (26) A B where T , T are the triplet lifetimes of A and B and A B T: A XkA + ka il T.=“‘ B B X _ + .k kb 1 combining eqs 25 and 26 would give 0T which can be rearranged to eq 27. B A. )I—L— + AI 1. 1 _ 1 {th a {kqu}(quQ, T ) kg‘Q*KbJ ®T ‘ . A B A 1 (kcfQT+ %—I(k I 1+ ?_) A B 1 B I B q A +<1—x>t Ink :010: {Q}+'~—-)+k_[Q] 2‘ q 8 TB q (27) inversion of eq 27 and multipy I on both side would yield 77 53— = (kEiQ1 + —%—)(kA[Q] + ———)/xtl {k8 {01(kqi }+—%—) + T i B q A LA A . A B 1 B . I _ I ___ + I kq1Qka. + (1 x)tZquIQIquIQI+ TB) kq.QIka} (28) The right—handed side of eq. 28 is multiplied by TATE/TATE to give __g__.-_— B 1 -A [ BT 1’23 ©T (quBIQIu)(quAm-m/xtl kq BIO (quA[Q]+l) + A T r. quA kaBfQ) + (1-X)t21k:TA[Q1(k§ B.041) + kBI k T (0)} (29) q B a A Now dividing the numerator and denominator of the right—hand side of eq. 29 by [012, we obtain a B l A A 1 ——=(kT+—-——)(kI+—-—-— )lB(/xt{kaT+————~—)+ 8T QB I01 qA I01 ‘38 8A IQI 8T _1__ 1 107(k8 TA kaB I} + (i- x) )t2 {k3 IA (kq B+[Q})+ {Q]quBkaTA (30) Eq. 30 gives the inverse of the total quantum yield for product formation as a function of 1/ (Q) . Since the numerator of eq. 30 is a polynomial linear in l/[Q], the sensitization plot will approach a straight line, i.e., an oblique asymptote, when l/[QI+00 The equation for this oblique asymptote can be deter— mined by mathematical procedures,83 and is given by Obique asymptote = —%~ —l + {(k r +kB B)C - kA kB [Q1— 2 q A q q A qT c {xt + (1- x)t 78 = B A _ A B where C xtl{quB+quAkaB}+(l x)t2{quA+quBkaTA} Therefore, . l A B A B =————f' r .. -+ _ intercept c2 ‘(quA+LqTB)C quAquB th (l X)t2} slope = —£— C intercept = —E—-{(kArA + kBTB)C _ kATAkBTBCth + (l-X)t2]} slope C q q q Case I. If k <> 2k? , and also kA = kB a . l b . 1 q q i 1 i.e. ka<< and kb =—l- TA TB 2, I ._ .» then C xtlquhB + TAkaB)} + (l x)t2 kq(TA + TBkaTA) = kq {xtlhA +TB) + (l-x)t2(TA + TBkaTA)} = quB {th(TA/TB + l) + (l-X)t2(TA/TB + kaTA)} Furthermore, if TA<> 7 >> 0 : If ka ZKi and kb in , kq kq i.e. ka = i and kb = —%— A B 'Then C = xtl-kq(TB +'lfiQ -t (l—X)t2'kq(TA + TB) : + — kq(TA TB)[th + (l x)t2] l = k (T + T )[xt + (1—X)t ] slope q A B l 2 2 + o + _ kq(TA TB) kq TATB[xtl (l x)t2] intercept = ' — 2 2 2 +T + —; - + , + — kq(TA B)[th (l x)tz] kq (TA TB) [ktl (l x)t2] T T l A B - — T xtl + (l x)t2 ( A+TB) 80 intercept = k (T + T )[1 _ A B J 31 9 q A B + T )2 09’ (TA B Case III. i'A B If ka<< iki and kb<< §ki i.e. ka<< ” and kb<< —£— LA TB then C = kqxtlLTB+ TA kb TB]+ kq(l-X)t2[TA+TBXaTA] = ___. ] kqfi AXt #[ A kb WB]+T (l— —X)t fl[ TB + k aTA } = kq{xtlTB + (l-X)t2TA; . TA + TB TATBEXt1+(l‘X)t2] intercept = 4— _ -1 2 + _ xtlTB (1 X)t2TA [xtlTB+(l- X) tZTA] r _ intercept : k (T + T ) _ EQTATBLth+(l x)t2] q A “B , + _. i slope thTB (1 X)t2:.A Furthermore,if TA>>TB then _ _ l + TB/TA T B/T A[xtl + (l—X)t2] intercept — ——#——— _ ______ 2 1'- T + - _‘ xtlLB/ A (l x)t2 ther /TA + (l X)t2] N l (l-X)t2 intercept : y (T +T ) _ 5qTATBEXt1+(l'X)t27 slope q A B (l-X)t2TA I? 7?‘ ,4 81 Now turns to the sensitization plot as l/(QJ+O The initial slope of this plot can be determined by evaluating the derivative of eq. 30 with respect to l/[Q] at l/(QJ = O, and is given by . .L. _ k; TATBLth+(l-X)t2].Kq(TA+TB) kq TA’B C initial slope — 2 2 (Lq TATBLth+(l—h)t21) ~ r +~ — ' — +T — 1 : (TA + [8) ‘{th(LB LATBRb)+(l X)t2(T: .BLAka) , 2 i 1 1_ T . | _ r . T quALBrxtl+(l x)t2 kqfxtl*(l X)t2] ATB l t + (l-X)t intercept X l 2 Case l- If ka < rk and Rh )> "k? l e ka —~— and kb = i then rA B , T T r rxt <—5—T1>+(1-x)t <—5~>} TA + TB 8 l *B 2 TB initial slope = - 2 ' ' T +’ -‘ ' + " T ‘ . quALfotl (1 X)t2] quth (l x)t2] A TB 2 __£_{ (TA+TB) _ th(TA/TB)+(1‘X)t2TA/IB quA‘TBFXtiHl‘XN21 [xtl+(l-X)t2]2 . gfieassg: ._. k T __i- ii 3'1 + $312.... ii. i_.- ” ‘ q A T f. T b A r J‘. \ 109“ (1+-5—» xt +(1—x)t ]- Xt (:n~+l)+(l-\)t (#L) . 1 2 l V 2 1 9 B B V‘ hf) f Y- ‘ - Fart“ rmore, i ‘A r8 . then + -v LEEPEEept _ k . {_%t1 iiifit 1 slope q A‘ xt1 + (l-X)t2 82 Case II. If k >> ZkA and k >> ng ——————— a l b i = , + r + — then C kq(TA TBLXtl (l x)t2] T + T T + T initial slope = A B - A B .4 + — ' _1 quATB[th (l x)tg quATBixtl(l X)t21 = O This means that there is only one triplet lifetime in this case i.e. 3A Z 3B reaches equilibrium before they are quenched. A B Case III. If k << 2k. and k << Zk. ———~——-— a l b i l 1 e k << and kb << a TA TB then C = kq{thTB + (l—X)t21A} T + T xt T + (l—X)t T initial slope = A B - 1 B 5 A T X + — ’ + — . PqTATBfi t1 (1 x)tél quth (l x)t2] TATE intercept = quATB Lth + (l—X)t2] slope (TA + TB)[th+(l-X)tzl - xtlTB+(l-X)t2TA DISCUSSION I. Effectscyfo-MethylSubstitutioncniTypeZEIPhotoreactivity Type II quantum yields for g—methylbutyrophenone, g-methyl— valerophenone, and o-methyl Y-methylvalerophenone are low and depend on y-hydrogen lability, as expected if another triplet- state reaction such as enolization (rate = ke) is competitive with Y—hydrogen abstration (rate = kr). The rate constants of type II reaction in benzene and t—BuOH can be calculated based on equation 31. max : _long QII “T kr TT (31) _ long -l — ®T kr (kr + ke) Since type II reaction comes only from the long-lived triplet, lon e g T is the yield of long—lived triplet measured by extrapo— lation of low concentration pentadiene data in a sensitization plot to [pentadiene}—l = 0. Table 14 contains the triplet lifetime data and rate constants for the g-alkyl phenyl ke- tones. Compared to the kr values of the unsubstituted ke— tones,14 the rate decrease produced by the ortho-methyl group is found to be a factor of 3.3. As indicated in Table 12 and 14, the solvent effect of t—butyl alcohol represents not only Pp maximizing, but also mT, kr and ke decreasing. Generally, ke in t—butyl alcohol is only l/S that in benzene. Similar 83 Table 14. Triplet Lifetime and Rate Constants for g-Alkyl Phenyl Ketones 0 A H Rléggtc—CR2R3 CH2R4 84 Ketones R1 R2 R3 R4 10 :{{a 107 “1;“? 85 In Benzene 32 2—CH3 H H CH3 3.4 0 3.0 33 2—CH3 H H CH2CH3 5.6 2 2.9 3g 2—CH3 H H CH(CH3)2 18.0 15 3.0 %% 2,3-(CH3)2 H H CH(CH3)2 4.0 2 2.0 3Q-h 2,4-(CH3)2 H H CH(CH3)2 3.6 1 2.4 3Q-d 2,4-(CD3)2 H H CH(CH3)2 4.9 2 2.3 31 2,5—(CH3)2 H H CH(CH3)2 4.3 1 2.7 3% 2,3,4,5-(CH3)4 H H CH(CH3)2 1.0 0 0.7 3% 2—CH3 CH3 H CHZCH3 4.0 2 1.6 ggb 2-CH3 CH3 CH3 CHZCH3 3.0 1 0.4 $1 2-CH2CH3 H H CHZCH3 5.3 3 2.3 In E-BuOH 3% 2—crI3 H H CH3 0.7 0 0.6 3% 2—CH3 H H CHZCH3 1.5 0 0.7 3% 2—CH3 H H CH(CH3)2 5.7 4 0.8 $2 2,3-(CH3)2 H H CH(CH3)2 0.9 0 0.4 gé-h 2,4-(CH3)2 H H CH(CH3)2 0.7 0 0.5 31 2,5-(CH3)2 H H CH(CH3)2 0.9 0 0.5 3% 2,3,4,5-(CH3)4 H H CH(cH3)2 0.3 0 0.2 Q% 2-CH2CH3 H H CH2CH3 2.0 1 0.7 85 Table 14. continued l/T k k ketone R1 R2 R3 R4 107 S_la 7 -l .7 —l In 3.0 M l,4-Dioxane — l: 33 2 CH3 H H CHZCH3 3.8 2.6 3.2 34 2-CH H H CH(CH ) 18.6 14.6 4.0 mm 3 3 2 akq = 5.0 x 109 M-1 S_l:hibenzene,2.32<109b{J'S-lin thuOH (C. Steel and L. Giering, unpablished work). bAlso undergoes type I cleavage reaction, kI = 0.6 x 107 S—i. decreases in triplet decay rates of g-methylacetophenone in 50 On the other polar solvents were reported by Lindqvist. hand, 3.0 M l,4-dioxane maximizes Pp without significantly changing ® and triplet lifetime values. T The effects of meta-and parafmethyl substitution on the photoreactivity of phenyl ketones have been thoroughly studied by Wagner, Thomas and Harris . 35 It has been shown that a me_t_a_— methyl decreases kr by a factor of 3, which is comparable to the reduction produced by an ortho—methyl. On the other hand, a factor of 8 rate decrease is observed for para-methyl. The fact that ortho—methyl has a rate decrease comparable to that of metafmethyl rather than para—methyl is not without prece— 84,85 . . . . Simple MO calculations85 of exc1ted state electron dent distribution in substituted benzenes indicate selective elec— tron transmission from the 9— and m—positions by electron- donating groups. This ortho-meta transmission in the lowest excited singlet of substituted benzene is obviously in 86 contrast to the ortho-para transmission in the ground state. The comparable rate constant for ortho-methyl and meta— methyl phenyl ketones also indicates that the assumed diffu- sion-controlled values of kqis reasonable. This assumption is further verified by the quenching study of g—methyl y- methylvalerophenone 34 in primary alcohols with various vis- cosities. It has been shown that in moderately viscous alco- is hols the diffusion-controlled rate of quenching, kdif' inversely proportional to viscosity 0, according to equation 80,86 32, a slightly modified Debye equation. As described k = k g dif = 8RT/2000 n (32) in Table 6 and Figure 4, exothermic energy transfer from trip- let 3% is just as ”diffusion-controlled" as for the unhidered valerophenone. This kind of sterically indifferent triplet energy transfer was observed before with d,d-dimethylvalero— phenone.79 Type II quantum yields and kr values demonstrate a pro- gressive decrease as more methyl groups are placed onto the ring, presumably because the energy difference AET between n,“* and N,“* triplet increases with additional methyl groups. It is interesting that no cyclobutanol products were detected for ketones 3% — 36, although cyclobutanols averge 18% of the total products from various unsubstituted phenyl ketones. This decreased tendency for cyclization versus elimination may be due to the steric interaction between the g—methyl group and l,4-biradical intermediates. On the other hand, cyclobutanols are formed during the irradiation of 87 ketones 3% and 40. The increase in the cyclizationzelimina- tion ratio (ketone 33 : 39 : 40 = 0 : 1.4 : 3.3) by d—methyl ’b’b ”\Jr‘v ’L’b substituents indicates a change in the behavior of the l,4- biradicals. The Lewis87 and Wagner79 groups have indepen— dently observed this d—methyl effect on cyclization: elimination ratios. It seems that G—methyl substitution destabilizes the conformation of the biradical which leads to cleavage relative to the conformation which leads to cyclization.79’87 The values of kr’ 2.4 x 107 sec_1 for o—methyl d-methyl— valerophenone 3% and 1.9 x 107 sec—1 for o—methyl d,d—di- methylvalerophenone 40, together with g—methylvalerophenone's kr = 2.7 X 107 sec—l, indicate that d—methyl substitution produces a small decrease in rates of y—hydrogen abstraction. Similarly, a slight decrease in type II reactivity was also observed in the case of 0:,d—dimethylvalerOphenone.76 These decreases are likely due to a weak inductive effect on the n,H* triplet rather than to a steric effect. It is known that strong electron—withdrawing groups on the a—carbon greatly enhance the reactivity of phenyl ketone triplets. 