N-Methylation of poorly nucleophilic aromatic amines with dimethyl carbonate

Article abstract of DOI:10.1007/s11164-015-2416-4

Abstract: Dimethyl carbonate (DMC), an environmentally friendly methylation agent, is a substitute for traditional methylation agents such as methyl halides (CH₃X, X?=?I, Br, Cl) or dimethyl sulfate. An efficient, convenient, and green method has been developed for N-methylation of poorly nucleophilic aromatic amines with DMC. It was found that the couple PEG400/K₂CO₃ provides good selectivity for the N-methylation product. Finally, the mechanism for reaction of amines with DMC was investigated, and a plausible multistep mechanism proposed and verified. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-015-2416-4

Res Chem Intermed
DOI 10.1007/s11164-015-2416-4
N-Methylation of poorly nucleophilic aromatic amines
with dimethyl carbonate
Huidong Yan1 Liufang Zeng1 Yaqiang Xie1
•
•
•
Yu Cui1 Liyi Ye1 Song Tu1
•
•
Received: 25 September 2015 / Accepted: 31 December 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract Dimethyl carbonate (DMC), an environmentally friendly methylation agent,
is a substitute for traditional methylation agents such as methyl halides (CH3X, X = I,
Br, Cl) or dimethyl sulfate. An efficient, convenient, and green method has been
developed for N-methylation of poorly nucleophilic aromatic amines with DMC. It was
found that the couple PEG400/K2CO3 provides good selectivity for the N-methylation
product. Finally, the mechanism for reaction of amines with DMC was investigated, and
a plausible multistep mechanism proposed and verified.
Graphical Abstract
Electronic supplementary material Theonlineversionofthisarticle(doi:10.1007/s11164-015-2416-4)
contains supplementary material, which is available to authorized users.
&
Song Tu
tusong@xmu.edu.cn
1
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, People’s Republic of China
123

H. Yan et al.
Keywords Dimethyl carbonate Á N-Methylation Á Poorly nucleophilic aromatic
amines Á Environmentally friendly
Introduction
N-Methylation of aromatic amines is an important reaction that is widely used in
industry for synthesis of dyes, pesticides, pharmaceuticals, etc. [1–3]. Generally, N-
methylation of aromatic amines is performed with methyl halides or dimethyl
sulfate, because of their high activity and mild reaction conditions. However, all
these methylation agents are toxic, hazardous, and corrosive compounds, posing a
great risk to the environment and operating facilities. In addition, they are not very
useful for preparing mono-N-methylanilines, because their poor selectivity leads to
mixtures of secondary and tertiary amines [4].
In recent decades, dimethyl carbonate (DMC) has often been reported as an
environmentally benign substitute for dimethyl sulfate (DMS) and methyl halides.
DMC is a well-known nontoxic reagent that has been used for many green
applications. In fact, DMC gives only methanol and carbon dioxide as byproducts
when used as a methylation agent. The carbonyl and methyl groups in DMC are the
two electrophilic reactive centers that can react with nucleophiles [5]. Thus,
nucleophiles undergo simultaneous N-methylation and carbamoylation, leading to
low selectivity for the desired N-methylation product [6].
Because of the low selectivity of DMC coupled with its significantly lower
reactivity at moderate temperatures compared with methyl halides or dimethyl
sulfate, its application must be combined with catalysts. Many different catalyst
systems have been proposed for this process, including zeolites [7–9], phosphonium
salts [10], Zn1-xCoxFe2O4 [11], 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) [12],
diphenylammonium triflate [13], and 18-crown-6 [14]. However, there are few
reports on simple and environmentally friendly methods specifically for N-
methylation of poorly nucleophilic aromatic amines using DMC. Herein, an
efficient, convenient, and ecofriendly method for N-methylation of poorly
nucleophilic aromatic amines with DMC is reported. Moreover, a plausible
multistep mechanism is proposed and verified.
Results and discussion
N-Methylation of p-aminoacetophenone (E) with DMC (Scheme 1) was chosen as
model reaction to explore the feasibility of use of DMC for N-methylation of poorly
nucleophilic aromatic compounds. Firstly, the reaction of p-aminoacetophenone and
DMC was performed at temperatures of 90–130 °C using tetrabutylammonium
bromide (TBAB)/K2CO3 as catalyst and dimethylformamide (DMF) as solvent. It
was noted that 130 °C was a suitable temperature for generation of the methylation
product. In contrast, DMC only acted as a methoxycarbonylation agent below
110 °C, failing to generate the methylation product.
123

