ja6b06144_si_002.cif (5.47 MB)
Download fileCobalt-Catalyzed C(sp2)‑H Borylation: Mechanistic Insights Inspire Catalyst Design
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posted on 2016-07-31, 00:00 authored by Jennifer
V. Obligacion, Scott P. Semproni, Iraklis Pappas, Paul J. ChirikA 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.