88 ’ 89 Therefore, methyl substitution should produce a week inductive effect in the opposite direction. The estimated quantum yield for type I cleavage of 68, tOgether with 0 and qu values, indicates a rate constant T '1‘) : [—‘(b of. BA 0" T k1 7 -l . , of type I cleavage at 0.58 x 10 sec . In this case, tne formation of a stable tertiary alkyl radical make the 1— cleavage process thermodynamically favorable and enables it 88 to compete with the type II process. The value of kI is some- what lower than that of a,d-dimethylvalerophenone (kI = 1.4 x 107).79 The reduced reactivity of 4% probably is due to the increased H,H* character in the lowest triplet state caused by o—methyl substitution, which is the same reason for lower reactivity in type II processes for these g—methyl substitu- ted phenyl ketones. II. Comparisons between Type II Reaction and Photoenoliza— tion Reaction At first glance the photoenolization process strongly resembles the type II process in that a y-hydrogen migrates to the carbonyl oxygen in the excited state of phenyl ketones. Compounds similar to g—methylvalerophenone 3% upon irradiation can potentially undergo both enolization and type II reaction. Bergmark52 showed that 33 does undergo type II photoelimi- nation in low yield (@max = 0.04). Assuming no significant radiationless decay for 33, Bergmark suggested a selectivity factor of 26 : l for abstraction of o—methyl versus d—methylene hydrogens. Furthermore, Bergmark demonstrated a 30 : 1 pre- ference for g—methyl : y—CH in the corresponding alkoxyl 3 radical. The closeness of these ratios Bergmark argued, affirms the usefulness of the alkoxy radical model in aryl ketone photochemistry and also demonstrates that no kinetic difference can be detected between photoenolization and the photoelimination reaction. Analysis of the kinetic data for 3% reveals two facts: (1) The low type II quantum yield actually results from a combination of low yield of triplet (®T = 0.21), low 89 probability for product formation from the diradical inter- mediate (Pp = 0.16), and competing enolization, not just from the latter as previously assumed.52 (2) The value of ke(2.9 x 107 sec—l) is comparable to that of kr(2.7 x 107 sec—1 ), which is quite different from the 30 : l preference ratio observed in the alkoxy radical system. Additional methyl substitution on the phenyl ring may have effects on both kr and ke' A comparison of kr and ke should provide more information about the mechanism of photo- enolization since the mechanism and kinetics of the type II reaction are well understood.14 For 2,3—dimethy1-, 2,4-di- methyl-,2,3-dimethy1-,enu32,3,4,5—tetramethylyimethylvalero- phenone the additional methyl groups lower kr progressively as shown in Table 14 and discussed in Section I , while ke remains unchanged (2-3 x 107 sec-1). The differences in kr and k8 indicate that different mechanisms might be involved in the photoenolization and type II reactions. This hypothesis is further supported by the following experimental data. First, o—ethylvalerophenone 4% has the same triplet lifetime as o—methylvalerophenone 3% and there is no increase in ke from 3% to 4% even though the C-H bond strength decreases from 3% (primary C—H)to;§1(secondary(}+n. On the other hand, the kr values increases by a factor of 16 when y C—H bond changes from primary to secondary.14 Equally interesting is the behavior of 2,4-dimethylvalerophenone 36h and 2,4-dimethyl-d —valerophenone 36d, which also display 6 comparable ke values. Therefore, there is no significant isotope effect on enolization, while a kH/kD value of 4.8 90 obtains in type II y—hydrogen abstraction.90 The fact that ke is independent of C-H bond strength, the lack of a kinetic isotope effect and the insensitivity of ke to additional methyl groups on the ring together indi- cate that the rate determining step for enolization is not hydrogen abstraction. This conclusion suggests that conformational factors may play an important role in the rate determing step for enolization reactions. Before a detailed correlation is made, it is appropriate first to examine the conformational analysis of these g-substituted phenyl ketones. III. Conformational Considerations Ortho-substituted phenyl ketones can exist both in syn 42a and anti conformations 42b. Montaudo and co-workers91 have studied the conformational preferences of ortho-substi— tuted acetophenones and benzophenones by proton nmr data and dipole moment measurements. The nearly identical chemical shift and dipole moment values of g—substituted ketones and the corresponding unsubstituted ketones suggested that these molecules exist preferentially in the conformation 62a rather than 62b. Conformations such as égb are disfavored in these CH CH 3 O 3 R // ——————> / o .. {—— o . a, \ \\ R 0 42a, syn 63b, anti compounds presumably because the steric interaction between the ortho—methyl and the R group would force the molecules 91 out of planarity. The use of electronic absorption spectra to interpret molecular geometries has been well recognized. 92 Specifically, it has been shown that introduction of a methyl substituent at the ortho position of acetophenone decreases the intensity Of the Wr"* Charge—transfer (La) band.93 The hypsochromic effect of the ortho substituent is explained as steric inhi- bition of resonance.93 Analysis of the uv spectra of the g- substituted ketones employed in this study (Table 9) reveals the following: (1) the La band intensities are essentially the same for g—methylbutyrophenone, g-methylvalerophenone, and g—methyl y—methylvalerophenone. This indicates that 1- substituents do not increase the steric hindrance between the Emmethyl and the side chain of the benzene ring. (2) An additional methyl group at the 3—position, such as in 2,3- dimethyl—, 2,3,4,5-tetramethyl—, and 2,3,S,6-tetramethyl- valerophenone, further depresses the La band intensity. The marked enhancement of the steric interference by meta-substi- tution can be attributed to the so-called buttressing effect. 94 (3) Substitution of methyl at the a position, such as in 9- methyl d—methylvalerophenone and g—methyl d,d—dimethy1valero- phenone, also remarkably depresses the La band intensity, presumably because of the bulkier size of the side chain. The disadvantage of using uv spectra to determine ground state conformations are twofold. First, it is difficult to obtain accurate band intensity measurements. Second, one is employing an energy difference between an electronically ex- cited state and ground state to evaluate a property of the 92 ground state. 13C nmr spectra measurements could avoid these drawbacks since l3C chemical shifts arise from the electronic ground state only and the possibility of overlapping bands is remote due to the characteristic low field resonance of the carbonyl carbon atom. It has been suggested70 that the angle of twist between the C=O group and the benzene ring (0) of a substituted ketone can be calculated using equation 33, where N o. l 05 x 900 000- v900 *c=o 6X is the carbonyl chemical shift of the ketone in question and 600 and 0900 are chemical shift values (carbonyl carbon) of model molecules which exist in planar (0 = 00) and ortho- O gonal (0 = 90 ) conformations, respectively. In this study l-tetralone (0 = 00) and 2,4,6—tri—tert— butylacetophenone (0 = 900) are chosen as the reference mo— lecules. Since acetophenone has previously been used as the model compound for 0 = 0 in a similar study,70 it is inte— resting to compare the 0 values based on the two different models. Table 15 contains the carbonyl carbon chemical shifts (downfield from internal TMS) and the angle of twist 0 for the ketones. There seems to be a good correlation between the angle of twist and steric hindrance to planarity in these ketones. Thus, the angle of twist increases from 00 to 25.40 and 45.9» in roceeding from l-tetralone to 2- P methyl- and 2,3—dimethyl y—methylvalerophenone. Additional 93 Table 15. 