Preferential α‐Hydrosilylation of Terminal Alkynes by Bis‐N‐Heterocyclic Carbene Rhodium(III) Catalysts

Abstract

Abstract. We describe a bis-N-heterocyclic carbene Rhodium(III) complex, featuring two trifluoroacetato ligands, that affords a variety of α-vinylsilanes in good yields by hydrosilylation of terminal alkynes. Selectivities around 7:1 α/β-(E) were reached, while the β-(Z) product was only marginally obtained. This example sharply contrasts with the β-(Z)-selectivity observed for its parent diiodido complex Vinylsilanes are valuable building blocks in organic synthesis and, therefore, new preparation methods that would pave the way to a more sustainable production of these compounds are of great interest.[1] Metal-catalyzed hydrosilylation of terminal alkynes is an efficient and atom economical route to vinylsilanes;[2] however, selectivity is a major issue for this reaction as three possible isomers may be obtained. The anti-Markovnikov syn-addition affords the  -(E)-vinylsilane, usually the major reaction product, while the anti-Markovnikov anti-addition gives the less frequent β-(Z)-vinylsilane.[3] On the other hand, Markovnikov additions to obtain selectively α-vinylsilanes are unusual. [3,4] Other noteworthy reports on the preparation of  -vinylsilanes include the hydrosilylation of terminal alkynes directed by hydroxy groups [5] or the silylcupration of terminal alkynes. [6] Although inner-sphere mechanisms seem to explain most of the selectivities hitherto reported, we recently proposed the first ionic outer-sphere mechanism for the hydrosilylation of terminal alkynes, substantiated by DFT calculations. The solvent, namely an acetone molecule, assists the heterolytic splitting of the Si-H bond by catalysts 1a and 1b (Figure 1). [7] Subsequently, the R3Si+ moiety is transferred by means of an oxocarbenium ion ([R3Si-O(CH3)2]+) to the substrate. Finally, nucleophilic attack of the hydrido ligand over the silylation product ([R3Si-C=C-R]+) affords selectively the β-(Z)-vinylsilane as a result of the steric interactions that govern the approach of R3Si-C=C-R+ to the hydrido ligand.

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

“…Template for SYNTHESIS © Thieme Stuttgart • New York 2021-08-22 page 8 of 14 reactions (Table 5, entries 4-7). [103][104][105][106] Rh catalyst based on a bis-NHC ligand (Table 5, entry 5) 104 achieved moderate αselective hydrosilylation of terminal alkynes, which was reversed in regioselectivity compared to the aforementioned Ir catalyst modified with the same ligand (Table 3, entry 16).…”

External Electric Field—Phenyl interaction boosts hydrosilylation of substituted alkynes to α‐vinylsilane

“…This hydrosilylation reaction involves the addition of a Si − H bond across a C − C triple bond, bringing an atom‐economical result with 100% atom efficiency. However, control of stereo‐ and regioselectivity is the major issue hampering the widespread use of this method, largely because three possible hydrosilylation products, namely α‐, β‐(Z)‐, and β‐(E)‐vinylsilanes, can be generated in the hydrosilylation reaction of terminal alkynes, with the former being rarely obtained selectively . By now, some works about the formation of α‐vinylsilane had been published by theorists and experimenters .…”

A Ligand‐Free Pt3Cluster Catalyzes the Markovnikov Hydrosilylation of Alkynes with up to 106Turnover Frequencies

“…Alkenylsilanes are ideally obtained by hydrosilylation of alkynes because this is the simplest transformation from the point of view of atom economy. [6] Extensive research has been performed to obtain and functionalize the b-products (anti-Markovnikova ddition), which can be formed in high yield and selectivity with ppm amounts of Pt catalyst or with otherm etal catalystsi ncluding, among others, Ir, [7] Ru, [8] Rh, [8a, 9] and Co. [9e, 10] In contrast, a-vinylsilanes (Markovnikova ddition) are only accessibleb yafewm ethods so far, involving Ru, [3,11] Cu, [12] Rh, [13] and Co complexes [14] as catalysts in amounts of approximately 1mol %, and the development of methodologies to access them in as implea nd cheap way wouldo pen the door to the preparationo fn ew compounds and to simplify existing synthetic procedures for other molecules. [6] Extensive research has been performed to obtain and functionalize the b-products (anti-Markovnikova ddition), which can be formed in high yield and selectivity with ppm amounts of Pt catalyst or with otherm etal catalystsi ncluding, among others, Ir, [7] Ru, [8] Rh, [8a, 9] and Co. [9e, 10] In contrast, a-vinylsilanes (Markovnikova ddition) are only accessibleb yafewm ethods so far, involving Ru, [3,11] Cu, [12] Rh, [13] and Co complexes [14] as catalysts in amounts of approximately 1mol %, and the development of methodologies to access them in as implea nd cheap way wouldo pen the door to the preparationo fn ew compounds and to simplify existing synthetic procedures for other molecules.…”

“…Extensive research has been performed to obtain and functionalize the β‐products ( anti ‐Markovnikov addition), which can be formed in high yield and selectivity with ppm amounts of Pt catalyst or with other metal catalysts including, among others, Ir, Ru, Rh, and Co . In contrast, α‐vinylsilanes (Markovnikov addition) are only accessible by a few methods so far, involving Ru, Cu, Rh, and Co complexes as catalysts in amounts of approximately 1 mol %, and the development of methodologies to access them in a simple and cheap way would open the door to the preparation of new compounds and to simplify existing synthetic procedures for other molecules.…”