Alkylation of ammonium salts catalyzed by imidazolium-based ionic liquid catalysts

Article abstract of DOI:10.1002/adsc.200600451

Quaternary ammonium salts were synthesized from ammonium salts and dialkyl carbonates over imidazolium ionic liquid catalysts. The reaction gave almost stoichiometric amounts of the quaternary ammonium salts for halides and nitrates. It was found that the electron-donating property of the alkyl moieties of ammonium cations, the electrophilic nature of the alkyl group of the carbonate, the acidity of the acid that the anion of the ammonium salt corresponds to, and the steric hindrance of the ammonium salts and the dialkyl carbonates are the key factors that influence the yields of quaternary ammonium salts. Strong electron-donating alkyl groups on the nitrogen atom of the ammonium salt, electron-withdrawing groups on the methylene carbon of dialkyl carbonate, and weaker steric hindrance of the starting ammonium salts and dialkyl carbonates favor the alkylation reaction of ammonium salts.

Full text of DOI:10.1002/adsc.200600451

FULL PAPERS
DOI: 10.1002/adsc.200600451
Alkylation of Ammonium Salts Catalyzed by Imidazolium-Based
Ionic Liquid Catalysts
Zhuo Qun Zheng,a Jie Wang,a Ting Hua Wu,a,* and Xiao Ping Zhoua,b,*
a
Institute of Physical Chemistry, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal
University, Jinhua 321004, Peopleꢀs Republic of China
Fax : (+86)-579-228-2595; e-mail: thwu@zjnu.cn
Department of Chemical Engineering, Hunan University, Changsha 410082, Peopleꢀs Republic of China
b
Fax : (+86)-731-882-1017; e-mail: hgx2002@hnu.cn
Received: September 2, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Quaternary ammonium salts were synthe- key factors that influence the yields of quaternary
sized from ammonium salts and dialkyl carbonates ammonium salts. Strong electron-donating alkyl
over imidazolium ionic liquid catalysts. The reaction groups on the nitrogen atom of the ammonium salt,
gave almost stoichiometric amounts of the quaterna- electron-withdrawing groups on the methylene
ry ammonium salts for halides and nitrates. It was carbon of dialkyl carbonate, and weaker steric hin-
found that the electron-donating property of the drance of the starting ammonium salts and dialkyl
alkyl moieties of ammonium cations, the electrophil- carbonates favor the alkylation reaction of ammoni-
ic nature of the alkyl group of the carbonate, the um salts.
acidity of the acid that the anion of the ammonium
salt corresponds to, and the steric hindrance of the Keywords: alkylation; ammonium salts; dialkyl car-
ammonium salts and the dialkyl carbonates are the bonates; ionic liquid; quaternary ammonium salts
Introduction
by reacting tertiary amines with alkyl halides. Wang
et al.[24] reported a facile method to synthesize quater-
Ionic liquids (ILs) or room-temperature molten salts nary ammonium salts by the direct reaction of amines
have attracted a lot of attention of chemists in recent with alcohols over acidic zeolites, such as H-Y and
years.[1–21] Representing a new class of non-molecular HZSM-5. When synthesizing quaternary ammonium
ionic compounds, ionic liquids were extensively inves- salts by these methods, the anions of the quaternary
tigated as reaction media (solvents),[14] phase-transfer ammonium salts are usually limited to halides.
