An Efficient Greener Approach for N-acylation of Amines in Water Using Benzotriazole Chemistry

Article abstract of DOI:10.3390/molecules25112501

A straightforward, mild and cost-efficient synthesis of various arylamides in water was accomplished using versatile benzotriazole chemistry. Acylation of various amines was achieved in water at room temperature as well as under microwave irradiation. The developed protocol unfolds the synthesis of amino acid aryl amides, drug conjugates and benzimidazoles. The environmentally friendly synthesis, short reaction time, simple workup, high yields, mild conditions and free of racemization are the key advantages of this protocol.

Full text of DOI:10.3390/molecules25112501

molecules
Article
An Efficient Greener Approach for N-acylation of
Amines in Water Using Benzotriazole Chemistry
Tarek S. Ibrahim 1,2, Israa A. Seliem 2,3 , Siva S. Panda 3,
*
, Amany M. M. Al-Mahmoudy 2,
Zakaria K. M. Abdel-Samii 2, Nabil A. Alhakamy 4 , Hani Z. Asfour 5 and Mohamed Elagawany 6
1
Department of Pharmaceutical Chemistry, Faculty of Pharmacy, King Abdulaziz University,
Jeddah 21589, Saudi Arabia; tmabrahem@kau.edu.sa
Department of Pharmaceutical Organic Chemistry, Faculty of Pharmacy, Zagazig University,
2
Zagazig 44519, Egypt; isliem@augusta.edu (I.A.S.); amanysinger77@gmail.com (A.M.M.A.-M.);
zakariaabdelsamii@yahoo.com (Z.K.M.A.-S.)
Department of Chemistry & Physics, Augusta University, Augusta, GA 30912, USA
3
4
Department of Pharmaceutics, Faculty of Pharmacy, King Abdulaziz University,
Jeddah 21589, Saudi Arabia; nalhakamy@kau.edu.sa
Department of Medical Microbiology and Parasitology, Faculty of Medicine, King Abdulaziz University,
5
Jeddah 21589, Saudi Arabia; hasfour@kau.edu.sa
Department of Pharmaceutical Chemistry, faculty of pharmacy, Damanhour University,
6
Damanhour 22511, Egypt; alfath_tours@yahoo.com
*
Correspondence: sipanda@augusta.edu; Tel.: +1-706-667-4022; Fax: +1-706-667-4519
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Ferenc Fulop and Dariusz Matosiuk
Received: 28 April 2020; Accepted: 26 May 2020; Published: 28 May 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: A straightforward, mild and cost-efficient synthesis of various arylamides in water was
accomplished using versatile benzotriazole chemistry. Acylation of various amines was achieved in
water at room temperature as well as under microwave irradiation. The developed protocol unfolds
the synthesis of amino acid aryl amides, drug conjugates and benzimidazoles. The environmentally
friendly synthesis, short reaction time, simple workup, high yields, mild conditions and free of
racemization are the key advantages of this protocol.
Keywords: green chemistry; one-pot synthesis; acylation; benzotriazole chemistry; benzimidazole;
microwave; aryl amide
1. Introduction
N-Acylation reactions are widely used in the organic chemistry, biology, pharmaceutical and
agricultural industries [1–3]. Chemically, they are a straightforward and powerful tool for the protection
of amino groups in multistep organic syntheses, for their convenient activation towards further chemical
transformations, or as widespread amide building blocks in biologically active targets, natural products
and pharmaceuticals [4,5].
The amide bond is exceptionally imperative in medicinal chemistry [
to the unique properties of peptides, proteins, and numerous other natural and synthetic compounds.
Most of the natural products and clinically used drugs contain an amide bond [ 14]. Approximately
6–8]. Amide groups contribute
9
–
25% of the pharmaceuticals present on the market contain at least one amide unit [15], and the
functional group was present in 2/3 of the drug candidates surveyed by three leading pharmaceutical
companies in 2006 [16]. A survey of the literature reveals that many drugs available in the market,
such as Penicillin (antibacterial), pyrazinamide (antitubercular), atorvastatin (antihyperlipidemic) [17
]
and valsartan (angiotensin receptor), possess their specific capabilities due to presence of amide linkage
in their structures [18].
Molecules 2020, 25, 2501; doi:10.3390/molecules25112501
www.mdpi.com/journal/molecules

