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.

Thiol-yne click polymerization

“…Besides the radical and amine-mediated reactions, the transition-metal complexes could catalyze the thiol-yne reaction and have been proven to be efficient catalysts for the preparation of regio-and stereo-regular vinyl sulfides. Several transition metal-based catalysts such as Rh, Ir, Ni, Pd, Pt, Au, and Zr have been developed [44][45][46]. Generally, the transition-metal-catalyzed thiol-yne reaction proceeds via a Migratory-Insertion mechanism [44].…”

“…Several transition metal-based catalysts such as Rh, Ir, Ni, Pd, Pt, Au, and Zr have been developed [44][45][46]. Generally, the transition-metal-catalyzed thiol-yne reaction proceeds via a Migratory-Insertion mechanism [44].…”

Emerging Organosulfonium Electrophiles as Unique Reagents for Carbon–Sulfur Bond Formation: Prospects in Synthetic Chemistry of Organosulfur Compounds

“…Metal-activated RSH can add to electronsufficient olefins such as vinyl ethers and enamines to give sulfide derivatives. 18 Similarly, transition-metal-catalyzed RSH addition to C=C double bonds proceeds with palladium, 19 rhodium, 20 actinoid, 21 and zirconium catalysts. 22 As a quite unfamiliar metal catalyst in organic synthesis, calcium-catalyzed RSH addition to alkyne gave dithioacetals (Scheme 4b).…”

Mechanism of the palladium-catalyzed hydrothiolation of alkynes to thioethers: a DFT study

“…Thus, while thiol hydrothiolation with aromatic or aliphatic alkynes would appear to provide ideal substrates for their activation by the transition metals, there has been relatively little investigation of this aspect of their chemistry. Furthermore, an experimental and theoretical investigation of rhodium-catalyzed hydrothiolation of ligandcontrolled regioselectivity also been performed recently by Castarlenas et al [46], and Frech et al [47] studied palladium-catalyzed hydrothiolation of aromatic alkynes and aliphatic or aromatic thiols into cis-configured antiMarkovnikov-type vinyl thioethers with excellent yields within a few minutes at 120°C (Scheme 1).…”