On the mechanism of sp 2 C-H borylation using ortho-N -substituted pyridinium cations.

ortho-N -Substituted pyridinium cations with the weakly coordinating anion [B(C 6 F 5 ) 4 ] - have been studied and crucial structural features in the sp 2 C-H borylation catalysis of 3-methylthiophene have been identified. The electron-deficiency of the aromatic core of the cation is essential for activity together with accessible protons. The spectroscopic yield of the borylation of 3-methylthiophene with catecholborane (CatBH) was optimized up to 86% and the method was further applied to other substrates such as N -alkylbenzenes. A mechanistic DFT study revealed the rate-limiting step in the catalysis to be the liberation of molecular H 2 (Δ G ‡ = 27.5 kcal mol -1 ), whereas the overall reaction was found to be exergonic by 5.1 kcal mol -1 .