Catalyst-free ambient temperature synthesis of isoquinoline-fused benzimidazoles from 2-alkynylbenzaldehydes via alkyne hydroamination
An efficient environmentally benign route for the synthesis of benzimidazo[2,1-a]isoquinoline has been developed by reacting 2-ethynylbenzaldehyde and related substituted alkynylbenzaldehydes with variously substituted ortho-phenylenediamines and aliphatic amines in ethanol. This method provides a convenient, room temperature, atom-economical, and catalyst-free access to diversely substituted isoquinoline fused benzimidazoles. Regioselectivity of the reaction, as referred to o-phenylenediamines, was confirmed by X-ray crystallography. The reaction was found to occur in three major steps (imine formations, cyclization, and aromatization) and a mechanism has been proposed.