Mechanistic Studies of Single-Step Styrene Production Using a Rhodium(I) Catalyst

The
direct and single-step conversion of benzene, ethylene, and
a Cu­(II) oxidant to styrene using the Rh­(I) catalyst (FlDAB)­Rh­(TFA)­(η2-C2H4) [FlDAB = N,N′-bis­(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene;
TFA = trifluoroacetate] has been reported to give quantitative yields
(with Cu­(II) as the limiting reagent) and selectivity combined with
turnover numbers >800. This report details mechanistic studies
of
this catalytic process using a combined experimental and computational
approach. Examining catalysis with the complex (FlDAB)­Rh­(OAc)­(η2-C2H4) shows that the reaction rate
has a dependence on catalyst concentration between first- and half-order
that varies with both temperature and ethylene concentration, a first-order
dependence on ethylene concentration with saturation at higher concentrations
of ethylene, and a zero-order dependence on the concentration of Cu­(II)
oxidant. The kinetic isotope effect was found to vary linearly with
the order in (FlDAB)­Rh­(OAc)­(η2-C2H4), exhibiting no KIE when [Rh] was in
the half-order regime, and a kH/kD value of 6.7(6) when [Rh] was in the first-order
regime. From these combined experimental and computational studies,
competing pathways, which involve all monomeric Rh intermediates and
a binuclear Rh intermediate in the other case, are proposed.