Search

Heck reactions of aryl bromides with cyclohexene and cyclopentene catalyzed by typical homogeneous as well as heterogeneous Pd catalysts (Pd/C, Pd/SiO 2 , Pd/MgO, Pd/Al 2 O 3 , and Pd(0), Pd(II) and [Pd(NH 3 ) 4 ] 2+ in zeolites Y or ZSM-5) have been studied in order to get detailed information on the reaction mechanism with regard to the catalyst. The focus of the present investigation was on correlations between selectivity (Heck products: double bond isomers of arylcycloalkenes, dehalogenation and double arylation products) and nature of the catalyst or active Pd species, respectively. The results indicate that dissolved molecular Pd species are responsible for the Heck coupling for both homogeneous and heterogeneous (solid) catalysts, whereas dehalogenation is due to a mechanism involving the surface of solid Pd metal particles and radical processes. The selectivity of the reactions can be controlled by the choice of catalyst and reaction conditions (base, solvent, temperature).

[Pd]-exchanged NaY zeolites have been prepared, characterized, and applied for the first time for catalytic carbon-carbon coupling reactions. The catalysts exhibit a high activity and selectivity toward the Heck reaction of aryl bromides with olefins for small palladium concentrations (< or =0.1 mol % of Pd). The catalysts can easily be separated from the reaction mixture and reused after washing without loss in activity. No limitation to the diffusion of adducts in the zeolite cages was observed (for linear alkenes). The electronic nature of the aryl bromides and the olefins has a dominating effect on the reaction yield and selectivity. The heterogeneous catalysts quantitatively convert all types of all aryl bromide (complete conversion of bromobenzene within 30 min) and activated aryl chlorides under standard reaction conditions. Product form selectivity is observed in the Heck reaction with cyclic olefins.