Thermal decomposition of diacyl peroxide. Part 11. 18O-Scrambling in carbonyl-18O-labelled phthaloyl peroxide, a cyclic Case III diacyl peroxide. Extremely large return of unescapable acyloxyl radical pair

Abstract

¹⁸ O-Scrambling in carbonyl-¹⁸O-labelled phthaloyl peroxide, a cyclic Case III diacyl peroxide, proceeds 42 times faster than its decomposition at 80° C. This is the only diacyl peroxide for which ¹⁸O-scrambling takes place faster than the decomposition. The activation parameters have been found to be Δ_dH^‡= 33.4 ± 0.7 kcal mol^–1 and Δ_dS^‡= 5.9 ± 1.9 cal mol^–1 K^–1 for decomposition and Δ_sH^‡= 31.9 ± 0.9 kcal mol^–1 and Δ_sS^‡= 9.5 ± 2.6 cal mol^–1 K^–1 for ¹⁸O-scrambling. The results are best explained in terms of 97.6% of return of the phthaloyloxyl radical intermediate which is generated by homolysis of the O–O bond of the peroxide affording the ¹⁸O-scrambled peroxide. Based on this mechanism, the transition state for degradation of the phthaloyloxyl radical intermediate is estimated to be 1.5 kcal mol^–1 greater than the recombination. Comparison of the kinetic data for phthaloyl peroxide with those for benzoyl peroxide reported by Martin and Hargis reveals that the extreme thermal stability of phthaloyl peroxide is not due to its cyclic structure which may hinder the O–O stretching vibration, but attributed to the facile phthaloyloxyl radical return. These observations clearly support our postulate concerning the structural effect on the ¹⁸O-scrambling of diacyl peroxide which indicates that the recombination of the aroyloxyl radical pair has to overcome a substantial activation energy barrier while a very small activation energy is necessary for aliphatic ones. The present kinetic data are discussed in comparison with those for the Cope rearrangement of 1,4-bis(dideuteriomethylidene)cyclohexane.