catalysts,[17] surfactants,[18] molecular probes and ionic Among them, quaternary ammonium chlorides and
channel blockers,[19] synthesis templates,[20] and IL quaternary ammonium bromides were the most com-
crystal applications.[21] The reason why ionic liquids monly reported. In the case of synthesizing quaterna-
attract the increasing attention of chemists is that ry ammonium salts with anions other than halides, an
they have found potential applications in chemical re- anion-exchange process is necessary.[25–27] In order to
actions as solvents and/or catalysts. 1-Ethyl-3-methyl- avoid this problem, Mori et al.[27] patented a process
imidazolium bromide ([EMIM]Br), which can be for their synthesis by reacting a tertiary amine with a
readily prepared from 1-methylimidazole and ethyl dialkyl carbonate to obtain a quaternary ammonium
bromide,[22] has found applications as reaction media methyl carbonate, and then reacting the quaternary
and/or catalysts for organic reactions. Hence, in the ammonium methyl carbonate with the corresponding
current study, we explore the possibility of using acid to obtain the quaternary ammonium salt with the
[EMIM]Br as a catalyst in the synthesis of quaternary right anion. Compared to the anion-exchange process
ammonium salts.
(from quaternary ammonium halides), Mori et al.ꢀs
Since quaternary ammonium salts are widely used method is more efficient.
as surfactants, phase-transfer catalysts, ionic liquid
In the current work, various quaternary ammonium
solvents, and liquid-crystalline amphiphiles,[21,23] there salts were synthesized from ammonium salts and di-
are many investigations devoted to their synthesis. alkyl carbonates in imidazolium ionic liquids. On
Quaternary ammonium salts are generally synthesized comparing the current method with the previously re-
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Table 2. Reactions of dimethyl carbonate with ammonium salts
ported ones, it is remarkable with respect to the use
of ammonium salts and dialkyl carbonates[28,29] as feed
stocks, which are less volatile and toxic than amines,
amides, alkyl halides, and alkyl sulfates. Although
there have also been reports to synthesize quaternary
ammonium salts by employing dialkyl carbonates as
alkylation reagents,[27,30] these methods are limited to
those cases where tertiary amines are used as reac-
tants. In the current studies, all kinds of salts of
amines, such as tertiary, primary, secondary amines,
and even ammonia can be alkylated by dialkyl carbo-
nates to furnish quaternary ammonium salts.
bearing different anions over [EMIM]Br catalyst.[a]
Entry Reactants
Product
Yield
[%]
1
NH4Cl/DMC
NH4Br/DMC
NH4NO3/DMC
96.0
96.5
96.4
70.4
47.6
44.3
44.8
43.1
>99
>99
>99
97.4
>99
98.3
54.7
73.2
78.2
48.6
>99
96.9
98.2
62.2
2
3
4
ACHTREUNG
Results and Discussion
5
ACHTREUNG
The alkylation results of ammonium bromide with di-
methyl carbonate (DMC) as alkylation reagent are
listed in Table 1. When the reaction was carried out in
the absence of catalyst, only a trace of tetramethylam-
monium bromide (yield: 0.4%) was formed. Among
the tested catalysts, an almost quantitative yield
(96.5%) of tetramethylammonium bromide was ob-
tained over 1-ethyl-3-methylimidazolium ([EMIM])
6
7
ACHTREUNG
8
ACHTREUNG
bromide.
When
1-butyl-3-methylimidazolium
9
ACHTREUNG
([BMIM])
bromide,
[EMIM]CH3CO2,
and
[BMIM]CH3CO2 were used as catalysts, the yields de-
creased to 84.7%, 72.5%, and 74.3%, respectively.
Even lower yields were obtained over [EMIM]BF4
and [EMIM]2SO4. Obviously, the alkylation reaction
of ammonium salts is accelerated by the IL catalysts,
but the catalytic activity is strongly dependent upon
the nature of the imidazolium cations and the coun-
teranions of the imidazolium ion. In the cases of imi-
dazolium bromides, the more bulky n-butyl-substitut-
ed imidazolium ring is relatively less active to the al-
kylation reaction of ammonium salts than the ethyl-
substituted ring.
