Oxidative Bromination of Alkenes
1503
with 2,3-dimethyl-2,3-dibromobutane. 2,3-Dimethyl-2,3-
dinitrobutane was detected as a small impurity only.
Analogous dinitro derivatives of 1-octene and 2-methyl-
propene were not detected. The detection of small amounts
of thermally unstable compounds, such as dinitro com-
pounds, by GC is a difficult problem. Meanwhile, the
formation of nitro compounds is very probable under the
bromination conditions [27]. Therefore, we suggested
conversion of NOx to the nitro compound to give rise to the
catalyst deactivation.
much lower intensity of the signals attributed to nitro
compounds even after repeated addition of HBr and a long-
time reaction with ethylene. No additional signals from
NOBr [21] or other possible intermediates were detected
during the oxidation of HBr in the absence of alkene. The
spectra recorded after the oxidation of HBr in the absence
of alkene indicated a complete conversion of nitrite to
nitrate. Although nitrate was used as the stoichiometric
oxidant or catalyst in oxidative halogenation at high tem-
perature and acidity of the medium [16, 18], NaNO3
showed no catalytic activity under our operating condi-
tions. That means nitrate is the second after nitro com-
pounds product of transformation of the NOx catalyst.
Deactivation of the catalysts owing to formation of the
nitro compounds proceeded readily in reaction with
nucleophilic alkenes, whereas oxidation of the catalyst was
facilitated by the lack of reactivity or absence of alkene. As
shown, formation of the nitro compounds can be avoided if
conducting the bromination in the stepwise mode (Table 3,
entry 7). On the other hand, deactivation does not create an
essential limitation for application of NOx catalyst. The
conversion of nitrite finally into nitrate can be regarded as a
positive process, because it prevents contacting with vol-
atile nitrogen oxides under processing the reaction
solutions.
A complete conversion of 2,3-dimethyl-2-butene was
achieved when the reaction was performed in the stepwise
mode. 2,3-Dimethyl-2,3-dibromobutane was obtained with
admixtures of tetramethyloxirane (Table 3, entry 6) and
unidentified compound with higher molecular mass. The
first one formed through conversion of alkene to bro-
mohydrin and then to epoxide, as suggested in paper [28].
The bromohydroxy compounds usually appeared in neutral
solutions, which is consistent with nearly complete con-
version of HBr (entry 7). The results obtained in bromin-
ation of 2,3-dimethyl-2-butene have shown that the
stepwise procedure is preferable in the case of strongly
nucleophilic alkenes.
3.3 14N NMR Analysis of the Catalyst Transformations
The 14N NMR spectrum of the NaNO2 solution without
3.4 Comparative Properties of BMImBr, BMImBF4
and HMImBr as Solvents for the HBr–NaNO2–O2
Brominating System
HBr showed the broad peaks of BMImBr (-197 ppm) and
-
NO2 anions (236 ppm) (Fig. 3, spectrum a). Dilution of
the solution with water decreased the viscosity; that
resulted in appearance of two narrow peaks from non-
equivalent nitrogen atoms of the BMIm? cation at -191
and -205 ppm (spectrum b). 2 min after addition of HBr
and starting the reaction with cyclohexene in an O2/
The brominating systems based on different ionic liquids
were tested in three cycles when equal portions of HBr
were added at the beginning of the process and after con-
suming the C2H4/O2 gas volume, which corresponded to
nearly complete conversion of the previous portion of HBr.
In the first cycle, the initial rates and durations of the
bromination were close in different imidazolium ionic
liquid solutions, but deactivation of the catalytic system
with each new portion of HBr proceeded more intensively
in HMImBr solution (Table 4). Similar to BMImBr,
HMImBr provided selectivity to 1,2-dibromoethane of over
98 % in the first cycle, and a small loss in the selectivity in
subsequent cycles. In BMImBF4 solution, the selectivity to
1,2-dibromoethane was lower due to increasing the portion
of 2-bromoethanol to 20 % in the first cycle, 22 % in the
third cycle, and 23 % in the stepwise bromination
(Table 4). As reported by Chiappe et al. [29], the bro-
mination of alkenes with Br2 in BMImBr and BMImBF4
ionic liquids produced dibromide, whereas two products—
bromohydrin and dibromide—were observed by Conte
et al. [23] in the oxidative bromination of styrene in water-
containing solutions. In the brominating systems tested
here, water content of 16–28 wt% did cause the appearance
-
Ar = 1/2 atmosphere, at 40 °C the signal of the NO2
anion disappered. Paramagnetic molecules NO and NO2,
which formed in the presence of acid, were not observed.
At the same time, low-intensity peaks appeared in the
region of 20 to -40 ppm, the intensity of which increased
by the end of the bromination (in 1 h, spectra c and d). The
narrow peak at about 0 ppm coincided with the peak of
NaNO3 added to the solution, therefore, it was attributed to
NO3- anions. The broad peaks appeared in the presence of
cyclohexene, but were absent when oxidation of HBr
(reaction (2)) proceeded without alkene. Therefore, the
broad peaks were tentatively assigned to the nitro com-
pounds formed from cyclohexene. If the peaks observed in
spectrum d, Fig. 3 were normalized to intensity of the
signal of BMImBr, ca. 30 % of nitrogen initially intro-
duced as NaNO2 was observed in the spectrum. The
remaining nitrogen was probably in the form of undetect-
able NO and NO2 molecules. Similar spectra of solution
were observed under bromination of ethylene, except for
123