Allylic Ionization Versus Oxidative Addition Into Vinyl C-X Bonds By Pd With Polyfunctional Olefin Templates

The chemoselectivity of activation by a (PPh3)(4)Pd catalyst on a series of small, olefin-based compounds that were substituted with a variety of allylic and vinylic functional groups was studied. Of particular note, the allylic acetate of 1-acetoxy-2-bromo-2-propene (7) was selectively ionized by Pd in the presence of a malonate nucleophile, while oxidative addition of the C-Br bond to Pd occurred exclusively in the presence of a boronic acid nucleophile. When the acetate nucleophile was used, no ionization of the acetate leaving group occurred at all, which was proven by the use of deuterium-labeled substrates (e.g., 11). This report demonstrates that the nucleophile interacts in some way with Pd prior to catalyst activation of the substrate. Certainly in the case of the malonate nucleophile, this is without precedent and contradicts the central dogma of how these proposed catalytic cycles operate.