Ruthenium/HI-catalyzed direct hydromethylation of indoles and quinolines in DME

A robust diphosphine-ruthenium(II) complex has been developed that can efficiently catalyze hydromethylation of N-heteroarenes to form N-methyl saturated N-heterocycles viz the reduction of N-heterocycles coupled with C–N bond formation. Both indoles and quinolines can be transformed to their corresponding N-methyl indolines and N-methyl 1,2,3,4-tetrahydroquinolines with high conversion (e.g., 12 indoles and 9 quinolines). On the basis of a series of experiments, a possible mechanism has been proposed to account for the hydrogenation and methylation processes involved in the catalytic pathway. More importantly, this approach provides a potential route for using DME as a methyl source for the production of N-methyl saturated N-heterocycles via hydromethylation.