N-Methylation of poorly nucleophilic aromatic amines with…
O
O
O
NH2
HN
O
N
B
O
HN
DMC, base
+
+
PTC, solvent
O
O
O
E
A
C
Scheme 1 N-Methylation of p-aminoacetophenone (E) with DMC
We proceeded to test the activity and selectivity of various catalyst systems for
methylation of p-aminoacetophenone with DMC. N,N-Dimethyl derivatives were
not formed under the reaction conditions; only the methoxycarbonylation byproduct
was generated.
Phase-transfer catalysts (PTCs) have been widely reported for O-methylation of
phenols with DMC [14]. In our research, we studied different kinds of PTCs for
methylation of poorly nucleophilic aromatic amines with DMC. Among the
catalysts studied, we noted that the couples TBAB/K2CO3 and PEG400/K2CO3
exhibited excellent selectivity for the N-methylation product (entries 1 and 8,
compared with entries 2–7, in Table 1). It should be noted that TBAB is expensive
and toxic. PEG400, on the other hand, is thermally stable, nontoxic, and
inexpensive. Thus, PEG400/K2CO3 was chosen as an ecofriendly catalyst system
for further research. Moreover, selectivity of 86.2 % was obtained in presence of
only K2CO3 (entry 9, Table 1). Further research results showed that K2CO3 is better
than KOH, KHCO3, or Na2CO3 (entry 8, compared with entries 10–12, in Table 1).
It was also found that both methylation and methoxycarbonylation products with
very low conversion and selectivity were formed through the reaction of p-
aminoacetophenone and DMC in absence of base (entry 13, Table 1). Other polar
aprotic solvents [dimethyl sulfoxide (DMSO), 1,3-dimethyl-2-imidazolidinone
(DMI), N-methylpyrrolidone (NMP), and sulfolane] were also investigated for this
reaction, and it was clear that DMF was the best among all the solvents studied
(entries 8 and 14–17, Table 1).
Other factors were also examined to optimize the reaction conditions. Table 2
shows the effect of the molar ratio of DMC, PEG400, and K2CO3 to p-
aminoacetophenone (E) on the conversion of E and the selectivity for the N-
methylated product (C) at 130 °C. It was found that an increase in the ratio of DMC
to p-aminoacetophenone from 1 to 3 equiv. led to an increase in the conversion of
E from 47.3 to 100 % (entries 1–3, Table 2). The selectivity for C also increased
from 50.4.3 to 90.1 % with increasing molar ratio of DMC to p-aminoacetophenone
from 1 to 3, while using larger excess of DMC had a negligible effect on the
selectivity for C (entry 4, Table 2). With increasing molar ratio of PEG400/E, the
selectivity for C also increased (entries 5–8, Table 2). The results obtained using
different K2CO3/E ratios are also included in Table 2. It was found that increasing
123

H. Yan et al.
Table 1 Effects of catalyst system and solvent on the reaction of p-aminoacetophenone with DMCa
b
Conversion /%
Entry
PTC
Base
Solvent
Selectivity
(C)b/%
1
TBABc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
KOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
DMI
100
100
100
100
100
86.9
100
100
100
47.3
95.7
89.7
14.2
91.8
25.1
74.7
57.4
90.5
75.0
85.2
77.0
77.2
54.8
87.1
90.1
86.2
13.1
90.8
60.4
10.8
61.8
18.1
28.3
11.9
2
TEBACc
CTMABc
TEABc
TMACLc
PEG6000d
PEG1000d
PEG400
–
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
a
PEG400
PEG400
PEG400
PEG400
PEG400
PEG400
PEG400
PEG400
KHCO3
–
K2CO3
K2CO3
K2CO3
K2CO3
NMP
Sulfolane
Reaction conditions: p-aminoacetophenone (10 mmol), DMC (3 equiv.), base (0.25 equiv.), PTC (0.1
equiv.), solvent (15 mL), 130 °C, 12 h
b
Determined by 1H NMR; Selectivity defined as N-methylated product/(N-methylated product ? car-
bamate ? N-methylcarbamate) 9 100
c
TBAB tetrabutylammonium bromide, TEBAC benzyltriethylammonium chloride, CTMAB hexade-
cyltrimethylammonium bromide, TEAB tetraethylammonium bromide, TMACL tetramethylammonium
chloride
d
0.5 g
the molar ratio of K2CO3 to p-aminoacetophenone from 0.1 to 0.3 had little effect
on the selectivity for C (entries 9–11, Table 2).
Methylation of p-aminoacetophenone (E) was chosen as model reaction for
mechanistic discussion. The reaction conditions were as follows: p-aminoacetophe-
none (10 mmol), DMC (3 equiv.), K2CO3 (0.25 equiv.), PEG400 (0.15 equiv.),
DMF (15 mL), 130 °C. In this work, 1H NMR spectroscopy was used to determine
the change in product composition with time (Fig. 1). p-MeCOC6H4NH(CO2Me)
(A) and p-MeCOC6H4N(CH3)(CO2Me) (B) are the intermediates in this process. As
shown in Fig. 2, the conversion of E increased with time up to 7 h, then remained
constant thereafter at 100 %. Meanwhile, the p-MeCOC6H4NH(CH3) (C) content
continued to increase until reaching conversion of 90.5 % in 11 h. These results
suggest that the ArNH2 does not follow a simple methylation reaction with the
methyl group of DMC. The concentration of B rose to a maximum of 58.2 % after
3 h and then started to fall, reaching 9.5 %, implying that B is just an intermediate
in the process of formation of C. Intermediate A appeared with low concentration at
123

