Methylation of amines, nitrobenzenes and aromatic nitriles with carbon dioxide and molecular hydrogen

Article abstract of DOI:10.1039/c3sc52676c

CO₂/H₂ was successfully employed in alkylation reactions by performing CO₂ reduction and amine N-methylation in one-pot. In the presence of a simple CuAlOx catalyst, N-methyl or N,N-dimethyl amines with different structures can be selectively synthesized with up to 96% yields by applying amine, nitrobenzene and nitrile as starting materials.

Full text of DOI:10.1039/c3sc52676c

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Methylation of amines, nitrobenzenes and aromatic
nitriles with carbon dioxide and molecular
hydrogen†
Cite this: Chem. Sci., 2014, 5, 649
a
ab
a
a
a
Xinjiang Cui, Xingchao Dai, Yan Zhang, Youquan Deng and Feng Shi*
Received 24th September 2013
Accepted 16th October 2013
2 2 2
CO /H was successfully employed in alkylation reactions by performing CO reduction and amine
N-methylation in one-pot. In the presence of a simple CuAlOx catalyst, N-methyl or N,N-dimethyl
amines with different structures can be selectively synthesized with up to 96% yields by applying amine,
nitrobenzene and nitrile as starting materials.
DOI: 10.1039/c3sc52676c
www.rsc.org/chemicalscience
the production of carbon-based chemicals, it should be an ideal
route to synthesize N-methyl or N,N-dimethyl amines by the
Introduction
The controllable activation and application of CO
2
as a building reductive methylation of amines, nitrobenzenes or nitriles with
block in catalytic transformations is one of the goals of the carbon dioxide and a reducing agent. This approach presents an
scientic community. Since the 1970's, two directions for economic and sustainable way for N-methyl or N,N-dimethyl
carbon dioxide utilization have been explored. One direction amine syntheses however, efficient catalyst systems are still
uses carbon dioxide as a carbonyl source in catalytic carbonyl- required. In order to develop such a catalyst, several crucial
1
ation reactions, such as the synthesis of carboxylic acid, ester factors are required; i.e. the catalyst must be highly active for
2
3
4
5
or lactone, formamide, urea or carbamate and polymer etc. carbon dioxide hydrogenation and amine N-methylation but
Another direction for CO
utilization is the incorporation of inactive for aromatic ring hydrogenation. Recently, it was
carbon dioxide into basic chemicals including methanol, light reported that amine methylation with CO /PhSiH was achieved
2
2
3
6
12
alkanes or olens. Although great developments have been using a homogeneous catalyst system, which provided a new
7
achieved on fundamental research of the CO
2
activation, few methodology for the utilization of CO
2
. Subsequently, the use of
industrial processes which utilize CO as a raw material were CO
2
/H
2
, as an alkylation reagent for amine methylation using
2
13
realized because CO is the most oxidized state of carbon and its homogeneous catalysts was reported. However, the one-pot
2
catalytic activation is difficult. Undoubtedly, the direct trans- reductive methylation of nitrobenzenes and aromatic nitriles
formation of CO
2
into valuable chemicals is still a challenge, with CO
2
has not previously been reported.
11a–d
and the development of new reactions for the efficient utiliza-
It is well known that nitrobenzene
and carbon dioxide
6,14
tion of CO2, as a building block in chemical synthesis, is highly reductions
are performed with hetero- or homogeneous
8
desirable.
