Enantioselective [4 + 1] Annulation Reactions of α‑Substituted Ammonium Ylides To Construct Spirocyclic Oxindoles

Ammonium ylides have a long history
in organic synthesis, but their
application in asymmetric catalysis is still underdeveloped in regard
to both substrate scope and reaction pathways compared with phosphorus
and sulfur ylides. Here a previously unreported asymmetric [4 + 1]
annulation reaction of 3-bromooxindoles and electron-deficient 1-azadienes
has been developed through ammonium ylide catalysis of a newly designed
2′-methyl α-isocupreine (α-MeIC), efficiently delivering
spirocyclic oxindole compounds incorporating a dihydropyrrole motif
in excellent enantioselectivity (up to 99% ee). To the best of our
knowledge, this work represents the first example of asymmetric catalysis
of ammonium ylides bearing α-substitutions, and the catalytic
[4 + 1] annulation pathway of ammonium ylides is also unprecedented.
Moreover, 1H NMR, mass spectroscopy, and computational
calculation studies were conducted, and the catalytic cycle and a
tentative explanation of the enantioselective mechanism have been
successfully elucidated.