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185
CHAPTER-6 (PART B)
Application of pyridinium based ionic liquids in the
selective N-methylation of anilines with dimethylcarbonate
Introduction: A Review of N-alkylation reactions
Aniline and aniline derivatives are compounds which possesses interesting
properties because of which chemists are attracted towards developing appropriate
synthetic protocol towards the synthesis of these important molecules. N-methylated
anilines have proved to be essential part in the field of agrochemical industries,
rubber industries, pharmaceutical industries and textile industries.1 It may also be
mentioned that N-methylamino acids acts as β-blockers and as amino acid residue
which terminates the synthesis of DNA in cancerous cells.
Due to the importance of N-alkylated anilines, some procedures for the
synthesis of these important compounds are reviewed here. Previously
N-methylanilines was synthesized by following the different synthetic pathway
which included (a) reduction of imines,2 amides,
3 oximes,
4 aziridine
5 and vinyl
derivatives6 (b) metal mediated alkylation,
7 (c) nucleophilic substitutions
8 and
others. Recently E. Byun et al. has reported the reductive N-alkylation of aniline
and nitroarene derivatives using different aldehydes catalysed by Pd/C in aqueous
2-propanol as a solvent and ammonium formate as the in situ hydrogen donor.9 This
reaction was carried out using premixing method to form imine first prior to
alkylation. The formation of the imines is essential because in the absence of this
reactive intermediate, the yield of the N-alkylated product would be very low and
side products would be numerous. The reaction is shown in Scheme 6B.1.
186
Scheme 6B.1
X
R
R1 H
O
R
HN R1
X= NH2, NO2
+HCOO-NH4
+/Pd/C
IPA/H2O
As a general procedure, the N-alkylated amines are prepared in liquid phase
using mineral acids as catalyst and alkyl halides or dimethylsulphate as alkylating
agents.10
W. Wang et al. had synthesized N-methylanilines from anilines on basic
zeolite CsOH/Cs, Na-Y under flow condition and used methanol as methylating
agent. Using in situ stopped-flow (SF) MAS NMR spectroscopy they had
successfully established that the intermediate formation of N-methyleneaniline as
the intermediate in the transformation.11
Another important method reported was the
noncatalytic N-methylation of anilines in supercritical methanol. The reaction was
carried out at high temperature which required long reaction time. This method of
alkylation invariably lead to the formation of the disubstituted product namely the
N,N-dimethylaniline unless reaction time and temperature were carefully controlled.
The authors had also carried out kinetic study and found that the rate of the reaction
increased with the addition of a small quantity of a base.12
T. Oku et al. had
investigated the activity of conventional solid acids such as H-mordenite, zeolite,
amorphous silica alumina and acid base bifunctional compound like Cs-P-Si mixed
oxide and alumina as catalysts for the selective N-methylation of bifunctionalized
amines with supercritical methanol in a continuous-flow, fixed-bed reactor.13
The
reaction is shown in Scheme6B.2.
187
Scheme 6B.2
N
OH
H
H
CH3OHN
OHH
N
OH
+ +Cat
NH
CH3OH
N
O
H
H
Cat
2-aminoethanolN-methylaminoethanol N,N-
dimethylaminoetahnol
Ethyleneamine 2-methoxyethylamine
Zn1-xNixFe2O4 types of catalysts were found to be effective in the selective
N-methylation of anilines with methanol. Optimization of the reaction conditions
showed that ZnFe2O4 is the most efficient compared to such other catalyst with the
same composition but different stoichiometry. ZnFe2O4 afforded products with 99%
selectivity for the formation of the mono alkylated N-methylaniline.14
In a later
communication, the same author had reported that N-alkylation of aniline using
Zn1-xCoxFe2O4 type of catalyst and compared the effectiveness of methanol and
dimethylcarbonate (DMC) as the alkylating agent in the reaction.15
B. Basu et al.
had successfully applied silica as promoter in the selective N-alkylation of amines
with different alkyl halide.16
From the results of this investigation, they concluded
that the selectivity of the reaction depended on the nature of silica used, the nature
and amount of the alkyl halide used and the temperature of the reaction. Another
synthetic procedure for N-alkylation of primary amines, diamines and polyamines
was developed by R. N. Salvatore et al., where they had used Cs-base as the
promoter and alkyl halide as alkylating agent.17
An efficient and easily recyclable
catalyst for this reaction is the unmodified magnetite, which was successfully
recycled as many as eight times without losing its effectiveness. The reactions were
completed within 7-10 days, through a hydrogen autotransfer process in presence of
188
potassium tert-butoxide and 1,4-dioxane.18
Other promoters which were used for
this specific synthesis are [Cp*IrCl2]2/NaHCO3 (Cp*= pentamethyl
cyclopentadienyl),19
Ph3P/DDQ,20
cationic ruthenium(II) compound
([(PPh3)2Ru(CH3CN)3Cl][BPh4]),21
Pd/Fe2O322
etc. In all these cases of alkylation,
the alcohols were used as the alkylating agent.
