Concerted Homolysis in Thermal Decomposition of Peresters from α-Hydroperoxydiazenes

Article abstract of DOI:10.1021/ja00387a042

Treatment of 2-(phenylazo)-2-propyl hydroperoxide with benzoyl chloride and pyridine in ether or in hydrocarbon at -5 deg C afforded the unstable 2-(phenylazo)-2-propyl perbenzoate (1).Thermolysis of 1 in p-chlorotoluene, chlorobenzene, or deuteriochloroform followed first-order kinetics with rate constants 6.4 +/- 0.7 x 10^-4 s^-1, 4.5 x 10^-4 s^-1, and 5.1 +/- 0.7 x 10^-4 s^-1, respectively, at 10 deg C.Those rate constants are about 10⁸ times as large as the rate constant for thermolysis of tert-butyl perbenzoate, which is known to react by one-bond scission.The much higher reactivity of 1 implies that it reacts by a concerted mechanism, to form the benzoyloxy radical, acetone, and phenyldiazenyl radical as initial fragments.Attempts to prepare 2-(tert-butylazo)-2-propyl perbenzoate (2), from 2-(tert-butylazo)-2-propyl hydroperoxide by the analogous procedure, failed.That failure suggests that 2 decomposes still more rapidly than 1, and the further enhancement of reactivity implies that 2 decomposes by concerted, 3-bond scission to yield benzoyloxy radical, acetone, nitrogen, and tert-butyl radical in the rate-determining step.The mechanisms of thermolysis of 1 and 2 are pertinent to the chemistry of α-hydroxydiazenes and α-hydroperoxydiazenes, for which concerted, induced decomposition pathways have been postulated earlier.