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Acylation using acetyl chloride and acetic anhydride is common among various reported strategies.
However, N-acylation through acyl chloride and/or acid anhydride has been associated with many
inherited disadvantages [19,20]. Further, for amino acid acylation, different coupling reagents are used,
which are mostly nonselective, hazardous and difficult to handle [21].
To overcome the challenges of acyl chloride- and acid anhydride-mediated N-acylation reactions
using acyl chlorides and/or acid anhydrides, numerous strategies have been investigated. Among them,
the metal-catalyzed or direct coupling of unactivated carboxylic acids [22
1,5-diazabicyclo[4.3.0]non-5-ene (DBN) tetraphenylborate salts [25], Beckman rearrangements using
mercury and ruthenium catalysts [26 27], copper-catalyzed oxidative amidation of aldehydes [28 29],
triazole- and imidazole-mediated acyl transfer reactions [30 32], and acylation method through
acylbenzotriazoles [33 35] are common. Benzotriazole chemistry has been explored, by the Katritzky
group [19 21 36 40] and others [41 42], in various types of reactions, including in the synthesis of amides.
–24], acylation through N-acyl
,
,
–
–
,
,
–
,
On the other hand, benzimidazoles are crucial core structures used to develop pharmaceuticals
and materials. Substituted benzimidazoles exhibit biological activities such as antitumor [43],
antihypertensive [44], antiulcer [45] and enzyme inhibition [46]. Some commonly employed synthetic
methods for benzimidazoles include: (i) reaction of 1,2-phenylenediamines with carboxylic acids or their
derivatives, like amidates, nitriles or orthoesters, in the presence of polyphosphoric acid [47] or mineral
acids [48]; (ii) cyclization of N-(N-arylbenzimidoyl)-1,4-benzoquinoneimines under a thermal or acidic
environment [49]; (iii) utilizing o-nitroanilines as intermediates [50]; (iv) oxidative cyclocondensation of
o-phenylenediamine with aldehydes [51].
Recently, the development of green synthetic methods has become an important strategy in organic
synthesis. Water has experienced increasing popularity due to being inexpensive, readily available,
and environmentally benign. In addition, water: (i) is cheap, nonflammable, non-toxic and safe for use;
(ii) eliminates additional efforts required to dry the substrates/reagents before use; (iii) offers unique
physical and chemical properties that often achieve the reactivity or selectivity unattainable in organic
solvents; and (iv) allows easy product isolation by filtration [52,53].
Benzotriazole chemistry has been practiced extensively in our group, and has often been found to
be superior to conventional routes for acylation [21]. Earlier, we reported the acylation of mesalazine [54
]
and the synthesis of benzothiazole [52] in water under microwave conditions. In this communication,
we extend the efficient synthetic protocol for the N-acylation of amines, which could be an important
tool for conjugate chemistry and also for the synthesis of 2-substituted benzimidazoles without any
catalyst, organic solvent or additional reagent. This protocol runs under both microwave and room
temperature and gives quantitative yields. To the best of our knowledge, this is the first environmentally
benign, catalyst- and organic solvent-free synthesis of N-acylated products of amines in water.
2. Results and Discussion
Carboxamines are important key intermediates, scaffolds for polymers, dendrimers and bioactive
molecules [55]. Among arylamines, amino acid arylamides are often used as substrates in fluorogenic,
chromogenic and amperogenic enzymatic assays [56]. For these applications, chirality is an important
factor. Several methods have been reported for arylamides, including the use of enzymes and flow
chemistry [57–71], and we believe we are reporting, for the first time, the synthesis of amino acid
arylamides in water.
We investigated the reaction conditions for the N-acylation of anilines with our in-house prepared,
protected aminoacylbenzotriazoles in water. Optimization of the reaction conditions showed the best
outcomes under microwave irradiation at 50 ◦C for 15–20 min, over conventional heating (Table 1).
We were also able to get the desired product by stirring the reactants at room temperature for 1–2 h.

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Table 1. Optimization of reaction condition.
Entry
Reaction Temp. (◦C)
20 (Room temp.)
20 (Room temp.)
50 (Conv.)
Reaction Time
Yield a (%)
1
2
3
4
5
1 h
2 h
74
86
77
82
96
30 min
1 h
50 (Conv.)
70 (MW)
15 min
a
Isolated yield.
There was no significant change in reaction time and yield when the above reaction was carried
out both in tap water and saturated brine solution separately. Furthermore, we also tried the reaction
in deionized water, to rule out the possibility of any metallic impurities from tap water catalyzing the
reaction. The results obtained were comparable in tap and deionized water, so to avoid the effort and
energy consumption needed to prepare deionized water, we chose tap water for our reactions.
Even though room temperature works for the N-acylation reactions, at a larger-scale, the reactions
proceed nearly to completion as some of the reactants are left unreacted. We carried out the above
reaction in both conventional heating and microwave irradiation conditions on a large scale. We got a
better yield with high purity under microwave conditions, in comparison with conventional heating.
We therefore ran the reactions of aromatic amines with benzotriazolides of protected amino acids
in water, under microwave irradiation for 15 to 20 min (Scheme 1). Our reaction condition yields pure
N-acylated products for all three types of protected amino acids (Boc, Cbz and Fmoc) with various
substituted anilines (Table 2). We believe the driving force of the reaction is controlled by diffusion,
since both of the reactants are water insoluble and form a heterogeneous reaction mixture. To justify
our hypothesis, we used hexanes, a non-polar solvent, as a reaction medium in which both reactants are
insoluble, and we found an equivalent outcome. We thus preferred nonflammable water over hexanes
in our reactions. To explore the use of our reaction condition, we used different amines with various
benzotriazolides. Our optimized reaction condition retains the chirality of the products, which was
confirmed by performing reactions with both the DL and L forms of amino acids. High-performance
liquid chromatography (HPLC) analysis of compound 9 (contains L-alanine) showed a single peak,
with nearly the same retention time (16.290 min) as that of one of the two peaks (16.813 and 18.407 min)
obtained from the mixture of the racemic compound 9+90 (contains DL-alanine) with the enantiopure
compound 9. The increase in height of one peak supports the retention of chiral integrity in our reaction
protocol (Supplementary Material).
In addition to the primary amines, we also tried our optimized reaction condition with secondary
amines. We were able to get the N-acylated secondary amines in good yields with high purity
(Scheme 2). Earlier, we reported these conjugates, which were synthesized by treating benzotriazolide
of boc-protected amino acids with secondary amines in the presence of triethylamine in tetrahydrofuran
(THF) [35].

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Scheme 1. Synthesis of N-acylated amines.
Table 2. Preparation of N-acylated amines.
Product
Entry
R-NH2
Yield (%)
Mp (◦C)
1
94
137–139
2
3
93
95
123–125
145–146
4
91
165–167

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Table 2. Cont.
Entry
R-NH2
Product
Yield (%)
Mp (◦C)
5
95
118–120
6
7
88
96
152–154
158–160
8
9
92
97
152–154
183–185
10
11
79
94
185–187
193–195
12
75
172–174

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Table 2. Cont.
Entry
R-NH2
Product
Yield (%)
Mp (◦C)
13
80
164–166
160–162
14
15
85
98
185–187
16
17
94
77
138–140
191–193
18
19
84
90
174–176
192–194

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Table 2. Cont.
Entry
R-NH2
Product
Yield (%)
Mp (◦C)
20
90
142–143
21
22
23
24
25
98
95
90
73
82
152–154
160–162
182–184
163–165
150–152
26
90
197–199

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Table 2. Cont.
Entry
R-NH2
Product
Yield (%)
Mp (◦C)
27
98
210–212
28
29
70
70
162–164
95–97
30
31
90
89
180–182
152–154
32
33
97
71
155–157
138–140
34
81
147–149

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Table 2. Cont.
Entry
R-NH2
Product
Yield (%)
Mp (◦C)
35
98
155–157
157–159
130–132
36
37
85
80
38
80
143–145
Scheme 2. Synthesis of Boc-protected amino acid with secondary heterocyclic amine conjugates.

