REACTION OF 2,3,4,5,6-PENTABROMOBENZYL BROMIDE
1401
pentabrombenzyl bromide II, pentabromotoluene VI,
1,2-bis(pentabromophenyl)ethane (V), and two uni-
a thermoionic detector and a glass column, 3000
3 mm, packed with 3% of OV-17 on Chromaton
dentified compounds. Probably, the latter are combina- N-Super (0.16 0.20 mm); carrier gas nitrogen, oven
tion products of pentabromobenzyl and triphenyl-
pyranyl radicals (VII and VIII). The mother liquor
contained triphenylpyrylium bromide and acetone (the
latter is formed by dehydrogenation of 2-propanol
with pentabromobenzyl radical). Evaporation of the
mother liquor gave 2,4,6-triphenylpyrylium bromide
(IV).
temperature 250 C, injector temperature 300 C. Thin-
layer chromatography was performed on Silufol UV-
254 plates; spots were visualized under UV light or
by treatment with iodine vapor. 2,4,6-Triphenyl-
pyranyl dimer was synthesized by the procedure
described in [6].
Reaction of 2,4,6-triphenylpyranyl (I) with
2,3,4,5,6-pentabromobenzyl bromide (II) in 2-
propanol. A mixture of 2.6 g of 2,3,4,5,6-pentabromo-
benzyl bromide and 70 ml 2-propanol was purged
with carbon dioxide to remove oxygen, and 1.4 g of
2,4,6-triphenylpyranyl dimers was quickly added. The
mixture was heated for 2 h while continuously bubbl-
ing carbon dioxide and cooled, and the precipitate was
filtered off (2.7 g) and treated with boiling 1-butanol.
The residue was 1,2-bis(pentabromophenyl)ethane,
yield 0.4 g (9%), grayish crystals, mp 334 337 C.
According to the TLC data, the extract contained
2,3,4,5,6-pentabrombenzyl bromide and 2,3,4,5,6-
pentabromotoluene (Rf 0.60 and 0.78, respectively;
eluent petroleum ether) and two unidentified com-
pounds (Rf 0.54 and 0.26; benzene carbon tetrachlo-
ride, 1:1). Evaporation of the mother liquor gave
1.2 g (65%) of 2,4,6- triphenylpyrylium bromide, mp
238 242 C.
Quantitative analysis of the products revealed some
interesting features. The pyranyl radical was con-
sumed completely, while the yield of triphenylpyry-
lium bromide IV was only 65%. Presumably, the
remaining part of I reacted with the pentabromobenzyl
radical III formed by reaction (1), leading to com-
pounds VII and VIII. Insofar as pentabromobenzyl
radical III is fairly stable, apart from reaction (3), it
undergoes dimerization to 1,2-bis(pentabromophenyl)-
ethane [yield 15%; reaction (2)] and abstracts hydro-
gen from the solvent (2-propanol), thus being con-
verted into pentabromotoluene VI (yield 7%).
The reaction of 2,4,6-triphenylpyranyl (I) with
2,3,4,5,6-pentabromobenzyl bromide (II) occurred at a
considerably lower rate: the mixture lost its purple
color only after heating for 2 h. When the reaction
was complete, the precipitate of 2,4,6-triphenylpyry-
lium bromide (IV) was separated (yield 27.5%), and
the mother liquor contained unreacted benzyl bromide
(II), 2,3,4,5,6-pentabromotoluene (VI), and com-
pounds VII and VIII (according to the TLC data, the
latter were identical to those formed in 2-propanol;
Silufol plates, benzene carbon tetrachloride, 1:1).
The quite a low yield of pyrylium bromide (IV)
(27.5% of the theoretical amount) must be noted.
Assuming that compound IV can be formed only by
reaction (1), it becomes clear why the reaction mixture
contains a large amount of unreacted pentabromo-
benzyl bromide ( 72.5%). The yield of pentabromo-
toluene was about 6%, oer reacted pentabromobenzyl
bromide. No dimerization product of pentabromo-
benzyl radical [reaction (2)] was detected in toluene.
Reaction of 2,4,6-triphenylpyranyl (I) with
2,3,4,5,6-pentabromobenzyl bromide (II) in toluene.
A mixture of 1.8 g of 2,3,4,5,6-pentabromobenzyl
bromide and 24 ml of anhydrous toluene was purged
with carbon dioxide to remove oxygen, and 1.0 g of
2,4,6-triphenylpyranyl dimer was quickly added. The
mixture was heated for 4 h while continuously bubbl-
ing carbon dioxide and cooled, and the precipitate was
filtered off. It was 2,4,6-triphenylpyrylium bromide,
0.35 g (28%), mp 238 242 C. Evaporation of the
mother liquor gave 2.4 g of yellow oily crystals which,
according to the TLC data, were a mixture of 2,3,4,-
5,6-pentabromobenzyl bromide, 2,3,4,5,6-pentabromo-
toluene (Rf 0.60 and 0.78, respectively; eluent petro-
leum ether) and two unidentified compounds with Rf
0.54 and 0.26 (benzene carbon tetrachloride, 1:1).
Presumably, the observed difference in the be-
havior of 2,4,6-triphenylpyranyl radical toward penta-
bromobenzyl bromide in 2-propanol and toluene re-
sults from different polarities of these solvents. More
polar 2-propanol favors electron transfer from radical
I with formation of a more polar species, triphenyl-
pyrylium bromide, while the formation of a polar
species in nonpolar toluene is difficult.
REFERENCES
1. Tanaseichuk, B.S., Tomilin, O.B., and Butin, K.P., Zh.
Org. Khim., 1982, vol. 18, no. 2, p. 241.
2. Tanaseichuk, B.S., Rumyantseva, K.S., Vasin, V.A.,
Shishkin, V.N., Rumyantsev, N.P., Shishkin, S.N., and
Butin, K.P., Zh. Org. Khim., 1981, vol. 17, no. 6,
p. 1270.
EXPERIMENTAL
The reaction mixtures and products were analyzed
by GLC on a Chrom-41 chromatograph equipped with
3. Shishkin, V.N., Vakaeva, S.S., Belozerov, A.I., Tana-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 9 2004
Burtasov
