During the last two decades, iridium catalyzed aromatic borylation has emerged as one of the most convenient methodologies for functionalizing arenes and heteroarenes. The regioselectivity of Ir-catalyzed borylations are typically governed by sterics, therefore it complements the regioselectivity found in electrophilic aromatic substitution or directed ortho metalation. This unique regioselectivity and broad functional group tolerance (ester, amide, halogen, etc.) allows for synthesis of novel synthetic intermediates, many of which were previously either unknown or difficult to make. Since these reactions are mainly driven by sterics, it is possible to install boronic ester group (Bpin) next to small substituents like hydrogen, cyano, or fluorine. This feature is helpful but can also create challenges, specially in cases like borylation of fluoro arenes. These fluoro arenes tend give 1:1 mixture of steric (meta to fluorine) and electronic (ortho to fluorine) products. Therefore, to overcome this problem, we introduced a two-step Ir-catalyzed borylation/Pd-catalyzed dehalogenation sequence that allows one to synthesize fluoroarenes where the boronic ester is ortho to fluorine (electronic). Here, a halogen para to the fluorine is used as a sacrificial blocking group allowing the Ir-catalyzed borylation to favor the electronic product exclusively. Then the chemoselective Pd-catalyzed dehalogenation by KF activated polymethylhydrosiloxane (PMHS) is used to remove the halogen without compromising the Bpin group. Halosubstituted aryl boronates have the potential for orthogonal reactivity in cross-coupling reactions. We began exploring cross-coupling of triorganoindiums with these arylhalides bearing boronic esters in collaboration with Prof. P. Sestelo at University of da Coruña, Spain. We were able to synthesize borylated biaryls by merging Ir-catalyzed C–H borylations with Pd-catalyzed organoindium cross-couplings.As a part of the Dow–MSU-GOALI collaborations, we were able to synthesize a cobalt catalyst for C-H borylations of alkyl arenes and heteroarenes. This catalyst enables selective monoborylation of the benzylic position of alkyl arenes using pinacolborane (HBpin) as the boron source. In 2016, an internship opportunity led to the screening of ligands for C-H borylations at the Dow chemicals company in Midland, MI. From this internship opportunity, we discovered the first ligand controlled synthesis of 1,2-di and 1,2,3-tri borylated arenes. Also, I investigated a recyclable iridium heterogeneous catalyst for borylations during the internship. Finally, a bulky terphenyl incorporated bipyridine ligand is synthesized for selective iridium catalyzed para C–H borylations.
Read
