An Improved and Efficient N-acetylation of Amines Using Choline Chloride Based Deep Eutectic Solvents

Full text of DOI:10.1080/00304948.2017.1320914

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
ISSN: 0030-4948 (Print) 1945-5453 (Online) Journal homepage: http://www.tandfonline.com/loi/uopp20
An Improved and Efficient N-acetylation of Amines
Using Choline Chloride Based Deep Eutectic
Solvents
Ana Amić & Maja Molnar
To cite this article: Ana Amić & Maja Molnar (2017) An Improved and Efficient N-acetylation
of Amines Using Choline Chloride Based Deep Eutectic Solvents, Organic Preparations and
Procedures International, 49:3, 249-257, DOI: 10.1080/00304948.2017.1320914
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Date: 01 June 2017, At: 12:43

Organic Preparations and Procedures International, 49:249–257, 2017
Copyright Ó Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2017.1320914
An Improved and Efficient N-acetylation of Amines
Using Choline Chloride Based Deep Eutectic
Solvents
2
ꢀ1
Ana Amic and Maja Molnar
1The Department of Biology, Josip Juraj Strossmayer University of Osijek, Cara
Hadrijana 8A, 31000 Osijek, Croatia
2Faculty of Food Technology Osijek, Josip Juraj Strossmayer University of
Osijek, F. Kuhaca 20, 31000 Osijek, Croatia
Amine acetylation is one of the most widely used reactions in organic chemistry and is
often used in synthesis of amides and protection of -NH group.1 Amine acetylation is
often performed using acetic anhydride2 or acetyl chloride1,3 in presence of acidic2 or
basic3 catalysts in organic medium. N-acetylation of amines was performed in aqueous
medium using sodium dodecyl sulphate with various anhydrides,4 with acetic anhydride
in acetic acid,5 acetic anhydride in aqueous sodium bicarbonate solution,2 in N,N-dime-
thylacetamide with N,N-carbonyldiimidazole,6 with acetic anhydride in the presence of
various catalysts such as RuCl3,7 La(NO3)3¢6H2O,8 NbCl5,9 NaHSO4¢SiO2,10 poly(4-
vinylpyridinium) perchlorate,11 BiFeO3 nanopowder,12 iodine,1 acylimidazoliumace-
tate,13 ZnAl2O4@SiO2nanocomposite.14 Saikia and coworkers15 performed N-acetylation
of amines using acetonitrile as an acylating agent and in situ generated trimethylsilyl
iodide as the catalyst under microwave heating conditions, while Zhang and Chen16
treated aryl amines with dimethylacetamide in the presence of hydrochloric acid to obtain
N-arylamide derivatives.
Many of these methods used in N-acetylation of amines are characterized either by
use of expensive and harmful reagents, solvents or catalysts, by harsh reaction conditions
or long reaction times. In our attempt to utilize choline chloride based deep eutectic sol-
vents in organic synthesis of some heterocyclic compounds, we noticed an immediate for-
mation of N-acetylated amines, which were easily isolated in very high yields, with no
need for further purification. Deep eutectic solvents (DES) became interesting to our
research group due to their applicability as green solvents in synthesis of heterocyclic
compounds, often used without any catalysts or acting as a catalysts themselves.17 So far
DESs have been utilized in organic synthesis of various heterocyclic compounds,18–22
alkylation of anilines23 and thiophenic compounds,24 as well as green solvents for extrac-
tion of various bioactive plant compounds.25 DESs have found many useful applications
in organic synthesis, which were extensively reviewed in the last few years.17,22,26 Deep
Received September 16, 2016; in final form January 31, 2017.
Address correspondence to Maja Molnar, Faculty of Food Technology Osijek, Josip Juraj
Strossmayer University of Osijek, F. Kuhaca 20, 31000 Osijek, Croatia. E-mail: maja.
molnar@ptfos.hr
249

