Deoxygenation of epoxides

To the best of our knowledge the first successful realization of this concept was achieved by Kochi in 1968, who described the deoxygenation of epoxides with Cr(II) reagents. However, no further attempts were undertaken to form C - C bonds with the pivotal /3-metaloxy radicals [25]. 

Olefins can be prepared by the deoxygenation of epoxides usually in good to excellent yields. However, the reaction is of synthetic use only if the epoxides employed as starting materials are prepared from other functional groups than olefins or if they can be isolated or readily accessed from natural sources. The issue of regioselectivity of deoxygenation is critical, however. 

It should not be forgotten, however, that titanocene(III) complexes are also excellent reagents for the deoxygenation of epoxides, as demonstrated independently by Schobert [13] and by Nugent and RajanBabu [5d[. An example of a reaction yielding a highly acid-sensitive product in reasonable yield is shown in Scheme 12.5. 

The deoxygenation of epoxides with the metal complexes mentioned above all seem to proceed via intermediate /i-melal oxy radicals. The reaction path after their trapping seems, however, to depend on the Lewis acidity of the ET reagent. 

Further examples of deoxygenation of epoxides by phosphine sulphides or selenides have appeared,41 as shown in Scheme 6 for simple epoxides (50). Incorporation of phosphorus into a five-membered ring appears to be responsible for the relatively rapid deoxygenations by the phosphole and A3-phospholen derivatives.41... 

A useful complement to the known methods for the deoxygenation of epoxides to alkenes is the reaction of epoxides with alkali (7,0-diethyl phosphorotellurolates. - ... 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

More frequent than the deoxygenation of epoxides to alkenes is reduction of epoxides to alcohols. Its regiospecificity and stereospecificity depend on the reducing agents. 

Deoxygenation of epoxides. Hpoxidcs, particularly a,/)-disubstituted ones, are deoxygenated by reaction with P2I4 or PI3 in CS2 at room temperature in 70 90% yield. The reaction occurs with retention (>97%) of configuration. In the case of terminal, trisubstituted, and tetrasubstituted epoxides, the yields with P2I4 or PI3 are low ( 50%) unless pyridine or triethylamine is added. 

Deoxygenation of epoxides. Finely dispersed lithium in refluxing THF converts epoxides into the corresponding alkenes, usually with retention of configuration.1 1 K. N. Gurudutt and B. Ravindranath, Tetrahedron Letters, 21, 1173 (1980). 

Deoxygenation of epoxides. Di-, tri-, and tetrasubstituted epoxides are converted into alkcncs by treatment with PI3 (or P2h). The reaction occurs most readily with a,/J-disubstiluted epoxides and with >98% retention of configuration. With more substituted epoxides addition of pyridine or triethylamine improves yields by preventing formation of alkyl iodides. 

Deoxygenation of epoxides. Complete details for preparation and use of the reagent for deoxygenation of epoxides, particularly of terminal epoxides, are available.1... 

Deoxygenation of epoxides.2 Epoxides are converted into alkenes stereo-specifically on reaction with benzeneselenocarboxamide at 0-20°. 

Stereoselective deoxygenation of epoxides. The ate complex [Bu3SnAl-(CH3)3] Li+ (1) formed from Bu3SnLi and A1(CH3)3 converts epoxides to alkenes with overall retention of stereochemistry. The results can be explained by inversion in the epoxide cleavage and antf-elimination of the Bu3Sn and O A1(CH3)3 groups. Example ... 

Electrocyclic rearrangements Organolithium reagents, 209 Elimination reactions to form carbon-carbon multiple bonds (see also Aromatization, Dehydrogenation, Deoxygenation of epoxides)... 

Deoxygenation of epoxides Oxiranes are converted into alkenes with the same configuration by ISi(CH3)3 generated in situ. 

Reduction and deoxygenation of epoxides. The radical formed on reaction of epoxides with Cp2TiCl (1) can be trapped by a H-atom donor such as cyclohexa- 1,4-diene (2) to provide an alcohol. This reduction is not useful for reduction of mono-... 

Deoxygenation of epoxides. The reagent effects deoxygenation of epoxides via an intermediate iodohydrin (11,30), which can be isolated as the trans-isomer in some cases. This reagent can effect selective reactions in the case of some diepoxides. 

In his early studies, Kagan showed that Sml2 can be used for the deoxygenation of epoxides.1 The deoxygenation of phosphine oxides and tin oxides is also effectively achieved using Sml2 provided that HMPA is used as an additive.59... 

DEOXYGENATION OF EPOXIDES WITH LOWER VALENT TUNGSTEN HALIDES trans-CYCLODODECENE... 

Tervalent phosphorus has a high affinity for oxygen and the P=0 bond once formed is very strong. This fact, which also provides the driving force for the Wittig and related reactions, has led to the widespread use of P(III) compounds for direct deoxygenation of epoxides, ozonides, carbonyl compounds, and both N- and 5-oxides.2... 

The deoxygenation of epoxides is not of great preparative value since it involves some loss of stereochemical integrity and the alkenes produced are more readily approached in other ways. Reductive cleavage of ozonides, for example, using triphenylphosphine, commonly forms part of the ozonolysis procedure for conversion of alkenes into carbonyl compounds. If a carbonyl compound is treated with an appropriate P(III) reagent then the reverse process may occur—reductive coupling to form a new C=C double bond. This has found a particularly important... 

The P(m) reagents have an even greater affinity for sulfur than they do for oxygen and there are a number of useful synthetic procedures based on desulfurization.9 In contrast to the deoxygenation of epoxides, the desulfurization of thiiranes is stereospecific and represents a potentially useful synthetic route to alkenes. Unfortunately, however, thiiranes were not easily obtained until the... 

Deoxygenation of epoxides. In the presence of rhodium(II) acetate, dimethyl diazomalonate converts epoxides into the corresponding alkcnes with formation of dimethyl oxomalonate. Alkcne isomerization or cyelopropanation is not observed. Yields arc generally >80%. 

Deoxygenation of epoxides. Trisubstituted steroidal epoxides undergo deoxygenation to the unsaturated steroids by reaction with PCQH,), and I, in CH,C1, (70-100% yield). 

The affinity of trivalent phosphorus for oxygen (and sulfur) has been put to use for the deoxygenation of epoxides and desulfurization of episulfides, which leads to alkenes. n vR... 

The system ReCp 03/PPh3 is not efficient in catalytic deoxygenation of epoxides but its analogue ReTpOs is and shows stereoselectivity. ... 

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