Cyclohexene allylic 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. 

There are some instances where the chlorination or bromination reaction can be used to good effect, however. Alkenes that have allylic hydrogens can sometimes be halogenated specifically at an allylic position in a process called allylic halogenation, another free radical chain reaction. For example, when low concentrations of bromine are photolyzed in the presence of the complicated cyclohexene shown in Figure 11.51, the product is exclusively brominated in one allylic position. 

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. 

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

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... 

Liquid-phase halogenation of hexachlorobenzene with chlorine trifluoride appears to proceed by a series of additions and vinylic and allylic substitutions until all of the hexachlorobenzene is converted into chlorofluorocyclohexenes, C6F (Clio ) (n = mainly 4, 5 and 6), and conversion to cyclohexane derivatives occurs only upon the passage of quite a large excess of chlorine trifluoride [177] (Figure 2.29). The cyclohexene derivatives produced mainly retain the structure —CC1=CC1—. 

Because allylic C—H bonds are weaker than other sp hybridized C-H bonds, the allylic carbon can be selectively halogenated by using iV-bromosuccinimide (NBS, Section 10.15) in the presence of light or peroxides. Under these conditions only the allylic C-H bond in cyclohexene reacts to form an allylic halide. 

Allylic conjugation stabilizes carbanions and p/f values of 43 (in cyclohexy-lamine) and 47 8 (in THF-HMPA) were determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene were found to have pK values of about 45 in cyclohexylamine. These data indicate that allylic positions have p/f 45. The hydrogens on the sp carbons in benzene and ethene are more acidic than the hydrogens in saturated hydrocarbons. A pK of 45 has been estimated for benzene on the basis of extrapolation from a series of halogenated... 

A more convenient method for halogenation of allylic and benzylic C-H moieties uses the readily available A-bromosuccinimide (NBS, 170) or NCS (173) with heat and/or light, in what is called the Wohl-Ziegler reaction. Reaction of NBS with cyclohexene in refluxing carbon tetrachloride gives bromocyclo-... 

The oxidation of cyclohexene with dioxygen in SCCO2 has also been reported. The iron(m)-halogenated-porphyrin-catalyzed reaction produced epoxide and allylic oxidation products (Scheme 14). The turnover number (TON) reached up to 1530 for cyclohexene in 24 h at 80°C. The activity is lower in SCCO2 than in organic solvents, but the selectivity for epoxidation (up to 34%) is higher in SCCO2. 

In a subsequent report, Ziegler and co-workers systematically studied the halogenation of several allylic substrates using a variety of reagents including A -bromo and A -chloro pthalimide, chloramine-T, N-bromo and A -chlorosaccharin, A -chloro-A -benzoyl-p-toluenesulfonamide, N-chloro-di-/ -toluylsulfonamide, and most notably, iV-bromo and N-chlorosuccinimide. Successful allylic bromination was reported for several substrates, among which were the conversion of cyclohexene (7) into bromocyclohexene (8) and the conversion of phenyl propene 9 into cinnamyl bromide (10). 

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