Ligand-Controlled Regioselectivity
in the Hydrothiolation
of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts
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Abstract
Rh–N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η<sup>2</sup>-olefin)]<sub>2</sub> and RhCl(IPr)(py)(η<sup>2</sup>-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<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 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