Comparative DFT study of metal-free Lewis acid-catalyzed C–H and N–H silylation of (hetero)arenes: mechanistic studies and expansion of catalyst and substrate scope

Direct selective dehydrogenative silylation of thiophenes, pyridines, indoles and anilines to synthesize silyl-substituted aromatic compounds catalyzed by metal-free Lewis acids was achieved recently. However, there is still insufficient mechanistic data for these transformations. Using density functional theory calculations, we conducted a detailed investigation of the mechanism of the B(C6F5)(3)-catalyzed dehydrogenative silylation of N-methylindole, N,N-dimethylaniline and N-methylaniline. We successfully located the most favourable reaction pathways that can explain the experimental observations notably well. The most favourable pathway for B(C6F5)(3)-catalyzed C-H silylation of N-methylindole includes nucleophilic attack, proton abstraction and hydride migration. The C-H silylation of N,N-dimethylaniline follows a similar pathway to N-methylindole rather than that proposed by Hou's group. Our mechanism successfully explains that the transformations of N-methylindoline to N-methylindole produce different products at different temperatures. For N-methylaniline bearing both N-H and para-phenyl C-H bonds, the N-H silylation reaction is more facile than the C-H silylation reaction. Our proposed mechanism of N-H silylation of N-methylaniline is different from that proposed by the groups of Paradies and Stephan. Lewis acids Al(C6F5)(3), Ga(C6F5)(3) and B(2,6-Cl2C6H3)(p-HC6F4)(2) can also catalyze the C-H silylation of N-methylindole like B(C6F5)(3), but the most favourable pathways are those promoted by N-methylindoline. Furthermore, we also found several other types of substrates that would undergo C-H or N-H silylation reactions under moderate conditions. These findings may facilitate the design of new catalysts for the dehydrogenative silylation of inactivated (hetero)arenes.