And radical bromination

The extent of substitution to be attained in unsaturated and brominated dodecahe-dranes through electrophilic and radical bromination have been elucidated as part of an effort directed at the C20 fullerene.19... 

Scheme 9 Barton decarboxylation and radical bromination in peptide formation by Taddei [7]...

Additional proof for the difference between Gif- and radical bromination was obtained from bromination of cyclohexyl bromide. Radical induced hydrogen atom abstraction occurrs at the p-position of bromoalkanes and in accordance with "Skell-Walling effect" results in the formation of the /ra 5-l,2-dibromide60. This was confirmed qualitatively for the radical bromination of cyclohexyl bromide. In contrast, this was not the case in the GoAggll bromination reaction. The ra 5-1,2-dibromide was found to be only a minor product, while trans-lA- and cw-l,3-dibromocyclohexanes were the major products. Thus all this data illustrates the different nature of Gif-reactions and radical reactions. Here it is worth mentioning that tertiary C-H bonds appeared to be the least reactive in the bromination process, as is found in the Gif- oxidation reactions. 

When this type of thermodynamic analysis is appHed to alkanes other than methane, similar results are obtained. For example, ethane will undergo both radical chlorination and radical bromination ... 

Energy diagrams for the two propagation steps of radical chlorination and radical bromination of ethane. 

In this chapter, we have seen that radical chlorination and radical bromination are both thermodynamically favorable processes. Bromination is slower but is more selective than chlorination. Both reactions are used in synthesis. When the starting compound has only one kind of hydrogen (all hydrogen atoms are equivalent), chlorination can be accompUshed without having to worry about regiochemical outcomes ... 

Podgorsek A, Stavber S, Zupan M et al (2009) Environmentally benign electrophilic and radical bromination on water H202-lTBr system versus A-bromosuccinimide. Tetrahedron 65 4429 39... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The azidomethyl ether, used to protect phenols and prepared by the displacement of azide on the chloromethylene group, is cleaved reductively with LiAH4 or by hydrogenolysis (Pd-C, H2). It is stable to strong acids, permanganate, and free-radical brominations. ... 

One possible interpretation is a change to a free radical chain mechanism. Bromine radical is first produced which then adds to the alkene. The resulting free radical reacts with hydrogen bromide to yield the final alkyl bromide and regenerate bromine radical. 

It is difficult to treat the effect of a heteroatom on the localization energies of aromatic systems, but Brown has derived molecular orbital parameters from which he has shown that the rates of attack of the phenyl radical at the three positions of pyridine relatively to benzene agree within 10% with the experimental results. He and his co-workers have shown that the formation of 1-bromoisoquinoline on free-radical bromination of isoquinoline is in agreement with predictions from localization energies for physically reasonable values of the Coulomb parameters, but the observed orientation of the phcnylation of quinoline cannot be correlated with localization ener-... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

The bromodihydrodibenz[/>,/]az.epine-5-carbonyl chloride 41, prepared by radical bromination of the 10,11-dihydro compound, on heating under pressure with ammonia undergoes dehydrobromination and amidation to yield Carbamazepine (42).122... 

When 10-phenylphenothiazine (104) (and 10-phenylphenoxazine) was brominated in acetic acid a number of products were isolated. Pyridine perbromide, though, only brominated the phenyl substituent (Scheme 47). The suggestion that acetic acid bromination might involve the radical cation of the substrate (104) was confirmed by generating the radical cation of the substrate (104) with perchloric acid prior to bromination. Again a 43% yield of the 3-bromo product and multiple bromination products were observed (Scheme 47). The reaction of 10-phenylphenoxazine with pyridine perbromide appeared to be at least partially electrophilic the products... 

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. 

Answer In this problem, we are starting with an alkane. There are no leaving groups, so we cannot do a substitution or an elimination reaction. There are also no double bonds, so we cannot do an addition. It seems that we are stuck, with nothing to do. Clearly, our only way out of this situation is to introduce a functional group into the compound, via radical bromination. Radical bromination will place a Br at the most substituted position (the tertiary position), and then we can eliminate ... 

Bromobenzyl groups were introduced into PPO by radical bromination of the methyl groups. The PPO bromobenzyl groups and PECH chloromethyl groups were then esterified under phase-transfer-catalyzed reaction conditions with the potassium carboxylates just described. This procedure has been described previously (29). The sodium salt of 4-methoxy-4 -hydroxybiphenyl was also reacted with PECH (no spacer). 

It is assumed that the reaction is initiated by a radical bromine abstraction to give 10-13, which after carbon monoxide insertion undergoes a rapid 5 -exo cycliza-tion onto the hydrazone moiety. The two diastereomeric hydrazinyl cyclopentanones 10-16 and 10-17 are formed with good yields, though with low stereoselectivity. 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

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