N-Methylation of poorly nucleophilic aromatic amines with…
Table 2 Effect of molar ratio
on selectivity of reaction of p-
Entry DMC/
PEG400/ K2CO3/
Conversionc/ Selectivity
Eb
Eb
Eb
%
(C)c/%
aminoacetophenone with DMCa
1
1
2
3
4
3
3
3
3
3
3
3
0.10
0.10
0.10
0.10
0.05
0.10
0.15
0.20
0.15
0.15
0.15
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.1
47.3
94.3
100
100
100
100
100
100
100
100
100
50.4
89.1
90.1
86.6
87.7
90.1
92.9
92.9
90.2
86.7
89.8
2
3
a
4
Reaction conditions: p-
aminoacetophenone (10 mmol),
5
DMF (15 mL), 130 °C, 12 h
6
b
mol/mol
Determined by 1H NMR.
7
c
8
Selectivity defined as N-
methylated product/(N-
methylated
9
10
11
0.2
product ? carbamate ? N-
methylcarbamate) 9 100
0.3
the beginning of the reaction but disappeared later, implying that, once intermediate
A is generated, it is soon converted into B. The reaction route is shown in Scheme 2.
Based on the above experimental results, a plausible mechanism for the N-
methylation of aromatic amines is proposed. As shown in Scheme 2, the ArNH2
does not follow a simple methylation reaction with the methyl group in DMC via
the SN2 mechanism. In fact, the ArNH2 undergoes two consecutive nucleophilic
displacements: firstly, the ArNH2 reacts with the carbonyl of DMC via the BAC
2
mechanism to yield the intermediate ArNHCO2Me [15–17], then ArNHCO2Me
reacts with the alkyl carbon in DMC to yield the intermediate ArNCH3CO2Me via
the BAl2 mechanism [18]. Subsequently, the final product ArNHCH3 is obtained by
demethoxycarbonylation of ArNCH3CO2Me.
Validation studies were also performed. It was found that the reaction of the
intermediate A with DMC afforded B with selectivity of 39.4 % along with E and
C for complete conversion after 12 h (Scheme 3). Moreover, when intermediate
B was reacted with methanol in absence of DMC, the yield of C was 100 %
(Scheme 4). However, when operating this reaction in absence of methanol, no
compound C was afforded (Scheme 4). These results indicate that the proposed
reaction mechanism is reasonable.
To the best of our knowledge, the carbonyl group of DMC is a harder electrophile
and the two methyl groups represent softer electrophiles; ArN-H represents a harder
nucleophile under experimental conditions. So, according to the principle of hard
and soft acids and bases (HSAB) [5], the hard–hard reaction occurs; as a result,
methoxycarbonylation is preferred over methylation initially, while the anion
ArN-CO2Me as a softer nucleophile produces the methyl derivative. It should be
pointed out that deprotonation of the carbamate probably aids methylation, while
steric effects probably prevent second methoxycarbonylation.
To expand the scope of the reaction, N-methylation of a variety of aromatic amines
with DMC was examined under the above optimal conditions, and the results are
presented in Table 3. According to the hard and soft acids and bases (HSAB)
principle, the presence of an electron-withdrawing group in the amine substrate makes
123

