A __’_._ ,._.____ may; PIES: 25¢ per day per item RETUMI’G LIBRARY MTERIALS: ”act in book return to name charge from circulation record: CHARGE TRANSFER INTERACTIONS IN THE PHOTOREDUCTION OF PHENYL KETONES BY Allen Edward Puchalski A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1980 O/Mp/JJ ABSTRACT CHARGE TRANSFER INTERACTIONS IN THE PHOTOREDUCTION OF PHENYL KETONES BY Allen Edward Puchalski This work provides experimental results that make it possible to combine two different pathways in the photo- reduction reaction into one general scheme. This scheme involves competing rate constants for the initial inter- action of excited ketones with substrate, and the sub- sequent reactions that lead to products. The initial in- teractions are direct hydrogen abstraction and exciplex formation. The selectivity of the reaction is shown to be related to the competition between these modes of inter- action and to the extent of charge-transfer interactions in the exciplex. The inefficiency of the reaction is dis- cussed as a combination of exciplex decay and radical disproportionation. The selectivity of the reaction was studied as a function of primary versus tertiary abstraction from p- cymene. For studies involving ring substitution of aceto- phenone, benzophenone cu: a,a,a-trifluoroacetophenone and for a-substitution of acetophenone a consistent pattern Allen Edward Puchalski emerged. This pattern showed an increase in the fraction of primary abstraction as the electron deficiency of the ketone increased. This is interpreted as the result of exciplex involvement in the reaction. This appears to be true for acetophenone as well as a,a,a-trifluoroaceto- phenone, even though exciplex may provide only a minor path to products for acetophenone. The reactions of the radicals formed in the photo- reduction reaction were modeled by generating the radicals independently using di-t-butyl peroxide. This resulted in almost identical product distribution for the two methods of radical formation, showing that cage reactions are un- important in the photoreduction reaction. The formation of disproportionation products in the peroxide experiments failed to account for all of the observed inefficiency of the photoreduction of either acetophenone or d,a,a-tri- fluoroacetophenone by alkylbenzenes. This suggested a source of inefficiency in these reactions prior to radical formation. The photoreduction of ketones by alcohols was shown to involve interaction of the excited ketone with the hydroxy group of the alcohol. For the reaction of aceto- phenone with l-phenylethanol the deuteration of the hydroxy group led to a decrease in the rate constant and an increase in the efficiency. This shows that interaction with the proton of the hydroxy group provides a quenching mechanism Allen Edward Puchalski for acetophenone. Propiophenone is shown to interact with acetophenone pinacol to give propiophenone pinacol and acetophenone. This is also explained by hydrogen abstrac- tion from the hydroxy group. The rate of hydrogen atom exchange from l-hydroxy-l-phenylalkyl radicals to ground state ketone was also studied using the photoreduction of ketones by alcohols. I would like to dedicate this thesis to my wife, Maureen, for her support and patient understanding. I would also like to dedicate this work to my parents, Helen and Walter Puchalski, for their constant encouragement and support. iiv ACKNOWLEDGEMENTS I wish to thank Professor Peter J. Wagner for his support in conducting this research and for sharing his knowledge and experience with me. I would also like to thank the members of the Wagner research group for many informative discussions. I would like to thank the Michigan State University Department of Chemistry for its financial support and the use of its facilities. Finally, I would like to express my gratitude to the National Science Foundation for their support through research assistantships from Dr. Peter J. Wagner's research grants. iii TABLE OF CONTENTS LIST OF TABLES . . . . . . . . . . . . . . . . LIST OF FIGURES . . . . . . . . . . . . . . . LIST OF SCHEMES . . . . . . . . . . . . . . . LIST OF ABBREVIATIONS . . . . . . . . . . . . INTRODUCTION 0 C O O I I O O O O O 0 O O O O 0 Historical Background-Early Product Studies Early Mechanistic Studies . . . . . . . . Kinetics . . . . . . . . . . . . . . Research Goals . . . . . . . . . Approach Used to Accomplish Goals . . MSULTS O I O O O O O O O O O O O O O I O O 0 Product Identification . . . . . . . Quantum.Yields . . . . . . . . . . . Quenching Studies . . . . . . . . . . Photoreduction of Ketones by p-Cymene Kinetics of Hydrogen Abstraction . . Hydrogen Abstraction from p-Cymene by t-Butoxy Radicals . . . . . . . . . Effects of Solvents on the Photoreduction Reaction . . . . . . . . . . . . Attempt to Maximize Quantum Yield of Photoreduction . . . . . . . . . . . Effects of Charge-Transfer Quenchers . . Effects of Acid . . . . . . . . . . . Generation of Radicals from di-t-Butyl Peroxide . . . . . . . . . . . . Photoreduction by Alcohols . . . . . . . DISCUSSION 0 O . C . O O C C C C O O O O C . . Selectivity of the Photoreduction Reaction Mechanistic Implications of Selectivity . Solvent Effects on Selectivity . . . . Fate of Radicals . . . . . . . Photoreduction by Alcohols . . Charge-Transfer Quenchers . . . iv Page vii xiv xvii xviii 11 16 17 21 21 23 24 24 38 47 48 52 56 64 70 76 90 90 103 105 110 116 CONCLUS ION O I O O C O O O O O O 0 Suggestions for Further Investigation . . EXPERIMENT“; O O O I O I O O O O 0 Preparation and Purification of Solvents . . . . . . . Acetonitrile Benzene . . . t-Butanol . . Pyridine . . Trifluoroacetic Internal Standards Undecane . . . Tetradecane . Pentadecane . Hexadecane . Octadecane . Nonadecane . Heneicosane . Docosane . . Quenchers . . . e 00.0w... 0 oooooooooP.ooo Q: Napthalene . . . DABCO . . . . . . p-Dimethoxyb nzene Hydrogen Donors . . . Toluene . . . . . . p-Cymene . . . . . p-Xylene . . . . . Cumene . . . . . . l-Phenylethanol . . l-Phenylethanol-O-d l-Phenylpropanol . l-Phenyl-2,2,2-Trifl o o Reactants . . . . . . Valerophenone . . . Acetophenone . . . p-Fluoroacetophenone m—Fluoroacetophenone u m-Trifluoromethylacetophenone p-Methoxyacetophenone p-Methylacetophenone Benzophenone . . . . 4,4'-Dimethoxybenzophenone 4,4'-Dichloroacetophenone . a,a,a-Trifluoroacetophenone r m-Trifluoromethyl-TFA . p-MethYI-TFA o o o 0 Chemicals t a o O O O O :0 O O O 0 O O O O O O O O O O O I I O O O O O O O O O oooomooooo coo00500000000000.0000.0000000 no...ooooooooof—loooooooooooooo Page 119 120 125 125 125 125 125 125 125 126 126 126 126 126 126 126 126 126 126 126 126 127 127 127 127 127 127 127 127 127 128 128 128 128 128 128 129 129 129 129 129 129 129 129 129 129 Page p-Methoxy-TFA . . . . . . . . . . . . . 129 m-Methyl-TFA . . . . . . . . . . . . 129 p-Chloro-TFA . . . . . . . . . . . . 130 d,a-Dif1uoroacetophenone . . . . . . 130 a-Fluoroacetophenone . . . . . . . . 130 Di-t-Butyl Peroxide . . . . . . . . . 131 t-Butyl Hypochlorite . . . . . . . . 131 Identification of Photoproducts . . . . 132 Bibenzyl . . . . . . . . . . . . . . 133 Bixylyl . . . . . . . . . . . . . . . 133 Dicumyl . . . . . . . . . . . . . . . 133 Acetophenone Pinacol . . . . . . . . 133 a,a,a-Trifluoroacetophenone Pinacol . 133 1,2-Diphenyl-2-propanol . . . . . . . 133 l,1,l-Trifluoro-Z-pheny1-3-methy1-3- (4-methylpheny1)-2-butanol . . . 133 l,l,l-Trifluoro-Z-phenyl-3-(4-i30- propylphenyl)-2-propanol . . . . 133 1,2-Bis-(4-isopropy1phenyl)-ethane . 133 1-(4-isopropy1phenyl)-2-(4- methylphenyl)-2-methylpropane . . 134 2,3-Bis-(4-methylphenyl)-2,3- dimethylbutane . . . . . . . . . 134 Techniques . . . . . . . . . . . . . . . . . 134 Glassware . . . . . . . . . . . . . . . 134 Preparation of Samples . . . . . . . . . 135 Degassing Procedure . . . . . . . . . . 135 Irradiation Procedure . . . . . . . . . 135 Analysis . . . . . . . . . . . . . . . . 136 Calculations of Quantum Yields . . . . . 137 Sample Calculation . . . . . . . . . . . 138 LIST OF “FEENGS O O O O O O O O O O O O O O O O 1 3 9 APPENDIX 0 O O O O O O O O O O O I O O O O O O O O 14 4 vi Table 1. 10. 11. 12. 13. 14. LIST OF TABLES Comparison of Rate Constants for Triplet Ketones and t-Butoxy Radicals . . . . . . Results for Reaction of Indicated Ketone with p-Cymene . . . . . . . . . . . . . . Maximum Quantum Yields for Reaction of TFA and p-Cymene o o o o o o o o o o o o o 0 Results of Photoreduction of Indicated Ketone with p-Cymene . . . . . . . . . . Results of Stern-Volmer Quenching of Acetophenone . . . . . . . . . . . . . . Results of Stern-Volmer Quenching of a-Fluoroacetophenone . . . . . . . . . . Results of Stern-Volmer Quenching of a,a-Difluoroacetophenone . . . . . . . . Results of Stern-Volmer Quenching of a,a,a-Trifluoroacetophenone . . . . . . . Rate Constants for Ketones from Stern— VOlmer Studies . . . . . . . . . . . . . 1 B Effect of Acetonitrile on the Reaction of TFA and Toluene in Benzene . . . . . . . Results from o; versus Toluene"1 . . . Variation of Product Ratiosa with p-Cymene (BH) Concentration, Reaction with TFA . . Product Distributions from Acetophenone and Various Substrates . . . . . . . . . Effect of Trifluoroacetic Acid (TEA) on Products from Indicated Ketone and p-Cymene O O O O O O O O I O O O O I O 0 vii Page 26 26 39 4O 40 40 41 46 46 50 51 S3 69 Table Page 15. Effect of Trifluoroacetic Acid (TFAA) on the Reaction of TFA and p-Cymene in Acetonitrile . . . . . . . . . . . . . . . 69 16. Quantum Yields for the Reaction of TFA and Toluene with Trifluoroacetic Acid (0.05M) in Benzene . . . . . . . . . . . . 71 17. Products from the Reaction of t-Butoxy Radicals with Toluene and l-Phenylethanol in Benzene . . . . . . . . . . . . . . . . 74 18. Products from the Reaction of t-Butoxy Radicals with Toluene and 1-Phenylethanol in Acetonitrile . . . . . . . . . . . . . 74 19. Products from the Reaction of t-Butoxy Radicals with Indicated Substrate and l-PhenYIethanOI O O O O O O O O O O O O O 74 20. Products from the Reaction of t-Butoxy Radicals with Indicated Substrate and l-Phenyl-Z,2,2-Trif1uoroethanol in Benzene 75 21. Quantum Yields for the Reaction of Propiophenone (PP) and l-Phenylethanol (APHz) o o o o o o o o o o o o o o o o o o 78 22. Quantum Yields for the Reaction of Acetophenone (AP) and 1-Pheny1propanol (PPHZ) O O O O C O O O O O O O O O C O O O 78 23. Results for Reaction of Acetophenone (AP) and Propiophenone (PP) with 2-Propanol in Benzene O O O I C O C O O O O O O O O O 8 o 24. Results for Reaction of Acetophenone (AP) and Indicated Ketone (K) with 2-Propanol in Benzene . . . . . . . . . . . . . . . . 82 25. Results from the Reaction of Propiophenone (PP) with Acetophenone Pinacol ((APH) 2) or l-Phenylethanol (APH2 ) . . . . . . . . 87 26. Effects of a-Substitution on Primary- Tertiary Ratio . . . . . . . . . . . . . . 91 27. Effects of Ring Substitution on Primary- Tertiary Ratio . . . . . . . . . . . . . . 91 viii Table Page 28. Rate Constants for Photoreduction . . . . . 92 29. Product Coupling Ratios-Photoreduction versus Peroxide Induced Reaction . . . . . 107 30. Disproportionation Results from Peroxide Experiments, Acetophenone and Toluene . . . 107 31. DisprOportionation Results from Peroxide Experiments, TFAH2 in Benzene . . . . . . . 109 32. Rate Constants for Charge Transfer Quenching of TFA and Toluene . . . . . . . . . . . . 117 33. Quantum Yield Data for Acetophenone and p-Cymene (BH) in Benzene . . . . . . . . . 145 34. Quantum Yield Data for a,a-Dif1uoroaceto- phenone and p-Cymene (BH) in Benzene . . . 147 35. Quantum Yield Data for a,a,a-Trifluoroaceto- phenone and p-Cymene (BH) in Benzene, Hydrocarbon Products . . . . . . . . . . . 149 36. Quantum Yield for a,a,d-Trif1uoroaceto- phenone and p-Cymene (BH) in Benzene, Alcohol Products . . . . . . . . . . . . . 150 37. Quantum Yield Data for a,a,a-Trifluoroaceto- phenone and p-Cymene (BH) in Acetonitrile . 151 38. Material Balance for the Reaction of a,a,a- Trifluoroacetophenone and p-Cymene (BH) in Benzene O O O O I O O O O O I I O O O O O O 152 39. Quantum Yield Data for AcetOphenone and Toluene (BH) in Benzene . . . . . . . . . . 153 40. Quantum Yield Data for a-Fluoroacetophenone and Toluene (BH) in Benzene . . . . . . . . 154 41. Quantum Yield Data for a,a-Dif1uoroaceto- phenone and Toluene (BH) in Benzene . . . . 155 42. Quantum Yield Data for a,a,a-Trifluoroaceto- phenone and Toluene (BH) in Benzene . . . . 156 43. Stern-Volmer Data for Acetophenone and .SM TOluene C O O I O O O O O O O O O O 0 O O O l 57 Table Page 44. Stern-VOlmer Data for Acetophenone and 1 0 0M Teluene I O O C O O O O O O O O O O O l 5 7 45. Stern-Volmer Data for Acetophenone and 1.5M Toluene . . . . . . . . . . . . . . . 158 46. Stern-VOlmer Data for Acetophenone and 2.0M Toluene . . . . . . . . . . . . . . . 158 47. Stern-Vblmer Data for Acetophenone and 2. SM TOIuene O O O O O O O O O I O O O O O 159 48. Stern-Volmer Data for a-Fluoroaceto- phenone and 0.40M Toluene . . . . . . . . 159 49. Stern-Volmer Data for a-Fluoroaceto- phenone and 0.80M Toluene . . . . . . . . 160 50. Stern-volmer Data for a-Fluoroaceto- phenone and 0.94M Toluene . . . . . . . . 160 51. Stern-VOlmer Data for a-Fluoroaceto- phenone and 1.50M Toluene . . . . . . . . 161 52. Stern-VOlmer Data for a-Fluoroaceto- phenone and 1.87M Toluene . . . . . . . . 161 53. Stern-Volmer Data for a,a-Difluoroaceto- phenone and .SM Toluene . . . . . . . . . 162 54. Stern-VOlmer Data for a,a-Difluoroaceto- phenone and 1.0M Toluene . . . . . . . . . 162 55. Stern-VOlmer Data for a,a-Difluoroaceto- phenone and 1.5M Toluene . . . . . . . . . 163 56. Stern-Volmer Data for a,a-Dif1uoroaceto- phenone and 2.0M Toluene . . . . . . . . . 163 57. Stern-Volmer Data for a,a-Dif1uoroaceto- phenone and 2.5M Toluene . . . . . . . . . 164 58. Stern-Volmer Data for a,a,a-Trifluoroaceto- phenone and .SM Toluene . . . . . . . . . 164 59. Stern-Volmer Data for a,a,a-Trif1uoroaceto- phenone and 1.0M Toluene . . . . . . . . . 165 60. Stern-Volmer Data for a,a,a-Trifluoroaceto- phenone and 1.5M Toluene . . . . . . . . . 165 x Table Page 61. Stern-VOlmer Data for a,a,a-Trifluoroaceto- phenone and 2.0M Toluene . . . . . . . . . 166 62. Stern-VOlmer Data for a,a,a-Trifluoroaceto- phenone and 2.5M Toluene . . . . . . . . . 166 63. Product Ratios from Reaction of Indicated Ketone with p-Cymene in Benzene . . . . . 167 64. Effect of Acetonitrile on Photoreduction of TFA by Toluene in Benzene . . . . . . . 168 65. Comparison of Products from Photoreduction of Acetophenone and Reaction of di-t- Buty1 PerOXide O O O O O O I O I O I O 0 O 168 66. Reaction of t-Butyl Hypochlorite (ROCl) and p-Cymene in Benzene . . . . . . . . . 169 67. Reaction of di-t-Butyl Peroxide and p-Cymene in Benzene . . . . . . . . . . . 169 68. Effect of Pyridine on the Photoreduction of TFA by Toluene in Benzene . . . . . . . 170 69. Effect of Pyridine on the Photoreduction of TFA by Toluene in Acetonitrile . . . . 171 70. Reaction of TFA and Toluene in Acetonitrile Quenched by DABCO . . . . . . . . . . . . 172 71. Reaction of TFA and Toluene in Benzene Quenched by DABCO . . . . . . . . . . . . 173 72. Reaction of TFA and p-Cymene in Benzene Quenched by DABCO . . . . . . . . . . . . 174 73. Reaction of TFA and Toluene in Aceto- nitrile Quenched by p-Dimethoxybenzene (Q) 175 74. Reaction of TFA and Toluene in Benzene Quenched by p-Dimethoxybenzene (Q) . . . . 175 75. Reaction of Acetophenone and Toluene in Acetonitrile Quenched by p-Dimethoxy- benzene (Q). . . . . . . . . . . . . . . . 176 76. Effect of Trifluoroacetic Acid on the Re- action of TFA and p-Cymene in Aceto- nitrile Run 1 . . . . . . . . . . . . . . 177 xi Table Page 77. Effect of Trifluoroacetic Acid on the Reaction of TFA and p-Cymene in Aceto- nitrile, Run 2 . . . . . . . . . . . . . . 178 78. Effect of Trifluoroacetic Acid on the Reaction of TFA and p-Cymene in Benzene . 179 79. Effect of Trifluoroacetic Acid on the Photoreduction on Acetophenone by p-Cymene in Benzene O O O O O O O 0 O O O O O O 0 I l 8 o 80. Effect of Trifluoroacetic Acid on the Photoreduction of TFA by Toluene (BH) in Benzene . . . . . . . . . . . . . . . . 180 81. Reaction of di-t-Butyl Peroxide (ROOR) with Toluene and 1-Pheny1ethanol (APHZ) . 181 82. Reaction of di-t-Butyl Peroxide with l-Phenylez,2,2-Trifluoroethanol (TFAH ) and Indicated Substrate (BH) in Benzefie 182 83. Quantum Yield Data for the Reaction of Propiophenone (PP) and 1-Pheny1ethanol (APHZ) in Benzene . . . . . . . . . . . . 183 84. Quantum Yield Data for the Reaction of Acetophenone (AP) and l-Phenylpropanol (PPH2) in Benzene . . . . . . . . . . . . 184 85. Quantum Yield Data for the Reaction of Both Acetophenone (AP) and PrOpio- phenone (PP) with Z-Propanol in Benzene . 185 ‘86. Reaction of Acetophenone (AP) and Indicated Ketone (K) with 2-Propanol in Benzene . . . . . . . . . . . . . . . . 186 87. Quantum Yield Data for Acetophenone and l-Phenylethanol (APHZ) in Benzene . . . . 187 88. Quantum Yield Data for the Reaction of Acetophenone with l-Phenylethanol (PhEtOH) and 1-Pheny1ethanol-O-d (PhEtOD) in Acetonitrilea, Run 1 . . . . . . . . . 