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 <i>trans</i>-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.