BENZYLDIBORONATES: EXPLORING THE REACTIVITY AND ITS APPLICATION IN KETONE SYNTHESIS & EFFECTS OF PINCER RIGIDIFICATION ON THE REACTIVITY OF COBALT COMPLEXES

Abstract

Geminal benzyldiboronates are attractive synthetic building blocks, as two boryl groups and a phenyl group attached to the same carbon, can offer a unique reactivity. While the understanding of their reactivity has been limited by the lack of methods for the preparation, recent advances in cobalt- and nickel-catalyzed C(sp3)–H perborylation allowed for development of new methodology employing gem-benzyldiboronates as well as investigations into activation modes and operative mechanisms of C–C bond-forming steps. The first part of this dissertation will describe an alkoxide-promoted transformation for the ketone synthesis from esters, multi-nuclear NMR studies to uncover the mechanism, and one-pot synthesis of elaborated ketones enabled by elucidation of the reaction mechanism.Organoboron compounds, especially those have C(sp2)–B bonds, are versatile reagents for the formation of C–C and C–heteroatom bonds. Our group has reported bis(phosphino)pyridine cobalt [(PNP)Co] complexes as precatalysts for C(sp2)–H borylation of fluoroarenes that outperform the bipyridine iridium catalysts for the regioselective functionalization of fluorinated arenes. DFT studies on [(PNP)Co]-catalyzed borylation of benzene support the crucial role of pincer ligand flexibility, in that pincer dynamics effectively relieve steric crowding around the cobalt center and lower activation barriers for the fundamental reaction steps. This pincer flexibility is also proposed to be the origin of reversible C–H activation, ultimately resulting in thermodynamic control of regioselectivity. Though intriguing, experimental data is lacking. To get further insights, more conformationally rigid PNP pincers were explored where the -CH2- spacers were replaced with -CMe2- or -NMe- spacers. In the following part of this dissertation, the preparation of cobalt complexes bearing modified pincers and evaluation of catalytic competence will be discussed. Indeed, such modifications have a detrimental effect on C–H borylation performance. During this investigation, it was discovered that some 4-coordinate cobalt alkyl pincers exhibit a high resistance to protonolysis of Co–C bonds upon the addition of alcohols, which is rare for Earth-abundant metals. This finding prompted the study of protonolysis reactivity of a series of pincer cobalt alkyl/aryl compounds. It turned out that compounds with more accessible triplet excited states tend to be more susceptible to hydrolytic decomposition.