188 89. Quantum Yield Data for the Reaction of Acetophenone with 1-Pheny1ethanol (PhEtOH) and 1-Phenylethanol-O-d (PhEtOD) in Acetonitrilea, Run 2 . . . . . . . . . 189 xii Table 90. 91. 92. Page Quantum Yield Data for TFA and 1-Pheny1- ethanol (APHZ) in Benzene . . . . . . . . . 190 Data for the Reaction of Pr0piophenone (PP) with Acetophenone Pinacol ((APH)2) or 1- Phenylethanol (APHZ) in Benzene . . . . . . 191 Quantum Yield Data for Formation of Acetophenone (AP) from the Reaction of a-Fluoroacetophenone with 2-Propanol . . . 192 xiii Figure l. 10. 11. 12. LIST OF FIGURES Results from.Reaction of AP and p-Cymene, PP Formation . . . . . . . . . . . . . . . Results from Reaction of AP and p-Cymene, PT (0) and TT (A0 Formation . . . . . . . Results from Reaction of DFA and p-Cymene in Benzene, PP (0), PT (+), and TT (A) Formation O O O O O O O O O O O O O O O 0 Results from Reaction of TFA and p-Cymone in Benzene, PP (o) and PT'(AQ Formation . Results from Reaction of TFA and p-Cymene in Benzene, TT Formation . . . . . . . . . Results from Reaction of TFA and p-Cymene in Benzene, KP (o) and KT (A) Formation . Results from Reaction of TFA and p-Cymene in Benzene, Formation of Pinacol 6+), Hydrocarbon (o) , and Cross-Coupled (A) Products . . . . . . . . . . . . . . . . . Results from Reaction of TFA and p-Cymene in Acetonitrile, PT (0) and PP 0A) Formation . . . . . . . . . . . . . . . . Results from Reaction of TFA and p-Cymene in Acetonitrile, TT Formation . . . . . . Results from Reaction of TFA and p-Cymene in Acetonitrile, KP (o) and KT (A) Formation I O O O O O O I O O O O C O O 0 Results from Reaction of TFA and p-Cymene in Acetonitrile, Hydrocarbon (A0 and Cross- Coupled (0) Product Formation . . . . . . Lifetimes of AP as a Function of Toluene Concentration . . . . . . . . . . . . . . xiv Page 27 28 29 30 31 32 33 34 35 36 37 42 Figure 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. Lifetimes of MFA as a Function of Toluene Concentration . . . . . . . . . . . . . . Lifetimes of DFA as a Function of Toluene Concentration . . . . . . . . . . . . . . Lifetimes of TFA as a Function of Toluene Concentration . . . . . . . . . . . . . . Effect of Pyridine on Reaction of TFA and Toluene in Benzene. Results for BB 6+), BK (A) ' and B. (.) O O O C O O O I O O O PYridine Quenching of the Reaction of TFA and Toluene in Acetonitrile. Results for BB ( . ) and BK (A) o o o o o o o o o o o o DABCO Quenching of the Reaction of TFA and Toluene in Acetonitrile. Results for BB (0), BK (+), and Pinacol (A) . . . . . . Effect of DABCO on the Radicals B- (o) and K- (40 Observed in the Reaction of TFA and Toluene in Acetonitrile . . . . . DABCO Quenching of the Reaction of TFA and Toluene in Benzene. Results for BB (0), BK (+) p B ‘ (A) o o o o o o o o o o o o o DABCO Quenching of the Reaction of TFA and p-Cymene in Benzene. Results for Hydro- carbon (0) and Cross-Coupled (+9 Products and Benzylic Radicals Q3) . . . . . . . . DABCO Quenching of the Reaction of TFA and p-Cymene in Benzene. Results for Primary (0) and Tertiary Radicals (A0 . . . . . . Quenching of the Reaction of TFA.and Toluene in Acetonitrile by p-Dimethoxy- benzene. Results for BB (0) and BK (A0 . Quenching of the Reaction of TFA and Toluene in Benzene by p-Dimethoxybenzene. Results for BB (0) and BK (A0 . . . . . . . . . . Quenching of the Reaction of AP and Toluene in Acetonitrile by p-Dimethoxybenzene . . XV Page 43 44 45 55 57 58 60 61 62 63 66 67 Figure Page 26. Results from Reaction of AP and l-Phenyl- ethanol in Benzene, Formation of Pinacol . 83 27. Results from Reaction of TFA and l-Phenyl- ethanol in Benzene, Formation 0f TFA PinaCOJ. O I O 0 O O O O O O O O O O O O O 84 28. Pinacol Formation from the Reaction of AP with Deuterated (A0 and Undeuterated (o) l-Phenylethanol . . . . . . . . . . . . . 85 29. Acetophenone Formation from the Reaction of a-Fluoroacetophenone and 2-Propanol . . 89 30. Potential Energy Diagram for the Photo- reduction Reaction . . . . . . . . . . . . loo xvi Scheme 1. 2. LIST OF‘SCHEMES Page Steps of the Photoreduction Reaction . . . 12 Possible Modes of Interaction of Excited Ketone with Substrate . . . . . . . . . . 15 Reaction Paths for Ketone with p-Cymene . 18 xvii PPH2 m-F-AP p-F-AP m-CF -AP 3 p-Me-AP m-Me-TFA p-Me-TFA m-CF3-TFA p-MeO-TFA p-Cl-TFA 4,4'-Me-BP 4,4'-MeO-BP 4,4'-C1-BP LIST OF ABBREVIATIONS Acetophenone l-Phenylethanol a-FlouroacetoPhenone a,a-Dif1uoroacetophenone a,a,a-Trifluoroacetophenone l-Phenyl-Z,2,2-Trifluoroethanol Benzophenone Propiophenone 1-Phenylpropanol m-Fluoroacetophenone p-Fluoroacetophenone m-Trifluoromethylacetophenone p-Methylacetophenone m-Methyl-a,a,a-Trifluoroacetophenone p-Methyl-d,a,a-Trifluoroacetophenone m-Trifluoromethyl-a,a,a-Trifluoroaceto- phenone p-Methoxy-a,a,a-Trifluoroacetophenone p-Chloro-a,a,a-Trifluoroacetophenone 4,4'-Dimethy1benzophenone 4,4'-Dimethoxybenzophenone 4,4'-Dichlorobenzophenone xviii INTRODUCTION The photoreduction reaction was first observed around the turn of the century and has periodically been the focus of intense interest ever since. This continued renewal of interest is due to an ever increasing knowledge of excited state processes and also advances in techni- ques which allow one to perform measurements that had pre- viously not been possible. New theories of excited state processes can not only increase the understanding of the photoreduction reaction, but the photoreduction reaction can be a valuable tool in developing and testing these theories. The primary goal of this research project has been to study the photoreduction reaction to see how it behaves with respect to theories of excited state behavior that are of current interest. The research focused on new ways to investigate the reaction that would result in the measurement of new parameters. These parameters could then be used to study individual steps of the reaction, leading to a more complete understanding of the overall process. The particular areas of excited state reactivity that are of current interest involve the interaction of excited molecules with ground state molecules to form excited complexes and the role of electron transfer in the formation of these complexes. Historical Background-Early Product Studies The first activity in the area of photoreduction started with the report of Ciamician and Silberl that the action of sunlight on a mixture of benzophenone and ethanol formed a precipitate identified as benzpinacol. This initial report led to the investigation of Other possible hydrogen donors, such as Paternd and Chieffi's study of hydrocarbons.2 They studied both aliphatic hydrocarbons such as pentane and decane, and alkylbenzenes such as toluene, ethylbenzene, cumene, p-xylene, diphenylmethane, and p-cymene. For the reaction of benzophenone with p- cymene, a compound that plays an important role in the work described here, the only products other than benz- pinacol that were observed were unsaturated hydrocarbons and resin. However, some products incorporating the hydrogen donor were identified in other systems, such as 1,1,2,2-tetraphenylethanol from the reaction of benzo— phenone and diphenylmethane and 2,3-dipheny1butane from ethylbenzene. These products are indicative of the types that are expected from the photoreduction reaction. The products are formed by coupling of the two radicals pro- duced in the initial hydrogen abstraction step: OH 0 " hv I AICR + BH -——h-ArCR + Bo OH OH OH OH I II I ArCR + B°-—-§- Arc—CAI“ + ArCB + BB R R R 3 4 Alcohols ' 3 and hydrocarbons are not the only hydrogen donors that have been used. Work has also been 5-11 tributylstannane,12 ethers,l3’14 16,17 18 done with amines, 9'15 These sulphides, mercaptans, and phenols. compounds, however, often lead to products that are dif- ficult to analyze or that undergo further reactions and complicate analysis and kinetic studies. Early Mechanistic Studies The mechanism of the reaction has been the subject of much interest. Bodenstein19 first suggested that the absorption of light by the carbonyl produces a biradical structure which abstracts a hydrogen atom from the donor to give a pair of radicals: 0° OH O " hv BH PhCPh ——-> PhCPh —> PhCPh + B- The predominant or exclusive formation of pinacol derived from the ketone with little, if any, crossed pinacol and pinacol derived from the alcohol was observed 1'2'20-22 Weizmann20 suggested in a number of studies. that this was due to the relative stability of radicals, some giving predominantly coupling products, others lead- ing to disproportionation. Pitts et a1.21 Proposed the exchange of a hydrogen atom from a hydroxy radical to ground state ketone to account for the high yield of benz- pinacol and acetone when benzophenone was photoreduced by 2-propanol: OH OH O 0 II | | || PhCPh + MeCMe ——pPhCPh + MeCMe 23,24 This exchange is a common reaction but usually goes unnoticed when the donor is a hydrocarbon since the only exchange between a hydroxy radical and ketone is a degen— erate one. Another major step in the understanding of the re- action came when excited states were characterized as 25-28 either singlets or triplets. The reactivity of com- pounds could then be correlated with this information. * * Whether the excited state is n, n or H," was shown to have a bearing on the course of the reaction.”-32 Photoreduction and the Norrish Type II reaction both involve abstraction of a hydrogen atom and have served as valuable probes in this particular respect. Both of these probes have led to the conclusion that n,w* triplets are normally much more reactive than w,w* toward abstraction of hydrogen atoms. They have also shown that n,w* triplets exhibit reactivity parallel to alkoxy radicals.3’33-36 This has been particularly evident for the comparison of t-butoxy radicals and the photoreduction of benzophenone and acetophenone (see Table 1). Not only are the relative reactivities very similar, but the absolute rate constants for these ketones are very close to those of t-butoxy radicals. These parallels break down for excited ketones that are significantly electron deficient and are believed to react through initial forma- tion of complexes which have strong charge-transfer char- acter to them.37'38 These excited state complexes, called exciplexes, do not exhibit the same trend in rate constants as alkoxy radicals. Alkoxy radicals and the triplets of acetophenone and benzophenone show reactivity patterns that correlate with carbon-hydrogen bond strength.33"35 Rate constants for more electron deficient ketones correlate better with the ionization potentials of donors38 than with carbon-hydrogen bond strengths. The major evidence for an intermediate complex being formed with electron deficient ketones is the study of a,a,d-trifluoroacetophenone (TFA) with toluene-a-d3 by I... 2... m5 8% -I E: 82 ox: I..- n..- ed 2: .8: n} ma m.m .3 m.~ m.m Rm III III Com Mocfl .2 NJ 2.4 m.~ nmcocozmoumomouosumwnalo.a.a nosocwsmoumod mosocmnmoNsom mamowpom wxousmlu A m z acne. T T m .mm moswumwwmn . m M OOCOHQM 0mm Houpmcncom Hocmmoumum Hosanna mcoamuwmwz mcoeso mcmncmnahnum mcwsaoa oumuumnsm mamowomm axouamlu com monouox uoamwua now museumsoo ovum mo semwuomsoo .H manna Wagner and Leavitt.38 They found the rate constants were the same for the reaction of TFA with toluene and toluene- a-d3, but the efficiency of the reaction changed signi- ficantly. This shows that the rate determining step does not involve breaking of a carbon-hydrogen bond. It also shows that there is a subsequent reaction involving car- bon-hydrogen bond breaking which is in competition with a decay mode. This competition is necessary to explain the deuterium isotOpe effect on product formation. For the reaction of acetophenone and toluene there is a deuterium isotope effect on both rate constants and quantum yields of product formation.38'39 The primary isotope effect on the rate constant shows that carbon-hydrogen bond breaking is involved in the rate-determining step of the reaction with acetophenone, unlike TFA. This can be interpreted as direct hydrogen abstraction. There also appears to be a deuterium isotope effect on the maximum quantum yield 38'40 This for the reaction of acetOphenone with toluene. suggests that there is a pathway that competes with direct hydrogen abstraction. If there is only one type of interaction the rate constant would vary, but the maximum efficiency should not change significantly. In fact the efficiency might be expected to increase if disproportionation were the major source of inefficiency since there would be a deuterium isotope effect for back hydrogen transfer. The exciplex proposed for the reaction of TFA with alkylbenzenes had a significant amount of charge- transfer to it. Weller41 studied the quenching of aromatic hydrocarbons by amines, a process which is due to an electron transfer from the amine to the excited hydro- carbon. He was able to relate the rate constants to the reduction potential of the acceptor, the ionization potential of the donor, and the energy of the excited state. Other work soon followed with ketones being quenched by 9'10'42'43 The ketones showed a much smaller change amines. in quenching rate constants as a function of donor ionization potential than observed for the hydrocarbons. Both systems have been shown to reach the diffusion con- trolled limit for quenching with donors of low enough 39'41'44'45 Amines are not the only 9,46-48 ionization potentials. compounds that can act as electron transfer quenchers. The major requirement is that the compound be easily oxidized. Therefore, it is not surprising that alkyl- 38 There will be a competi- benzenes can form complexes. tion between complex formation and other processes, such as direct hydrogen abstraction, with the relative rates of the two processes determining which one predominates. What happens once these complexes are formed is a major concern of this research project. 41 The treatment of Weller relates the rate con- stant for quenching to the change in free energy for electron transfer. To do this he assumed the change in free energy of activation was proportional to the change in free energy for the reaction and that the change in free energy of the reaction is equal to the change in either the excited state reduction potential of the acceptor, or the oxidation potential of the donor. While other workers49'50 have suggested different equations for relating the free energy of activation to the free energy of the reaction, most of them predict similar results for a wide range of free energy changes. These relationships hold very well when the reaction is a pure endothermic electron transfer reaction. When an exciplex with only partial charge-transfer is formed, which is the rule for triplet ketones,10 the change in free energy for reaction is no longer equal to the change in free energy for electron transfer. The relationship between free energy of exciplex formation and free energy of electron transfer for these systems depends on the contribution from charge-transfer to the stability of the exciplex. The stability of an exciplex is due to the electronic interactions between the excited and ground state molecules involved. These electronic interactions can be described by a wavefunction which is a combination of the wavefunc- tions for the locally excited states and the wavefunctions for the electron transfer states.51'52 l"exciplex = a“’01:! + bwD+A' + cwD*A + de'A+ 10 For the exciplexes of interest here the coefficients "c" and "d" are negligible compared to "a" and "b"; therefore the wavefunction for the exciplex can be described as: wexciplex = a'J’DA‘” + bwD+A‘ The character of the exciplex is then determined by the relative importance of the two coefficients "a" and "b". When "a” is negligible compared to "b" the complex can be considered as a pair of ions or radical ions. These ions . . . 