Dynamics of the Wolff rearrangement of six-membered ring o-diazo ketones by laser flash photolysis

The Wolff rearrangement of six-membered ring o-diazo ketones, 1-oxo-2-diazo-1,2-dihydronaphthalene (DNQ) and its four 4- and 5-sulfonate analogues, 1-oxo-2-diazo-1,2-dihydrobenzene (DBQ), and diazoMeldrum (DM), are studied in solution at room temperature by laser flash photolysis. A presursor to a ketene is observed in most cases. It is assigned to be an oxirene on the basis of reactivity, kinetic results, and thermodynamic results. The Wolff rearrangement does not involve a concerted mechanism but a stepwise reaction for these types of diazo ketones. Thermodynamic analyses indicate that the activation barriers for the transformation of the oxirenes into ketenes are very low, typically 10-20 kJ/mol, in good agreement with theoretical predictions in the literature. The observation of oxirene in solution in nanosecond time scales, and longer, is attributed to the low preexponential factors due to the negative entropy of solvation. The important role of the solvent is further confirmed in the kinetic analysis of ketene formation in various water-methanol mixtures.