Dehydrohalogenation dienes

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Conjugated-Diene Butyl. CDB can be obtained by the controlled dehydrohalogenation of halogenated butyl mbbet (27). This product concept remains in the development stage. 

Both the above potentiating modifications were next included in a single molecule. Catalytic reduction of 173 affords the corresponding 5a-derivative. This is then taken on to the 2,4-dibromo compound (see the transformation of 12 to 21 for discussion) dehydrohalogenation gives the 1,4 diene, 9a-fluoropred nisolone acetate (176), ) a potent antiinflammatory agent. 

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

Dibromo-7-oxabicyclo[4.1.0]heptanes can be obtained by bromination of the respective 7-oxabicyclo[4.1.0]hept-3-ene, the monoepoxide of cyclohcxa-1,4-diene, and when submitted directly to the dehydrohalogenation reaction give products 3 and 4.2 149 150... 

A rather broadly applicable method leading to products which can be further modified starts from 3,8-diaryl-l,2-diazacycloocta-2,8-dienes 4.23,24 With /V-bromosuccinimide dihalogenation occurs which, after a double dehydrohalogenation, gives 3,8-diphenyl-1,2-di-azocine (5 Ar = Ph, X = H) in low yield. The reaction with sulfuryl chloride to give tetra-... 

Sulfonyl imides (78) are, like sulfenes, prepared by dehydrohalogenation of the corresponding sulfonyl chlorides (79) (usually called sulfamoyl chlorides). Like sulfenes, they take part in [2 + 2] and [4 + 2] cycloaddition reactions with electron-rich alkenes or with 1,3-dienes, yielding 1,2-thia-zetidine 1,1-dioxides (80)104 or dihydro-1,2-thiazines (81),105 respectively. 

Attempts were reported119,129 to synthesize the ortho-linked diphenyl cyclo-propenone phenanthreno cyclopropenone 173 by dehydrohalogenation of the dibromo derivative 174 of dibenzo cyclohepta-l,3-diene-6-one. The only product isolated was the anhydride 175 of phenanthrene-9-carboxilic acid, which was shown not to arise from 173l29 ... 

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. 

Fluoro-l-haloalka-l,3-dienes are prepared in a similar manner by treatment of substituted 2,2-(dihalocyclopropyl)methanols with a hydrogen fluoride/pyridine/diisopropylamine/ sodium fluoride system to give compound 2, followed by dehydrohalogenation with potassium /< r/-butoxide.80... 

Preparation of cathylates, 414 Preparation of cholesta-3,5-diene, 331 Preparation of 5 -cholest-2-ene, 343 Preparation of 5a-cholest-3-ene, 347 Preparation of 5a-cholest-6-ene, 354 Preparation of cyanohydrins, 412 Preparation and dehydrohalogenation of a-halo ketals, 301... 

Dehydrohalogenation of 2//,2//-3-chlorooctafiuoro-l-iodopentane (14) using a sodium hy-droxide/potassium hydroxide mixture gives perfluoropenta-1,2-diene (15).50... 

Indirect methods, which are primarily essentially of academic interest, nevertheless may find use under special circumstances. One of these, involving the brominative decarboxylation (Borodin-Huns-diecker reaction) of silver a-fluorocyclopropanecarboxylate, provides a convenient and stereospecific route to the synthesis of fluorobromocyclopropyl compounds (equation 26).96 Another approach makes use of the Diels-Alder reaction between tetrachlorocyclopropene and dienes (equation 27).97,98 The adducts can undergo facile dehydrohalogenation or ring opening. 

To aiq>ly (478) as a dienophile in Diels-Alder reactions it is, therefore, prepared in situ in the presence of a suitable diene, while maintaining its stationary concentration as low as possible. The most popular methods of preparation (Scheme 112) include dehydrohalogenations of halobenzenes, e.g. (479), me-talladon of o-dihalobenzenes, e.g. (4M)," as well as thermal decompositions of benzenediazonium-2-... 

The preparation of this class of dienes has received considerable attention during the last few years. Scheme 127 depicts the current approaches, such as 1,4-eliminations of o-xylene derivatives via reduction (544) - (M2), thermal dehydrohalogenation ( 5) (542), and fluoride-induced desilylation... 

If the cyclobutene-vinylketene equilibrium is established in the presence of a 1,3-diene such as cy-clopentadiene, cyclohexadiene or ( )-l,3-pentadiene, the ketene is trapped via a [4 + 2] cycloaddition reaction to furnish a 2,3-divinylcyclobutanone (127). When such reactions are carried out above 120 C a further 3,3-sigmatropic rearrangement takes place to yield cyclooctadienones (128) in overall yields ranging from 30-90%. The intermediate divinylcyclobutanes can also be obtain by dehydrohalogenation of crotyl chloride in the presence of the dienes. ... 

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. 

The approach to the diene system of silicon is related to the halogena-tion-dehydrohalogenation reactions discussed previously. The presence of two benzylic positions introduces additional complications. 

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