45,53-55 may diffuse apart, espec1ally in polar solvents. Because of this it is possible that many excited state electron transfer reactions proceed via exciplexes with 56 finite lifetimes. For the reactions of triplet ketones with alkylbenzenes that will be studied here, there is no 54,57 It evidence for formation of solvated radical ions. is likely that the exciplexes discussed here have a significant amount of character derived from the excited ketone (coefficient "a" in equation XV is important). The contribution from the charge transfer component (co- efficient "b") is also important, but depends on the ketone and substrate. For example, there should be a much greater contribution from the charge-transfer interaction for TFA with a substrate than for acetOphenone with the same sub- strate because of the relative excited state reduction potentials. Because of this there are two factors to take into consideration with respect to reactions occurring via 11 exciplexes. The first factor that has to be determined is whether all of the reaction occurs through exciplex. The second is whether a change in the charge-transfer character of the exciplex influences the course of the reaction. If there are other paths for reaction besides exciplex forma- tion systematic changes in either ketone or substrate may change the results in a predictable manner, but it may not be possible to determine which of these factors are responsible. One of the major goals of this research is to explore systems that may enable one to separate these two effects so that they can be measured independently. Kinetics To understand excited state reactivity it is necessary to make quantitative measurements of quantum yields, lifetimes and rate constants. This information is necessary to compare a large number of systems since chemical yields do not accurately reflect quantum yields or rates of reactions. The quantum yield (0) is defined as: = number of molecules that react number of photons of light absorbed Scheme 1 shows the reactions that are involved in the photoreduction of a ketone (K) by substrate (BH). The reaction proceeds through an intermediate which goes on to products with an efficiency a. This intermediate may be an exciplex or a pair of radicals formed from hydrogen 12 Scheme 1. Steps of the Photoreduction Reaction II. III. IV. VI. Reaction Rate hv kisc kisc kd isc d1 1 3 kc1 3 K*-———e K kd[ K*] 3 k 3 3 K* + Q 41.; K + Q* kqu][ K*] 3 kr 3 K* + BH -——————>[Intermediates] krlBHJI K*] [Intermediates]-—E—9»Products [Intermediates] 3.29..) K + BH 13 abstraction. The rates for each step of the reaction are also given. The efficiency of the reaction can be broken down into the efficiency for each step of the reaction. 1. The efficiency of triplet formation (415C) ¢ = kisc isc k. + k isc d1 2. The efficiency of intermediate formation from triplet ketone is: kr[BH] krlBH] + kq[Q] + kd 3. The efficiency of intermediate going on to give product is a. The overall quantum yield is a product of these three efficiencies: kr[BH] Quantum Yield = ¢ = a0 (VII) of Product Prod isc krlBH] + Ed + kq[Q] Without quencher the quantum yield (0°) is: k [BH] 0° = a4. r 18C (VIII) krlBH] + kd Dividing equation VIII by equation VII one obtains: k [BH] + k + R [Q] ¢o _ r d q _ 14 where 1-1 = kr[BH] + k A plot of ¢°/¢ versus [Q] d. gives a slope of qu. The value of kq is known for triplet quenchers in various solvents;58-60 therefore, the value of T can be easily obtained. A plot of 1-1 as a function of substrate concentration ([BH]) gives an intercept of kd and slope of kr. By inverting equation VIII one obtains a linear plot of (")-l versus [BH]-1: k 0-1_ ‘1 d (9 ) - (a¢isc) (1 + E;T§§T) The slope of the plot is kd(a¢isckr)-l, and the intercept is (a¢iSC)-l. Dividing the slope of the intercept gives kd/kr' This ratio can be used as an independent check for the values of kd and kr obtained by quenching studies. The intercept is the maximum quantum yield, the value ex- pected for infinite substrate concentration. The efficiency (a) of the intermediate going on to give product can be calculated from this value and the inter- system crossing yield. The possibility of more than one type of inter- action of ketone with substrate, as shown in Scheme 2, is also consistent with this kinetic derivation. The rate constant for interaction of ketone with substrate (kr in Scheme 2. x. 3K* + BH XI. 3K~~~BH* XII. 3K---BH* XIII. 3K* + BH XIV. E:fi_¥"§T XV. R:§'I‘§7 15 with Substrate Reaction k ex E 3 KoooBH* kd ____E§_, K + BH k ____E_9 K-H + B- kH ________ -—————> K-H + B. l ~——£L—; Products _ l .£_2_9. K + BH Possible Modes of Interaction of Excited Ketone Rate 3 keXIBHJI K*1 kd [Exciplex] ex kaExcipleX] kHIBHII3K*] 16 Scheme 1) may actually be the sum of rate constants for exciplex formation (kex) and direct hydrogen abstraction (kH)‘ The efficiency of the reaction would then be a function of the efficiency with which exciplex forms radicals going on to products. Unfortunately there is no easy method to measure the individual rate constants as given in Scheme 2. The rate constants for interaction of ketone with substrate reported here are the kr values according to Scheme 1. Other methods must be used to estimate the contributions from kH and kex to kr’ Research Goals The major objectives of this research were: 1. The study of the selectivity of reactions occurring from exciplexes. 2. The study of the influence of external factors, such as solvents and additives, on exciplex reactions. 3. The investigation of the initial interaction of excited state ketones with substrate. This includes direct hydrogen abstraction and exciplex formation. 4. To investigate the reactions of the radicals that are generated in the photoreduction re- action by independent methods in order to determine their contribution to the overall pattern of reactivity. 17 5. To coordinate the information for individual steps of the reaction into a comprehensive mechanism for the photoreduction reaction. Approach used to Accomplish Goals The primary system studied was the photoreduction of a series of ketones by p-cymene. This substrate was used since an exciplex involving p-cymene has two reaction pathways that give products (Scheme 3). These two path- ways, transfer of primary and tertiary hydrogen atoms (or protons), can be easily monitored from the products formed. Other substrates were used when other facets of the reaction, other than exciplex selectivity, were being investigated. Ketones that react primarily, if not entirely, through exciplex formation were studied using p-cymene as substrate to investigate exciplex selectivity and to monitor the effects of solvents and additives on exciplex behavior. Ketones which are not believed to re- act entirely by exciplex were also investigated using p-cymene as substrate to determine if there was any evidence for exciplex formation with these ketones and if it would be possible to determine the fraction of re- action proceeding by such exciplex formation. In addition to p-cymene and other alkylbenzenes the photoreduction of various ketones by alcohols was investigated. These experiments were performed to obtain 18 Scheme 3. Reaction Paths for Ketone with p-Cymene. CH3 CH3 1 3K* + CH3H CH3 3 k d kH H1 3 CH3 CH2 c 3. H3 \\\\\\\\If/////’ c 3H H3 Products 19 information on disproportionation between hydroxy radicals and to investigate the exchange of hydrogen atoms from hydroxy radicals to ground state ketones. Although pre- vious attempts had been made to investigate individual aspects of the photoreduction of ketones by alcoholsZI’zz' 61’62 no complete description of the reaction could be formulated. One major question that remained was the extent of disproportionation that actually occurs and how it relates to reaction efficiency. Quantum efficiency studies63 suggest disproportionation is high for aceto- phenone and 1-phenylethanol, while studies with optically active alcohols20 suggest it isn't important. Since half of the radicals produced in the reaction of ketones with alkylbenzenes are hydroxy radicals their interactions play an important part in the understanding of the overall photoreduction reaction. Other aspects of radical reactions were in- vestigated by comparing product ratios from the photore- duction reaction to those from radicals generated in- dependently by decomposition of di-t-butyl peroxide. Abstraction of a hydrogen from a.substrate by t-butoxy radical would produce the same radical as abstraction by an excited ketone. The hydroxy radical formed from the ketone would be formed by hydrogen abstraction from the corresponding alcohol. In this way the radicals would be formed separately, not in a solvent cage, and ketone formed 20 from the disproportionation could also be measured. In this way important information regarding relative rates of coupling and disproportionation reactions could be obtained. Although there have been numerous studies of radical- 64'65 they have not necessarily been aimed radical reactions, at the type of information that is important to understand the photoreduction reaction. RESULTS Product Identification The products of photoreduction of a,a,a-trifluoro- acetophenone (TFA) by p-cymene were isolated by a combina- tion of column chromatography and sublimation. The two cross-coupled alcohols (KP and KT) and one of the diastereo- meric pinacols (KK) were readily separated on an alumna KP KT R = Me x = OH Y = CF3 column. One of the pinacol products could not be isolated in a pure form, but the infrared and n.m.r. spectra of a 50:50 mixture of the two pinacols were consistent with only pinacol product present. 21 22 The three hydrocarbon coupling products (PP, PT and TT) were isolated by first separating them from other TT products by column chromatography (alumina, hexane as eluting solvent), and then by subjecting them to fractional sublimation. The two symmetrically coupled products (PP and TT) sublimed more readily than the crossed (PT) product. Therefore, the two symmetrical products could be obtained by sublimation of mixtures rich in either the PP product, from photoreduction of TFA, or rich in TT product, from photoreduction of acetophenone. The coupling products from other substrates, such as toluene, cumene, and p-xylene, were identified by come parison of gc retention times with the known compounds. The cross-coupling products from the reactions of these 23 substrates with a variety of ketones, as well as the pinacol products, were identified either by comparison with authentic samples or by comparison to similar systems. The retention times of the cross-coupled products were consistently be- tween the retention times of the products from the self- coupling of radicals. Which self-coupling product had the shorter retention time depended on ketone, substrate and the g.c. column used. For example, on a SE-30 column the cross-coupled alcohol had a retention time shorter than acetophenone pinacol but longer than bibenzyl. For the reaction of TFA and p-cymene the pinacol was the first product observed, followed by the two cross-coupled alcohols and finally the three hydrocarbon products. The three hydrocarbon products are an exception to this trend, with the cross-coupled product (PT) having the shortest retention time, followed by TT and PP. The hydrocarbon and pinacol products were always formed in roughly equal amounts, and their sum was usually close to the amount of cross-coupled product. However this latter situation was dependent on substrate, ketone and the solvent. Quantum Yields Quantum yields were determined by parallel irradia— tion at 313nm of degassed sample solutions and an actino- meter in a merry-go-round apparatus at 25°C. Samples con- tained 0.1M ketone and the appropriate concentration of 24 donor. The solvent was usually benzene, although acetonitrile was also used for some compounds. The actinometer was a 0.1M solution of valerophenone in benzene.66 Percent con- version was kept as low as possible, usually 10% or less for systems other than p-cymene. Due to low yield of some pro- ducts in the p-cymene system, it was difficult to obtain quantum yields for all products at conversion below 20%. The product to standard ratios were obtained by v.p.c. analysis. Quenching78tudies Stern-Volmer quenching runs were performed in benzene at 366nm irradiation using napthalene as quencher. Conversion was kept below 10% for the unquenched solution. Plots were linear for the range studied, usually to ¢°/¢ values of 3 or 4. The ¢°/¢ values were identical for both bibenzyl and cross-coupled alcohols for all cases studied. Quenching by amines was studied at 313nm in either benzene or acetonitrile solutions. The Stern-Volmer quenching plots were not always linear for these quenchers, and all products did not show the same quenching efficiency. Photoreduction of Ketones by p-Cymene Quantum yields for the three hydrocarbon coupling products (PP, PT, and TT) were determined for the photore- duction of acetophenone (AP), d,a-dif1uoroacetophenone 25 (DFA), and a,a,a-trifluoroacetophenone (TFA) with p-cymene in benzene (see Table 2). In all cases all three products 1 vs. [p-cymeneJ-1 (Figures 1 gave linear plots of 4— through 5). For the photoreduction of TFA quantum yields for all expected products (three hydrocarbons (PP, PT, and TT), two cross-coupled alcohols (KP, KT), and pinacol products (KK)) were calculated (Table 3, Figures 6 and 7). Although at some of the lower concentrations of p-cymene there was slightly less hydrocarbon than pinacol, yields of the two products were fairly close at the higher p- cymene concentrations. The quantum yield for cross- coupled alcohols at the higher concentrations is equal to the sum of the other two products. The material balance for the TFA and the p-cymene is good (greater than 80%). The photoreduction of a-fluoroacetOphenone (MFA) by p- cymene was also investigated, but the ratio of products varied with p-cymene concentration. Other results that will be presented later point to secondary radical re- actions influencing the course of this reaction. Results obtained for the reaction of TFA and p- cymene in acetonitrile solution (Table 2, Figures 8 through 11) showed a maximum quantum yield of primary radicals higher than in benzene. The products from tertiary radicals were formed in the same efficiency, how- ever, and the result is a significant change in the ratio of tertiary products. 26 consumes uozm m mmo.o emoo.c mac.o mvo.o mma.o mmo.o maeuueeoumoe mefl.o vqo.o ceoc.o mHo.o Hmo.o eeo.o emo.c «museum x. ex as am mm as ex vamp—me Maw—=6 wage NOE—0 K050 NMEO XMEO UCO> H 0m meoEMOIQ one «he no :oHuommm Mom moaoww Educmso Essflxmz .m manna mafiuuwcouwomo osoucomm Ha.m mm.m ~m.~ omm mm mm «NH m.HH v.m name oo.~ mo.~ ow.~ 0mm mm mm mma m.v~ m.~a mama mh.m m~.m Hm.m pea om vm em m.m m.m memo hm.o nm.o mn.o mm we oom mm m.om o.m>~ med lime am we as (we (lop! emr .mm mm A2. omonNHQoououcH ammoumuaH Aauzv macaw msouox ocoESOIm sues monouox msoflum> mo cowuomom How muasmom .N manme 600 PP 500 400 300 200 100 Figure l. d 27 o o T I T 44] 0 4 0.8 1 2 1 6 2.0M l [p-Cymene] Results from Reaction of AP and p-Cymene, RP formation. 28 2% [p-Cymene] Figure 2. Results from Reaction of AP and p-Cymene, PT (0) and TT (A) formation. 29 300*— OIH 250 — 200«4 150 — 1001— [p-Cymene] Figure 3. Results from Reaction of DFA and p-Cymene in Benzene, PP (0), PT 6+9, and TT (AD Formation. 30 300 l 76 250 200 150 100 50 T I T F l 1 2 4 6 8 10M" 1 fp-Cymenej Figure 4. Results from Reaction of TFA and p-Cymene in Benzene, PP (o) and PT (A) Formation. 31 1500 lo ¢TT 1250 1000 750 500 250 r l T ‘1 1 2 4 6 8 10M- 1 [p-Cymene] Figure 5. Results from Reaction of TFA and p-Cymene in Benzene, TT Formation. 32 120 - OIH 100 _ 80 — 4A 60 _ 40.. 20.. l I I I “*1 2 4 6 3 10M"1 1 [p-Cymenej Figure 6. Results from Reaction of TFA and p-Cymene in Benzene, KP (o) and KT (40 Formation. 33 120 - one 100 - 80H 60'- 40-4 20.. 2 4 6 3 10M"1 [p-Cymene] Figure 7. Results from Reaction of TFA and p-Cymene in Benzene, Formation of Pinacol (+9, Hydrocarbon (o), and Cross-Coupled.(A0 Products. 240‘— ell-4 200 - 160 — 120.- 0 80.. 40 - I I I I 2 4 6 8 10M” 1 [p-Cymene] _ Figure 8. Results from Reaction of TFA and p-Cymene in Acetonitrile, PT (0) and PP (£0 Formation. 35 3000 ‘ TT 2500 ' 2000 ‘ 1500 ‘ 1000 ‘ 500 ‘ I I I I I 2 4 6 8 10M.“ 1 Ep-Cymene] Figure 9. Results from Reaction of TFA and p-Cymene in Acetonitrile, TT Formation. 36 60- OIH 50 u 40.. 30«— 20‘— I I I I II 2 4 6 8 10M 1 fb-Cymenéj Figure 10. Results from Reaction of TFA and p-Cymene in Acetonitrile, KP (o) and KT (A0 Formation. 37 90‘— .1. ¢ 75 — A 60 — 45‘— A 30,. A 15- ._, _____..___——’_i ——-"—'W I I I I I 2 4 6 8 10M"1 1 [p-Cymene] Figure 11. Results from Reaction of TFA and p-Cymene in Acetonitrile, Hydrocarbon CA) and Cross- Coupled (0) Product Formation. 38 The primary-tertiary ratios for a variety of sub- stituted acetophenones, benzophenones and a,a,a-tri- fluoroacetophenones were obtained by analyzing the three hydrocarbon products. For these calculations no internal standard was used and the ratios were obtained from relative product areas. For the m-CF3-TFA only the PP/PT ratio could be obtained due to the small amount of TT product. For a number of other ketones the PP product could not be determined either because it was a minor pro- duct or because other products such as pinacol and cross- coupled alcohols interfere with the analysis. The re- maining ketones were analyzed for all three hydrocarbon products. The results are listed in Table 4. Kinetics of Hydroqen Abstraction Kinetic parameters were determined for AP, MFA, DFA, and TFA from quenching studies with napthalene at 366m. The lifetimes were determined from Stern-VOlmer plots at several toluene concentrations for each ketone. These lifetimes are listed in Tables 5 through 8. The plots of (lifetime).1 versus toluene concentration (Figures 12 through 15) give values for kr and kd from the slope and intercept, respectively. These results are given in Table 9. The ratio of kd to kr was also obtained from the slope divided by the intercept of a plot of (quantum yield).1 versus (toluene concentration).1 (Table 10). 