Pd(Ⅱ)-Catalyzed MIZOROKI-HECK Reaction

Arylalkene is one of main constructions easily found in dyes, medicines, natural products, pesticides, cinnamic acid type spices and daily chemicals, as well as in polymer materials. The Mizoroki-Heck reaction which usually refers to a coupling reaction of unsaturated halides (or triflates) with alkenes in the presence of bases and palladium catalysts to form the substituted alkenes, has become one of the most important synthetic methods especially for the preparation of arylalkenes since its discovery in 1971.In the past four decades, the palladium-catalyzed Mizoroki-Heck coupling reaction has been developed significantly in its excellent catalytic efficiency including both high turnover numbers (TON) and turnover frequency (TOF) under mild conditions. However, ligands such as triarylphosphine ligands are normally necessary to enhance the reactivity and stability of Pd(0) catalysts. On the other hand, reductants such as alcohols, amines, or amides, etc are also required to convert the initial Pd(II) species to Pd(0) catalysts for igniting the coupling reactions. Furthermore, the selectivity to aryl C-I bonds vs. aryl C-Br bonds is not good in many cases. On the basis of the previous reports in our group on Pd(II)-catalyzed cross-couplings of aryl iodides with simple arenes and the catalytic Ullmann coupling reactions of aryl iodides in the absence of additional reductants, herein we have established successfully a Pd(II)-catalyzed Mizoroki-Heck reaction specially to aryl iodides without any external reductants in acetone. In the investigation of the reaction system, we could exclude that acetone, olefins or additives acting as the reductants under the reaction conditions. Thus a mechanism was proposed and partly confirmed by a series of experiments, involving the oxidative additions of Pd(II) catalysts with aryl iodides followed by the aryl-Pd(II) intermediates formed with the iodine anions from aryl iodides might playing the role of reductants to couple with olefins.Overall, an effective and convenient palladium-catalyzed Mizoroki-Heck reaction with high selectivity to aryl C-I bonds without any ligands and additional reductants has been developed. The catalytic cycle is still Pd(0)/Ar-Pd(II)/Pd(0) even though the initial catalysts are Pd(II) species.