Bases in elimination reactions

Alkali metal oiganometallics, when used as bases in elimination reactions, often afford many by-products however, with triphenylmethylpotassium good results were obtained. On the other hand, some weak bases, such as alkali halides, sometimes in the presence of the corresponding carbonates, in DMF or HMPA have been used successfully for eliminations. In some cases, dehydrohalogenation has... 

Alkyl halides react with nucleophiles in substitution reactions and with bases in elimination reactions. 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Tabushi reported that dichlorocarbene CC12 reacts with benzyl alcohol to form an O-H insertion product, i.e., benzyl dichloromethyl ether as the primary product, which undergoes a further base-catalyzed elimination reaction to give benzyl chloride as the final product (Scheme 3, Eq. I).14 In contrast to this... 

Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Kluger and Brandi (1986b) also studied the decarboxylation and base-catalysed elimination reactions of lactylthiamin, the adduct of pyruvate and thiamin (Scheme 2). These reactions are nonenzymic models for reactions of the intermediates formed during the reaction catalysed by the enzyme pyruvate decarboxylase. The secondary j3-deuterium KIE for the decarboxylation was found to be 1.09 at pH 3.8 in 0.5 mol dm-3 sodium acetate at 25°C. In the less polar medium, 38% ethanolic aqueous sodium acetate, chosen to mimic the nonpolar reactive site in the enzyme, the reaction is significantly faster but the KIE was, within experimental error, identical to the KIE found in water. This clearly demonstrates that the stabilization of the transition state by hyperconjugation is unaffected by the change in solvent. 

In the formation of the carbon-to-carbon double bond, the Br atom, together with an H atom on an adjacent carbon atom, has been removed or eliminated from the haloalkane and not replaced. This reaction is often referred to as a base-induced elimination reaction (see p. 62). 

This reaction is achieved by heating the monohaloalkane under reflux with ethanollc potassium (or sodium) hydroxide, i.e. potassium (or sodium) hydroxide dissolved in ethanol. The reaction is referred to as a base-induced elimination reaction and to demonstrate more clearly the role of the base in the reaction, it is worth considering the mechanism involved. This is illustrated for the reaction between 2-bromopropane and ethanolic potassium hydroxide ... 

The 3-0-benzyl derivative underwent a rapid reaction with TASF at reflux temperature to give methyl 3-0-benzyl-A,6-0-benzylidene-2-deoxy-2-fluoro-p- -glucopyranoside (J ) in A5% yield, and a minor product (19% yield) tentatively assigned the structure of methyl 3-0-benzvl-A.6-0-benzvlldene-2-deoxv-B-D-ervthro-hex-2-enopyranoslde. Base-catalyzed elimination reactions with trlflyl derivatives are uncommon (, ), but have been observed In certain furanoid (A0,A2) and, recently, in pyranoid (33,35) ring systems (see also Table I). Eliminations in glycopyranosldes occurred (33,35) under conditions which decreased the ease of nucleophilic substitution (33,A3,AA). 

Clerici et al. in their extensive work on cyclic sulfonamides have demonstrated that the compounds 69 and 70 undergo base-induced ring cleavage via elimination of diethylcyanamide and SO2 to afford 71 (Equation 14) <2001TL5455>. Various mechanisms are feasible, however, the necessity for base in the reaction medium implies deprotonation, either at nitrogen or carbon, followed by fragmentation as outlined below (Scheme 7). 

A base-catalyzed, elimination reaction was a key step in a synthesis of D-ribose from L-glutamic acid.188 In that work, L-glutamic acid was converted, by a series of reactions, into 5-0-benzyl-2,3-dideoxy-D-glycero-pentofuranose (157) from compound 157, a mixture of glycosides was obtained which, on treatment with bromine and calcium carbonate, gave the monobromo derivative 158 as a mixture of diastereoisomers. Base-catalyzed dehydrobromination of 158 afforded the unsaturated derivative 159. Hydroxylation of 159 with potassium permanganate or with osmium tetraoxide gave a mixture of methyl 5-0-benzyl-/3-D-ribofuranoside and methyl 5-O-benzyl-a-D-lyxofuranoside. 

Alkyl halides typically undergo elimination reactions when heated with strong bases, typically hydroxides and afkoxides, to generate alkenes. Removal of a proton and a halide ion is called dehydrohalogenation. Any base in the reaction mixture (H2O, HS04 ) can remove the proton in the elimination reaction. 

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