Selective N-alkylation of aromatic primary amines catalyzed by bio-catalyst or deep eutectic solvent

Article abstract of DOI:10.1007/s10562-010-0479-9

Biocatalysts or deep eutectic solvents (DES) are effective for selective N-alkylation of various aromatic primary amines. These methods avoided complexity of multiple alkylations giving products in good yields. Both DES and lipase can be recycled and re-used at least five times. In addition, these catalysts are biodegradable, non-toxic and cost-effective.

Full text of DOI:10.1007/s10562-010-0479-9

Catal Lett (2011) 141:178–182
DOI 10.1007/s10562-010-0479-9
Selective N-Alkylation of Aromatic Primary Amines Catalyzed
by Bio-catalyst or Deep Eutectic Solvent
•
Balvant Singh Hyacintha Lobo
•
Ganapati Shankarling
Received: 25 August 2010 / Accepted: 19 October 2010 / Published online: 2 November 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Biocatalysts or deep eutectic solvents (DES)
are effective for selective N-alkylation of various aromatic
primary amines. These methods avoided complexity of
multiple alkylations giving products in good yields. Both
DES and lipase can be recycled and re-used at least five
times. In addition, these catalysts are biodegradable,
non-toxic and cost-effective.
However, most of the traditional methods require polar
organic solvents, high reaction temperature and lack
selectivity, thus resulting in multiple alkylations, even with
the use of limited amounts of the alkylating agent. Hence,
these factors greatly limit the scope of selective procedures
for mono N-alkylation of aromatic primary amines.
Biocatalysis is an efficient and green tool for organic
synthesis that is selective and requires mild reaction con-
ditions [11]. Lipases are ubiquitous enzymes of consider-
able industrial importance. Several lipases are active in
organic solvents and catalyze many important reactions,
including esterification [12], transesterification [13], reg-
ioselective acylation of glycols and menthols [14], and
peptide synthesis [15].
Keywords N-alkylation Á Aromatic primary amines Á
Deep eutectic solvent Á Lipase
1 Introduction
Amines and their derivatives are noteworthy intermediates
in different applications, such as fine chemicals, agro-
chemicals, dye intermediates, pharmaceuticals and also as
catalysts for polymerization [1, 2]. Aromatic and aliphatic
secondary amines are also used as antioxidants in petro-
chemicals, polymers, and rubber [3]. Due to their unique
biological properties, substituted amines are widely used
in antihypertensive, antihistamine, and anti-inflammatory
drugs [4, 5].
Besides enzymatic catalysis, green methods such as the
use of ionic liquids (ILs) have recently attracted considerable
attention due to their unique properties. ILs, however, are
neither biodegradable nor cost-effective [16]. Deep eutectic
solvents (DES) are similar to conventional ILs in terms of
low vapor pressure and low flammability. In addition, DES
are biodegradable, non-toxic, and inexpensive. DES are
mainly prepared by combining choline chloride with dif-
ferent hydrogen bond donors, such as urea, or lewis acids,
such as zinc chloride [16]. The mixing of choline chloride
and urea at 74 °C results in significant depression of freezing
point that arises from an interaction between urea molecules
and the chloride ion [17]. Choline is a naturally occurring
biocompatible compound and choline chloride is commer-
cially produced on a large scale as a chicken feed additive
[18]. DES are useful in electrochemical applications [19, 20],
including electropolishing of metal alloys, as electrolytes in
dye-sensitized solar cells, etc. but their ability to serve as
catalysts as well as solvents has not been adequately
explored in the field of synthetic organic chemistry except a
few reported examples [21–23].
Synthetic approaches to prepare aromatic secondary
amines include reductive alkylation [6], direct N-alkylation
by SN2 displacement [7], and preparation in the presence of
cesium bases in DMF [8]. Recently, some methods have
been developed for selective N-alkylation involving sup-
ported catalysts [9] or metal complexes [10].
