Gold(Ⅰ)-Catalyzed Allenyl Cope Rearrangement: Evolution from Asynchronicity to Trappable Intermediates Assisted by Stereoelectronic Switching

摘要

Pericydic reactions bypass high-energy reactive intermediates by synchronizing bond formation and bond cleavage. The present work offers two strategies for uncoupling these two processes and converting concerted processes into their "interrupted" versions by combining Au(Ⅰ) catalysis with electronic and stereoelectronic factors. First, we show how the alignment of the C3-C4 bond with the adjacent n systems can control the reactivity and how the concerted scission of the central σ bond is prevented in the boat conformation. Second, the introduction of a fluorine atom at C3 also interrupts the sigmatropic shift and changes the rate-determining step of the interrupted cascade from the 6-endo-dig nucleophilic attack to the fragmentation of the central C3-C4 bond. Furthermore, this effect strongly depends on the relative orientation of the C-F bond toward the developing cationic center. The equatorial C-F bond has a much greater destabilizing effect on TS2 due to the more efficient through-bond interaction between the acceptor and the cationic it system. In contrast, the axial C-F bond is not aligned with the bridging C-C bonds and does not impose an equally strong deactivating stereoelectronic effect. These differences illustrate that the competition between concerted and interrupted pericyclic pathways can be finely tuned via a combination of structural and electronic effects modulated by conformational equilibria. The combination of Au(Ⅰ) catalysis and C-F-mediated stereoelectronic gating delays the central bond scission, opening access to the interrupted Cope rearrangements and expanding the scope of this classic reaction to the design of new cascade transformations.