In the previous chapter (Chapter 6: Part A) a discussion regarding the
disadvantages of using alkyl halide and dimethyl sulphate as the alkylating agent in
contrast to the advantages of using dimethyl carbonate as the alternative green
alkylating agent have been reported. Attempts to use the user friendly DMC for
N-methylation of anilines to N-methylanilines have already been reported in several
publications. M. Selva et al. had used NaY Faujasite for the synthesis of
N-methylanilines from anilines using symmetrical and unsymmetrical alkyl
carbonate as methylating agents. They had also performed the reactions using zeolite
catalyst in triglyme solvent.23
The authors in a subsequent communication had
reported another method for the mono N-methylation of primary amines with the
unsymmetrical alkyl methyl carbonate. In this study they had used Na-exchanged Y
faujasite as promoter and also examined the kinetic effect of the reactions in
different solvents .The results of the study indicated that the reaction is inhibited in
more polar solvent like DMF.24
Another important method developed by the same
authors is the chemoselective methylation of aminophenols, aminobenzylalcohols,
aminobenzoic acids and aminobenzamides. It was NaY faujasite which promoted
the reactions and selectively only N-methyl compounds were formed rather than the
formation of other concurrent compounds such as O-methylation product and or
N-/O- methoxy carbonylation products.25
An interesting procedure reported is the
use of the methylcarbamates as the methylating agent. The carbamates were
synthesized from primary aliphatic amines and DMC in supercritical CO2.26
In
addition to the methods reported above, there are other elegent and selective
procedures reported for the selective synthesis of N-alkylation of anilines.27
189
In 2003, N. Nagaraju and his co-author had studied the catalytic activity of
amorphous AlPO4 and Metal-AlPO4 in the vapour phase alkylation of aniline with
methanol or DMC. During investigations it was observed that V-AlPO4 and
Co-AlPO4 showed high selectivity towards N-monomethylation of aniline.28
A
subsequent report was published by C.-P. Xu and co-workers where they had
successfully alkylated not only secondary amines such as piperazines but also
amino acids and amino alcohols in by a process of hydrogenation in the presence of
Pd/C or Pd(OH)2/C as catalyst in the presence of a variety of alcohols as the
alkylating agents.29
The reaction is summarized in Scheme 6B.3.
Scheme 6B.3
NNH
O
Ph
NNR
O
Ph
Pd/C; Pd(OH)2/C
H2; ROH
Alkylation of amines with alcohols
N
BnO OBn
HBnO
N
HO OH
MeHOOH
OH
H2; Pd/C
MeOH; rt
One-pot debenzylation-N-methylation of pyrrolidine
N
BnO
BnHO
N
HO
MeHO
H2; Pd(OH)2/C
MeOH; rt
COOHCOOH
One-pot O-debenzylation-N-transalkylation of N, O dibenzylbulgecinine
190
It is evident from the above mentioned references that the commonly used
alkylating agents are alkyl halide, alcohol and DMC. Another alkylating agent
which is successfully applied for N-alkylation of aniline is the nitrile. H. Sajiki et al.
had used nitrile for N-alkylation of aromatic and aliphatic amines promoted by the
catalysts Pd/C or Rh/C.30
The same authors have reported using the same catalyst and the nitrile in the
intermolecular cyclization leading to the synthesis of indole and indole derivatives
they have used the same procedure for a direct transformation of monoalkylated
aromatic secondary amines from the aromatic nitro compounds.31
Reductive
monoalkylation of aliphatic and aromatic nitro and amino compounds were carried
out by R. Nacario and co-author using ammonium formate as the hydrogen source,
Pd/C as the hydrogen transfer catalyst and nitrile as alkylating agent.32
The reactions
are summarized in Scheme 6B.4
Scheme 6B.4
X
R R' CN
NHCH2R'
R
N(CH2R')2
R
X=NO2, NH2
NH4HCO2/H2O
Pd/C, CH3OH+ +
NO2
NH2
OMeO2NH2N OMe
N
HN
CH3
EtHN OMe
N
HN
CH3
+Et3NH+formate
Pd/C, CH3CN
191
In the previous chapters, the advantages of MW assisted synthesis have been
reported. It may be mentioned that the use of microwave technique can be
conveniently used for the N-alkylation of amines and the reactions were found to be
highly efficient. In 2009, G. Marzaro et al. successfuly applied the microwave
technique for this reaction in water medium without any catalyst and alkyl halide
was used as the alkylating agent.33
A similar procedure was reported by M. C.
