Branch-selective reductive coupling of 2-vinyl pyridines and imines via rhodium catalyzed C-C bond forming hydrogenation

Hydrogenation of 2-vinyl azines 1a-1e in the presence of arylsulfonyl imines 2a-2I at ambient temperature and pressure employing cationic rhodium catalysts ligated by tri-2-furylphosphine results in regioselective reductive coupling to furnish branched products of imine addition 3a-3v, which embody modest to high levels of syn-diastereoselectivity. Catalytic coupling of 6-bromo-2-vinylpyridine 1a to imine 2I under an atmosphere of elemental deuterium provides deuterio-3I, with deuterium exclusively at the former P-position of the vinyl moiety. These data are consistent with a catalytic mechanism involving oxidative coupling of the vinyl azine and imine partners to furnish a cationic aza-rhodacyclopentane, which upon deuteriolytic cleavage releases the adduct and regenerates cationic rhodium(l) to close the catalytic cycle. These studies represent the first metal catalyzed reductive C-C couplings of vinyl azines.