Heterogeneous Pd-catalyzed biphenyl synthesis under moderate conditions in a solid-liquid two-phase system

The coupling of substituted halobenzenes to form biphenyls is effected at moderate temperature (65°C) using a reducing agent such as a formate salt and a base (NaOH) in the presence of a catalytic amount of phase-transfer catalyst and 5% Pd/C catalyst. The reaction conditions can be optimized to give reasonable selectivity, and the competing reduction process is minimized. The roles of temperature, catalyst loading, reducing agents, the base, and the phase-transfer catalyst are discussed. The catalyst can be efficiently recycled. However, an entirely different course of reaction occurs when a mixture of halobenzenes is utilized.