Dihaloindium hydride as a novel reducing agent.

Dihaloindium hydrides (X2InH) are novel reducing reagents, which act in both an ionic and a radical manner. The hydrides were easily generated from InX3 and Bu3SnH to reduce a variety of functionalities such as aldehydes, ketones, enones, and imines. The combination of a phosphine and Cl2InH accomplished the selective transformation from acid chlorides to aldehydes. One-pot treatment of Cl2InH, enones, and aldehydes achieved reductive aldol reactions, in which the predominant reduction of enones was followed by an aldol reaction between the resulting indium enolates and the remaining aldehydes. It is noteworthy that both anti- and syn-selective aldols were obtained by the use of THF and an aqueous solvent, respectively. The replacement of Bu3SnH with Et3SiH as a hydride source allowed the catalytic use of InBr3 to give the syn-selective aldols. The dehalogenation of alkyl halides was achieved by a catalytic amount of InCl3 in the presence of Bu3SnH. This procedure was applied to some representative cyclizations as radical proof. A simple and non-toxic system, NaBH4/InCl3, also promoted dehalogenation, intramolecular cyclization, and intermolecular coupling reactions. In addition, the Et3SiH/InCl3 system was found applicable to an effective intramolecular cyclization of enynes. 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.