Cobalt-Catalyzed C(sp~2)-H Borylation: Mechanistic Insights Inspire Catalyst Design

摘要

A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B_2Pin_2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic Isotope effects, and identification of the catalyst resting state support turnover limiting C-H activation from a fully characterized cobalt(Ⅰ) boryl intermediate. Monitoring the catalytic reaction as a function of time revealed that borylation of the 4-position of the pincer in the cobalt catalyst was faster than arene borylation. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which slows the rate of C-H oxidative addition and hence overall catalytic turnover. This mechanistic insight inspired the next generation of 4-substituted PNP cobalt catalysts with electron donating and sterically blocking methyl and pyrrolidinyl substituents that exhibited increased activity for the C-H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B_2Pin_2. Kinetic studies on the improved catalyst, 4-(H)_2BPin, established a change in turnover limiting step from C-H oxidative addition to C-B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)_2BPin, was prepared and crystallographically characterized and proved inactive for C-H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(Ⅲ) complexes.