The asymmetric 1,2-Markovnikov hydrosilylation of conjugated dienes with primary silanes catalyzed by a quinolineoxazoline 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 pyridineoxazoline cobalt-catalyzed step-growth polymerization with terephthalaldehyde. The result of a deuterium-labeling experiment involving the reaction of PhSiD 3 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.
The strategic carbon-to-silicon substitution at a stereogenic center can produce chiral silanes with significantly improved properties relative to their carbon congeners. We herein report an unprecedented cobalt-catalyzed asymmetric hydrosilylation of unsymmetric alkynes with dihydrosilanes that furnishes silicon-stereogenic vinylhydrosilanes with high regio- and enantioselectivity. The absolute configurations of the products were determined by chiroptical methods in combination with DFT calculations. The synthetic versatility of the vinylhydrosilanes as chiral building blocks was further demonstrated by asymmetric Si-H insertion and catalytic hydroboration reactions.
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