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(mu-OH)(NHC)(eta(2)-olefin)](2) (coe, IPr (3), IMes (4); ethylene, IPr (5)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward alpha-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex 3 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)(eta(2)-coe)(py) (6) and Rh(SCH2Ph)(IPr)(eta(2)-HC CCH2Ph)(py) (7), and the Rh-III-hydride-dithiolato derivative RhH(SCH2Ph)(2)(IPr)(py) (8) 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.