Haloalkenes dehydrohalogenation

Among the fluoride ion promoted reactions which occur in dipolar non-HBD solvents are alkylations of alcohols and ketones, esterifications, Michael additions, aldol and Knoevenagel condensations as well as eliminations for a review, see reference [600]. In particular, ionic fluorides are useful in the dehydrohalogenation of haloalkanes and haloalkenes to give alkenes and alkynes (order of reactivity R4N F > K ([18]crown-6) F > Cs F K F ). For example, tetra-n-butylammonium fluoride in AjA-dimethylformamide is an effective base for the dehydrohalogenation of 2-bromo-and 2-iodobutane under mild conditions [641] cf Eq. (5-123). 

Haloalkenes can be prepared by dehydrohalogenating saturated hydrogen-containing polyhalocarbons using liquid alkali metal acid fluoride and/or alkali metal fluoride compositions [75], HCFC-133a can be converted to CF2=CHC1 using these catalyst systems as shown in eq 21. 

Dehydrohatogenatioa. This base in benzene, toluene, or methylene chloride is a satisfactory reagent for dehydrohalogenation of haloalkenes to form alkynes. One advantage over KOH or NaOH is that an aprotic solvent can be used. Examples ... 

With a strong base such as sodium amide, both dehydrohalogenations occur readily. However, with weaker bases such as sodium hydroxide or potassium hydroxide in ethanol, it is often possible to stop the reaction after the first dehydrohalogenation and isolate the haloalkene. 

In dehydrohalogenation of a haloalkene with at least one hydrogen on each adjacent carbon, a side reaction occurs, namely the formation of an allene. 

Haloalkenes are versatile synthons for the synthesis of functionalized olefins and dienes, and are often prepared by the dehydrohalogenation of polyhalo-alkanes. The method often suffers from poor yields or low conversion, but Chin-Hsien has demonstrated that phase-transfer conditions allow the reaction to proceed cleanly, in high yield (79— 90%), ... 

Dehydrohalogenation of dihaloalkanes proceeds through the intermediacy of haloalkenes, also called alkenyl halides. Althongh mixtures of E- and Z-haloalkenes are in principle possible, with diastereomerically pnre vicinal dihaloalkanes only one product is formed because elimination proceeds stereospeciflcally anti (Section 11-6). 

We have encountered haloalkenes—alkenyl halides—as intermediates in both the preparation of alkynes by dehydrohalogenation and also the addition to alkynes of hydrogen halides. Alkenyl halides have become increasingly important as synthetic intermediates in recent years as a result of developments in organometalUc chemistry. These systems do not, however, follow the mechanisms familiar to us from our survey of the haloalkanes (Chapters 6 and 7). This section discusses their reactivity. 

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