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

Abstract

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) 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 α-vinylsilanes with high yields and high α/β selectivity. Mechanistic study revealed that the stoichiometric reactions of [(IPr) 2 Co 2 (CO) 6 ] with PhCCH and HSiEt 3 can furnish the dinuclear cobalt alkyne and mononuclear cobalt silyl complexes [(IPr)(CO) 2 Co(μ−η 2 :η 2 -HCCPh)Co(CO) 3 ], [(IPr)(CO) 2 Co(μ-η 2 :η 2 -HCCPh)Co(CO) 2 (IPr)], and [(IPr)Co(CO) 3 (SiEt 3 )], respectively. Both dicobalt bridging alkyne complexes can react with HSiEt 3 to yield α-triethylsilyl styrene and effect the catalytic Markovnikov hydrosilylation reaction. However, the mono(NHC) dicobalt complex [(IPr)(CO) 2 Co(μ-η 2 :η 2 -HCCPh)Co(CO) 3 ] exhibits higher catalytic activity over the di(NHC)-dicobalt complexes. The cobalt silyl complex [(IPr)Co(CO) 3 (SiEt 3 )] is ineffective in catalyzing the hydrosilylation reaction. Deuterium labeling experiments with PhCCD and DSiEt 3 indicates the synaddition 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 DSiEt 3 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 Co 2 C 2 -butterfly core in the structure of [(IPr)(CO) 2 Co(μ-η 2 :η 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 α selectivity of the catalytic system originates from the joint play of the dicobalt carbonyl species to coordinate alkynes in the Co(μ-η 2 :η 2 -HCCR′)Co mode and the steric demanding nature of IPr ligand.

The Influence of Silane Steric Bulk on the Formation and Dynamic Behavior of Silyl Palladium Hydrides

“…In hydrosilylation reactions, the steric hindrance of silane substituents is observed to impact the regio- and stereoselectivity of the reaction for a wide range of metal catalysts including Pt, − Pd, − Ni, Rh, − and other late transition metals. − Nonetheless, despite the proposed role of silane steric hindrance in transformations involving silyl metal hydride intermediates, focused studies on the impact of silane steric bulk in oxidative addition with late transition metals are scarce. In examples of oxidative addition using Ni, , Pd, − and Pt complexes, − tertiary silanes are observed to yield discrete silyl metal hydrides, while reacting the same complexes with secondary or primary silanes gives metal bis­(silyl) species; however, some silyl metal hydride complexes of Pd and Pt have been isolated from very bulky primary silanes. ,, In contrast, systematic variation of silane substituents in the oxidative addition of tertiary silanes to Rh complexes reveal a pronounced decrease in the rate of oxidative addition (and stability of the formed products) with increasing steric bulk at silicon. , Deeper understanding of the impact of silane steric bulk on the formation and stability of silyl metal complexes is necessary to more fully understand hydrosilylation reactions.…”

Transition-Metal-Catalyzed Stereo- and Regioselective Hydrosilylation of Unsymmetrical Alkynes

“…107-109, 111, 113 A binuclear carbonyl Co complex with NHC ligation was found effective for α-selective hydrosilylation of terminal alkynes with trisubstituted hydrosilanes, having good functional group tolerance (Table 5, entries 15). 114 The 2,9diaryl-1,10-phenanthroline modified Fe exhibited excellent αselectivity in the hydrosilylation of various terminal alkynes with high activity (TON up to 4500, Table 5, entry 16). 68 Interestingly, when the 2,9-diaryls of phenanthroline ligand were 3,5-disubstituted aryls, the Fe catalyst gave α-selectivity; when the 2,9-diaryls of phenanthroline ligand were 2,4,6trisubstituted aryls, the Fe catalyst gave β-selectivity (Table 2, entry 12).…”

Recent Advances in the Functionalization of Terminal and Internal Alkynes

“…Markovnikov hydrosilylation reactions of terminal alkynes with tertiary silanes to afford a series of α‐vinylsilanes using a dicobalt carbonyl N‐heterocyclic carbene (NHC) complex as the catalyst was reported by Liang Deng and co‐workers (Scheme 30). [30] In this protocol, hydrosilylation of both alkyl‐ and aryl‐substituted terminal alkynes with a variety of tertiary silanes were achieved in high yields along with high α/β selectivity. The highlights of this protocol include high Markovnikov addition selectivity, broad substrate scope in terms of alkynes and silanes and excellent functional group compatibility.…”