Alkene-forming eliminations

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

Excessive heating of the reaction mixture must be avoided otherwise an alkene-forming elimination reaction is induced this is particularly the case with secondary and especially tertiary alcohols. 

A. Thibblin, Mechanisms Of Sol volytic Alkene-Forming Elimination-Reactions, Chem. Soc. Rev. 1993, 22, 427. 

The two most common alkene-forming elimination reactions are dehy-drohalogenation—the loss of HX from an alkyl halide—and dehydration—the loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base, such as potassium hydroxide. For example, bromocyclohexane yields cyclohexene when treated with KOH in ethanol solution ... 

Mixture of nitric and sulfuric acid (mixed acid) used extensively in nitration of aromatic hydrocarbons, is generally unsuitable for nitration of alkanes, since primary nitroalkanes are rapidly hydrolyzed by hot sulfuric acid and secondary and tertiary nitroalkanes form tars (in all probability via rapid alkene-forming elimination and subsequent polycondensation, polymerization, etc.). It is, however, significant to point out that it is not necessarily the lack of reactivity of paraffins with mixed acid that makes the nitration of saturated hydrocarbons unsuitable, but that fast secondary reactions of any nitro products formed (as well as oxidative side reactions) can take place. This difficulty can be, at least in part, overcome by using preformed nitronium salts as nitrating agents. 

Reactions of Saturated Sulphoxides and Selenoxides.—The main reactions of saturated sulphoxides and selenoxides are associated with the nucleophilic properties of the sulphinyl and seleninyl groups, and their activating effect on the adjacent saturated carbon atoms. The Pummerer rearrangement (as well as rearrangements of unsaturated sulphoxides discussed in a later section) and alkene-forming eliminations are instances of the former property the main consequence of the latter property, the formation of sulphinyl-stabilized carbanions, is covered in Chapter 2. 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

In 1875 Alexander M Zaitsev of the University of Kazan (Russia) set forth a gen erahzation describing the regioselectivity of p eliminations Zaitsev s rule summarizes the results of numerous experiments m which alkene mixtures were produced by p elim matron In its original form Zaitsev s rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the f3 carbon hav mg the fewest hydrogens... 

When optically pure 2 3 dimethyl 2 pentanol was subjected to dehydration a mixture of two alkenes was obtained Hydrogenation of this alkene mixture gave 2 3 dimethylpentane which was 50% optically pure What were the two alkenes formed in the elimination reaction and what were the relative amounts of each" ... 

As would be expected in a substrate with a poor leaving group the predominant elimination process is of the Hofmann type. However, the authors did find some unexpected reactions. For example, the p-nitrophenyl sulphone was completely consumed within six hours of heating with the glycolate system, yet alkenes formed only a minor part of the products, the major part not being clearly identified. The p-dimethylaminophenyl sulphone was three to six times more reactive than the other sulphones, but it also underwent elimination at a significant rate with either solvent, even in the absence of alkoxides. The reasons for this are obscure. 

TABLE VI.2 The Effect of a and p Branching on the Rate of E2 Elimination and the Amount of Alkene Formed ... 

The unique feature of the Horner-Wittig reaction is that the addition intermediate can be isolated and purified, which provides a means for control of the reaction s stereochemistry. It is possible to separate the two diastereomeric adducts in order to prepare the pure alkenes. The elimination process is syn, so the stereochemistry of the alkene that is formed depends on the stereochemistry of the adduct. Usually the anti adduct is the major product, so it is the Z-alkene that is favored. The syn adduct is most easily obtained by reduction of (3-ketophosphine oxides.269... 

The alkenyl iodide or triflate 369 reacts in the absence of Cul with two moles of terminal alkyne 370 to form the substituted fulvene 371. The reaction can be explained by the intermolecular insertion of the alkyne twice, followed by the intramolecular insertion of the alkene, and / -elimination to form the fulvene 371(268]. 

Problem 7.53 Give structures of all alkenes formed and underline the major product expected from E2 elimination of (a) 1-chloropentane, (fc) 2-chloropentane. 

In another example710, the phosphonium salt 76 decomposes competitively by the two mechanisms of substitution and elimination (reaction 211). This salt behaves in a manner intermediate between salts 74 and 75 indeed, the SN(P) mechanism is not completely excluded as for the salt 74 but the EHfi mechanism is still favoured with regard to the salt 75 since the alkene formed is stabilized. 

Another example of oligomer preparation by C-C bond formation is outlined in Figure 16.30. In this synthesis, nitroalkyl phenyl selenides are converted into nitrile oxides in the presence of support-bound terminal alkenes, forming isoxazolines. Oxidative elimination of the selenide yields a new alkene, which can then be subjected to further 1,3-dipolar cycloaddition with a new nitrile oxide. Although this synthesis is short and easy to perform, the cycloadditions proceed with low diastereoselectivity... 

By Wittig and related reactions (3-Dimethylaminopropyl)-triphenylphosphorane, 119 Sodium amide, 278 Vinyl(triphenyl)phosphonium bromide, 343 (E)-Alkenes By elimination reactions Arylselenocarboxamides, 22 Dichlorobis(cyclopentadienyl)-titanium, 102 Hydrogen peroxide, 145 From three-membered heterocycles Tributyltinlithium-Trimethylalu-minum, 320 Trisubstituted alkenes Chloromethyldiphenylsilane, 74 Organocopper reagents, 207 Alkenes (Methods to form alkenes)... 

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