39 Table 4. Results of Photoreduction of Indicated Ketone with p-Cymene in Benzene Ketone %PP %PT %TT m—CF3-TFA 71 29 a m-Me-TFA b 81 19 Benzophenone b 45 55 4,4'-MeO-BP b 40 60 4,4'-Me-BP b 28 72 m-F-AP b 48 52 m-CF3-AP b 50 50 p-Me-AP b 38 62 TFA 62 30 8 p-MeO-TFA 31 45 24 p-Me-TFA 40 45 15 p-Cl-TFA 40 44 16 Propiophenone 6 41 53 Acetophenone 8 39 53 p-F-AP 20 35 45 4,4'-Cl-BP 16 42 42 aMinor Product bPP was not analyzed, percentage is for products analyzed even though PP may be significant. 40 Table 5. Results of Stern-Volmer Quenching of Acetophenone [Toluene](M) qu(M-l) 1(10-63) 0.50 6620 1.32 1.00 6000 1.20 1.50 5800 1.16 2.00 5300 1.06 2.50 4700 0.94 Table 6. Results of Stern-Vblmer Quenching of a-Fluoroacetophenone [Toluene](M) qu(M-l) 1(10-75) 0.40 2500 5.00 0.80 2050 4.10 0.94 1725 3.45 1.50 1300 2.60 1.87 1150 2.30 Table 7. Results of Stern-Vblmer Quenching of a,a-Difluoroacetophenone [Toluene](M) qu(M-1) 1(10-73) 0.50 660 1.32 1.00 524 1.05 1.50 420 0.84 2.00 364 0.73 2.50 300 0.60 41 Table 8. Results of Stern-Volmer Quenching of 0,0,a-TrifluoroacetOphenone 1 7 [Toluene](M) qu(M- ) 1(10’ 3) 0.50 700 1.40 1.00 480 0.96 1.50 370 0.74 2.00 ' 300 0.60 2.50 233 0.47 42 0.64— 0.41- 0.2 — I I I I I1 0.5 1.0 1.5 2.0 2 [Toluene] Figure 12. Lifetimes of AP as a Function of Toluene Concentration. 43 I I I I I 0.4 0.8 1.2 1.6 2.0M [Toluene] Figure 13. Lifetimes of MFA as a Function of Toluene Concentration. 44 18 ‘- .1. T (us-1) 15 '- o 12 - 9 - 5 .. 3 c— l l I I '7 0.5 1.0 1.5 2.0 2.5M [Toluene] Figure 14. Lifetimes of DFA as a Function of Toluene Concentration. 45 24 ‘— l T . (us-l) 20 ‘- C 16 “ 12 ‘- 8 cut 4 - I I I I I 0.5 1.0 1.5 2.0 2.5M [Toluene] Figure 15. Lifetimes of TFA as a Function of Toluene Concentration. 46 Table 9. Rate Constants for Ketones from Stern-Volmer Studies Ketone kd(1063-l) in Benzene AP 0.70 (0.50)a MFA 1.32 DFA 5.40 TFA 3.50 (7.00)a aReference 38 Table 10. Results from 0-1 Ketone Slope (M) AP 45.0 (45.0)a MFA 21.5 DFA 20.2 TFA 66.0 (22.6)a aReference 38 kr(106M’1s’1) kd/kr (M) with Toluene 0.12 (0.12)a 5.83 (4.17)a 1.64 0.80 4.30 1.26 6.80 (7.30)a 0.51 (0.96)a versus [Toluene]-l Intercept Slope/Intercept (M) 12.0 (7.7)3 3.75 (5.84)a 9.5 2.26 17.7 1.14 13.0 (18.9)a 5.08 (1.20)a 47 Hydrogen Abstraction from_p-Cymene by t-Butoxy Radicals To study the selectivity of alkoxy radicals, t- butyl hypochlorite and di-t-butyl peroxide were both used as sources of t-butoxy radicals. For the reaction of t-butyl hypochlorite, benzene solutions containing 1.0M p-cymene and varying concentrations of t-butyl hypochlorite were degassed by passing a stream of nitrogen through them; they were then irradiated at 366nm. Two products were observed by g.c., the primary chloride (l-chloromethyl- 4-isopropy1benzene) and a,p-dimethylstyrene. The latter product was due to the quantitative dehydrochlorination of the tertiary chloride in the injector port to the gc.. When cumene was used in place of p-cymene, both a-cumyl chloride and a-methylstyrene were observed and there was an irregular baseline between these two products that is indicative of decomposition. The peroxide experiment was performed with 0.1M di-t-butyl peroxide and 0.6M p-cymene in benzene. The samples were degassed by three freeze-thaw cycles. The reaction was initiated by irradiation at 313nm. The pro- ducts observed were the three hydrocarbon coupling pro- ducts (PP, PT and TT) previously described for the photo- reduction of ketones by p-cymene. The results for the t-butyl hypochlorite reaction varied with hypochlorite concentration, possibly as a result of abstraction by chlorine atoms or high concentrations of hydrogen chloride. 48 At low hypochlorite concentrations the primary-tertiary ratio reached a constant value, giving a ratio of 2.4 to 1 in favor of tertiary abstraction. The results from di- t-butyl peroxide show relative product formation for TTzPTzPP to be 7.5:5.9:1, corresponding to a 2.7 to 1 ratio in favor of tertiary abstraction. Effects of Solvents on the Photoreduction Reaction To study the effects of solvents on product dis- tribution in the photoreduction reaction, substrates such as toluene and cumene were used in preference to p-cymene. The reason for using the simpler substrates was to reduce the number of products formed and thus to facilitate analysis. This simplification is shown to be justified for systems in which toluene and p-cymene were both used as substrates and produced similar results. The experi- ments were carried out using solutions of either 0.05M or 0.10M ketone and the appropriate substrate, usually 0.50M, in the solvent system indicated. Samples were irradiated at 313nm. Analysis for the coupling products was per- formed by g.c. in the usual manner. Self-coupling pro— ducts of the radicals formed by hydrogen abstraction from the substrate are listed as BB. Pinacol products are listed as RR and cross-coupled products are listed as BK. To compare acetonitrile and benzene as solvents for the photoreduction of TFA, the ratio of cross-coupled alcohol to bibenzyl as a function of acetonitrile 49 concentration was looked at using toluene as the hydrogen donor. The results in Table 11 show that the ratio of cross-coupled product (BK) to bibenzyl (BB) increased when 5.0M acetonitrile was added to benzene. Increasing the acetonitrile about 5.0M had no further effect on the ratio. The results for TFA and p-cymene are very similar to those obtained with toluene. The ratios were not ob- tained as a function of acetonitrile concentration in this case, but as a function of p-cymene concentration in either pure benzene or pure acetonitrile as solvent. The results are given in Table 12. The pinacol was analyzed in the p-cymene system and therefore it is also possible to look at the ratio of cross-coupled product to pinacol. The amount of product incorporating radicals derived from p- cymene and ketone can be used to obtain the relative amounts of these radicals found. It was observed that bibenzyls increase relative to cross-coupling products and the pinacols decrease relative to them as p-cymene con- centration increases. This is found in both benzene and acetonitrile. It is also observed that the ratio of the two types of radicals is a function of p-cymene concentra- tion. In benzene solution both types of radicals de- crease as p-cymene concentration decreases: however, the radicals derived from p-cymene decrease faster than the hydroxy radicals. At higher p-cymene concentrations the 50 Table 11. Effect of Acetonitrile on the Reaction of TFA and Toluene in Benzene [MeCN] [BB] [BK] [BKl/[BB] (M) (10’3M) (10‘3M) 0.0 1.45 3.19 2.20 5.0 1.52 4.78 3.14 10.0 1.70 5.35 3.15 15.0 1.95 6.14 3.15 51 mamowtnn aumwuuwu 0cm mumeum mo Esm man we .m moms» man no saw on» ma mm ..B& 0:0 mm. muosooua toadsoolmmouo 03» on» no son may ma xmm .muosooum :H 6:50m ..69 can .Bm .mm. muostoum conumooutmn sam.o eva.o m.~ H.m mno.o mmo.o m.H m.m oa.o m-.o va.o m.~ o.m NHH.o smo.o m.H m.m o~.c s-.o e-.o m.m m.m ova.c mHH.o m.H o.m oq.o nnnnnnnnnn nun nun mea.o mma.o m.~ s.~ om.o om~.o Hv~.o m.m q.m ema.o mma.o ~.~ v.~ oo.~ .m .m mm mm mm mm .s .m as mm mm mm .2. 0 (I70 0\ 0 ON 9 .9 e\ (b . 6\ 0 "mm. QHfiHUfiCOUmO< mquCQm «he sufi3 sofluommm .GOwumuucmocou .mm. mcmEhUIm nufl3 mofiumm uosooum mo cowHMAHm> .NH manna m 52 radicals are found in roughly equal amounts. In acetonitrile solution the radicals derived from p-cymene decrease as p-cymene concentration decreases, as expected, but there is very little dependence on p-cymene concentration for quantum yield of hydroxy radicals. A comparison of the fraction of radicals that give cross-coupled product in each solvent can be made. The results show that in benzene the cross—coupled alcohols account for just over half of the products. In acetonitrile the cross-coupled alcohols account for two-thirds of the observed products. For the photoreduction of acetophenone the solvent effect on product ratios is very similar to that found for TFA. The results in Table 13 show that for p-xylene and cumene the fraction of radicals that couple to give cross- coupled product is higher in acetonitrile than in benzene. This difference is not as significant for cumene as it is for p-xylene. Toluene was studied in t-butanol and showed results similar to cmmene in acetonitrile. Attempt to Maximize_ggantum Yield of Photoreduction The possibility that a hydrogen bonding polar solvent could be used to maximize the quantum yield by solvating radical pairs and preventing disproportionation was investigated using pyridine. The ability of pyridine to increase the quantum yield of the Norrish Type II re- action is well lcnown.68-7o The experiments to study the Table 13. Substrate p-Xylene p-Xylene Cumene Cumene Toluene Tolueneb Toluened p-Xylened Cumened 310' M bReference 67 cMillimole of Product d Reference 38 Solvent MeCN Benzene MeCN Benzene t-BuOH Toluene Benzene Benzene Benzene 53 [331a 2.68 3.36 1.28 1.54 0.83 2.03c 1.00e 1.00‘3 1.003 eRelative Product Formation [3x] a 5.49 4.66 3.10 3.26 2.77 4.19 1.64e 1.323 1.59e [KK] 3.45 4.26 1.39 1.87 1.62 2.37 a C [8°] 10.90 11.40 5.66 6.34 4.40 8.25 a C Product Distributions from Acetophenone and Various Substrates [K-la 12.40 13.20 5.88 7.00 6.00 8.93° --'-- 54 effects of pyridine were performed in the same manner as those in the previous section on solvent effects. When pyridine was added to a benzene solution there was no substantial increase in benzyl radical formation for the photoreduction of TFA by toluene. However at concentrations of pyridine about 0.01M the ratio of cross- coupled product-to bibenzyl began to increase (see Figure .16). Initially this increase was rapid and accompanied by an increase in cross-coupled product and a decrease in bibenzyl. At higher pyridine concentrations (about 1.0M) both products showed quenching by pyridine. Although a material balance was not done comparing results with and without pyridine present it seems unlikely that pyridine changes the material balance significantly. To account for the change in product ratio but not a change in benzyl radicals, while also accounting for a change in material balance, would require a much more complicated explanation than simply assuming that a polar solvent, such as pyridine or acetonitrile, increases the fraction of radicals that give cross-coupled product. The results in acetonitrile show no change in pro- duct distribution, suggesting that both pyridine and acetonitrile have the same solvation effect on the coupling reactions. Both bibenzyl and the cross-coupled product are quenched by pyridine at almost identical rates. This quenching is expected since pyridine has an ionization 55 3.0 - (b0 7(3— 2.5 - + 2.0 .4 1.5 - + + 1.0 1» A A A 0.5 - I I I :1 fl 0.4 0.8 1.2 1.6 2.0M [Pyridine] Figure 16. Effect of Pyridine on Reaction of TFA and Toluene in Benzene. Results for BB (+9, BK (A), and B- (o). 56 potential comparable to benzene, which quenches by exciplex 38 formation. The results in acetonitrile are shown in Figure 17. Effects of Charge-Transfer_Quenchers Quenching by compounds that are not capable of triplet energy transfer, but are capable of quenching by exciplex formation or electron transfer to give radical ions, was found to have a significant effect on product ratios. This effect was studied in a manner similar to the investigation of solvent effects. The quencher was added to solutions containing 0.05M ketone and substrate, usually 0.50M, in either benzene or acetonitrile. The samples were then degassed by four freeze-thaw cycles and irradiated in parallel at 313nm. Comparison of pro- ducts to an internal standard by g.c. analysis provides relative product formation for different quencher concentra- tions. For the photoreduction of TFA with 1.0M toluene in acetonitrile, quenching by DABCO leads to an initial increase in all three products, followed by quenching of all three products at higher DABCO concentrations (see Figure 18). At the higher DABCO concentrations bibenzyl is quenched more than the cross-coupled alcohol, although both plots are curved. When a plot of ¢°/¢ versus DABCO concentrations is made for benzyl radicals and hydroxy radicals, instead of the three products, the results 57 15.0 -— ¢0 F 12.5 - 4A 10.0 ‘— 7.5 - 5.0 — A 2.5 - I I I I ‘7 0.4 0.8 1.2 1.6 2.0M [Pyridine] Figure 17. Pyridine Quenching of the Reaction of TFA and Toluene in Acetonitrile. Results for BB (0) and BK (A0. 58 6 - ¢0 4— 5'- o 4-3 3— O 2.. + + A ‘ A 1* ++t + in AAA A I I I I *1 0.0006 0.0012 0.0018 0.0024 0.0030M [DABCO] Figure 18. 'DABCO Quenching of the Reaction of TFA and Toluene in Acetonitrile. Results for BB (9), BK (+) , and Pinacol (A). 59 are much better. Even though these plots, shown in Figure 19, initially curve below one they straighten out. Al- though they do not give the same slope, both plots give the same intercept (0.4) within experimental error. The values of qu derived from the slope divided by intercept 1 for benzyl radicals and 1040M"1 for hydroxy is 1670M' radicals. When the experiment (0.5M toluene) is performend in benzene instead of acetonitrile there is no enhancement of benzyl radical yield. At low DABCO concentrations the bibenzyl yield is increasedlnnzthe cross-coupled product is decreased, as shown in Figure 20. This leads to an over- all linear quenching of benzyl radicals with an intercept of 1.0 and a qu value of 410M-1. In benzene solution the quenching of the reaction of TFA and p-cymene by DABCO is essentially the same as the reaction using toluene. Low concentrations of DABCO lead to an increase in bibenzyl coupled products (PP, PT and TT) and a corresponding decrease in cross-coupled pro- ducts, as shown in Figure 21. The overall quenching of the benzylic radical formation (primary and tertiary) is linear. There is no change in primary-tertiary ratio. The plots for quenching of primary and tertiary radicals are shown in Figure 22. The quenching of the reaction between TFA and 0.5M toluene by p-dimethoxybenzene in acetonitrile, shown 60 3.0‘— (Do 7(3— 2.5'- 2.0 — 1.5!— A 1.0-4 7. ° -° , AA A 0.51- I I I I I1 0.0006 0.0012 0.0018 0.0024 0.0030M [DABCO] Figure 19. Effect of DABCO on the Radicals B- (o) and K- (40 Observed in the Reaction of TFA and Toluene in Acetonitrile. 61 1.8.. 9| + 1.5 .. " 1.2.— 0.9.. 0.6.— I I I I [I7 0.0004 0.0008 0.0012 0.0016 0.0020” [DABCO] Figure 20. DABCO Quenching of the Reaction of TFA and Toluene in Benzene. Results for BB (0), BK (+). and Bo (A). era . 305 -' 3.0.. 2.0 _ 1.5 1.0 62 Figure 21. I I I I I 1 0.002 0.004 0.006 0.008 0.010 0.012M [DABCO] DABCO Quenching of the Reaction of TFA and p-Cymene in Benzene. Results for Hydrocarbon (o) and Cross-coupled (+9 Products and Benzylic Radicals (A0. 2.4 _. 6T9 O 2.01— 1.8.. 1.6‘- 1.2‘- 1.0 63 O> Figure 22. I I I I I I 0.002 0.004 0.006 0.008 0.010 0.012M [DABCO] DABCO Quenching of the Reaction of TFA and p-Cymene in Benzene. Results for Primary (0) and Tertiary Radicals (A) . 64 in Figure 23, did not show any of the product enhancement observed with DABCO. Instead quenching was linear for both bibenzyl and cross-coupled alcohol, both giving an inter- cept of one. The qu value of bibenzyl was three times 1 1 that of the cross-coupled product (3040M- and 1000M- respectively). The qu for benzyl radical was 1650M‘1. In benzene solution p-dimethoxybenzene quenching (Figure 24) did not show such a large difference in qu values 1 and S63M-l for bibenzyl and cross-coupled alcohol (722M- respectively). The qu value for benzyl radicals in benzene is 600M-l. For a comparison of ketones, the re- action of acetophenone and toluene in acetonitrile was quenched by p-dimethoxybenzene. Unfortunately both bibenzyl and cross-coupled alcohol curve above ¢°/¢ = 2, as can be seen in Figure 25. The initial slope for both products gives a qu value of 480M-1. Effects of Acid Since the basicity of amines may be responsible for a change in product distribution the effect of acid was also investigated. The acid used was trifluoroacetic acid, a strong acid that is soluble in both benzene and acetonitrile. To study the effect of acid on selectivity, experiments were run with 0.05M ketone and 0.50M p-cymene in either benzene or acetonitrile with varying concentra- tions of trifluoroacetic acid. To study the overall efficiency of the reaction, quantum yields were determined 65 2.0 — 1.5 - I I I II I 0.0001 0.0002 0.0003 0.0004 0.0005M [p—DMOB] Figure 23. Quenching of the Reaction of TFA and Toluene in Acetonitrile by p-Dimethoxybenzene. Results for BB (0) and BK (A) . 66 2.25 ‘- °l 2.00 “ 1.75 “ A 1.50 "' 1.25 ‘ I I I I 7 0.0003 0.0006 0.0009 0.0012 0.0015M [p- DMOB] Figure 24. Quenching of the Reaction of TFA and Toluene in Benzene by p-Dimethoxybenzene. Results for BB (0) and BK (AD. 67 II I7 II I 7 0.002 0.004 0.006 0.008 0.010M E p- 011013 1 Figure 25. Quenching of the Reaction of AP and Toluene in Acetonitrile by p-Dimethoxybenzene. Results for BB (0) and BK (A0. 