Dye-sensitized photooxygenation of the C=N bond.: 5.: Substituent effects on the cleavage of the C=N bond of C-aryl-N-aryl-N-methylhydrazones

The title compounds are cleaved cleanly at the C = N bond by singlet oxygen (O-1(2), (1)Delta(g),) yielding arylaldehydes and N-aryl-N-methylnitrosamines. These reactions take place more rapidly at -78 degrees C than at room temperature. The effects of substituent variation at both the C-aryl and N-aryl groups were studied using a competitive method. Good correlations of the resulting rate ratios with substituent constants W or e) were obtained yielding small to very small p values indicative of small to very small changes in charge distribution between the reactant and the rate determining transition state. Electron withdrawing groups on the C-aryl moiety retard reaction somewhat by preferential stabilization of the hydrazone. Electron donors on the other hand slightly stabilize the rate determining transition state. Substituents on the N-aryl group have almost no effect. Inhibition by 3,5-di-tert-butylphenol was not observed showing that free (uncaged) radical intermediates are not involved in the mechanism. We postulate a mechanism in which the initial event is exothermic electron transfer from the hydrazone to O-1(2) leading to an ion-radical caged pair. Subsequent covalent bond formation between the hydrazone carbon and an oxygen atom is rate controlling. The transition state for this step also has a lower enthalpy than the starting reactants, but the free energy of activation is dominated by a large negative T Delta S-double dagger term leading to the negative temperature dependence. Direct formation of a C-O bond in the initial step is not unambiguously ruled out. Subsequent steps lead to C-N cleavage.