250
Amicꢀ and Molnar
eutectic solvents have been described as environmentally benign mixtures of readily
available, biodegradable, recyclable components with low vapor pressure.17,22 DESs are
non-toxic, formed with 100% atom economy and their high solubility in water implies a
simple reaction workup.17 Choline chloride is often used in the formation of DESs due to
its low toxicity, biodegradability and low cost, in combination with urea, thiourea, glyc-
erol, carboxylic acid, etc.27
The purpose of this work was to investigate the influence of various DESs on yield,
reaction time and temperature applied for N-acetylation of various amines. We also inves-
tigated the influence of DES recycling on product yield.
An optimization on amine:acetic acid anhydride (AAA) ratio was performed on
anthranilic acid in a DES made of choline chloride (ChCl) and malonic acid (Table 1).
The best ratio was shown to be 1:2 (amine:acetic anhydride), since reaction can be per-
formed at room temperature in only 10 minutes. If the amount of AAA is to be decreased,
the reaction time should be increased. Thus, all reactions with different amines were per-
formed at the ratio of amine:acetic anhydride (1:2) at room temperature, except for the
DES derived from ChCl:adipic acid and ChCl:thiourea which were performed at higher
temperatures (140ꢀC and 110ꢀC) due to a higher melting point of the corresponding
DESs.
After the optimization of the ratio of reactants, N-acetylation of anthranilic acid and
4-chloroaniline amine was performed in different DESs as shown in Table 2. The best
DES for acetylation for both compounds at room temperature, in terms of yield and reac-
tion time, was shown to be ChCl:urea (1:2) and ChCl:malonic acid (1:1). Reactions were
monitored on TLC and finished when all the reactants were consumed. Anthranilic acid
was acetylated in ChCl:urea DES in 15 minutes at room temperature, with isolated yield
of 68%. For acetylation of anthranilic acid in ChCl:malonic acid DES, less time
(10 minutes) was required for the reaction to be completed (yield 57%). Combination of
ChCl and adipic acid did not yield any products for anthranilic acid.
The best reaction time and yield (at room temperature) for 4-chloroaniline was
in the DES made of ChCl and malonic acid (88% yield in 15 minutes), as well as
urea (98% yield in 30 minutes), but for choline chloride/urea DES it took more time
for the reactants to be completely consumed. Thiourea in combination with ChCl
was also proven to be very efficient, but higher temperature was required. This sug-
gests that the effect will be highly dependent on the combination of compounds
mixed to produce the DES.
Table 1
Optimization of Anthranilic Acid:Acetic Acid Anhydride Ratio for N-acetylation
Anthranilic acid in DES ChCl:malonic acid (1:1)
Ratio anthranilic acid:
acetic anhydride
reaction
time (min)
yield (%)
1: 3
1: 2
1–3
10
57
57
52
49
15
1: 1.5
1: 1.2
1: 1
60
120
180

An Improved and Efficient N-acetylation of Amines
251
Table 2
Optimization of Conditions for N-acetylation of Anthranilic Acid and 4-chloroaniline
Anthranilic acid
Solvent
T(ꢀC)
Yield (%)
Reaction time (min)
ChCl:urea (1:2)
r.t.
r.t.
68
57
56
52
—
15
10
60
60
—
ChCl:malonic acid (1:1)
ChCl:oxalic acid (1:1)
ChCl:thiourea (1:2)
ChCl:adipic acid (1:1)
r.t.
109
140
4-chloroaniline
Solvent
T(ꢀC)
Yield (%)
Reaction time (min)
ChCl:urea (1:2)
r.t.
r.t.
98
88
62
87
64
30
15
30
15
30
ChCl:malonic acid (1:1)
ChCl:oxalic acid (1:1)
ChCl:thiourea (1:2)
ChCl:adipic acid (1:1)
r.t.
109
140
All of the above indicated that the best choice for N-acetylation of amines are
two DESs, ChCl:urea and ChCl:malonic acid. Therefore, the rest of amines were
acetylated in these two DESs and the results are shown in Table 3. All the reactions
were monitored on TLC and finished when all the reactants were consumed or after
60 minutes.
When aniline was acetylated, TLC showed full consumption of aniline, but iso-
lation required an extraction with dichloromethane (DCM) after water was added to
the mixture. ChCl:urea DES yielded 91% of acetanilide in only 30 minutes. 1-Naph-
thylamine was acetylated in the highest yield (86%) and in the shortest time
(15 min) in ChCl:urea DES. The same procedure was also applied on 2-((4-methyl-
2-oxo-2H-chromen-7-yl)oxy)acetohydrazide and the same results were achieved for
ChCl:urea (52%) and ChCl:malonic acid (54%) DESs. Acetylation of 2-nitro-4-(tri-
fluoromethyl)aniline yielded no acetylated product, while acetylation of 4-nitro-3-
(trifluoromethyl)aniline yielded acetylated product; but after 60 minutes, some of the
reactant was still in the mixture. This indicates the significance of the nitro group
position. 4-(Trifluoromethyl)aniline was easily acetylated in both solvents in
60 minutes in 88% and 87% yields. Combination of ChCl:urea or malonic acid was
also suitable for 2,3,4-trifluoroaniline (58% and 31%) and cyclopropylamine (36%
and 44%). 2-Aminothiophenol was successfully acetylated by obtaining diacetylated
product S-(2-acetamidophenyl)ethanethioate.
An influence of DES recycling on final yield of the product was also investigated. An
acetylation of 4-chloroaniline was performed in two eutectic solvents CC/U and CC/
malonic acid and results are shown in Table 4. It is evident that recycling of DES up to
five times does not influence the yield of the final product.
The overall data shows that the best DES for N-acetylation of amines is ChCl:
urea (1:2) in terms of reaction time, temperature, yield and purity of final com-
pounds. This solvent is easily prepared from biodegradable components and final

