α-Alkylation of arylacetonitriles with primary alcohols catalyzed by backbone modified N-heterocyclic carbene iridium(I) complexes

A series of backbone-modified N-heterocyclic carbene (NHC) complexes of iridium(I) (1d-f) have been synthesized and characterized. The electronic properties of the NHC ligands have been assessed by comparison of the IR carbonyl stretching frequencies of the in situ prepared [IrCl(CO)(2)(NHC)] complexes in CH2Cl2. These new complexes (1d-f), together with previously prepared 1a-c, were applied as catalysts for the alpha-alkylation of arylacetonitriles with an equimolar amount of primary alcohols or 2-aminobenzyl alcohol. The catalytic activities of these complexes could be controlled by modifying the N-substituents and backbone of the NHC ligands. The NHC-Ir-I complex 1f bearing 4-methoxybenzyl substituents on the N-atoms and 4-methoxyphenyl groups at the 4,5-positions of imidazole exhibited the highest catalytic activity in the alpha-alkylation of arylacetonitriles with primary alcohols. Various alpha-alkylated nitriles and aminoquinolines were obtained in high yields through a borrowing hydrogen pathway by using 0.1 mol% 1f and a catalytic amount of KOH (5 mol%) under an air atmosphere within significantly short reaction times.