Highly Selective Pd-Catalyzed Reductive Coupling of Substituted Haloarenes with Supported Phase-Transfer Catalyst using Zn as the Reducing Agent

Highly selective reductive coupling of substituted haloarenes to biaryls is accomplished by Zn in N,N-dimethylformamide (DMF) and in the presence of a catalytic amount of PdCl₂, PPh₃, and a carbon-supported phase-transfer catalyst (PTC). Selectivity as high as 100% is achieved with chlorotoluenes. It is realized that the supported PTC has a predominant role in minimizing the rate of the hydrodehalogenation reaction. The reaction is found to be selective only when homogeneous PdCl₂ is applied as the catalyst, whereas heterogeneous Pd/C-catalyst selectively reduces chloroarenes to arenes under similar conditions. The role of PPh₃ is discussed and the effects of different process parameters such as temperature, PdCl₂ loading, PPh₃ to PdCl₂ ratio, amount of supported PTC, and solvents have been examined. A mechanism is proposed which is in good agreement with the experimental results obtained.