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Despite tremendous success in the synthesis of 2-substituted benzimidazoles, many of the
methodologies suffer from one or more limitations, such as long reaction times, the formation of
several side products, harsh reaction conditions, low yields, complicated work-up procedures and the
generation of acidic and metallic wastes. As a consequence, the development of a new method, or
technical improvement of the existing methods, is still an important experimental challenge. To expand
the range of applicability of our optimized greener protocol, we treated benzotriazole-activated
substituted benzoic acids with o-phenylenediamine in water, under microwave conditions. We obtained
our desired product in 1 h (Scheme 3).
Scheme 3. Synthesis of 2-substituted benzimidazoles.
To elucidate the use of water in our reaction protocol, we added 5 mol% of a phase-transfer catalyst
(Aliquat 336), which lowered the yields of the products, again supporting our proposed reaction
mechanism of diffusion. The physical state of the reactants is also important: the use of microwave
irradiation over conventional heating significantly improved the yields and purity, with retention
of the chiral integrity. All the synthesized compounds were fully characterized by spectral studies
(Supplementary Material).
3. Conclusions
In conclusion, we report mild, fast, efficient, facile and green conditions for the N-acylation of amines
in water, without the use of catalyst or reagent. The heterogeneous reaction runs in water and forms the
N-acylated products without loss of chirality and with high yields. The optimized reaction conditions

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work well at room temperature as well as under microwave irradiation for small-scale reactions, but for
large-scale reactions microwave conditions are preferred. The application of microwaves and the concept
of a heterogeneous reaction mixture expands the use of the reaction condition for the synthesis of
2-substituted benzimidazoles. Given its qualities of being racemization-free, high yield, catalyst- and
solvent-free and ecofriendly, as well as the possibility of it scaling-up, the reaction has substantial
potential for implementation by the pharmaceutical and agriculture industries.
4. Experimental Section
Melting points were determined on a capillary point apparatus equipped with a digital thermometer.
NMR spectra were recorded in (DMSO-d6) on Bruker NMR spectrometers operating at 500 MHz for 1H
[with tetramethysilane (TMS) as an internal standard] and 125 MHz for 13C. All microwave-assisted
reactions were carried out with a single mode cavity Discover Microwave Synthesizer (CEM Corporation,
Charlotte, NC, USA). The reaction mixtures were transferred into a 10 mL glass pressure microwave
tube equipped with a magnetic stirrer bar. The tube was closed with a silicon septum and the reaction
mixture was subjected to microwave irradiation (Discover mode; run time: 60 s; Power Max-cooling
mode). HPLC analysis was carried out on Agilent 6120 LCMS instrument with Chirobiotic T column.
4.1. General Methods for N-acylation
In a typical procedure, a mixture of amine (1 equiv.) and N-protected aminoacylbenzotriazole or
arylylbenzotriazole (1 equiv.) was subjected to microwave irradiation (20 W, 50 ◦C) in water (3 mL)
for 15–20 min. After completion of the reaction, aqueous Na2CO3 or 4N HCl was added and the
mixture was extracted with ethyl acetate or filter the precipitates, followed by washing with water.
In most of the cases the isolated products were in pure form, and some were recrystallized in ethanol.
Benzotriazoles could be recovered from the aqueous layer by pH-controlled acidification.
tert-Butyl (S)-(1-oxo-3-phenyl-1-(phenylamino)propan-2-yl)carbamate (3). White microcrystals (94%);
m.p. 137–139 ◦C (Lit. m.p. 138–139 ◦C [57]). 1H NMR (500 MHz, DMSO-d6)
δ
: 10.01 (s, 1H), 7.58
(d, J = 7.8 Hz, 1H), 7.44–6.90 (m, 10H), 4.32 (s, 1H), 3.49–1.79 (m, 2H), 2.01–0.59 (m, 9H). 13C NMR
(125 MHz, DMSO-d6) δ: 171.2, 155.9, 139.4, 138.4, 129.9, 129.7, 129.6, 129.3, 129.2, 128.6, 128.5, 126.7,
126.7, 123.8, 119.9, 119.8, 78.6, 57.0, 37.9, 28.6. HRMS m/z calcd for C20H24N2O3 [M + H]+ 341.1787,
found 341.1789.
tert-Butyl (S)-(3-methyl-1-oxo-1-(phenylamino)butan-2-yl)carbamate (4). White microcrystals (93%); m.p.
1
123–125 ◦C (Lit. m.p. 120–121 ◦C [58]). H NMR (500 MHz, DMSO-d6)
δ
9.95 (s, 1H), 7.60 (d, J = 7.4 Hz
,
2H), 7.30 (t, J = 7.2 Hz, 2H), 7.05 (t, J = 7.5 Hz, 1H), 6.82 (d, J = 6.7 Hz, 1H), 3.94 (t, J = 8.6 Hz, 1H),
2.06–1.94 (m, 1H), 1.39 (s, 9H), 0.90 (d, J = 5.1 Hz, 6H); 13C NMR (125 MHz, DMSO-d6)
δ171.2, 156.1, 139.3,
129.2, 123.8, 119.7, 78.5, 61.0, 30.9, 28.7, 19.7. HRMS m/z calcd for C16H24N2O3 [M + H]+ 293.1787, found
293.1786.
◦
Benzyl (2-oxo-2-(phenylamino)ethyl)carbamate (5). White microcrystals (95%); m.p. 145–146 C
(Lit. m.p. 148–149 ◦C [59]). 1H NMR (500 MHz, DMSO-d6)
δ
9.99 (s, 1H), 7.61 (d, J = 7.6 Hz, 2H),
7.55 (t, J = 6.8 Hz, 1H), 7.43–7.23 (m, 7H), 7.05 (t, J = 6.8 Hz, 1H), 5.07 (s, 2H), 3.84 (d, J = 5.7 Hz, 2H);
13C NMR (125 MHz, DMSO-d6)
δ 168.4, 157.1, 139.4, 137.5, 129.2, 128.8, 128.3, 128.2, 123.7, 119.6, 66.0,
44.6. HRMS m/z calcd for C16H16N2O3 [M + H]+ 285.1161, found 285.1169.
(9H-Fluoren-9-yl)methyl (2-oxo-2-(phenylamino)ethyl)carbamate (6). White microcrystals (91%); m.p.
◦
1
165–167 C [60]. H NMR (500 MHz, DMSO-d6)
d, J = 7.3 Hz, 2H), 7.66–7.58 (m, 3H), 7.42 (t, J = 7.2 Hz, 2H), 7.36–7.29 (m, 4H), 7.05 (t, J = 6.4 Hz, 1H),
4.33 (d, J = 6.7 Hz, 2H), 4.25 (t, J = 6.5 Hz, 1H), 3.84 (d, J = 5.7 Hz, 2H).; 13C NMR (125 MHz, DMSO-d6)
δ 10.02 (s, 1H), 7.89 (d, J = 7.4 Hz, 2H), 7.74
(
δ
168.4, 157.1, 144.3, 141.2, 139.4, 129.9, 129.2, 128.1, 127.6, 125.7, 123.7, 120.6, 119.6, 66.2, 47.1, 44.5.
HRMS m/z calcd for C23H20N2O3 [M + H]+ 373.1474, found 373.1477.
tert-Butyl (R)-(3-methyl-1-oxo-1-(p-tolylamino)butan-2-yl)carbamate (7). Yellow microcrystals (95%);
◦
◦
1
m.p. 118–120 C (Lit. m.p. 115–117 C [58]). H NMR (500 MHz, DMSO-d6)
δ 9.05 (s, 1H), 7.39
(d, J = 7.6 Hz, 2H), 6.99 (d, J = 5.0 Hz, 2H), 5.79 (d, J = 9.2 Hz, 1H), 4.25 (t, J = 7.9 Hz, 1H), 2.28 (s, 3H),

