Rhodium-Catalyzed Arene Alkenylation Using Benzoquinone Derivatives as Oxidants

The Rh-catalyzed conversion of olefins and arenes to alkenyl arenes using [(η²-C₂H₄)₂Rh-(μ-OPiv)]₂ as the catalyst precursor and 12 ortho- and para-substituted benzoquinone derivatives as the in situ oxidant is reported. Included are comparative studies of the quinone derivatives for (1) rate of styrene production from benzene and ethylene, (2) Markovnikov to anti-Markovnikov selectivity for reactions of benzene and propylene, and (3) ortho/meta/para selectivity when using <i>tert</i>-butylbenzene as the arene. Cyclic voltammetry was utilized to measure reduction potentials for each quinone to determine any possible influence of the quinone redox potential on arene alkenylation rate and selectivity. While significant differences in selectivity are observed between <i>ortho</i>-quinone derivatives, such differences are minimal when para-substituted quinones are utilized. These results suggest that <i>ortho</i>-benzoquinone derivatives likely serve as bidentate ligands, which explains the stronger influence on catalyst activity of <i>ortho</i>-benzoquinone identity compared to <i>para</i>-benzoquinones. Although <i>ortho</i>-benzoquinones generally give styrene production rates faster than those of <i>para</i>-benzoquinones, 3,5-di-<i>tert</i>-butyl-<i>ortho</i>-benzoquinone and <i>ortho</i>-chloranil react with ethylene to form bicyclo[2.2.2]-oct-5-ene-2,3-dione derivatives as a significant side product.