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R. Ghorbani-Vaghei et al. / Tetrahedron Letters 50 (2009) 1861–1865
Br
N
Br
Br
N
Br
N
Br
Br
N
n
N
Br
CH
Br
N
2 n
S
H2C
S
S
S
S
S
O
O
O
O
O
O
O
O
O
O
O
O
PBBS
TBBDA
PBPS
Figure 1.
CH3
250 MHz): d 8.21–7.55 (br, aromatic), 7.15 (s, NH); Anal. Calcd for
[C12H10N2O4S2: (310.22)n]: C, 46.46; H, 3.22; N, 9.03; S, 20.67.
Found: C, 46.18; H, 3.47; N, 8.34; S, 19.62.
CH2Br
TBBDA or PBBS or PBPS
(PhCO2)2, EtOAc, reflux
2.2. Poly(N,N0-dibromo-N-phenyl-benzene-1,3-disulfonamide)
I
II
Poly(N-phenyl-benzene-1,3-disulfonamide) (1.00 g) was dis-
solved in chilled aq NaOH (2 mL, 3 M) at rt and the solution was
transferred to a beaker. Br2 (58.4 mmol) was added to the solution
with vigorous stirring and a precipitate formed immediately. The
product was collected by vacuum filtration, washed with distilled
coldwater (30 ml), andthendriedina vacuumdesiccatoratrt for5 h.
Scheme 1.
ious alkyl benzenes were cleanly converted to the corresponding
products with good to excellent yields (Table 1, compounds a–n).
In Table 1 (compound b), we showed that the benzylic bromin-
ation occurred in the benzylic position. From the peak of 5.08 ppm
for brominated ethyl benzene it is obvious that there is no dibro-
mination observed under this procedure. Also, Table 1 shows ben-
zylic bromination of the 3-methylbenzaldehyde (compound i)
without over oxidation.
In this reaction, we observed that by control of the reaction
time, the bromination of p-xylene afforded mainly a product that
is preferentially brominated at only one of the methyl groups.
The major products were monobrominated with the best selectiv-
ity in 73% yields (Table 1, compound c). On prolonging the reaction
time, we obtained two methyl groups that were monobrominated
(Table 1, compound d). Similar results were obtained for com-
pounds f and h listed in Table 1.
It seems interesting that 4-methylacetophenone (Table 1, com-
pound l) was exclusively brominated by a free radical process at
the benzyl position leading to benzyl bromide.
We used two solvents in this work, non-chlorinated and chlori-
nated solvents. We found (Table 1) that two solvents afforded
approximately equal results. Therefore, we selected ethyl acetate
as a green solvent instead of carbon tetrachloride because carbon
tetrachloride is toxic and is not to be used as a green solvent due
its effects on the environment.
Yield: (1.2 g, 80%), mp: 180 °C; IR (KBr):
m 1609 (aromatic),
1380 and 1172 (SO2) cmꢀ1 1H NMR (DMSO-d6, FT-250 MHz): d
;
8.20–7.50 (br, aromatic); 13C NMR (DMSO-d6, FT-400 MHz): d
147.25, 140.70, 138.51, 133.28, 131.63, 131.12, 130.34, 129.52,
126.44, 123.81, 123.38, 119.03. Anal. Calcd for [C12H8N2O4S2Br2:
(470.02)n]: C, 30.66; H, 1.70; N, 5.96; S, 13.64. Found: C, 31.10;
H, 1.17; N, 5.50; S, 13.73.
2.3. Typical procedure: benzylic bromination of toluene with
TBBDA
N,N,N0,N0-Tetrabromobenzene-1,3-disulfonamide (0.30 g, 0.54
mmol), benzoyl peroxide (0.1 g, 0.413 mmol), and EtOAc (10 mL)
were placed in a 25 mL flask. The flask was cooled in ice-water
and toluene (2 mmol) was added dropwise with stirring. The mix-
ture was refluxed for 90 min. The reaction was monitored by TLC
(n-hexane/acetone, 9:1). Then, the flask was cooled, the sulfon-
amide was filtered off under vacuum filtration, solvent was
removed under reduced pressure, and benzyl bromide was col-
lected as a single product; yield: 93%. For less volatile compounds,
thin layer chromatography (4:1, n-hexane/acetone) was used for
purification.
The advantages of our reagents over common reagents in the
benzylic bromination of toluene are listed in Table 2.
2.4. Typical procedure: benzylic bromination of toluene with
PBBS
2. Experimental
Poly(N,N0-dibromo-N-ethyl-benzene-1,3-disulfonamide) (0.50 g),
benzoyl peroxide (0.1 g, 0.413 mmol), and EtOAc (10 mL) were
placed in a 25 mL flask. The flask was cooled in ice-water, and tolu-
ene (2 mmol) was added dropwise with stirring. The mixture was
refluxed for 120 min. The reaction was monitored by TLC (n-hex-
ane/acetone, 9:1). Then, the flask was cooled, the polysulfonamide
was removed by vacuum filtration, solvent was removed under re-
duced pressure, and benzyl bromide was collected as a single prod-
uct; yield: 90%. For less volatile compounds, thin layer
chromatography (4:1, n-hexane/acetone) was used for purification.
2.1. Poly(N-phenyl-benzene-1,3-disulfonamide)
PCl5 (16.5 mmol), as chlorination agent, was added to the sodium
salt of benzene-1,3-disulfonic acid (5.00 g, 18 mmol). To initiate the
reaction, the vessel was heated (40–50 °C), then the reaction contin-
ued autonomously. After complete conversion (2 h), crushed ice
(100 g) and CHCl3 (100 mL) were added. Removal of the solvent un-
der reduced pressure gave benzene-1,3-disulfonylchloride (4.2 g
84%), which was placed in a beaker. The beaker was heated on an
oil bath (80 °C) until the benzene-1,3-disulfonylchloride liquefied.
The molten 1,3-phenylenediamine (9 mmol, 0.972 g) was added
dropwise to the reactant, and the mixture was stirred with a glass
rod. The mixture was heated (80 °C) and stirred for 20 min and then
cooled, and water (50 ml) was added. The product was collected by
vacuum filtration and washed with a little cold water (10 mL).
Recrystallization from acetone gave the title compound.
2.5. Typical procedure: benzylic bromination of toluene with
PBPS
Poly(N,N0-dibromo-N-phenyl-benzene-1,3-disulfonamide) (0.50 g),
benzoyl peroxide (0.1 g, 0.413 mmol), and EtOAc (10 mL) were
placed in a 25 mL flask. The flask was cooled in ice-water, and tol-
uene (2 mmol) was added dropwise with stirring. The mixture was
Yield: (3.2 g, 85%), mp: 200 °C; IR (KBr):
m 3419 (NH), 1609 (aro-
matic), 1385 and 1170 (SO2) cmꢀ1 1H NMR (DMSO-d6, FT-
;