Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate–rhodium(I)–N-heterocyclic carbene complexes

RhI–NHC–olefin complexes bearing a N,O-quinolinolate bidentate ligand have been prepared from [Rh(μ-OH)(NHC)(η2-olefin)]2 precursors (olefin = cyclooctene, ethylene). The disposition of the chelate ligand with regard to the carbene in the square planar derivatives is strongly influenced by the steric hindrance exerted by the coordinated olefin. These complexes efficiently catalyzed the addition of thiophenol to phenylacetylene with good selectivity to α-vinyl sulfides, which can be increased up to 97% by addition of pyridine. Several key intermediates have been detected including the η1-alkenyl species resulting from alkyne insertion into a Rh–H bond. DFT calculations on the mechanism support a hydrometallation pathway that entails the oxidative addition of thiol, 2,1-insertion of the alkyne into the Rh–H bond, and reductive elimination as the rate-determining step. Remarkably, coordination of pyridine to the β-alkenyl intermediate but not to the α-alkenyl, which results in a net stabilization, is the key for the Markovnikov selectivity.Financial support from the Ministerio de Economía y Competitividad (MEC/FEDER) of Spain (Project CTQ2013-42532-P), the Diputación General de Aragón (E07) and CONSOLIDER INGENIO-2010, under the Project MULTICAT (CSD2009-00050) is gratefully acknowledged. The support under the KFUPM–University of Zaragoza research agreement is also highly appreciated. ADG thanks the “Subprograma de Formación Posdoctoral” from the Ministerio de Economia y Competitividad.Peer Reviewe