Markovnikov Hydrosilylation of Alkynes with Tertiary Silanes Catalyzed by Dinuclear Cobalt Carbonyl Complexes with NHC Ligation.

Metal-catalyzed hydrosilylation of alkynes is an ideal atom-economic method to prepare vinylsilanes that are useful reagents in the organic synthesis and silicone industry. Although great success has been made in the preparation of β-vinylsilanes by metal-catalyzed hydrosilylation reactions of alkynes, reported metal-catalyzed reactions for the synthesis of α-vinylsilanes suffer from narrow substrate scope and/or poor selectivity. Herein, we present selective Markovnikov hydrosilylation reactions of terminal alkynes with tertiary silanes using a dicobalt carbonyl N-heterocyclic carbene (NHC) complex [(IPr)2Co2(CO)6] (IPr = 1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) as catalyst. This cobalt catalyst effects the hydrosilylation of both alkyl- and aryl-substituted terminal alkynes with a variety of tertiary silanes with good functional group compatibility, furnishing α-vinylsilanes with high yields and high α/β selectivity. Mechanistic study revealed that the stoichiometric reactions of [(IPr)2Co2(CO)6] with PhC≡CH and HSiEt3 can furnish the dinuclear cobalt alkyne and mononuclear cobalt silyl complexes [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)2(IPr)], and [(IPr)Co(CO)3(SiEt3)], respectively. Both dicobalt bridging alkyne complexes can react with HSiEt3 to yield α-triethylsilyl styrene and effect the catalytic Markovnikov hydrosilylation reaction. However, the mono(NHC) dicobalt complex [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3] exhibits higher catalytic activity over the di(NHC)-dicobalt complexes. The cobalt silyl complex [(IPr)Co(CO)3(SiEt3)] is ineffective in catalyzing the hydrosilylation reaction. Deuterium labeling experiments with PhC≡CD and DSiEt3 indicates the syn-addition nature of the hydrosilylation reaction. The absence of deuterium scrambling in the hydrosilylation products formed from the catalytic reaction of PhC≡CH with a mixture of DSiEt3 and HSi(OEt)3 hints that mononuclear cobalt species are less likely the in-cycle species. These observations, in addition to the evident of nonsymmetric Co2C2-butterfly core in the structure of [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], point out that mono(IPr)-dicobalt species are the genuine catalysts for the cobalt-catalyzed hydrosilylation reaction and that the high α selectivity of the catalytic system originates from the joint play of the dicobalt carbonyl species to coordinate alkynes in the Co(μ-η2:η2-HCCR')Co mode and the steric demanding nature of IPr ligand.