A New Insight into the Stereoelectronic Control of the Pd0 -Catalyzed Allylic Substitution: Application for the Synthesis of Multisubstituted Pyran-2-ones via an Unusual 1,3-Transposition

Abstract

Pyran-2-ones 3 undergo a novel Pd⁰ -catalyzed 1,3-rearrangement to afford isomers 6. The reaction proceeds via an η² -Pd complex, the pyramidalization of which (confirmed by quantum chemistry calculations) offers a favorable antiperiplanar alignment of the Pd-C and allylic C-O bonds (C), thus allowing the formation of an η³ -Pd intermediate. Subsequent rotation and rate-limiting recombination with the carboxylate arm then gives isomeric pyran-2-ones 6. The calculated free energies reproduce the observed kinetics semi-quantitatively.

Keywords: allylic rearrangement; catalysis; palladium; quantum chemistry calculations; stereoelectronic effects.