Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh­(μ-OH)­(NHC)­(η²-olefin)]₂ (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex <b>3</b> has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including Rh^I-thiolato species Rh­(SCH₂Ph)­(IPr)­(η²-coe)­(py) (<b>6</b>) and Rh­(SCH₂Ph)­(IPr)­(η²-HCCCH₂Ph)­(py) (<b>7</b>), and the Rh^III-hydride-dithiolato derivative RhH­(SCH₂Ph)₂(IPr)­(py) (<b>8</b>) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity