Gold catalysis : cyanosilylation Reaction

Abstract

I. Gold Catalysis: Cyanosilylation Reaction Gold had been considered to be an extremely inert metal, but recently it was found that nanometer-sized gold particles on metal oxide supports acted as catalysts for simple organic reactions, such as oxidation and hydrogenation, even at or below room temperature. In Chapter 1, we report that gold nanoparticles (AuNPs) of zero oxidation state $(Au^o)$ are catalytically active for a C-C bond forming reaction, cyanosilylation of aldehydes. Cyanosilylation is one of the most useful reactions for transforming carbonyl compounds into the corresponding α-hydroxy nitriles, which can be further manipulated to versatile building blocks, such as α-hydroxy acids, α-hydroxy carbonyl compounds, and β-amino alcohols. The AuNP-catalyzed cyanosilylation proceeded smoothly at room temperature with 0.2 wt% loading of AuNPs. The reactions of aromatic aldehydes were almost quantitative except benzaldehyde derivatives containing the electron-withdrawing $NO_2$ group, and α,β-unsaturated aromatic aldehydes were most reactive substrates for the reactions. The reactions also went smoothly for aliphatic aldehydes. Mechanistic studies indicate that the reactions proceeded both homogeneously and heterogeneously: homogenous catalysis by leached gold species and heterogeneous catalysis by the adsorption of the reactants (aldehydes and trimethylsilyl cyanide) onto AuNPs. The ratio of homogeneous and heterogeneous catalysis was estimated to be about 4 to 1. This mechanistic finding led us to examine the reaction catalyzed by homogeneous cationic gold species more thoroughly. In Chapter 2, we describe that gold(III) chloride was a highly efficient catalyst for cyanosilylation of various ketones and aldehydes. The reactions were complete within 30 min at room temperature with only 1 mol% AuCl_3, yielding the corresponding cyanohydrin trimethylsilyl ethers in very good yields. The isolated yields for ketones were up to 98%, and the reactions w...