B. Singh Á H. Lobo Á G. Shankarling (&)
Dyestuff Technology, Institute of Chemical Technology,
N. P. Marg, Matunga, Mumbai, Maharashtra 400019, India
e-mail: gs.shankarling@ictmumbai.edu.in;
gsshankarling@gmail.com
123

Selective N-Alkylation of Aromatic Primary Amines
179
Herein, we report selective mono N-alkylation of aro-
matic primary amines using deep eutectic solvent of cho-
line chloride and urea that plays a dual role as efficient
catalyst as well as a recyclable solvent and lipase as a
biocatalyst in ethanolic medium. Both of these methods
were conducted under mild conditions, avoiding the use of
a strong base or highly polar organic solvents.
formation. Another deep eutectic mixture used during
optimization was prepared similarly by combining choline
chloride and glycerol in 1:2 ratio respectively, maintaining
the temperature at 80 °C.
2.2 Procedure for Lipase-Catalysed N-Alkylation
In a 50 mL round bottom flask, a mixture of aniline (1.0 mL,
10.7 mmol) and hexyl bromide (2.3 mL, 16 mmol) in eth-
anol (6 mL) with lipase as catalyst (50 mg, 5% by weight of
amine) was added (Scheme 2) and stirred at 45 °C for 4 h.
The progress of the reaction was monitored by TLC [solvent
system—hexane:ethyl acetate (8:2)]. After cooling, the
catalyst was filtered off through a Whatmann-42 filter paper
and washed with ethanol. The filtrate was distilled under
vacuum using rotary evaporator to recover ethanol and the
residue thus obtained was washed with water and extracted
with ethyl acetate. The residue was chromatographed on
silica gel column and eluted with hexane:ethyl acetate (9:1),
to afford pure hexyl aniline.
2 Experimental
The enzyme lipase was acquired from Sigma-Aldrich. It
was obtained from Pseudomonas sp. strain with particle
size of less than 60–100 mesh and activity of 30,000 u/g.
FT-IR spectrums were recorded on a Bomem Hartmann
and Braun MB-Series FT-IR spectrometer. 1H NMR
spectrums were recorded on Varian 300 MHz mercury plus
spectrometer and mass spectral data were obtained with a
micromass-Q-TOF (YA105) spectrometer. Common
reagent grade chemicals were procured from M/s S.D. Fine
Chemical Ltd. India and were used without further
purification.
This batch was scaled up for the reaction of aniline with
hexyl bromide that gave product with a yield of 84%. The
lipase catalyst recovered by filtration during scale-up batch
was used for recycling studies.
2.1 Preparation of Deep Eutectic Solvent
In this study, two deep eutectic solvents (DES) were syn-
thesized according to the procedures reported in the liter-
ature [17]. The preparation involved reaction of choline
chloride 1 (1 mol) with urea 2 (2 mol) at 74 °C (Scheme 1)
till a clear solution was obtained which was used for
reactions without any purification. This method gave deep
eutectic solvent 3 with 100% atom economy since it
completely forms a eutectic mixture with no by-product
2.3 Deep Eutectic Solvent Catalyzed Reaction
In a 50 mL round bottom flask, a mixture of aniline
(1.0 mL, 10.7 mmol) and hexyl bromide (2.3 mL,
16 mmol) in deep eutectic solvent (7.0 mL) was added
(Scheme 2) and stirred at 50 °C for 4 h. The progress of
the reaction was monitored by TLC [solvent system—
hexane:ethyl acetate (8:2)]. After cooling, ethyl acetate was
Scheme 1 Preparation of deep
eutectic mixture from choline
chloride and urea
Scheme 2 Selective
N-alkylation of aromatic amines
with alkyl bromide in lipase-
catalyzed medium and deep
eutectic solvent medium
123

180
B. Singh et al.
added to the reaction mixture and the DES layer was
separated off. The ethyl acetate layer was washed with
water and concentrated. The residue was chromatographed
on silica gel column and eluted with hexane:ethyl acetate
(9:1), to afford pure hexyl aniline.
Table 1 Influence of organic solvents for mono N-alkylation of
aniline with hexyl bromide in presence of lipase as a bio-catalyst
Sr. no.
