Donor-Functionalised Expanded Ring N-Heterocyclic Carbenes: Highly Effective Ligands in Ir-Catalysed Transfer Hydrogenation

The performances of a number of Rh-I and Ir-I complexes of type [M(NHC)(COD)Cl] in the transfer hydrogenation of ketones were tested under a variety of reaction conditions, and with a variety of substrates, allowing comparison of Rh- and Ir-NHC complexes, and also comparison of the influence of the NHC ligand on catalytic performance. Notably, of the RhI and IrI complexes with symmetrically substituted NHCs only those bearing cyclohexyl substituents were active, with RhI complexes of saturated 5-, 6- and 7-NHCs with N-Mes substituents, [Rh(5,6,7-Mes)(COD)Cl], showing no activity in transfer hydrogenation under the test conditions. RhI and IrI complexes of unsymmetrical o-methoxyphenyl donor-functionalised NHCs (df-NHC) with differing carbene ring sizes were also tested in transfer hydrogenations, with the Rh-I complexes displaying no catalytic activity. However, the corresponding df-NHC Ir-I complexes were found to be extremely effective catalysts. Catalyst tests also demonstrated the excellent stability of these complexes.