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(C 6 F 5 ) 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(C 6 F 5 ) 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(C 6 F 5 ) 3 , Ga(C 6 F 5 ) 3 and B(2,6-Cl 2 C 6 H 3 )( p -HC 6 F 4 ) 2 can also catalyze the C-H silylation of N -methylindole like B(C 6 F 5 ) 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.