Mechanism of the Direct Reaction of Phosphite Ozonides with Olefins

Article abstract of DOI:10.1021/jo01303a017

The direct reaction of triphenyl phosphite ozonide (TPPO) with olefins, occurring at low temperatures where no thermal generation of singlet oxygen occurs, is observed in identical fashion with a caged-ring and two o-phenylene phosphite ozonides whose thermal generation of singlet oxygen is very slow compared to TPPO.Biadamantylidene, which is quite reactive toward singlet oxygen, appears totally hindered against the direct reaction with TPPO.In the case of Δ²-dihydropyran the direct reaction contrasts strongly with the behavior toward singlet oxygen in that (1) ene reaction is only one-tenth as great as dioxetane formation, (2) the importance of ene reaction is not a function of solvent polarity, and (3) ene reaction is inhibited by free-radical scavengers.In the case of tricyclopropylethylene, the formation of allylic hydroperoxides in the case of singlet oxygen is replaced in the direct TPPO reaction by cleavage to ketone and aldehyde under conditions where the hydroperoxides are stable and hence cannot be intermediates.The direct reaction of TPPO with 1,2-dimethylcyclohexene and 2-methyl-2-butene gave mixtures of ene products similar to those from singlet oxygen and, in the case of the former hydrocarbon, quite different from the product of chain oxidation.Dabco, tetramethyl-p-phenylenediamine, and other tertiary amines rapidly decompose phosphite ozonides to phosphate and ground-state oxygen, with evidence of electron transfer from the amines.In this and in the direct oxygenation of dihydropyran, the 31P NMR signal goes directly from 63 to 17 ppm, showing that there is no detectable intermediate state of phosphorus between TPPO and phosphate.The facts are accommodated by a mechanism involving initial bond formation between the olefin and thecenter oxygen atom of the ozonide, with chemical and sterochemical sequels which are discussed.The freezing point of chloroform is depressed to the same extent by TPPO as by triphenylphosphate, showing that TPPO is monomeric.