13C nmr Chemical Shifts and Angle of Twist O H ' \ C-R R by 2 l Ketone Substituent 6c=o* angle of twist 0 on Ar ppm acetophenone l—tetra~ lone 34 2—CH3 201.5 20.8 25.4 3% 2,3-(CH1)2 205.9 43.1 45.9 36 2,4-(CH3)2 204.2 35.4 38.8 37 2,5-(CH3)2 204.0 34.8 38.2 38 2,3,4,5-(CH3)4 207.1 48.2 50.5 39 2-CH3, d—CH3 208.3 49.2 51.4 40 2—CH3, a,d-(CH3)2 213.8 61.8 63.5 41 2-CH2CH3 205.1 39.2 42.4 46 2,4,6—(CH3)3 211.0 68.0 69.4 47 2,3,5,6-(CH3)4 209.2 65.3 66.5 48 2,3,4,5,6-(CH3)5 209.5 66.2 67.6 4% 2,4,6-(CH3)3, 217.4 70.4 71.6 Ot-CH 3 59 2,4,6-[CH(CH7)2]3 211.9 73.1 74.3 él 2.4,6-(CH3)3, 218.9 87.3 87.8 a ,OL- (CH3) 2 Acetophenone 197.6 0 0b d—tetralone 197.8 0b 0 2,4,6-tri-t— 212.9a 90b 90b butylacetOphenone a from reference 70- b assumed value, 94 d-methyl substitution, such as in g—methyl a-methylvalero- phenone and g—methyl d,d—dimethylvalerophenone also makes the ketones out of plane more, presumably due to the severe steric interference between the g—methyl group and the bulky a-methyl substituted side chain. All of the 2,6-disubstituted ketones are highly twisted with 2,4,6,d,d—pentamethyl substitution resulting in the greatest out of plane twist. It should be noted that the conformational distribution and behavior in the excited state may not be the same as in the ground state and that the alkoxy radical obtained from 52 actually is not a perfect the corresponding hypochlorite model for the excited ketone. The alkoxy radical has a freely rotating acyl group whereas the excited ketones do not, pre— sumably because population of the H*Horbita1 enhances the bond order between the benzene ring and the carbonyl carbon . . . 95 increases the barrier to rotation, and makes the excited state more planar than the ground state. Therefore, the rate of rotation from anti to syn (and vice versa) in the excited ' <——-——> o :l/ \ G \. state would be expected to be slower than that in the ground 95 state. In other words, the rapid equilibrium existing between these conformers in the ground state may not be established in the excited state if some rapid reactions other than rota- tion are available for one or both of the excited conformers. IV. Conformational Effect on Photoenolization This investigation of n—alkyl substituted ketones was undertaken primarily to determine how conformational factors effect the photoenolization reaction and to provide more in— formation about the mechanism and kinetics of this reaction. Previous discussion indicates that hydrogen abstraction is not rate-determining in photoenolization and some other pro- cess must be involved in the rate-determining step. In order to substantiate the involvement of conforma— tional factors in the rate determining step, the sensitiza- tion efficiencies of n—methylacetophenone 4% and 8—methy1-l- tetralone 44 were measured. The difference between 4% and 44 is that 43 can exist both in syn and nnni conformations where- as 44 is fixed in the gyn conformation and can therefore serve as a model for syn-43. It should be pointed out, however, that CH 0 —‘ "W CH CH3 CH 0 ( 3 // 3 / I 3 'l /\{/C\ /C:O o o 3 1 L\\//’ \\V// 4.2-ail: 4469.05}. 44, this nomenclature does not imply two planar conformers but rather two rotational minima in which the ortho methyl and the carbonyl oxygen are either on the same side or on oppo- site sides of the plane perpendicular to the benzene ring and bisecting the para-carbon and the carbonyl carbon. 96 As shown in Figure 16, the sensitization plot for 4% indi- cates the presence of two distinct triplets. Kinetic analysis of this plot, which is similar to that derived by Dalton and l for the triplet formed Synder,96 yields qu values of 158 M- in 21% yield and 1.0 M_1 for a triplet formed in 31% yield. Since the quenching rate is diffusion controlled, as indicated 9 -1 8-1 for s—methyl y-methylvalerophenone, Kq = 5 x 10 M in benzene.97 The two triplets are then calculated to have decay rate of 3 x 107 and 5 x 109 8.1, respectively. The former apparently is the one observed by Lindqvist50 while no subnano— second triplet has been detected before. On the other hand, 8-methyl-l—tetralone 44, displays a linear sensitization plot which indicates a triplet yield of 0.28 and a triplet decay rate of 3 x 109 5’1. If we make the reasonable assumption that only rapid enolization can be responsible for the unusually rapid triplet decay of both 43 and 44, we conclude that the very short—lived triplet of 43 is the syn conformer while the long-lived triplet is the 3231 conformer. The comparable triplet decay rates for 44 and the short—lived triplet of 4% are certainly consistent with this conclusion. Therefore, study of the type II photoelimination of various g££ng_alkyl ketones (32 - 41), together with the sensitization studies of 4% and 44, shows that the triplet state snn£+syn rotation is most likely the rate-determining step in photo- enolization. The following scheme summarizes what we believe to be the mechanism for photoenolization of s—tolyl alkyl 97 CH3 ——-—9 79% ""“"“ 21% \\ O hv hv syn* -—————* ' lanti* m30% 3 3 x 107sec_l 3 kr syn* S anti*‘———* type II ketones. The process competing with normal triplet reactions of the anti triplet is irreversible rotation into a syn con— formation which enolizes so rapidly that other reactions with rate constants 3.107 S_1 cannot compete. The lOO—fold greater reactivity of the n—methyl relative to the y—methylene is . . . . . . . . 9 actually expected on the baSis of their intrinSic labilities and the two frozen rotations in the former, and is close to what Bergmark observe for the n—methyl alkoxy radical.52 Pre- vioualy, Lewis, Johnson and Ruden99 have shown that endo-2-ben- zoylnorbornane has two frozen rotations and has a rate constant of Y-hydrogen abstraction (7 x 109 sec—l) similar to that for syn-4%. The ®$ong values determined from the sensitization study in fact measure the percentage of anti ground states in what 98 is presumably a rapid conformational equilibrium. If this argument holds true, then the more sterically hindered the ketone is, the lower the percentage of EEEE conformers would -1ong be and consquently the lower @T should be. The relatively low Giong values of s—ethylvalerophenone, n—methyl d—methyl- valerophenone, and s—methyl a,a—dimethylvalerOphenone and their relatively large ground state angles of twist, as indicated by their 13C nmr and uv spectra, are consistent with this line of reasoning. The «.107 S_1 rate of rotation in the triplet would indi- cate a barrier of some 8 Kcal, if a normal preexponential fac- tor of 1012—13 is assumed. Because of severe nonbonded inter- actions in the totally planar forms, this barrier would cer- tainly be expected to be smaller than that in the excited benzaldehyde, for which a value of some 20 Kcal has been cal— culated.100 The decrease in ke caused by d-methyl substitu- tion may indicate that the rotation process is slowed down somewhat by interference between the n—methyl.and the bulky d-methyl substituted side chain. It should be noted that the triplet enole has been suggested 37’43 to intervene between the triplet of ketone and by Ullman thegflxnnuistatecnftheeumfl.intjmzphotoenolization11 nsec) andcnuashort-lived VWLJD nsec). n—Methylbenzophenone undergoes photoinduced deuterium incorporation more efficiently than n- methylacetophenone (0D = 0.38), again from two triplets with a total ®T of unity. If deuterium incorporation can serve as a 100 quantitative measure of enol formation, the low quantum yields of both triplet formation and deuterium incorporation in s- methylacetophenone indicate that the majority of singlet reac- tion is strictly quenching rather than enolization. It is quite possible that radiationless decay occurs via partial hydrogen abstraction for the singlet excited state of g—methyl- phenyl ketones,as is now established for singlet state y-hydro— gen abstraction.104’105 As pointed out earlier, ke in EsEE-butyl alcohol is only 1/5 that in benzene. This decrease in triplet decay rate was interpreted50 as being due to solvent-induced shifts in the relative energy levels of the closely spaced n,H* and H,H* configurations. This possibility is very real for the rate 106 decreases in kr' but is unlikely for the long—lived anti triplet since the rate is the measure of snns+syn rotation rather than of hydrogen abstraction and does not change with ring substitution. It is possible that the decrease in ke in nsnn-butyl alcohol is due to the higher viscosity and/or to the bulkier size of the 33:1 triplet resulting from hydrogen bonding between the nsnnsbutyl alcohol and the triplet of ketone. Sensitization studies revealed an approximate five— fold rate decrease for enolization by the syn triplet when the solvent is changed from benzene to Eggs-butyl alcohol. This change presumably occurs because the pepulation of the n,H* triplet state is lowered by a polar solvent. Irradiation of g-methyl m—methyloxyacetophenone 45 gives both the type II cleavage product and oxetanols. The percen- tage yield of cyclobutanol is 15%, which is lower that the 43% 101 reported for d—methoxyacetophenone.76 ching plot of 45 is nonlinear and two qu values of 4.9 M- and 1.4 M_1 are obtained based on eq. The Stern-Volmer quen- 1 23. Notice that both triplets are extremely short-lived(Tl==lrmn T2==0.3ns) and no long—lived triplet is observed. The very rapid y-hydro- gen abstraction for both syn-4% and anti-45 triplets would re- sult in measurable reaction from both conformers. A rate cons- tant of 3. 2 x 109 sec“1 for y-methoxyacetophenone has previously been reported by Lewis and Turro.76 As shown in the following scheme, anti-45 triplet undergoes only type II reaction while CH CH 3 O 3 Chi» —“<-— o A... 3 CH3 0 hv lsyn* 3 7 type 114_____. syn* <3_X 10 109 3 x 109 enol hv l . anti* ' * anti type 11 10 Syn-£5 triplet can undergo both type II reaction and enoliza- tion competitively. Therefore, the longer-lived triplet would C301:respond to anti—43 which gives kr 1.0 x 109 sec _1. The 102 same 3-fold decrease in kr is observed upon n-methyl substi- tution. The shorter-lived triplet would correspond to syn-45 ”b ”b 9 and a ke of 2.6 x 10 sec-liscalculatedtasedCNiequation 34. This value compares favorably with that measured for the 8- methyltetralones. 