Mechanism of Thermolysis of Endoperoxides of Aromatic Compounds. Activation Parameters, Magnetic Field, and Magnetic Isotope Effecs

Article abstract of DOI:10.1021/ja00414a029

A mechanistic investigation has been made of the thermolysis of several endoperoxides of anthracenes and naphthalenes which produce molecular oxygen and the parent aromatic species quantitatively.Qualitative thermochemical measurements in the solid state indicate that in all cases studied, the reactions were endothermic.This situation appears to be valid in solution also.Clean first-order kinetics were observed for these thermolyses.Activation parameters were derived from the temparature dependence of the first-order rate constants.The primary yields of singlet oxygen (¹O2) from the several endoperoxides were determined, and a correlation was discovered between the A factors (ΔS<*> values) for thermolysis and the yield of ¹O2.It was found that high A factors (positive ΔS<*> values) correlated with relatively low yields of ¹O2 and that low A factors (slightly negative or near zero ΔS<*> values) correlated with nearly quantitative yields of ¹O2.These two results are interpreted in terms of a diradical mechanism which leads to low yield of ¹O2 and a concerted mechanism which leads to quantitative yields of ¹O2.This interpretation is consistent with the observation of a magnetic field effect on the yield of ¹O2 from endoperoxides whose thermolyses proceed with positive ΔS<*> values and the absence of a magnetic field effect on the yield of ¹O_i endoperoxides whose thermolyses proceed with near zero ΔS<*> values.Further support for the occurrence of a diradical mechanism is available from the demonstration of a special 17O isotope effect on the thermolysis of an endoperoxide which is postulated to undergo thermolysis principally via a diradical intermediate.The thermolysis of endoperoxides which decompose mainly by a diradical mechanism yields triplet molecular oxygen that is selectively enriched in 17O.