COMMUNICATION
mum temperature and decided to vary the solvent and
metal salt. NaVO3 showed a similar yield compared with the
ammonium counterpart (Table 1, entry 5). [VOACTHNURGTNEUNG(acac)2] and
VOSO4 gave similar NMR yields to NH4VO3 (Table 1, en-
tries 7 and 8). WO3, known to catalyze the oxidative bromi-
nation with H2O2,[11] did not function as a catalyst (Table 1,
NH4VO3 and 200 mol% PTS·H2O and Bu4NBr in acetoni-
trile (method C). The bromination reaction of a-methylstyr-
ene gave 32% of the dibromide and 62% of the bromohy-
drin regioselectively (Table 2, entry 4). On the other hand,
using 5 mol% NH4VO3, 900 mol% Bu4NBr, and 900 mol%
TFA in 1,4-dioxane at 808C (method B), only the bromohy-
drin was produced in 60% yield (Table 2, entry 3). In the
case of trans-b-methylstyrene, the ketone was obtained in
addition to the erythro-dibromide and bromohydrin
(Table 2, entry 5).[12] The bromination reaction of cis-b-
methylstyrene led to the threo-dibromide together with
small amounts of the erythro isomer, bromohydrin, and
ketone (Table 2, entry 6). Starting from the styrene deriva-
tives, the regioisomeric bromohydrins and ketones were not
formed. Dibromides were produced in the reaction of ali-
phatic alkenes by using method B. Allylbenzene and 1-
decene were converted into the dibromides in 94% and
entry 9). The oxo-metal complexes including [TiO
ACHTUNGTNER(NUNG acac)2],
[MoO2A(acac)2], and MnO2 displayed no promising results
CHTUNGTRENNUNG
(Table 1, entries 10–12). Acetonitrile, dimethylformamide,
acetic acid, and dimethoxyethane were found to be less ef-
fective than 1,4-dioxane (Table 1, entries 13–16). Among the
various conditions examined, the reaction with 5 mol%
NH4VO3, Bu4NBr (300 mol%), and TFA (300 mol%) in
1,4-dioxane at 808C was superior (method A).
To explore the validity of this method, a variety of arenes,
alkenes, and alkynes were examined in the bromination re-
action. Some selected results are listed in Table 2. The bro-
97%
yields,
respectively
(Table 2, entries 7 and 8). In the
case of 5-hexene-1-ol, the di-
bromide was obtained, with the
hydroxy group intact, giving a
94% yield (Table 2, entry 9).
Moreover, the present catalytic
system could be applied to the
bromination of alkynes. The
bromination reaction of 1-phe-
nylpropyne and 1,4-dimethoxy-
2-butyne provided only the
Table 2. Bromination reaction of arenes, alkenes, and alkynes.
Entry
Substrate
Method[a]
Products, yield of isolated product [%]
1
A
73
2
A
72
3
B
C
0
33
60
62
4[b]
5[b]
C
C
59
18
17
15
11
trans-1,2-dibromoalkenes
in
98% and 77% yields, respec-
tively (Table 2, entries 10 and
11).
6[b]
51[c]
It should be noted that the
amount of the catalyst could be
7
8
9
B
B
B
94
successfully
reduced
to
97
94
1 mol%, although a longer re-
action time was required
(Table 3, entry 1). Futhermore,
the bromination reaction pro-
ceeded well, even under air, in
the presence of 600 mol%
Bu4NBr and TFA to give 70%
of the bromination compound
(Table 3, entry 2). The bromina-
tion depended on the amounts
of the bromide source and acid.
Use of 900 mol% of the bro-
mide source and acid led to the
dibromide in 70% yield with
10
11
B
B
94
77
[a] Method A: 0.4 mmol substrate, 5 mol% NH4VO3, 300 mol% Bu4NBr, 300 mol% TFA, 1.5 mL 1,4-dioxane,
under atmospheric oxygen, 808C, 18 h. Method B: 0.4 mmol substrate, 5 mol% NH4VO3, 900 mol% Bu4NBr,
900 mol% TFA, 1.5 mL 1,4-dioxane, under atmospheric oxygen, 808C, 12 h. Method C: 0.4 mmol substrate,
1 mol% NH4VO3, 200 mol% Bu4NBr, 200 mol% PTS·H2O, 1.5 mL acetonitrile, under atmospheric oxygen,
room temperature, 6 h. [b] NMR yield. [c] Together with the erythro isomer (9%).
mination reaction of the phenol derivatives, such as 2,6-xyl-
enol and resorcinol, using method A gave the monobromi-
nated compounds regioselectively in similar yields (Table 2,
entries 1 and 2). This bromination reaction could also be ap-
plied to the bromination of alkenes. In the case of styrene,
the reaction proceeded smoothly at high conversion rates,
even at room temperature, in the presence of 1 mol%
concomitant formation of the monobromide in 28% yield
(Table 3, entry 3). When a catalytic amount of TFA was
used, the bromination yield was reduced significantly
(Table 3, entry 4). Moreover, this bromination was not effec-
tively performed under argon (Table 3, entry 5), although
the desired compound was obtained quantitatively by using
of excess amounts of NH4VO3, Bu4NBr, and TFA, even
1214
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 1213 – 1216