Allylic bromination, mechanism

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 ... 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

It is generally supported that the bromination with NBS proceeded by a radical (ref. 11) or an ionic mechanism via bromine molecule. For instance, the former was suggested in benzylic and allylic bromination with NBS for Whol-Ziegler reaction (ref. 12). Calo et al. (ref. 5) accounted NBS brominated phenol by the latter mechanism. 

That the mechanism of allylic bromination is of the free-radical type was demonstrated by Dauben and McCoy, who showed that the reaction is veiy sensitive to free-radical initiators and inhibitors and indeed does not proceed at all unless at least a trace of initiator is present. Subsequent work indicated that the species that actually abstracts hydrogen from the substrate is the bromine atom. The reaction is initiated by small amounts of Br. Once it is formed, the main propagation steps are... 

The mechanism of allylic bromination by N-bromosuccini-mide (NBS, 1) (p T 198) is so often given wrongly in textbooks that the correct route is worth repeating here. NBS is usually used in refluxing carbon tetrachloride and the inltation step Is either thermolysis... 

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. " ... 

Cyclohexadienol was prepared by Rickborn in 1970 from reaction of the epoxide of 1,4-cyclohexadiene with methyl lithium.100 A hydrate of naphthalene, 1-hydroxy-1,2-dihydro-naphthalene was prepared by Bamberger in 1895 by allylic bromination of O-acylated tetralol (1-hydroxy-l,2,3,4-tetrahydronaphthalene) followed by reaction with base.101 Hydrates of naphthalene and other polycylic aromatics are also available from oxidative fermentation of dihydroaromatic molecules, which occurs particularly efficiently with a mutant strain (UV4) of Pseudomonas putida.102,103 The hydrates are alcohols and they undergo acid-catalyzed dehydration to form the aromatic molecule by the same mechanism as other alcohols, except that the thermodynamic driving force provided by the aromatic product makes deprotonation of the carbocation (arenonium ion) a fast reaction, so that in contrast to simple alcohols, formation of the carbocation is rate-determining (Scheme 6).104,105... 

Fig. 1.26. Mechanism for the allylic bromination of cyctohex-ene according to the Wohl-Ziegter process.

Mechanism 6-1 Allylic Bromination 228 Summary Methods for Preparing Alkyl Halides 229 6-7 Reactions of Alkyl Halides Substitution and Elimination 231 6-8 Second-Order Nucleophilic Substitution The Sn2 Reaction 232 Key Mechanism 6-2 The S j2 Reaction 233 6-9 Generality of the SN2 Reaction 234... 

The general mechanism for allylic bromination shows that either end of the resonance-stabilized allylic radical can react with bromine to give products. In one of the products, the bromine atom appears in the same position where the hydrogen atom was abstracted. The other product results from reaction at the carbon atom that bears the radical in the second resonance form of the allylic radical. This second compound is said to be the product of an allylic shift. 

Overall, CH3 has been replaced by CH2OH or CH-0 in an allylic position, a transformation similar to the NBS allylic bromination reaction that you met in Chapter 39, but with a very different mechanism. The by-product of the oxidation is a selenium(II) compound, and it can be more practical to carry out the reaction with only a catalytic amount of Se02, with a further oxidizing agent,... 

Structures I and II differ only in the lactonic C-O bond. This seemingly simple transformation conceals complications derived from the oxidative process involved in the formation of this bond. Necessarily, the oxidant must be an external agent since the enone II does not show the traces (reduced fragments) that are always produced during any intramolecular redox operation. This oxidizing agent in all probability is bromine. In fact, one could predict, in the absence of any further evidence, that the reaction mechanism must proceed by way of an allylic bromination at the 7 carbon of enone I, which would provide the required functionalization for an intramolecular nucleophilic displacement of bromide by the carboxylate anion in a second step. This is shown in structure... 

Dauben and McCoy demonstrated that the mechanism of allylic bromination is of the free-radical type, showing that the reaction is very sensitive to free-radical... 

Figure 2.15 shows a different and more sophisticated approach to the combination of cationic and radical mechanisms, since in this case both processes bear a living character. Allylic brominated poly(pPIN) with Br/p-pinene unit ratios of 1.0 and 0.5 (Mn of 2 810 and 2 420, respectively and a similar MWD of 1.3) were obtained by treatment with A-bromosuccinimide/AIBN and then used as macroinitiators in conjunction with CuBr and 2,2 -bipyridine for the atom transfer radical polymerization (ATRP) of acrylic monomers [52, 53]. 

Allylic bromination and chlorination proceed by a radical chain mechanism involving the same type of chain initiation, chain propagation, and chain termination steps involved in the radical halogenation of alkanes. 

The mechanism we described for allylic bromination by NBS poses the following problem. NBS is the indirect source of Rxy which then takes part in chain propagation. But if Bt2 is present in the reaction mixture, why does it not react instead with the carbon-carbon double bond by electrophilic addition In other words, why is the observed reaction allylic substitution rather than addition to... 

Halogenation of a larger alkyl side chain is highly regioselective, as illustrated by the halogenation of ethylbenzene. When treated with NBS, the only monobromo organic product formed is 1-bromo-l-phenylethane.This regioselectivity is dictated by the resonance stabilization of the benzylic radical intermediate. The mechanism of radical bro-mination at a benzylic position is identical to that for allylic bromination (Section 8.6A). 

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