Scavenging of intermediates formed in photolysis of alpha-diazocarbonyl compounds and hydroxycyclopropenones. Implication on the mechanism of the photo-Wolff reaction

The question of whether alpha-carbonylcarbene intermediates are formed as k etene precursors in the photo-Wolff reaction of alpha-diazocarbonyl compoun ds was addressed by comparing pyridine scavenging of transients generated b y laser flash photolysis of benzoyldiazomethane and 1-naphthoyldiazomethane with pyridine scavenging of transients formed by flash photolysis of the c orresponding arylhydroxycyclopropenones. The rate laws for the scavenging p rocess in the two different photoreactions in aqueous solution were found t o be exactly the same, and since ketene intermediates are common to both re actions but alpha-carbonylcarbenes are not, the transients were thus identi fied as ketenes. Formation of these ketene intermediates in the photo-Wolff reaction took place within the duration of the laser pulse (ca. 20 ns), an d alpha-carbonylcarbene intermediates, if formed at all, must have lifetime s shorter than this.