68 for samples containing 0.10M ketone, 0.05M trifluoroacetic acid and varying concentrations of toluene in benzene. The addition of trifluoroacetic acid to a photo- reduction in benzene increases product yield, changes product distribution and also changes selectivity when p-cymene is the substrate. This occurs when either aceto- phenone or TFA is the ketone (see Table 14). The changes in primary-tertiary ratio for the two ketones are in opposite directions. Addition of 0.05M trifluoroacetic acid to benzene cuts the tertiary-primary ratio for acetophenone in half while doubling the same ratio of TFA. Since the tertiary-primary ratio is approximately ten times greater for acetophenone than for TFA in pure benzene, the overall result with acid is a much smaller difference in selectivity. With 0.05M acid in benzene the tertiary- primary ratio for acetophenone is slightly greater than twice what it is for TFA. In acetonitrile TFA shows neither enhancement of products nor any change in primary-tertiary ratios when trifluoroacetic acid is added (Table 15). In both benzene and acetonitrile there is quenching of products when the acid concentration exceeds 0.05M. To see how quantum yields varied with substrate concentration, TFA was photoreduced by varying concentra- tions of toluene in benzene with 0.05M trifluoroacetic acid added. The results for 1.0M to 2.5M toluene are Ml m cm~.o mm.H ¢¢H.o mmo.o vo.H smm.o mm.a mmm.o mea.o om.o ~mm.o hm.~ meo.o Hm~.o cH.o 24¢.o em.m ous.o sm~.o oc.o .2. 6.92. n.ms. n.mm. 6.9m. .<.H bmc.o zoo: dme oo.m m.m n.ma mh.q w~.o mm.a oo.o 2002 «he om.H o.m H.m om.~ Hm.o oo.~ mo.o mcmucmm 4&8 om.m h.H h.m oo.~ NH.o mm.o oo.o mcmucwm «he va.c III: IIII mm.H ma.m om.m mo.o msmucmm ad mm.c I--- I--- o~.o ov.a ~m.o oo.o cumucmm me .2. B\m 0.92. n.mx. n.mm. 6.99. 6.9m. .mcme. uco>aom 020002 mcosaolm 02m QGOHGM UGHMOHUGH EOHH mHUSOOHm GO Afldhfiv UHUQ OHHTOTOHcflflmfiHE HO HUOHHQ .vH OHDMB 70 shown in Table 16. The quantum yields for bibenzyl and cross-coupled alcohol are almost identical to the maximum quantum yields without acid present. Generation of Radicals from di-t-Butyl Peroxide Radicals were generated by irradiation of di-t- butyl peroxide at 313nm to produce t-butoxy radicals, which subsequently abstracted hydrogen atoms from sub- strate to produce the radicals of interest. For example benzyl radicals were formed by hydrogen abstraction from toluene and l-phenyl-l-hydroxyethyl radicals were formed by abstraction from l-phenylethanol. The concentrations of substrates were adjusted on the basis of rate constants for hydrogen abstraction by t-butoxy radicals so that the radicals were formed in roughly equal amounts. This means that the model system for the photoreduction of aceto- phenone by toluene should contain approximately ten times the concentration of toluene as of 1-phenylethanol. In this way t-butoxy radicals would abstract an equal number of hydrogen atoms from each substrate since the rate con- stant for abstraction from 1-pheny1ethanol is roughly ten times the rate constant for abstraction from toluene.36 The samples containing the substrates and di-t-butyl peroxide in either benzene or acetonitrile were degassed by four freeze-thaw cycles. After irradiation at 313nm, analysis was performed by g.c. in the same manner as the photoreduction reaction. In addition to the three coupling 71 Table 16. Quantum Yields for the Reaction of TFA and Toluene with Trifluoroacetic Acid (0.05M) in Benzene [Tolgene] 033 ¢BK 03. 1.00 0.070 0.155 0.295 1.52 0.071 0.154 0.295 2.01 0.071 0.154 0.296 2.50 0.072 0.157 0.301 72 products (BB, BK and KK) the formation of ketone produced by disproportionation was also monitored. Due to the hydroxy radical (K-) inducing decomposition of the peroxide,71 the concentration of the peroxide was varied to study the relative amount of ketone formed. Radical Formation t-BuO- + PhCH ———¥- t-BuOH + PhCH ' 3 2 OH OH | l t-BuO- + PhCHR —. t—BuOH + PhCR Coupling OH (K-) (B-) Disproportionation OH 0 ll PhCR + PhCH ° -——t-PhCR + PhCH 2 3 Induced Decomposition OH O I ll PhCR + (t-Bu0)2‘———&»PhCR + t-BuOH + t-BuO° The coupling and diSpr0portionation products for benzyl and hydroxy radicals formed from toluene and 73 l-phenylethanol were measured in both benzene (Table 17) and acetonitrile (Table 18). In acetonitrile there was less of a dependence on peroxide concentration for forma- tion of acetophenone. Other substrates were also studied for a direct comparison with acetophenone photoreduction. Cumene and p-xylene were studied in both benzene and acetonitrile, while toluene was studied in t-butanol also. The peroxide experiment, the results of which are shown in Table 19, were run in parallel with the photoreduction reaction, the results of which are in Table 13. There is a larger fraction of cross-coupled product formed in acetonitrile than there is in benzene and the percentage of cross-coupled product is greater for cumene than for p-xylene in either solvent. Since there is a possibility that the substitution of fluorine on the methyl group of the ketone might effect the radical reactions, the peroxide was decomposed in benzene in the presence of l-phenyl-Z,2,2-trifluoroethanol and alkylbenzenes. The alkylbenzenes used were toluene, cumene and p-xylene. The results in Table 20 show that the relative amount of cross-coupled product is approximately the same for all three substrates. The concentration of peroxide was not varied for this study, however the amount of TFA measured for the reaction with toluene or p-xylene is only a small fraction of the amount of cross-coupled product. 74 Table 17. Products from the Reaction of t-Butoxy Radicals with Toluene and l-Phenylethanol in Benzene [Peroxide] [AP]a [PP]a [BKJa [KKJa [B-la [K-la (M) 0.020 0.96 1.20 2.53 3.30 4.93 9.13 0.030 2.21 1.52 2.82 3.17 5.86 9.16 0.051 2.10 1.01 1.63 1.54 3.65 4.71 0.101 4.53 1.81 2.65 2.21 6.27 7.07 0.152 7.19 2.47 3.42 2.69 8.36 8.80 a10'3M Table 18. Products from the Reaction of t-Butoxy Radicals with Toluene and l-Phenylethanol in Acetonitrile [Peroxide] [AP]a [BB]a [BK]a [KK]a [B-Ja [K-Ja (M) 0.050 1.00 1.09 1.86 1.04 4.04 3.94 0.100 1.90 1.84 3.02 1.67 6.70 6.36 0.150 2.67 2.28 4.07 2.29 8.63 8.65 a10'3M Table 19. Products from the Reaction of t-Butoxy Radicals with Indicated Substrate and 1-Phenylethanol Substrate Solvent [BB]a [BK]a [KK]a [B-]a [K- Cumene MeCN 5.52 9.80 3.90 20.8 17.6 Cumene Benzene 5.03 8.36 4.90 18.4 18.2 p-Xylene MeCN 4.87 8.26 3.69 18.1 15.7 p-Xylene Benzene 4.09 5.64 5.26 13.8 16.2 Toluene t-BuOH 4.05 8.50 3.27 16.7 15.1 a -3 10 M Table 20. Substrate Toluene Cumene p-Xylene a10'3M [TFA]a 2.83 0.58 1.27 0.24 75 [88]a 1.90 0.63 1.96 [BK]a 3.80 2.09 4.03 [RR 2.51 1.14 1.05 1.48 ]a 7.60 3.35 7.95 ]a Products from the Reaction of t-Butoxy Radicals with Indicated Substrate and 1-Phenyl-2,2,2- Trifluoroethanol in Benzene [K- 5.02 6.08 4.19 6.99 76 Photoreduction by Alcohols The low quantum yields of acetophenone photoreduc- tion by alcohols, such as 1-phenylethanol and 2-propanol,63 prompted a re-examination of the interaction of excited acetophenone with alcohols in an attempt to determine the source of the inefficiency. The experiments were carried out in either benzene or acetonitrile solutions of ketone and alcohol. For maximum quantum yield studies the ketone concentration was kept constant (0.10M) and the alcohol concentration was varied. For the hydrogen exchange studies used to determine disproportionation and cage re- actions the alcohol concentration was kept constant and the ketone concentration was varied. The products that were measured were the alcohol corresponding to reduction of the starting ketone, the ketone corresponding to oxida- tion of the starting alcohol, and the pinacols formed from the coupling of two hydroxy radicals. The product alcohol should be formed only by disproportionation, while the product ketone can be formed by disproportionation and exchange of a hydrOgen from the hydroxy radical to the ground state starting ketone. This hydrogen exchange accounts for the majority of the ketone produced. The first experiment was used to determine the extent of radical disproportionation. This was accomplished using two complementary systems. One system was the photo- reduction of prOpiophenone by 1-pheny1ethanol and the 77 other was photoreduction of acetophenone by 1-phenylpropanol. Both systems will produce the same radicals in the primary reaction and therefore disproportionation can be measured in both directions. The disproportionation products measured were 0 OH OH OH OH O I I I PhCEt + PhCHMe .33.". PhCEt + PhCMe +2- PhCHEt + PhCMe \/ \/ Disproportionation Disproportionation Coupling Products 1-phenylpropanol and 1-pheny1ethanol. By increasing ketone concentration it is possible to make use of the exchange of hydrOgen atoms from hydroxy radicals to ground state ketones to study cage coupling and the disproportiona- tion of a particular hydroxy radical. The results of this can be clearly seen in Tables 21 and 22. The photoreduc- tion of varying concentrations of acetophenone by 1-pheny1- propanol gave the best results in regard to this. For the highest concentration of acetophenone studied (0.3M) not only is the quantum yield of l-phenylethanol less than 3% of the quantum yield of acetophenone pinacol, but also the quantum yield of crossed pinacol is less than 1% of the total quantum yield of all pinacols formed. The experi- ment with propiophenone and l-phenylethanol shows similar trends. 78 0 NH 0m 00 .>cou w Hocmmoumamconmla tom .md. ococondoumom mo cofiuommm on» How moamwm 3202050 ha 0N .>:oo w Hoconumahcmnmla new .mm. msocanOAQOHm mo :ofluommm may now moawflw Educmso m w m .mmm. e 0m50.0 0H50.0 0mv0.0 00H0.0 m .mmm.e ocwucmm 2a Zo~.0 n HH00.0 Hm00.0 H5H0.0 ma00.0 .mmm..mmd.e mma.0 00H.0 bNN.0 m0m.0 ~.2e«. moancmm cw sz.0 va0.0 00m0.0 0000.0 00m0.0 .2mm..=m<.e bNH0.0 m0~0.0 N .mmm.e 0NN.0 vom.0 mm .ousmmoe on 30a 0090 .Hocmmoumahsmnmua. bN00.0 mm00.0 «000.0 m500.0 N mmde N :3... .Hocmaumasemamue. mNH.0 MHH.0 0m0.0 0HH0.0 «000.0 0000.0 .mmmm. .NN .HN NOM.0 HmH.0 000.0 0m0.0 .2. .mm. 0.269 00N.0 m¢H.0 mm0.0 0N0.0 a... OHQMB 79 The two hydroxy radicals were also generated by using 2-propanol as the hydrogen donor and by having equal amounts of acetophenone and propiophenone in solution (Table 23). In these experiments both l-phenylethanol and l-phenylpropanol were measured. The ratio of pinacol products is a function of the steady-state concentration of the two hydroxy radicals and the ratio of their self- coupling rate constants. The observed ratio of aceto- phenone pinacol to propiophenone pinacol is approximately nine. The ratio of hydroxy radicals from acetophenone to those from propiophenone is therefore approximately three. These ratios change slightly with increased ketone con- centration. The total of the disproportionation products, 1-phenylethanol and l-phenylpropanol, account for about 3% of the products, with the other 97% being the pinacol products. When ketones other than propiophenone were used in the above experiment, electron withdrawing groups in- creased the fraction of hydroxy radicals formed from.that ketone relative to hydroxy radicals formed from aceto- phenone. Electron donating groups had the opposite effect. Thus when a solution equimolar in acetophenone and p- methoxyacetophenone was photoreduced by 2-propanol in benzene, the only product observed was acetophenone pinacol. When acetophenone and m-trifluoromethylacetophenone were used the acetophenone pinacol accounted for less than 4% 80 Table 23. Results for Reaction of Acetophenone (AP) and Propiophenone (PP) with 2-Pr0panol in Benzene a Run °:oo w muosooum mucmuommm .meme. Hoamaumesemamne no .~.ma<.. Hoomcwm osocmnmoumom £903 .00. cocoonQOAQOHm Ho cowuommm way How muaommm .mm manna 88 could be observed. The reaction of MFA with toluene pro- duced bibenzyl, acetophenone and an unidentified com- pound that is probably the cross-coupled alcohol. This latter compound was produced in smaller quantities than either bibenzyl or acetOphenone. It could not be deter- mined if fluorine was lost as a fluorine atom or as fluoride ion, and whether it occurred in the initial photoreduction or after formation of the hydroxy radical. Since the hydroxy radical could be formed by hydrogen transfer when 2-propanol is the substrate, whether MFA pinacol is formed or not could be useful in answering this question. The quantum yields for acetophenone formation are high and extrapolate to a maximum quantum yield of 3.3 (see Figure 29). A quantum yield greater than two suggests a chain reaction other than just hydrOgen exchange. In addition, only a minor long retention time product was observed. This product could be MFA pinacol or 1,2- dibenzoylethane. The latter produce could be formed from loss of a fluorine atom from the ketone and subsequent coupling of the radicals. Whatever this product is, it is clearly a minor product compared to acetophenone. 89 6 — _l__ ¢AP 5 .. 4 .. 3 .. 2 .. l .7 Figure 29. l I I I 4 8 12 16 _.___1__.___ 2-Propanol Acetophenone Formation from the Reaction of a-Fluoroacetophenone and 2-Propanol. DISCUSSION Selectivity of the Photoreduction Reaction Comparing primary-tertiary ratios for systemat- ically substituted ketones shows that electron withdrawing groups decrease, and electron donating groups increase, preference for tertiary hydrogen abstraction from p-cymene. This effect is observed for acetophenone substituted in the a-position (Table 26) and for ring substituted aceto- phenones, benzophenones and 0,0,a-trifluoroacetophenones (Table 27). A literature report for benzophenone and p-cymene gives a result favoring tertiary over primary with a ratio of 3.8 to 1.72 While this is higher than ob- tained here (2.4 to l), the conditions were not given and could account for the difference. For the sequence of acetophenone substituted by fluorine in the a-position (AP, DFA, and TFA) the rate con- stants increase as primary preference increases (see Table 28). This can be interpreted as a decrease in selectivity corresponding to an increase in reactivity. The results for TFA, a very reactive ketone, give a ratio of primary to tertiary products of 3.4 to 1. This corresponds to a primary to tertiary preference of 1.1 to 1 per hydrogen 90 91 Table 26. Effects of a-Substitution on Primary-Tertiary Ratio Ketone Tertiary : Primary Propiophenone 2.7 : 1.0 Acetophenone 2.8 : 1.0 a,a-Dif1uoroacetophenone 1.0 : 1.8 a,a,a-Trifluoroacetophenone 1.0 : 3.4 Table 27. Effects of Ring Substitution on Primary- Tertiary Ratio Ketone Tertiary : Primary Benzophenones 4,4'-Me-BP 5.2 : 1.0 4,4'-MeO-BP 3.0 : 1.0 BP 2.4 : 1.0 4,4'-C1-BP 1.7 : 1.0 Acetophenones p-Me-AP 3.3 : 1.0 AP 2.8 : 1.0 m-F-AP 2.3 : 1.0 M-CF3-AP 1 . 9 : 1 . 0 p-F-AP 1.7 : 1.0 a,a,a-TrifluoroacetOphenones p-MeO-TFA 1 . 0 : 1 . 