9 -l 9 -l I" =- : : A 1/ 2 kr + ke 1.0 X 10 S + ke 3.6 x 10 S (3-) Thus, the reactions of n—substituted phenyl ketones are totally controlled by ground state conformations for ketones which undergo type II reaction extremely fast (e.g. 45) whereas rotationally-controlled excited-state reactions take over for ketones which undergo type II reaction with a rate comparable to the rate of rotation(e.g. s—methylvalerophenone). V. 2,6—Disubstituted Ketones In an extensive study of 2,4,6-trialkylphenyl ketones, Kitaura and Matsuura4S noted that cyclobutenol formation was often the preferred course of reaction. In no case could ground state dienols be trapped by dienophiles, but deuterium exchange processes do occur. "Anomalous" photochemical be— havior has also been noted for 2,6-disubstituted benzophe- nones.107’108 No detailed kinetics for the photoenolization of these 2,6-disubstituted ketones have been reported. As far as ground state conformation is concerned, 2,6- disubstituted phenyl ketones are highly twisted, as indicated earlier by 13C nmr and uv spectra. They cannot have syn and snni conformers because of symmetry. Nonetheless, two kine- tically distinct triplets were observed in the sensitization studies. Triplet yields and decay rates for some 103 2,6-disubstituted acetophenones and 8—methyltetralones are listed in Table 16. The long-lived triplets have a decay rate of 5 x 107 sec_1 which is almost double that of mono- substituted acetophenones. The short-lived triplets have a decay rate similar to their tetralone analogs which produce only one short-lived triplet. Table 16. Triplet Yield and Lifetimes for 2,6-Disubstituted Acetophenones and Tetralones. Ketone long-lived ' short—lived iéong l/I,107S_l aihort 1/1~,107s'l Acetophenone 2,4,6—trimethyl- 0.34 5.2 0.28 125 2,3,5,6—tetramethyl- 0.24 5.0 0.20 100 2,3,4,5,6-pentamethyl- 0.26 4.5 0.12 66 l-Tetralone 8-methyl — - 0.28 300 3,3,6,8-tetramethyl- - - 0.26 150 5,6,7,8-tetramethyl- - — 0.14 48 l-Indanone 4,7-dimethy1- - - 0.21 220 For 2,4,6-trimethyl y-methylvalerophenone, 2,3,5,6-tetra- methyl- and 2,3,4,5,6—pentamethylvalerophenone, both aceto- phenone and benzocyclobutenol products are formed (equation 35) . The type II product is readily quenched with diene whereas O 104 (0 We (3)—(3+ off (35) benzocyclobutenol is not. Use of the ketones to photosensitize theisomerization of l, 3-pentadiene also indicates two triplets with lifetimes similar to those of the 2 ,6-disubstituted aceto- phenones. The short-lived triplet with a lifetime of 0.8 ns is probably the precursor of benzocyclobutenol. The rate constant of type II reaction can be calculated based on equation 31. Table 17 contains the long—lived triplet lifetime data and rate constants for these 2,6-disubstituted ketones. Table 17. Long-lived Triplet Lifetime and Rate Constants for 2,6-Disubstituted Ketones in Benzene. 9 i2 ‘33 R1 -C-CH-CH2-CH-CH3 long kSA k Ketone R1 R2 R3 l/T7 —1' 7r_l 7e_l _- 10 S 10 S 10 S 46 4-CH3 H CH3 5.5 0.43 5.1 47 3,5-(CH ) H II 5.0 0.06 4.9 ’L’b .3 2 48 3,4,5-(CH3)3 H H 4.6 0.01 4.6 49 4-CH CH H 1.1 0.09 1.0 ’L’b 3 3 .——- - Therefore, the type II reaction rates constant for ketone 105 46, 41, 48, and 49 is only 1/104, 1/135, 1/800 and 1/90 of the corresponding unsubstituted phenyl ketones. These ratio are actually expected considering the methyl ring substitution effect, i.e. 1/8 for each nsns methyl and 1/3 for each ortho and ESE? methyl group. The ke values for ketone 46, 43 and 48 are close to the long—lived triplet decay rate for the corres- ponding acetophenone obtained from sensitization studies. Although the acyclic ketones have only one triplet ground- state conformer, they may well form two triplets with different twist angles and different lifetimes. The more twised one can undergo type II hydrogen abstraction or rotate with a rate cons- tant 5 x 107 sec-1 to the more planar form, which can readily abstract an ortho benzylic hydrogen. The proposed mechanism for 2,6-disubstituted ketones is summarized in the following scheme. As shown in Table 16, the short-lived triplet decay rate decreaSes as more methyl groups are added to the phenyl ring. The triplet decay rate for 2,3,4,5,6-pentamethylacetophenone is only 1/8 that for 2-methylacetophenone, wheareas a lOO-fold decrease is realized in the type II reaction. The rate de— creases presumably are due to less population of the reactive n,n* triplet state caused by electron-donating methyl group. There seems to be no difference in short-lived decay rate between methyl-substituted l—tetralone and corresponding inda— none indicating the carbonyl group in both ketones has roughly the same proximity to the benzylic hydrogen, i.e. both carbo- nyl groups are fixed in the syn conformation to the enolizable 106 Mi 8* 3m > So ”4% / / 265:, 0* 7 1 x 10 . f i _ 3x100s1 l>10951 W V C / OH O 2 1. 0H // 0H /\ - .—... _-'.-_ —.—.._. , 107 methyl group. This conclusion is supported by the uv spectra, in that both ketones show normalLa band intensities. When the ring size become larger than eight the carbonyl group might start to twist out of plane109 and conformational factors may well be important again. Finally, it is noticable that a ten-fold increase of quantum yield for benzocyclobutenol formation is obtained upon addition of 3 M dioxane. This very large solvent effect can be explained as stabilization by a Lewis base of an interme- diate which leads to cyclobutenol product. This intermediate could either be a hydroxy diradical similar to that involved in type II reaction14 or a dienol species. VI. Summary The studies conducted with the s—alkyl phenyl ketones have significant implications for both photoenolization re- actions and type II reactions. The effect of s—methyl subs— tituents on the type II reaction has been shown to be compa- rable to that of memethyl, in that both lower reactivity by a factor of 3. The steric effect of an g-methyl group on exothermic energy transfer is small as indicated by quenching of s—methyl y—methylvalerophenone being "diffusion—controlled.” Ortho—methyl substitution does not seem to lower rates of type I a-cleavage beyond the factor of three caused by inversion of triplet states. The low type II quantum yields for n—methyl substituted phenyl ketones bearing a y hydrogen result from a combination of low yield of triplet (eT = 0.21), low probability for pro- duct formation from the diradial intermediate (P = 0. 08 - 0. 18) , 108 and competing photoenolization reaction. Analysis of type II quantum yields and triplet lifetimes of. ortho-tolyl alkyl ketones indicates a rate for triplet state enolization of 3 x 107 sec—l, similar to rates which have been measured by flash spectroscopic kinetics. Although the rate of y-hydrogen abstraction decreases as additional methyl group are substituted onto the benzene ring, the rate of triplet state enolization remains unchanged. The rate is also the same 3, and CD3. Sensitization studies reveal that ortho-methylphenyl alkyl ketones yield two triplets with for ortho-ethyl, CH distinctly different lifetimes, while their cyclic analogs, namely 8—methy1 substituted l-tetralones yield only one short— lived triplet. Therefore, this work suggests that syn trip— lets enolize rapidly (k = 4 x 109 sec-l) such that formation of this conformer is rate—determining in enolization of snni triplets. Although the 2,6-disubstituted phenyl ketones probably have only one principal ground state conformation, two kine- tically distinct triplets have been observed in sensitization studies. The long-lived triplet decays with a rate of 5 x 107 sec_1 but does not produce benzocyclobutenol; the short—lived one forms benzocyclobutenol with a rate of m 109 sec—1 for 2,4,6—trimethy1—, 2,3,5,6-tetramethyl—, and 2,3,4,5,6-penta- methylvalerophenone. In conclusion, the photochemistry of nnnnnfalkylphenyl ketones is dominated by conformational factors, in particular the ground state syn/anti ratio and the rate for antiesyn ro- tation in the excited state. Although the importance of 109 ground-state conformations in photochemical processes is now well documented,110 photoenolization is a rare case111 where an excited—state conformational change appears to be rate limiting. VII. Suggestions for Further Research 1. Effects of n—Alkyl Substituents on the Lifetime of l,4-Biradical Intermediates. s—Alkyl substituents effect the l,4—biradicals since 9- methyl phenyl ketones have relatively low probability of pro— duct formation. Determination of the biradical lifetimes should provide valuable information about the origin of such changes. This can be done by comparing the quenching effi- ciencies with which a given thiol quenches type II elimina- tion from valerophenone (VP) and s—methylvalerophenone 3%. The relative biradical lifetimes can be calculated from the ratio of the quenching slopes. Do/¢ ['RSH] 2. n—Methylphenyl d-Diketones Photocyclization of 1-(s—tolyl)-l,2—propanedione 46 to 2—hydroxy—2—methylindanone 47 has been reported.112u114 Pro- duct 47 can either be formed directly by a 1,6—hydrogen migra— tion from the ortho methyl to the excited acetyl carbonyl 110 if 41 oxygen or by a 1,5-hydrogen shift (to give an enol) followed an aldol cyclization. Ogata and TakagillS provided some evi— dence of the intermediacy of the enol by trapping it with di- methyl acetylenedicarboxylate. However, no detailed kinetic data on this reaction have been reported. It would be inteo resting to study this raction by a combination of quenching and sensitization studies similar to that conducted in the monoketone system and to explore the possibility of con- formational involvements in these n—methylphenyl a—diketone molecules. 