Palladium(II)-Catalyzed Ullmann Reaction Of Aryl Iodides

Biaryl motifs are easily found in many natural products, medicines and organic electronical materials. One century ago, Ullmann discovered successfully that biaryls were prepared through homocoupling reactions of aryl halides in the presence of stoichiometric copper under high temperature. After that, transition metal-catalyzed couplings of aryl halides were developed, but additional reductants were also necessary in these reactions.On the other hand, the formation of unsymmetrical biaryls, the more important products in organic synthesis by cross-coupling reactions is always a challenge to chemists. Although a few examples on the cross-couplings between two different aryl halides have been disclosed, the catalytic cross-couplings of only aryl iodides are rarely reported because enhancing the reaction selectivity on the formation of unsymmetrical products is not easy using the current systems.Herein, we describe a novel Pd(II)-catalyzed Ullamnn coupling reaction of various aryl iodides in the absence of special reductants to afford symmetrical or unsymmetrical biaryls respectively. Especially in the cross-coupling reactions, both selecting a pair of aryl iodides with different reactivity and tuning the ratio of them are crucial to obtain the unsymmetrical biaryls as major products. Furthermore, a new mechanism of these Pd(II)-coupling reactions is also proposed. In a series of tests to investigate the actual reductants in these processes, we found that the aryl iodides could play the role of reductant and the initial catalysts should be Pd(II) complexes. Thus, Pd(IV) species involved in the catalytic cycles of these couplings are reasonable.In summary, a new effective and convenient method has been established for the preparation of symmetrical and unsymmetrical biaryls through Pd(II)-catalyzed coupling reactions of various aryl iodides without additional reductants. This process is not only green in organic synthesis but also significant in basic chemistry for exploring more new reactions.