Photo-bromination

R. J. Ferrier and P. C. Tyler, Photo-bromination of carbohydrate derivatives. Part 3. C-5 bromination of penta-O-benzoyl-ff- and -/i-1)-gl ucopyranosc a route to D-xylo-hexos-5-ulose derivatives and a-L-idopyranosides, J. Chem. Soc. Perkin I, (1980) 1528-1534. 

Acylated 2-oxoglycosyl bromides of type II may efficiently be generated from hydroxyglycal esters by either of two ways, i. e. a high-yield, three-step procedure involving hydroxylaminolysis (29), deoximation (30), and photo-bromination (31), or, alternately, by a one-step process, simply consisting of exposure of hydroxyglycal ester I, in dichloromethane solution, to NBS or... 

Photochlorination of 1,1-dichlorocyclopropane (1) yielded a mixture of 1,1,1,3-tetrachloro-propane (2) and 1,14,3,3-pentachloropropane (3), the latter by subsequent chlorination of the primary product tetrachloropropane. The ring opening by a chlorine radical occurred with inversion as demonstrated by the use of deuterium-labeled starting material. Photo-bromination of 1,1-dichlorocyclopropane gave l,3-dibromo-l,l-dichloropropane as the only product. ... 

This same influence is exhibited not only in the dark, but also in photo-brominations. From the preceding data, Bruner has concluded that solvents such as nitrobenzene and acetic acid, which have strong ionizing (dissociating) powers, favor substitution in the nucleus. 

Imidazoline-2-thione derivatives such as 43 were synthesized by condensation of 1-amino-l-deoxy-D-fructose (or its iV-alkylated derivatives) with glycosyl isothiocyanates and converted to dehydration products or spiro-cyclic derivatives such as 44 and 45, respectively, under different acidic conditions. The spiro-thiohydantoin analogues 47 and 48 were synthesized from the photo-bromination product 46 (Scheme 9), the latter being able to interfere with the formation of glucose from glycogen in rats. ... 

The halogens include fluorine, chlorine (shown in the first photo), bromine, iodine (shown in the second photo), and astatine. 

C02-0097. The element bromine exists as diatomic molecules and is a liquid under normal conditions. Bromine evaporates easily, however, giving a red-brown color to the gas phase above liquid bromine, as shown in the photo. Draw molecular pictures showing liquid bromine, the gas above it, and the dynamic equilibrium between the phases. 

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

Marchand and co-workers ° synthesis of 5,5,9,9-tetranitropentacyclo[5.3.0.0 .0 °.0 ] decane (52) reqnired the dioxime of pentacyclo[5.3.0.0 .0 °.0 ]decane-5,9-dione (49) for the incorporation of the four nitro groups. Synthesis of the diketone precursor (48) was achieved in only five steps from cyclopentanone. Thus, acetal protection of cyclopentanone with ethylene glycol, followed by a-bromination, and dehydrobromination with sodium in methanol, yielded the reactive intermediate (45), which underwent a spontaneous Diels-Alder cycloaddition to give (46). Selective acetal deprotection of (46) was followed by a photo-initiated intramolecular cyclization and final acetal deprotection with aqueous mineral acid to give the diketone (48). Derivatization of the diketone (48) to the corresponding dioxime (49) was followed by conversion of the oxime groups to gem-dinitro functionality using standard literature procedures. 

In addition, mechanisms for regeneration of photo-chemically active bromine that involve aerosol particles or reactions on the snowpack have been proposed. For example, McConnell et al. (1992) and Tang and McConnell (1996) proposed that HBr and organobromine compounds could be converted to Br2 through adsorption and reaction on ice and aerosol particles. Fan and Jacob (1992) suggested that HOBr, formed by the reaction of BrO with H02,... 

Eriksson J, Jakobsson E, Marsh G, et al. 2001. Photo decomposition of brominated diphenylethers in methanol/water. BFR 203-206. 

