The Pd-catalyzed coupling of allyl halides and tin aryls: why the catalytic reaction works and the stoichiometric reaction does not.

Arylallylpalladium complexes [Pd(5-C6F5-eta3-cyclohexenyl)(C6Cl2F3)(NCMe)] (10) and [Pd2(mu-C6Cl2F3)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (13) have been synthesized. Complex 13 is an example of a rare class of metal complexes with aryl bridges and its X-ray crystal diffraction structure has been determined. These arylallylpalladium complexes are involved in the coupling of Bu3SnRf (1, Rf = dichlorotrifluorophenyl) and [Pd2(mu-Br)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (2); complex 10 has been detected in the course of the stoichiometric coupling reaction in acetonitrile. Decomposition experiments of 10 and 13 in different conditions, and comparison with the reactions of 1 and 2, allow us to determine that reductive elimination does not occur in the absence of additives. p-Benzoquinone coordinates to Pd to give complex 15 and promotes reductive elimination to give the coupling products selectively. The outcome of the coupling reaction is controlled by the reductive elimination step, but the overall rate is controlled by the faster preequilibrium, which determines the concentration of 10 or 13. Palladium-catalyzed coupling of allyl halides and tin aryls works better than the stoichiometric allyl-aryl reductive coupling on isolated allylarylpalladium complexes, because they benefit from the presence in the solution of substrate allylic halides acting as electron-withdrawing olefins and promoting reductive elimination. More efficient allyl-aryl couplings, whether stoichiometric or catalytic, can be achieved upon addition of p-benzoquinone to the reaction mixture in a noncoordinating solvent.