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

-Substituted pyridinium cations with the weakly coordinating anion [B(CF)] have been studied and crucial structural features in the sp 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 -alkylbenzenes. A mechanistic DFT study revealed the rate-limiting step in the catalysis to be the liberation of molecular H (Δ = 27.5 kcal mol), whereas the overall reaction was found to be exergonic by 5.1 kcal mol.