1 p-Me-TFA 1.0 : 1.7 m-Me-TFA 1.0 : 2.1 TFA 1.0 : 3.4 m-CF -TFA 1.0 : 4.7 3 92 Table 28. Rate Constants for Photoreduction aSum of PP, PT and TT b APH2 is l-Phenylethanol cQuantum yield for pinacol of the ketone .d Acetophenone formation Ketone Substrate kd/kr(M) kr(106M-ls-1) ¢ggx AP Toluene 3.75 0.12 0.083 AP p-Cymene 0.94 0.48 0.058a AP APHZb 0.31 1.46 0.590c MFA Toluene 2.26 1.64 0.105 MFA 2-Propanol 1.08 3.43 3.300d DFA Toluene 1.14 4.30 0.056 DFA p-Cymene 0.20 24.50 0.045a TFA Toluene 5.08 6.80 0.077 TFA p-Cymene 0.42 82.20 0.051a ' TFA APHZb 2.57 13.4 1.000c 93 and can be interpreted as lacking any selectivity. Other results suggest selectivity is not directly related to reactivity. A primary to tertiary preference of 4.9 to 1 is obtained for m-trifluoromethyl-a,a,a-trifluoroaceto- phenone. This is a per hydrogen preference to 1.6 to l for primary product. This is large enough to be considered a definite preference and not just a lack of selectivity. This suggests that there is a change in selectivity with electron-withdrawing substituents, not a decrease in selectivity related to an increase in reactivity as men- tioned above. The selectivity for ketones without electron- withdrawing groups is in favor of tertiary hydrogens, which would be expected on the basis of bond strength. The rate constants for these ketones show a primary 38'39 which is expected for a re- deuterium isotOpe effect, action involving carbon-hydrogen bond cleavage in the rate determining step. For significantly electron deficient ketones, the selectivity is in favor of the primary hydrogens. The rate determining step in the reaction has been shown to be formation of an exciplex.38 It is possible to con- clude that electron-withdrawing substituents increase the likelihood of exciplex formation, and that reaction via exciplexes show a preference for abstraction of primary hydrogens. The magnitude of this primary preference depends on the extent of charge transfer in the exciplex. For example the rate determining step for the reaction of TFA 94 and toluene is exciplex formation.38'40 Since m-CF3-TFA is more electron deficient and reacts faster than TFA with toluene?3 it is likely that it too reacts entirely via exciplex formation with toluene. If both of these ketones react with toluene solely by exciplex formation it is likely that they also react with p-cymene entirely by exciplex formation, yet their selectivities are different (3.4 to 1 for TFA versus 4.9 to 1 for m-CF -TFA). A 3 direct hydrogen abstraction pathway would be in competi- tion with exciplex formation. Since p-cymene is extremely sensitive to sub- stituent effects it could prove to be useful in determining the fraction of reaction proceeding through an exciplex and the amount of charge transfer in a particular exciplex. To do this, however, the change in ratio due to extent of charge transfer has to be separated from the change in ratio due to percent of reaction proceeding through exciplex. To illustrate the difficulty of separating these effects and to underscore the utility of the p-cymene system it is informative to compare rate constants and selectivities for acetophenone and p-MeO-TFA. The rate constant for interaction with toluene is larger for acetophenone (kr = 1.9 x lOSM-ls-l in acetonitrile74) than for p-MeO-TFA (kr = 5.1 x 104M'1s'l in acetonitrile73). Yet acetophenone shows greater preference for tertiary abstraction, 2.8 to 1 versus 1 to 1.1 for p-MeO-TFA. Based 95 on rate constants alone very little could have been said as to whether an exciplex was involved in this reaction or not. The para methoxy group changes excited state reduction potential, thereby decreasing the rate constant for exciplex formation. It also changes the lowest triplet state from 73 which decreases n,w* for TFA to n,n* for p-MeO-TFA, the rate constant for any direct hydrogen abstraction. Therefore, this is a case where a substituent slows down both pathways, possibly by comparable amounts. The primary- tertiary ratio for p-MeO-TFA is a third of the ratio of TFA, yet the rate constants for reaction with toluene dif- fer by a factor of 200. It is not possible to determine how much of the change in selectivity is due to a change in exciplex selectivity and how much may be due to some direct hydrogen abstraction. A number of systems can be used as models for the behavior of the photoreduction reaction. The reaction of t-butoxy radicals with p-cymene was used as a model for direct hydrogen abstraction. Both n,n* triplet states and t-butoxy radicals have an unpaired electron in a non- bonding orbital on oxygen. This half-filled orbital is responsible for direct hydrOgen abstraction. The tertiary- primary preferences for reaction of p-cymene with aceto- phenone (2.8 to 1) and benzophenone (2.4 to 1) are very close to the results for t-butoxy radicals (2.7 to 1). Rate constants are also very similar for these three 96 compounds (Table 1). This shows that t-butoxy radicals are a good model for the triplets of acetOphenone and benzophenone, but does not necessarily mean that all pro- ducts come from direct hydrogen abstraction. Since the effects of substituents suggest that there is some exciplex formation with acetophenone and benzophenone, it is possible that t-butoxy radicals can also form a complex with p-cymene. However this complex may not be as important for product formation as it is for explaining quantum inefficiency for acetOphenone and benzophenone. Oxidation by cobalt (III) acetate can be used as a model for the two step reaction where electron transfer is followed by proton transfer, the former step being 75 showed that the rate determining. Onopchenko and Schulz methyl hydrogens are preferred over the isopropyl hydrogen by nine to one for this reaction. Addition of lithium chloride changes this to a 3.2 to l preference for the tertiary hydrogens. The electrochemical oxidation of p-cymene in methanol shows oxidation of the tertiary center to be approximately twice that of the primary center.76 However the effect of supporting electrolyte and base would have to be carefully investigated since they should both affect the deprotonation of the radical cation.77 This is clearly evident from the effect lithium chloride had on the oxidation by cobalt mentioned above. 97 Some photochemical systems that are believed to proceed by electron transfer show similar preference for primary hydrogens. In the photoreduction of esters by p-cymene78 there appears to be a primary preference. How- ever, the products observed suggest there may be some secondary reactions occurring making it difficult to de- termine exact ratios. Cohen8 has looked at the photo- reduction of benzophenone by tertiary amines and studied the products of oxidation cf the amines to find a pre- ference for primary or secondary proton transfer over tertiary proton transfer. Thus for N,N-dimethyl-Z- butylamine there is more than twelve times more formaldehyde from oxidation of the methyl group than 2-butanone from oxidation of the 2-buty1 group. This corresponds to a greater than 2 to l preference for the methyl protons when corrected for the number of hydrogens on each group. Similarly there is more than two times more acetaldehyde than acetone formed from the reaction of benzophenone with diisoprOpylethylamine. For the reaction of excited stilbene with amines the product ratio appears to depend on the statistical number of each type of hydrogen except for a few cases where a definite preference for primary versus tertiary hydrogen transfer (20 to l corrected for the number of hydrogens) is observed.79 This large primary preference was attributed to steric effects. Davidson5 studied the disappearance of benzophenone when photoreduced 98 by amines with N-methyl or N-benzyl groups and found a greater disappearance for the N-methyl amines. This is not as useful as Cohen's study, however, since the two groups were not present in the same amine. This would be similar to comparing quantum yields of photoreduction by toluene and cumene instead of the primary and tertiary hydrogens of p-cymene. Mechanistic Implications of Selectivity The above results are consistent with the argument that charge transfer in the exciplex leads to greater reactivity of the primary hydrogen. The results also support a competition between direct hydrogen abstraction and exciplex formation for the photoreduction of aceto- phenone by alkybenzenes. The rate constants for aceto- phenone photoreduction show interactions with l-phenyl- ethanol to be greater than with toluene, cumene and p- xylene. This is consistent with direct hydrogen abstrac- tion dependent on carbon-hydrogen bond-strengths. Be- cause of relative bond strengths and ionization potentials the alkylbenzenes should show smaller rate constants for direct hydrOgen abstraction and larger rate constants for exciplex formation than l-phenylethanol. Therefore, it is possible that there is some exciplex formation between acetophenone and alkylbenzenes, although deuterium isotope effects rule out all products coming from an irreversibly formed exciplex. 99 One way to describe the competition between path- ways would be to construct a three dimensional energy dia- gram as in Figure 30. The z-axis would be potential energy, the x-axis would be the movement of the hydrogen atom, and the Y-axis would be a measure of electron trans- fer from the substrate to the ketone. The origin would be the encounter between ketone and substrate with no complexa- tion (Point A). Moving in the direction of charge transfer one would go from point A over a transition state for exciplex formation (Point B) to the exciplex (Point C). Moving in the direction of hydrogen transfer from point A one would proceed up in energy to the transition state for direct hydrogen abstraction (Point D) and then down to the radical pair (Point G). From the exciplex (Point C) one could proceed up in energy toward point F, which is the transition state for transfer of a hydrogen (or proton) from the exciplex. This path combines movement along the axis for charge separation as well as the hydrogen transfer axis. This is in keeping with the transfer of a proton, which would neutralize the charge separation and lead to the same radical pair as direct abstraction. The two reaction paths can now be described by this diagram. Direct hydrogen abstraction is represented by the path A-D-G, and reaction via exciplex is described by path A-B-C-F-G. The relative energies of B and D deter- mine whether an exciplex will be formed or not. If an exciplex is formed the relative energy of points B and F 100 mwélx .GOauomom cofluoovmnouonm ozu H00 Emummao amumcm aneucmuom .om whomam mexmuu 101 determine if it will be formed reversibly. For TFA and toluene all reaction proceeds via an irreversibly formed exciplex. This means that for TFA and toluene point F is lower than point B which, in turn, is lower than point D. In addition to reaction through point F and reversal of exciplex formation the exciplex has another path not shown. That path is decay to ground state reactants and leads to much of the inefficiency in photochemical reactions pro- ceeding through exciplex formation. It is possible that there are systems where points B and F are significantly higher in energy than the barrier for radiationless decay of the exciplex and points B and D are comparable in energy. For this case product formation would be primarily by direct hydrogen abstraction even though there would be comparable amounts of direct hydrogen abstraction and exciplex formation. The exciplex would mainly be a quench- ing reaction contributing little to product formation. Such a situation may be occurring in the reaction of acetophenone with alkylbenzenes. The selectivity results from p-cymene suggest that point D is higher for a primary hydrogen abstraction than for a tertiary hydrogen abstraction, while the Opposite seems to be true for point F. The extent of charge trans- fer in the exciplex would change the position of point C along the Y-axis and would also be expected to change the energies of points B, C, and F while points A and D should 102 be unaffected. The relative energies of point P for primary and tertiary abstraction may also be affected. These changes would have a significant effect on the course of the reaction. An increase in charge separation corresponding to a decrease in the energy of points B and C would lead to an increase in the rate of exciplex forma- tion. This is consistent with the observed relationship between rate constant for exciplex formation and AG for electron transfer. An increase in charge transfer may also lower the energies of the transition state for re- action from exciplex (Point F) and the barrier for radiationless decay. There is no evidence for either of these changing very much, however. The reaction of TFA and l-phenylethanol is an interesting situation. The rate constant for this re- action is comparable to that of the reaction of TFA and toluene, even though toluene would be expected to be more reactive toward exciplex formation. This suggests that TFA may be reacting partially via direct hydrogen abstrac- tion with l-phenylethanol. The above interpretations, suggesting that aceto- phenone can form exciplexes with alkylbenzenes and TFA can react with 1-phenylethanol by direct hydrogen abstraction, are not unreasonable. Most reactions that proceed through more than one mechanism have cases where only one mechanism is predominant as well as cases where the mechanisms are 103 in competition. Determining when only one mechanism is Operating is a major step in understanding reactions that occur by more than one mechanism. The results discussed here have laid the foundation for investigating competing pathways in the photoreduction reaction. Solvent Effects on Selectivity The increase in primary preference in acetonitrile compared to benzene is large enough to be of interest, but does not suggest a major change in mechanism. The dif- ference in maximum quantum yields for photoreduction of TFA by p-cymene in benzene and acetonitrile are not large enough to suggest a major interaction of solvent with the exciplex. The increase in primary preference can be attributed to a combination of more charge-transfer in the exciplex, due to a polar solvent, and a decrease in dis- proportionation of radicals. The change in selectivity when trifluoroacetic acid is added to benzene but not acetonitrile suggests the acid is reacting with the ketone-benzene exciplex. The formation of an exciplex is the main path for radiation- 38 If acid reacts with less decay (kd) for TFA in benzene. this exciplex to form another intermediate it is possible that this intermediate can react with p-cymene with a dif- ferent selectivity than the triplet ketone. The reaction of trifluoroacetic acid with a TFA-benzene exciplex was 80 suggested by Bryce-Smith to explain the addition of TFA 104 to benzene to form 1,1-diphenyl-2,2,2-trifluoroethanol. The protonation of an exciplex by a protic solvent has also been suggested for an intramolecular exciplex forma- tion.81 The lack of dependence of quantum yield on sub- strate concentration for the reaction of TFA with toluene in benzene with 0.05M trifluoroacetic acid further supports this mechanism. The slope divided by the intercept of the double reciprocal plot, which is equal to kd/kr’ is close to zero. Therefore kr should be much greater than kd. This means that either kr has increased dramatically or kd has decreased. Lifetimes obtained from quenching studies indicate that neither has changed, however. The reaction of acid with the TFA-benzene exciplex to form another reactive intermediate which can react with toluene can be used to explain these results. This means that reaction with benzene, which had been the major component of kd' leads to reaction. This effectively reduces the kd and in- creases the kr' leading to a very small ratio of kd/kr from the double reciprocal plot. However the rate constants for reaction of triplet ketone with benzene and toluene do not change, and therefore the lifetimes of ketone are the same with and without acid present. Further information is needed to detenmine the nature of the intermediate formed when acid interacts with the TFA-benzene exciplex. It is also interesting to note that maximum quantum yields are the same whether acid is present or not, although the 105 selectivity of product formation with p—cymene changes noticeably. The former result suggests a common inter- mediate while the latter suggests different intermediates. Fate of Radicals Comparison of the coupling products from radicals generated from.the photoreduction reaction to those formed by abstraction by t-butoxy radicals show no significant differences. Since the photoreduction reaction produces radicals in-cage and peroxide decomposition can only pro- duce them out-of-cage, there must not be a significant cage reaction occurring in the photoreduction reaction. A correction for different amounts of the two radicals was made by assuming equal rate constants for the three coupling reactions. This method predicts half of the products to be cross-coupled product and the other half equally divided between the two self-coupling pro- ducts (BB:BK:KK equal to 1:2:1) when the two types of radicals are formed in equal amounts. When there are unequal amounts of benzyl radicals (B-) and hydroxy radicals (K-) the cross-coupled product concentration is predicted as follows: __ [B'] [K-J [BKJpredicted - [B-] + [KJT The concentrations of B- and K- are calculated from the observed coupling products. Comparing the experimentally 106 observed values to the predicted values shows no significant difference between coupling ratios from photoreduction and peroxide initiated reaction (Table 29). This comparison also shows that the ratio is a function of the particular radicals involved and the solvent used. Changing relative ratios of the two types of radicals does not appear to change the ratio of BK observed to BK predicted. The amount of ketone formed from hydroxy radicals in the peroxide reaction gives an upper limit to the amount of diSproportionation occurring. To calculate the amount of disproportionation it was assumed that all the ketone measured was a result of disproportionation between a hydroxy radical and a benzyl radical. This neglect of any disproportionation between two hydroxy radicals is justified on the basis of the results of the photoreduction of acetOphenone by various alcohols, which resulted in only 3% disprOportionation compared to 97% coupling. The oxidation of hydroxy radicals by peroxide is also ignored even though it is not negligible. To relate these results to the photoreduction reaction the fraction of ketones to cross-coupled product was used in conjunCtion with the product distribution for the coupling products to calculate the maximum fraction of hydroxy radicals that can be ex- pected to disprOportionate (Tables 30 and 31). This value (K- DiSprop./K- Formed) is the maximum inefficiency that can be expected in the photoreduction reaction as a result of radical diSproportionation. 