3. Competition between Photoenolization and Type II Reaction in 2—Alkyl—8-methyl-l-tetralone Investigation of the photochemistry of ketones such as 48-50 would provide information about the competition between ‘5; 65%;, 48 82 28 PhotoenolizationenuitypeIKEreaction. Lewisandco-workers116 have studied the photochemistry of 2—propyl-l-tetralone and measured a rate constant of 5. 9 x 108 sec-1 for type II hydrogen 111 abstraction. Therefore, it is expected that the type II reaction will be able to compete with enolization reaction (3 x 109 sec-l) more efficiently in %8 than in syn—n-meth- ylvalerophenone and even more so in compound 49. In this fashion we can monitor the photoenolization reaction of syn triplet by type II product formation, which is directly measurable. Photochemical study of compound 50 would prove even more beneficial. In this molecule both the benzylic hydro- gen and the side chain y-hydrogen are fixed in the same proximity to the carbonyl oxygen. The rate of type II reaction should be very rapid and the rate difference, if any, between type II reaction and enolization would re- flect the intrinsic lability98 of the benzilic hydrogen. It is also of interest to investigate the phoeokin— etics of 2,4,6-trimethyl g,a—dimethylvalerophenone which would be extremely hindered such that a planar triplet may not be formed at all. 4. Temperature-dependent Studies It has been shown in this study that EEE1+§XE triplet rotation seems to be rate-limiting in photenolization for acyclic s-methyl-substituted ketones. A temperature—depen— dent study on ke would give us a more accurate determination of the barrier to rotation in the excited state. The confor- mational equilibrium between syn and snni conformers is faster in the ground state than in the excited state. Al- though both uv and 13C nmr spectra indicate predominately 112 syn conformations, conformer distribution in the ground state have not been determined quantitatively. Use of dynamic nmr techniques, either through 1H or 13C nmr spec- troscopy, may resolve this problem and display the distinct existence of these two isomers at very low temperature. The ratio and the rate of interconversion of these two isomers at various temperatures can also be derived from such a study. EXPERIMENTAL I. Chemicals 1. Solvents a. Benzene Analytical reagent grade benzene supplied by Mallinckrodt Chemical Works was purified by stirring gallon quantities with 300-ml portions of concentrated sulfuric acid several times. The sulfuric acid was discarded and a new portion added until the acid layer remained colorless. The benzene was then stirred over 10% aqueous sodium hydroxide for 24 hours, washed with saturated aqueous sodium chloride, and dried over anhydrous magnesium sulfate. The benzene was distilled from 100 g of phosphorous pentoxide through a 95 cm column packed with glass helices. A high reflux ratio was maintained, and the central cut (m80%) was collected for use.'fluaboilingp©int was 79.8 t 0.2 C, uncorrected. b. t—Butyl Alcohol The E-butyl alcohol (J. T. Baker Co.) was distilled from freshly cut sodium at atmospheric pressure (bp 82.0 i 0.2 C, uncor.) C. Pyridine "Certified A.C. S. grade” pyridine from Fisher Scientific Co. was distilled from barium oxide and the middle fraction retained. The boiling point was 115.6 t 0.2 C, uncor. 113 114 d. l—Propanol Fisher Scientific Co. l-propanol was distilled from cal- cium hydride at atmospheric pressure (bp 97.8 i 0.2 C, uncor). e. l-Pentanol l-Pentanol (Fisher Scientific Co.) was purified in the same manner as 1—propanol. The boiling point was 138.1 3: 0 .2 C, uncor. f. l—Heptanol Matheson Coleman Bell l—heptanol was purified in the same manner as l-propanol. The boilling point was 176.5 f 0.3 C, uncor. g. l,4-Dioxane "Scintanalyzed" l,4—dioxane (Fisher Scientific Co.) was allowed to stand over ferrous sulfate for two days, then mixed with concentrated hydrochloric acid (5 : 2 ether : acid), and then refluxed at 120 — 130°C for 10 h under nitrogen.117 A resinous material was separated from the cooled liquid by fil- tration and the filtrate was neutralized with sodium carbonate. The l,4-dioxane was distilled twice from sodium hydroxide and once from sodium under nitrogen. Thetmfilinggxfljmzwa5101.4 i 0.2 C, uncor. h. Methyl Alcohol—d Methyl alcohol—d, 99% pure, supplied by Aldrich Co. was used as received. 1. n-Heptane n-Heptane (J. T. Baker Co.) was purified in the same manner as benzene, except on a smaller acale. The boiling point was 98.4 t 0.3 C, uncor. 115 2. Ketones Several methods of preparation were used depending on the availability of the starting materials and the characteristics of the reaction. The general methods employed are listed below with the ketones prepared by that method. The structure of each ketone was verified by its spectroscopic properties. Mass,'H nmr, l3C nmr, ir and uv spectra were obtained on a Hitachi Perkin—Elmer RMU-6 mass spectrometer operated by Mrs. Lorraine A. Guile, a Varian T-60 nmr spectrometer; a Varian CFT—20 nmr spectrometer; a Perkin-Elmer model 237-B infrared spectrometer; and a Cary 17 spectrometer, respectively. The purity of each ketone was carefully checked by VPC analysis. Unless other- wise indicated, all the ketones used were obtained with >99% purity. Method A The ketone was prepared by dropwise addition of 0.12 mole of the Grignard of an s-alkylbromobenzene to 0.1 mole of the appropriate aliphatic nitrile in 200 ml of ether.118 The re- sulting solution of imine salt was carefully stirred into 300 g of ice containing 30 ml of concentrated HCl (4.8 mole). The aqueous layer was extracted with ether to remove any soulble impurities, such as the starting nitrile and biphenyls. The imine hydrochloride was then hydrolyzed to the desired ketone by heating the solution on a steam bath for one hour. The ketone was then extracted into ether and the organic layer was dried over anhydrous magnesium sulfate. The solvent was removed and the ketone was purified by vacuum distillation. For further purification, the ketone was run through a short 116 column of alumina and then vacuum distilled. The following ketones were prepared by this method: a. o-Methylbutyrophenone Made form n—bromotoluene (Eastman) and butyronitrile (Aldrich): bp 93°C (3 Torr); MS (70 eV) m/e 162, 134, 119, 91; 'H nmr (CDC13) 6 0.99 (t, 3 H), 1.74 (m, 2 H), 2.40 (s, 3 H), 2.84 (t, 2 H), 7.38, 7.75 (4 H, aromatic H); ir (neat) 1685 cm-1, 1602 cm-1, 1450 cm- b. o-Methylvalerophenone Made from s—bromotoluene and valeronitrile (Eastman): bp 84°C (3 Torr); MS (70 eV) m/e 176, 148, 134, 119, 91; nmr ) 0 0.99 (t, 3 H), 1.57 (m, 4 H), 2.38 (s, 3 H), 2.58 3 (t, 2 H), 7.38, 7.75 (4 H, aromatic H); ir (CC14) 1683 cm‘l, 1600 cm'l, 1452 cm-1. (CDCl c. o—Ethylvslerophenone Made from n—ethylbromobenzene (Aldrich) and valeroni— trile: bp 134°C (7 Torr); MS (70 eV) m/e 190, 162, 105, 91; nmr (CDC13) 6 1.05 (t, 3 H), 1.30 (t, 3 H), 1.58 (m, 4 H), 2.84 (m, 4 H), 7.34, 7.65 (4 H, aromatic H); ir (CC14) 1680 cm—1, 1602 cm‘l, 1450 cm—1 d. 8—Methy1«l‘tetralone n-Methyl-Y-Chlorobutyrophenone was prepared from s- bromotaluene and 4—chlorobutyronitrile (Aldrich) as described in Method A. bp 126—1290C (5 Torr); nmr 0 2.11 (m, 2 H), 2.37 (s, 3 H), 2.91 (t, 3 H), 3.48 (t, 3 H), 7.12 and 7.44 (4 H, aromatic H). Aluminum chloride (0.05 mole) in CS2 (50 m1) Was added slowly to a cooled solution of the chloroketone (0.1 mOle) in C82 (80 m1). When the addition was complete the 117 mixture was stirred at room temperature for one hour and then poured into a solution of dilute HCl. The mixture was extracted with 125 m1 ether. The extract was washed first with 20 ml dilute aqueous sodium hydroxide and then with water (20 m1) and dried over sodium sulfate. The ether was removed, and 8-methyl- l-tetralone was obtained by vacuum distillation: bp 121°C (3 Torr); MS (70 eV) m/e 160, 145, 132, 119, 91; nmr (CDC13) 6 2.04 (t, 2 H), 2.39 (s, 3 H), 2.92 (m, 2 H), 3.50 (t, 2 H), 1 7.04, 7.16, 7.44 (3 H, aromatic H); ir (CC1 ) 1689 cm‘ , 1606 cm—1, 1446 cm—1 4 e. 4,7—Dimethyl-l—indanone Made from 2,4-dimethy1 3-chlorobutyrophenone which was prepared from p—xylene and 3—chloropropionitrile (Aldrich): mp 46°C; MS (70 eV) m/e 160, 145, 132, 105; nmr (CDC13)0 2.05 (s, 3 H), 2.34 (s, 3 H), 2.69 (t, 2 H), 3.05 (t, 2 H), 7.10, 1 7.25 (2 H, aromatic H); ir (CC1 1690 cm_ , 1604 cm-1, 1435 4) -1 cm . Method B The ketone was prepared by drOpwise addition of 0.12 mole of the Gridnard of 3-methy1butylchloride to 0.1 mole of the apprOpriate benzonitrile in 200 ml of ether. The work-up pro- cedure is the same as described in Method A. The following ketones were prepared by this method. f. o-Methyl y—methylvalerophenone Made:Emnngftolunitrileamui3—methylbutylchloride(Aldrich): bp 98°C (4 Torr); MS (70 eV) m/e 190, 162, 134, 119, 91; nmr (CDC13) 6 0.90 (a, 6 H), 1.59 (m, 3 H), 2.40 (s, 3 H), 2.85 (t, 2 H), 7.31 and 7.85 (4 H, aromatic H); ir (CC14) 1683 cm—{ 118 - —1 1600 cm 1, 1452 cm g. 2,3—Dimethyl y—methylvalerophenone