Organic solvents
Reaction time (h)
Yield (%)a
1
2
3
4
Ethanol
4
85
81
75
50
Chloroform
Dichloromethane
Hexane
5
This batch was also scaled up for the reaction of aniline
with hexyl bromide that gave product with a yield of 79%.
The deep eutectic solvent layer obtained during extraction
in the scale-up batches was recovered and used for recy-
cling studies.
6–8
12
Reaction conditions: aniline (1.0 mL, 10.7 mmol), hexyl bromide
(2.3 mL, 16 mmol), lipase (50 mg, 5% by weight of amine), organic
solvent (6 mL), temp. = 45 °C
a
Isolated yields
2.4 Selected Spectral Data
selected as a model. Ethanolic medium with lipase catalyst
gave the desired product in a shorter period of time with
comparatively greater yield, indicating that ethanol is
efficient compared to other solvents in terms of perfor-
mance and green aspect, due to its biomass origin.
Three different biodegradable media were tested for
their dual activity as catalysts and solvents in the selective
N-alkylation reaction of aromatic amines (Table 2). The
results for N-alkylation of aniline with hexyl bromide
produced comparatively better results in DES made from
choline chloride and urea.
6a: kmax (methanol)/nm 247 and 301; mmax =cmÀ1 3434 (NH),
1270 (C–N), 1643 (N–H bend); 1H NMR (300 MHz:
CDCl3, Me4Si) 0.89–1.61(13H, m, aliphatic CH), 3.27(1H,
s, NH), 6.62–7.26 (5H, m, aromatic CH); 13C NMR
(300 MHz: CDCl3, Me4Si) 131.9, 129.3, 114.3, 113.1,
44.4, 31.7, 29.8, 26.9, 22.7, 14.1; m/z (EI) 178.1 (M ? 1);
C12H19N calculated m/z: 177.1
6c: kmax (methanol)/nm 241 and 295; mmax =cmÀ1 3434
(NH), 1370 (C–N), 1235 (C–O stretching),1610 (N–H
bend); 1H NMR (300 MHz: CDCl3, Me4Si) 0.89–2.16
(11H, m, aliphatic CH), 3.22 (3H, m, CH2 and NH),3.78
(3H, s, OCH3), 6.18–7.24 (4H, m, aromatic CH); 13C NMR
(300 MHz: CDCl3, Me4Si) 161.6, 130.7, 105.4, 102.6,
96.8, 55.8, 45.2, 30.5, 26.4, 31.7, 22.8,14.1; m/z (EI) 208.2
(M ? 1); C13H21NO calculated m/z: 207.2
Lipase or DES-catalyzed reactions were further
explored in mono N-alkylation reactions of different aro-
matic amines, as summarized in Table 3. A wide range of
structurally varied aromatic primary amines with different
functional groups underwent mono N-alkylation with var-
ious alkyl bromides under mild reaction conditions.
Selective N-alkylation reactions of aromatic amines with
electron donating groups gave good yields in both the
reaction media and their reaction times were shorter in
comparison to aromatic amines with electron withdrawing
groups. The reactions gave selectively mono products as a
major component with very faint spot of di-product as
observed on TLC plate.
6g: kmax (methanol)/nm 238 and 340; mmax =cmÀ1 above
3000 (NH), 1662 (N–H bend), 1369 (C–N), 1596, 1498
(C=C); 1H NMR (300 MHz: CDCl3, Me4Si) 0.89–1.54
(10H, m, aliphatic CH), 3.21–3.40 (4H, m, CH and NH),
6.52–7.25 (4H, m, aromatic CH); 13C NMR (300 MHz:
CDCl3, Me4Si) 145.6, 129.8, 122.8,114.9, 45.3, 31.7, 30.1,
26.7, 22.7, 14.1; m/z (EI) 212.1 (M ? 1);C12H18ClN cal-
culated m/z: 211.1
6i: kmax (methanol)/nm 399 and 423; mmax =cmÀ1 3344
(NH), 1600 (N–H bend), 1282 (C–N), 1461, 1537 (C=C);
1H NMR (300 MHz: CDCl3, Me4Si) 0.90–2.17 (11H, m,
aliphatic CH), 3.19 (2H, m, aliphatic CH), 4.63 (1H, b s,
NH), 6.51–8.07 (4H, m, aromatic CH); 13C NMR
(300 MHz: CDCl3, Me4Si) 153.6, 137.7, 141.5, 114.9,
43.5, 31.5, 29.1, 26.7, 22.6, 14.0 m/z (EI) 221.1 (M - 1);
C12H18N2O2 calculated m/z: 222.1
Although the role of DES and lipase in the present work
is yet to be confirmed, we propose the following mecha-
nism (Schemes 3, 4) for DES (choline chloride and urea)
Table 2 Influence of biodegradable media for mono N-alkylation of
aniline with hexyl bromide
Sr.
