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 beta-vinylsilanes by metal-catalyzed hydrosilylation reactions of alkynes, reported metal-catalyzed reactions for the synthesis of alpha-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)(2)Co-2(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 a-vinylsilanes with high yields and high alpha/beta selectivity. Mechanistic study revealed that the stoichiometric reactions of [(IPr)(2)Co-2(CO)(6)] with PhC equivalent to CH and HSiEt3 can furnish the dinuclear cobalt alkyne and mononuclear cobalt silyl complexes [(IPr)(CO)(2)Co(mu-eta(2):eta(2)-HCCPh)Co(CO)(3)], [(IPr)(CO)(2)Co(mu-eta(2):eta(2)-HCCPh)Co(CO)(2)(IPr)], and [(IPr) Co(CO)(3)(SiEt3)], respectively. Both dicobalt bridging alkyne complexes can react with HSiEt3 to yield alpha-triethylsilyl styrene and effect the catalytic Markovnikov hydrosilylation reaction. However, the mono(NHC) dicobalt complex [(IPr)(CO)(2)Co(mu-eta(2):eta(2)-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 Ph equivalent to CCD and DSiEt3 indicates the synaddition nature of the hydrosilylation reaction. The absence of deuterium scrambling in the hydrosilylation products formed from the catalytic reaction of PhC equivalent to 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)(2)Co(mu-eta(2):eta(2)-HCCPh)Co(CO)(3)], point out that mono(IPr)-dicobalt species are the genuine catalysts for the cobaltcatalyzed hydrosilylation reaction and that the high a selectivity of the catalytic system originates from the joint play of the dicobalt carbonyl species to coordinate alkynes in the Co(mu-eta(2):eta(2)-HCCR')Co mode and the steric demanding nature of IPr ligand.