C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles.

Abstract

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L′)][BArF4] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH2C(Me)2)C6H4]; L/L′ = pyridine, pivalonitrile, or the vacant site, BAr4F = B[(3,5-(CF3)2)C6H3]4). Each of these compounds promoted facile directed C(sp2)–H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C–H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C–H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C–H bond cleavage and C–C bond reductive elimination from isolable cobalt(III) precursors.