ALKENES deoxygenations

Appropriate reagents remove the heteroatoms to form alkenes (deoxygenation, desulfonation, deamination, dediazoniation). 

As another example of nitrene formation, the reaction of o-nitrostilbene (96) with CO in the presence of SnCU affords 2-phenylindole (97). The reaction is explained by nitrene formation by deoxygenation of the nitro group with CO, followed by the addition of the nitrene to alkene. Similarly, the 2//-indazole derivative 99 was prepared by reductive cyclization of the A-(2-nitrobenzyli-dene)amine 98[89]. 

Attack on oxiranes by trivalent phosphorus (64HC(19-l)43l) provides a method of deoxygenation to alkenes with inversion (c/. Section 5.05.3.4.3(hY)) and this makes possible the interconversion of (Z)- and (f)-alkenes (Scheme 58) (B-74MI50505). Silicon nucleophiles behave analogously (76JA1265, 76S199). 

Chidsey and coworkers made pioneering works in preparing covalently bonded monolayer films on silicon surfaces by the radical-initiated reaction of 1-alkenes with the H-Si(lll) surfaces. Reactions were carried out in neat deoxygenated alkenes using thermal decomposition of diacyl peroxides as the... 

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... 

Unsymmetrical alkenes can be prepared from a mixture of two ketones, if one is in excess. " The mechanism consists of initial coupling of two radical species to give a 1,2-dioxygen compound (a titanium pinacolate), which is then deoxygenated. " ... 

Another particularly reactive form of titanium is generated by including 0.25 equivalent of I2. This reagent permits low-temperature reductive deoxygenation to alkenes.241... 

As vicinal dibromides are usually made by bromination of alkenes, their utility for synthesis is limited, except for temporary masking of a double bond. Much more frequently it is desirable to convert a diol to an alkene, and several useful procedures have been developed. The reductive deoxygenation of diols via thiono carbonates was... 

The deoxygenation of cyclic ethers means the cleavage of both C-O bonds and leads to alkenes or alkanes depending on the hydrogenating activity of the catalyst (Scheme 4.2). 

Usually, various quantities of alkane are found during the hydrogenolyses of oxacycloalkanes. This is especially so for the oxiranes on platinum metals in the presence of hydrogen. These alkanes presumably form from alkenes, which are formed from deoxygenation (Scheme 4.67). 

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

Hydrogenolysis of tert-alcohols.1 2 3 This deoxygenation can be effected with Raney Ni slurry (Aldrich 50% slurry in water) that has been washed eight times with distilled water and twice with 1-propanol. Thus reaction of a tertiary alcohol with washed Raney Ni for a short time yields a mixture of alkenes that furnish a single alkane on hydrogenation catalyzed by Pd/C. 

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. - ... 

Divalent chromium reduces triple bonds to double bonds (trans where applicable) [195], enediones to diones [196], epoxides to alkenes [192] and aromatic nitroso, nitro and azoxy compounds to amines [190], deoxygenates amine oxides [191], and replaces halogens by hydrogen [197,198],... 

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,... 

Epoxides (oxiranes) can be reduced in different ways. So-called deoxygenation converts epoxides to alkenes. Such a reaction is very useful since it is the reversal of epoxidation of alkenes, and since both these reactions combined represent temporary protection of a double bond. 

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

Reduction of aromatic aldehydes to pinacols using sodium amalgam is quite rare. Equally rare is conversion of aromatic aldehydes to alkenes formed by deoxygenation and coupling and accomplished by treatment of the aldehyde with a reagent obtained by reduction of titanium trichloride with lithium in dimethoxyethane. Benzaldehyde thus afforded /ra/is-stilbene in 97% yield [206, 209]. 

An interesting deoxygenation of ketones takes place on treatment with low valence state titanium. Reagents prepared by treatment of titanium trichloride in tetrahydrofuran with lithium aluminum hydride [205], with potassium [206], with magnesium [207], or in dimethoxyethane with lithium [206] or zinc-copper couple [206,209] convert ketones to alkenes formed by coupling of the ketone carbon skeleton at the carbonyl carbon. Diisopropyl ketone thus gave tetraisopropylethylene (yield 37%) [206], and cyclic and aromatic ketones afforded much better yields of symmetrical or mixed coupled products [206,207,209]. The formation of the alkene may be preceded by pinacol coupling. In some cases a pinacol was actually isolated and reduced by low valence state titanium to the alkene [206] (p. 118). 

Titanium in a low valence state, as prepared by treatment of solutions of titanium trichloride with potassium [206] or magnesium [207] in tetrahydro-furan or with lithium in dimethoxyethane [206], deoxygenates ketones and effects coupling of two molecules at the carbonyl carbon to form alkenes, usually a mixture of both stereoisomers. If a mixture of acetone with other ketones is treated with titanium trichloride and lithium, the alkene formed by combination of acetone with the other ketone predominates over the symmetrical alkene produced from the other ketone [20(5] Procedure 39, p.215). 

Aliphatic alcohols are not reducible under electrochemical conditions. Conversion to a suitable anionic leaving group however does allow carbon-oxygen bond cleavage. Thus, methanesulphonates are reduced at a lead electrode under constent current conditions and this affords an overall tw o step process for the conversion of alcohols to alkanes [9].Deoxygenation of alcohols by this route has been applied successMly in the presence of other functional groups which are difficult to reduce such as alkene, epoxide, ester and nitrile. Cyclopropanes are formed in 50-97 %... 

Both unsymmetrical diols and alkenes can be prepared by applying these methods to mixtures of two different carbonyl compounds. An excess of one component can be used to achieve a high conversion of the more valuable reactant. A mixed reductive deoxygenation using TiCl4/Zn has been used to prepare 4-hydroxytamoxifen, the active antiestrogenic metabolite of tamoxifen. 

Radicals formed by fragmentation of xanthate and related thiono esters can also be trapped by reactive alkenes.217 The mechanism of radical generation from thiono esters was discussed in connection with the Barton deoxygenation method in Section 5.4. 

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