Competing Epoxidation and Allylic Hydrogen Activation: trans-β-Methylstyrene Oxidation on Au(111)

The reaction of trans-beta-methylstyrene with atomic oxygen oil Au(111) has been investigated under ultrahigh vacuum conditions. Temperature programmed reactions, isothermal reactions, and infrared reflection absorption spectroscopy have been employed in order to probe the competing reactivity of the bifunctional allylic olefins. Partial oxidation products, including epoxide, benzoic acid, and cinnamic acid, are observed, in addition to combustion to CO2 and water. These products can be understood in terms of the competing oxidation of the C=C bond and the allylic C-H bond. In particular, we show that cinnamic acid is formed via oxygen-insertion into the allylic C-H bond to form the phenylallyloxy intermediate. This latter pathway appears unique to gold and, specifically, has not been reported for silver or copper Surfaces. Epoxidation and allylic H activation proceed with it selectivity of approximately 60 and 27%, respectively, at in initial oxygen coverage of 0.2 ML. Our results provide direct evidence that oxidation of both the C=C bond and the allylic carbon in trans-beta-methylstyrene is promoted by atomic oxygen on Au(111) to yield partiol oxidation products.