Intramolecular Palladium(II)-Catalyzed 1,2-Addition to Allenes

Abstract

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 (π-allene)palladium complex to produce a (π-allyl)palladium intermediate. Subsequent intramolecular attack by the second internal nucleophile gives the product. The intermediate (π-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.