Mechanistic Study on the Reaction of Phenyldiazomethanes with Singlet Oxygen: Formation and Cycloreversion of 1,2,3,4-Dioxadiazole Intermediates

The formation of carbonyl oxides in the singlet oxygen (1O2) oxidation of phenyldiazomethanes 1 has been investigated mechanistically. Product ratios of N2/N2O, which are indicative of the selectivity in the carbonyl oxide/ketone formation, were determined by gas chromatography/mass spectrometry (GC/MS). The yields of carbonyl oxides were not affected by changing solvents but were significantly increased with the increasing electron-donating ability of substituents on diazomethanes. Rate constants for the quenching of 1O2 by 1 as determined by monitoring the emission of 1O2 at 1270 nm were also insensitive to solvents. These results suggest the predominant formation of 1,2,3,4-dioxadiazole intermediates by the cycloaddition of 1O2 to 1 followed by its cycloreversion, the selectivity of which is controlled by the relative stability of resulting carbonyl oxides. The formation of carbonyl oxides, observable from its transient absorption at ∼400 nm, was in good agreement with the decay of 1O2 within the exper...