Photochemical bromination

Bromine reacts direcdy with alkanes but this reaction has Httle value because mixtures are obtained. However, photochemical bromination of alkyl bromides can be quite selective (23). 

Photochemical brominations offluorobenzene or difluorobenzenes produce hexabromocyclohexane derivatives in low yield. Tri- and tetrafluorobenzenes are unreactive under these conditions [17 (Table 6). Analogous free-radical chlorination of benzotrifluoride produces a 9% yield of the hexachloro adduct [iS]... 

Scheme 7.4 Pilot scale example of a photochemical bromination...

C. H. Langford, and E. M. J. Templeton, Photochemical Bromine Production Implicated in Arctic Boundary-Layer Ozone Depletion, Nature, 355, 150-152 (1992). 

Bromination of allylic positions cannot be achieved specifically by using elementary bromine, unless electrophilic addition to the rc-bond (Eq. 18) is unfavorable because the substituents have a high negative inductive effect. Efficiency of electrophilic addition of Br2 may also be diminished by steric effects. In addition to these secondary thermal reactions, the specificity of photochemical brominations of allylic positions using Br2 will also suffer from the competing (radical) addition of Br to the double bond (Eq. 19) [31]. 

Rather specific thermal or photochemical bromination of allylic positions is, however, possible by using N-bromosuccinimide as brominating agent. Both procedures produce, however, variable quantities of Br2 depending on reaction conditions [32]. In thermal procedures, the concentration of intermediate Br2, and, hence, the importance of secondary addition products, can be controlled by the relative quantity of radical initiator (e.g., A1BN) and by reaction temperature. The appearance of addition products in a photochemical procedure would be evidence for the mechanism proposed by Adam et al. [2, 3, 33] which includes the intermediate production of Br2 (Eq. 21). 

In a similar attempt, Vernekar et al. [134] carried out photochemical bromination of polyolefin surfaces. It was observed that the gas phase photochemical bromination occurred with high degree of regioselectivity. The bromination of LDPE surface yielded four different species (a) secondary Br, (b) allyl Br, (c) vinyl Br and (d) dibromide, in varying proportions depending upon the underlying mechanism. However, the formation of allyl Br moieties was predominant. The proposed mechanism of photochemical bromination is presented in Scheme 10. 

Catalysis of these reactions by potassium bromide has been observed.232,236 Aralkyl bromides are formed as intermediates that undergo acetolysis to the corresponding acetates under the reaction conditions. Results236 were consistent with a free radical mechanism, even with reactive toluenes, in contrast to the electron transfer mechanism observed in the absence of potassium bromide. Relative reactivities236 corresponded closely to those observed in photochemical brominations, suggesting that bromine atoms, formed by electron transfer oxida-... 

Phenylbenzo[c ]furan is considered to be an intermediate in the reaction between the photochemical bromination product of 2-methylbenzophenone and a series of dienophiles to give the corresponding Diels-Alder adducts (76JCS(P1)336). l-Methyl-3-phenyl-benzo[c]furan is similarly assumed to be an intermediate in the reaction between the photobromination product of 2-ethylbenzophenone and dienophiles. 

Other recent oxidative methods have yet to make a major impact. Photochemical bromination can be used for the oxidative cleavage of a benzyl ether on a substantial scale.290-291 An intermediate a-bromo ether hydrolyses to a hemiace-tal intermediate that collapses to give benzaldehyde and the liberated alcohol [Scheme 4 156] ... 

Methylbenzophenone gives, on photochemical bromination (refluxing CCI4), 2-bromomethylbenzophenone in 80% yield. When the bromination was conducted at room temperature and the crude product treated in refluxing CHCI3 with iV-phenylmaleimide, dimethyl maleate, diethyl fu-marate, methyl vinyl ketone, dimethyl acetylenedicarboxylate, and 1,4-naphthoquinone adducts 92 (mp 227-228°C, 75%), 93 (mp 120-121°C, 65%), 94 (mp 129-130°C, 51%), 95 (mp 81-82 C, 23% an unidentified product was also isolated), 96 (mp 149-150°C, 62%) and 97 (mp 315-316°C, 19%) were obtained. l-Phenylbenzo[c]furan is assumed to be an intermediate in these reactions. Whereas the formation of 92-96 can be simply... 

McConnell J. C., Henderson G. S., Barrie L., Bottenheim J., Niki H., Langford C. H., and Templeton E. M. J. (1992) Photochemical bromine production impheated in Arctic boundary-layer ozone depletion. Nature 355, 150-152. 

Dibromo-2-pyrone was first reported by Pirkle and coworkers <69JOC2239> in 1969, and was prepared from 2-pyrone 1, via either a 3-step sequence that involved two successive brominations, followed by elimination of HBr, or a four-step process comprised of a bromination, HBr elimination and photochemical bromination reaction, followed by a second HBr elimination (Scheme 12). 

Interox has developed an indirect approach to this range of products based on a photochemical bromination followed by hydrolysis and oxidation steps (Fig. 12). The bromine a potentially environmentally unfriendly reagent in its own right, is generated in situ from H202 and HBr. 

A patent has been lodged covering the sulfochlorination of gaseous alkanes. " Photochemical bromination using N-bromosuccinimide of the arenes (136) has been examined under a nitrogen atmosphere. The remote functionalization of the alcohol (137) has been carried out using DIB/Ii/cyclohexane. The product obtained from this, after oxidation, is the iodoketone (138, 84%). ... 

Photochemical reaction of 4-tcrt-butyltoluene with HBr/HjOj gives benzyl bromide easily hydrolyzed to 4-tert-butylbenzaldehyde [18c]. Thermal and photochemical bromination of decalin gives mainly tetrabromooctalin [18d] ... 

Step CF2 CF2 O CF2 CF2 NF + Av CF2 CF2 0 CF2 CF2-N. -f F-). Pyrolysis (700 °Q of the cyclobutyl derivative (32) provides perfluoro-(iV-vinyl-morpholine) (35) (86%), photochemical bromination and hydrobromination of which yields the ethyl derivatives (33) and (34), respectively. Reinvestigation of the thmmal decomposition of perfluoro-iV-fluoromorpholine in platinum at 600 °C has shown that the product comprises perfluoro-fiV-methyloxa2 lidine) (37) (17%),... 

Photochemical bromination of 1, d+4.21°, affords 2, which is optically active, D - 3.23°, but 3 under the same conditions gives 4, which is optically inactive. Explain. 

Specific photochemical bromination of (423) has been used in a new synthesis of L-ascorbic acid (Scheme 77). ... 

Radical substitutions in paraffin derivatives do not take place by attack on carbon as in the 5 and reactions. Instead, they involve abstraction of an atom, usually hydrogen, by a radical reagent to give an alkyl radical which then reacts in some way to form the product. This secondary reaction also usually involves transfer of an atom to the alkyl radical from some normal molecule which is thus in turn converted to a radical. The result is a radical chain reaction in which radicals effectively catalyze substitution. A good example is the photochemical bromination of paraffins, which takes place in the following way ... 

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