Bromine accessibility method

Straight-line relationships were obtained between the accessibilities by the bromine method and the IR crystallinity indices and the wide angle x-ray scattering (WAXS) indices for 16 native and regenerated celluloses with accessibilities ranging from 70 to 6% (See Table 5.3 and Figure 5.21 and Figure 5.22). 

The accessibilities determined by the bromine method involve only the noncrystalline, less-ordered regions (LORs) and do not include the surfaces of the crystalline regions. The glycosidic oxygen on these surfaces is buried half a molecule deep within the crystallite [260]... 

Bromine-82 has a half-life of about 36 hours this is not sufficient for the isotope to be used conveniently in tracer work especially if labelled reagents have first to be prepared and purified. Low concentrations of bromine in small specimens of organic materials, such as polymers, can be determined by the method of neutron activation analysis (2). The various substances are prepared using ordinary bromine and then samples are bombarded with thermal neutrons so that the nuclear reaction 81Br(n, y)82Br occurs. Activity is therefore induced in the samples comparison with standards treated similarly permits determination of the bromine contents of the unknowns. For this technique to be applicable, it is necessary to have access to a powerful source of thermal neutrons. Neutron activation analysis can be used for the determination of very low concentrations of many elements and its general features have been fully discussed (3). 

Alternatively, a,a -dibromo ketones are reductively debrominated with concomitant cyclization by [Cr(C0)4N0][PPN] at -78 or — 95°C in dichloromethane. This method is particularly suited for NMR tube scale experiments, but is limited to tri- and tetrasubstituted cyclopropanones. Bicyclo[1.1.0]butanones are also accessible by this synthetic procedure which utilizes 1,3-elimination of two bromine atoms from 2,4-dibromocyclobutanones. ... 

There are some examples of nucleophilic displacement of a suitably activated bromine by fluoride, but there have been instances where such reactions are unsuccessful <93AHC(57)291>. Electrophilic methods do not include direct fluorination, but metallic derivatives provide carbanions which react with fluorine sources. Thus, 2- and 4-fluoroimidazoles have been made from the lithioimidazoles, for example, perchloryl fluoride converts 2-lithio-l-methylimidazole into the 2-fluoro derivative in more than 50% yield. Access to 4- and 5-fluoroimidazoles is even more convenient from the trimethylstannyl derivatives using fluorine or caesium fluoroxysulfate as the electrophile . Mercury groups react in much the same way, but they are more difficult to prepare and purify <86BSF930>. 

The third method closed the third oxazole ring having a bromomethyl group at C(7) (44) by a short heating of 7-(2,3-dibromopropyl)-8-hydroxyxanthines 46 (accessible by addition of bromine to 7-allyl-8-hydroxyxanthines 45 in alkali) (62MI1) (Scheme 12). 

Bromine-lithium exchange in the various isomeric pyridines [152] and quinolines [153] is the method of choice for generating lithiated intermediates free from other isomers. This method is particularly attractive if a bromo compound is easily accessible, e.g. 3-bromoquinoline [156],... 

Finally, a-bromo ketones, like x-bromo aldehydes, can also be obtained by way of the isolated enol acetates 703 exactly the calculated amount of bromine is added, with ice-cooling, to the acetate in CC14 and the mixture is then mixed with dry methanol and set aside for two days. This method is valuable when the enol acetate required is readily accessible. A ketone R RCHCOCH3 can afford two isomeric enol acetates A or B which of them is formed may depend on the natures of R and R but also on the reaction conditions. 

When the desired aryllithium or heteroaryllithium species is not accessible using directed proton abstraction, then halogen-lithium exchange provides an efficient method for its formation. Bromine-lithium exchange is particularly popular and allows a regiospecific formation of the desired organolithium compound, which can then be used for subsequent carbon-carbon bond formation. For example, 3-lithiofuran can be obtained by treatment of 3-bromofiiran with n-butyllithium addition of an aldehyde gives the 3-substituted furan product (1.123). 

Macor also exploited the Mori-Ban indole synthesis to synthesize several anti-migraine analogues of sumatriptan and homo-tiyptamines as potent and selective serotonin reuptake inhibitors (SSRIs). Noticeably, the presence of the second bromine (the bromine passenger ) on the substrate was not significantly deleterious to the reaction although a small amount of the 7-bromoindole might be sacrificed at the end of the reaction to consume the active palladium catalyst. The approach to 7-bromoindole could provide a general method to access 7-bromoindoles (a rare class of indole derivatives), which then could be further manipulated for the synthesis of more complex 7-substituted indoles. 

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