Oxirane biradical

The visible light excitation of diene - NO reactant pairs has been also found to cause an oxygen-atom transfer from NO to diene to form the oxirane biradical [77], which is similar to the photochemical reactions observed in the alkene - NO - Ar systems. Two isomers are considered for the nitrite radical of butadiene ... 

Fitzmaurice, D.J. and Frei, H., Photochemistry of cycloalkene NOj coUisional pairs in a cryogenic matrix chemical trapping of cycloalkene oxirane biradical conformers and comparison of product control for excitation above and below the NOj dissociation threshold, /. Phys. Chem., 95, 2652, 1991. 

The ring strain associated with the three-membered ring facilitates the generation of reactive species such as biradicals (cyclopropanes) and ylides (oxiranes and azir-idines) upon photoexcitation. These intermediates can undergo cycloaddition reaction with olefins to give five-membered rings. Selected examples from each reactive intermediate are presented in this section. 

Irradiation (56) of tetraphenyloxetanone [122] at -196°C results in the formation of a blue intermediate identified as the ring-opened oxirane. The species is described as a ylid rather than a biradical because no ESR spectrum of the material could be detected at -196°C. 

From the excited-state oxirane, mainly propanal is formed by hydrogen abstraction via a biradical. With the increase of pressure, the excited state is quenched and, at the same time, there is a rise in the quantity of acetone. The two products are therefore not formed from the same intermediate. Acetone formation is conceived... 

The photocatalytic isomerization is explained by intramolecular hydrogen migration. The reactive state is an n-n triplet and the products arise from a common biradical intermediate. The cis isomer does not react. Irradiation of oxiranes not containing the phenyl group gives rise to cyclobutanol epimers (Eq. 350). ... 

Flowers et al. have dealt with the thermal gas-phase reactions of methyl-oxirane, other methyl-substituted oxiranes, and ethyloxirane. The kinetics of the processes have been compared. Pyrolysis of these compounds is a first-order, homogeneous, nonradical process the reaction rate is not affected by radical scavengers. A biradical mechanism holds. The thermochemical behavior of cyclopentene oxide and cyclohexene oxide is similar. The primary products are the corresponding carbonyl compounds and unsaturated alcohols. Two mechanistic possibilities have been discussed they are obtained from a common biradical intermediate or the alcohol is formed directly from the oxirane in a concerted manner. Thermolysis of spirooxiranes leads to ketone derivatives via biradicals with homolytic bond cleavage (Eqs. 376, 377). ... 

Oxirane formation can also occur via free radical mechanisms, as in the reaction of certain fluoroalkenes with oxygen. Under pressure at elevated temperatures, oxygen alone can suffice, but activation is frequently provided in the form of radical initiators (e.g., tribromofluoromethane) and ultraviolet light. Thermolysis of dioxole 5, comonomer from which DuPont s Teflon-AF is made, offers an unusual route to an oxirane. Rearrangement of the heterocycle presumably takes place via a biradical intermediate. ... 

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