10
11
12
13
14
15
16
17
18
19
20
21
22
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
When ammonium salts having different N-substitu-
ent groups and different anions are reacted with
DMC in the presence of [EMIM]Br catalyst, the re-
sults listed in Table 2 are obtained. In those cases
DMC
AHCTREUNG
AHCTREUNG
Table 1. Reactions of ammonium bromide with dimethyl
carbonate over different imidazolium ionic liquid catalysts.[a]
AHCTRE[NUG (CH2CH3)2H2N] PO4/
3
Entry
Reactants
Catalyst
Yield [%]
DMC
[(CH2CH3)3HN]Cl/DMC
[(CH2CH3)3HN]Br/DMC
1
2
3
4
5
6
7
8
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
NH4Br/DMC
N
96.5
84.7
72.5
74.3
41.6
52.4
11.3
0.4
AHCTREUNG
AHCTREUNG
AHCTREUNG
DMC
AHCTREUNG
[a]
Reaction conditions: ammonium bromide (2.0 mmol),
DMC (8.0 mmol), catalyst (0.8 mmol), 443 K, 8 h.
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Alkylation of Ammonium Salts
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À
where an ammonium salt bearing ClÀ, BrÀ, or NO3
Table 2. (Continued)
anions was alkylated by DMC, a yield in excess of
96.0% of the quaternary ammonium salt was ob-
tained. However, in the cases of DMC reacting with
the sulfates, oxalates, acetates, and phosphates of am-
monia, diethylamine, or triethylamine (Table 2 en-
tries 4 to 8, 15 to 18, 22 to 23), relatively lower yields
of the quaternary ammonium salts were obtained. For
even bulkier alkyl-substituted ammonium salts, such
as those with n-C12H25 and cyclohexyl substituents
(Table 2, entries 27 to 29 and 33 to 37) even when the
Entry Reactants
Product
Yield
[%]
ACHTREUNG
23
24
25
26
27
28
29
73.3
>99
>99
>99
97.3
84.4
88.7
A
3ACHTREUNG
R
À
A
N
anions were SO42À, CH3CO2 , C2O42À, and PO43À, the
reaction still gave high yields of the quaternary am-
monium salts (from 84.4 to 99.0%). Except for the
cases of tertiary ammonium salts, carbamates were
formed as by-products in our reactions. In Deng
et al.ꢀs work[31] (also at 443 K), by employing imidazo-
lium ILs as solvents and catalysts, primary and secon-
dary aliphatic amines could react with DMC to give
alkyl carbamates in good yields. What is worth men-
tion is that in the current work, the methylation of
ammonium halides and nitrates by DMC gave almost
stoichiometric amounts of quaternary ammonium
salts.
A
CHTREUNG
DMC
A
CHTREUNG
DMC
A
CHTREUNG
DMC
30
31
32
33
34
35
36
37
38
>99
>99
>99
>99
92.3
95.7
91.5
95.8
Comparing the yields of quaternary ammonium salt
obtained from the reactions of DMC with
(NH4)2C2O4, NH4CH3CO2, or (NH4)3PO4 to those ob-
tained from the reactions of DMC with the alkyl-sub-
stituted ammonium oxalates, acetates, or phosphates
(Table 2 entries 5, 6, 8, 16, 17, 18, 23, 28, 29, 34, 35
and 37), it was found that the substituted ammonium
salts gave higher yields of quaternary ammonium salt
+
yields than the simple NH4 salts. This might be at-
tributed to the fact that alkyl-substituted amines are
better nucleophilic attacking reagents than ammonia.
It also explains the higher yields in the cases of am-
monium salts bearing stronger donor groups, such as
n-C12H25 and cyclohexyl. On the other hand, the am-
monium salt could be deactivated by substitution with
electron-withdrawing group. For instance, aniline
chloride (Table 2 entry 38) cannot be alkylated by
DMC. Based on these results, we suggest that the al-
kylation of ammonium salts is realized through nucle-
ophilic attack of the ammonium nitrogen on the
methyl group of DMC. Among all of the tested am-
monium salts, the behavior of sulfates is intricate. Re-
search work in this aspect is still in progress.
