Halogenation allylic bromination

Aside from the use of the wax in cosmetics and specialty lubricant applications, the cost of the oil has limited its scope of incorporation into the industrial market. However, efforts by Shani et al. (9-12) have provided some degree of functionalization of jojoba, particularly at the olefin positions. These functionalizations fall into the classes of halogenation, allylic bromination-dehydrohalogenation, sulfurization, phosphorylation, ozonolysis, amination, oxidation, and pyrolysis. These derivatives are intended to boost the polarity of the wax or provide functionalization for binding to polymers. 

Neutral HX addition X = P, S, Se, Si Allylic bromination Carbon-halogen addition... 

The fact that the bromine concentration remains at veiy low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must... 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. " Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), " and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step " " " instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide. " ... 

Although allylic brominations and chlorinations offer a method for attaching a reactive functional group to a hydrocarbon framework, we need to be aware of two important limitations. For allylic halogenation to be effective in a particular synthesis ... 

The mechanism is similar to other free-radical halogenations. A bromine radical abstracts an allylic hydrogen atom to give a resonance-stabilized allylic radical. This radical reacts with Br2, regenerating a bromine radical that continues the chain reaction. 

Olefins, addition—Continued of diazoacetic ester, 498 of halo ethers, 232 of halogen, 106 of hydrogen cyanide, 603 of hydrogen halide, 105 of hypohalous acid, 109 of polyhalides, 107 to cyanides, 571 allylic bromination, 36, 104 condensation, with halides, 108 with phenols, 179 conversion to amides (Wi 11 gerodt),... 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

Compounds of tropolone type have a duplicitous nature in that the molecule can behave as either an aromatic or an allylic functionality. Tropolones can be brominated according to a method similar to that for alkenes, with tautomerisation restoring the cycloheptatriene backbone. In contrast to this simple halogenation with bromine, other halogenated products form less readily, requiring more advanced organic synthetic pathways. 

The fact that the Br2 concentration remains at very low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must compete with polar addition of bromine via a bromonium ion intermediate. The reactions differ in their dependence on bromine concentration. The allylic substitution is one-half order in bromine, whereas the addition reaction follows a first- or second-order dependence on [Br2] (see Section 5.3). Therefore a low concentration of Br2 favors substitution over addition. 

Allylic bromination by NBS is a radical-chain reaction occurring on the surface of the NBS crystals. A solution of NBS in tetrachloroethane or nitro-methane adds bromide to a C=C group 354 solvents are therefore used in which NBS and, if possible, also the succinimide formed, are difficultly soluble dry CC14 is usually chosen, but for reactive substances cyclohexane or benzene may also be used. The more polarized the N-halogen is, i.e.9 the more positive the Br is rendered, the more easily does the A-halogenated compound cause addition to the C=C bond NBS is particularly suitable for allylic bromination because of its steric structure and the almost apolar nature of its N-Br bond. 

Bromination of jojoba oil in carbon tetrachloride yielded tetrabromojojoba derivatives at 20°C (9). When treated with excess base, these bromides yielded the corresponding acetylenes from Z,Z olefins of jojoba or allenes from the E,E isomerized jojoba (Scheme 2) with the expected hydrolysis to acid and alcohol. Allylic bromination (nonregiospecific) with A-bromosuccsinimide (NBS) followed by dehydro-halogenation yielded polyunsaturated oils with degrees of unsaturation up to the hexaenoic jojoba derivative (10). These highly unsaturated materials were envisioned... 

---