A Well-Defined (POCOP)Rh Catalyst for the Coupling of Aryl Halides with Thiols

This article describes a well-defined
pincer-Rh catalyst for C–S
cross-coupling reactions. (POCOP)­Rh­(H)­(Cl) serves as an active precatalyst
for the coupling of aryl chlorides and bromides with aryl and alkyl
thiols under reasonable conditions (3% mol cat., 110 °C, 2–24
h, >90% yield). For select substrates, >90% yields were obtained
with
catalyst loading as low as 0.1%. Key mechanistic intermediates have
been isolated and fully characterized, including (POCOP)­Rh­(Ph)­(SPh)
(6a) and (POCOP)­Rh­(SPh2) (6b).
The aryl/bis­(phosphinite) (POCOP)Rh system has been shown to favor
aryl thiolate reductive elimination at elevated temperatures and in
some cases at room temperature, compared with the analogous diarylamido/bis­(phosphine)
(PNP)Rh pincer system. Concerted reductive elimination has been studied
with 6a directly and in the presence of aryl bromide
and aryl chloride traps. This investigation demonstrates a clear rate
dependence on aryl chloride concentration during catalysis, a dependence
that is absent when using aryl bromides. The rate of catalysis is
dramatically reduced or brought to zero for ortho-tolyl halides, which can be traced to slower C–S coupling
and slower carbon–halogen oxidative addition for ortho-substituted aryls. The influence of the sterics in the thiol component
is less straightforward. The S–H oxidative addition product
(POCOP)­Rh­(H)­(SPh) (16) has been fully characterized and
its reactivity has been examined, resulting in the isolation of the
sodium-thiolate adduct (POCOP)­Rh­(NaSPh) (19). The solid-state
structure of 19 shows Na interactions not only with sulfur,
but also with a neighboring Rh and the chelating aryl carbon of the
pincer framework. The reactivity of 16 and 19 indicates that these potential side products should not hinder catalysis.