Photobrominations

Photobromination of anhydride (74) results in 56% trans (75) and 36% cis (76). Compound (76) is thought to result from intermediate radical (77) ... 

Figure 7.10 Repetitive nature of the propagation cycle in photobromination of a hydrocarbon (RH) by a free-radical chain reaction...

Photobromination of the N-acetyl derivative (124 R1=Ac R2 = H) with NBS as the brqminating agent produces the 10-bromo derivative (124 R1 = Ac, R2 = Br) whereas with an acyl group at the 4-position, a mixture of the 2-bromo, 8-bromo and 2,8-dibromo derivatives is formed (75JMC976). 

In batch procedures, liquid phase photobrominations halt in general at conversions between 50 and 70%, the concentration of HBr reaching a level where the back reaction (Eq. -15) becomes more efficient than the productforming step (Eq. 16). Particular reaction conditions and, hence, particular reactor geometries and/or accessories are then required to remove continuously the HBr produced. Photobrominations are preferentially developed at boiling point temperatures, as has been beautifully demonstrated in the industrial synthesis of 1-bromo-diethylcarbonate (Eq. 17) [30]. 

Kistiakowsky and Sternberg (64) have studied in 1953 the gas phase photobromination of ethylene and obtained results in general consistent with the earlier results of Schmitz and co-workers (92). They found the quantum yields of the photobromination to be independent of wavelength between 4800 and 6800 A., but observed no bromination at 7150 A., where insufficient energy was available for dissociation into two bromine atoms, although this radiation was absorbed by bromine. The mechanism proposed by Schmitz and co-workers... 

Vaughan et al. (103) studied the photobromination of ethylene, propylene, 1-butene, isobutene, and vinyl chloride in the gas phase and found in every case where a distinction could be made that the product was almost entirely the so-called abnormal addition product in terms of the Markownikoff s rule. However, the more recent work of Skell et al. (95) shows that 2-bromo-w-propyl radical may rearrange very rapidly to 1-bromo-isopropyl radical. In view of this the observed exclusive terminal addition of Br atoms in the case of propylene could be in part due to rapid rearrangement. 

This article deals with the state of knowledge of ring photobromination of carbohydrate derivatives about 12 years after the phenomenon was first observed. The reaction is very unusual in providing means by which hydrogen atoms bonded to carbon atoms of cyclic carbohydrate derivatives may be substituted directly and, in many cases, affording selectivity at ether carbon atoms in preference to the dioxygenated anomeric centers. Some aspects of it have been reviewed from the standpoint of the operation of the captodative effect.24... 

Photobromination of methyl tetra-O-acetyl-a-L-idopyranuronate (11) afforded the 5-bromo-/3-D-gluco compound 10 in 63% isolated yield, indicating30 that epimers 9 and 11 react by way of a common C-5 radical intermediate (see Scheme 3). Methyl tetra-0-acetyl-/ -D-galactopyranuronate may be converted into the analogous (5i )-bromide by use of bromine or N-bro-mosuccinimide. In the former case, small proportions of a bromoacetyl product are also formed.32... 

Previous studies had indicated that, in general, although some differences were sometimes observable, the products formed by photobromination of carbohydrate derivatives using bromine or yV-bromosuccinimide were similar. When compound 31 or 32, however, was treated with the latter reagent, a major difference was found, and the main product (74% isolated) was the orthoamide 35, formed, it was concluded, by way of the bromides 33 and 34 and, thence, a cyclic 3,4-benzoxonium ion.35 Support for this route was obtained by observing that treatment of a mixture of the bromides with yV-bromosuccinimide in refluxing carbon tetrachloride without irradiation resulted in their complete conversion into the orthoamide 35. yV-Bromoacet-... 

An example of the photobromination of a nucleoside derivative is given in Section II, 11. 

The negative results obtained on photobromination of 1,6-anhydro-2-0-benzoyl-3,4-0-isopropylidene-/ -D-galactose and its 3,4-0-cyclohexylidene analog are more difficult to rationalize, especially as the authors used W-bro-... 

The photobromination of a relevant cyclohexane derivative is noted in Section 11,11. 

Photobromination of the enolone 69 is of interest, because radicals formed at C-1 or C-5 (carbohydrate numbering) are both subject to captoda-... 

Somewhat surprisingly, particularly in view of the proposed means of formation of the glycosid-3-ulose derivative 2,1,5-anhydro-2,3,4,6-tetra-O-benzoyl-D-arah/no-hex-1 -enitol (tetra-O-benzoyl-2-hydroxy-D-glucal) does not undergo bromination at the allylic C-3 atom the only products isolated following attempted photobromination with N-bromosucdnimide were two dibromides produced by addition reactions.50... 

Because the aromatic rings of compounds of this class render the anomeric centers benzylic, and also conceivably subject to captodative radical stabilization, photobromination results might be expected to correlate with those observed for the glycosyl cyanides (see Section 11,6). 

Configurational inversion at C-2 of compound 82 has no major effect on the products of photobromination, tetra-0-acetyl-/ -D-mannopyranosyl chloride giving the C-2 epimers of compounds 84 and 86 in 72 and 12% yield, respectively.25... 

Carbohydrate analogs that have been produced by direct photobromination reactions are the nucleoside derivative l,A-dibenzoyl-2, 3, 5 -tri-0-benzoyl-4 -bromoadenosine38 (98) (compare Section II,3b) and the cyclohexane derivative (15,25,37 ,45,55,7/ )- l-acetoxy-2,3,4-tetra- >-benzoyl-oxy-7-bromo-6-oxabicyclo[3.2.1]octane (99) (compare Section II,4a). 

Flo. 1.—Representative compounds which, on photobromination, afford isolable products with bromine atoms in place of the identified hydrogen atoms, [(a) R1 H.OMe.OAc R2 = Me,Ac p anomers much more reactive same products are formed from C-5 epimers (b) R — Ac,Bz p anomers much more reactive same products are formed from C-5 epimers (c) mixed C-S epimeric bromides are formed (d) mixed 04 epimeric bromides are formed (e) R — Ac,Bz configurations at C-2—C4 may vary (f) X OJVOBz (g) react at 01 and 05 the a anomers react much less readily and give no products of C-l bromination (h) R1 — H,N02 R2 - Ac,Bz],... 

Given the presence of stable protecting groups on the hydroxyl functions of cyclic carbohydrate derivatives, photobrominations occur at the ether ring positions with facilities which depend upon the relative rates of hydrogen abstractions from the available sites67 (see Scheme 16). These, in turn,... 

The many carbohydrate compounds to have been examined (see Section II) clearly raise the interesting question of the combined effects of the ring-oxygen atoms and of the other substituents on radicals formed at C-l and at C-4 and C-5 for furanoid and pyranoid compounds, respectively. The several acetylated / -D-glucopyranosyl derivatives (100) to have been studied are particularly suitable compounds for examination in this regard, since, because of their near-symmetry, the effects of the anomeric substituent (X) in respect of its ability to facilitate photobromination at C-l can be compared closely with the effect of the substituent (CH2OAc) at C-5. 

Not enough data are available for the complete assessment of the chemical reactions undergone by the products of photobromination of carbohydrate derivatives, but the following observations permit an appreciation of the current state of knowledge of their chemistry and of the potential of these compounds as synthetic intermediates. 

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