Lubinu et al. where they had used Pd catalyst and tertiary amine to enhance the
reaction of N- alkylation using similar reagents.34
The use of ILs for the synthesis of
N-alkylated amines is rare and only a few methods are available in literature.
Imidazolium based ILs [bmim][PF6], [bmim][NTf2], [emim][NTf2] were used as a
solvent for N-alkylation of anilines and both the N-methyl anilines as well as
N,N-dimethyl anilines were obtained as the end products. C. Chiappe et al. had
reported the use of ILs for the N-alkylation using a variety of alkyl precursors such
as examined with many alkyl halides such a MeI, EtI, BuCl, BuBr, CH2=CH-CH2Br,
BnBr.35
The same authors had reported the successful application of [bmim][PF6]
for the alkylation of primary amines using simply alkyl halides or the tosylates.36
High chemoselective alkylation of multifunctional amines such as benzylamine and
amino acid derivative were successfully carried out by them. Monopoli et al.,
reported the use of molten quaternary ammonium salt as the medium for
N-alkylation with alkyl chloride as the alkylating agent.37
In continuation with the
investigation carried out with DMC as the methylating agent in ILs, it was found
worthwhile to study the N-methylation reaction with the same reagent under
identical reaction conditions.
6B.1. Materials and method
From literature reviewed above, it is clear that selective mono N-alkylation
of amines is not an easy task to achieve as over methylation to give both the N-alkyl
amine and N,N-dialkylamines is a possibility. Over alkylation as well as
C-alkylated products were often formed in addition to the desired N-alkylated
products. Further the use of toxic methylating/alkylating agents also makes the
reaction unfavourable from the point of view of Green Chemistry. It may also be
192
mentioned that some of the catalysts used suffer from deactivation during the course
of the reaction and hence these cannot be recycled. It is found that with most of the
catalyst the reactions need long reaction time which is not energy efficient as per the
requirement of green chemistry technique.
In this chapter, an attempt had been successfully made to use the pyridinium
based ILs as catalyst cum solvent for the selective mono-N-methylation of anilines
and its derivatives using DMC as the source of the methyl group. Results indicate
that this reagent and reaction medium used is much more efficient for
N-methylation and has a distinct advantage over other reported methods. In
Chapter 2 the preparation and characterization of three different type of pyridinium
based ILs has already been discussed. Here, 1-butyl-4-methyl pyridinium bromide
was used as catalyst cum solvent and DMC was used as the methylating agent for
the synthesis of N-methylated anilines. The main aim of the reaction was to study
the selectivity towards the formation of N-mono methylated product of anilines
which, depended on the amount of IL used and the temperature of the reaction.
Experiments carried out indicated that when 1 equivalent of IL was used, both the
N-monomethyl as well as N,N-dimethyl anilines were formed in more or less equal
amounts. One redeeming observation was that the reaction proceeded to completion
and unreacted anilines were not observed. On the other hand when 0.5 equivalent of
IL was used, the N-methyl aniline was the predominant product and
N,N-dimethylanilines was obtained in negligible amount. The reactions were found
to proceed to complete conversion of the reactant. The temperature dependence of
the reaction was similarly studied and it was observed that at 170 °C both the
products were formed in more or less equal proportions and the percentage of N-
monomethyl aniline significantly improved on decreasing the reaction temperature.
The reactions were carried out at 130-150°C and it was found that complete
conversion to give the mono-N-methylated aniline occurred at the time range of 0.5-
1.5 hrs. The yield of mono N-methylated anilines were found to be to the extent of
70-75%.
193
As in the case of phenols, the reactivity of anilines towards N-methylation
also depended on the nature of the substituents present on the phenyl ring. In case of
2-nitroaniline the reaction was complete within 40 minutes and in case of 2-bromo
aniline and 3-bromo aniline the reaction time required for complete conversion was
1.5 hr. The results of the reaction with different substrates are summarized in Table
6B.1.