107 Table 29. Product Coupling Ratios-Photoreduction versus Peroxide Induced Reaction [BK]([B] 4' [KJI Substrate Solvent Reaction [Kl/[B] B x K p-Xylene Benzene Ketone 1.16 0.76 p-Xylene Benzene Peroxide 1.17 0.75 Cumene Benzene Ketone 1.10 0.98 Cumene Benzene Peroxide 0.99 0.92 p-Xylene MeCN Ketone 1.14 0.95 p-Xylene MeCN Peroxide 0.87 0.99 Cumene MeCN Ketone 1.04 1.07 Cumene MeCN Peroxide 0.85 1.03 Toluene t-BuOH Ketone 1.36 1.09 Toluene t-BuOH Peroxide 0.90 1.07 Table 30. DisprOportionation Results from Peroxide Experiments, Acetophenone and Toluene [Persfiide] Solvent [APIAillBK] EégégfiggE’ 0.020 Benzene 0.28 0.14 0.030 Benzene 0.44 0.25 0.051 Benzene 0.56 0.35 0.101 Benzene 0.63 0.42 0.152 Benzene 0.68 0.48 0.050 MeCN 0.35 0.21 0.100 MeCN 0.39 0.24 0.150 MeCN 0.40 0.25 108 Unfortunately the di-t-butylperoxide oxidizes the hydroxy radicals from l-phenylethanol very rapidly in benzene. This results in a large amount of acetophenone formed from this reaction in addition to the acetophenone formed from disproportionation. Conversions have to be kept very low to prevent competitive absorption of light by acetophenone. When low peroxide concentrations are used this causes difficulties in analyzing for acetophenone. Because of this, results in benzene are not as useful as the results in acetonitrile. However, it does appear as if disproportionation is responsible for less than 25% of the reaction in acetonitrile and less than 35% of the re- action in benzene. Since the photoreduction of aceto- phenone by toluene is less than 50% efficient in benzene these results suggest pathways for inefficiency other than disproportionation. .The most likely source of this in- efficiency is exciplex formation. For the reaction of peroxide with l-phenyl-2,2,2- trifluoroethanol and either toluene or p-xylene (Table 31) the amount of ketone (TFA) formed is very small. Apparently this hydroxy radical is not oxidized as rapidly as the one formed from l-phenylethanol. The maximum.amount of in- efficiency expected from disproportionation is less than 15%. The inefficiency observed for the photoreduction of TFA by toluene and p-xylene is greater than 70%.38 Since the photoreduction of TFA by toluene and p-xylene proceed ‘Table 31. Substrate Toluene Cumene p-Xylene 109 DiSproportionation Results from Experiments, TFAH2 in Benzene [TFA] ITFAI + IBKI 0.13 0.38 0.06 Peroxide K -D.'I.8 r0 0 K-Formed 0.08 0.25 0.03 110 entirely by exciplex formation, this inefficiency is easily explained by decay of the exciplex to ground state reactants. Photoreduction by Alcohols The minor amounts of l-phenylethanol and 2,3- diphenyl-Z,3,-pentanediol formed when 0.3M acetophenone is photoreduced by l-phenylpropanol to give almost entirely acetophenone pinacol is inconsistent with in-cage coupling or significant dispr0portionation. The fraction of in-cage coupling has to be less than the fraction of 2,3-diphenyl- 2,3-pentanediol found. Since this product accounts for less than 1% of the products there cannot be significant in-cage coupling. Since the l-phenylethanol accounts for only 3% of the products and the acetophenone pinacol accounts for roughly 97%, disproportionation must not be responsible for the greater than 40% inefficiency observed in the photoreduction of acetophenone by l-phenylethanol. The results of the complementary experiment using propio— phenone and l-phenylethanol lead to the same conclusion. The photoreduction of TFA by l-phenylethanol is more efficient than the photoreduction of acetophenone. Since the TFA should be expected to result in more exciplex formation it is unlikely that exciplex decay is the cause of the inefficiency for acetOphenone. In addition to information on disprOportionation and cage reactions, these experiments also provide a method of estimating the rate constant for exchange of 111 a hydrogen from a hydroxy radical to a ground state ketone. It is possible to calculate the steady state concentra- tion of hydroxy radicals if the rate constant for coupling, the light intensity, and the quantum yield for reaction are known. The rate of formation of radicals is the quantum yields times the light intensity. The rate of dis- appearance is the rate constant for coupling times the radical concentration squared. For the hydroxy radicals from the reaction of acetophenone and l-phenylethanol the rate constant for coupling in benzene is 2 x 109M-ls-1.82 The light intensity is approximately 0.008E/l.hr. for these experiments, and the quantum yields for reaction are approximately 0.20. This gives a steady—state concentra- tion of hydroxy radicals of approximately 1.5 x lO-BM. In the acetophenone and l-phenylpropanol experiment about 80% of the radicals from the alcohol have exchanged when the acetophenone concentration is 0.03M. This means that at this concentration exchange is four times faster than coupling. Using the rate constants for coupling and the steady state concentration of radicals the rate constant for exchange can be estimated to be approximately 4000M-ls-1. The exchange in the opposite direction when propiophenone is photoreduced by l-phenylethanol is slower, with a rate constant on the order of 103M-1s-1. These rate constants are considerably slower than the rate constant of 2.75 x lOBM-is-l for the exchange of a hydrogen atom from 112 2-hydroxy-2-propyl radicals to benzophenone in the photo- 83 This is not reduction of bezophenone by 2-propanol. surprising since the system studied here involves almost isoenergetic radicals, while the radicals involved in the photoreduction of benzophenone by 2-pr0panol are not as close in energy. A better example is the degenerate ex- change of a hydrogen from a hydroxybenzyl radical to benzaldehyde. The rate constant for this exchange was de- 84 to be 8 x 104M-ls-l. termined from CIDNP experiments The fact that the exchange is slower for the ketones and alcohols used here allows the measurement of the exchange by chemical methods such as product studies. For faster exchange the reactions have to be studied by physical methods such as flash photolysis and CIDNP.83 The comparison of 1-phenylethanol and l-phenyl- ethanol-O-d as the substrate for photoreduction of acetophenone in acetonitrile suggests the hydroxy proton influences the reaction. Even though the l-phenylethanol- O-d used was only 65% deuterated, the increase in maximum quantum yield was significant. The deuterated alcohol showed a maximum quantum yield of 0.71 compared to 0.49 for the undeuterated alcohol, a 45% increase. The exact cause of the interaction is not clear, but it may involve interaction with the a-protons of acetOphenone. Both benzophenone and TFA are photoreduced by alcohols more efficiently than is acetophenone. Neither TFA nor 113 benzophenone have a-protons. It is possible that the triplet enol of acetophenone is formed when acetophenone interacts with alcohols. This reaction may occur by initial abstraction of the hydroxyl proton since deutera- tion increases the efficiency of the reaction. 3fi* OH OH OH PhCMe + PhCHMe ——pPhCMe + PhCMe —.. Products C-H ' ' 3 0* OH OH O- PhCMe + PhCHMe ———.- PhCMe + PhCHMe O-H ’ r- H 3 * OH OH _.._.. I PhC=CH2 + PhCHMe 0 .. The reaction of propiophenone and the pinacol of aceto- phenone to give propiophenone pinacol and acet0phenone supports the possibility of hydrogen abstraction of a hydroxyl proton. In this case the alkoxy radical that is formed can cleave to give ketone and a hydroxy radical. Decomposition of pinacols by ketones was observed by cl)- (IH I OH I PhC —C|IPH ——» PhCMe + PhCMe Me Me Schonberg and Mustafa, who attributed it to sensitized bond cleavage instead of a hydrogen abstraction mechanism. 114 The reaction has also been studied by CIDNP for the benzo- phenone-benzpinacol reaction.83 The results for the reaction of a-fluoroaceto- phenone (MFA) and 2-propanol suggests a chain mechanism involving abstraction of a hydrogen, exchange of hydrogens to ground state MFA from 2-hydroxy-2-propyl radicals, loss of fluorine atoms from the hydroxy radicals of MFA, and hydrogen abstraction from 2-propanol by the fluorine atoms. The loss of fluorine atoms and their abstraction of hydrogen from 2-pr0panol is necessary to account for the high maximum quantum yield (3.3) of acetophenone. If fluorine were lost from the hydroxy radical to give the enol of acetophenone but did not abstract hydrogen from 2-propanol, the maximum quantum yield should be less than two. Loss of fluorine from the excited state, a process which is common for a-chloro and a-bromo ketones, with sub- sequent hydrogen abstraction from 2-propanol by fluorine, followed by disproportionation of the resulting radicals, could also give acetophenone. This mechanism would give at best a maximum quantum yield of one, however. Loss of fluorine from a radical would not normally be expected and was not observed for the difluoro and trifluoro ketones. The concentration effect observed for selectivity with p-cymene could be due to a change in the amount of abstraction by fluorine. At higher p-cymene concentration there would be higher radical concentration, and therefore 115 Primary PhotoreactiOns (Initiation) 3 0* fl PhCCHzF ——_—.. PhCCH2 + F- 30* OH OH OH PhCCHzF + MeCHMe -—————.. PhCCHzF + MeCMe Secondary Radical Reactions (Chain PrOpagation Steps) OH OH F ————-- PhC=CH + F- PhOCH 2 2 OH OH F- + MeCHMe -————0- HF + MeCMe OH O E OH MeCMe + PhCCHZF -—————~. MeCMe + PhCCHzF Radical Coupling pH on gs 2 PhCCHzF ———.» PhC— CPh CHZF CHZF 0 0 0 II II II 2 PhCCI-IZ ____.. PhCCHZCHZCPh Disproportionation OH O ‘ I II I PhCCEH2 + MeCMe ——> PhCCH + MeCMe 3 H 0 ll PhCCH2 + PhCCH2P____,.PhCCH + PhCCHZF 3 116 bimolecular coupling of the hydroxy radicals could more effectively compete with the unimolecular loss of fluorine. Charge-Transferguenchers Quenching of the reaction of TFA and toluene by DABCO or p-dimethoxybenzene led to rate constants for quenching that are all within a factor of two of diffusion controlled (Table 32). Although there may be changes in product ratios and plots for particular products may curve, it is possible to get rate constants by looking at total formation of a particular radical. This is necessary because the ratio of hydroxy radicals to benzyl radicals increases with increased quenching, causing a significant difference in quenching of bibenzyl versus cross-coupled alcohol. By looking at quenching of total benzyl radicals this effect can be corrected for. The increased product formation observed at low DABCO con- centrations in acetonitrile, leading to an intercept of 0.4 with TFA and 1.0M toluene, depends on both solvent and the basicity of the amine. In benzene the product ratio changes, but there is no product enhancement. When p-dimethoxybenzene is the quencher there is no pro- duct enhancement in either benzene or acetonitrile. The reason for the product enhancement could be due to DABCO reacting with either the exciplex or the triplet ketone before exciplex formation. Reaction of DABCO with the radicals would not be expected to have such a large effect 117 IIII IIII m~.¢ com IIII IIII on... owe. IIII IIII mm.~ ode a.~. oeoa om.o~ one. a v a v P . H .HImHI2aoH. x .HI2. x .HanIzmoH. x .HI2. x .M .m 00.H Hv.H 00.H 00.0 Am 0 .3: 00.0 00.0 00.0 00.H .2. .mcmsaoe. mcmncwnaxozuweflolmm GCONGOQ 2002 GCGNGGQ 2002 ucm>aom momsHos can «he no 020202020 memsmne mmumsu H00 mucmumcoo 0000 mm2oId mmzoIm oom.o Hmm.o oam¢.o m~.~ omm.o mm.m Hm.~ mhfl.o moH.o Han.o «Hp.o nmm¢.o oo.~ Ho¢.o ~H.m mm.a vma.o «mo.o mmo.o mam.o omo¢.o mn.H oov.o m~.m -.~ maa.o Hmo.o oem.o mmm.o omm¢.o om.H nem.o ma.~ ma.a «ma.o Hao.o Gmm.c vam.o cmmm.o m~.H ocv.o p~.~ mm.a mha.c mac.o Hem.c 44m.o vosm.o oo.a oom.o H~.~ mm.a «ma.o Hmo.o mmv.o ovv.o omm~.o ma.o A: -osc is -osc 12m-osc m 125 _mma .99. Ham. Aauame H baaoxmm nuaoxae usaoxae tho\am .mm. wcmncmm cw Ammv mcmE>UIm Ucm maocmnaoumod How sumo onHM Educmoo .mm manna .Hs ma .2mmco.o HNHUHG .unHH .zamoo.o _aao_o ommaw m CEDHOUQ oomdw ¢ GESHOUM Hm.o H mm .Ecmam .20a.o u .mcocmsmoumo¢_ 145 omm.o no.4 ma.~ maa.o mas.o mma.o Hom.o om¢m.o om.~ 84¢.c Ge.m m~.~ vma.o Hmo.o aaa.o Hmm.o ommq.o m~.~ cmm.o mm.m Hm.~ mna.o moa.o Hah.o ~Hh.° nmm¢.o oo.~ Hoq.o ma.m mm.a «mH.o ~mo.o ~mo.o mac.o omoe.o ma.a om4.o mm.m -.~ mha.o Hmo.o ovm.o mmm.o ommq.o om.H hem.o ma.~ mh.a «ma.o Hao.o omm.o 44m.o ommm.o m~.H oov.o an.~ mw.a m~H.o mac.o Hem.o svm.o noam.o oo.H oom.o H~.~ mm.a ama.o Hmo.o mmq.o owq.o omm~.o mn.o E -o: E -s: E -0: 2: _mm_ _99_ Ham_ AanamcmH nuao\mm apaoxae maso\ea maso\am _mm_ mcmucmm cw Ammv mcmfihunm 0cm mcocosmoumo< How sumo onwM Esucmoo .mm magma Table 33 (continued) [BH] (M) 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 (DPT 0.0090 0.0108 0.0116 0.0127 0.0129 0.0144 0.0147 0.0159 ¢TT 0.0143 0.0158 0.0177 0.0186 0.0202 0.0207 0.0225 0.0231 ¢PP 0.00195 0.00230 0.00226 0.00260 0.00261 0.00300 0.00290 0.00330 Zomoo.o _mHU.Q zmmoo.o .m.o.m oonae d 265.00 .o.q .mm.o H mm .EcMHM .zoa.o u .mcocmcmoumomouosauflalc.5. 147 mm.~ 99.0 99.~ 4m..o 999.o mm~.o n9~m.o om.~ 99.. oo.o Go.~ o...o m~9.o m-.o mmmh.o m~.~ m~.~ m9.o mq.~ em..o om9.o .m~.c n~99.o oo.~ «9.. om.o mo.~ o...o ~99.o .m~.o amm9.o m9.. m~.~ ~9.o mv.~ qm..o om9.o ov~.o amHm.o om.. «9.. 9m.o mm.. o...o «o9.o n.m.o um¢9.o m~.. m..~ 9o.o mm.m vm.-o m~9.o m-.o nm99.o oo.. «9.. .m.o .m.. o...o Gom.o mm..o n.mo.o m9.o .mnoH. .muo.. .mno.. m .2. .mm. .99. .99. . .m. H m.o\mm m.o\99 m.o\9m .mm. HI mcwucmm :9 .mm. macawolm com mcocmnmoumomouocamwaaa.c How mumn oamww Esusmoo .vm manna Table 34 (continued) [BH] (M) 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 ¢PT 0.0164 0.0172 0.0177 0.0181 0.0187 0.0181 0.0187 0.0185 ¢TT 0.0046 0.0050 0.0052 0.0054 0.0055 0.0056 0.0054 0.0058 (DPP 0.0158 0.0162 0.0167 0.0169 0.0176 0.0171 0.0172 0.0174 149 .0: «.9 “.0: «mm 1.“: mvm “ammoo.o u .Hmo.o “ZHNoo.o u ..~o.o N23.8... n .Hmo.n “SmIOHM oo.. u 9m .om9.@ m assHoo .o.m .e:m.m .20..o n .mcoqmnmoumomouos.uflu9uc.a.s. mmmo.o o4moo.o sm.o.o oo..o mm.~ mm.o 9a.. «9.9 m...o m~¢.o m.nm~.~ mvmo.o o.moo.o mm.o.o oo..o m¢.~ .m.o -.. mm.o 9o..o mo¢.o 9.699.. Gamo.o oumoo.o mm.o.o oo..o @9.~ mm.o mm.. mm.o mo..o 9m¢.o m.nm~.. m9.o.o o.~oo.o 9moo.o ocm.o m~.m mm.o .m.~ mm.~ G.m.o oo¢.. w.omo.. ~m~o.o o9~oo.o o..o.o oo..o ~m.~ 9~.o a... 99.o omo.o oom.o m.nm9.o 99.9.o .m.oo.o 9999.9 oom.c m~.m mm.o 9m.~ .m.~ .m~.o oqm.. m.ooo.o mm.o.o .m.oo.o ~9oo.o mom.o .o.9 G9.o mo.m ~m.m «mm.o ovm.. 8.09m.o 9o.o.o 99ooo.o mwoo.o mao.o No.9 9o.o 9¢.o m..o v.o.o mmo.o m.nom.o «soo.o nqooo.o .mco.o 9-.o m4.. ...o o9.o m~.o omo.o qm..o m.no..o age 99o 999 ..\9.89 n.mm. m.99.. m.9... .~o\mm .~o\99 .~o\9m .mmw muooooum nonsmoouvm: .mcmucmm :9 .mm. camsholm com moocmnmoumomouooamwualc.d.c now sumo camfiw Eoucmoo .mm manna 150 memo.o mhmo.o mNmo.o mmwo.o mmmo.o mxe vmao.o Nch.o mmHo.o MNHo.o smoo.o 8M0 .o.~® o as:.oo .o.m mma.c oom.o mmo.o oom.o 890.0 mom.o mmo.o mmo.o «mo.o 9-.o axe ..\m.mH 0.9m. .ECMHm ~20H... mh.m wv.m hm.h hH.H hm.H o.9m. o.oa m.oa 0.xx. Zamoo.o Emmoo.o wo.H Nm.H .0: «mm .9: me :8... _hHUHG II In U) II In U) .mcocwcmoumom0905HMHnelc.c.s. mo.m om.h oom.m omv.N 9m... .omm.m oh.o qv.a 9.o\mm HHN.O mmm.o oam.m omn.m ovm.m mam.o NNb.o 9.o\9x M99o\zx muosooum HonooHd .mcmucmm c9 .mm. macawoum 0cm wcococmouwomouooHM9ualc.c.s How upon camfiw Esucmso m.wNo.H m.moo.o M.82... usgomoo “\UOHOO .2. .mm. .mm OHQMB 151 SMIOHW .mamEmm omumwomuuw may you concasoamo mum ocuuommu mcoflumuucmocoo was .60995 0903 .2m0.o.o. ~90 ocm .ZOMNo.o. 9.0 mc9c9oucoo mcmuco; mo uosq9.m .E H m can mHmEMm mo uosqwam HE m m coqu9omuu9 Hmumfim “.0090 m 020.00 .o.0 N00.. u 00 .00.. 00 1.0.90 o 055.00 .0.9 .99.. u 90 m o o n m .9: 0. .sc0.0 .20..0 u .mcocmcmoumomouos.m.u9n0.0.0. 0000.0 90.00.0 09.0.0 09..0 0090.0 000.0 009.0 . 99.9 000.0 90.0 0.90 00.. 9090.0 90.00.0 ...0.0 09..0 0090.0 000.0 009.0 00.0 000.0 .0.9 . ..90 00.0 09.0.0 00.00.0 9000.0 0...0 ..90.0 000.0 009.0 09.0 909.0 0..9 9.09 09.0 0000.0 .0000.0 0000.0 00..0 09.0.0 000.0 009.0 09.9 ....0 00.. 0.09 0..0 090 990 990 900 900 900 ..\9.0. -. .90. .. .99. . .99. .99. .wmw o m o m m m m.“ 90.0 ..0. 0.0.0 0000.0 009.0 00.9 900.0 90.0 00.. 09.0 ..0. 009.0 0000.0 909.0 90.9 000.0 00.0 00.0 00.0 0... ..0.0 9000.0 009.0 .0.9 900.0 09.0 09.0 .0.0 0.9. 009.0 90.0.0 00..0 00.9 .00.0 00.0 0..0 .2. 0.0.99. 0.0.00. 0.0990\99 0.099o\99 069909.. o.n9.o\99 o.n9.o\99 m9.o\00 .00. oawuuwcoumod :9 .mm. momewoam pom maocmnaou00009059999910.0.0 909 name v.09» Educmso .9m manna 152 .wocmummmmmmwo mcoumx momnm> ooEHOM mcmE>OIQ 8099 0.009009 90 ucsosm co ommmn moccamn .mwumumzo “Enloao .oo.N® U 000.00 .o.0 .00.. u 00 0.0090 0 000.00 .o.0 .00.. u 000 0.090 0 020.00 .o.0 .00.. u 000 .00 0 .200.0 .209000.0 u ..90. .Ewmoo.o u .hHU. .Zwmoo.o u _HHU_ .Emmmo.o u chocmsmoumOMOHosdmwualo.006. 0.0 90.9 9... 00.0 00.0 0..9 ..0.0 00.9 000 00.9 9... 00.0 00.0 9..9 0.0.0 00.. 9.0 90.0 0... 09.0 00.0 .0.9 900.0 00.0 wocmamm .2. 0.0.0000: 0.90. 0.00. 0.00. 0.99. 0.90. o9.o\90 .00. .0.. 99.. 00..0 990.0 90 0.00 00.9 09.. 99.9 00.9 90.. 09.. 09..0 0.0.0 90 0.09 00.9 9... 90.0 00.. 00.. 09.. 09..0 000.0 00 ..09 00.9 0... 00.9 00.0 00004 000000 .2. 09.000 0.9o\00 0.9o\99 0.9o\90 so.mum>:oo0 .009.0 m..o\<090 m..o\009 0..o\009 .00. mcmucmm :9 .mm. mcmE>Ulm 6cm wcocmsmouwomonosamwuald.0.0 mo GOHuommm mcu Mom wocmamm Hawumumz .mm @0309 153 Table 39. Quantum Yield Data for Acetophenone and Toluene (BH) in Benzene [BH] (M) 133/€14a [BB] (10-3M @BB 1.02 0.198 0.469 0.0182 1.53 0.263 0.623 0.0241 2.02 0.312 0.739 0.0286 2.50 0.363 0.860 0.0333 [Acetophenone] = .lOM, [C14] = .00230M, 313nm, 7.5 hr., Ia = .0258E/l, g.c. column B @140° aSF = 1.03 154 20-0.0 009.0 0 000.00 .0.0 .00.9 n 000 .00.0 0 000.00 .0.0 .00.. u 000 .00.0 0 000.00 .0.0 .09.. u 000 M .