Bromination of cyclohexene

In the two examples cited so far the chlorination of propene and the bromination of cyclohexene both criteria are met... 

Table 2 Third-order Rate Constants for the Bromination of Cyclohexene in 1,2-Dichloroethane a... 

Enantioselective bromination of cyclohexene (11) in an inclusion complex with the optically active host compound, (i, i )-(-)-trans-4,5-bis(hydroxy-diphenylmethyl)-2,2-dimethyl-l,3-dioxacyclopentane (10a) was accomplished. 

Wohl-Ziegler bromination of cyclohexene 2a and treatment of the resulting 3-bromocyclohexene with tri(acetonitrile)tricarbonylmolybdenum, followed... 

Novel results were reported for allylic bromination. In radical bromination of cyclohexene in CCI4 under light the selectivity of substitution over addition was shown to be controlled by bromine concentration.304 Substitution via the corresponding allyl radical, while relatively slow, is irreversible and fast enough to maintain the concentration of bromine at a sufficiently low level to prevent significant addition. The reaction of two strained alkenes, fZ)-1,2-dimethyl-1,2-di-ferf-butylethylene and the -isomer (14), leads to the corresponding bromosubstituted product, instead of addition 305... 

Kinetic studies92 of the bromination of cyclohexene, alken-l-ols 41 and 4-pentenoic acid, by the bromonium ion of adamantylideneadamantane bear out the findings shown in Scheme 16. 

Fig. 1.24. Mechanism for the allylic bromination of cyclohexene according to the Wohl-Ziegler process. 

Under certain conditions, the bromination of cyclohexene follows an unusual rate law ... 

Like allylic cations, allylic radicals are stabilized by resonance delocalization. For example, Mechanism 15-2 shows the mechanism of free-radical bromination of cyclohexene. Substitution occurs entirely at the allylic position, where abstraction of a hydrogen gives a resonance-stabilized allylic radical as the intermediate. 

The bromine atom has replaced an atom of hydrogen and so this is a substitution reaction. The reagent is electrophilic bromine and the molecule is aromatic so the reaction is electrophilic aromatic substitution and that is the subject of this chapter. We can compare the bromination of cyclohexene and of benzene directly. 

Semiconductor-mediated photoelectrochemical oxidation of halide salts provides a procedure for the halogenation of alkenes via excitated halide species. For example, bromination of cyclohexene has been performed in a Ti02/Bu4NBr (or PhaPMeBr) /O2 system. The reactive bromine species probably arise from a one-electron oxidation of adsorbed bromide ions on the semiconductor by photoin ation, which produces surface-bound bromine atoms (equation 40). 

McMillen, D. W., Grutzner, J. B. Radical Bromination of Cyclohexene in CCI4 by Bromine Addition versus Substitution. J. Org. Chem. 1994, 59, 4516 528. 

The reagent has been shown to be effective for allylic bromination, the reaction being markedly catalyzed by light. Thus bromination of cyclohexene was effected under illumination with a 275-watt Sylvania sunlamp. The reaction can be carried... 

PROBLEM 10.4 Assume that A-bromosuccinimide serves as a source of Br2, and I write equations for the propagation steps in the formation of 3-bromocyclohex- I ene by allylic bromination of cyclohexene. J... 

Bromination of cyclohexene with NBS (N-bromosuccinimide) in the presence of benzoyl peroxide gives a monobrominated product Write down the product, along with a suitable mechanism. 

---