Hydroxo–Rhodium–N-Heterocyclic Carbene
Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation
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Abstract
The new Rh–hydroxo dinuclear
complexes stabilized by an
N-heterocyclic carbene (NHC) ligand of type [Rh(μ-OH)(NHC)(η<sup>2</sup>-olefin)]<sub>2</sub> (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst
precursors for alkyne hydrothiolation under mild conditions, presenting
high selectivity toward α-vinyl sulfides for a varied set of
substrates, which is enhanced by pyridine addition. The structure
of complex <b>3</b> has been determined by X-ray diffraction
analysis. Several intermediates relevant for the catalytic process
have been identified, including Rh<sup>I</sup>-thiolato species Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-coe)(py) (<b>6</b>) and
Rh(SCH<sub>2</sub>Ph)(IPr)(η<sup>2</sup>-HCCCH<sub>2</sub>Ph)(py) (<b>7</b>), and the Rh<sup>III</sup>-hydride-dithiolato
derivative RhH(SCH<sub>2</sub>Ph)<sub>2</sub>(IPr)(py) (<b>8</b>) as the catalytically active species. Computational DFT studies
reveal an operational mechanism consisting of sequential thiol deprotonation
by the hydroxo ligand, subsequent S–H oxidative addition, alkyne
insertion, and reductive elimination. The insertion step is rate-limiting
with a 1,2 thiometalation of the alkyne as the more favorable pathway
in accordance with the observed Markovnikov-type selectivity
