Dehydrohalogenation alkene synthesis

Elimination teactions are the most important means for synthesizing alkenes. In this chapter we shall study two methods for alkene synthesis based on elimination reactions dehydrohalogenation of alkyl halides and dehydration of alcohols. 

Synthesis of an alkene by dehydrohalogenation is almost always better achieved by an E2 reaction ... 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

There are four main general methods [40 2] for the preparation of perfluorinated alkenes, namely dehydrohalogenation, dehalogenation, pyrolysis and halogen exchange reactions of appropriate fluorinated precursors. The overall features of the mechanisms of each of these processes have already been discussed (Chapters 6 and 7, Sections 1 and 11). Representative examples of each of these types of synthesis are collated in Table 7.5 clearly the method of choice for the synthesis of a particular fluoroalkene will depend... 

Dehydrohalogenation and ether cleavage of tetrahydrofurfuryl chloride by sodium sand produces 4-penten-l-ol in 82% yield. Likewise, 4-octen-l-ol is obtained from 3-chloro-2- -propyltetrahydropyran. This synthesis is general for 4-alken-l-ols from the commercially available di-hydropyran (cf. method 21). ... 

The direct catalytic carbonylation of halides to aldehydes is not readily achieved. Aryl, heterocyclic and vinyl halides, for example, in the presence of [Pd(PPh3)2Ch], a stoichiometric quantity of tertiary amine and synthesis gas (CO/H2), are converted to aldehydes, but the conditions are somewhat drastic (80-100 bar, 80-150 Alkyl halides are even less suitable for this reaction as they tend to undergo dehydrohalogenation to form alkenes, rather than carbonylation. However, using the platinum catalyst [PtCh(PPh3)2], primary alkyl iodides can be successfully carbonylated to aldehydes in good yield under moderate conditions (equation 5). °... 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes. The principal methods by which alkenes are prepared in the laboratory are two other P eliminations the dehydration of alcohols and the dehydrohalogenation of alkyl halides. A discussion of these two methods makes up the remainder of this chapter. 

With a regioselectivity opposite to that of the Zaitsev rule, the Hofmann elimination is sometimes used in synthesis to prepare alkenes not accessible by dehydrohalogenation of alkyl halides. This application decreased in importance once the Wittig reaction (Section 17.12) became established as a synthetic method. Similarly, most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods. 

The radical addition of SF5CI (SFsBr) to double or triple bonds and subsequent dehydrohalogenation of corresponding adducts provides access to SFs-substituted alkenes or alkynes, which as shown below are valuable precursors for the synthesis of different types of SFs-substituted generally... 

In this chapter we described methods for the synthesis of alkenes using dehydrohalogenation, dehydration of alcohols, and reduction of alkynes. We also introduced the alkylation of alkynide anions as a method for forming new carbon-carbon bonds, and we introduced retrosynthetic analysis as a means of logically planning an organic synthesis. 

In the early 1950s, the synthesis of alkenes with complete control of the position of the carbon-carbon double bond was not possible. Mixtures of alkenes, sometimes having rearranged carbon frameworks, were often formed upon base-induced dehydrohalogenation... 

This reaction illustrates the double dehydrohalogenation of a vicinal dibromo compound to form an alkyne. It is a useful reaction for the synthesis of alkynes, because the starting dibromides are readily available from alkenes (see, e.g.. Experiment [A2b]). 

Synthesis of an Aikyne from an Aikene (Section 7.5B) Treating an alkene with Bij or CI2 gives a dihaloalkane. Treating the dihaloalkane with NaNHj or another strong base results in two successive dehydrohalogenations to give an alkyne. 

With a regioselectivity opposite to that of the Zaitsev rule, the Hofmann elimination is sometimes used in synthesis to prepare alkenes not accessible by dehydrohalogenation... 

The resurgent interest in the synthetic utility of 1-nitro-alkenes is evidenced by the variety of recent reports concerning their preparation. " Thus, the dehydration of 2-nitro-alcohols with sodium hydride or dicyclohexylcarbo-di-imide, the dehydrohalogenation of a-halogeno-oximes, and the nitro-selenylation of alkenes followed by oxidative deselenylation all provide 1-nitro-alkenes in moderate to excellent yield. In particular, these methods are all applicable to the synthesis of conjugated cyclic nitro-alkenes and complement those procedures previously reported for the preparation of both cyclic and acyclic nitro-alkenes (Vol. 3, p. 175 Vol. 4, p. 183 Vol. 5, p. 196 Vol. 6, p. 208). 

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