Halides, alkyl, base induced elimination

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) ... 

Another method of seeking evidence of the EIcBirr mechanism is to exam heavy-atom isotope effects in the leaving group. Of course, these should be much more significant in an E2 process because the bond is breaking in the transition state. For example, Thibblin and co-workersfound that in the base-induced elimination of an alkyl halide in which the p-carbon is unusually acidic (indene derivative, 12), moderately strong bases (triethylamine and methoxide) lead to a significant Cl/ Cl isotope effect = 1.010 1.009, where a maximum effect of... 

The synthesis of stereodefined acyclic alkenes via 3-elimination reactions—such as (1) dehydration of alcohols, (2) base-induced eliminations of alkyl halides or sulfonates (tosyl or mesyl esters), and (3) Hofmann eliminations of quaternary ammonium salts—often suffers from a lack of regio- and stereoselectivity, producing mixtures of isomeric alkenes. 

In a so-called vicarious nucleophilic substitution of hydrogen,75 2,3-diphenylpyrido[2,3-6]-pyrazine is alkylated in the 8-position by [(chloromethyl)sulfonyl]benzene. This reaction proceeds by addition of the carbanion to the electron-deficient ring position of a nitroarene or electrophilic heteroaromatic system, followed by base-induced -elimination of the corresponding hydrogen halide.76,77 As with quinoxalines and naphthyridines, the reaction with pyrido[2,3-6]pyrazines also affords products bisannulated at the pyrazine or the pyridine moiety, depending on the kind of 2/3-substitution (cf. Section 7.2.3.1.2.2.2.). 

One of the most commoidy used methods for forming carbon-carbon double bonds is by -elimination reactions of the types shown in Scheme 2.1, where X = e.g. OH, OCOR, halogen, OSO2R, NRa, etc. Included among these reactions are acid-catalysed dehydrations of alcohols, solvolytic and base-induced eliminations from alkyl halides or sulfonates and the Hofmann elimination from quaternary ammonium salts. They proceed by both E2 (elimination bimolecular) and E1 (elimination... 

The Julia olefination allows the generation of a double bond by alkylation of a suUbne, followed by a base-induced elimination of ben-zenesulfinic add. The water-soluble sulfinate can be separated off, and recovered as the free sulfinic add. The sulfone is then obtained by reaction ofthe sulfinic acid with the appropriate alkyl halide. [73]... 

Geometrical Considerations in the E2 Elimination Reaction. Base-induced elimination of alkyl halides (dehydrohalogenation) is a general reaction and is an excellent method for preparing alkenes.This process is often referred to as -elimination, since a hydrogen atom is always removed p to the halide (leaving group) ... 

There are also a series of more complex eliminations that you should be aware of (Eqs. 10.63-10.67), although we are not going to look at these in any detail. One is the elimination of 1,2-dihaloalkanes and 1,4-dihaloalkanes (the Grob fragmentation) using Zn to create al-kenes or dienes (Eqs. 10.63 and 10.64, respectively). The first step in both reactions involves the oxidative addition of Zn to a C-X bond, a reaction we will cover in detail in Chapter 12. Other eliminations involve y-amino alkyl halides, which can spontaneously undergo elimination (Eq. 10.65), and the base-induced eliminations of both (3-hydroxyketones (Eq. 10.66, the reverse aldol reaction) and 8-ketoketones (Eq. 10.67, the reverse Michael addition). 

The reaction of an alkyl halide or tosylate with a nucleophile/base results either in substitution or in elimination. The resultant nucleophilic substitution and base-induced elimination reactions are two of the most widely occurring and versatile reaction types in organic chemistry, both in the laboratory and in biological pathways. 

Here too, a second alkylation can be made to take place yielding RC=CR or R C=CR. It should, however, be remembered that the above carbanions—particularly the acetylide anion (57)—are the anions of very weak acids, and are thus themselves strong bases, as well as powerful nucleophiles. They can thus induce elimination (p. 260) as well as displacement, and reaction with tertiary halides is often found to result in alkene formation to the exclusion of alkylation. 

Because of the harsh conditions and high basicity of fluoride anions, these reactions are often accompanied by base-induced y3-elimination and decomposition of the TAA catalyst via the Hoffmann-type degradation. Particularly, secondary alkyl halides, cyclohexyl chloride, and 2-octyl bromide, in the reaction with KF under these conditions undergo -elimination. 

For efficient action of Y as cocatalysts, it is necessary that in reactions with alkyl halides they behave as bases abstracting proton not as nucleophiles entering undesired nucleophilic substitution. From a variety of possible Y-H, only OH acids—alcohols and phenols, and to smaller extent some NH acids—are shown to be proper cocatalysts. In a model studies of base-induced PTC j8-elimination of HBr from cyclohexyl bromide, it was shown that benzyl and benzhydryl alcohols, 2,2,2-trifluoroethanol, trifluoromethylphenyl carbinol, and mesitol are particularly efficient cocatalysts (71) as shown in equation 159. 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

Rappoport has presented a detailed outline of the mechanisms of the reactions of vinyl halides with nucleophilic reagents. Modena et /. " have provided further evidence in support of a spectrum of transition states for elimination from activated vinyl halides induced by alkoxide bases. Cristol and Whittemore have shown that the stereoselectivity of elimination from vinyl halides is largely determined by the choice of basic reagent alkoxide bases encourage am/-elimination, whereas syn-elimination and alpha-elimination become dominant with lithium alkyls. 

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