catalysts, which involve the activation of hydrogen molecules
Methylation reactions, such as the N-methylation and N,N- through adsorption on the catalytic active site to form hydride,
dimethylation of amines, which use methanol as an alkylation and the transfer of the hydride to the nitro group or carbon
reagent have a vital function in the chemical industry, and they dioxide. These results, combined with the recent progress of
represent a series of key intermediates for the synthesis of dyes, amine alkylation with alcohols, provide inspiration for the
9
perfumes, pesticides and others. Currently, these compounds direct use of CO
2
and H
2
for the alkylation of amines. It was
are obtained through the N-methylation of amines with meth- shown that the reaction can take a hydride formation and
1
0
15
anol or formaldehyde while the amines are produced via transfer route, too. Thus, according to the known catalyst
11
nitrobenzene or nitrile hydrogenation. Considering the systems for nitrobenzene and carbon dioxide reduction, and
importance of applying carbon dioxide as the carbon source for aniline alkylation, it may be possible to design and prepare a
2
catalyst capable of utilizing H for the reductive N-methylation
of nitrobenzene with CO . Moreover, in consideration of the
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Green catalytic conditions of the carbon dioxide reduction, a non-
2
a
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy
noble metal catalyst might be a suitable candidate, because
of Sciences, Lanzhou, 730000, China. E-mail: fshi@licp.cas.cn; Fax: +86-931-
aromatic ring reduction oen happens in the presence of a
8
b
277088; Tel: +86-931-4968142
16
noble metal catalyst. Based on the above discussions, copper
should be a good choice to achieve the target because it is the
University of the Chinese Academy of Sciences, Beijing, 100049, China
†
Electronic supplementary information (ESI) available: Characterization of
11
common active species in nitrobenzene hydrogenation,
catalyst and products. See DOI: 10.1039/c3sc52676c
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6,14
15
carbon dioxide reduction and amine alkylation, but is less
active in aromatic ring hydrogenation.
a
2 2
Table 1 Selective methylation of primary amine with CO /H
Results and discussion
The catalyst used here was prepared by a co-precipitation method
by adding an aqueous solution of Na CO into a Cu(NO ) and
Entry
1
Substrates
Products
Yields/%
2
3
3 2
Al(NO ) solution. The CuAlOx catalyst was obtained aer sepa-
3
3
ration, washing, drying, calcination and reduction under
b
c
86 (81 )/82
hydrogen ow of the precipitate. XPS characterization of the
2
catalyst suggested the formation of metallic copper and Cu O,
Fig. S1.† This result was conrmed by XRD characterization,
which showed the diffraction patterns of Cu(111), (200), (220),
2
3
79
(
311) and Cu O(111), Fig. S2.† Regarding the HR-TEM charac-
2
b
80 (74 )
terization, crystal lattices of Cu(111) and Cu(200) can be observed
clearly. Line-scan EDS analysis of the catalyst sample suggested it
was composed of nano-CuOx particles deposited on the plane of
AlOx, Fig. 1. BET analysis showed the surface area of the catalyst
4
5
65
80
2
ꢀ1
to be 138 m g , Fig. S3.†
The catalyst screening was performed using mono-methyla-
tion of aniline as the model reaction. First, an 86% yield of
N-methyl aniline with 95% aniline conversion was obtained
using the prepared CuAlOx catalyst (Table 1, Entry 1). The
methylation reaction did not occur in the absence of the cata-
6
7
8
70
43
lysts and <2% N-methyl aniline was formed if using Al
Cu O, CuO or the mixture of Al /Cu, Al /CuO, Al
and Al /Cu/Cu O as catalysts directly (Table S1†). Therefore,
2
O
3
, Cu,
2
2
O
3
2
O
3
2
O
3
2
/Cu O
2
O
3
2
the synergy between copper and aluminium generated during
the catalyst preparation should be the major reason for its high
activity. Importantly, the conversion and yield of the reaction
b
83 (77 )
rd
was maintained at 97% and 82% at the 3 run, suggesting that
the catalyst is stable during the reaction. XRD characterization
of the used catalyst sample showed almost the same crystal
9
80
structure while some CuCO
catalyst according to XPS analysis, Fig. S1 and 2.† TEM and 10
3
formed on the surface of the
b
85 (82 )
11
64
1
1
2
3
81
71
b
14
75 (69 )
15
83
a
Reaction conditions: Entries 1–7, 1.0 mmol amine, 50 mg catalyst, 2 mL
ꢁ
hexane, 3.0 MPa CO
amine, 50 mg catalyst, 2 mL hexane, 3.0 MPa CO
2
, 6.0 MPa H
2
, 160 C, 24 h; Entries 8–15, 1.0 mmol
ꢁ
2
, 7.0 MPa H
2
, 160 C,
4
8 h. The yields were obtained by GC-FID using biphenyl as the external
b
c
standard material. Isolated yield. The yield at the 3rd run.
