Rhodium(I) Complexes with Ligands Based on N-Heterocyclic Carbene and Hemilabile Pyridine Donors as Highly E Stereoselective Alkyne Hydrosilylation Catalysts

Abstract

Cationic rhodium(I) complexes containing picolyl-NHC (NHC = N-heterocyclic carbene) ligands that differ in the substitution at the 6-position of the pyridine donor serve as efficient E-selective alkyne hydrosilylation catalyst precursors. Particularly, when the steric hindrance of the picolyl fragment is increased, a catalyst precursor exhibiting high catalytic activities (TOF up to 500 h −1 at S/C ratios of 1000) and excellent E selectivities (E/α ratio ≥95/5) in the hydrosilylation of a series of aryl, alkyl, and functionalized terminal alkynes with both carbo-and alkoxysilanes has been obtained. The picolyl-NHC ligands in the Rh complexes exhibit a dynamic behavior in solution due to the hemilabile coordination of the pyridine fragment. Preliminary mechanistic studies support the involvement of Rh silyl hydrido species, which are generated in low concentrations from Rh complexes and the silane, in the hydrosilylation of alkynes in agreement with the assumption of Chalk−Harrod-type mechanisms.

Iridium-catalyzed Markovnikov hydrosilylation of terminal alkynes achieved by using a trimethylsilyl-protected trihydroxysilane

“…The bulky (TMSO) 3 SiH, which is easily prepared from HSiCl 3 and (TMS) 2 O, was chosen as the silane partner to verify our anticipation. It is noteworthy that though in contrast with the wide utilization of Et 3 SiH, (EtO) 3 SiH and other common hydrosilanes in hydrosilylation of unsaturated molecules, examples that use (TMSO) 3 SiH as hydrosilane source are still limited [58][59][60][61][62][63][64] , it showed its unique function in promoting the reaction selectivity 63 or inducing different selectivities 64 . Several simple iridium catalysts were tested firstly under a mild condition (entry 1-4).…”

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

“…46 The researchers also developed some other ligands, such as N/O coordinative ligands as well as NHCs, for Rh-catalyzed β-E selective hydrosilylation of terminal alkynes (Table 1, entries 20-22). [49][50][51] Among these, a pyridine-NHC-Rh catalyst enabled β-E selective hydrosilylation of aryl acetylenes, alkyl acetylenes, and alkenyl acetylenes in high yield and good selectivity (Table 1, entry 22). 51 Scheme 4 Rh-catalyzed alkyne hydrosilylation for the synthesis of alkenyl silicon polymers.…”

“…[49][50][51] Among these, a pyridine-NHC-Rh catalyst enabled β-E selective hydrosilylation of aryl acetylenes, alkyl acetylenes, and alkenyl acetylenes in high yield and good selectivity (Table 1, entry 22). 51 Scheme 4 Rh-catalyzed alkyne hydrosilylation for the synthesis of alkenyl silicon polymers.…”

“…46 The researchers also developed other ligands, such as N/O coordinative ligands as well as NHCs, for Rhcatalyzed -E-selective hydrosilylation of terminal alkynes (entries 20-22). [49][50][51] Among these, a pyridine-NHC-Rh catalyst enabled -E-selective hydrosilylation of aryl acetylenes, alkyl acetylenes, and alkenyl acetylenes in high yield and good selectivity (entry 22). 51 In addition to Pt and Rh, catalysts based on other noble metals (e.g., Ir, Os, Ru, Pd) were also successfully applied in the -E-selective hydrosilylation of terminal alkynes (entries 23-28).…”

Rh(I) Complexes in Catalysis: A Five-Year Trend

“…New complexes using previously reported ligands include a series of picolyl-NHCcontaining Rh(I) species differing only in the steric hindrance at the 6-position of the hemilabile pyridine moiety. These complexes are able to reach high catalytic activities and E selectivities in the hydrosilylation of a vast gamut of terminal alkynes with different silanes [54] (Figure 2). Following this trend, an annulated NHC bearing a fused naphthyridine ring was prepared by a different research group, whose Rh(I) complex is analogous to that just mentioned and to others previously reported [55].…”