Molecules 2020, 25, 2501
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2.21–2.13 (m, 1H), 1.43 (s, 9H), 1.05 (d, J = 7.1 Hz, 6H); 13C NMR (125 MHz, DMSO-d6)
δ
170.8, 156.6,
135.4, 133.5, 129.2, 120.2, 80.0, 60.8, 31.3, 28.4, 20.8, 19.3. HRMS m/z calcd for C17H26N2O3 [M + H]+
307.1943, found 307.1944.
Benzyl (2-oxo-2-(p-tolylamino)ethyl)carbamate (8). White microcrystals (88%); m.p. 152–154 ◦C (Lit. m.p.
1
153–154 ◦C [61]). H NMR (500 MHz, DMSO-d6)
δ
: 9.86 (s, 1H), 7.52 (t, J = 5.8 Hz, 1H), 7.46 (d, J = 8.2 Hz
,
1H), 7.40–7.29 (m, 6H), 7.10 (d, J = 8.2 Hz, 2H), 5.05 (s, 2H), 3.79 (d, J = 6.1 Hz, 2H), 2.24 (s, 3H). 13C NMR
(125 MHz, DMSO-d6) δ: 167.6, 156.6, 137.0, 136.4, 132.1, 129.1, 128.4, 128.3, 128.2, 127.9, 127.8, 127.7, 126.4,
119.1, 65.5, 43.9, 20.4. HRMS m/z calcd for C17H18N2O3 [M + H]+ 299.1317, found 299.1319.
Benzyl (S)-(1-oxo-1-(p-tolylamino) propan-2-yl) carbamate (9). White microcrystals (96%); m.p. 158–160
◦C (Lit. m.p. 160–162 ◦C [59]). 1H NMR (500 MHz, DMSO-d6)
δ
: 9.86 (s, 1H), 7.56 (d, J = 7.2 Hz, 1H),
7.47 (d, J = 8.0 Hz, 1H), 7.41–7.24 (m, 6H), 7.10 (d, J = 8.1 Hz, 2H), 5.02 (q, J = 12.6 Hz, 2H), 4.37–3.96
(m, 1H), 2.25 (s, 3H), 1.28 (d, J = 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
: 171.2, 155.7, 136.9, 136.5,
δ
132.1, 129.0, 129.0, 128.4, 128.3, 127.8, 127.7, 127.5, 119.2, 118.4, 65.4, 50.7, 20.4, 18.1. HRMS m/z calcd for
C18H20N2O3 [M + H]+ 313.1474, found 313.1481.
Benzyl (RS)-(1-oxo-1-(p-tolylamino) propan-2-yl) carbamate (9+90). White solid (92%), m.p. 152–154 ◦C.
1H NMR (500 MHz, DMSO-d6)
7.10 (d, J = 8.2 Hz, 2H), 5.02 (q, J = 12.6 Hz, 2H), 4.34–4.01 (m, 1H), 2.25 (s, 3H), 1.28 (d, J = 7.1 Hz,
3H). 13C NMR (125 MHz, DMSO-d6) : 171.2, 155.7, 136.9, 136.5, 132.1, 129.1, 129.0, 128.4, 128.3, 127.8,
δ: 9.86 (s, 1H), 7.56 (s, 1H), 7.47 (d, J = 8.0 Hz, 2H), 7.39–7.26 (m, 5H),
δ
127.8, 127.7, 119.2, 119.2, 65.3, 50.7, 20.4, 18.1. HRMS m/z calcd for C18H20N2O3 [M + H]+ 313.1474,
found 313.1488.
Benzyl (S)-(3-methyl-1-oxo-1-(p-tolylamino)butan-2-yl)carbamate (10). White microcrystals (97%);
1
m.p. 183–185 ◦C [62]. H NMR (500 MHz, DMSO-d6)
δ
9.98 (s, 1H), 7.60–7.25 (m, 8H), 7.15 (d, J = 6.4 Hz
,
2H), 5.09 (s, 2H), 4.03 (t, J = 6.4 Hz, 1H), 2.29 (s, 3H), 2.15–1.94 (m, 1H), 0.95 (d, J = 3.9 Hz, 6H); 13C NMR
(125 MHz, DMSO-d6) δ 170.7, 156.8, 137.5, 136.8, 132.8, 129.6, 128.9, 128.3, 128.2, 119.8, 66.0, 61.5, 30.9,
20.9, 19.7. HRMS m/z calcd for C20H24N2O3 [M + H]+ 341.1787, found 341.1788.
Benzyl (S)-(4-(methylthio)-1-oxo-1-(p-tolylamino)butan-2-yl)carbamate (11). White microcrystals (79%);
m.p. 185–187 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ
: 9.94 (s, 1H), 7.63 (d, J = 7.8 Hz, 1H), 7.48 (d, J = 8.2
Hz, 2H), 7.42–7.25 (m, 5H), 7.11 (d, J = 8.2 Hz, 2H), 5.07–4.99 (m, 2H), 4.22 (dd, J = 13.2, 8.5 Hz, 1H),
2.66–2.32 (m, 2H), 2.25 (s, 3H), 2.04 (s, 3H), 2.05–1.69 (m, 2H). 13C NMR (125 MHz, DMSO-d6)
: 170.2,
δ
156.1, 136.9, 136.3, 132.2, 129.1, 129.0, 128.3, 128.3, 127.8, 127.8, 127.7, 119.3, 119.3, 65.4, 54.6, 31.6, 29.7,
20.4, 14.6. HRMS m/z calcd for C20H24N2O3S [M + H]+ 372.1508, found 372.1512.
(9H-Fluoren-9-yl)methyl(S)-(1-oxo-3-phenyl-1-(p-tolylamino)propan-2-yl)carbamate(12).Whitemicrocrystals
(94%); m.p. 193–195 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ 10.03 (s, 1H), 7.86 (d, J = 7.3 Hz, 2H), 7.73 (d,
J = 8.3 Hz, 1H), 7.64 (t, J = 7.5 Hz, 2H), 7.47 (d, J = 7.8 Hz, 2H), 7.42–7.24 (m, 9H), 7.11 (d, J = 7.7 Hz,
2H), 4.48–4.35 (m, 1H), 4.23–4.14 (m, 3H), 3.08–3.00 (m, 1H), 2.96–2.84 (m, 1H), 2.24 (s, 3H); 13C NMR
(125 MHz, DMSO-d6)
δ 170.7, 156.4, 144.1, 141.1, 138.3, 136.7, 133.0, 129.7, 129.6, 128.6, 128.1, 127.5,
126.9, 125.7, 120.5, 119.9, 66.2, 57.3, 47.0, 38.0, 20.9. HRMS m/z calcd for C31H28N2O3 [M + H]+ 477.2100,
found 477.2109.
(9H-Fluoren-9-yl)methyl (1-oxo-1-(p-tolylamino)propan-2-yl)carbamate (13). White microcrystals (75%);
m.p. 172–174 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ: 9.87 (s, 1H), 7.89 (d, J = 7.5 Hz, 2H), 7.76–7.54
(m, 3H), 7.51–7.43 (m, 2H), 7.44–7.29 (m, 4H), 7.10 (d, J = 8.2 Hz, 2H), 4.39–4.09 (m, 4H), 2.25 (s, 3H),
1.43–1.21 (m, 3H). 13C NMR (125 MHz, DMSO-d6)
δ: 171.3, 155.8, 143.9, 140.7, 136.5, 132.1, 129.0, 127.6,
127.0, 125.3, 120.1, 119.1, 65.6, 50.7, 46.6, 20.4, 18.1. HRMS m/z calcd for C25H24N2O3 [M + H]+ 401.1787,
found 401.1785.
Benzyl (S)-(1-((4-methoxyphenyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (14). White microcrystals
◦
◦
1
(80%); m.p. 164–166 C [Lit. m.p. 167 C [63]]. H NMR (500 MHz, DMSO-d6) δ: 9.94 (s, 1H), 7.64
(d, J = 8.4 Hz, 1H), 7.48 (d, J = 8.9 Hz, 2H), 7.36–7.20 (m, 10H), 6.88 (d, J = 9.0 Hz, 2H), 4.96 (s, 2H), 4.38
(td, J = 9.7, 4.8 Hz, 1H), 3.72 (s, 3H), 3.02 (dd, J = 13.6, 4.6 Hz, 1H), 2.85 (dd, J = 15.5, 8.2 Hz, 1H). 13C NMR
(125 MHz, DMSO-d6) δ: 169.9, 155.9, 155.3, 137.9, 136.9, 131.9, 129.8, 129.21, 129.1, 128.3, 128.2, 128.0, 127.7,

