Oxirane reactions ring strain

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. 

The occurrence of these reactions is always determined by thermodynamic factors. Oxirane has a large ring strain. Its polymerization around room temperature exhibits AGp<0. For 1,4-dioxane under the same conditions, AGp > 0. In other words, polyoxirane will split off 1,4-dioxane because the Gibbs energy of its depolymerization is negative. Actually the polymer should depolymerize completely. That this is not the case, is caused by kinetic factors. Termination of depolymerization need not coincide with termination of polymerization. 

The release of ring strain in epoxides is probably responsible for the high reactivity of these special ethers. HI opens epoxides under mild conditions stereospecifically to iodohydrins (Scheme 26). The mechanism is similar to the reaction of bromide with epoxides (see Section 1.7.3.3). It should be noted, however, that reduction of epoxides to alkenes may occur if vicinal diiodides are intermediately formed, which can lose I2 under the reaction conditions. With the combination of acyl chloride and Nal unstable diiodides are avoided and 2-iodoethyl esters are formed from oxiranes (Scheme 27). ° ... 

There is no unique isodesmic reaction for a particular problem. For example, to compare the ring strain in oxacyclopropane (oxirane, ethylene oxide) with that in cyclopropane, one might calculate the reaction energy ofthe oxygen exchange reaction (remember that isodesmic reactions do not have to be experimentally achievable) ... 

The properties of the thiiranes are primarily due to ring strain. In spite of the smaller strain enthalpy, thiirane is thermally less stable than oxirane. Even at room temperature, linear macromolecules are formed because of polymerization of ring-opened products. Substituted thiiranes are thermally more stable. The following reactions are typical for thiiranes [13] ... 

Several bi- and tri-cyclic thiepan derivatives were obtained by autoxidation of the cyclobutadiene (111) or by various reactions with strained sulphur-containing acetylenes, e.g.. the dithiet (112). The ylides derived from cis- and trans-ill) were alkylated stereoselectively at low temperatures, and they reacted with carbonyl compounds to give ring-opened oxirans (113). Reduction of the 6-methoxy-carbonyl derivative of (72) afforded the ten-membered cyclic sulphide (114). ... 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

On the contrary, a-lithiated epoxides have found wide application in syntheses . The existence of this type of intermediate as well as its carbenoid character became obvious from a transannular reaction of cyclooctene oxide 89 observed by Cope and coworkers. Thus, deuterium-labeling studies revealed that the lithiated epoxide 90 is formed upon treatment of the oxirane 89 with bases like lithium diethylamide. Then, a transannular C—H insertion occurs and the bicyclic carbinol 92 forms after protonation (equation 51). This result can be interpreted as a C—H insertion reaction of the lithium carbenoid 90 itself. On the other hand, this transformation could proceed via the a-alkoxy carbene 91. In both cases, the release of strain due to the opening of the oxirane ring is a significant driving force of the reaction. 

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