Halogenation cyclohexene

Reagent combinations lor additions of the halogen fluorides to cyclohexene to form trans 1 halo 2 fluorocyclohcxane (equation 1) are shown in Table 1... 

Bromoamidation of cyclic olefins allowed the synthesis of bicyclic oxazolines. For instance, treatment of cyclohexene with A-bromoacctamidc as the halogen source and different nitriles at 0 °C, in the presence of SnCU or BF3 Et20 and water, led to oxazolines 128 through intermediate traw.v-bromoamides 127. The scope of the bromoamidation appears quite broad with regard to olefinic and nitrile components <06JA9644>. 

The radical addition of halogen to an alkene has been referred to briefly in Section 9.3.2. We saw an example of bromination of the double bond in cyclohexene as an unwanted side-reaction in some allylic substitution reactions. The mechanism is quite straightforward, and follows a sequence we should now be able to predict. 

In 1996, Detty and coworkers reported on the oxidation of sodium halides to positive halogens with hydrogen peroxide in two-phase systems of dichloromethane and pH 6 phosphate buffer, catalyzed by organoteUurium catalysts 237 (Scheme 181). Mixtures of 1,2-dihalocyclohexane (238) and 2-halocyclohexanol (239) were formed upon reaction of the positive halogens formed with cyclohexene. From the three tellurium catalysts... 

Under high temperature or UV light and in the gas phase, cyclohexene can undergo free radical substitution by halogens. A common reagent for allylic... 

Fluoroalkylations arc undesirable side-reactions of hydrogen fluoride catalyzed alkylations of alkenes.208 However, selective chain elongation can be achieved in moderate yields with ethene and its halogen derivatives. Thus, various alcohols (terl-butyl, 1,1-dimethylpropyl. benzyl) and alkenes, such as cyclohexene, have been used in fluoroalkylations with ethene derivatives in the presence of hydrogen fluoride (e.g., generation of 1 and 2, respectively).205,400... 

Methylene appears to be formed by dehydro-halogenation of CH3C1 with strong bases.47 Methyl chloride and phenyl sodium, in the presence of cyclohexene, isobutylene, and cf.s-2-butene, yielded norcarane, 1,1-dimethylcyclopropane, and cis-l,2-dimethylcyclopropane, respectively. 

Cyclohexene was shown to react partly by a radical mechanism when chlorinated in the absence of oxygen even in the dark.247 The reaction is slightly less stereoselective than the ionic process (96% trans-1,2-dichlorocyclohexane vs. 99%). Isomeric butenes under similar conditions give products nonstereoselec-tively.248 Branched alkenes are less prone to undergo free-radical halogenation. 

Bromo- and chloro-iodinanes (29 and 31) behave as free radical halogenating agents (79JA3060). They give photoinitiated benzylic halogenation of toluene or allylic halogenation of cyclohexene in high yield. Cyclic 10-C1-3 and 10-F-3 species have not yet been reported. 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

This is true for the addition of hydrogen halide (H-X), halogens (X-X) and other electrophilic reagents (NO —Cl, I — Nj, etc.) to cyclohexenes. The oxymercuration reaction (Hg(OAc>2) in the presence of several nucleophiles (H20, AcOH, ROH) is another well known example. 

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