Unsymmetrical Biaryls by Palladium‐Catalyzed Coupling of Aryl Halides with Internal Reduction

Aryl bromides containing a (1,4,5,6‐tetrahydro‐6‐oxopyridin‐3‐yl)methyl substituent can be coupled with aryl halides yielding unsummetrical biaryls. In the course of this process, the cyclic enamide is oxidized to the 2(1 H )‐pyridinone. The reaction proceeds by a six‐membered palladacycle resulting from allylic C–H bond activation of the cyclic enamide, leading to a six‐membered palladacycle. A subsequent transmetalation reaction with an aryl‐Pd‐X intermediate eventually leads to mixed biaryls by reductive elimination and β‐hydride elimination. The best yields were obtained with 2 equiv. of aryl halide. The required substrates were prepared from bromoiodobenzenes by a sequence consisting of a Heck reaction with allyl alcohol, enamine formation, Michael addition to ethyl acrylate, and heterocycle formation with benzylamine.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)