Ligand‐Free Hydrosilylation of Aldehydes Mediated by Highly Active Zinc Metal

Abstract

The reduction of carbonyl compounds has been generally considered to be a fundamental protocol for the preparation of the corresponding alcohols. Catalytic hydrogenation of carbonyls has long been a traditional method for achieving this. 1 Aside from this protocol, metal-hydride, 2 boron-based reagents, 3 and bio-catalytic reduction 4 are also considered to be suitable methodologies. In addition, an emerging tool is reduction of the carbonyl group in the presence of a transition-metal catalyst. Numerous transition-metals have been extensively examined to successfully complete this transformation, 5 especially in transfer hydrogenation which is a safe and operationally simple protocol utilizing either element hydrogen or metal-hydrides. 5i, j Along with these protocols, a zinc-based reduction process has been extensively applied for reduction of carbonyl compounds, mainly due to the environmental friendliness and economic benefits. Furthermore, the zinc-based reduction protocol could be readily extended to hydrosilylation of carbonyl compounds, thereby providing the corresponding alcohols. Since the first report by Caubere, 6 and pioneering work by Minoun, 7 it has emerged as an attractive synthetic method to reduce carbonyl compounds. 8 When using this approach, zinc complexes such as zinc acetate and diethylzinc, which are particularly activated by nitrogen-chelating ligands, have frequently been used as pre-catalysts. In early studies of zinc-catalyzed hydrosilylation of aldehydes and ketones, both zinc(II) salt and zinc (0) powder worked well to obtain the corresponding alcohols in the presence of metal-hydride and chlorotrimethylsilane. In 2009, Nishiyama et al. reported a successful method for hydrosilylation of carbonyl compounds using zinc acetate as a catalyst without any assistance from ligands. 9 From a synthetic point of view, reactions of carbonyl compounds with zinc metal also have value in novel applicability. A recent study by Len et al. used zinc metal and carbonyl compound for selective reductive pinacol coupling. 10 The reaction was carried out with zinc metal and NH 4 Cl in an aqueous media, yielding the diols as a major product along with small amounts of reduced alcohols. More recently, Dash's group has described a relatively simple process for selective reduction of carbonyls using a combination of zinc metal-powder with an aqueous NH 4 Cl solution as a proton source in THF. The corresponding product alcohols were exclusively produced along with a trace amount of the pinacol product diols (Scheme 1). 11 Inspired by the above results, we were interested in exploring the nature of highly active zinc (Zn*) in the hydrosilylation of carbonyl compounds. To investigate the potential of our strategy, carbonyl compounds were treated with a mixture of highly active zinc and silane in the absence of any other ligands. Herein, we describe the successful development of Zn*-mediated hydrosilylation of aldehydes and ketones, producing the corresponding alcohols as a major product.To elucidate our stra...