Reactivity of sydnones with singlet oxygen

Abstract

The rate constants k_r for the reaction of several sydnones with singlet oxygen (¹O₂^∗) have been measured by the competition technique based on the 485 nm laser flash photolysis of a ¹O₂^∗ sensitizer and the kinetics of depletion of 1,3-diphenylisobenzofuran. The sydnone with a methyl substituent at the 4-position is particularly reactive (k_r = (1 - 5) × 10⁷ M^-1 s^-1 in various solvents). In contrast, the sydnones that are unsubstituted at the 4-position exhibit only poor reactivity towards ¹O₂^∗ (k_r ≤ 5 × 10⁵ M^-1 s^-1 in methanol and benzene). The solvent polarity has a small positive influence on k_r; for example, on going from carbon tetrachloride to acetone or acetonitrile, the increase in k_r for 4-methyl-3-p-tolylsydnone is only twofold to threefold. This can ben explained by a slight increase in dipole moment on achieving a transition state from a sydnone substrate that is already considerably polar.