Fig. 1 TEM (a), HR-TEM (b) and line-scan EDS analysis (c) images of
the CuAlOx catalyst sample (d).
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a
line-scan EDS analysis of the used sample also showed a similar Table 2 Selective methylation of secondary amine with CO
structure to the fresh catalyst, Fig. S4 and 5.† Furthermore, the
2 2
/H
reaction stopped if the catalyst was ltered off during the
middle stage of the reaction and ICP-AES analysis showed that
the metals which leached into the ltrate were below the
detection limit. These ndings suggest that the leached metals
do not contribute signicantly to the catalytic activity. In addi-
tion, if 10 mmol aniline was used as a starting material and
Entry
1
Substrates
Products
Yields/%
86
150 mg CuAlOx was used as the catalyst, a 64% yield of N-methyl
aniline was obtained with an 80% aniline conversion, Scheme
S1.† This suggests that scaling up of the reaction is possible
however, reducing the catalyst loading results in a low yield.
Following these initial studies, the N-methylation reactions
2
3
4
89
91
45
2 2
of additional amines with CO /H were explored. The trans-
formation of anilines, which contain electron-donating as well
as electron-withdrawing substituents at different positions, was
carried out smoothly to obtain the corresponding products with
65–80% yields (Entries 2–4). During the methylation reaction,
5
79
hydro-dechlorination of p-Cl-aniline was observed, which
decreased the yield of the desired product. Biphenyl-2-amine
and naphthalen-2-amine afforded the corresponding products
with yields of 80% and 70% (Entries 5–6). However, dodecan-1-
amine was converted into the corresponding product with a
yield of only 43%, and the reason for its low activity is still
currently unclear (Entry 7). The formation of tertiary amine
products was observed to be the sole by-product of the mono-
methylation reactions. We were encouraged by this result to
further investigate the possibility of obtaining tertiary amines
by tuning the reaction conditions. N,N-Dimethylaniline was
6
7
8
9
83
87
b
96 (87 )
92
84
86
1
0
produced with an 83% yield just by increasing the H
2
pressure
from 6 MPa to 7 MPa and reacting for 48 h (Entry 8). Following
this, we explored other N,N-dimethylation reactions to examine 11
the scope and limitation of the current methodology. Anilines
with either electron-donating or electron-withdrawing groups
b
1
2
79 (75 )
yielded the N,N-dimethyl amine products in moderate to good
yields (Entries 9–11). The dechlorination also occurred and
about 5% N,N-dimethylaniline was detectable if using p-Cl-
aniline as a starting material. There are many pharmaceuticals
containing the dimethyamino-methylene group, and we
focused our attention on the synthesis of such compounds
a
Reaction conditions: 1.0 mmol amine, 50 mg catalyst, 2 mL hexane, 3.0
ꢁ
MPa CO
2
, 7.0 MPa H
2
, 160 C, 24 h. The yields were obtained by GC-FID
b
using biphenyl as the external standard material. Isolated yield.
using CO
methylation reaction of benzylamine and full conversion of atoms didn't retard the catalytic activity (Entries 5–12).
benzylamine was observed with a yield of 81% (Entry 12). Ben-
Since the amine methylation and dimethylation reactions
2 2
/H as the alkylating agent. First, we studied the yields of 79–96% and the heterocycles containing N and O
zylamines with different substituents such as methyl, methoxy can be achieved successfully, it was interesting to investigate
and isopropyl groups were converted into the corresponding the possibility of methylation reactions of nitrobenzenes and
tertiary amines with yields of 71–83% (Entries 13–15).