Molecules 2020, 25, 2501
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127.5, 127.4, 126.5, 126.3, 124.9, 120.9, 113.8, 65.3, 56.8, 55.1, 37.6. HRMS m/z calcd for C24H24N2O4 [M + H]+
405.1736, found 405.1737.
Benzyl (S)-(1-((4-methoxyphenyl)amino)-1-oxopropan-2-yl)carbamate (15). White microcrystals (85%);
m.p. 160–162 ◦C (Lit. m.p. 161.5–162.5 ◦C [64]). 1H NMR (500 MHz, DMSO-d6)
δ
: 9.88 (s, 1H), 7.63–7.45
(m, 3H), 7.42–7.27 (m, 5H), 6.88 (d, J = 9.0 Hz, 2H), 5.03 (q, J = 12.6 Hz, 2H), 4.35–4.04 (m, 1H), 3.72
(s, 3H), 1.29 (d, J = 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
: 171.0, 155.7, 154.8, 137.0, 132.2,
δ
132.1, 128.4, 128.3, 127.8, 127.7, 125.4, 125.3, 120.7, 113.8, 65.4, 55.2, 50.7, 18.2. HRMS m/z calcd for
C18H20N2O4 [M + H]+ 329.1423, found 329.1431.
Benzyl (S)-(1-((2-methoxyphenyl)amino)-1-oxopropan-2-yl)carbamate (16). White microcrystals (98%);
m.p. 185–187 ◦C [65]. 1H NMR (500 MHz, DMSO-d6)
δ
: 9.04 (s, 1H), 8.03 (d, J = 7.4 Hz, 1H), 7.75
(d, J = 6.7 Hz, 1H), 7.47–7.21 (m, 5H), 7.16–6.97 (m, 2H), 6.91 (t, J = 8.2 Hz, 1H), 5.06 (q, J = 12.4 Hz,
2H), 4.45–4.18 (m, 1H), 3.81 (s, 3H), 1.29 (d, J = 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
: 171.3,
δ
155.9, 148.9, 136.9, 128.4, 128.4, 127.8, 127.7, 127.1, 124.2, 124.2, 120.6, 120.3, 111.1, 65.5, 55.8, 50.9, 17.8.
HRMS m/z calcd for C18H20N2O4 [M + H]+ 329.1423, found 329.1422.
tert-Butyl (S)-(1-((4-fluorophenyl)amino)-3-methyl-1-oxobutan-2-yl)carbamate (17). Yellow microcrystals
1
(94%); m.p. 138–140 ◦C (Lit. m.p. 141–142 ◦C [58]). H NMR (500 MHz, DMSO-d6)
δ
10.02 (s, 1H), 7.62
(dd, J = 7.9, 5.1 Hz, 2H), 7.13 (t, J = 8.7 Hz, 2H), 6.83 (d, J = 8.2 Hz, 1H), 3.91 (t, J = 7.6 Hz, 1H), 2.04–1.94
(m, 1H), 1.39 (s, 9H), 0.90 (d, J = 6.3 Hz, 6H); 13C NMR (125 MHz, DMSO-d6)
δ171.1, 159.4, 157.5, 156.1,
135.7, 121.5, 121.4, 115.8, 115.6, 78.5, 61.0, 30.8, 28.6, 19.6. HRMS m/z calcd for C16H23FN2O3 [M + H]+
311.1693, found 311.1699.
Benzyl (S)-(1-((4-fluorophenyl) amino)-1-oxo-3-phenylpropan-2-yl) carbamate (18). White microcrystals
1
(77%); m.p. 191–193 ◦C (Lit. m.p. 195 ◦C [63]). H NMR (500 MHz, DMSO-d6)
δ: 9.94 (s, 1H), 7.64
(
d, J = 8.4 Hz, 1H), 7.48 (d, J = 8.9 Hz, 2H), 7.36–7.20 (m, 10H), 6.88 (d, J = 9.0 Hz, 2H), 4.96 (s, 2H),
4.38 (td, J = 9.7, 4.8 Hz, 1H), 3.72 (s, 3H), 3.02 (dd, J = 13.6, 4.6 Hz, 1H), 2.85 (dd, J = 15.5, 8.2 Hz, 1H).
13C NMR (125 MHz, DMSO-d6)
: 169.9, 155.9, 155.3, 137.9, 136.9, 131.9, 129.8, 129.21, 129.1, 128.3,
δ
