Rhodium-Catalyzed Arene Alkenylation Using Only Dioxygen as the Oxidant
Weihao Zhu, T. Brent Gunnoe
Abstract
We report the oxidative conversion of unactivated arenes and alkenes to alkenyl arenes using unpurified air or purified dioxygen as the only oxidant. This method uses RhCl3 salt as the catalyst precursor and avoids the use of co-oxidants such as Cu(II). The use of dioxygen as the in situ oxidant gives water as the only byproduct of the alkenylation reaction. Conditions to achieve >1000 turnovers of alkenyl benzene products have been developed. As the catalysis progresses, oxidation of styrene product to form benzaldehyde becomes competitive. Compared to the Rh catalysis using Cu(II) oxidants, the aerobic reactions give decreased reaction rate and reduced anti-Markovnikov/Markovnikov selectivity when using α-olefins. For styrene formation, the reaction rate shows a first-order dependence on catalyst concentration, ethylene concentration (with saturation at higher ethylene concentrations), and dioxygen. An intermolecular kinetic isotope effect value of 2.7(6) was determined from parallel reactions with C6H6 versus C6D6. Synthesis of trans-stilbene and pentenyltoluenes has been demonstrated using this Rh-catalyzed aerobic alkenylation reaction.