Bromo radicals

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

Bromination of (Z)-2-methyl-3-alkenal acetal 98 with NBS in DMSO under irradiation affords selectively bromohydrin 99. The reaction is assumed to proceed via a bromo-radical (generated by irradiation of NBS) that reacts with DMSO to give a yellow intermediate (possibly BrO—SMe2) which may further react either with olefin to give the adduct 99 or with water, furnishing hypobromous acid185. 

The irradiation of 674 leads to a number of products, where the progress of the reaction is much slower compared with 670. While an intermediate as discussed above may be formulated once again, here, bromo radicals are formed which lead to 2-bromomethyl-3,4-dibromo-5-methylthiophene (675) potentially as well as to 676 by the addition of a bromo radical to 677 with subsequent cleavage of the bromohydrin. 

Radical reactions require an initiation step. In this example, a bromo radical is formed. 

In some cases the addition of bromide to alkene radical cations is reversible. For example, the addition of bromide to the p-methyl-4-methoxystyrene radical cation occurs reversibly, as demonstrated by the formation of the radical cation when the P-bromo radical is generated independently by photolysis of l-(4-methoxyphenyl)-l,2-dibtomopropane (Eq. 18). An equilibrium constant of 2 x 10 M has been measured for the loss of bromide from this radical in acetonitrile. The apparent lack of reactivity of 1,3-dioxole radical cations with bromide ion in water has also been explained on the basis of reversible addition with rapid loss of bromide from the product radical. However, on the basis of the solvent effects noted above, it is also possible that the lack of reactivity in water is a solvent effect since decreases in reactivity of 4 to 5 orders of magnitude have been observed for reactions of bromide ion with styrene radical cations in largely aqueous solvent mixtures. - ... 

Another Sml2-mediated domino process was reported by the Baran group in the synthesis of (+)-Cortistatin A 51 [22]. When the bromoketone 47 was treated with Sml2 in the presence of the additive DMPU (l,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone), a cyclopropane fragmentation occurred to form an a-bromo radical 48, which underwent subsequent reduction by Smij and then bromide elimination to give 49 with a newly formed C=C double bond and a Sm(lll) enolate. After a few minutes, the addition of 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) resulted in the a-brominated product 50, which was converted into cortistatin A 51 (Scheme 5.13). 

Halogen Displacement. Poly(phenylene oxide)s can also be prepared from 4-halo-2,6-disubstituted phenols by displacement of the halogen to form the ether linkage (48). A trace of an oxidizing agent or free radical initiates the displacement reaction. With 4-bromo-2,6-dimethylphenol, the reaction can be represented as in equation 10 ... 

Hydrogen hahdes normally add to form 1,2-dihaLides, though an abnormal addition of hydrogen bromide is known, leading to 3-bromo-l-chloropropane [109-70-6], the reaction is beUeved to proceed by a free-radical mechanism. Water can be added by treatment with sulfuric acid at ambient or lower temperatures, followed by dilution with water. The product is l-chloro-2-propanol [127-00-4]. 

As mentioned above (Section 2.13.2.1.3), bipyrimidine photoproducts can arise, probably by reaction between two radicals. Thus, irradiation of an aqueous solution of 5-bromouracil (ill R=Br) in the absence of oxygen produces a variety of products including uracil, barbituric acid, 5-carboxyuracil (111 R = CO2H), several non-pyrimidine compounds and, as a stable end-product, the biuracil (114 R = H). A similar product (114 R = Me) is formed from 5-bromo-l,3-dimethyluracil (ilS). When two such related uracil derivatives are irradiated together, a mixed bipyrimidine product is formed, inter alia (B-76MI21302). 

In the pyrrole series, ester groups a to nitrogen are more readily hydrolyzed by alkali, but those in a /3 position more readily by acid. A methoxycarbonyl group in the 2-positlon is meta directing thus bromination yields mainly 4-bromo-2-methoxycarbonylpyrrole. Free radical chlorination with f-butylhypochlorite gives the 5-chloro derivative. 

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

Irradiation of the A-bromo- or N- chloro-azetidin-2-ones (71) in the presence of alkenes, alkynes or radical donors induces rearrangement to the /3-haloalkyl isocyanates (72) via a... 

Decaitxjxylallon of a mixed anhydride (Ihlohydtoxamlc-cartwxylic) and interception of radicals as a sulfide, selenide or bromo denvative. 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

Functionalized rubbers. Butyl rubber (isobutylene with about 2% iso-prene) has been functionalized through the residual double bonds via the bro-mobutyl intermediate to produce a material with 2% conjugated diene (see Fig. 19). This resin shows high reactivity towards e-beam or UV (free radical or cationic [53]). The bromo butyl intermediate has also been used to attach acrylate or photoinitiator groups to the butyl backbone [54]. 

This ionic mechanism is supported by kinetic data. A free radical mechanism has been proposed for the unique bromination (with bromine) of 3a, 12a-diacetoxypregnan-20-one ethylene ketal, which gives the 21-bromo derivative in excellent yield. ... 

A -Halogenated compounds such as iV-chlorotnfluoroacetamide, A -chloro-imidosulfuryl fluonde and N N dichlorotnfluoromethylamine add across C=C bonds to form saturated amides [14] tmidosulfury I fluorides [15] and amines [16], respectively Allylic halogenation also occurs with the use of A-bromo- or A-chIo roperfluoroamides The primary amine A,A-dichlorotrifluororaethylamine selectively affords 11 or 2 1 adducts with either tetrafluoroethylene or chlorotrifluoroethylene [16] (equation 7) The reaction mechanism is believed to involve thermal free radicals, with control achieved principally by reaction temperature The 1 1 adduct is formed even in the presence of a large excess of olefin... 

This ether is formed from the alcohol and the benzyl bromide (NaH, DMF, 94% yield). It was used primarily because of its stability toward NBS during the radical-promoted conversion of a 4,6-benzylidenepyranoside to the 6-bromo-4-benzoate. Cleavage occurs quantitatively upon hydrogenolyis (Pd/C, H2). ... 

Examine spin density surfaces for l-bromo-2-propyl radical and 2-bromo-l-propyl radical (resulting from bromine atom addition to propene). Eor which is the unpaired electron more delocalized Compare energies for the two radicals. Is the more delocalized radical also the lower-energy radical Could this result have been anticipated using resonance arguments ... 

Spin density surface for l-bromo-2-propyl radical shows location of the unpaired electron. 

Bromination at 450°C hardly occurs, but when the pumice is impregnated with ferrous or cuprous bromide a much better yield of 2,4,6-tribromopyridine is obtained. When pyridine is brominated at 180°C in the presence or absence of impregnated pumice only 2-bromo- and 2,6-dibromopyridine are formed. These facts are not consistent with an electrophilic substitution, which should take place at the 3-position, On the other hand, the high temperature coefficient of the reaction (cf. Table XV) and the fact that ultraviolet light has no effect on the reaction argue against a simple free-radical sub-stitutiond ... 

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