title: Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts.
abstract: Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(η(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize  formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs  coordination of pyridine trans to it, consequently blocking  access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring  formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where  migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step.
authors.affiliation: Instituto de Síntesis Química y Catálisis Homogénea (ISQCH)-Departamento de Química Inórganica, CSIC-Universidad de Zaragoza, Pl. S. Francisco S/N 50009 Zaragoza, Spain.
authors.forename: Andrea
authors.initials: A
authors.lastname: Di Giuseppe
authors.forename: Ricardo
authors.initials: R
authors.lastname: Castarlenas
authors.forename: Jesús J
authors.initials: JJ
authors.lastname: Pérez-Torrente
authors.forename: Marcello
authors.initials: M
authors.lastname: Crucianelli
authors.forename: Victor
authors.initials: V
authors.lastname: Polo
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Sancho
authors.forename: Fernando J
authors.initials: FJ
authors.lastname: Lahoz
authors.forename: Luis A
authors.initials: LA
authors.lastname: Oro
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D002384:Catalysis; D006571:Heterocyclic Compounds / Q000737:chemistry*; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism; D013379:Substrate Specificity; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry
chemical_list: D000480:Alkynes; D006571:Heterocyclic Compounds; D008024:Ligands; D009942:Organometallic Compounds; D013438:Sulfhydryl Compounds; D013440:Sulfides; C030011:carbene; D012238:Rhodium; D008697:Methane