2 research outputs found
Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts
RhâN-heterocyclic carbene compounds [RhÂ(ÎŒ-Cl)Â(IPr)Â(η<sup>2</sup>-olefin)]<sub>2</sub> and RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) (IPr = 1,3-bisÂ(2,6-diisopropylphenyl)Âimidazol-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 RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-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 thiolateâhydride
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 thiolateâalkyne
disposition, favoring formation of 2,2-disubstituted metalâalkenyl
species and subsequently the Markovnikov vinyl sulfides via alkenylâhydride
reductive elimination. DFT calculations support a plausible reaction
pathway where migratory insertion of the alkyne into the rhodiumâthiolate
bond is the rate-determining step
Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts
RhâN-heterocyclic carbene compounds [RhÂ(ÎŒ-Cl)Â(IPr)Â(η<sup>2</sup>-olefin)]<sub>2</sub> and RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-olefin) (IPr = 1,3-bisÂ(2,6-diisopropylphenyl)Âimidazol-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 RhClÂ(IPr)Â(py)Â(η<sup>2</sup>-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 thiolateâhydride
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 thiolateâalkyne
disposition, favoring formation of 2,2-disubstituted metalâalkenyl
species and subsequently the Markovnikov vinyl sulfides via alkenylâhydride
reductive elimination. DFT calculations support a plausible reaction
pathway where migratory insertion of the alkyne into the rhodiumâthiolate
bond is the rate-determining step