Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane Formation

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((PDI)-P-Mes)CoCH3 ((PDI)-P-Mes = 2,6-(2,4,6-Me3C6H2-N=CMe)(2)C3H3N), promotes the catalytic dehydrogenative silylation of linear alpha-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)(2)MeSiH and (EtO)(3)SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive alpha-olefins to serve as functional equivalents of the more valuable alpha, omega-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective beta-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.