Deoxygenation, epoxides

Transition-metal atoms have been shown to deoxygenate epoxides to alkenes (36). Chromium and titanium atoms emerged as the most effective species in this regard, abstracting over two equivalents of oxygen. By studying the reaction of a wide range of epoxides with chromium atoms, the reaction... 

A final possibility was raised by the observation that (Cp Re0)2(jU.-0)2 deoxygenated epoxides. Migration of carbon not to rhenium but to oxygen would result in a Re(V)-epoxide complex ... 

Organophosphorus reagents based on triphenylphosphine, or trimethylsilyl iodide, may be used to deoxygenate epoxides to re-form the parent alkene. Reactions based on this, or on a related scheme using the reduction of an iodohydrin, have been used in the synthesis and protection of alkenes as their epoxides (Scheme 2.23). 

Sodium (cyclopentadienyl)dicarbonylferrate, dicobaltoctacarbonyl, and iron pentacarbonyl have been reported to deoxygenate epoxides efficiently to the corresponding alkenes. The reaction with (cyclopentadienyl)dicarbonylfeirate proceeds with inversion of stereochemistry, whereas Fe(CO)5 shows low stereoselectivity. Both Co2(CO)8 and Fe(CO)s are applicable to epoxides having carbonyl groups (equations 45 and 46). 

Carbenes or carbenoids have also been reported to deoxygenate epoxides. To generate these species, dimethyl diazomalonate/rhodium(II) acetate and 9-diazofluorene 44 have been employed both reagents show high stereoselectivity. For example, ciJ-2-butene oxide is converted to cir-alkene with more than 93% retention under irradiation (350 nm) using 9-diazofluorene, and functional groups, such as carbonyl, can survive using diazomalonate and rhodium(II) acetate (equation 49). 

Trifluoroacetyl iodide, generated in situ from trifluoroacetic anhydride and sodium iodide, deoxygenates epoxides.The reaction begins by anti opening of the epoxide, followed by the formation of an... 

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