Alkene preparation

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

This reaction is known as the Hofmann elimination it was developed by August W. Hofmann in the middle of the nineteenth century and is both a synthetic method to prepare alkenes and an analytical tool for structure detennination. 

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. 

Ziegler-Natta catalyst (Section 31.2) A catalyst of an alkylaluminum and a litanium compound used for preparing alkene polymers. 

For a list of methods of preparing alkenes using boron reagents, with references, see Ref. 

The Hofmann elimination is useful synthetically for preparing alkenes since it gives the least substituted alkene. The reaction involves thermal elimination of a tertiary amine from a quaternary ammonium hydroxide these are often formed by alkylation of a primary amine with methyl iodide followed by reaction with silver oxide. The mechanism of the elimination is shown in Scheme 1.13 in this synthesis of 1-methyl-1-... 

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

M,For a list of methods of preparing alkenes using boron reagents, with references, see Ref. 106, pp. 218-222. MJZweifel Fisher Snow Whitney J Am. Chem. Soc. 1971, 93, 6309. 

Many of the synthetic routes parallel those used to prepare alkene complexes c platinum(II). Replacement of chloride ion in PtClJ- by a water soluble alkyne is a frequentl used method (equation 266), 809 812 or the reaction can be assisted by the use of a silver salt t facilitate halide displacement (equation 267).813 With hexafluorobutyne-2 the five-coordinat adduct can be isolated before it converts into the vinyl complex (89 equation 268).814 Alkyne displace alkenes from platinum(ll) complexes. 

Alkenes are relatively low oxidation level hydrocarbons. The most common way to prepare alkenes is to cany out the elimination of a small molecule from between vicinal carbon atoms. However, this is only a viable strategy if the regiochemistry of elimination can be controlled. That is, traditional dehydrohalo-genations or dehydrations often are regioselective but not regiospecific, so that mixtures of structurally isomeric olefins are formed. For example,... 

The E2 elimination occurs with a strong base (like a hydroxide or ethoxide ion) in a protic solvent (like ethanol or water). The E2 reaction is more common than the El elimination and more useful. All types of alkyl halide can undergo the E2 elimination and the method is useful for preparing alkenes. 

The fact that the Julia-Lythgoe olefination requires more than one step to prepare alkenes has generally been accepted as an inconvenient and inevitable part of the procedure developed by Marc Julia and Basil Lythgoe. This flaw kept nagging at Marc Julia s brother Sylvestre, who would not rest until he had found the one-step (Sylvestre) Julia olefination. The (Sylvestre) Julia-Kocienski olefination has become the state-of-the-art-variant of this olefination (Figure 11.23). It may be applied to any kind of aldehyde. 

Another problem that occurs with eliminations is the regiochemistry of the reaction. As we saw in Chapter 9, most eliminations follow Zaitsev s rule and produce the more highly substituted alkene as the major product. However, a significant amount of the less highly substituted product is also formed. In addition, mixtures of ds and trans isomers are produced when possible, further complicating the product mixture. Because separating a mixture of such isomers is usually a difficult task, elimination reactions are often not the best way to prepare alkenes. (Other methods will be described in subsequent chapters.) However, if only one product can be formed, or if one is expected to greatly predominate in the reaction mixture, then these elimination reactions can be quite useful. 

In Chapters 9 and 10 the use of elimination reactions to prepare alkenes was described. The major problem with that method is that a mixture of alkenes is often produced, resulting in lower yields and separation problems. The Wittig reaction provides an alter-... 

Fig. 11.28. Aldehyde alkyne chain elongation via [1 -rearrangement of a vinyl carbene (Seyferth procedure). First, a Horner-Wadsworth-Emmons olefination of the aldehyde is carried out to prepare alkene A. Upon warming to room temperature, alkene A decomposes and gives the vinyl carbene B. From that, the alkyne is formed by way of a [1,2]-rearrangement.

One method of preparing alkenes is a reductive elimination using sodium amalgam of a substituted 2-phenylsulfonylethanol (XX) obtained by addition of phenylalkylsulfone to a ketone using a basic organometallic catalyst [131]. 

We encountered dehydrohalogenation in Sec.. 5.12 as one of the best methods of preparing alkenes. A. that time we said that the mechanism involves a single step base pulls a hydrogen ion away from carbon, and simultaneously a halide ion separates-aided, of course, by solvation. 

Chapter 5 continues the chemistry of alcohols and alkyl halides by showing how they can be nsed to prepare alkenes by elimination reactions. Here, the students see a second example of the formation of carbocation intermediates from alcohols, but in this case, the carbocation travels a different pathway to a different destination. 

An important method for preparing alkenes involves the elimination (El or E2) of HX from alkyl halides (see Section 5.3.2). Alcohols can also be converted to alkenes by activating the OH group (e.g. by protonation or conversion to a tosylate) to make this into a better leaving group (see Section 5.2.2). 

---