no.
Bio-degradable media
Reaction
time (h)
Yield
(%)a
1
2
3
Glycerol
10
8
55
65
78
3 Results and Discussion
Deep eutectic solvent [ChCl:glycerol]
Deep eutectic solvent [ChCl:urea]
4
Several organic solvents were screened in the initial
experiments for lipase-catalysed selective N-alkylation of
aromatic primary amines, as summarized in Table 1.
Herein, the reaction of aniline and hexyl bromide was
Reaction conditions: aniline (1.0 mL, 10.7 mmol), hexyl bromide
(2.3 mL, 16 mmol), biodegradable media (7.0 mL), temp. = 50 °C
a
Isolated yields
123

Selective N-Alkylation of Aromatic Primary Amines
181
Table 3 Mono N-alkylation of aromatic amines with alkyl bromide by using lipase as bio-catalyst in ethanolic media or deep eutectic solvent
(choline chloride:urea)
Entry
Ar
Alkyl group
Lipase-catalyzed reaction
Reaction time (h)
Deep eutectic solvent
Reaction time (h)
Yielda (%)
Yieldb (%)
6a
6b
6c
6d
6e
6f
–C6H5
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Hexyl
Butyl
4
85
84
82
82
82
78
80
77
74
77
80
80
79
75
85
83
81
78
4
5
78
73
80
70
77
70
75
73
70
72
74
80
77
73
89
87
80
79
–C10H7
4.5
2.5
4
m-MeOC6H5
o-MeC6H5
p-MeC6H5
o-ClC6H5
p-ClC6H5
3,4-diClC6H5
p-NO2C6H5
m-NO2C6H5
p-FC6H5
3
2.5
3
4
6
6
6g
6h
6i
5
5
4
8
5
10
9
6j
5
6k
6l
5
7
–C6H5
2
2
6m
6n
6o
6p
6q
6r
p-ClC6H5
p-NO2C6H5
–C6H5
Butyl
4.5
5
3
Butyl
6
Benzyl
Benzyl
Benzyl
Benzyl
3
3
m-MeOC6H5
p-ClC6H5
p-NO2C6H5
1.2
2
1.5
2.5
4
3.5
a
Isolated yield obtained from lipase-catalyzed reaction at 45 °C
Isolated yield from reaction in deep eutectic solvent at 50 °C
b
Scheme 3 Proposed
mechanism for mono
N-alkylation of aromatic
primary amines in deep eutectic
solvent (choline chloride:urea)
Scheme 4 Proposed
mechanism for mono
N-alkylation of aromatic
primary amines in lipase-
catalyzed medium
and lipase as a catalyst in selective mono N-alkylation of
aromatic amines. Lipase catalysts are known to function by
means of proton abstraction by Asp-His dyad [24] that
assists in increasing the nucleophilicity of aromatic amine.
In case of DES, the hydrogen bonding interactions between
DES and aromatic amino group could be possibly
responsible for increasing the nucleophilicity of the latter
thus leading to its faster attack on alkyl bromide.
123

182
B. Singh et al.
yields. The DES is biodegradable, non-toxic, inexpensive,
and recyclable. Hence, the findings of the present study
demonstrate the potential use of lipase or DES as efficient
and recyclable catalysts/solvents.
