Vinyl halide dehydrohalogenation

The twofold dehydrohalogenation takes place through a vinylic halide intermediate, which suggests that vinylic halides themselves should give alkynes when treated with strong base. (Recall A vinylic substituent is one that is attached to a double-bond carbon.) This is indeed the case. For example ... 

AJkynes can be made by dehydrohalogenation of vinylic halides in a reaction that is essentially an E2 process. In studying the stereochemistry of this elimination, it was found that (Z)-2-chloro-2-butenedioic acid reacts 50 times as fast as the corresponding isomer. What conclusion can you draw about the stereochemistry of eliminations in vinylic halides How does this result compare with eliminations of alkyl halides ... 

Pyrolysis of bisquaternary ammonium hydroxides 7-12 Cleavage of selenoxides 7-13 Dehydrohalogenation of dihalides or vinylic halides... 

In contrast to the (E)-isomer, (Z)-alkenyl(phenyl)-A3-iodane 41 is labile and decomposes with a half-life time of 20 min to terminal alkynes in chloroform solution at room temperature [64]. Stereo electronically preferable reductive anti / -elimination accounts for this facile decomposition. In fact, the kinetic results for E2-type dehydrohalogenation of vinyl halides show that the relative rates of elimination decrease in the order anti /3->syn / - a-elimination [65]. Similar anti -elimination of vinyl-A3-iodane was proposed in the oxidation of methoxyallene with (diacetoxyiodo)benzene 4 to 3-acetoxy-3-methoxypropyne [66]. 

In some cases, we can generate a carbon-carbon triple bond by eliminating two molecules of HX from a dihalide. Dehydrohalogenation of a geminal or vicinal dihalide gives a vinyl halide. Under strongly basic conditions, a second dehydrohalogenation may occur to form an alkyne. 

We have already seen (Section 7-9A) many examples of dehydrohalogenation of alkyl halides. The second step is new, however, because it involves dehydrohalogenation of a vinyl halide to give an alkyne. This second dehydrohalogenation occurs only under extremely basic conditions—for example, fused (molten) KOH or alcoholic KOH in a sealed tube, usually heated to temperatures close to 200 °C. Sodium amide is also used for the double dehydrohalogenation. Since the amide ion ( NH2) is a much stronger base than hydroxide, the amide reaction takes place at a lower temperature. 

Vinylsilanes. Vinyl halides are converted into vinyltrimethylsilanes on reaction with Na (2.5 equiv.) and chlorotrimethylsilane in ether (25°, 2 hours). Yields are 65-85%. The configuration of the original bond is almost completely retained. cw-Vinyl halides can give rise to 1-trimethylsilylalkynes, formed by dehydrohalogenation followed by silylation, as by-products. This Wurtz-Fittig reaction is a convenient route to vinylsilanes when the vinyl halide is readily available. 

Hydrogen halide eliminations take place from 1,1- and 1,2-dihaloalkanes (equations 5 and 6), as w ell as from vinyl halides (equations 7 and 8). Dehydrohalogenation... 

The mechanism and stereochemsitry of dehydrohalogenations from vinyl halides have been extensively reviewed - It should however be pointed out that the effects of particular bases and solvents and of temperature cannot always be predicted with confidence as to rate of reaction and product distribution (acetylene, allene, diene). Therefore a variety of combinations of base, solvent and reaction conditions should be tried in order to obtain satisfactory results. 

Dehydrohalogenation by oxygen bases from vinyl halides proceeds readily by trans elimination via an 2-type of mechanism. c/s-Elimination is sluggish or does not occur at all. Thus trans elimination from bromovinyl ethers 2 (R = alkyl) furnishes the acetylenes 3 in a fast reaction in about 90% yield, whereas elimination from 4 (R = alkyl) is very sluggish and yields a mixture of the acetylene 3 and allene 5 (equations 17 and 18). Similar observations were shown in Schemes 1 and... 

The kinetics of dehydrohalogenation from the configurational isomers of vinyl halides have been determined for numerous reactions. Thus cw-/7-nitro-P-bromo-styrene in the presence of ethanolic NaOH is converted quantitatively by trans elimination to /7-nitrophenylacetylene within a few minutes (equation 21), whereas the trans isomer hardly reacts at all in that short time. However, the latter affords l,l-diethoxy-2-p-nitrophenylethane in high yield when kept under the above conditions for 20 days (equation 22). The mechanism of the latter reaction could not... 

The acetylide so formed can be directly alkylated to an internal acetylene without work-up. A further advantage of dehydrohalogenation of vinyl halides with NaNH in liquid ammonia or in a dipolar aprotic solvent (e.g. DMSO) is that both the cis and trans haloolehns furnish the acetylene, in contrast to reaction with oxygen bases (see Section II.A.2, equation 19). Bromoolefins 9 and cis-10 are converted to... 

The unstable acetylene 11 is obtained by dehydrohalogenation of the appropriate vinyl halide with BuLi (equation 62) . A series of mono- and dihaloacetylenes are similarly obtained in low to moderate yields on using PhLi in ether at 0 °C (e.g. 

DBU and its salts have been patented and used as dehydrohalogenation agents for fluoropolymers (83JAP(K)219202), fluororubbers (78MI3), and poly(vinyl halide) in the preparation of polarizing films (83JAP(K)21929), and as dissociation catalysts for blocked isocyanates (83JAP(K)65764). 

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). ... 

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). °... 

Vinyl halides are very unreactive so that under mild conditions the dehydrohalogenation stops here. Only under more vigorous conditions - use of a strong base (NaNH2) is the alkyne generated. 

This alkyne can be produced from the corresponding alkyl dihalide by dehydrogalogenation. The dehydrohalogen-ation is carried out in two steps with KOH (ale) that produces a vinyl halide and NaNH2, which is a stronger base, that gives the alkyne. To obtain the dihalide. 

The dehydrohalogenation of vinyl halides in the s5mthesis of alk3mes shows steric preferences analogous to those of alkyl halides. The reaction of (Z)-)3-bromostyrene with hydroxide ion in isopropyl alcohol at 43°C was 2.1 X 10 faster than the reaction of the ( ) isomer. The results were interpreted in terms of differing mechanisms for the eliminations of the two compounds. As shown in equation 10.28, the (Z) isomer can undergo concerted elimination of hydrogen and bromine because they are in the proper orientation for anti-coplanar elimination. 

The double dehydrohalogenation reaction is usually run in the presence of a strong base, such as KOH or NaNH2, and proceeds in two stages. In the first, an intermediate bromoalkene is formed, which can be isolated under more mildly basic conditions. In fact, this reaction is a valuable route to vinyl halides. The mechanism of elimination involves the abstraction of the proton on the carbon atom p to the halogen. The E2 mechanism, which operates under these strongjy basic conditions, is fastest when it involves removal of a proton, antiperipla-... 

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