nitriles using CO /H as the alkylation reagent. Under the same
2 2
Inspired by the results in Table 1, we investigated whether reaction conditions as in Table 2, N,N-dimethyl aniline can be
tertiary amines can be synthesized from secondary amines. synthesized with 50% yield using nitrobenzene as the starting
ꢁ
Clearly, N-methyl aniline was converted into N,N-dimethylani- material. When performing the reaction at 170 C for 48 h, the
line successfully with a yield of 86% (Table 2, Entry 1). The yield of N,N-dimethyl aniline reached 86% (Table 3, Entry 1).
reaction was also successful for the methylation of N-methyl-1- The scope of nitrobenzenes bearing different groups was
phenylmethanamine and dibenzylamine (Entries 2–3). Unfor- further investigated and good results were obtained (Entries
tunately, a dialkyl amine such as dioctylamine was difficult to 2–4). The yields of the corresponding N,N-dimethyl amines were
be N-methylated. The yield of the desired product was only 45% 79–86%. Next, the dimethylation reactions of nitriles were
(Entry 4). Notably, cyclic secondary amines including 1,2,3,4- investigated under the same reaction conditions. To our
tetrahydroisoquinoline, piperazine, and morpholine derivatives delight, the dimethylation of benzonitrile derivatives can also
can be transformed into the corresponding tertiary amines with be carried out with moderate yields (Entries 5–7). The major
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Table 3 Selective methylation of nitrobenzene and nitrile with CO
2
/
a
H
2
Entry
1
Substrates
Products
Yields/%
b
86 (80 )
2
3
79
b
86 (82 )
4
83
55
5
6
b
45 (39 )
7
51
a
Reaction conditions: 1.0 mmol nitrobenzene or nitrile, 50 mg catalyst, 2
ꢁ
mL hexane, 3.0 MPa CO
2
, 7.0 MPa H
2
, 170 C, 48 h. The yields were
obtained by GC-FID using biphenyl as the external standard material.
b
Isolated yield.
byproducts were an aldehyde intermediate and phenyl-
methanol, suggesting that the reductive hydrolysis of nitriles
occurred. Meanwhile, nitrobenzene can be completely reduced
to aniline however, 35% benzylamine and 65% dibenzylamine
were obtained for benzonitrile hydrogenation without CO2
(Scheme S2†).
Finally, the reaction mechanism with CO
2
/H
2
as the Scheme
1
Reaction mechanism exploration of the N-methylation
2 2 2 2
alkylation reagent was explored for the synthesis of N-methyl reaction with CO /H . (I) 1 mmol aniline, 3.0 MPa CO , 6.0 MPa H , 50 mg
ꢁ
CuAlOx catalyst, 160 C, 12 h; (II) 1 mmol aniline, 16 mmol MeOH, 50 mg
or N,N-dimethyl aniline with nine parallel reactions, Scheme
. All the reactions were progressed for 12 h and checked by
GC-MS. In reaction (I), the products were 65% N-methyl
ꢁ
CuAlOx catalyst, 160 C, 12 h; (III) 1 mmol aniline, 16 mmol MeOH,
1
ꢁ
3
.0 MPa CO
2
, 50 mg CuAlOx catalyst, 160 C, 12 h; (IV) 1 mmol aniline,
, 6.0 MPa H , 16 mmol methanol, 50 mg CuAlOx catalyst,
2
aniline, 2% N-methyl-N-phenylformamide and 10% N,N- 160 C, 12 h; (V) 1 mmol aniline, 3.0 MPa CO, 6.0 MPa H , 50 mg CuAlOx
3
.0 MPa CO
2
ꢁ
2
ꢁ
dimethyl aniline. In reaction (II), although 16 equivalents of catalyst, 160 C, 12 h; (VI) 1 mmol aniline, 3.0 MPa CO
2
, 1.0 MPa CO,
ꢁ
6
.0 MPa H
formamide, 6.0 MPa H
mmol N-phenyl-N-methyl formamide, 6.0 MPa H
2
, 50 mg CuAlOx catalyst, 160 C, 12 h; (VII) 1 mmol phenyl
methanol were applied, the conversion of aniline was 60%
with the generation of 50% N-methyl aniline, 2% N-methyl-N-
phenylformamide and 5% N,N-dimethyl aniline. Similar
ꢁ
2
, 50 mg CuAlOx catalyst, 160 C, 12 h; (VIII)
, 50 mg CuAlOx
1
2
ꢁ
catalyst, 160 C, 12 h; (VIIII) 1 mmol diphenyl urea, 6.0 MPa H
2
, 50 mg
ꢁ
results were obtained if CO
2
or CO
2
/H
2
was added, (III) and CuAlOx catalyst, 160 C, 12 h. Y ¼ yield and the numbers were obtained
(
IV), which means the presence of CO
2
or CO
2
/H
2
does not by the peak areas of GC-MS without correction.
