Direct Synthesis of Styrene by Rhodium-Catalyzed Oxidative Arylation of Ethylene with Benzene

Various rhodium complexes were examined for oxidative arylation of ethylene with benzene to directly produce styrene. Using Rh(ppy)2(OAc) (1) (ppyH=2-phenylpyridine), the reaction of benzene with ethylene gives styrene and vinyl acetate in 77 and 23% selectivities, respectively, in contrast to the selectivities using Pd(OAc)2, which are 47% styrene and 53% vinyl acetate. The observation that complex 1 is an active catalyst for both styrene formation and H–D exchange between CH3CO2D and C6H6 suggests that styrene formation involves a Rh-mediated, benzene CH bond activation process. The crystal structures of complex 1 and Rh(ppy)2(acac–O,O′) (2) (acac=acetylacetonato) are also reported. Rh(acac)(CO)2 also works as catalyst for styrene formation by addition of acacH and O2 without any oxidizing agent, such as Cu salt. In this system, vinyl acetate is not formed at all in spite of the presence of acetic acid.