Nickel-catalyzed mizoroki-heck- versus Michael-Type addition of organoboronic acids to alpha,beta-unsaturated Alkenes through fine-tuning of Ligands

Various arylboronic acids reacted with activated alkenes in the presence of [Ni(dppe)Br-2], ZnCl2, and H2O in CH3CN at 80 degrees C to give the corresponding Mizoroki-Heck-type addition products in good to excellent yields. Furthermore, 1 equivalent of the hydrogenation product of the activated alkene was also produced. By tuning the ligands of the nickel complexes and the reaction conditions, Michael-type addition was achieved in a very selective manner. Thus, various p- and o-substituted arylboronic acids or alkenylboronic acid reacted smoothly with activated alkenes in CH3CN at 80 degrees C for 12 h catalyzed by Ni(acac)(2), P(o-anisyl)(3), and K2CO3 to give the corresponding Michael-type addition products in excellent yields. However, for m-substituted arylboronic acids, the yields of Michael-type addition products are very low. The cause of this unusual meta-substitution effect is not clear. By altering the solvent or phosphine ligand, the product yields for m-substituted arylboronic acids were greatly improved. In contrast to previous results in the literature, the present catalytic reactions required water for Mizoroki-Heck-type products and dry reaction conditions for Michael-type addition products. Possible mechanistic pathways for both addition reactions are proposed.