Ru-catalyzed oxidative coupling of arenes with olefins using O₂ [4]

Catalytic carbon−carbon bond formation by C−H activation is a topic of much current interest. Significant progress has been made in recent years in the development of synthetically useful catalytic addition of arenes to alkenes to give the saturated alkyl arenes.1,2 Catalytic oxidative coupling of arenes with alkenes to give aryl alkenes, in which the double bond is preserved, is a highly desirable goal. Such a reaction, which does not require the utilization of a reactive substituent and does not produce waste, may have an advantage over other methods for the preparation of aromatic alkenes, such as the well-known Heck reaction for the vinylation of aryl halides. Stoichiometric coupling of olefins with arenes promoted by Pd(II) is well-known.3a Tsuji et al.,4 and lately Fujiwara et al.5 and others,6 have demonstrated catalysis by utilizing peroxides as oxidants in their systems. While good catalytic activity was achieved with some alkenes, acrylates resulted in low activity (∼10 turnovers). The use of peroxide oxidants and acetic acid solvent in these systems is problematic from the industrial point of view. Attempts to use O2 or air resulted in low activity in intra-7 and intermolecular3b-d,8 coupling, and alkene oxidation took place in the intermolecular reaction. Another approach utilizing Rh carbonyl clusters under high (20−30 atm) CO pressure9 resulted in modest catalytic activity with concomitant hydrocarbonylation of the alkene. Low catalytic activity was reported for the Rh-catalyzed photochemical coupling of arenes with alkenes, in which concomitant hydrogenation of the alkene took place and biaryls were formed as byproducts.10 A low yield, Rh-catalyzed oxidative phenylation of ethylene was reported very recently.11