On the mechanism of sp2 C–H borylation using ortho-N-substituted pyridinium cations

Abstract

ortho-N-Substituted pyridinium cations with the weakly coordinating anion [B(C₆F₅)₄]⁻ have been studied and crucial structural features in the sp² 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 H₂ (ΔG^‡ = 27.5 kcal mol⁻¹), whereas the overall reaction was found to be exergonic by 5.1 kcal mol⁻¹.