Rhodacarborane complexes with a triphenylphosphonium substituent as efficient catalysts for on-water transfer hydrogenation of carbonyl compounds.

Two isomeric rhodacarborane chloride complexes [1,8-Bn2-2,2-Cl2-X-PPh3-2,1,8-closo-RhC2B9H8]2 (X = 7 and 11) were synthesized. One of them (X = 7) exhibits high catalytic activity in the on-water reduction of aromatic and aliphatic aldehydes and ketones with ammonium formate to the corresponding alcohols with a 0.2 mol% loading. The enantiopure form of a planar chiral catalyst was obtained via isolation of a cyclometallated complex with (S)-proline. The enantiopure form of the catalyst affords alcohols with enantiomeric excess when prochiral ketones are used in the reaction. The same catalyst is active in the Oppenauer-type oxidation of (±)-1-phenylethanol with acetone in the presence of K2CO3. The transfer hydrogenation is accompanied by in situ reversible cyclometallation of the catalytic species engaging the ortho-CH bonds of PPh3.