title: Cobalt-Catalyzed C(sp(2))-H Borylation: Mechanistic Insights Inspire Catalyst Design.
abstract: A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B2Pin2 (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 B2Pin2. 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.
authors.affiliation: Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Jennifer V
authors.initials: JV
authors.lastname: Obligacion
authors.affiliation: Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Scott P
authors.initials: SP
authors.lastname: Semproni
authors.affiliation: Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Iraklis
authors.initials: I
authors.lastname: Pappas
authors.affiliation: Department of Chemistry, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Chirik