Theoretical Studies on a New Class of C–C Bond Formation: Palladium-Catalyzed Reactions of α‑Diazocarbonyl Compounds with Allylic Esters

Abstract

The Pd-catalyzed reactions of α-diazocarbonyl compounds with allylic esters have been comprehensively studied with the aid of DFT calculations. The reaction can be divided into two catalytic processes. The first process is Pd-catalyzed allylic ester decarboxylation leading to formation of the intermediate product 1,4-enyne (<b>P1</b>). In this process, the ester oxidative addition proceeds via an ion-pair mechanism rather than a concerted mechanism. The decarboxylative step from the oxidative addition product (<b>4</b>) proceeds via an (η²-CC)→Pd-coordinated ion-pair intermediate (<b>9</b>). The rate-determining step for this catalytic process is found to be the 1,4-enyne reductive elimination step. The second process is the Pd-catalyzed reaction between 1,4-enyne (<b>P1</b>) and the diazo substrate, leading to the final product. In this process, the coupling between 1,4-enyne (<b>P1</b>) and the metal carbene generated from the reaction of PdL₂ with the diazo substrate forms a four-membered metallacyclic intermediate, from which β-phenylethynyl elimination and subsequent reductive elimination step are followed to produce the final product, a quaternary carbon compound. Our study revealed that the terminal allylic carbon atom instead of the internal atom is linked to the carbon center of the diazo substrate