J. Y. Jeon, J. K. Varghese, J. H. Park, S.-H. Lee, B. Y. Lee
SHORT COMMUNICATION
are off-white-colored solids and the melting points are in
agreement with the reported values. When acetic acid is
used instead of nitric acid, conversion of bromide anions
into bromomethane is accelerated, but the generated di-
methyl phosphate anion is not protonated by acetic acid.
Complete evacuation after the reaction eliminates the added
acetic acid to give the pure tetrabutylammonium salt of the
dimethyl phosphate anion {[Bu4N]+[(MeO)2P(O)O]–} in
high yield (96%). Tetrabutylphosphonium bromide is also
converted into tetrabutylphosphonium salts of various
anions without any problems under the same procedure and
conditions (Table 1, Entries 12–14).
Conclusions
The halide anion in a quaternary onium salt attacks tri-
methyl phosphate in the presence of an equivalent amount
of strong acid under neat and mild conditions (60 °C, 15 h).
Through this reaction, the halide anion in the quaternary
onium salt can be efficiently exchanged with various anions
–
–
–
–
–
–
[NO3 , BF4 , PF6 , CF3SO3 , CH3SO3 , ClO4 , p-
–
–
CH3C6H4SO3 , CF3CO2 , 2,4-(NO2)2C6H3O–]. This proto-
col can be applied to large-scale syntheses, allowing for eco-
nomical preparation of key intermediates for a highly active
catalyst for CO2/epoxide copolymerizations.
Experimental Section
Typical Procedure for Conversion of Q+X– into Q+A– (Small Scale):
HA (3.10 mmol) was slowly added to a flask containing trimethyl
phosphate (5.21 g, 37.2 mmol) at 0 °C. Q+X– (3.10 mmol) was
added to this solution with stirring. After the mixture was stirred
at 60 °C for 15 h under weak N2 bubbling, the excess amount of
trimethyl phosphate was removed by vacuum distillation (37 °C/
0.07 Torr). The residue was dissolved in dichloromethane (5 mL)
and 10% aqueous NH4OH solution (3.10 mmol, ≈4 mL) was added
dropwise to make the solution neutral. NaA (3.10 mmol) was dis-
solved in the water phase. The two phases were stirred at room
temperature for 2 h. The organic phase was collected, and the aque-
ous layer was extracted with an additional amount of dichloro-
methane (2 mL). The organic phases were combined, and the sol-
vent was removed with a rotary evaporator. A small amount of
residual trimethyl phosphate was completely removed through
evacuation at 100 °C for 2 h.
Table 1. Conversion of quaternary onium halides into their onium
salts of various anions.[a]
Entry
Q+
X–
A–
Yield
[%]
M.p.[b]
[°C]
–
–
–
1
2
3
4
5
6
7
8
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4N+
Bu4P+
Bu4P+
Bu4P+
bbim+[e]
bbim+
bbim+
I–
NO3
NO3
NO3
BF4
PF6
96
117–118 (116–118)
Br–
Cl–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
Br–
95 (98.5[c]) 117–118 (116–118)
90
90
96
93
90
95
95
92
91
90
94
96
92
96
91
117–118 (116–118)
161–162 (155–161)
246–247 (244–246)
111–112 (112–113)
76–77 (70–80)
214–215 (211–215)
70–71 (70–72)
57–58
101–103
71–72
95–98 (96–99)
107–108
–
–
–
–
CF3SO3
CH3SO3
–
ClO4
9
pTsO–
–
10
11
12
13
14
15
16
17
CF3CO2
–
Conversion of Bu4N+Br– into Bu4N+NO3 (30-g Scale): Nitric acid
DNP–[d]
–
NO3
(69%, 8.50 g, 93.1 mmol) was slowly added to a flask containing
trimethyl phosphate (30.0 g, 214 mmol) at 0 °C. Bu4N+Br– (30.0 g,
93.1 mmol) was added to this solution with stirring. After the mix-
ture was stirred at 60 °C for 15 h under weak N2 bubbling, the
resulting mixture was dissolved in dichloromethane (150 mL) and
10% aqueous NH4OH solution (93.1 mmol, ≈120 mL) was added
dropwise to make the solution neutral. NaNO3 (7.91 g, 93.1 mmol)
was dissolved in the water phase. The two phases were stirred at
room temperature for 2 h. The organic phase was collected, and
the solvent was removed with a rotary evaporator. A small amount
of residual trimethyl phosphate was completely removed through
evacuation at 100 °C for 2 h. The product was an off-white solid
and the yield was 97.4% (27.6 g).
–
BF4
–
CF3SO3
–
NO3
liquid
liquid
liquid
–
BF4
–
CF3SO3
[a] Reaction conditions: Q+X– (3.10 mmol), HA (3.10 mmol), tri-
methyl phosphate (37 mmol), 60 °C, 15 h. [b] Values in parentheses
are the ones reported in the Aldrich catalogue. [c] Yield for 30-g
scale synthesis. [d] 2,4-Dinitrophenolate. [e] 1,3-Dibutylimid-
azolium.
When 1-butyl-3-methylimidazolium bromide ([bmim]+-
Br–) is treated with trimethyl phosphate in the presence of
nitric acid, the bromide anions are completely converted
Conversion of 1 into 2: Nitric acid (69%, 2.10 g, 23.0 mmol) was
slowly added to a flask containing trimethyl phosphate (10.0 g,
71.4 mmol) at 0 °C. {3-Methyl-5-[{Bu3N+(CH2)3}2CMe]salicyl-
aldehyde}(I–)2 (1; 10.0 g, 11.5 mmol) was added to this solution
with stirring. After the mixture was stirred at 60 °C for 20 h under
weak N2 bubbling, the resulting oily mixture was dissolved in
dichloromethane (50 mL) and 10% aqueous NH4OH solution
(23.0 mmol, ≈30 mL) was added dropwise to make the solution
neutral. NaNO3 (1.95 g, 23.0 mmol) was dissolved in the water
phase. The two phases were stirred at room temperature for 2 h.
The organic phase was collected, and the solvent was removed with
a rotary evaporator to obtain a yellowish oil. A small amount of
residual trimethyl phosphate was completely removed through
–
into volatile bromomethane, but the product [bmim]+NO3
is highly soluble in water, hampering its isolation through
extraction with dichloromethane. The byproduct, dimethyl
phosphoric acid, cannot be completely removed by evacua-
tion even at a temperature as high as 120 °C. However,
hydrophobic 1,3-dibutylimidazolium ([bbim]+) salts of vari-
ous anions can be prepared from [bbim]+Br– according to
this protocol with high yields (Ͼ90%; Table 1, Entries 15–
17). For other hydrophilic quaternary ammonium halides
such as N-butylpyridium bromide or tetraethylammonium
bromide, the reaction with trimethyl phosphate in the pres- evacuation at 100 °C for 2 h. The yield was 97% (8.50 g).
ence of nitric acid is successful, but isolation of the nitrate
salt is impossible because of the high solubility of the prod-
uct in water.
Supporting Information (see footnote on the first page of this arti-
cle): Copies of the NMR spectra of new compounds and elemental
analysis data.
3568
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Eur. J. Org. Chem. 2012, 3566–3569