N-heterocyclic carbenes bearing a naphthyl substituent and their metal complexes: synthesis, structure, and application in catalytic transfer hydrogenation

A series of unsymmetrical imidazolinium bromides (3a–d) bearing naphthyl and benzyl groups (R’ = CH2C6H2(CH3)3-2,4,6 (a); CH2C6H(CH3)4-2,3,5,6 (b); CH2C6(CH3)5 (c); CH2C6F5 (d)) at the N1 and N3 positions were successfully synthesized. [RuCl2(NHC(p-cymene)] (NHC= N-heterocyclic carbene) complexes (4a–d) were prepared by the reaction of [RuCl2(p-cymene)]2 with imidazolinium salts (3a–d). The new salts (3a–d) and their ruthenium(II) complexes (4a–d) were characterized by 1 H, 13 C, 19F NMR, and elemental analysis. The ruthenium(II) complexes (4a–d) were employed as catalysts for the transfer hydrogenation (TH) of ketones in the presence of KOH using 2-propanol as a hydrogen source and the results were compared.