128.2, 128.0, 127.7, 127.5, 127.4, 126.5, 126.3, 124.9, 120.9, 113.8, 65.3, 56.8, 55.1, 37.6. HRMS m/z calcd for
C23H21FN2O3 [M + H]+ 393.1536, found 393.1533.
Benzyl (2-((4-fluorophenyl)amino)-2-oxoethyl)carbamate (19). White microcrystals (84%); m.p. 174–176 ◦C.
1H NMR (500 MHz, DMSO-d6)
δ
: 10.02 (s, 1H), 7.61 (dd, J = 8.8, 5.0 Hz, 2H), 7.56 (t, J = 6.0 Hz, 1H),
7.41–7.32 (m, 6H), 7.15 (t, J = 8.9 Hz, 1H), 5.06 (s, 2H), 3.80 (d, J = 6.1 Hz, 2H). 13C NMR (125 MHz,
DMSO-d6) δ: 167.8, 158.9, 156.6, 137.0, 135.3, 128.4, 128.3, 128.2, 127.8, 127.7, 120.8, 120.8, 115.4, 115.2, 65.5,
43.9. HRMS m/z calcd for C16H15FN2O3 [M + H]+ 303.1067, found 303.1066.
Benzyl (S)-(1-((4-fluorophenyl)amino)-1-oxopropan-2-yl)carbamate (20). White microcrystals (90%);
1
m.p. 192–194 ◦C [65]. H NMR (500 MHz, DMSO-d6)
δ
: 10.03 (s, 1H), 7.69–7.51 (m, 3H), 7.44–7.25
(m, 5H), 7.23–7.04 (m, 2H), 5.03 (q, J = 12.7 Hz, 2H), 4.32–3.96 (m, 1H), 1.29 (d, J = 7.1 Hz, 3H). 13C NMR
(125 MHz, DMSO-d6) δ: 171.4, 158.9, 155.8, 136.9, 135.4, 128.3, 128.30, 127.8, 127.71, 127.7, 120.9, 120.9,
115.3, 115.1, 65.4, 50.7, 17.9. HRMS m/z calcd for C17H17FN2O3 [M + H]+ 317.1223, found 317.1228.
(9H-Fluoren-9-yl)methyl (2-((4-fluorophenyl)amino)-2-oxoethyl)carbamate (21). Yellow microcrystals
◦
1
(90%); m.p. 142–143 C. H NMR (500 MHz, DMSO-d6)
7.64–7.54 (m, 2H), 7.36–7.19 (m, 10H), 7.15 (t, J = 8.4 Hz, 2H), 4.97 (s, 2H), 4.45–4.36 (m, 1H), 3.07–3.00
(m, 1H), 2.90–2.82 (m, 1H); 13C NMR (125 MHz, DMSO-d6)
170.9, 159.6, 157.6, 156.5, 138.1, 137.4,
δ 10.20 (s, 1H), 7.66 (d, J = 8.0 Hz, 1H),
δ
135.6, 129.7, 128.8, 128.6, 128.2, 128.0, 126.9, 121.7, 121.7, 115.8, 115.7, 65.8, 57.4, 37.9. HRMS m/z calcd
for C23H19FN2O3 [M + H]+ 391.1380, found 391.1393.
tert-Butyl (S)-(1-((4-chlorophenyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (22). White microcrystals
(98%); m.p. 152–154 ◦C [66]. 1H NMR (500 MHz, DMSO-d6)
δ
: 10.17 (s, 1H), 7.61 (d, J = 8.8 Hz, 2H),
7.50–7.00 (m, 8H), 4.31 (dd, J = 12.8, 9.2 Hz, 1H), 3.26–2.65 (m, 2H), 1.59–1.11 (m, 9H). 13C NMR (125 MHz,
DMSO-d6) δ: 170.9, 155.4, 138.0, 137.8, 129.3, 129.2, 129.0, 128.8, 128.6, 128.1, 128.0, 126.8, 126.3, 126.3, 120.8,
78.1, 56.6, 55.1, 37.3, 28.1. HRMS m/z calcd for C20H23ClN2O3 [M + H]+ 375.1397, found 375.1389.
Benzyl (2-((4-chlorophenyl)amino)-2-oxoethyl)carbamate (23). White microcrystals (95%); m.p. 160–162 ◦C.
1H NMR (500 MHz, DMSO-d6)
δ: 10.10 (s, 1H), 7.62 (d, J = 8.7 Hz, 2H), 7.56 (t, J = 5.9 Hz, 1H), 7.42–7.29

