The integral role of halide ions as a ligand in the reactions of stoichiometric arylpalladium reagents and Pd(II)-catalyzed reaction of phenylmercuric acetate with various allylic compounds were studied. The halide ion was found to inhibit β-H elimination and promote β-heteroatom elimination in acidic media. In this reaction, a C-Pd intermediate with a β-heteroatom (including halogen, acetoxy, alkoxy, and hydroxyl groups) gives only β-heteroatom elimination product in the presence of halide ions, while β-H elimination is effectively blocked.

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Pd(OAc)(2) combined with nitrogen-containing ligands (e.g., 2,2'-bipyridine) catalyzed the cyclization of (Z)-4'-acetoxy-2'-butenyl 2-alkynoates (1) in acetic acid to afford the alpha-(Z)-acetoxyalkylidene-beta-vinyl-gamma-butyrolactones (2) with high efficiency and high stereoselectivity. The nitrogen-containing ligands, like halides, served to favor beta-heteroatom elimination over beta-hydride elimination in Pd(II)-mediated reactions. The generality of this ligand effect was probed in both stoichiometric and catalytic reactions. With these results in hand, the catalytic asymmetric protocol was achieved with high enantioselectivity (up to 92% ee) when pymox (pyridyl monooxazoline) or bisoxazoline was used. The absolute configuration of the products and the synthetic utility of this asymmetric transformation were established through the convenient synthesis of (3S)-(+)-A-factor.