Tandem one-pot palladium-catalyzed reductive and oxidative coupling of benzene and chlorobenzene

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 E(a) = 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.