Platinum-catalyzed formation of quinolines from anilines.: Aliphatic α-C-H activation of alkylamines and aromatic ortho-C-H activation of anilines

Platinum(II) bromide catalyzes the reaction of aniline with n-Bu3N to give N-butylaniline and 2-propyl-3- ethylquinoline. The formation of the quinoline derivative is strongly favored ( TON up to 40) by the presence of both n-Bu4PBr and 1-hexene. The same reaction products are formed by reaction of aniline with either n-Bu2NH or n-BuNH2 (PtBr2 catalyst). Under similar experimental conditions, the " PtBr2-n- Bu4PBr-1-hexene" system catalyzes the transformation of N-hexylaniline into 2-pentyl-3-butylquinoline ( TON up to 42). Similarly, N-ethylaniline affords 2-methylquinoline ( TON of 22). In both cases, the presence of 1-hexene strongly favors the conversion of the N-alkylaniline. On the basis of both complementary experiments and literature data, these unprecendented results are interpreted by an alpha-C-H activation of n-Bu3N (iminium species as intermediate), leading to N-butylaniline ( butyl group transfer to aniline). The "PtBr2-n-Bu4PBr-1-hexene" system also catalyzes an alpha-C-H activation of the alkyl part of N-(n-alkyl) anilines to generate the corresponding imines, which dimerize. A heterocyclization then occurs, via a Pt(II)-catalyzed aromatic ortho-C-H activation, followed by deamination and dehydrogenation (1-hexene as hydrogen acceptor). Although not completely clear, the role of an alkene (1-hexene or ethylene) is highlighted.