A-Elimination reactions

This mechanism of a -elimination reaction is supported by experimental findings with " S- and C-labeled starting materials." The Chugaev reaction is analogous to the ester pyrolysis, but allows for milder reaction conditions—i.e. it occurs at lower temperatures. It is less prone to side reactions, e.g. the formation of rearranged products, and is therefore the preferred method. 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

Both phase transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.153 The carbene can be trapped by alkenes to form dichlorocyclopropanes. 

One of the most interesting aspects of the mechanism shown in equation 10 is the last step, an a-elimination reaction to give the new alkylldene hydride complex. Our results do not imply that p-elimination to give an olefin hydride 1ntermediate is relatively slow. It is possible that although K2 > Kj, ki > k2 (equation 11), i.e., p-elimination is still faster. If this is true, it must also... 

Hydroxylaminolysis, treatment with stronger alkali (0.5 m NaOH, 2 h, 100°C) and alkaline methanolysis (0.25 m NaOMe, 1 h, 50°C) lead to complete O-deacylation of LPS and lipid A (176). Particularly in the case of alkaline methanolysis, ester-linked 3-acyloxyacyl residues undergo, in addition to transmethylation, a -elimination reaction, whereby the (R)-3-hy-droxy fatty acid ester is first transformed into the a,/ -unsaturated and then into the (S.-R -methoxy fatty acid methyl ester. The acyl substituent, on the other hand, is eliminated in the form of the free fatty acid (176). In fact, the presence of a 3-methoxyacyl derivative in the fatty acid spectrum of a given LPS is a strong indication for the presence of an ester-bound 3-acyloxyacyl... 

The w-chloroalkyldiphenylphosphines (11) have been prepared by the reaction of equimolar quantities of sodium diphenylphosphide with aco-dichloroalkanes. Whereas the phosphine (11 n = 3) can be converted into the Grignard reagent (12), which reacts with dimethylchlorophosphine to form the unsymmetrical diphosphine (13), the Grignard reagent (14) undergoes a -elimination reaction to regenerate diphenylphosphide ion.13... 

Involvement of a-elimination reactions for in situ prepared catalysts from WC16 and Me4Sn was demonstrated by the use of 13C in tetramethyltin. The norbomene polymers formed contained the 13CH2 alkene moiety as the end-group. Also unstable C14W=CH2 and Cl4W=13CH2 species were observed by H NMR spectroscopy [14],... 

The mechanism of this remarkable a-elimination reaction has been scrutinized by several research groups [17,49,51,396-404]. From the experimental data obtained this process is best described as an intramolecular deprotonation of one neopentyl ligand by another, the latter being released as neopentane (Figure 3.4). 

Figure 14 (a) Elimination reaction catalyzed by antibodies 43D4 and 3D12 raised against the quaternary ammonium 17 and (b) Kemp reaction catalyzed by antibody 34E4 raised against the amidinium 18. 

The described oxidative a-elimination reactions have been rationalized as proceeding... 

As would be expected from their electron-deficient nature, carbenes are highly reactive. Carbenes can be generated by a-elimination reactions. 

The proposed mechanism of this reaction is composed by an initial S v2-type nucleophilic substitution reaction of 113 with the nucleophilic a-sulfonyl lithium carbanion to give the alkylmagnesium species (114) having a sulfonyl group at the / -position. Then, a -elimination reaction of magnesium sulfinate from the intermediate (114) occurs... 

A. Elimination Reactions Involving the Alkoxy Substituents of Ortho Esters... 

On the basis of the close correlation between the number of hydroxy-amino acids in bovine submaxillary mucin and the number of hexosamine and sialic acid residues, Hashimoto and Pigman (34) proposed that an O-glycosidic linkage was the only one that could be present in large amounts. Anderson et al. (38) reported a disappearance of much of the serine after incubation of the proteoglycan of chondroitin sulfate with alkali and suggested that this loss resulted from a -elimination reaction. 

The most common routes to carbenes27 are also the major ones to generate silylcar-benes, namely dediazoniation of aliphatic diazo compounds and a-elimination reactions (Scheme 1). The extrusion of N2 from silyl-substituted diazo compounds can be achieved by UV-irradiation or thermally. The thermal decomposition, however, is of less importance since these diazo compounds are thermally much more stable than their nonsilylated counterparts, so that thermal impact may stimulate noncarbene pathways. 

Suitable candidates for a-elimination reactions are silylmethyl halides (— base-induced elimination of H-Hal), silylmethyl dihalides (— halide/metal exchange followed by elimination of a metal halide) and stable carbenoid-type compounds such as (a-halo-a-silylalkyl)mercury compounds (— thermal elimination of mercury(II) halide). Bis(phenylthio)(trimethylsilyl)methyl lithium (— elimination of LiSPh) represents a borderline case (see Section III.E.8). 

Diisopropylethylamine is a useful base for this purpose, especially when further transformations of the onium salts are possible. a-Elimination reactions of this type have recently been employed for the synthesis of alkenyl(diaryl)-and alkenyl(triaryl)onium salts of Group 15 and Group 16 elements (Scheme 46)... 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

The reaction illustrated in equation 24 involves a series of 1,3-alkyl, silyl and germyl migrations between the metal-silylene and germylene transients formed subsequent to an a-elimination reaction initiated by the elimination of CO109. 

Okuno, T., Kubota, T., Kuroda, T., Ueno, H., and Nakamuro, K. 2001. Contribution of an enzymatic a, -elimination reaction in detoxification pathway of selenomethionine in mouse liver. Toxicol. 

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