Aspects of catalysis. Part III. Soluble iridium-containing catalysts for the reduction of cyclohexanones by propan-2-ol and for the formation of ethers from cyclohexanones (or enol ethers) and alcohols

Abstract

Transfer of hydrogen from propan-2-ol (containing water) to relatively unhindered cyclohexanones is catalysed by some soluble iridium compounds. Stereoselectivity in the reduction of a substituted cyclohexanon depends on the ligands associated with the iridium. The proportion of axial alcohol from the reduction of cholestan-3-one or 4-t-butylcyclohexanone ranges from 30–23%(piperidine present) to 66–78%(dimethyl sulphoxide present). Preformed iridium–sulphoxide complexes also give the latter degree of stereoselectivity. In the absence of water these complexes catalyse another reaction between cyclohexanones and alcohols, ethers being formed by reductive etherification of the ketones. The reaction favours the production of axial ethers and higher yields of ether can be obtained by starting with the ketone in the form of an enol ether.