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

Abstract

Cationic rhodium(I) complexes containing picolyl-NHC&#13;\n(NHC = N-heterocyclic carbene) ligands that differ in the substitution at&#13;\nthe 6-position of the pyridine donor serve as efficient E-selective alkyne&#13;\nhydrosilylation catalyst precursors. Particularly, when the steric&#13;\nhindrance of the picolyl fragment is increased, a catalyst precursor&#13;\nexhibiting high catalytic activities (TOF up to 500 h−1 at S/C ratios of&#13;\n1000) and excellent E selectivities (E/α ratio ≥95/5) in the&#13;\nhydrosilylation of a series of aryl, alkyl, and functionalized terminal&#13;\nalkynes with both carbo- and alkoxysilanes has been obtained. The&#13;\npicolyl-NHC ligands in the Rh complexes exhibit a dynamic behavior in&#13;\nsolution due to the hemilabile coordination of the pyridine fragment.&#13;\nPreliminary mechanistic studies support the involvement of Rh silyl hydrido species, which are generated in low concentrations&#13;\nfrom Rh complexes and the silane, in the hydrosilylation of alkynes in agreement with the assumption of Chalk−Harrod-type&#13;\nmechanisms.