Hydroxo-Rhodium-N-Heterocyclic Carbene as Efficient Catalyst Precursors for Alkyne Hidrothiolation
Autores: Laura Palacios, Maria J. Artigas, Victor Polo, Fernando J. Lahoz, Ricardo Castarlenas, Jesús J. Pérez-Torrente y Luis Oro
Ref. revista: ACS Catalysis, 3, 2910-2919 (2013)
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 (3), IMes (4); ethylene, IPr (5)) 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 3 has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including RhI-thiolato species Rh(SCH2Ph)(IPr)(η2-coe)(py) (6) and Rh(SCH2Ph)(IPr)(η2-HCCCH2Ph)(py) (7), and the RhIII-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.
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 (3), IMes (4); ethylene, IPr (5)) 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 3 has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including RhI-thiolato species Rh(SCH2Ph)(IPr)(η2-coe)(py) (6) and Rh(SCH2Ph)(IPr)(η2-HCCCH2Ph)(py) (7), and the RhIII-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.