Base-induced elimination

The reaction of 3-methoxy-1,2,4-triazine 1-oxide 20 with the carbanion generated from chloromethyl phenyl sulfone proceeds as the vicarious nucleophilic substitution (VNS) of hydrogen (Scheme 1, path B) via addition of the carbanion at position 5 of the heterocycle. Following base-induced elimination of HCl and protonation, 3-methoxy-5-phenylsulfonyl-1,2,4-triazine 4-oxides 65 result (88LA627). 

Dichlorocarbene can be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction, and use curved arrows to indicate the movement of electrons in each step. What relationship does your mechanism bear to the base-induced elimination of HC1 from chloroform ... 

Nucleophilic substitution and base-induced elimination are two of the most widely occurring and versatile reaction types in organic chemistry, both in the laboratory and in biological pathways. We ll look at them closely in this chapter to see how they occur, what their characteristics are, and how they can be used. 

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. 

A second piece of evidence in support of the E2 mechanism is provided by a phenomenon known as the deuterium isotope effect. For reasons that we won t go into, a carbon-hydrogen bond is weaker by about 5 kj/mol (1.2 kcal/mol) than the corresponding carbon-rfaiiferiwm bond. Thus, a C-H bond is more easily broken than an equivalent C-D bond, and the rate of C-H bond cleavage is faster. For instance, the base-induced elimination of HBv from l-bromo-2-phenylethane proceeds 7.11 times as fast as the corresponding... 

Assume that you are carrying out the base-induced dehydrobromination of 3-bromo-3-methylpentane (Section 11.7) to yield an alkene. How could you use 1R spectroscopy to tell which of two possible elimination products is formed ... 

Conjugated dienes can be prepared by some of the methods previously discussed for preparing alkenes (Sections 11.7-11.10). The base-induced elimination of HX from an allylic halide is one such reaction. 

Hell-VoLhard-Zelinskii (HVZ) reaction, in which an acid is treated with Bc2 and PBr3. The a-halogenated products can then undergo base-induced E2 elimination to yield a,j6-unsaturated carbonyl compounds. 

Curran s synthesis of ( )-A9(l2)-capnellene [( )-2] is detailed in Schemes 30 and 31. This synthesis commences with the preparation of racemic bicyclic vinyl lactone 147 from ( )-norbomenone [( )-145] by a well-known route.61 Thus, Baeyer-Villiger oxidation of (+)-145 provides unsaturated bicyclic lactone 146, a compound that can be converted to the isomeric fused bicyclic lactone 147 by acid-catalyzed rearrangement. Reaction of 147 with methylmagne-sium bromide/CuBr SMe2 in THF at -20 °C takes the desired course and affords unsaturated carboxylic acid 148 in nearly quantitative yield. Iodolactonization of 148 to 149, followed by base-induced elimination, then provides the methyl-substituted bicyclic vinyl lactone 150 as a single regioisomer in 66% overall yield from 147. 

Note. With non-cyclic /i-silylketones, /3-bromoketones are obtained these undergo facile base-induced elimination to the corresponding enones. 

Base-induced elimination (2). KH (5 g, 40% dispersion in oil, 60 mmol) was washed with hexane. To a stirred slurry of the residue in THF (50ml) was added the above alcohol (50 mmol) within a few minutes, hydrogen evolution was complete. The mixture was stirred for 6h at ambient temperature, and then carefully poured on to ice-cold saturated ammonium... 

Base-induced elimination. KH (0.1 g, 50% slurry in oil, 1.25mmol) was washed with pentane (4 ml). To the residue was added THF (5 ml) and the alcohol (0.378mmol), and the mixture was stirred for lh at ambient temperature. It was then partitioned between ether and saturated ammonium chloride solution, and the ethereal layer was separated, dried and concentrated to give the alkenes as a 95 5 mixture of ( ) (Z) isomers, in 96% yield (g.l.c.). 

The elimination of sulfur dioxide from thiirane dioxides leading to the corresponding alkenes is not the only result of base-induced reactions other products are also formed. This fact raises the question of the mechanistic pathway of this reaction. In general, the thiirane dioxide is treated with a large excess of the base in an appropriate solvent for several hours at room temperature or below. Bases commonly used are 2n NaOH (in water), NaOCH3 (in methanol), t-BuO-K + (in f-BuOH) and BuLi (in tetrahydrofuran) or KOH-CCU (in t-BuOH)16-19"112 113. 

Base-induced eliminative ring fission, in which both the double bond and the sulfone function take part, has been observed in thiete dioxides253. The reaction can be rationalized in terms of initial Michael-type addition to the double bond of the ring vinyl sulfone, followed by a reverse aldol condensation with ring opening. The isolation of the ether 270c in the treatment of 6c with potassium ethoxide (since the transformation 267 -> 268 is not possible in this case) is in agreement with the reaction mechanism outlined in equation 101253. 

The procedure described here 1 is a convenient two-step reaction that relics on the base-induced elimination-isomerization reactions of gem-dichlorooyclopropanes.8-15 The reaction mechanism has been studied.16 The principal advantage of this method is the ready availability of necessary reagents. 

Base-induced elimination of sulphinate from homoallylic sulphones , from y-ketosulphones , and from 1,2-bissulphones has been used in synthetic sequences ranging from the preparation of retinoic acid and of its methyl ester , to a novel pentannulation sequence that leads to a range of cross-conjugated dienes , as exemplified by equation (69). The overall yield for the two steps was 63%. 

The stereoselectivity of these reactions arises from a base-induced anti elimination after the migration. The elimination is induced by addition of methoxide to the boron, generating an anionic center. 

In 1,2-eliminations involving carbon atoms (i.e. most), the atom from which Y is lost is usually designated as the l-(a-) carbon and that losing (usually) H as the 2-(/ -) carbon in the older a/J-terminology, the a- is commonly omitted, and the reactions are referred to as p-eliminations. Among the most familiar examples are base-induced elimination of hydrogen halide from alkyl halides—this almost certainly the most common elimination of all—particularly from bromides (1) ... 

Other examples of the ElcB pathway are benzyne formation from C6HsF (cf. p. 174), reversal of simple nucleophilic addition to 0=0, e.g. base-induced elimination of HCN from cyanohydrins (20 cf. p. 212),... 

By far the commonest elimination mechanism is the one-step concerted (E2) pathway exhibiting, e.g. for the base-induced elimination of HBr from the halide RCH2CH2Br (21), the rate law ... 

Finally, there are base-induced rearrangements involving carban-ions that proceed via 1,3-elimination to form cyclopropanone inter-... 

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