0
Based on the results of the current investigation,
there is no doubt that [EMIM]Br is an active catalyst
for the alkylation of ammonium salts, although in
some cases the product yields are still lower than
60%. Nevertheless, since that the catalyst bears
BrÀanions, these could contaminate the final quater-
nary ammonium salt products. There are two ap-
proaches to resolve this problem. One approach is to
use the traditional anion-exchange resin to clear out
BrÀanions (since BrÀanions in our cases are impurities
39
>99
[a]
Reaction conditions: entries 1–8, ammonium cations
(2.0 mmol), entries 9–39, hydrogen atoms on the nitrogen
atom of each kind of the ammonium salts (4.0 mmol), DMC
(8.0 mmol), [EMIM]Br (0.8 mmol), 443 K, 8 h.
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introduced from the relatively small amount of cata- catalyst. However, for some reactions, such as the
lyst, the current synthesis pathway still shows advan- cases of quaternary ammonium sulfates and acetates,
tages over the traditional methods that need to use an the yields were low. Further research work for the op-
anion-exchange resin to remove a large amount of timization of the reaction conditions to improve prod-
halogen anions). The other approach is to use the imi- uct yield is needed.
dazolium catalyst bearing the same anion as that of
In order to explore the impact of the higher homo-
the final quaternary ammonium salt. In order to test logues of DMC on the reaction, diethyl carbonate, di-
the second approach, some the experiments were per- benzyl carbonate, and diphenyl carbonate were tested
formed and the results are presented in Table 3. as the alkylation or arylation reagents for ammonium
These investigations confirmed that it is possible to salts. As shown in Table 4, the reactivity of the carbo-
avoid the anion contamination introduced from the nates decreases in the order dimethyl carbonate
Table 3. Reactions of dimethyl carbonate with ammonium salts having the same anions as that of catalysts.[a]
Entry
Ammonium salt
Catalyst
Product
Yield [%]
1
A
>99
2
3
4
C
(n-C12H25)]Br
[EMIM]Br
>99
21.6
52.6
59.8
5
[a]
Reaction conditions: the hydrogen atoms on nitrogen atom of each kind of ammonium salt (4.0 mmol), DMC (8.0 mmol),
catalyst (0.8 mmol), 443 K, 8 h. [EIM] refers to 1-ethyl-3-hydrogen imidazolium cation.
Table 4. Reactions of ammonium salts with dialkyl carbonates substituted by different groups.[a]
Entry
Ammonium salt
Carbonate
Product
Yield (%)
71.3
1
AHCTRE[NUG (CH3)3HN]Cl
DBC
2
3
C
DBC
DBC
72.0
90.1
4
5
C
DEC
DEC
42.9
53.6
6
A
DPC
0
[a]
Reaction conditions: the hydrogen atoms on nitrogen atom of each kind of ammonium salts (4.0 mmol), dialkyl carbonate
(8.0 mmol, for DBC 4.0 mmol), [EMIM]Br (0.8 mmol), 443 K, 8 h. DBC denotes dibenzyl carbonate, DEC denotes dieth-
yl carbonate and DPC denotes diphenyl carbonate.
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(DMC)>dibenzyl carbonate (DBC)>diethyl carbon- um salts, such as those listed in Table 2. It becomes
ate (DEC)>diphenyl carbonate (DPC). We predict complicated when one applies this mechanism to ex-
that there are two possible reasons that lead to this plain the behavior of the anions of ammonium salts.