Acknowledgments Authors are thankful to UGC-CAS for provid-
ing fellowship and Institute of Chemical Technology, SAIF IIT—
Bombay and Institute of Science, Mumbai for recording 1H NMR, IR
and Mass spectra.
References
1. Seayad AMA, Klein H, Jackstell R, Gross T, Beller M (2002)
Science 297:1676
2. Bhattcharyya AK, Nandi DK (1975) lnd Eng Chem Prod Res Dev
14:162
3. Pillai RBC (1993) J Mol Catal 84:125
4. Bradshaw JS, Krakowiak KE, Izatt RM (1992) Tetrahedron
48:4475
Fig. 1 Recyclability of catalysts. a Isolated yield obtained from
lipase-catalyzed reaction at 45 °C. b Isolated yield from reaction in
deep eutectic solvent at 50 °C
5. Bergeron RJ, Feng Y, Weimar WR, McManis JS, Dimova H,
Porter C, Raisler B, Phanstiel O (1997) J Med Chem 40:1475
6. Szardenings AK, Burkoth TS, Look GC, Campbell DA (1996)
J Org Chem 61:6720
7. Koh K, Ben RN, Durts T (1993) Tetrahedron Lett 34:4473
8. Salvatore RN, Nagle AS, Jung KW (2002) J Org Chem 67:674
9. Won KJ, Kazuya Y, Noritaka M (2009) J Catal 263:205
10. Benoit B, Stefan M, Rhett K (2009) Chem Eur J 15:3790
11. Schmid A, Dordick JS, Hauer B, Kiener A, Wubbolts M, Witholt
B (2001) Nature 409:258
12. Kiran KR, Divakar S (2001) J Biotechnol 87:109
13. Yadav GD, Manjula Devi K (2004) Chem Eng Sci 59:373
14. Therisod M, Klibanov AM (1987) J Am Chem Soc 109:3977
15. Zhang LQ, Zhang YD, Xu L, Li XL, Yang XC, Xu GL (2001)
Enzym Microb Technol 29:129
The mono N-alkylation reaction of aniline with hexyl
bromide is considered as a standard for recycling purposes.
Recycling experiments of the lipase catalyst were per-
formed by direct filtration of the reaction mass. The pro-
cess was followed for five successive cycles and decrease
in yield was observed after the third cycle (Fig. 1). This
reduction in yield might be due to inactivation of enzyme
as the cycles are increased or loss of the enzyme during
handling. The lipase catalyst as well as the ethanol were
recovered and successfully reused.
In recycling experiments, DES was recovered by simple
biphasic extraction with ethyl acetate. The process was
followed for five successive cycles and only a slight
decrease in the catalytic activity of DES was observed after
the fourth cycle (Fig. 1).
16. Abbott AP, Davies DL, Capper G, Rasheed RK, Tambyrajah V
(2007) US Patent 7,183,433
17. Abbott AP, Capper G, Davies DL, Rasheed RK, Tambyrajah V
(2003) Chem Commun 1:70
18. Gorke JT, Srienc F, Kazlauskas RJ (2008) Chem Commun
10:1235
19. Jhong H, Wong D, Wan C, Wang Y, Wei T (2009) Electrochem
Commun 11:209
20. Abbott AP, Capper G, McKenzie KJ, Ryder KS (2007) J Elec-
troanal Chem 599:288
4 Conclusion
In conclusion, the novel methods described in this paper
offer a new scope for mono N-alkylation of various aro-
matic primary amines using green catalyst/solvent. The
lipase-catalysed medium gave good yields in short reaction
times, and maximum recovery of lipase as well as ethanol,
thus reducing the toxic wastes. The second method using a
DES as the catalyst and solvent also gave moderate to good
21. Abbott AP, Bell TJ, Handa S, Stoddart B (2005) Green Chem
7:705
22. Morales RC, Tambyrajah V, Jenkins PR, Davies DL, Abbott AP
(2004) Chem Commun 2:158
23. Sonawane YA, Phadtare SB, Borse BN, Jagtap AR, Shankarling
GS (2010) Org Lett 12:1456
24. Li C, Feng X, Wang N, Zhou Y, Yu X (2008) Green Chem
10:616
123