Made from 2,3-dimethy1benzonitrile and 3—methy1buty1- chloride. bp 1460C (6 Torr); MS (70 eV) m/e 204, 176, 161,105; nmr (CDC13) 6 1.05 (d, 6 H), 1.62 Un,3ffl, 2.40 h3,31fl, 2.48 (s, 3 H), 2.83 (t, 2 H), 7.18 and 7.41 (3 H, aromatic H); ir -l -l -l (CC14) 1681 cm , 1604 cm , 1452 cm . Method C The ketone was prepared by Fridel-Crafts acylation of appropriate substituted benzenes. In a typical preparation 0.2 mole of 4—methylvalery1 chloride was slowly added to a cooled solution of msns—xytene (0.2 mole) in 1,2-dichloro— ethane in the presence of aluminum chloride (0.21 mole). When the addition was complete the mixture was stirred at room temperature for an hour and then poured into a stirred solution of ice water and hydrochloric acid. The mixture was extracted with ether. The combined extracts were washed with dilute aqueous sodium hydroxide and then with water and dried. The ether was removed, and the residue was vacuum distilled. The purification method was the same as in method A. The following ketones were prepared by this method: h. 2,4—Dimethy1—y—methylvalerophenone Made from r_n—xylene (Matheson Coleman & Bell) and 4-methyl- valeryl chloride. The latter was prepared from 4—methy1va1eric acid (Eastman) and thionyl chloride (Fisher Scientific Co.) : bp 1220C (3 Torr); MS (70 eV) m/e 204, 176, 161, 133, 105; nmr (CDCl ) 0 0.91 (d, 6 H), 1.59 (m, 3 H), 2.31 (S, 3 H), 2.48 3 (S, 3 H), 2.77 (t, 2 H), /.01, 7.32 (3 H, aromatic H); ir 119 l (CC14) 1685 cm—1, 1600 cm_ , 1450 cm—1 i. 2,4~Dimethyl~dfieY-methylvalerophenone Made from rnxylene-d6 (Mercks) and 4-methylvalery1 chloride: bp 1450C (6 Torr); MS (70 eV) m/e 210, 182, 164, 139, 111; nmr (CDC13) 6 0.93 (d, 6 H), 1.58 (m, 3 H), 2.75 (t, 2 H), 7.01, l, 1600 cm’l'1450 cm'l. 7.32 (3 H, aromatic H);ir(CCl )1685cxm- 4 j. 2,5-Dimethyl—yfmethy1va1er0phenone Made from p-xylene (Mallincrodt) and 4—methylva1ery1 chlo- ride: bp 1460C (5.5 Torr); MS (70 eV) m/e 204, 176, 161, 133, 105; nmr (CDC13) 0 0.90 (d, 6 H), 1.57 (m, 3 H), 2.28 (S, 3 H), 2,38 (s, 3 H), 2.73 (t, 2 H), 7.11, 7.28 (3 H, aromatic H); ir 1 ) 1685 cm_1 , 1448 cm-1 (CC1 , 1601 cm- 4 k. 2,3,4,5—Tetramethyl y—methylvalerophenone Made from 1,2,3,4—tetramethylbenzene (Aldrich) and 4- methylvaleryl chloride: bp 1390C (1 Torr); MS(70 eV) m/e 232, 204, 289, 262, 246, 233; nmr (CDCl 6 0.94 (d, 6 H), 1.58 3) On, 3 PM , 2.18 Us, 6 EU , 2.26 Us, 6 EU , 2.71. (t, 2 EU , 7.01 1 1 l (s, 1 H); ir (CC1 1678 cm‘ , 1602 cm’ , 1441 cm— 4> 1. 2,4,6-Trimethyl y-methylvalerophsnone Made from mesitylene (Matheson Coleman & Bell) and 4- methylvaleryl chloride: bp 114°C (1 Torr); MS (70 eV) m/e 218, 190, 175, 147, 132, 119; nmr (CDC13) 6 0.90 (d, 6 H), 1.55 (m, 3 H), 2.13 (s, 6 H0, 2.21 (s, 3 H),2.68(tq 2H),6.90 (s,22H); . —l -1 —l lr (CC14) 1695 cm , 1598 cm , 1450 cm m. 2,3,5,6-Tetramsthylvalerophenone Madeifixml1,2,4,5-tetramethylbenzene(Aldrich)emuivaleryl chloride (Eastman): bp 130°C (3 Torr); MS (70 eV) m/e 218, 190, 175, 147, 133, 118; nmr (CDCl 0 0.97 (d, 3H), 1.55 (m, 4 H), 3) 120 2.12 (s, 6 H), 2.20 (s, 3 H), 2.81 (t, 2 H), 7.23 (s, 1 H); 1 ir (CC1 1687 cm- , 1598 cm_l, 1450 cm—1, 4) n. 2,3,4,5,6—Pentamethylvalerophenone Made from pentamethylbenzene (Eastman) and valeryl chlo- ride: bp 128°C (2 Torr); MS (70 eV) m/e 232, 204, 189, 161, 147; nmr (CDC13) 0 0.99 (d, 3 H), 1.57 (m, 4 H), 2.14 (s, 6 H), 2.21 (s, 9 H), 2.84 (t, 2 H); ir (CC14) 1685 cm—1, 1600 cm-1, 1450 cm-1. o. 5,6,7,8-Tetramethyl-1-tetralone Made from 1,2,3,4-tetramethy1benzene and 4-chlorobutyry1 chloride: bp 154°C (2 Torr); MS (70 eV) m/e 202, 174, 162, 147, 131, 115; nmr (CDC13) 0 2.22 (m, 13 H), 2.38 (s, 3 H), 3.12 (t, 2 H); ir (CC14) 1689 cm—1, 1604 cm71, 1442 cm—1 p. 2,4,6-Triisepropylacetophenone Made from 2,4,6-trii50propy1benzene (Aldrich) and acetyl chloride (Mallincrodt): mp 580C; MS (70 eV) m/e 246, 218, 203, 0 1.3]. U1, 18 H), 2.05 (S,Z3H), 2.76 00,13H), 1680 cm’l, 1588 cm-1, 1450 cm-1. 160; nmr (CDC13) 7.01 (s, 2 H); ir (CC14) q. 2,4,6-Triisopropy1 Y-methylvalerophenone Made from 2,4,6—triisopropy1benzene and 4-methy1va1eryl chloride: bp 134°C (1 Torr); MS (70 eV) m/e 302, 274, 231, 203; nmr (CDC13) 6 0.99 (d, 6 H), 1.30 (d, 18 H), 1.60 (m, 3 H), . — -1 2.83 (t, 5 H), 7.02 (s, 2 H); 1r (CC1 ) 1680 cm 1, 1586 cm , 4 1452 cm-1 Method D The ketone was prepared by the coupling reaction between a phenyl magnesium bromide and aliphatic acid chloride. The work-up procedure is the same as that used in method B. The 121 following ketones were prepared by this method. r. o—Methyl a-methylvalerOphenone Made from n—bromotoluene and a-methylvaleryl chloride. The latter was prepared by Dr. J. McGrath and was distilled before use: bp 72°C (0.3 Torr); MS (70 eV) m/e 190, 162, 147, 119, 91; nmr (CDCl ) 0 0.6-1.6 (m, 10 H), 2.38 (S, 3 H), 3.32 3 (m, 1 H), 7.02, 7.28 (4EL.aromaticPU, the]l)protonnmdtiplet includes 0 1.05 (d, 3 H, -CH3), 0.91 (t, 3 H, -CH3) and 4 methylene protons; ir (CC14) 1675 cm-1, 1602 cm—1, 1443 cm_l. s. o-Methylunjd—dimethy1valerophenone Made from s—bromotoluene and d,d-dimethylvalery1 chloride. The latter was prepared by Dr. J. McGrath and was distilled before use: bp 74°C (0.3 Torr); MS (70 eV) m/e 204, 176, 161, 119, 91; nmr (CDCl ) 0 0.82 (m, 5 H), 1.21 (S, 6 H), 1.51 (m, 3 2 H), 2.20 (s, 3 H), 7.06 (4 H, aromatic H); ir (CC14) 1670 cm—1, 1598 cm-1, 1450 cm_l' This compound was purified via its oxime, which was prepared by refluxing 0.1 mol of the ketone and 0.75 mol of hydroxylamine hydrochloride in 200 m1 of 10% aqueous sodium hydroxide solution for 2 days. Upon cooling the oxime was obtained as a white solid. The oxime was then purified by recrystallization from ethanol, mp 135- 60C (uncor.). Hydrolysis of 10 g of the oxime by refluxing with 120 m1 of 10 8 aqueous hydrochloric acid for 4 hr was followed by steam distillation. The distillate was extracted with 200 m1 ether. The combined extracts were washed with 20 ml water and dried over 10 g sodium sulfate. The n-methyl 0 a,G-dimethylvalerophenone was then vacuum distilled. bp 74 C (0.3 Torr); MS (70 eV) m/e 204, 176, 161, 119, 91; nmr (CDC13) 122 6 0.82 (m, 5 H), 1.21 (S, 6 H), 1.51 (m, 2 H), 2.20 (S, 3 H), 1 1 7.06 (4 H, aromatic H) : ir (CC1 1670 cm—1, 1598 cm“ , 1450 cm- 4) °The following ketones were purchased and were purified before use: t. o-Methylscetophenons (Aldrich). This ketone was purified by passing through a short column of alumina and then distilled. u. o-Methylbenzophenone (Aldrich). Purification method is the same as that of n—methylacetophenone. v. 2,4,6-Trimethylacetophenone (Aldrich). Purified as above two ketones. w. 3,3,6,8-Tetramethyl-l-tetralone (Aldrich). This ketone was purified by recrystalizing it from methanol twice, mp 58.5-59OC (uncor.). 3. Quenchers a.1,3-Pentadiene When l,3-pentadiene was used in obtaining Stern-Volmer plots, a mixture of the sis and trans isomers was used. Be- fore use, the 1,3—pentadiene (Aldrich Chemical Co ) was run through a 50 cm column of neutral alumina and then distilled. When the isomerization of l,3-pentadiene was studied, sis-1,3- pentadiene (Chemical Samples Co.) was used as received. VPC analysis indiCated that this sis-1 , 3-pentadiene was > 99 . 7% pure. b. 2,5-Dimenhy1-2,4-hexadiene 2,5-Dimethyl-2,4-hexadiene (Chemical Samples Co.) sub- limed in the bottle at refrigerator temperature (%0°C) and atmospheric pressure. The sublimed compound was used. 123 c. 1-Msthylnaphnna1ene l—Methylnaphthalene (Aldrich Chemical Co.) was distilled twice prior to use. The boiling point was 107.3 f 0.5 C (10 Torr). 4. Internal Standards The internal standards used were all high molecular weight alkanes which were previously purified.119 Standard Supplier bp or mp . . 119— Tetradecane (C14) Columbia Organic Chem. 1200C at 10mm Hg Pentadecane (C15) Columbia Organic Chem. 1320C at 10mm Hg Hexadecane (C16) Aldrich Chemical Co. 146°C at 10mm Hg Heptadecane (C17) Aldrich Chemical Co. 158°C at 8mm Hg Octadecane (C18) Aldrich Chemical Co. mp = 29-300C Eicosane (C20) Matheson Coleman & Bell mp = 35-33.50C II. Methods 1. Preparation of Samples for Photolysis Using class A volumetric glassware and an analytical ba- lance sensitive to tenths of milligrams, solutions containing the requisite amounts of ketone, internal standard, quencher, and other additives were prepared at room temperature (about 250). Generally, duplicate 2.8 m1 samples of each solution were syringed into Pyrex photolysis tubes. These photolysis tubes were prepared from carefully sorted and cleaned 100 x 13mm Pyres culture tubes by heating them 3 cm from the neck and drawing out the softened tubes to a length of approximately 18 cm. The samples were attached to the stopcocks of a vacuum line using 1 hole rubber stoppers and degassed by freezing them 124 in liquid nitrogen followed by evacuating them to 1 x 10—3 torr for 30 min; the samples were then isolated from the va- cuum pump by closing the stopcocks and allowed to thaw. This freeze-pump—thaw sequence was carried out two more times, the tubes being sealed with a torch before the final thawing.Each tube was inverted several times after being sealed to ensure adequate mixing. 2. Photolysis The sample tubes were irradiated in parrallel using a water bath immersed merry-go—round apparatus120 to insure that all the samples received the same amount of incident light and that the temperature remained constant (22—24OC). A 450 watt Hanovia medium pressure mercury lamp was used as light source, and the 3130 A region was isolated by a 1 cm path of a 0.002 M potassium Chromate - 1% potassium carbonate aqueous filter solution. The 3660 A line, used in quenching studies with l—methylnaphthalene, was isolated by a set of Corning No. 7083 filters. The samples were generally irra— diated until %5% of the starting ketone was converted to products. 