Dehydrohalogenation alkyne synthesis

A Halogenation-Double Dehydrohalogenation Used in Alkyne Synthesis... 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert... 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

Halogenation-dehydrohalogenation Synthesis of an alkyne through an alkenyl halide intermediate. Hantzsch pyridine synthesis Condensation of two 3-dicarbonyl molecules, an aldehyde, and ammonia... 

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert trans-2,5-dimethyl-3-hexene to 2,5-dimethyl-3-hexyne by adding bromine across the double bond, then doing a double elimination. The infrared and mass spectra of the major product are shown here. 

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... 

Synthesis of Alkynes by Elimination Reaotions 314 [ A MECHANISM FOR THE REACTION ] Dehydrohalogenation of w c-Dibromides to Form Alkynes 315... 

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. 

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. 

Illustrate the use of halogenation-double dehydrohalogenation in the synthesis of the alkynes... 

Note that the synthesis of this terminal alkyne requires a total of rAree equivalents of base. The double dehydrohalogenation itself requires two equivalents. However, a third equivalent is required because, as the alkyne forms, its acidic hydrogen atom reacts with the amide ion to convert the alkyne to its conjugate base. Without the extra equivalent of amide ion, the reaction would not go to completion. The water in the workup step acts as an acid to protonate the alkynide and to convert any excess amide ion to ammonia. 

Although many transition metal complexes containing T) -bonded substituted acetylides are known, few are available in more than moderate yields via conventional reactions of metal halides with an anionic aUcynyl compound of an alkali metal, magnesium, or copper(I) or hy dehydrohalogenation in a reaction between the metal halide and a 1-alkyne. More recently, reactions between many 1-alkynes and RuCl(l Ph3)2(Tj -CsHs) have been shown to give cationic vinylidene complexes, which are readily deprotonated to give the corresponding substituted Tj -acetylides. The synthesis of the phenylethynyl derivative is typical the intermediate phenylvinylidene complex is not isolated. 

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