Highly Stereoselective [2 + 4] Annulation of Phosphenes and Enones with β‑Electron-Donating Groups

Enones with β-electron-donating groups can exist
in their
zwitterionic resonance forms, which favor nucleophilic attack to phosphenes
generated from phosphoryl (aryl)diazomethanes under blue light irradiation
to yield zwitterionic intermediates. The intermediates cyclize to
afford highly stereoselective trans-3,4-dihydro-1,2-oxaphosphinine
2-oxides in good to excellent yields. In the cyclic transition state
of the final cyclization, the stereoelectronic effect and the Coulomb
force control the high stereoselectivity. The electron-donating groups
of enones play double roles in both nucleophilic attack and cyclization.
The reaction features readily available starting materials, high atom-economy,
no catalyst, a fast reaction rate, broad functional group-tolerance
and substrate scope, high yields and stereoselectivity, and mild conditions.