H. Yan et al.
O
O
O
O
NH2
N
B
O
HB2
HN
O
HA2
HN
HE2
HE1
HE2
HE1
HB2
HB1
HA2
HA1
HC2
HC1
HC2
HC1
+
+
HB1
HA1
O
E
O
A
C
Fig. 1 1H NMR spectrum (400 MHz) observed during reaction of p-aminoacetophenone with DMC
NH2
Scheme 2 Process of reaction
of p-aminoacetophenone with
DMC
NH
SN2
E
C
O
O
O
O
HN
O
N
O
A
B
O
O
123

N-Methylation of poorly nucleophilic aromatic amines with…
Fig. 2 Composition changes of products in reaction of p-aminoacetophenone with DMC during the
reaction time
O
O
HN
O
N
O
NH2
HN
DMC,TBAB,K2CO3
DMF,130 C,12h
O
O
O
O
E
A
B
C
21.2%
39.4%
39.4%
Scheme 3 Reaction of intermediate A with DMC
the nitrogen atom become a softer nucleophile. As shown by the results of these
experiments, the reaction of p-nitroaniline and p-cyanoaniline with DMC produced
methylation products in high yield (entries 1 and 2, Table 3). However, when o-
nitroaniline and 2-methyl-4-nitroaniline were tested, a significant decrease in the
selectivity for the N-methylated product was observed (entries 3 and 4, Table 3). The
reaction also worked well for N-methylation of arylsulfonamides with DMC, and
N,N-dimethylated products were obtained in high yield (entries 5–7, Table 3).
Experimental
All reagents and solvents were obtained from commercial sources and used without
further purification. NMR spectra were recorded in CDCl3 solution on a Bruker
400 MHz spectrometer with tetramethylsilane (TMS) as internal standard. Mass
spectra were recorded on a Bruker Dalton Esquire 3000 plus LC–MS apparatus.
123

H. Yan et al.
O
O
N
O
NH
NH
TBAB, K2CO3
CH3OH, TBAB, K2CO3
DMF, 130°C, 12h
DMF, 130°C, 12h
O
O
C
C
B
100%
Scheme 4 Reaction of intermediate B with methanol
General procedure for N-methylation of aromatic amines with DMC: A reaction
flask was charged with amine substrate, PTC, base, solvent, and DMC. The mixture
was heated to 130 °C, and the reaction was monitored by thin-layer chromatography
(TLC). After reaction completion, the mixture was extracted with EtOAc and
washed with H2O. The combined organic extracts were dried over anhydrous
MgSO4 and concentrated under vacuum. The residue was purified by flash
chromatography (silica gel, petroleum ether/EtOAc) to afford the desired
compound.
The structures of all the published compounds were confirmed by previously
published protocols.
1-(4-(Methylamino)phenyl)ethanone [19]: 1H NMR (CDCl3, 400 MHz) d 2.50 (s,
3H), 2.90 (s, 3H), 4.34 (s, 1H), 6.55–6.57 (m, 2H), 7.82–7.85 (m, 2H).
Methyl 4-acetylphenylcarbamate [20]: 1H NMR (CDCl3, 400 MHz) d 2.57 (s,
3H), 3.80 (s, 3H), 7.48–7.51 (m, 2H), 7.92–7.94 (m, 2H).
Methyl 4-acetylphenylmethylcarbamate: 1H NMR (CDCl3, 400 MHz) d 2.59 (s,
3H), 3.35 (s, 3H), 3.76 (s, 3H), 7.37–7.40 (m, 2H), 7.95–7.98 (m, 2H); 13C NMR
(CDCl3, 400 MHz) d 26.51, 37.12, 53.10, 124.57, 129.03, 133.99, 147.44, 155.56,
196.97. MS (ESI) calcd. for [C11H14NO3]? (M ? H?) 208.1, found 207.8; calcd.
for [C11H13NNaO3]? (M ? Na?) 230.1, found 229.8; calcd. for [C22H26N2NaO6]?
(2M ? Na?) 437.2, found 436.8.
N-Methyl-4-nitrobenzenamine [21]: 1H NMR (CDCl3, 400 MHz) d 2.94 (d, 3H,
J = 4.8 Hz), 4.65 (s, 1H), 6.51–6.54 (m, 2H), 8.07-8.10 (m, 2H).
N,N-Dimethyl-4-nitrobenzenamine [22]: 1H NMR (CDCl3, 400 MHz) d 3.11 (s,
6H), 6.59–6.62 (m, 2H), 8.11–8.14 (m, 2H).
Methyl 4-nitrophenylcarbamate [23]: 1H NMR (CDCl3, 400 MHz) d 3.82 (s, 3H),
7.16 (s, 1H), 7.56–7.58 (m, 2H), 8.19–8.21 (m, 2H).
N-Methyl-2-nitrobenzenamine [24]: 1H NMR (CDCl3, 400 MHz) d 3.02 (t, 3H,
J = 3.6 Hz), 6.63–6.66 (m, 1H), 6.82–6.85 (m, 1H), 7.44–7.47 (m, 1H), 8.03 (s,
1H), 8.15–8.17 (m, 1H).
123

