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

Abstract

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 α-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 α-alkylation of arylacetonitriles with primary alcohols. Various α-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.