Scavenging of Intermediates Formed in Photolysis of α-Diazocarbonyl Compounds and Hydroxycyclopropenones. Implication on the Mechanism of the Photo-Wolff Reaction

The question of whether α-carbonylcarbene intermediates are formed as ketene precursors in the photo-Wolff reaction of α-diazocarbonyl compounds was addressed by comparing pyridine scavenging of transients generated by laser flash photolysis of benzoyldiazomethane and 1-naphthoyldiazomethane with pyridine scavenging of transients formed by flash photolysis of the corresponding arylhydroxycyclopropenones. The rate laws for the scavenging process in the two different photoreactions in aqueous solution were found to be exactly the same, and since ketene intermediates are common to both reactions but α-carbonylcarbenes are not, the transients were thus identified as ketenes. Formation of these ketene intermediates in the photo-Wolff reaction took place within the duration of the laser pulse (ca. 20 ns), and α-carbonylcarbene intermediates, if formed at all, must have lifetimes shorter than this.