Intramolecular palladium(II)-catalyzed 1,2-addition to allenes

Palladium(II)-catalyzed intramolecular 1,2-additions to allenes substituted with an internal nucleophile have been developed. Carboxylic acids, alcohols, N-substituted amides, and carbamates were used as internal nucleophiles in the palladium-catalyzed reaction, which afforded lactones, tetrahydropyrans, tetrahydrofurans, pyrrolidines, and oxazolidinones in good isolated yields. The reactions were performed in the presence of LiBr with Pd(OAc)(2) as the catalyst. Two different reoxidants, p-benzoquinone or Cu(OAc)(2), were used, the choice of oxidant being dependent on the substrate. The reaction proceeds through an external nucleophilic attack (Br-) on a (pi -allene)palladium complex to produce a (pi -allyl)palladium intermediate. Subsequent intramolecular attack by the second internal nucleophile gives the product. The intermediate (pi -allyl)palladium complexes were isolated and characterized. The scope and limitation of the reaction were studied together with its mechanism and selectivity, under different reaction conditions.