Allylic halides, dehydrohalogenation

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. 

The base-promoted dehydrohalogenation of vinyl or allylic halides affords poor yields of allenes because of the competing side reaction leading to acetylenes [21,22], The presence of tertiary hydrogens a to the halogen also helps to... 

The general reaction for the dehydrohalogenation of vinylic and allylic halides is shown in Scheme 65. 

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 majority of grafting from syntheses have been accomplished by converting polymeric halides into macro-initiators. Allyl halides provide suitably stable carbocation derivatives and consequently the partial dehydrohalogenation of poly(vinyl chloride) has been found to enhance the rate of styrene grafting in the presence of aluminium chloride. Similar sites available in chloroprene were activated by silver hexafluorophosphate to induce grafting of IBVE. Nitrosyl and... 

Dehydrohalogenation cf Vinylic or Allylic Halides Dehalogenation of23-Dihalopropenes Dehydration ef Allylic Alcohols Dehalogenation of gem-Dibromocyclopropanes... 

Under phase transfer conditions (boiling benzene, 50% aquous sodium hydroxide solution and tetia-n-butylammonium hydrogenphosphate) benzamides are monoalkylated. In the solid-liquid PTC system potassium hydride, tetrahydrofuran and 18-crown-6, acidic trifluoroacetamides are rapidly deprotonated and alkylated with lower kyl, allyl and benzyl halogenides in high yield. Methylation of trifluoroacetamides is carried out under less basic conditions in KOH/acetone. A similar reaction in KOH/DMSO generally allows the efficient reaction of benzamide and acetanilide with primary alkyl halides with secondary halides only moderate yields are accomplished, whereas tertiary halogenides are not alkylated due to dehydrohalogenation. ... 

Phenylacetonitrile reacts with alkyl, allyl, and benzyl halides as one would expect (see Eq. 10.1). In the presence of excess alkylating agent and base, dialkylation is usually observed. The yield of alkylation product is usually high with normal alkyl halides (Cl, Br, I) and somewhat lower for secondary halides. There is no report of the successful alkylation of phenylacetonitrile by a tertiary alkyl halide under phase transfer conditions. This is undoubtedly due to the facile dehydrohalogenation reaction of such haloalkanes. It is probably the elimination problem which keeps 2-phenylethyl chloride from alkylating phenylacetonitrile in high yield [23], despite the fact that it is a primary halide. It is not clear why 1-phenylethyl chloride should alkylate the same substrate almost quantitatively under the same conditions [23], although the fact that the latter is a benzyl halide no doubt is part of the reason. 

In addition to the cross-coupling reactions of alkyl halides or sulfonates with organometallic nucleophiles, the former can also directly undergo C-C bond formation with nonactivated C-H bonds of alkenes or arenes in a Heck-type reaction. Iron-catalyzed intramolecular dehydrohalogenation of 2-iodoethanal alkenyl acetals with phenyl-magnesium bromide in the presence of a catalytic amount of iron(II) chloride provides tetrahydrofliran derivatives in moderate yields (Scheme 4-240). Allyl 2-halophenyl ethers provide dihydrobenzofiiran derivatives under these conditions. ... 

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