Design of Highly Selective Alkyne Hydrothiolation RhI-NHC Catalysts: Carbonyl-Triggered Nonoxidative Mechanism

New Rh-I-IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-carbene) complexes bearing an N,O-pyridine-2-methanolato (N-O) bidentate ligand have been prepared. The carbonyl complex Rh(N-O)(IPr)(CO) efficiently catalyzes the hydrothiolation of a range of alkynes with high selectivity to alpha-vinyl sulfides. Reactivity studies and DFT calculations have revealed a new nonoxidative catalytic pathway, passing through Rh-I catalytic intermediates, which is driven by the interplay between the pyridine-2-methanolato and carbonyl ligands. The basic alkoxo ligand promotes the deprotonation of the thiol to generate the Rh-I active species, whereas the pi-acceptor character of the carbonyl ligand hinders the oxidative addition process. In addition, the stereochemistry of the key thiolate-pi-alkyne intermediate, which is determined by the electronic preference of the carbonyl ligand to coordinate cis to IPr, facilitates the rate-limiting alkyne thiometalation step.