Cyclic alkenes => allylic halides

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. 

Blaser and Spencer used aroyl halides in place of aryl halides, with aroyl chlorides being of specific interest as ubiquitous, relatively cheap compounds ( Blaser reaction ) [24], This latter reaction is normally conducted in aromatic solvents phosphines are not used here as catalyst ligands since they fully inhibit the reaction. In the same way, benzoic acid anhydrides can be used as the aryl source in combination with PdCl2 and catalytic amounts of NaBr [79]. In this reaction, one of the arenes is used in the coupling reaction by elimination of CO, whereas the other benzoate serves as the base. The benzoic acid thus formed can easily be recycled into the anhydride. The use of aryl and vinyl triflates according to Cacchi [25] and Stille [26] extends the scope of the Heck coupling to carbonyl compounds phenol derivatives act via triflate functionalization as synthetic equivalents of the aryl halides. The arylation of cyclic alkenes [27], electron-rich vinyl ethers [28], and allylic alcohols [29] is accessible through Heck reactions. Allylic alcohols yield C-C-saturated carbonyl compounds (aldehydes) for mechanistic reasons (y9-H elimination), as exemplified in eq. (6). 

It is often difficult to make a comparison between the various results obtained for the same polyenes as different reaction conditions (ratio of reactants, temperature, time) were used in each case. The addition of dichlorocarbene (chloroform/base/phase-transfer catalysis) to straight chain and cyclic unconjugated di- and trienes, carried out under identical conditions but varying the catalysts, showed the peculiar properties of tetramethylammonium chloride. Under precisely tailored conditions, either highly selective mono- or polyaddition of dichlorocarbene to the polyenes is possible tetramethylammonium chloride was the most efficient catalyst for monocyclopropanation. (For the unusual properties of tetramethylammonium salts on the phase-transfer catalyzed reaction of chloroform with electrophilic alkenes see Section 1.2.1.4.2.1.8.2. and likewise for the reaction of bromoform with allylic halides, see Section 1.2.1.4.3.1.5.1.). For example, cyclopropanation of 2 with various phase-transfer catalysts to give mixtures of 3, 4, and 5, ° of 6 to give 7 and 8, ° and of 9 to give 10 and 11. °... 

Cross-couplings. The enynes are formed from allyl halides and alkynylstannanes. The Ni(acac)2-catalyzed cross-coupling of alkyl iodides and diaUcylzincs is particularly useful because many functionalities are tolerated. An intramolecular addition to an alkene can intervene in this coupling thereby creating functionalized cyclic compounds. ... 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or alkenes (by hydride extraction) and SbFs in SO2 or S02C1F. Divinylmethyl... 

Benzylic and allylic positions are hydroxylated by CPO in halide-dependent catalytic transformations. Toluene and p-xylene are oxidized to the respective aldehydes and carboxylic acids [247, 248]. Ethylbenzene and other substrates with longer alkyl chains form the respective benzylic/allylic alcohols with high enantio-selectivity. Straight-chain aliphatic and cyclic (Z)-alkenes are hydroxylated, favoring small unsubstituted substrates in which the double bond is not more than two carbon atoms from the terminus. Steric control is observed for benzylic hydroxylations. 

When the alkene is cyclic, or the insertion step forms a quaternary center, a substitution product is not obtained. For example, stereospecific syn addition of an arylpalladium halide 13 to cyclohexene generates cyclohexylpalladium(II) intermediate 14 (Scheme 6-3). The C—Pd cr-bond in this intermediate is anti to H and syn elimination to form a substitution product is not possible. However, elimination of cis hydrogen H is possible and generates allylic product 15. This pathway of the Heck reaction is particularly important in complex molecule construction since a new stereogenic center is produced. 

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