According to C. F. Barwald and A. Monheim (1835), the decomposition is accelerated by the presence of organic substances. J. Milbauer tried the effect of thirty-two metal chlorides of sodium tungstate and molybdate of uranyl sulphate and of sulphuric, selenic, arsenic, and boric acids on the photo-decomposition of chlorine water, and found. that none accelerated but that most retarded the action. Chlorine catalyzes the decomposition of bromine water and bromine, chlorine water while iodine does not accelerate, but rather retards the reaction, probably by forming relatively stable iodine compounds. A. Bcnrath and H. Tuchel found the temp, coeff. of the velocity of the reaction with chlorine water between 5° and 30° increases in the ratio 1 1 395 per 10°. 

Fig. 3.11 Agelas sp. An example of a very common marine sponge that produces a wide variety of halogenated metabolites such as the brominated fatty acids 733 and 734 (Photo J. R. Pawlik)...

Fig. 3.19 Flustrafoliacea, a North Sea bryozoan and a producer of many bromotryptamines and brominated indole alkaloids, such as 1343-1346 (Photo A. D. Wright)...

Fig. 3.32 Acarospora gobiensis, a Central Asian lichen that contains the novel brominated depsidones, acarogobiens A and B (2146 and 2147) (Photo T. Rezanka)...

Fig. 3.33 Dysidea dendyi, an Australian sponge that contains the brominated dioxins, spongia-dioxins A-C and related methyl ethers (2235-2239) (Photo N. Utkina)...

Buser, H.R. (1988) Rapid photo lytic decomposition of brominated and brominated/chlorinated dibenzodioxins and dibenzofurans. Chemosphere 17, 889-903. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

In these compounds, the intermediate arylmethyl radical is long-lived. However, further excitation of the radicals in benzene results in production of the bromine atom-benzene charge transfer complex ( max 550 nm), an indication of photo-induced cleavage of the remaining C-Br bond. This is further supported by observation of the corresponding vinyl products. 

Reaction (IV) is possible and in fact quite likely. The breaking of a carbon-bromine bond involves the absorption of 58,000 calories and this minimum is close enough to the 55,000 calories to be within the limit of uncertainty of the constants involved. This reaction can account for the experimental results and is in line with spectroscopic evidence that ethyl halides dissociate photo-chemically into the halogen atom and the free radical. 

The conversions of various steroidal l,4-dien-3-ones (109) into 5-ene-la,3/3-diols (111) via deconjugation to the l,5-dien-3-one, reduction to the l,5-dien-3/8-ol (110), and hydroboration have been described in a series of papers.125 The corresponding 5,7-diene-la,3j8-diols (118), required for photo-isomerization as a key step in the preparation of la-hydroxylated derivatives of vitamin D, have been obtained either by allylic bromination-dehydrobromination of the 5-ene-la,3/3-diols or by the alternative route outlined in Scheme 2. The key step in this reaction sequence is the protection of the 5,7-diene system as the adduct (116) with 4-phenyl-... 

Two straightforward preparations of androstadienes have been described. When testosterone tosylhydrazone is treated with methyl-lithium in dimethyl digol, androsta-2,4-dien-17/3-ol is formed in 91% yield,44 and when 5a-androst-16-ene is sequentially brominated and dehydrobrominated the expected 5a-androsta-14,16-diene is produced.45 Photo-oxidation of the latter diene in the presence of Rose Bengal afforded the hydroxyandrostenone (119). 

Treatment of the conanine (372) with bromine-aqueous sodium carbonate yields192 (100%) the conenium salt (375) which is readily converted into (376) (80%). Compound (376) is also formed starting from (373). When C-20 is substituted by two methyl groups as in (374) the product is (377). The conanine (373) is photo-oxygenated,193 in the presence of methylene blue, to (376). The nitrone (378) is quantitatively deoxygenated by triphenyl phosphite with trimethyl phosphite the reaction is more complex.194... 

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