252
Amicꢀ and Molnar
Table 3
N-Acetylation of Different Amines in Choline Chloride:urea and Choline Chloride:
malonic Acid DESs
Solvent
T(ꢀC)
Yield (%)
Reaction time (min)
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
34
69
60
60
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
91
60
30
30
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
86
53
15
60
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
52
54
30
30
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
–
–
–
–
(Continued on next page)

An Improved and Efficient N-acetylation of Amines
253
Table 3
N-Acetylation of Different Amines in Choline Chloride:urea and Choline Chloride:
malonic Acid DESs (Continued)
Solvent
T(ꢀC)
Yield (%)
Reaction time (min)
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
88
87
60
60
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
65
57
60
60
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
58
31
15
15
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
36
44
60
60
ChCl:urea
ChCl:malonic acid
r.t.
r.t.
74
69
60
60

254
Amicꢀ and Molnar
Table 4
Influence of DES Recycle on Final Product Yield Shown for Acetylation of 4-
chloroaniline
Solvent
Yield (%)
Choline chloride:urea
1st recycle
2nd recycle
98
94
97
97
97
96
88
88
87
87
86
87
3rd recycle
4th recycle
5th recycle
Choline chloride:malonic acid
1st recycle
2nd recycle
3rd recycle
4th recycle
5th recycle
products are easily isolated by addition of water or extraction with suitable solvents.
Apart from being effective, this procedure is also environmentally friendly.
Experimental Section
Melting points were determined on a capillary melting point apparatus (Electroter-
mal, Rochford, UK) and are uncorrected. Thin-layer chromatography was performed
with fluorescent silica gel plates F254 (Merck, Darmstadt, Germany), which were
checked under UV (254 and 365 nm) light, using benzene: acetone: acetic acid (8:
1: 1) as a solvent. The MS spectra were recorded on LC/MS/MS API 2000 (Applied
Biosystems/MDS SCIEX, CA, USA). 1 H NMR spectra were recorded on a Bruker
Avance 600 MHz NMR Spectrometer (Bruker Biospin GmbH, Rheinstetten, Ger-
many) at 293 K in DMSO-d6. The elemental analysis for C, H and N was done on a
Perkin-Elmer Analyzer 2400 Series II (Perkin–Elmer, Boston, MA, USA). Infrared
spectra (nmax-cm-1) were recorded on a Beckmann FT-IR 3303, using KBr disks.
All solvents and reagents were supplied by commercial suppliers.
Preparation of Deep Eutectic Solvents
All deep eutectic solvents were prepared according to the procedures described in the
literature.20
Synthetic Procedures for N-acetylation of Amines
0.0025 Moles of the respective amine was added to DES and stirred at different tempera-
tures (depending on the solvent) until dissolved. 0.005 Moles of acetic anhydride was
added to the mixture and stirred until the completion of reaction, which was monitored
by TLC. Upon completion, water was added to the reaction mixture; separated solid was

