Tandem One-Pot Palladium-Catalyzed Reductive and Oxidative Coupling of Benzene and Chlorobenzene

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Abstract
The in situ combination of oxidative coupling of benzene to biphenyl and reductive coupling of chlorobenzene (also to biphenyl) using palladium catalysts (Pd2+/Pd0) is described. In each cycle, the reductive process regenerates the catalyst for the oxidative process and vice versa. Kinetic investigations show that the reaction rate depends on [C6H6], [C6H6Cl], and catalyst loading, with Ea = 13 kcal mol-1. The reduced palladium catalyst undergoes deactivation through aggregation and precipitation, but it is observed that during this deactivation process the Pd0 becomes an active catalyst for the reductive coupling of chlorobenzene. Accordingly, while Pd0/C particles are inactive, Pd0 colloids do catalyze the tandem reaction. Conversion is increased in the presence of a phase-transfer catalyst, presumably due to stabilization of the active Pd0 clusters. The two halves of the catalytic cycle are examined in the light of previous research, regarding analogous oxidative and reductive coupling reactions, using stoichiometric amounts of PdCl2 and Pd0, respectively. The roles of homogeneous PdCl2 and Pd0 clusters are discussed.