3. Photolysate Analysis a. Gas Chromatography The analysis of products or of the disappearance of re- actants in photolyzed solutions was carried out in all cases by gas-liquid partition chromatography. Two Varian Aerograph gas chromatograph models with flame ionization detecters were used: Hy-Fi III Series 1200; and Hy—Fi Model 600 C. An Infor— tronics Model CRS-208 Automatic Digital Integrator was used to H—hfl— ._.__...._ ____- __._._..__ , 125 integrate peaks. Nitrogen (about 25 ml/min) was used as the carrier gas, and hydrogen (about 25 ml/min) and air (about 250 ml/min) were used for the flame. Ten foot x 1/8" aluminum columns packed with a mixture of 4% QF-l and 1% carbowax 20 M on chromosorb G usually separate the photolysate mixture effectively. A 20' x 1/8" aluminum column packed with 25% l,2,3-tris(2-cyano- ethoxy) propane on 60/80 chromosorb P in an Aerograph Hy~Fi Model 600 D was used to analyze cis—trans isomerization of l,3-penta- diene. Further information regarding the VPC conditions employed to analyze a particular run may be found in the tables of pho— tokinetic data. b. Identification of PhotOproducts The photoproducts, n—methyl—, n—ethyl—, 2,3,-dimethyl-, 2,4—dimethyl—, 2,5—dimethy1—, and 2,3,4,S—tetramethylaceto- phenone were identified by comparing their retention time with authentic compounds on analytical VPC columns. Two pho— toproducts were formed from irradiation of s—methyl d-methyl— valerophenone: (l) s—methylpropiophenone was identified by comparing retention time on analytical VPC columns with authen— tic sample. (2) Cyclization product, the cyclobutanol, was identifiedemsl-(n—methylphenyl)—2,4-dimethy1cyclobutanolvflfixfli had the following spectral data: 0 2.28 (s, 6 H), 1.04 (d, 6 H), 1.64 (d, 2 H), 2.87 (m, 2 H), 3.20 (s, 1 H), 7.25 (m, 4 H); ir (CC1 3485 chl. Peaks in the VPC traces of the photolysates 4) cflfn—methyl(ipa—dimethylvalerophenoneINN:assignedeatimzparent ketone or to n—methylphenyl E—butyl ketones were assumed to be 126 the cyclobutanols on the basis of their expected and observed proximity to the parent ketone peak. The photoproduct of 2,4,6-trimethylacetophenone was iso- lated and identified as 2,4-dimethy1benzocyclobutenol based on the following spectral data: IR (Nujol) 3250 cm-l(OH); nmr (CDCl 5 6.68 (s, 2, aromatic protons), 3.15 (g, 2, ~CH2), 3) 2.27 (s, 3, benzylic -CH3), 2.22 (s, 3, aromatic CH3) and 1.64 (s, 3, aliphatic CH ). Mass spectrum (70 eV): m/e 162 (parent 3 peak). Two products were formed during the photolysis of 2,4,6-trimethyl—, 2,3,5,6-tetramethyl—, 2,3,4,5,6-pentamethy1- valerophenone: (1) Type II photoproducts, 2,4,5—trimethyl-, 2,3,5,6-tetramethyi—, 2,3,4,5,6-pentamethylacetophenone, were identified by comparing retention times on analytical VPC co- lumnSTVith authentic compounds which were independently syn- thesized. (2) Cyclization products, the benzocyclobutenols, were identified by their IR spectra (3250 cm-1, —OH) and by their VPC retention times relative to the parent ketone peak. Two products were formed during the irradiation of s— methylahethoxyacetophenone4éa Zibenzemesolutioncontaining 0.5 g ii was irradiated for 2 days with a 450-W Hanovia medium— pressure mercury vapor lamp through Pyrex. The products were separatedenuicollectedkn/preparative\nxioneiS’><3/8"column packed with 20% SE 30 on Chromosorb W at 1650. The first pro— duct was identified as s—methylacetophenone which had an nmr spectrum identical with that of a commercial sample. The se- cond product was identified as 1-(s—methylphenyl)-1-hydroxy- 3-oxetane from the following spectral data: nmr (CDC13) 6 2.42 127 (s, 3 H), 3.72 (s, 1 H), 5.21 (s, 4 H) and 7.24 (m, 4 H); MS (70 eV) m/e 146, 91; ir (CC14) 3480 cm'1 c. Standardization Factors for Internal Standards In order to obtain the concentrations of the various pho- toproducts by VPC, known concentrations of internal standards were used in the photolysis solutions. To determine the con— centration of a photoproduct P, the response of the VPC de- tector to a given measured concentration of P was compared to the response to a similar concentration of internal standard, S. Thus, a standardization factor, SF was calculated accord— ing to equation 25. = [P] area S peak SF [S] X area P peak (25) In the deuterium incorporation experiments 0.1 M degassed solutions of s—methylacetophenone in CH3OD With various con— centrations of l,3—pentadiene were irradiated at 313 nm for 2 hr. After the irradiation was complete the tubes containing the photolysate were broken at the top and CH3OD was let eva- porate in the hood. The mass spectral was then taken for each tube sample. The toluene ion peak (m/e = 91) was used as re- ference peak for the determination of deuterium incorporation based on the following equation o,_r_n._:il__ri_:.l_ NILE- °D“(m) (m) +2L(m) E H.0 ”hypeeocho< 0.ma mm0.0 m000.0 00.0 00.0 HH.> Hm0.0 em00.0 m0m.0 m0.0 000.m m.ea 0N0.0 0000.0 em.0 00.0 mm.0 0m0.0 m000.0 Hem.0 00.0 00m.H e.mH mm0.0 vm00.0 em.0 H0.0 00.0 >m0.0 v000.0 mmm.0 00.0 000.H 0.0a 0m0.0 m000.0 mm.0 00.0 00.m ve0.0 0e00.0 00m.0 m0.w 00m.0 0.0 m00.0 0000.0 0m.0 00.0 om.v 0m0.0 m000.0 Hmm.0 00.0 00N.0 m.m m00.0 0ma0.c 0m.0 00.0 mm.m 000.0 0HH0.0 00m.0 m0.0 m0a.0 >.N HVH.0 mem0.0 00.0 na.m 0e.m 000.0 mma0.0 00m.0 H0.0 Hm0.0 H Hmm.0 m000.0 «0.0 e0.0 H mmm.0 e0m0.0 me0.0 0m.0 000.0 .mp5 How scepMHpmpuH popmm 0 0 0m.0 00.m 0 0 em.0 00.0 0 coHpmepmpHH oHOMmm Q , D e o z E e - 8 ice ....:. .1.) a 3 .Ce.;. ..em;. . ye o 1,N+s H+z me e are. m+z H+z 8.00 Amaanzv ococmsdoNsmpa>cpezlo Amaauzv ecocecQOpeomH>£pmzlo ecocmcmoNcenamnmeHm Use ecocmLQOpoomamcpezlm 2 00.0 CH coflpMHOQHoocH Esflumpsmo I’I'llllll’ 4" .40m maflme 0 x 0012\0 + 20 u 12\0 + 200 + 0022\0 + 20 I 2\0 + 200 u o 0 @0030 :0 CO0pe0pmpMH 0:0050 poEHOM maowox popQMOpseo Q eco0pmpcemlm.0m )0 0 000.0 Hassoc donors 144 0 mm.o n Hes .mo.m n .s.m 000.0 u e0o\msocoreouoo< 00:000 E 0000.0 + ococozmouo0m> E 0.0 ”0pmeOC0po< 0.00 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 0.00 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 0.00 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 0.00 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 00.0 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 00.0 0000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 00.0 000.0 0000.0 00.0 00.0 00.0 000.0 0000.0 00.0 00.0 000.0 0 000.0 0000.0 000.0 00.0 0 000.0 0000.0 00.0 00.0 0 .mp: 00 How c00000tmnp0 Hmp0< 0 0 00.0 00.0 0 0 00.0 00.0 0 :00p8006000 eyewom {WWI e s 2 0e 0 : z , .lllll .lalli .4 0 .llhll I!!! me 200. 0+: MHE me e nae. 0+: 0+2 8000 000020 ecocezmoNch0wsmeIO 000020 ecocmSQOpeDM0mcpozlo ocosozmowcep0>£pozlm ppm msocosm000000>£umznm 2 00.0 :0 CO0pmuomHooC0 ESHMmpsmo .000 @0280 145 Table 35. n—Methyl a-methoxyacetophenone in Benzene with 1,3- pentadiene as Quencher {Quencher}, M Ci: 3:2: 2::: {AP} 6 ®:: 0.104 1.284 0.0306 0.170 1 0.251 0.769 0.0183 0.102 1.67 0.502 0.568 0.0135 0.075 2.26 0.753 0.447 0.0107 0.059 2.87 1.004 0.395 0.0094 0.052 3.25 1.506 0.333 0.0079 0.044 3.86 2.008 0.283 0.0068 0.038 4.53 2.510 0.247 0.0059 0.033 5.20 photon count: 0.180 g 1‘:L Internal standard: 0.0121 M C SF = 1.97 015; Analytical conditions: Column A at 1350C —— u- 146 Table 36. g-Methyl d—methylvalerophenone in Benzene —-—— “--————— .m“ ___ ————"—_ éggitiye AP peak area CB peak areaa ©CB CB‘ {AP‘ cen C peak area C peak area 4 ’ " J tration 16 20 CB 0 0.456 0.723 1 0.0014 0.001 [Quencherb}, M 0.010 fi~ 0.332 2.18 0.0006 ——— 0.020 —— 0.204 3.54 0.0004 ——— 0.030 —— 0.160 4.52 0.0003 ——— 0.040 ‘— 0.123 5.90 0.0002 —~— {l,4—Dioxane}, M 1.0 2.055 2.745 ———- 0.0053 0.005 2.0 2.574 3.112 ~—— 0.0060 0.006 3.0 2.827 3.408 ———- 0.0066 0.006 Photon count: 0.201 g 1—1 Internal standard: 0.00124 M C16; SF = 1.77 0.00118 M C SF = 1.65 20’ Analytical conditions: 10% QF—l, 1% carbowax 1550c (column D) at aType II cyclization product, cyclobutanoles. bl,3-Pentadiene. 147 .Hoflzuocmoopouola Q .Aémv ooxcmpHMNcmoawzuoEIN .HUSGOHQ H wm>em AmU .HO o ..| ~ON .4 o no om H 1 mm . o r moaoo o Ao< Home mm.H H mm “mao z NHHoo.o Afluflpp< 83 .mm GHQGB mcomcmm Ce ow mcoumm wo mflmxaouosm 148 Table 38. 2,4,6—Trimethyl y-methylvalerophenone in Benzene Additive AP peak area BCBa peak area fég [APIX [BCB}X concentration Cl8 peak area C2O peak area ®AP 103 103 0 0.218 1.251 1 0.41 2.29 [Quencherb], M 0.0023 0.181 1.260 1.205 0.34 2.30 0.0046 0.152 . 1.248 1.432 0.29 2.28 0.0069 0.134 1.253 1.628 0.25 2.29 0.0092 0.119 1.241 1.828 0.22 2.27 0.0115 0.108 1.240 2.012 0.20 2.27 [Dioxane], M 2.0 1.357 9.47 2.55 17.33 3.0 1.591 10.18 2.99 18.63 photon count: 0.1116 g 1—1 Internal standard: 0.00105 M C SF 1.80 for AP 18’ SF 0.00125 M C 1.47 for BCB 20’ Analytical conditions: Column A at 1450C -—— —~—— aCyclization product, benzocyclobutenol. b2,5-Dimethyl-2,4—hexadiene. 149 Table 39. 2,3,5,6-Tetramethylvalerophenone in Benzene Additive 5:21: 2:2: 5:21: 2:2: :2. “P“; [.3ng tration 18 '18 'AP 10 10 0 0.0952 5.825 1 0.168 8.43 [Quencherb], M 0.0031 0.0728 5.816 1.307 0.128 8.42 0.0062 0.0577 5.827 1.651 0.102 8.43 0.0093 0.0471 5.814 2.023 0.083 8.41 0.0124 0.0432 5.808 2.203 0.076 8.40 [1,4—Dioxane}, M 2.0 0.594 51.8 1.05 74.9 3.0 0.714 53.6 1.26 77.5 Photon count: 0.4215 C 1—1 Internal standard: 0.00108 M C18; SF = 1.64 for AP SF 1.34 for CB . . 