An Improved and Efficient N-acetylation of Amines
255
filtered off and dried. DES was recycled by evaporation of water. Water soluble acety-
lated amines were extracted with a suitable organic solvent.
N-Acetylanthranilic acid. White-yellow solid (68%, 0.30 g, 1.7 mmol); Mp D 180ꢀC
(lit.28,29 184ꢀC); Rf D 0.67; 1H NMR (300 MHz, DMSO-d6) d: 11.08 (s, 1 H), 8.49–8.46
(d, 1 H, J D 8.29), 8.00–7.97 (dd, 1 H), 7.59 (t, 1 H, J D 7.91, J D 1.51), 7.16 (t, 1 H),
2.16 (s, 3 H); 13C NMR (75 MHz, DMSO-d6) d: 170.0, 168.9, 141.3, 134.4, 131.5, 122.9,
120.4, 117.0, 25.4; MS (ESI): m/z D 177.90 [M-HC].
N-(4-Chlorophenyl)acetamide. White solid (98%, 0.42 g, 2.5 mmol); Mp D 179ꢀC–
1
181ꢀC (lit.29 178ꢀC); Rf D 0.59; H NMR (600 MHz, DMSO-d6) d: 10.09 (br.s, 1 H),
7.62–7.60 (d, J D 8.80, 1 H), 7.34–7.35 (d, J D 8.80, 1 H), 2.05 (s, 3 H); 13C NMR
(150 MHz, DMSO-d6) d: 168.4, 138.2, 128.5, 126.7, 120.4, 24.0; MS (ESI): m/z D
167.80 [M-HC].
5-Iodo-N-acetylanthranilic acid. Off white solid (69%, 0.53 g, 1.7 mmol); Mp D
1
155ꢀC (lit.28 160ꢀC); Rf D 0.68; H NMR (300 MHz, DMSO-d6) d: 11.07 (br.s., 1 H),
8.29–8.22 (d, 1 H), 7.95–7.90 (d, 1 H), 1.25 (s, 3 H); 13C NMR (75 MHz, DMSO-d6) d:
169.0, 151.4, 142.5, 141.9, 139.4, 122.6, 119.5, 74.6, 25.5; MS (ESI): m/z D 304.00 [M-
HC].
N-Phenylacetamide. Off white solid (91%, 0.31 g, 2.3 mmol); Mp D 114ꢀC–115ꢀC
1
(lit.29 114ꢀC); Rf D 0.41; H NMR (300 MHz, DMSO-d6) d: 11.06 (br.s., 1 H), 8.47–
8.44 (d, J D 8.29, 1 H), 7.98–7.95 (d, J D 7.91, J D 1.51, 1 H), 7.56 (t, 1 H), 7.13 (t,
1 H), 2.13 (s, 3 H); 13C NMR (75 MHz, DMSO-d6) d: 169.9, 141.3, 134.4, 131.5, 123.0,
120.4, 116.9, 25.4; MS (ESI): m/z D 133.90 [M-HC].
N-(Naphthalen-1-yl)acetamide. Pale purple solid (86%, 0.40 g, 2.2 mmol); Mp D
1
157ꢀC (lit.29 158–160ꢀC); Rf D 0.67; H NMR (300 MHz, DMSO-d6) d: 9.94 (br.s.,
1 H), 8.13–7.46 (m, J D 9.42, J D 9.04, 7 H), 2.21 (s, 3 H); 13C NMR (75 MHz, DMSO-
d6) d: 169.4, 134.3, 134.2, 128.6, 126.4, 126.2, 126.0, 125.5, 123.2, 122.0, 24.0; MS
(ESI): m/z D 184.00 [M-HC].
N’-Acetyl-2-((4-methyl-2-oxo-2H-chromen-7-yl)oxy)acetohydrazide. Off-white
1
solid (54%, 0.39 g, 1.35 mmol); Mp D 237ꢀC–239ꢀC; Rf D 0.14; H NMR (300 MHz,
DMSO-d6) d: 10.1 (br.s., 1 H), 9.85 (br.s., 1 H), 7.72–7.69 (d, J D 9.04, 1 H), 7.04–6.98
(m, 2 H), 6.23 (s, 1 H), 4.74 (s, 2 H), 2.40 (s, 3 H), 1.88 (s, 3 H); 13C NMR (75 MHz,
DMSO-d6) d: 168.6, 166.4, 161.1, 160.5, 154.9, 153.7, 127.0, 126.9, 114.1, 113.0, 111.9,
102.1, 66.7, 20.9, 18.6; IR (KBr): 3173 (amide -NH stretching), 1724 (ketone –C D
O),1610 (-NH banding), 1263 (-C-N stretching) cm–1; MS (ESI): m/z D 289.10 [M-HC].
Anal. Calcd. for C14H14N2O5 (290.2): C, 59.93; H, 4.86; N, 9.65. Found: C, 60.19; H,
4.74; N, 9.81.
N-(4-(Trifluoromethyl)phenyl)acetamide. White solid (88%, 0.45 g, 2.20 mmol); Rf
D 0.61; 1H NMR (600 MHz, DMSO-d6) d: 10.33 (br.s, 1 H), 7.80–7.64 (d, J D 8.80 Hz,
4 H), 2.09 (s, 3 H); 13C NMR (150 MHz, DMSO-d6) d: 168.66, 142.8, 125.9, 125.8,
123.0, 122.9, 118.7, 24.0; MS (ESI): m/z D 209.90 [M-HC].
Commercially available (Matrix Scientific, USA) under CAS number: 349–97-3 and
characterized by Mp D 152ꢀC–153ꢀC.

