Strained olefins epoxidation

Strain-related features in epoxidation of strained olefins by peracids were first discussed for the cis/trans-isomers of cyclooctene (189). The rate... 

The conjugate addition of nucleophiles to activated halogeno-olefins has been shown to result in a Darzens intermediate, which in the presence of a carbonyl group affords an epoxide. In cases where excessive strain prevents epoxide formation, formation of a cyclopropane ring ensues (Scheme 4). ... 

When lithiated, the ring strain of the three-membered heterocycle remains important, and this strain, combined with a weakening of the a-C-O bond, due to its greater polarization, make metalated epoxides highly electrophilic species [2], They react with strong nucleophiles (often the base that was used to perform the a-deprotonation) to give olefins following the elimination of M2O (Scheme 5.2, Path B), a process often referred to as reductive alkylation . 

Dioxiranes are three-membered cyclic ring peroxides that are expected to be very unstable owing to ring strain. They are effective oxygenating agents for epoxidations of olefins, allenes, polycyclic aromatic hydrocarbons, enols. and a, /i-unsaturated ketones for insertions of oxygen into X—H... 

Other Addition Reactions.—A quantitative study of the epoxidation of 3-substituted cholest-5-enes with peroxy-acid shows that both the rate and the epimer ratio vary according to the C(3)-substituent. " The epoxidation clearly has some electrophilic character. o-Sulpho-perbenzoic acid, which may be used in aqueous-organic solvents, converted cholesterol efficiently into the a-epoxide (89%). The A -olefinic bond in cholestan-5,16-dien-3 -ol is sufficiently reactive, perhaps as a consequence of ring strain, to permit selective 16a, 17a-epoxidation. " ... 

The combination of a (tetraarylporphyrinato)Fe photocatalyst and molecular oxygen transforms strained alkenes to (preferentially) epoxides, whereas unstrained olefins lead to allylic oxygenation products [89]. The use of water-soluble metal porphyrin complexes (Mn , Fe ) facilitates the separation of substrates and products in aqueous solvent systems [90]. Copper(II) chloride induces chemo- and regioselectivity in the photooxyclorination of olefins (eq. (11)) [91]. 

This is not the case for a Rhodococcus rhodochrous strain that affords 1,2-epoxyalkanes from short-chain terminal olefins. Indeed, in the latter case, no product inhibition has been observed [106]. However, the ee s obtained were not determined, and it seems probable that they were quite low. On the other hand, a Nocardia corallina strain was described that afforded the corresponding 1,2-(i )-epoxy-2-methylalkane with ee s as high as 90% depending on the chain length [ 107]. These epoxides were used as chiral building blocks to prepare prostaglandin co-chains. 

It is not possible to state whether the more favorable C-H bond angle or the lower strain energy of the allylic structure (after C-H bond rupture) is responsible for the lower selectivity of bicyclo[2,2,2]oct-2-ene. However, comparison of these two bicyclic olefins indicate that relatively small changes in structure can have dramatic effects on selectivities to corresponding epoxides. Prevention of allylic C-H bond breaking is critical for epoxide formation. 

In many examples the reactivity has been discussed in terms of strain release. Whereas it is apparent that some strain is released in addition reactions, other factors have to be considered. Thus, a linear correlation between the rate of epoxidation and the ionization potential of bridgehead olefins has been observed (227). Other factors such as the polarizability and hyperpolarizability, which are associated with the outer region of the electronic structure of molecules as well as with intermolecular forces and chemical reactivity have not yet been considered (277). 

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