\00.00.0 u 9 ..00 0 .000.0 .009000.0 u .9.0. .2..0 u .0000000000000000.0u0. 090.0 0000.0 900.0 00.9 09.. 99.9 099.0 000.0 0.0.0 90.9 000.0 0900.0 .00.0 09.9 99.. 90.9 .09.0 900.0 090.0 90.. 900.0 0000.0 000.0 00.9 00.. 00.9 ..9.0 009.0 .900.0 00.. 000.0 0990.0 090.0 90.. .... 00.. 00..0 00..0 909.0 90.0 00 00 00 .2. 0 0 0 0.00. 0.00. 0.00. 09.o\00 09.o\00 09.o\00 .00. mcmncmm :9 .mm. 0:05.09 pom mcocmnmoumomouonamla How name @900» Eoucmso .ov magma Table 41. [BB] (M) 1.01 1.49 2.00 2.52 155 Quantum Yield Data for a,a-Difluoroacetophenone and Toluene (BH) in Benzene 813/014a [BB] (10'3M) 033 0.285 0.687 0.0269 0.334 0.805 0.0316 0.380 0.916 0.0359 0.412 0.993 0.0389 [0,a-Difluoroacetophenone] = .lOM, [C14] = .00234M, 313nm, 7.5 hr., a SF = 1.03 a = .0255E/l, g.c. Column B @140° 156 mo H I hmn mo.H u mmm omeaw m cesaoo .0.m .H\mm5mo.o u mH ..H: h .ECMHM .Emmmoo.o u _vHU_ .ZH.o u _mcocmSQOumOMOHOSHMflHBId.ded_ osoo.o memo.o Hm.~ Hom.o omc.a amm.o Hm.m «vmo.c mmmo.o oe.~ emm.o mum.o mmm.o oo.~ mmmo.o enao.o oo.~ Hom.o nmm.o qm~.o om.H deco.o mmao.o mm.~ n~¢.o mow.o mo~.o Ho.H Azmuoav A: nod. Ase am mm o 6 ”mm. _mm_ nqao\mm 6¢Ho\mm _mm_ wcmucmm cw Ammv mcmzaoa can mcocmnmoumomouosamwuala.d.d How mama Udmfiw Esucmso .Nv manme 157 Table 43. Stern-Volmer Data for Acetophenone and .SM Toluene [Q] (10’3M) BB/Cl4 ¢°BB/QBB 0.0 0.131 1.00 0.083 0.0861 1.52 0.166 0.0629 2.08 0.414 ' 0.0347 3.78 [Acetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0050M, 366nm, 36 hr., g.c. Column B @l40° Table 44. Stern-Volmer Data for Acetophenone and 1.0M Toluene [Q] (10'3M) BB/Cl4 ¢°BB/¢BB 0.0 0.174 1.00 0.082 0.122 1.43 0.165 0.087 2.00 0.412 0.050 3.48 [Acetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0045M, 366nm, 36 hr., g.c. Column B @140° 158 Table 45. Stern-Volmer Data for Acetophenone and 1.5M Toluene [Q] (10'3M) BB/Cl4 ¢°BB/¢BB 0.0 0.149 1.00 0.070 0.107 1.39 0.139 0.081 1.83 0.349 0.049 3.04 [Acetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0051M, 366nm, 36 hr., g.c. Column B @140° Table 46. Stern-Volmer Data for Acetophenone and 2.0M Toluene [Q] (10'3M) BB/C14 ¢°BB/¢BB 0.0 0.186 1.00 0.068 0.137 1.36 0.136 0.108 1.72 0.340 0.0067 2.80 [Acetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0052M, 366nm, 36 hr., g.c. Column B @140° 159 Table 47. Stern-Vblmer Data for Acetophenone and 2.5M 'Toluene [01 (10'3M) 88/014 4°BB/4BB 0.0 0.221 1.00 0.072 0.166 1.33 0.147 0.133 1.66 0.361 0.080 2.70 [Acetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0051, 366nm, 36 hr., g.c. Column B @140° Table 48. Stern-Vblmer Data for a-Fluoroacetophenone and 0.40M Toluene [01 (10'3M) BB/c16 4°BB/4BB 0.0 0.0878 1.00 0.69 0.0323 2.72 1.39 0.0195 4.50 2.78 0.0110 7.98 [a-Fluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) Quencher, [C16] = 0.00589M, 313nm, 10 hr., g.c. Column B @ 145°! 160 Table 49. Stern-Volmer Data for a-Fluoroacetophenone and 0.80M Toluene [Q] (10'3M) BB/C16 0°BB/0BB 0.0 0.1480 1.00 0.69 0.0618 2.39 1.39 0.0368 4.02 2.78 0.0226 6.55 [a-Fluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C16] = 0.00589M, 313nm, 10 hr., g.c. Column B @ 145° Table 50. Stern—VOlmer Data for a-Fluoroacetophenone and 0.94M Toluene [Q] (10'3M) BB/C16 0°BB/¢BB 0.00 0.224 1.00 0.47 0.125 1.79 0.94 0.084 2.67 1.87 0.054 4.13 [a-Fluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C16] = 0.00300M, 313nm 11 hr., g.c. Column B @ 145° 161 Table 51. Stern-Volmer Data for a-Fluoroacetophenone and 1.50M Toluene [Q] (10‘3M) BB/Cl4 (DOBB/(pBB 0.000 0.641 1.00 0.129 0.561 1.14 0.258 0.487 1.32 0.646 0.345 1.86 [a-Fluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.00470M, 366nm, 14 hr., g.c. Column B @ 140° Table 52. Stern-Volmer Data for a-Fluoroacetophenone and 1.87M Toluene [Q] (10’3M) BB/C16 4°BB/0BB 0.00 0.351 1.00 0.47 0.233 1.51 0.94 0.173 2.03 1.87 0.111 3.17 [a-Fluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) Quencher, [C16] = 0.00300M, 313nm, 11 hr., g.c. Column B @ 145° 162 Table 53. Stern-Volmer Data for a,a-Difluoroacetophenone and .SM Toluene [Q] (10'3M) BB/Cl4 0°BB/0BB 0.0 0.464 1.00 0.462 0.357 1.30 0.924 0.294 1.58 2.310 0.181 2.56 [a,a-Difluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0049M, 266nm, 36 hr., g.c. Column B @ 140° Table 54. Stern-Volmer Data for a,a-Difluoroacetophenone and 1.0M Toluene [Q] (10'3M) BB/Cl4 ¢°BB/4BB 0.0 0.495 1.00 0.449 0.403 1.23 0.899 0.345 1.43 2.250 0.227 2.18 [0,a-Difluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) Quencher, [C14] = 0.0049M, 366nm, 36 hr., g.c. Column B @ 140° 163 Table 55. Stern-Volmer Data for a,a-Dif1uoroacetophenone and 1.5M Toluene [Q] (10'3m) BB/C14 0°BB/0BB 0.0 0.339 1.00 0.465 0.290 1.17 0.930 0.246 1.38 2.320 0.169 2.01 [a,a-Difluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0049M, 366nm, 36 hr., g.c. Column B @ 140° Table 56. Stern-Volmer Data for a,a-Difluoroacetophenone and 2.0M Toluene [Q] (10'3M) BB/C14 0°BB/0BB 0.0 0.350 1.00 0.465 0.298 1.17 0.930 0.264 1.33 2.320 0.187 1.87 Ia,a-Difluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0048M, 366nm, 36 hr., g.c. Column B @140° 164 Table 57. Stern-velmer Data for a,a-Dif1uoroacetophenone and 2.5M Toluene [Q] (10’3M) BB/C14 6°BB/4BB 0.0 0.424 1.00 0.477 0.375 1.13 0.955 0.329 1.29 2.390 0.243 1.74 [a,a-Difluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0045M, 366nm, 36 hr., g.c. Column B @140° Table 58. Stern-volmer Data for a,c,a-Trifluoroacetophenone and .SM Toluene [01 (10‘3M) BB/C14 4°BB/4BB 0.0 0.0852 1.00 0.99 0.0516 1.65 1.99 0.0360 2.37 4.98 0.0184 4.63 [a,a,a-Trifluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0052M, 366nm, 38 hr., g.c. Column B @140° 165 Table S9. Stern-Volmer Data for 0,0,a-Trifluoroacetophenone and 1.0M Toluene [Q] (10’3M) BB/C14 WEB/03B 0.0 0.199 1.00 1.00 0.139 1.43 2.00 0.100 1.99 5.00 0.058 3.41 [a,a,a-Trifluoroacetophenone] = 0.10M, benzene solvent napthalene (Q) quencher, [C14] 8 0.0049M, 366nm, 36 hr., g.c. Column B @140° Table 60. Stern-VOlmer Data for 0,0,a-Trifluoroacetophenone and 1.5M Toluene [Q] (10‘3M) BB/C14 ¢°BB/0BB 0.0 0.185 1.00 1.00 0.134 1.38 2.01 0.103 ‘ 1.80 5.02 0.062 2.98 (a,a,a-Trifluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0047, 366nm, 40 hr., g.c. Column B @140° 166 Table 61. Stern-Volmer Data for 0,0,a-Trifluoroacetophenone and 2.0M Toluene [Q] (10'3M) BB/C14 6o BB/¢BB 0.0 0.224 1.00 0.99 0.176 1.27 1.98 0.139 1.61 4.96 0.089 2.52 [a,a,a-Trifluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0053, 366nm, 38 hr., g.c. Column B @140° Table 62. Stern-Volmer Data for 0,0,a-Trifluoroacetophenone and 2.5M Toluene -3 [Q] (10 M) BB/C14 ¢°BB/¢BB 0.0 0.180 1.00 1.05 0.145 1.24 5.23 0.079 2.29 [0,a,a-Trifluoroacetophenone] = 0.10M, benzene solvent, napthalene (Q) quencher, [C14] = 0.0057, 366nm, 36 hr., g.c. Column B @140° 167 Table 63. Product Ratios from Reaction of Indicated Ketone with p-Cymene in Benzene Ketone TT/PT PP/PT Column Used m-CF3-TFAa ----- 2.40 B m-Me-TFAa 0.24 ----- A Benzophenoneb 1.20 ----- B, 4,4'-Meo-BPc 1.50 ----- B 4,4'-Me-BPb 2.60 ----- A m-F-APb 1.10 ----- A m-CF3-APb 1.00 ----- A p-Me-APa 1.60 ----- A p-MeO-TFAd 0.53 0.68 B p-Me-TFAa 0.33 0.90 A p-Cl-TFAa 0.37 0.90 A PropiOphenonee 1.30 0.14 B p-F-APa 1.30 0.56 A 4,4'-c1-BPa 1.00 0.38 A g.c. Column A @155°, g.c. Column B @175°, 313nm, [Ketone] = 0.10M, [p-Cymene] = 1.0M a17 hr. b23 hr. C52 hr. d59 hr. e11 hr. 168 Table 64. Reaction of t-Butyl Hypochlorite (ROCl) and p-Cymene in Benzene [3001] A/Ba [Al/[81b 0.015 2.26 2.40 0.030 2.28 2.42 0.060 2.05 2.17 g.c. Column B @180°, 366nm, 1.5 hr. aProduct A is a,p-dimethylstyrene from quantitative elimination of the tertiary chloride. Product B is 4- isopropylbenzyl chloride. bResponse factors for the two products were estimated by comparing cumene to a-methylstyrene and B-chloroethyl- benzene to ethylbenzene. Table 65. Reaction of di-t-Butyl Peroxide and p-Cymene in Benzene Product Relative Peak Area PP 1.00 PT 5.89 TT 7.61 [di-t-butyl peroxide] = 0.10M, [p-cymene] = 0.60M, 313nm, 16 hr., g.c. Column B @180° 169 Table 66. Effect of Pyridine on the Photoreduction of TFA by Toluene in Benzene [Pyridine] 138/C16a BK/C16b [BBlc [axle 0° /0 0° /¢ BB BB BK BK 0.0 0.091 0.21 0.67 1.63 1.00 1.00 0.088 0.71 0.30 0.52 2.33 1.28 0.70 0.52 0.059 0.30 0.43 2.33 1.54 0.70 2.01 0.43 0.25 0.31 1.94 2.12 0.84 [C16] = 0.0062M, g.c. Column B @140°, 313nm, 5 hr., [Toluene] = 0.50M, [TFA] = 0.050M a SP = 1.18 bsr = 1.25 C10'3M Table 67. Effect of Acetonitrile on Photoreduction of TFA by Toluene in Benzene [MeCN] 138/C14a BK/C14b [BB] [BK] (M) (M) (M) 0.0 0.351 0.70 0.00145 0.00319 5.0 0.368 1.09 0.00152 0.00478 10.0 0.411 1.22 0.00170 0.00535 15.0 0.472 1.40 0.00195 0.00614- [Cl4] = 0.0040M, 313nm, 10 hr., g.c. Column B @140°, [Toluene] = 0.50M, [TFA] = 0.050M a SF 1.03 bSF 1.09 ..omaw 4 cesaoo .0.m «mo.H u mmn “vo.H u ems x Nom.o u mmm 1mm.o 0 mm 1: IOH0 1m~.a u mm .NH H H mm m m c n.ohaw m cesaoo .0.mo .Hocmnumflacmndua on.o can moflxonmm Hausnlulflp 23H.o mo cofiuommu ma m umcocmnmouwom Zomo.o mo cofluosomuouozm ma Mn .HOCMUDQIU mH U “QGTNCQQ mfi m «wdwuuflcoumom mH £0 2 .HS ma .Ecmam 170 hm.m om.w mo.v hm.a M.£ho.m m.£mm.m m U A2°o.Hv DGQDHOB om.v om.m mo.m mm.~ who.o m¢H.¢ m m AZHm.ov QGDEDU om.m om.m Nm.m mN.N nNH.h mvm.v m < Ava.cv wcwfido m~.m vo.m mo.v mo.m amo.v mma.m m m “Smm.ov wcoamxnm mm.m 0N.m hm.v 5H.N www.m mom.m m d ASmm.cv mamawxlm N¢.H hh.m mm.o mm.o M.£mm.H Smm.o M U AZCG.HV TCDSHOB hm.H mN.m em.a OH.H nfim.m m>~.~ M m Atom.ov mcmfidu mm.H oa.m m~.H mm.o flm~.~ mmo.H M d Axom.ov mcwfifiu mN.v mm.¢ mm.m h¢.~ Hmm.m mmo.~ M m Axom.cv mamahxlm mv.m av.m mo.~ mo.m Mam.m mmo.~ M d Atom.ov wcmHhxlm mMMM_ mHMm_ Gamma U.0maU\MM UmHU\Mm omHU\mm neoHuommm mucm>aom A2..ocoov mumuumnom weflxoumm Hausmuuuflo mo GOwuommm new ozocmnmoumod mo :ofiuooomuouonm Eoum muosooum mo cemwnmmsou .mo magma 171 Table 69. Effect of Pyridine on the Photoreduction of TFA by Toluene in Acetonitrile . . a b c c o o [nyfidine] BB/C16 BK/C16 [BB] [BK] 038/483 ¢BB/¢BK 0.0 0.313 0.98 1.51 5.02 1.00 1.00 0.065 0.230 0.76 1.11 3.90 1.36 12.9 0.50 0.076 0.30 0.37 1.54 4.12 3.27 2.05 0.024 0.09 0.12 0.46 13.00 11.00 [C16] = 0.0041M, g.c. Column B @140°, 313nm, 5 hr., [Toluene] = 0.50M, [TFA] = 0.050M asp = 1.18 bSF = 1.25 -3 c10 M 172 .HS 5 om.H ¢O.H Hm.o mm.o cm.c hm.o no.o oo.H eso\sme sme\eme mme\mme .EGMHM Hw.H mm.H No.H om.o mm.c mm.c mm.o co.H eo.m am.~ hH.H vm.c mh.c bh.o cm.c cc.H Hm.m hm.v Hm.m om.m mo.m v¢.m oa.m ov.m MMMMH ..owaw m assHoo .0.m .zmmoo.o h.a h.NH N.hH N.mH a.ma m.hH a.hH m.pa 83:: H San: oc.H Hm.H vv.¢ ma.m mo.m mh.m om.m mH.m mamm_ 6 42o.H mm.o ch.c vN.H mm.H mm.a mm.H va.H mh.o oHo\mm HmfimH—HOBU— m¢.H Na.d om.m mh.N mm.~ mw.~ on.m vm.m omao\xm n .Zomo.o wH.o mN.o Ah.c mm.o oo.d mc.H vo.H mm.o oao\mm oomwuommmmn on» mum so new .meo «.momo o aesaoo .o.m .mo.H u mmo “.mhaw m :EsHoo .0.m .mc.a u awn rzmuoam .0: NH .eamflm .amco.o u Mano. .zvvcc.o n _hHo1 .zom.o u .mcmeao161 .zomo.o u ”may. ma.~ m~.~ ~m.a qm.m oh.oa na.a «H.H «~.H mH.H on.a mo.~ ec.a mH.H ma.o em.o oo.H oo.H oo.H oo.H oo.o c.ae\.wo e.me\.mo sme\eme mme\mme ”conga. mm.o mq.m voo.o Hm.o ~m.o ma.c vu.o ch.o oa.o nfl.o ow.oa oo.a mn.q Hoa.o em.o cm.a m~.o Ho.a mmo.o ~m.o mo.o o>.H ~H.H mm.¢ hna.o «H.H ov.~ q~.o pm.o mmc.o mm.o om.o em.o mm.H o~.m mea.o >o.~ om.~ om.o HH.H meo.o om.o on.o oo.o «Mex. MMmMH m_ae_ 6.9m. mama. ohao\am ohao\mm n-o\ea namo\em nH~o\mm m_oom.H qo.a -.m om.a mmo.o ~mm.o hmo.o H~.m p.oa mh.q m~.o mm.H ~m.o mn.v oo.H mmo.o mvo.o ooo.o 121 o_ee_ omms_ 0Mmm_ 0.99. o_em_ namo\as namo\mm 6H~o\mm 6H~o\aa ma~o\em ”once. a com .oafiuuwcoumom :H ocmEMolm can «he no cofiuomom on» so ofiofi owuoomouosHmaua mo uomwum .wn manna 178 zmuoao .momw o assaoo .o.m .mo.H u mun omega m aesaoo .o.m .oo.H u mmm .0: on .eamam .hmco.o u Mano. .mmoo.o u _hao. .zo.H u gunmeaoua_ .zomo.o n .4291 oo~.o ~m.H «va.o amo.o omo.o Hmm.o omo.o «mo.o vo.a emm.c mm.a mmm.o ~o~.o mmo.o omv.o «Ha.o omo.o om.o ~mm.o hm.~ mem.c Hm~.c mma.o oos.o m-.o hmo.o oa.o He¢.o pm.~ mph.o ha~.o noa.o -m.o mu~.o moH.o oo.o :3 0_9s_ 0Hmm1 omen. omen. nhao\mm nuao\ms mamoxmm mHmo\em _nflomu N com .mawuuflcouoofi ca mamfiuolm now «me no cofiuommm may :0 ofiom capoomouosamwus mo pommmm .mn wanna 179 .momw o candoo .0.m .mo.H u mm .mhaw m :ssaoo .0.m .mo.a 0 mm .0: cm .scmam .omoc.o u MH~o1 .mmcc.o u _hao1 .zom.o u Hmcmeaoum_ .zomo.o u 1:691 mo.m mo.m mm.a Nam.o ma.a mm.H HH.~ mnv.o «po.o mam.o ~HH.° oo.m mo.m ma.~ mam.c mm.H ~m.a mm.~ eem.o mma.o mam.o ~mo.o mo.a mo.m em.a mma.o «m.o v¢.o mo.a nam.o mmo.o mH~.o ooo.o :21 0_es_ onmm_ 0.0:. 0.99. o_em_ :6Ho\sx :pao\ms 6H~o\mm mamo\ea 6H~o\am .8204. ocmncmm cw ocmshonm can fins Ho :ofiuommm on» :0 wand Daumomouosamana mo uomwmm .mh manna 180 Table 79. Effect of Trifluoroacetic Acid on the Photo- reduction of AcetOphenone by p-Cymene in Benzene b b [Acid] P'r/c17a TT/C17 [PT]c [TT]C [PPJC (M) PP/C17 0.000 0.260 0.395 0.058 0.92 1.40 0.20 0.050 0.933 0.620 0.449 3.30 2.19 1.59 0.110 0.590 0.460 0.221 2.09 1.63 0.78 [C17] = 0.0044M, SF = 0.81, 313nm, 13 hr. [Acetophenone] 0.050M, [p-Cymene] = 0.50M ag.c. Column A @150° bg.c. Column B @185° c10'3M Table 80. Effect of Trifluoroacetic Acid on the Photo- reduction of TFA by Toluene (BH) in Benzene [BH] 138/C14a BK/c14b [BB]c [axle 0 B 0 (M) B BK 1.00 0.601 1.26 3.36 7.44 0.070 0.155 1.52 0.608 1.25 3.39 7.38 0.071 0.154 2.01 0.610 1.25 3.41 7.38 0.071 0.154 2.50 0.620 1.28 3.46 7.56 0.072 0.157 [C14] = 0.0054M, g.c. Column B @140°, 313nm, 9 hr., [TFA] = 0.10M, [Trifluoroacetic Acid] = 0.050M, Ia = 0.048lE/1 asr II ..a O U.) bSF ll ..- O O \D c10.314 181 .H: mu .thco.o _mHoHA 1.“: ma .2mmcc.o “oonao m aesaoo .0.m .m~.H .0.m .HH.~ u m~.~ hc.q 56.: ~o.n «o.a mm.H mo.~ ~¢.m :~.~ mw.~ em.~ mo.H 5H.m ~m.~ om.m mm.~ m_es_ u_sm_ mN.N ¢Q.H mo.H h¢.N Hm.H Ho.H mm.H oN.H «Mam. hw.m om.H co.H mH.h mm.v OH.N HN.N mm.o Manda .mm 0 ESMHM .Zod.o N~m.o vmw.o ova.o mov.o mmm.c vmm.o mmv.c mhv.o 0mao\xx n HmHU_£ u.H£ mm .meco.o u ”Hocmnumahcmnmlaa oam.c mmv.c th.o mmm.o om¢.o vwm.o hmv.c mam.o mHo\mm «.oqflw a :EsHoo .o.m .BH.H emu “.maw 4 assaoo .o.m .om.~ n Hmm.o HmN.o MbH.o NNv.c cam.o NBH.o mvN.o oma.o omao\mm vom.o qu.o who.o mmm.o «hm.o mna.c th.o who.o mao\m< .Zo.H " mmHUH m as -oa m H “oovaw a :Esaoo «oaauuacouoom aw 4 .mcmucmn mH mm o n HDCQSHOBH 4 CD a: mm 00 fl fl macm>aom mmmv Hocmnumaacmcmna 0cm osmsHoa saws Amoomv wowxoumm Hausmlulwo mo cowuommm omH.o ooa.o omo.o NmH.o HOH.o Hmo.o omo.o oNo.o sz Hmoom_ .Hm manna mo.H u mm: om.o n mm: mm.o u emu mm.o u emu 182 me.: no.3 om.a vm.o mq~.o A~¢A.o :mmm.c Hmo.c HH.° mo.H mo.~ mo.c -.H mpa.o m-4.o mmvfl.o o:a.o HH.o 4H.H om.m om.H mm.o mma.o 0:mm.o 64mm.o omo.o mm.o Hm.~ -- -- mm.~ oae.o ------------ mv~.c -- sz 0_sx_ omen. onmm_ 0.4691 mHo\ss mHo\xm mao\mm amao\:m:m-H :uns mcflxoumm Hausa-u-«c mo :owuommm HH.H u mm: zm-oH0 mH.H 0 mm: m~.~ u emu admoo.o u :mHo_ .mu=::s\.~ :6 .ONH 0: .cm 800: cmeemumoum < aesaoo .0.m ..u: m: .samam .zmmc.o u .moflxouom Hanan-u-A61 .zom.c u _Hocm:uwouosam«uu-.~.-H»:0:d-H_ mcwahxlm meoEso ocmsaoa 0202 mm .mm manna 183 z IoH4 «Hoomcam commouo on» ma deM can MmM .Hoomcam ococonmoumom ma «MmMg «m~.H u mmm “mm.a H mm «oohaw m assaoo .0.m01~e;: n emu 13.4 u mmo N.23 4 :EsHoo .o.m: .mo.~ mm: .Hoomcwm mcocmnmoflmoum ma m m m . H\mmomo.o u H ..u: a .e:mam .zaqoo.c n “Ado: .zavoo.c 0 “mac. .zam.c.._Hoam:umam:0:a-H_ omho.o vmac.o IIIII I mNH.c caao.o mm.v hH.H IIII mm.h hw.o ooN.c oauo.o momo.o IIIIII maa.c Nmoc.o mN.v mm.H IIII mm.m mm.o mVH.o omvo.o memo.c hNHo.o mmo.o «mco.o Nh.~ mo.m hh.o hm.m Hm.o Nmo.o mmao.o mono.o mmmo.o ----------- «4.4 mo.m 55.4 -- -- omo.o m m m 4 4 4 m :21 4 Me a me s so :49 name “Human. 4.4444. an4s4x_ 4Hm4_< H.Nmm41 M441 cmm.o mo~.° -- mom.o om.a mmh.o omo.o oom.o vom.o mmm.c -- m~n.c 44.4 pm~.o hmo.c mva.o Ham.o avm.¢ Hm4.o msm.o mc.a ~cp.c amo.o ~mo.o 4H~.o mqm.o ~om.c -- Hem.o mam.o ----- omo.o m.0>~o\msmx .w.0nao\me4x 0.mpao\4x4s umum4 muoumm N :21 .I :mmmomcflm mHo\m4< 0.:mHo\m4 0.:mao\m4 n.mmao\ mam ”mm. mcmucwm :H Ammmdv doomsuoamcmnmla can Addy ococonm0wmonm mo coauomom on» How mama puma» Bounded .mm wanna 184 2 ca «0:020:4044044 new ml 6 mcoamnmouoom no 4004444 commouo on» 44 4M4M .60449 m 425400 .o.m .mm.4 u mmo 44004444 0:046:400004 04 4444 ..0440 m 454400 .0.4 .40.4 n 44 N.000 4 ass4oo .0.0 .04.4 u 44 n m .4: 4 .24444 .24400.0 n 4440. .24000.0 n 4m4o_ .204.0 0 4404040444440:4-4_ 4400.0 044.0 4400.0 4400.0 404.0 4400.0 404.0 4400.0 4400.0 444.0 4440.0 444.0 0000.0 4400.0 000.0 4400.0 404.0 4400.0 4400.0 040.0 44444 44444 44444 44xmcm4 444. 40.0 04.0 444.0 440.0 44.4 0440.0 444.0 404.0 404.0 44.0 00.4 444.0 440.0 44.4 4440.0 0040.0 4400.0 444.0 44.0 04.04 004.0 444.0 04.4 0040.0 0040.0 0000.0 000.0 44.4 44.4 444.0 044.0 44.4 4040.0 4400.0 0440.0 040.0 umum< mHOHmm 044444. 044444. 4044444.0 o44ox4444 :44o\4444 m4o\44440 mm4ox4444 mm4ox4444 444. . mcmuamm :4 Ammmmv accomoumahcwnmla UnmAm