promote the reaction of aniline and methanol. At the same
time, the methanol concentration in the reaction mixture was
<
1% aer the reaction of CO2 and H2 with or without the aniline and CO/H , with a 10% yield of N-methyl aniline and a
2
addition of aniline. Therefore the possibility of methanol as 2% yield of N,N-dimethyl aniline (V). Surprisingly, only 12%
the reaction intermediate is much less. Interestingly, only N-methyl aniline was produced even if aniline was reacted
1
2 2 2
3% conversion of aniline was obtained in the reaction of with CO/CO /H (VI), suggesting that the presence of CO did
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not improve the reaction of aniline with CO/H
presence of CO inhibited the reaction of aniline with CO /H . analyser and a 5 keV Ar ion-gun. All spectra were recorded
2
and that the with a dual Mg/Al anode X-ray source, a hemispherical capacitor
+
2
2
Based on this data, CO does not appear to be an important using non-monochromatic Mg Ka (1253.6 eV) radiation.
reaction intermediate. It is noteworthy that there was Nitrogen adsorption–desorption isotherms were measured at
no detectable N-phenylformamide and N-methyl-N-phenyl- 77 K using a Micromeritics 2010 instrument. The pore-size
formamide. Therefore, the hydrogenation reactivities of distribution was calculated by the Barrett, Joyner and Halenda
N-phenylformamide and N-methyl-N-phenylformamide were (BJH) method from the desorption isotherm. The Cu and Al
then tested. Clearly, as the results show for reaction (VII) and content of the catalyst was measured by inductively coupled
(
VIII), N-phenylformamide and N-methyl-N-phenylformamide plasma-atomic emission spectrometry (ICP-AES), using an Iris
were quantitatively converted into N-methyl aniline and N,N- advantage Thermo Jarrel Ash device.
dimethyl aniline. These results suggest that formamide was
the possible intermediate for the N-methylation reaction.
However, the formamide was not derived from carbon
Typical procedure for catalyst preparation
monoxide due to the poor results given in reaction (V). In 1.45 g Cu(NO
addition, only 30% N-methyl aniline and 7% N,N-dimethyl (6.0 mmol) were added into 100 mL deionized water and agitated
aniline were produced if using N,N-diphenyl urea as the until complete dissolution. Then, 20 mL Na CO solution (0.94 M)
3 2 2 3 2 2
) $3H O (6.0 mmol), 2.25 g Al(NO ) $9H O
2
3
starting material (VIIII), which suggests that di-substituted was added in a dropwise manner and the mixture was further
urea was not the reaction intermediate. According to the stirred for 4 h at room temperature. The reaction mixture was
discussions above, a reaction mechanism is given in Scheme centrifuged and washed with water to remove the base until the
1
. First, aniline was carbonylated with CO /H to generate pH value of the aqueous solution was ꢂ7.0. Subsequently, the
2
2
ꢁ
ꢁ
N-phenylformamide and it was then quickly hydrogenated solid was dried at 120 C for 4 h, calcined in static air at 350 C for
into N-methyl aniline. Following this, the N-methyl aniline 12 h and reduced under hydrogen ow at 350 C for 3 h. The
ꢁ
was again reacted with CO
2 2
/H to form N-methyl-N-phenyl- catalyst was denoted as CuAlOx. According to the nitrogen
formamide and following further hydrogenation, N,N- absorption and desorption analysis, the BET surface area of the
2
ꢀ1
dimethyl aniline was produced.
