Olefins allylic bromination

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

Several conjugated diolefins have been made by heating bromo olefins with solid potassium hydroxide or excess quinoline. In the latter case, the bromo olefins were made available by allylic bromination of olefins with N-bromosuccinimide. /S-phenylbutadiene is obtained in 46% yield by the action of pyridine on the corresponding secondary chloride. Chlorination of n-butyl chloride gives an isomeric mixture of dichlorides from which low yields (18-30%) of butadiene are obtained by passing the vapors over soda lime at about 700°. ... 

Allylic bromination of olefins, 104 Amides, acidolysis, 569 acylation, 576 N-alkylation, 572, 678 condensation with aldehydes, 575, 576... 

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),... 

Reaction with olefins. Wohl in 1919 briefly examined the reaction of tetra-methylcthylene with NBA and tentatively suggested that the reaction involved allylic bromination. Wolfe and Awang have examined the reaction of NBA with olefins and find that the reaction does not involve allylic bromination but furnishes 2-bromo-N-bromoacetimidates, a new class of compounds. Thus the reaction of cyclohexenc (I)... 

Wohl-Ziegler bromination The allylic bromination of an olefin using /V-bromo-succinimide (NBS). 

Brominations are, in most cases, carried out by methods similar to those for the preparation of chlorine derivatives. Sampey (127) gives a history of the photobromination of benzene and toluene. Davis (123) made a thorough examination of the relative rates of bromination of the olefins, concentrating particularly on ethylene. An unusual method for producing bromine compounds is by the use of bromosuccinimide or related compounds. This method is called the Wohl-Ziegler reaction and causes allylic bromination. It has been the subject of a couple of reviews (124,1 0). 

Reaction with olefins. Study of the reaction of cvclohexenc with N-bromoacetam-ide in carbon tetrachloride solution under ultraviolet illumination established that the initial product is (rans-acetamidocyclohexyl bromide.12 There was no evidence of allylic bromination. 

The following regularities have been observed in allylic bromination of mono-, di-, and alicyclic olefins by NBS.352,354 In absence of activation methylene groups are brominated more easily than methyl groups, and these somewhat more easily than methine groups. Straight-chain and branched-chain olefins are brominated by NBS only once at each allyl position two bromine... 

Allylic bromination by NBS can be applied with success not only to olefins but also to <%,/ -unsaturated ketones, carboxylic esters, nitriles, and lactones. Radical-formers must be added when NBS is used for allylic bromination of conjugated dienes, for side-chain bromination of aromatic or heterocyclic compounds (see p. 198), or for replacement of tertiary hydrogen atoms next to a C=C bond. 

Olefins 316 and 317 were extensively employed by R.K. Brown and his coworkers as substrates for the syntheses of DL-hexoses. Allylic bromination of 316 and 317 led to the same single compound, l,6-anhydro-2-bromo-2,3,4-trideoxy-P-DL-erythro-hexopyranose (318). This result indicated that the allylic... 

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. 

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... 

Allylic bromination of olefins with A-bromosuccinimide. Peroxides or ultraviolet light are used as initiators ... 

The strained S-lactam was opened smoothly to yield fS-ketoester 324, which on treatment with sodium methylate provided lactam 323. A highly selective Borch reduction finally led to (+)-eburnamonine. Olefination of both enantiomers followed by allylic bromination and oxidation eventlessly afforded apovincamine, which had been converted into vincamine already. 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

Since NBS is conveniently handled and since it is unreactive towards a variety of other functional groups, it is a valuable reagent for brominating in allylic position in olefines, acetylenes or even aromatic compounds. 

Bromination of olefines by NBS is also known as Wohl-Ziegler bromination and as already written, the reaction is specific at allylic position and good yields are obtained. 

Allylic and vinylic ethers via methoxytelluration of olefins (general procedure) To a solntion of phenyltellnrinm tribromide (2.22 g, 5 mmol), prepared from diphenyl diteUnride and bromine in methanol (5 mL), is added the olefin (10 mmol) and the mixture refluxed for 1 h. On cooling, the ()3-methoxy)alkyl or cycloalkylphenyltellnrinm dibromide precipitates and is separated by filtration. 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

If the above assumptions are correct, initial attack by positive bromine must occur on the olefin in accord with Markownikov s rule. The bromine is thus placed as required by the structure of the product, at the end of the side chain in both the vinylphenyl and allyl compounds. 

Olefins. When the substrate molecule contains a double bond, treatment with chlorine or bromine usually leads to addition rather than substitution. However, for other radicals (and even for chlorine or bromine atoms when they do abstract a hydrogen) the position of attack is perfectly clear. Vinylic hydrogens are practically never abstracted, and allylic hydrogens are greatly preferred to other positions of the molecule, This is generally attributed41 to resonance stabilization of the allylic radical ... 

The other bromine atom comes from another bromine-containing molecule or ion. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an olefin in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated.126... 

The mechanism is usually electrophilic (see p. 737), but when free-radical initiators (or uv light) are present, addition can occur by a free-radical mechanism.624 Once Br or Cl radicals are formed, however, substitution may compete (4-1 and 4-2). This is especially important when the olefin has allylic hydrogens. Under free-radical conditions (uv light) bromine or chlorine adds to the benzene ring to give, respectively, hexabromo- and hex-achlorocydohexane. These are mixtures of stereoisomers (see p. 131).625... 

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