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

• Published in: Journal of the American Chemical Society 2017-02, Vol.139 (4), p.1485-1498
• Main Authors: Vaughan, Benjamin A, Khani, Sarah K, Gary, J. Brannon, Kammert, James D, Webster-Gardiner, Michael S, McKeown, Bradley A, Davis, Robert J, Cundari, Thomas R, Gunnoe, T. Brent
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.6b10658

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 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.