A DFT study on the mechanism of rhodium-catalyzed regioselective hydrothiolation of the allyl amine

Abstract

The reaction mechanisms of Rh-catalyzed regioselective hydrothiolation of the allyl amine employing four bidentate phosphine ligands are investigated with DFT calculations. The free energy profiles of anti-Markovnikov and Markovnikov pathways arising from different alkene insertion types are computed to elucidate the ligand-controlled regioselectivity. For 1,2-bis(diphenylphosphino)benzene (dppbz) and 1,3-bis(diphenylphosphino)propane (dppp) ligands with small nature bite angle (βn ≤ 86°), the anti-Markovnikov pathway that features the 1,2-alkene insertion into Rh-H bond is favored by 2 ˜ 4 kcal/mol in barriers of elementary steps. While for 1,4-bis(diphenylphosphino)butane (dppb) and bis(2-diphenylphosphinophenyl)ether (DPEphos) ligands with large nature bite angle (βn ≥ 99°), the Markovnikov pathway with 1,2-alkene insertion into Rh-S bond is preferential by 2 ˜ 7 kcal/mol in barriers. The P-Rh-P bite angle is a reliable predictor and regulator of the regioselectivity of reaction as evidenced by good correlations between reaction barrier and P-Rh-P bite angle. Smaller P-Rh-P bite angle in TSs is generally found for small nature bite angle ligand dppbz and dppp in preferential anti-Markovnikov pathway, while TSs with larger P-Rh-P bite angle are favored by large nature bite angle ligand DPEphos and dppb. Larger difference in P-Rh-P bite angles of TSs between Markovnikov and anti-Markovnikov pathway generally leads to the greater disparity in barrier heights of two pathways, and hence greater regiodivergency of reaction.