CuAlOx catalyst was 138 m g .
Conclusion
Typical reaction procedure for the N-methylation of amines
In conclusion, the direct use of CO /H in N-methylation reac- 1.0 mmol amine, 50 mg CuAlOx catalyst and 2 mL hexane were
2
2
2
tions was achieved successfully. The results showed that by added into an 80 mL autoclave. It was then exchanged with CO ,
applying a simple CuAlOx catalyst, primary amines, secondary and 3.0 MPa CO
2
and 6.0 MPa H
2
were introduced. The reaction
ꢁ
amines, nitrobenzene and aromatic nitrile derivatives can be was reacted at 160 C for 24 h under magnetic stirring. Subse-
efficiently and selectively transformed into the corresponding quently, the autoclave was cooled to room temperature, and
N-methyl or N,N-dimethyl products with excellent yields. It 40 mg biphenyl and 10 mL ethanol were added for quantitative
offers a clean and economic method for the synthesis of analysis by GC-FID (Agilent 6890A).
0
N-methyl and N,N -dimethyl amines. Furthermore, it also
promotes the use and recycling of CO2.
Typical reaction procedure for the N,N-dimethylation of
amines
Experimental section
Generals
1.0 mmol amine, 50 mg CuAlOx catalyst and 2 mL hexane were
added into an 80 mL autoclave. Then it was exchanged with
All solvents and chemicals were obtained commercially and CO
2
, and 3.0 MPa CO
2
and 7.0 MPa H
2
were introduced. The
ꢁ
were used as received. Mass spectra were in general recorded on reaction took place at 160 C for 48 h under magnetic stirring.
an HP 6890/5973 GC-MS. High-resolution TEM analysis was Subsequently, the autoclave was cooled to room temperature,
carried out on a JEM 2010 operating at 200 keV. The catalyst and 40 mg biphenyl and 10 mL ethanol were added for quan-
samples aer pretreatment were dispersed in methanol, and titative analysis by GC-FID (Agilent 6890A).
the solution was mixed ultrasonically at r.t. A part of the solu-
tion was dropped on the grid for the measurement of the TEM
images. XRD measurements are conducted by a STADI P auto-
mated transmission diffractometer (STOE) equipped with an
Typical reaction procedure for the N-methylation of secondary
amines
incident beam curved germanium monochromator selecting Cu 1.0 mmol amine, 50 mg CuAlOx catalyst and 2 mL hexane were
ꢁ
Ka1 radiation and a 6 position sensitive detector (PSD). The added into an 80 mL autoclave. It was then exchanged with CO
2
,
ꢁ
XRD patterns are scanned in the 2q range of 20–90 . For the data and 3.0 MPa CO
and 7.0 MPa H
were introduced. The reaction
2
2
ꢁ
interpretation WinXpow (STOE) soware and the Powder took place at 160 C for 24 h under magnetic stirring. Subse-
Diffraction File (PDF) database of the International Centre of quently, the autoclave was cooled to room temperature, and
Diffraction Data (ICDD) were used. The XPS measurements 40 mg biphenyl and 10 mL ethanol were added for quantitative
were performed with a VG ESCALAB 210 instrument provided analysis by GC-FID (Agilent 6890A).
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Typical reaction procedure for the N,N-dimethylation of
nitrobenzenes and nitriles
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, and 3.0 MPa CO
2
and 7.0 MPa H
2
were
ꢁ
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