N-Methylation of poorly nucleophilic aromatic amines with…
Table 3 N-Methylation of anilines and amides with DMC
Entry
1
ArNH2
Time/h
12
Conversiona/%
S(MNM)a/%
S(DNM)a/%
NH2
100
85.4
14.6
NO2
NH2
2
12
100
95.3
0
CN
NH2
3
4
24
12
100
56.5
14.9
0
0
O2
N
NH2
NO2
85.1
O
O
5
6
12
12
100
100
0
0
100
100
S
NH2
Cl
O
O
S
NH2
F
F
F
O
O
7
24
100
15.3
84.7
S
NH2
Reaction conditions: substrate (10 mmol), DMC (3 equiv.), K2CO3 (0.25 equiv.), PEG400 (0.15 equiv.),
DMF (15 mL), 130 °C, 12 h
a
Determined by 1H NMR; MNM: mono-N-methylated product; DNM: N,N-dimethylated product;
S(MNM) = [MNM/(MNM ? DNM ? carbamate ? N-methylcarbamate)] 9 100; S(DNM) = [DNM/
(MNM ? DNM ? carbamate ? N-methylcarbamate)] 9 100
Methyl 2-nitrophenylcarbamate [25]: 1H NMR (CDCl3, 400 MHz) d 3.84 (s, 3H),
7.11–7.15 (m, 1H), 7.62–7.66 (m, 1H), 8.02 (s, 1H), 8.20–8.22 (m, 1H), 8.55–8.58
(m, 1H).
Methyl methyl 2-nitrophenylcarbamate [26]: 1H NMR (CDCl3, 400 MHz) d 3.33
(s, 3H), 3.59 (s, 2H), 3.78 (s, 1H), 7.35–7.37 (m, 1H), 7.41–7.45 (m, 1H), 7.61–7.65
(m, 1H), 7.95–7.97 (m, 1H).
4-(Methylamino)benzonitrile [27]: 1H NMR (CDCl3, 400 MHz) d 2.88 (d, 3H,
J = 5.2 Hz), 4.27 (s, 1H), 6.54–6.57 (m, 2H), 7.42–7.44 (m, 2H).
N,N-Dimethyl-4-chlorobenzenesulfonamide [28]: 1H NMR (CDCl3, 400 MHz) d
2.72 (s, 6H), 7.51–7.55 (m, 2H), 7.71–7.74 (m, 2H).
N,N-Dimethylbenzenesulfonamide [29]: 1H NMR (CDCl3, 400 MHz) d 2.71 (s,
6H), 7.54–7.58 (m, 2H), 7.60–7.64 (m, 1H), 7.77–7.79 (m, 2H).
N,N-Dimethyl-4-(trifluoromethyl)benzenesulfonamide [30]:
400 MHz) d 2.76 (s, 6H), 7.82–7.84 (m, 2H), 7.91–7.93 (m, 2H).
1H NMR (CDCl3,
123

H. Yan et al.
Conclusions
We present an efficient, convenient, and ecofriendly method for N-methylation of
poorly nucleophilic aromatic amines with DMC. The reaction takes place at 130 °C
and atmospheric pressure by employing the couple PEG400/K2CO3 as catalyst. This
method avoids use of toxic methyl halides and complicated catalytic systems, and
provides a green process for some important chemical transformations.
Acknowledgments The authors acknowledge the Key Planning Project of Science and Technology of
Fujian Province, China (Grant Number 2014N0020) for financial support.
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