result. One is the stability of carbon cations, the other In the case of strong acid salts, such as chloride, bro-
is the steric hindrance of the substituent groups of the mide, and nitrate, the protons might be able to effec-
carbonates. The stability of carbon cations follows the tively polarize the oxygen atoms of the ester as shown
order:
in Scheme 1, which results in easier alkylation of the
ammonium ions. In the reactions of DMC with am-
monium sulfate and ammonium phosphate, the lower
yields of quaternary ammonium salts might be attrib-
uted to the reason that there are two or three cations
surrounding each anion, so the strong steric hindrance
around the ammonium sulfate or phosphate might
and the strength of the steric hindrance of the sub- impair the access of the dialkyl carbonate to the am-
stituent groups of the carbonates follows order: monium ions. In the case of ammonium acetates, am-
monium oxalates, and ammonium carbonates, one
reason for the relatively lower yield of quaternary
ammonium salts might be that they are weak acid
salts and the proton shown in Scheme 1 cannot effec-
tively protonize the oxygen of the ester; the other
Concerning the stability of carbon cations, dibenzyl reason could be that, in the ammonium oxalate and
carbonate should give the highest yield of quaternary ammonium carbonate cases, the strong steric hin-
ammonium salt. However, from the steric hindrance drance around the ammonium ions might also retard
point of view, dimethyl carbonate should give the the reaction and lead to low yields of quaternary am-
highest yield of quaternary ammonium salt. As a monium salt.
result of both of the factors, some change in the
yields of quaternary ammonium salts with these car-
bonates is expected. Hence, the reactivity order di- Conclusions
methyl carbonate>dibenzyl carbonate>diethyl car-
bonate>diphenyl carbonate is reasonable. Again, the In summary, we have developed a new method for
results further verified that the reaction is a nucleo- the synthesis of quaternary ammonium salts. Quater-
philic substitution of ammonium nitrogen on the alkyl nary ammonium salts having different anions could
group of the dialkyl carbonate.
be directly synthesized by reacting ammonium salts
From the results of our investigation, a possible re- with dialkyl carbonates over imidazolium IL catalysts.
action mechanism is suggested (Scheme 1). It is sup- Our results show that the yield of the quaternary am-
posed that, at the reaction temperature (443 K), the monium salt is correlated to the electron-donating
ammonium salt might release free ammonia or amine, property of the alkyl moieties of the ammonium ions,
which could attack the alkyl group of the dialkyl car- the nature of the anions, and the nature of the alkyl
bonate. The suggested mechanism can explain most of chains of the dialkyl carbonate. In the case that DMC
the reaction results. If the ammonium cations bear reacts with strong acid ammonium salts, such as am-
electron-donation alkyl moieties, the carbonates bear monium halides and nitrates, almost stoichiometric
good leaving groups (i.e., benzyl), and the geometri- yields of quaternary ammonium salts were reached
cal hindrance of both of the reactants is weak, the re- (not including the cases where the ammonium nitro-
action should give high yields of quaternary ammoni- gen atoms bear phenyl groups). For ammonium salts
bearing bulky alkyl groups on the nitrogen atoms,
such as cyclohexyl and n-C12H25, even when the am-
monium salts are weak acid salts, high yields of qua-
ternary ammonium salts were achievable.The current
synthesis of quaternary ammonium salts, in contrast
to the reported methods[27,30] which work only in the
case of tertiary amines, works for almost all kinds of
ammonium salts (not including ammonium ions bear-
ing phenyl groups on the nitrogen atom). In the cur-
rent synthesis reaction, alcohol and carbon dioxide
are formed as by-products, which could be converted
back to dialkyl carbonate through a reported pro-
Scheme 1. Possible reaction mechanism.
cess.[32]
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action, thus the amount of tetramethylammonium cation
(MTC) can be calculated based on the ratio of the peak areas
of peak a (AHa) to that of peak b (AHb) according to Eq. (1)
below:
Experimental Section
General Remarks
The dialkyl carbonates were commercial products. Some of
the ammonium salts were purchased and some were pre-
pared by neutralizing amines with a stoichiometric amount
of the corresponding acids, and then dried at room tempera-
ture in vacuum. The N,N-dialkylimidazolium ionic liquids
were prepared according to the procedure described in liter-
ature.[22,33]
M
TC =AHb/AHa ꢂMIL ꢂ1/4
(1)
Product Isolation
In some of the scale-up reactions, the product quaternary
ammonium salts were purified. Since both of the catalyst
and product are ionic compounds, the complete isolation of
the quaternary ammonium salt from the catalyst could be a
challenge. Based on the differences in solubility of the cata-
lyst and the product (quaternary ammonium salt) in ace-
tone, quaternary ammonium salts of high purity can be re-
covered by an acetone washing process as follows: 1) after
the reaction, alcohol and dialkyl carbonate were removed
from the reaction mixture by distillation, 2) acetone was
added and stirred for 2 h, 3) the mixture was filtrated to
obtain the solid quaternary ammonium salt, and 4) the solid
quaternary ammonium salt was washed two times with ace-
tone, and then dried under vacuum.
