Alkyne Hydrosilylation Catalyzed by a Cationic Ruthenium Complex:  Efficient and General Trans Addition

Abstract

The complex [Cp*Ru(MeCN) 3 ]PF 6 is shown to catalyze the hydrosilylation of a wide range of alkynes. Terminal alkynes afford access to α-vinylsilane products with good regioselectivity. Deuterium labeling studies indicate a clean trans addition process is at work. The same complex is active in internal alkyne hydrosilylation, where absolute selectivity for the trans addition process is maintained. Several internal alkyne substrate classes, including propargylic alcohols and α,β-alkynyl carbonyl compounds, allow regioselective vinylsilane formation. The tolerance of a wide range of silanes is noteworthy, including alkyl-, aryl-, alkoxy-, and halosilanes. This advantage is demonstrated in the direct synthesis of triene substrates for silicon-tethered intramolecular DielsAlder cycloadditions.Metal-catalyzed simple addition reactions of alkynes are important synthetic processes and are desirable from the standpoint of synthetic efficiency and atom economy. Questions of regioand stereoselectivity become important considerations, and pathways affording syn addition are plentiful and well studied. Metal catalysis of trans addition processes-where new bonds must be formed on opposite sides of the alkyne-have only rarely been observed. Indeed, the possibility of creating direct trans addition processes which do not involve initial cis addition followed by E/Z isomerization is an exciting target. This paper describes our work studying trans addition processes in alkyne hydrosilylation with ruthenium catalysts to afford vinylsilane products.Vinyl-metal species are extremely important building blocks in organic synthesis. 1,2 Among these, vinylsilanes play a growing role due to their low cost, low toxicity, ease of handling, and simplicity of byproduct removal. Particularly significant is the potential of vinylsilanes as nucleophilic partners in palladium-catalyzed cross-coupling reactions. 3 Vinylsilanes are also useful as acceptors in conjugate addition reactions, as masked ketones through Tamao-Fleming oxidation, 4 and as terminators for cation cyclizations. 5 Unlike most other organometallic reagents-including in many cases organoboranes and organostannanes-vinylsilanes can be readily carried through many synthetic operations.However, the utility of vinylsilanes has been inhibited by the inconvenience of accessing stereo-and regio-defined vinylmetal compounds. Among the possible routes to these compounds, hydrosilylation of alkynes represents the most straightforward, atom-economical access. 6 Classical platinum catalysis 7 provides clean cis addition to (E)-vinylsilanes (2), and ligand tuning has made this a very regioselective process as well. 8 In addition, there has been significant progress using a variety of metal-catalyzed approaches to provide stereodefined, 1,2-substituted vinylsilanes of the form 1 or 2. There has been no reported general access to bmtrost@stanford.edu. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript 1,1-disubstituted vinylsilanes 9 (3) a...

Two Enabling Strategies for the Stereoselective Conversion of Internal Alkynes into Trisubstituted Alkenes

“…[31] Ongoing investigations will trytoc larify this point. [32] C-Methylation of a-hydroxy alkenylsilanes of type B( E= Si): Cooperativity between the protic functionality in the substrate and ac hloride ligand in ar uthenium catalyst has recently been shown to be strong enough to overturn the inherentb ias of ruthenium-catalyzed trans-hydrosilylations of propargyl alcohols for silyl delivery distal to the -OH group: [15,33] rather,hydrosilylations of propargyl alcohols A catalyzed by [Cp*RuCl] 4 afford products of type B in which the silyl group resides proximal to the steering substituent in good yields and selectivities (Scheme 3). [12,13,16] Since the reaction also tolerates many functional groups, such compounds deserve consideration as readily accessible and more benign substrates forC-alkylation.…”

[NiX2(NHC)2] Complexes in the Hydrosilylation of Internal Alkynes

“…The selectivity towards any of these products depends upon several factors, such as the substituents on the alkyne and the silane, the catalyst, and the reaction conditions. The hydrosilylation of alkynes may be catalyzed by a number of different metal complexes, including rhodium, [5][6][7] iridium, [6,8] ruthenium [9,10] and platinum. [11][12][13] Complexes of the less precious metals nickel, [14] cobalt, [15] and titanium [16] have also [a] Leiden Institute of Chemistry, Leiden University, P. O.…”

Chemoselective Catalytic Hydrosilylation of Nitriles

“…[1] In the case of ketones and imines, it also serves as a convenient reduction method in that it provides protected alcohols and amines in one step. [2] Whereas diverse procedures have been developed for the hydrosilylation of C = X (X = C, N, O), [1] P = O, [3] S = O, [4] and CC bonds, [1,5] SiÀH addition to the NC triple bond remains a great synthetic challenge.…”