In a typical procedure, anilines, DMC and the IL namely the 1-butyl-4-
methyl pyridinium bromide were mixed and heated to 130-150 °C for 0.5-1.5 hrs
(Table 6B.1). The anilines were completely converted and N-monomethyl products
were obtained in high percentage yield and the byproduct N,N-dimethylanilines
were obtained in negligible amount. The isolation of the products was simple and
the IL could be recovered and reused. The reaction carried out is shown in Scheme
6B.5. All the synthesized products were characterized by NMR spectra and GC-Ms
spectra. In Figure 6B.(1-2) presented the 1H and
13C NMR spectra of one product
and in Figure 6B.3 the GC-Ms spectra of another product is shown.
Scheme 6B.5
NH2
R
H3CO O
CH3
O
NHCH3
R
CH3OH CO2
+ +
+
NBr
N(CH3)2
R
+
1(a-e) 2(a-e)
Yield= 1(a-e)/70-75%, 2(a-e)/25-30%
R= H, NO2, Br, Cl(NO2)
194
Table 6B.1: Methylation of anilines with DMC catalysed by IL
Entry Substrate Time
(min)
Product 1 Yielda
(%)
Product 2 Yielda
(%)
1 Aniline 60 (1a) N-
methylaniline
75 (2a) N,N-
dimethylaniline
25
2 2-nitroaniline 40 (1b) N-methyl-2-
nitroanilne
75 (2b) N,N-dimethyl-
2-nitroaniline
25
3 3-
bromoaniline
90 (1c) N-methyl-3-
bromoaniline
70 (2c) N,N-dimethyl-
3-bromoaniline
30
4 2-
bromoanilne
90 (1d) N-methyl-2-
bromoaniline
70 (2d) N,N-dimethyl-
2-bromoaniline
30
5 4-chloro-2-
nitroanilne
90 (1e) N-methyl-4-
chloro-2-
nitroaniline
70 (2e) N,N-dimethyl-
4-chloro-2-
nitroaniline
30
a. Yields refers to pure isolated products.
195
Figure 6B.1
1H NMR Spectra of N-methyl-2-Bromoaniline
Figure 6B.2
13
C NMR Spectra of N-methyl-2-Bromoaniline
NH
H3C
Br
NH
H3C
Br
196
Figure 6B.3: Mass Spectra of N-methyl-4-chloro-2-nitroaniline
NHH3C
Cl
NO2
197
6B.2. Conclusion
From the experiments performed and reported herein, it was observed that
selective mono-N-methylation of anilines can be carried out under environmentally
benign condition by using the DMC as the methyl group donor. Rigorous control of
reaction conditions in terms of the amount of IL used and the reaction temperature
gave predominantly the monomethylated products and the possible C-methylated
products were totally absent. The reaction can be performed by a simple procedure
by using the pyridinium based ILs which acts not only as a solvent for the reaction
but also as a catalyst. The reaction conditions are simple and environment friendly
and no VOCs need to be used. The best advantage of the reaction was that the IL
could be recycled and reused.
198
6B.3. Experimental section
Melting points were recorded in a VMP-D model Melting point apparatus
and are uncorrected. 1H-NMR and
13C-NMR spectra were recorded in a Bruker
Advance digital 300 MHz spectrometer in CDCl3. In both the recordings TMS was
used as the internal standard. Mass spectra were recorded in a Perkin Elmer Clarus
600 Gas Chromatograph and Clarus 600C Mass Spectrometer (Column used Elite
5MS).
6B.3.1. Selective N-mono methylation of anilines: Synthetic
procedure
In a 5 mL RBF, 1mmol anilines, 1 mL DMC and 0.5 mmol of IL were added
and placed in an oil bath fitted with a reflux condenser. The reaction mixture was
heated to 130-150°C for the period indicated in Table 6.1. On completion of
reaction, as monitored by TLC using ethyl acetate and petroleum ether (60-80 °C),
the reaction mixture was extracted with diethyl ether (5mL x 3), washed with water,
dried with anhydrous Na2SO4 and solvent removed by distillation. The crude
product was purified by column chromatography on silica gel column and ethyl
acetate-petroleum ether as eluent.
6B.3.2. Reusability of ionic liquid
After completion of the reaction, water was added and the crude products
extracted with diethyl ether. The IL being water soluble was recovered by
evaporation of the water extract in a rotary evaporator and then stored in desiccator
for further use. The recycled catalyst could be used for three successive runs
without appreciable loss in its catalytic activity.
199
6B.3.3. Spectral data
N-methylaniline
Colourless Liquid.
1H NMR (300 MHz, CDCl3, TMS) δH: 7.276 (2H, t, J = 7.5 Hz,
ArH), 6.795 (1H, t, J = 7.2 Hz, ArH), 6.684 (2H, d, J = 7.8 Hz,
ArH), 3.523 (1H, s, NH), 2.882 (3H, s, NCH3).
