Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

Abstract

The BX₃ promoted transition-metal-free borylative cyclizations have recently attracted increasing attention. In 2015, the Li group reported a BCl₃ mediated aminoboration of unfunctionalized olefins. In their originally proposed reaction pathways, the overall reaction barrier was calculated to be about 35 kcal mol⁻¹, which is too high to be reasonable for a reaction that smoothly occurs at room temperature. Through DFT calculations, we have reinvestigated the mechanism of this reaction and found a much more favorable reaction pathway with a computed activation free energy of about 20 kcal mol⁻¹. Based on this new mechanism, we then calculated the energy profiles for the BCl₃ promoted corresponding aminoboration of alkynes. According to the computational prediction, this transformation could easily take place at room temperature. Later experiments show that the aminoboration product is indeed generated, but the major product of this reaction is the hydroamination product. Through further calculations, we found that an unexpected protodeboronation is quite favored both kinetically and thermodynamically at the post-aminoborylation stage. Finally, we optimized the conditions for the newly discovered metal-free hydroamination reaction and found it suitable for a set of alkyne substrates.