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.