Transfer hydrogenation of ketones with [Ru₄H₃(CO)₁₂]^- as the precatalyst

Abstract

The cluster [N(PPh₃)2][Ru₄H₃(CO)₁₂]1a has been found to be an efficient precatalyst for the transfer hydrogenations of ketones and α,β-unsaturated ketones. With substrates such as (5S)-carvone [2-methyl-5-(1-methylethenyl)cyclohex-2-en-1-one], (3R)-methylcyclopentanone and (3R)-methylcyclohexanone, moderate to high diastereoselectivities were observed for reduction of the conjugated olefinic and ketonic functionalities respectively. Aromatisation of carvone to 5-isopropyl-2-methylphenol and disproportionation of cyclohex-2-en-1-one to phenol and cyclohexanone have also been found to be catalysed by 1a. Studies with radical inhibitors and other evidence suggest a radical mechanism for the transfer-hydrogenation and aromatisation reactions. In the transfer hydrogenation of cyclohex-2-en-1-one, the rate of conversion of 1a into other soluble species can be modelled accurately if autocatalysis is assumed. The time-dependent concentration profiles of cyclohex-2-en-1-one, cyclohexanone and cyclohexanol are simulated well if autocatalytic formation of an active intermediate followed by consecutive reactions leading to the formation of products is assumed. Such a model is also consistent with the proposed radical mechanism.