A highly efficient route to C-3 alkyl-substituted indoles via a metal-free transfer hydrogenation

A highly efficient route to C-3 alkyl-substituted indoles via completely metal-free catalytic transfer hydrogenation of 3-indolemethanols was developed. This process proceeds via vinylogous iminium intermediates formed in situ in the presence of Brّnsted acids, and Hantzsch ester is used as the reductant. The reduction works extremely well with a large substrate scope, and the yields exceed 90% in almost all cases.