Heterogeneous Palladium-Catalysed Heck Reaction of Aryl Chlorides and Styrene in Water under Mild Conditions

The Heck reaction of chlorobenzene and substituted chlorobenzenes with styrene is performed in water using catalytic palladium on carbon. The presence of a reducing agent, a mild base, and a phase-transfer catalyst enhances the reactivity of chloroarenes. Transfer hydrogenation of the stilbene double bond to yield 1,2-diphenylethane is also observed in situ. The competing reactions in this system are haloaryl homocoupling and hydrodehalogenation. The effects of various process parameters on substrate conversion and product selectivity are examined for PhCl and PhBr and the unique role of the reducing agent is discussed. A series of filtration experiments indicate that, in this system, the catalysis is heterogeneous and not due to palladium leaching.