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

Abstract

The direct and single-step conversion of benzene, ethylene, and a Cu(II) oxidant to styrene using the Rh(I) catalyst ( Fl DAB)Rh(TFA)(η 2 -C 2 H 4 ) [ Fl DAB = 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 ( Fl DAB)Rh(OAc)(η 2 -C 2 H 4 ) 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 ( Fl DAB)Rh(OAc)(η 2 -C 2 H 4 ), exhibiting no KIE when [Rh] was in the half-order regime, and a k H / k D 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.