0 Analytical conditions: Column A at 153 C ——— “*— aCyclization product, benzocyclobutenol. b2,5-Dimethyl—2,4—hexadiene. 150 Talbe 40. 2,3,4,5,6-Pentamethylvalerophenone in Benzene ‘ 0 Additive AP peak area BCBa peak area @AP [AP1x [BCB]X Concen— C2O peak area C20 peak area @AP 103 103 tration 0 0.0374 0.293 1 0.075 0.449 b. {Quencher ;, M 0,0042 0.0258 0.291 1.449 0.052 0.441 0.0084 0.0193 0.290 1.941 0.039 0.440 0.0126 0.0162 0.293 2.310 0.032 0.449 0.0168 0.0129 0.287 2 898 0.026 0.439 2,0 0.0949 2.984 0.190 4.57 3.0 0.1122 3.174 0.225 4.86 t _ -l Photon count: 0.3750 g 1 Internal standard: 0.00118 M C20, SF = 1.70 for AP SF = 1.30 for CB . 0 Analytical conditions: Column B at 158 C aCyclization product, benzocyclobutenol. b 2,5—Dimethy1—2,4uhexadiene. 151 Table 41. Stern-Volmer quenching of Benzocyclobutenol Forma- tion from 2,4,6—trimethylacetophenone in Benzene Irradiated at 366 nm [Quenchera], M _%CB Peak areab EBCB_ 18 peak area ”BCB 0 0.945 1 0.204 0.532 1.775 0.408 0.425 2.224 0.612 0.319 2.958 0.716 0.292 3.233 a1-Methylnaphtha1ene. bBenzocyclobutenol formation. Table 42. Stern—Volmer Quenching of Benzocyclobutenol For— mation from 2,4,6—triisopropylacetOphenone in Benzene Irradiated at 366 nm (Quenchera1, M BCB peak areab '?%CB C20 peak area PBCB 0 1.475 1 0.174 1.283 1.150 0.348 1.169 1.261 0.522 1.001 1.475 0.696 0.941 1.568 0.879 0.857 1.721 a b . 1—Methylnaphtha1ene. Benzocyclobutenol formation. 152 Table 43. 2,4,6-Trimethylpivalophenone in Benzene . . o 888666;:ation 318 [BA] X 10 ® 7%“ 0 0.874 6.81 .0909 1 [Quencherb], M 0.010 0.109 0.86 .0114 7.96 0.020 0.056 0.44 .0059 15.48 0.030 0.041 0.32 .0042 21.47 0.040 0.031 0.24 .0032 27.94 [RSHC1, M 0.01 1.767 13.77 .1838 0.02 1.768 13.78 .1839 Photon count: 0.0749 5 1—1 Internal standard: 0.00402 M C18; Analyatical conditions: 4% QF—l. SF 1% carbowax at 1450C aType I product, 2,4,6-trimethy1benzaldehyde. bl,3-Pentadiene. C1-Dodecanethiol. 153 Table 44. 2,4,6-Trimethyl d—methylvalerophenone Additive AP peak area BCB peak area {APIX IBCB}x©gp concentration C18 peak area C20 peak area 103 7AP 0 0.197 3.427 0.35 5. 1 {Quenchera], M 0.0011 0.138 3.490 0.25 5. -424 0.0022 0.103 3.421 0.18 5. .903 0.0033 0.086 3.409 0.15 5. .276 0.0044 0.071 3.394 0.13 4. .778 {1,4—Dioxane], M 2.0 1.098 32.96 1.95 48. 3.0 1.204 35.72 2.14 52. 4.0 1.206 35.71 2.14 52. Photon count: 0.1936 g 1"1 Internal standard: 0.00112 M C18; SF = 1.58 for AP 0.00108 M C20; SF 2 1.36 for BCB Analytical condition: Column A at 155 C —-— ___———— -——_—_—.—_—_ a2,5-Dimethy1-2,4—hexadiene. 154 II. Sensitization Studies Table 45. Various Ketones with 0.2 M cis—1,3—pentadiene in Benzene Ketone [c-P] % t-P {t—P}a , M 0 O corr. T 02 0.202 2.28 0.00472 0.208 33 0.198 2.39 0.00477 0.210 ’L’L 34 0.201 2.38 0.00479 0.211 WW 35 0.198 1.86 0.00370 0.163 %% gé-h 0.203 2.57 0.00522 0.230 36-d 0.202 2.54 0.00515 0.227 mm 37 0.210 2.11 0.00445 0.196 mm 38 0.208 1.73 0.00361 0.159 %% 41 0.214 1.29 0.00277 0.122 mm Acetophe- 0.207 9.45 0.0227 1.00 none Analytical conditions: 25' x 1/8" aluminum column packed with 25% 1,2,3-tri(2-cyanoethoxy)propane on 60/80 chromosorb P at 580C (column C) aCorrected for back reaction according to: % t—P corr. 0.55 0.55 — 8 t—P - UDCOEL. 0.55 1n 155 Table 46. Various Ketones with 0.15 M cis—1,3—pentadiene in E-BuOH Ketone [c—P1O % t—P {t-P180rr.’ M QT 32 0.198 2.81 0.00558 0.110 3% 0.204 2.71 0.00553 0.109 34 0.206 2.65 0.00547 0.108 3% 0.194 2.48 0.00482 0.095 éé—h 0.189 3.79 0.00725 0.143 36-6 0.196 3.28 0.00654 0.129 37 0.203 2.79 0.00568 0.112 38 0.206 2.26 0.00466 0.092 41 0.211 1.70 0.00359 0.071 Acetophe— 0.198 20.50 0.0507 1.00 none Analytical conditions: Column C at 580C a . Corrected for back reaction. b0.1 M in benzene. Table 47. 0.05 M o—Methylacetophenone in Benzene 156 [c—P}—l, M71 9 t—P a [t-P), M 19_§§§_ corr. 0 c+t 0.202 0.60 0.0297 2.21 0.410 1.10 0.0269 2.44 0.503 1.22 0.02433 2.70 0.801 1.74 0.02176 3.02 0.998 1.93 0.01938 3.39 1.981 3.33 0.01685 3.90 2.994 4.45 0.01487 4.42 4.371 6.08 0.01392 4.72 6.012 8.08 0.01344 4.89 7.658 10.18 0.01330 4.94 8.383 10.99 0.01311 5.01 9.182 11.88 0.01293 5.08 10.012 12.90 0.01288 5.10 1 Photon count: 0.1184 5 1- Analytical concitions: As 1_‘~ in Table 45 aCorrected for back reaction and also for ..5_ the increases in isomerization quantum yield above 1 M diene. 157 Table 48. 0.05 M o—MethylbenZOphenone in Benzene {c-Pl"l % —p (t-p], M 0.555 corr. 5 ' 'c t 0.387 1.88 .0486 1.165 0.597 2.59 .0435 1.302 0.789 3.06 .0388 1.458 0.992 3.39 .0342 1.653 1.984 5.33 .0269 2.102 2.330 5.41 .0232 2.436 2.586 5.34 .0206 2.744 4.285 8.22 .0192 2.950 6.092 11.21 .0184 3.074 7.986 14.27 .0179 3.168 8.404 14.65 .0174 3.248 8.694 15.08 0173 3.263 10.042 15.66 0156 3.631 Photon count: 0.1020 E 1‘1 Analytical conditions: As in Table 45 _ —..——__—-‘—~_———._— 158 Table 49. 0.05 M g—Methylvalerophenone in Benzene and E—BuOH [c-97773 Benzene 7.7 t—BuOH — M_1 8 t—p {t—p], M 0&555 8 t-p [t-P],M 05555 c+t cat 0.404 0.76 0.0190 2.266 0.24 0.0058 4.120 1.006 0.90 0.0091 2.667 0.52 0.0052 4.601 1.598 1.22 0.0076 3.133 0.78 0.0048 4.932 3.196 1.88 0.0059 4.064 1.21 0.0038 6.293 4.461 2.30 0.0052 4.639 1.42 0.0032 7.533 5.840 2.83 0.0048 4.933 1.70 0.0029 8.227 7.203 3.38 0.0047 5.094 1.89 0.0026 9.119 8.406 3.87 0.0046 5.201 2.11 0.0025 9.533 9.912 4.22 0.0046 5.213 2.26 0.0025 9.706 10.02 4.54 0.0045 5.279 2.45 0.0024 9.799 photon count: 0.0431 g 1’1 Analytical conditions: As in Table 45 .—-- .-—_. _ 159 Table 50. 0.05 M g—Methyl y—methylvalerOphenone in Benzene [c—P1—l, M-1 8 —p a [t—P], M —9;§§§— corr. ®c+t 10.02 5.29 0.00528 5.28 15.91 7.07 0.00444 6.27 31.92 9.87 0.00309 9.01 49.02 10.91 0.00223 12.52 63.73 11.99 0.00188 14.81 79.81 13.24 0.00166 16.79 98.04 13.38 0.00136 20.41 Photon count: 0.0502 i 1—1 Analytical conditions: As in Table 45 a 1 . Corrected ror back reaction. 160 Table 51. o—Ethylvalerophenone in Benzene [c-P], M_1 8 t-pcorr [t—p), M 0&555 c+t 0.201 1.31 0.0653 3.782 0.394 2.37 0.0602 4.102 0.411 2.31 0.0561 4.401 0.975 4.16 0.0427 5.788 1.725 5.78 0.0335 7.374 3.402 10.20 0.0299 8.236 5.001 14.44 0.0289 8.550 7.182 19.94 0.0278 8.894 8.179 22.39 0.0273 9.021 9.004 23.97 0.0266 9.274 9.946 26.99 0.0271 9.102 Photon count: 0.445 g 1—1 Analytical conditions: As in Table 45 ——~ -__.‘ ____._. 161 Table 52. o-Methyl d-methylvalerophenone (39) and o—Methyl “J d,a—dimethylvalerophenone (4w) in Benzene 1 -1 Ketone 39 Ketone 40 ;' [C7P] ’M 8 t—p [t—p},M 05555 8 t—p [t-p],M 03553 cct cct 0.202 0.86 0.0426 3.204 0 69 0.0391 4.002 0.501 2.01 0.0401 3.402 1.42 0.0284 4.803 1.004 3.10 0.0310 4.398 2.18 0.0217 6.301 2.002 4.71 0.0235 5.802 2.97 0.0149 9.189 2.704 4.80 0.0177 7.694 2.98 0.0110 12.392 4.598 7.38 0.0161 8.502 4.72 0.0103 13.301 6.104 8.85 0.0145 9.411 5.79 0.0095 14.402 7.805 10.87 0.0139 9.796 7.24 0.0093 14.702 8.405 11.47 0.0136 10.001 7.64 0.0091 14.996 9.152 12.36 0.0135 10.110 8.33 0.0091 15.001 10.004 13.38 0.0134 10.205 8.98 0.0089 15.204 Photon count: 0.246 E 171 Analytical conditions: As in Table 45 162 Table 53. 2,4,6—Trimethy1 Y—methylvalerophenone (46) and 2,3,4,5,6-Pentamethylphenone (48) in Benzene 1-1 —1 Ketone 46 Ketone 48 _ [C—P’ ’M 8 t-P [t-p],M 0°555 8 t—P {t—P],M 0-555 corr. ®c+t ©c+t 0.402 1.16 0.0289 1.884 0.81 0.0203 2.684 0 581 1.58 .0272 .998 1.13 .0194 2 802 0.989 2.56 .0259 .101 1.85 .0188 2.901 1.201 3.03 .0253 .153 2.19 .0183 2.976 2.002 4.59 .0229 .372 3.41 .0170 3.189 4.004 4.81 .0195 .786 6.05 .0151 3.602 6.006 10.94 .0182 .984 8.71 .0145 3.752 8.003 14.09 .0176 .089 11.02 30138 3.950 9.989 16.96 .0169 .204 13.51 .0135 4.022 10.568 17.70 .0167 .248 14.20 .0134 4.047 11.201 18.74 .0167 .251 15.01 .0134 4.058 Photon count: 0.098 E 1- Analytical conditions: As in Table 45 163 Table 54. 8-Methy1-1-tetralone (51) and 5,6,7,8—Tetramethy1—1— tetralone (52) in Benzene r 1-1 -1 Ketone 51 1- Keotne 52 _ ‘C-PJ ’M 8 t--pCorr {t—p},M 05555 8 t—p [t-p],M 0&355 ° c+t c1; 0.801 1.75 0.0218 5.667 1.28 0.0159 7.766 0.987 2.04 0.0206 5.999 1.52 0.0154 8.033 1.582 2.79 0.0177 7.001 2.34 0.0148 8.349 1.994 3.22 0.0161 7.667 2.84 0.0142 8.694 2.601 3.39 0.0130 9.490 3.59 0.0138 8.945 3.372 3.79 0.0113 10.998 4.39 0.0130 9.499 5.752 4.24 0.0074 16.799 6.33 0.0110 11.233 7.801 4.66 0.0060 20.702 7.63 0.0098 12.658 -1 Photon count: 0.223 E 1 Analytical conditions: As in Table 45 164 Table 55. 2.4.6-TriiSOpropy1 y—methylvalerophenone in Benzene fc-Pl-l, M71 8 t—P Et-P), M 0°555 corr. (DC->1: 0.315 0.55 0.0175 18.62 0 682 0.88 0.0129 25.27 1 269 1.09 0.0086 37.79 1.921 1.16 0.0060 53.92 2.802 1.27 0.0045 71.85 3.784 1.30 0.0034 94.98 4.903 1.38 0.0028 116.01 I ’7 -l PhOton tount: 0.58/ E 1 Analytical conditions: As in Table 45 -—_ 165 Table 56. Reciprocal Quenching Study for the Unknown Product Formation from 2,4,6-Trimethy1acetophenone in Benzenea - _ Y- - d) iQuencherb},M fQuencherf 1,M l P“OdUC: giaZrzged pro- -1 ’18 ye duct 0 09 0 00 0.4265 2.344 0.00816 .00139 716 0 5541 1.805 0.01031 ,00175 513 0 6559 1 325 0.01244 ,00213 471 0 9467 1.056 0.01899 .00324 308 a . . . . 0.95 M ketone, 313 nm irradiation. 1,3~Pentadiene as quencher. OGRAPHY 10. 11. 12. 13. 14. 15. 16. 17. 18. BIBLIOGRAPHY U) A. Jablonski, Z. Phy ik, a“, 38(1935). J E. F. Ullman, Accts. Chem. Res., 1, 353(1968). C. H. Bamford and R. G. W. Norrish, J. Chem. Soc. , 1531(1938) . H. —G. Heine, Justus Liebigs Ann. Chem., 732, 165(1970). U P. Yates, Pure Appl. Chem., k6, 93(1968). J P. Yates and R. 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