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

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/alpha 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.