Photolysis of 4-diazo-2-pyrazolin-5-ones in methanol gives a mixture of stereoisomeric methyl 3-aryl-(or alkyl-) azo-2-alkenoates. A study of the absorption spectra of the species generated upon steady-state photolyses in glasses at 77 K, in solution at 296 K, and by laser and lamp flash photolysis experiments suggests that a long-lived heterocyclic intermediate that we formulate as a carbene or its valence isomer, as well as a ketene generated by ...
