Exploring the Catalytic Activity of Ge(II) and Sn(II) Cationic Crown Ether Complexes: Hydrosilylation.

Lewis acid-catalyzed hydrosilylation of carbonyl compounds was explored using cationic germanium(II) and tin(II) crown ether complexes as catalysts. Only the germanium(II) crown ether complexes were found to reduce aldehydes efficiently and selectively to one product, the symmetric ether. The scope of the reaction was explored using [Ge([12]crown-4)2][OTf]2 as the catalyst revealing tolerance for a variety of functional groups, excluding those containing nitrogen. Lower conversions and less selectivity were observed in ketone hydrosilylations. The mechanism of aldehyde reduction catalyzed by [Ge([12]crown-4)2][OTf]2 was probed using several experimental techniques, including the stoichiometric addition of silane and aldehyde, deuterium-labelling studies, a Hammett analysis and variable time normalization analysis. The etherification of aldehydes using [Ge([12]crown-4)2][OTf]2 is believed to proceed through a carbonyl-activated pathway. The active catalyst likely loses at least one crown ether ligand providing an open coordination face for the substrate(s). The reaction pathway proceeds through two distinct steps. The first is the hydrosilylation to give a coordinated silyl ether that then undergoes etherification in the second step. The open coordination face of the cationic Ge(II) species facilitates the coordination of more than one substrate/reagent.