Development of Cobalt Catalysts for themeta-Selective C(sp2)-H Borylation of Fluorinated Arenes

Cobalt precatalysts for themeta-selective borylation offluorinated arenes are described. Initial screening andstoichiometric reactivity studies culminated in the preparation of a cobalt alkyl precatalyst supported by the sterically protectedterpyridine (5,5 ''-Me2ArTpy=4 '-(4-N,N '-dimethylaminophenyl)-5,5 ''-dimethyl-2,2 ':6 ',2 ''-terpyridine). Under the optimizedconditions, borylation with this precatalyst afforded up to 16 turnovers and near-exclusivemetaregioselectivity with a range ofsubstitutedfluoroarenes in cyclopentyl methyl ether solvent at room temperature. Deuterium kinetic isotope effects of 2.9(2) at 23 degrees C support a turnover-limiting and selectivity-determining C(sp2)-H activation step, and stoichiometric C-H activationexperiments provided insights into the identity of the C-H activating intermediate in catalysis. Analysis of the relevant Co-C andC-H bond thermodynamics support that the thermodynamics of C-H activation favorortho-to-fluorine selectivity, providingadditional, indirect support for kinetic control of C-H activation as the origin ofmetaselectivity