Molecules 2020, 25, 2501
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(m, 7H), 5.05 (s, 2H), 3.81 (d, J = 6.1 Hz, 2H). 13C NMR (125 MHz, DMSO-d6)
δ: 168.6, 157.1, 138.3,
137.5, 131.5, 129.1, 128.9, 128.8, 128.7, 128.3, 128.2, 127.2, 127.1, 121.1, 65.9, 44.6. HRMS m/z calcd for
C16H15ClN2O3 [M + H]+ 319.0771, found 319.0777.
Benzyl (S)-(1-((4-chlorophenyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (24). White microcrystal
(90%); m.p. 182–184 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ
: 10.29 (s, 1H), 7.76 (d, J = 7.8 Hz, 1H), 7.63
(d, J = 8.8 Hz, 2H), 7.44–7.23 (m, 11H), 7.21 (d, J = 7.2 Hz, 1H), 4.96 (s, 2H), 4.80–3.81 (m, 1H), 3.02
dd, J = 13.7, 4.6 Hz, 1H), 2.86 (dd, J = 13.6, 10.3 Hz, 1H). 13C NMR (125 MHz, DMSO-d6)
: 171.2, 156.4,
(
δ
138.3, 138.2, 137.4, 130.0, 129.8, 129.7, 129.1, 128.8, 128.8, 128.7, 128.6, 128.2, 128.0, 127.8, 127.4, 127.3,
126.9, 121.3, 65.8, 57.5, 37.9. HRMS m/z calcd for C23H21ClN2O3 [M + H]+ 409.1241, found 409.1244.
Benzyl (S)-(1-((4-chlorophenyl)amino)-1-oxopropan-2-yl)carbamate (25). White microcrystal (73%);
1
m.p. 163–165 ◦C (Lit. m.p. 165–166 ◦C [67]). H NMR (500 MHz, DMSO-d6)
δ
: 10.11 (s, 1H), 7.72–7.55
(m, 3H), 7.42–7.28 (m, 7H), 5.53–4.69 (m, 2H), 4.50–3.73 (m, 1H), 1.29 (d, J = 7.1 Hz, 3H). 13C NMR
(125 MHz, DMSO-d6) δ: 171.7, 155.78, 137.9, 136.9, 128.6, 128.6, 128.3, 128.3, 127.8, 127.7, 126.8, 126.7,
120.8, 120.7, 65.4, 50.8, 17.9. HRMS m/z calcd for C17H17ClN2O3 [M + H]+ 333.0928, found 333.0923.
Benzyl (S)-(1-((4-chlorophenyl)amino)-4-(methylthio)-1-oxobutan-2-yl)carbamate (26). White microcrystal
1
(82%); m.p. 150–152 ◦C [68]. H NMR (500 MHz, DMSO-d6)
δ: 10.20 (s, 1H), 8.28–7.57 (m, 3H), 7.47–7.27
(m, 7H), 5.11–4.97 (m, 2H), 4.24 (dd, J = 12.9, 8.5 Hz, 1H), 2.84–2.29 (m, 2H), 2.06 (s, 3H), 1.98–1.82 (m, 2H).
13C NMR (125 MHz, DMSO-d6)
δ: 170.7, 156.1, 137.8, 136.9, 131.7, 128.6, 128.3, 128.3, 127.8, 127.7, 127.7
126.9, 120.9, 119.7, 65.5, 54.7, 31.4, 29.7, 14.6. HRMS m/z calcd for C19H21ClN2O3S [M + H]+ 393.0961,
found 393.0969.
(9H-Fluoren-9-yl)methyl (1-((4-chlorophenyl)amino)-1-oxopropan-2-yl)carbamate (27). White microcrystal
(90%); m.p. 197–199 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ
: 10.13 (s, 1H), 7.89 (d, J = 7.4 Hz, 2H), 7.82–7.54
(m, 5H), 7.48–7.25 (m, 6H), 4.65–3.89 (m, 4H), 1.30 (d, J = 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
δ:
171.7, 155.8, 143.8, 143.8, 140.7, 137.9, 128.6, 127.6, 127.0, 126.7, 125.3, 125.3, 125.2, 120.7, 120.1, 65.6, 50.8,
46.6, 17.9. HRMS m/z calcd for C24H21ClN2O3 [M + H]+ 421.1241, found 421.1235.
(9H-Fluoren-9-yl)methyl (S)-(1-((4-chlorophenyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (28)
.
◦
1
White microcrystal; (98%); m.p. 210–212 C. H NMR (500 MHz, DMSO-d6)
δ: 7.92–7.80 (m, 4H),
7.75–7.59 (m, 2H), 7.41 (dd, J = 11.1, 4.6 Hz, 3H), 7.37–7.02 (m, 10H), 6.27 (s, 1H), 4.64–3.86 (m, 3H),
3.48–2.68 (m, 2H). 13C NMR (125 MHz, DMSO-d6)
δ: 172.2, 143.9, 142.6, 139.40, 137.4, 135.4, 133.2, 129.5,
129.4, 129.2, 128.9, 128.5, 128.0, 127.8, 127.3, 126.1, 125.7, 123.9, 122.3, 122.2, 121.4, 121.2, 120.7, 120.0,
115.3, 109.7, 79.2, 57.2, 53.8, 37.9. HRMS m/z calcd for C30H25ClN2O3 [M + H]+ 497.1554, found 497.1553.
Benzyl (S)-(1-((2-chlorophenyl)amino)-1-oxopropan-2-yl)carbamate (29). White microcrystal (70%);
m.p. 162–164 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ
: 9.43 (s, 1H), 7.76 (d, J = 7.8 Hz, 1H), 7.70 (d, J = 6.8 Hz
1H), 7.49 (d, J = 7.9 Hz, 1H), 7.42–7.28 (m, 6H), 7.19 (t, J = 7.5 Hz, 1H), 5.05 (s, 2H), 4.45–4.14 (m, 1H),
1.33 (d, J = 7.1 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
: 171.72, 155.9, 136.9, 134.56, 129.4, 129.4,
,
δ
128.3, 128.3, 127.8, 127.7, 127.5, 126.2, 126.0, 125.3, 65.5, 50.6, 17.8. HRMS m/z calcd for C17H17ClN2O3
[M + H]+ 333.0928, found 333.0941.
Benzyl (S)-(1-((4-nitrophenyl)amino)-1-oxopropan-2-yl)carbamate (30). White microcrystal (70%); m.p.
1
95–97 ◦C (Lit. m.p. 99 ◦C [69]). H NMR (500 MHz, DMSO-d6)
δ
: 8.93–8.01 (m, 3H), 7.82 (t, J = 7.6 Hz
,
1H), 7.65 (t, J = 7.7 Hz, 1H), 7.54–7.20 (m, 4H), 7.16–6.81 (m, 1H), 5.59–5.38 (m, 1H), 5.12–4.95 (m, 2H),
1.54 (d, J = 7.2 Hz, 3H). 13C NMR (125 MHz, DMSO-d6)
δ: 172.4, 156.0, 145.3, 136.7, 131.2, 130.7, 128.4,
128.3, 127.9, 127.8, 127.7, 126.8, 120.2, 113.9, 65.8, 50.1, 16.8. HRMS m/z calcd for C17H17N3O5 [M + H]+
344.1168, found 344.1160.
Benzyl(2-((2-hydroxyphenyl)amino)-2-oxoethyl)carbamate(31). Whitemicrocrystals(90%); m.p. 180–182◦C
(Lit. m.p. 174–176 ◦C [65]). 1H NMR (500 MHz, DMSO-d6)
δ
9.93 (s, 1H), 9.11 (s, 1H), 7.98–7.83 (m, 1H),
7.82–7.64 (m, 1H), 7.52–7.23 (m, 5H), 6.98–6.73 (m, 2H), 5.08 (s, 2H), 3.88 (d, J = 4.7 Hz, 2H); 13C NMR
(125 MHz, DMSO-d6) δ168.6, 157.2, 147.7, 137.4, 128.9, 128.3, 128.1, 126.5, 124.8, 121.6, 119.5, 115.7, 66.1, 44.9.
HRMS m/z calcd for C16H16N2O4 [M + H]+ 301.1110, found 301.1117.
Benzyl (S)-(1-oxo-1-(pyridin-4-ylamino)propan-2-yl)carbamate (32). White microcrystals (89%); m.p.
◦
1
152–154 C [70]. H NMR (500 MHz, DMSO-d6) δ: 8.43–8.14 (m, 4H), 7.82 (t, J = 7.7 Hz, 1H), 7.65

