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

Abstract

Abstract 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 Rh I and Ir I complexes with symmetrically substituted NHCs only those bearing cyclohexyl substituents were active, with Rh I 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. Rh I and Ir I 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.