256
Amicꢀ and Molnar
S-(2-Acetamidophenyl)ethanethioate. Yellowish oil (65%, 0.34 g, 1.63 mmol); Rf D
0.63; 1H NMR (300 MHz, DMSO-d6) d: 9.35 (br.s., 1 H), 7.73–7.70 (m, 1 H), 7.48–7.46
(m, 1 H), 7.44–7.41 (m, 1 H), 7.19–7.25 (m, 1 H), 2.40 (s, 3 H), 2.06 (s, 3 H); 13C NMR
(75 MHz, DMSO-d6) d: 193.6, 140.2, 137.0, 130.9, 125.8, 125.6, 30.7, 23.9; MS (ESI):
m/z D 210.20 [MCHC].
Commercially available (CheMall Corporation, USA) under CAS number: 1204–55-3.
N-(2,3,4-Trifluorophenyl)acetamide. White solid (58%, 0.27 g, 1.45 mmol); Mp D
89ꢀC–91ꢀC (lit.30 92ꢀC); Rf D 0.58; 1H NMR (300 MHz, DMSO-d6) d: 9.91 (br.s, 1 H),
7.59–7.28 (m, 1 H), 7.27–7.25 (m, 1 H), 2.09 (s, 3 H); 13C NMR (75 MHz, DMSO-d6) d:
169.3, 145.5, 141.3, 138.0, 124.5, 119.3, 112.1, 23.6; MS (ESI): m/z D 187.90 [M-HC].
N-Cyclopropylacetamide. Yellowish liquid (44%, 0.11 g, 1.1 mmol); Rf D 0.84
(DCM:ethyl acetate 20:1); 1H NMR (300 MHz, DMSO-d6) d: 7.83 (br.s, 1 H), 2.64–2.53
(m, 1 H), 1.91–1.74 (s, 3 H), 0.59–0.56 (m, 2 H), 0.36–0.34 (m, 2 H)31; 13C NMR
(75 MHz, DMSO-d6) d: 170.67, 23.0, 22.7, 21.5, 6.0; MS (ESI): m/z D 100.1 [MCHC].
N-(4-Nitro-3-(trifluoromethyl)phenyl)acetamide. Yellow solid (74%, 0.46 g,
1.85 mmol); Mp D 95–97ꢀC; Rf D 0.48; 1H NMR (400 MHz, DMSO-d6) d: 10.93 (br.s.,
1 H), 8.28–8.04 (m, J D 9.54, J D 8.80, 3 H), 2.15 (s, 3 H); 13C NMR (150 MHz,
DMSO-d6) d: 169.7, 143.9, 129.7, 127.7, 121.8, 116.9, 114.4, 53.1, 24.2; MS (ESI): m/z
D 247.00 [M-HC].
Commercially available (Matrix Scientific, USA) under CAS number: 393–12-4 and
characterized by Mp D 108ꢀC–109ꢀC as pale yellow solid.
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