Cobalt-Catalyzed Regio- and Enantioselective Markovnikov 1,2-Hydrosilylation of Conjugated Dienes

The asymmetric 1,2-Markovnikov hydrosilylation of conjugated dienes with primary silanes catalyzed by a quinoline-oxazoline cobalt complex has been described. This protocol provides an efficient approach to chiral allyl dihydrosilanes with high regioselectivity and enantioselectivity (up to 96% ee). The catalyst system is effective for a wide array of conjugated dienes, including mono- and 1,2-disubstituted dienes with aryl and/or alkyl substituents. Further, the products are applied to the synthesis of polyorganosiloxanes (organo-silicone copolymers) containing side chains of enantioenriched allylic functionalities using a pyridine-oxazoline cobalt-catalyzed step-growth polymerization with terephthalaldehyde. The result of a deuterium-labeling experiment involving the reaction of PhSiD3 and 1,3-pentadiene suggests that the hydrosilylation most likely proceeds through a modified Chalk-Harrod mechanism involving the 1,2-insertion of the terminal double bond of the diene into the Co-Si bond.