Reactions and Product Analysis
In a typical reaction, ammonium salt, dialkyl carbonate, and
the catalyst (the amount of every reagent is described in the
corresponding sections) were charged into a stainless steel
autoclave (inside volume 3.0 mL), and then the reactor was
heated to 443 Kin an oven and kept at this temperature for
8 h (without stirring).
For product quantification, in most cases, we did not sepa-
rate the products for every reaction mixture. The mixture
was directly dissolved in D2O to prepare a solution and ana-
lyzed on an NMR spectrometer. SiACHTRE(UNG CH3)4 was used as refer-
ence. Calculations of the quaternary ammonium salt yield
were based on the peak areas of the hydrogen atoms of the
product and then calibrated by using standard samples, as
indicated below.
Typical Reactions
DMC (3.600 g, 40.0 mmol) was reacted with trimethylammo-
nium bromide (2.800 g, 20.0 mmol) over [EMIM]Br (0.764 g,
4.0 mmol) in a stainless steel autoclave lined with polytetra-
fluoroethylene (inside volume 50 mL) at 443 Kfor 8 h. The
product tetramethylammonium bromide was purified ac-
cording to the above described process; yield: 2.973 g
(19.3 mmol, 96.5%). The 1H NMR analysis did not show
any [EMIM]+ contamination.
In another example, DMC (10.507 g, 116.6 mmol) was re-
acted with N-cyclohexylammonium bromide (3.584 g,
19.9 mmol) over [EMIM]Br (2.027 g, 10.6 mmol) in an auto-
clave (inside volume 75 mL) under the same reaction condi-
tions as described in the previous example. After methanol
and the unreacted dimethyl carbonate had been removed by
distillation, the product was washed with acetone (150 mL
of acetone were used in the first washing and 10 mL of ace-
tone were used in the second and the third washings, respec-
1H NMR Quantification of Quaternary Ammonium
Salts
1
As an example, Figure 1a shows one of the H NMR spectra
from the reaction mixture of ammonium bromide with
DMC in the presence of [EMIM]Br. The peak a with a
chemical shift at 1.42 ppm is assigned to the hydrogen of the
methyl group on the ethyl group of [EMIM]Br. The peak b,
which shows the same chemical shift of 3.12 ppm as that of
the standard tetramethylammonium bromide (Figure 1b), is
assigned to the hydrogen atoms of the methyl groups on the
tetramethylammonium cation. Since [EMIM]Br acts as a
catalyst in the reaction system. the amount (MIL) of
[EMIM]Br should remain constant before and after the re-
1
Figure 1. a) The H NMR spectrum of the mixture from the reaction of NH4Br and dimethyl carbonate in the presence of
[EMIM]Br. b) The 1H NMR spectrum of the standard sample tetramethylammonium bromide.
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Alkylation of Ammonium Salts
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tively) to obtain the solid N-cyclohexyl-N,N,N-trimethyl am-
[14] M. Picquet, D. Poinsot, S. Stutzmann, I. Tkatchenko, I.
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1
yield 4.084 g (18.4 mmol, 92.4%). The H NMR analysis did
not show any [EMIM]Br contamination.
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Feng Professor” Foundation of Zhejiang Normal University
(KYJ01Y05002). We thank Mr. Ren Wei Zheng for NMR
analysis.
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