13C NMR (75 MHz, CDCl3, TMS) δ: 149.40, 129.27, 117.28, 112.48, 30.77.
GC/Ms m/z (relative intensity): 107 ([M]+) (32), 106 (45), 88 (16), 86 (94), 84 (100),
77 (12), 51 (69).
N-methyl-2-nitroaniline
Mp: 39-40 °C.
1H NMR (300 MHz, CDCl3, TMS) δH: 8.181 (1H, d, J = 8.4 Hz,
ArH), 8.073 (1H, s, NH), 7.473 (1H, t, J = 8.1 Hz, ArH), 6.850
(1H, d, J = 8.7 Hz, ArH), 6.661 (1H, t, J = 7.8 Hz, ArH), 3.034
(3H, d, J = 5.4 Hz, CH3).
13C NMR (75 MHz, CDCl3, TMS) δ: 146.36, 136.33, 126.85, 115.21, 113.37, 29.75.
GC/Ms m/z (relative intensity): 152 ([M]+) (56), 135 (7), 119 (12), 107 (8), 106 (32),
105 (61), 104 (29), 91 (13), 79 (71), 77 (100), 63 (13), 51 (25).
N-methyl-3-bromoaniline
Liquid.
1H NMR (300 MHz, CDCl3, TMS) δH: 7.027 (1H, t, J = 8.1 Hz,
ArH), 6.814 (1H, d, J = 8.4 Hz, ArH), 6.739 (1H, s, ArH), 6.519
(1H, d, J = 8.4 Hz, ArH), 2.822 (3H, s, CH3).
HN
Me
HN
Me
NO2
HN
Me
Br
200
13C NMR (75 MHz, CDCl3, TMS) δ: 150.37, 139.77, 130.26, 119.75, 114.59,
111.10, 30.38.
GC/Ms m/z (relative intensity): 187 ([M+2]+) (83), 186 (85), 185 ([M]
+) (100), 184
(97), 173 (5), 171 (6), 105 (51), 104 (28), 92 (13), 91 (21), 77 (61), 63 (22), 52 (41).
N, N-dimethyl-3-bromoaniline
Liquid.
1H NMR (300 MHz, CDCl3, TMS) δH: 7.083 (1H, t, J = 7.8 Hz,
ArH), 6.835-6.809 (2H, m, ArH), 6.631 (1H, d, J = 7.8 Hz,
ArH), 2.947 (6H, s, N(CH3)2).
13C NMR (75 MHz, CDCl3, TMS) δ: 130.19, 119.01, 116.23, 115.01, 113.12,
110.84, 40.35, 40.21.
GC/Ms m/z (relative intensity): 201 ([M+2]+) (75), 200 (95), 199 ([M]
+) (83), 198
(100), 187 (23), 186 (23), 185 (30), 184 (28), 157 (9), 155 (9), 118 (48), 105 (23),
104 (23), 77 (30).
N-methyl-2-bromoaniline
Liquid.
1H NMR (300 MHz, CDCl3, TMS) δH: 7.422 (1H, d, J = 7.8 Hz,
ArH), 7.216 (1H, t, J = 7.5 Hz, ArH), 6.646-6.558 (2H, m,
ArH), 4.357 (1H, s, NH), 2.903 (3H, s, NCH3).
13C NMR (75 MHz, CDCl3, TMS) δ: 132.21, 128.50, 117.54, 110.67, 109.55, 30.57.
GC/Ms m/z (relative intensity): 187 ([M+2]+) (63), 186 (100), 185 ([M]
+) (65), 184
(90), 105 (62), 104 (30), 91 (23), 77 (63), 64 (16), 63 (24), 52 (41).
N
Me
Br
Me
HN
Me
Br
201
N-methyl-4-chloro-2-nitroaniline
Mp: 107-109 °C.
1H NMR (300 MHz, CDCl3, TMS) δH: 8.191 (1H, s, ArH),
8.046 (1H, s, NH), 7.420 (1H, d, J = 9.6 Hz, ArH), 6.821 (1H,
d, J = 9.3 Hz, ArH), 3.036 (3H, d, J = 5.4 Hz, CH3).
13C NMR (75 MHz, CDCl3, TMS) δ: 145.19, 136.30, 125.76, 114.67, 29.74.
GC/Ms m/z (relative intensity): 188 ([M+2]+) (15), 186 ([M]
+) (49), 141 (11), 140
(13), 139 (16), 138 (14), 125 (13), 113 (33), 111 (31), 105 (100), 77 (40), 75 (28).
HN
Me
NO2
Cl
202
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