Brominations allylic, olefins

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. 

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... 

An important question now arises here. If bromine is the reacting species why does it not react with the double bond either by ionic or free radical mechanism The answer is that its concentration is too low and this slows the rate of addition and in the circumstances only allylic substitution successfully takes place. This means that if there is regular, slow and steady supply of bromine and if somehow HBr formed, be removed to check addition, then it should be possible to brominate an olefine in allylic position even in absence of NBS and this has been demonstrated by Tedder et al. 

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... 

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

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. 

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 ... 

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. 

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 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... 

Although the reaction in Scheme 10 is a highly efficient procedure, a two-step process was required to prepare aziridines from olefins. Two more convenient methods for the one-step aziridination using CT were discovered by the authors in 1998, one of which involves the iodine-catalyzed aziridination of unfunctionalyzed olefins with CT trihydrate [7b] (Scheme 11). The bromine-catalyzed aziridination of unfunctionalyzed olefins and allylic alcohols with anhydrous CT was reported at the same time [7c], though in this case phenyltrimethylammonium tribromide (PTAB), and not Br2, was used as a catalyst (Scheme 12). These two reactions are applicable to a wide range of olefins, and are considered to proceed by almost the same pathway. 

This reaction is unique because it permits one to place a bromine atom on a molecule containing an olefin linkage and the product retains the C=C. For this reaction to take place there must always be a H on an a— or allylic carbon (carbon adjacent to OC). This is the H that is replaced by the Br. If there are several a-carbons each containing H, then the Br may go to any of the a-carbons, i.c.,... 

In the first propagation step of the Wohl-Ziegler bromination, the bromine atom abstracts a hydrogen atom from the allylic position of the olefin and thereby initiates a substitution. This is not the only reaction mode conceivable under these conditions. As an alternative, the bromine atom could attack the C=C double bond and thereby start a radical addition to it (Figure 1.25). Such an addition is indeed observed when cyclohexene is reacted with a Br2/AIBN mixture. 

Bromination of an olefin in the allylic position with N-bromoimides has become a valuable method for the preparation of unsaturated halo-genated compounds. In general, it consists in heating the unsaturated compound in anhydrous carbon tetrachloride under reflux with N-bromo-succimide (or N-bromophthalimide). As the bromination proceeds, suc-cinimide collects at the surface of the mixture. After the completion of... 

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)... 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

Intramolecular peroxymercuration (Eq. 18) followed by brominolysis afforded the dioxetane (10). Since the allylic hydroperoxides are readily available via ene-reaction of a suitably alkyl-substituted olefin (Eq. 16), this method has synthetic potential, especially if electrophiles other than bromine can be employed to provide dioxetanes with other substituents. 

The research team of J. Tadanier prepared a series of C8-modified 3-deoxy-P-D-manno-2-octulosonic acid analogues as potential inhibitors of CMP-Kdo synthetase. One of the derivatives was prepared from a functionalized olefinic carbohydrate substrate by means of the Wohl-Ziegler bromination. The stereochemistry of the double bond was (Z), however, under the reaction conditions a cis-trans isomerization took place in addition to the bromination at the allylic position (no yield was reported for this step). It is worth noting that the authors did not use a radical initiator for this transformation, the reaction mixture was simply irradiated with a 150W flood lamp. Subsequently the allylic bromide was converted to an allylic azide, which was then subjected to the Staudinger reaction to obtain the corresponding allylic amine. 

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

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