Molecules 2020, 25, 2501
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(t, J = 7.7 Hz, 1H), 7.47–7.22 (m, 5H), 5.51–5.49 (m, 1H), 5.07–5.02 (m, 2H), 1.54 (d, J = 7.2 Hz, 3H).
13C NMR (125 MHz, DMSO-d6)
δ: 172.4, 156.0, 145.3, 136.7, 131.1, 130.6, 128.4, 128.3, 127.9, 127.8, 126.7,
120.2, 113.9, 65.8, 50.1, 16.8. HRMS m/z calcd for C16H17N3O3 [M + H]+ 300.1270, found 300.1274.
Benzyl (S)-(1-oxo-1-(pyridin-3-ylamino)propan-2-yl)carbamate (33). White microcrystals (97%);
m.p. 155–157 ◦C. 1H NMR (500 MHz, DMSO-d6)
δ
: 8.39–8.13 (m, 4H), 7.82 (t, J = 7.7 Hz, 1H), 7.65
(t, J = 7.7 Hz, 1H), 7.46–7.24 (m, 5H), 5.53–5.47 (m, 1H), 5.07–5.02 (m, 2H), 1.54 (d, J = 7.3 Hz, 3H).
13C NMR (125 MHz, DMSO-d6)
δ: 172.4, 156.0, 145.3, 137.0, 136.7, 131.1, 130.6, 128.4, 127.9, 127.8, 126.8,
120.2, 113.9, 65.8, 50.1, 16.8. HRMS m/z calcd for C16H17N3O3 [M + H]+ 300.1270, found 300.1265.
N-(p-Tolyl)pyrazine-2-carboxamide (35). White microcrystals (71%); m.p. 138–140 ◦C (Lit. m.p. 148 ◦C [72]).
1H NMR (500 MHz, DMSO-d6)
δ
: 10.62 (s, 1H), 9.28 (d, J = 1.4 Hz, 1H), 8.92 (d, J = 2.5 Hz, 1H), 8.80 (dd,
J = 2.5, 1.4 Hz, 1H), 7.77 (d, J = 8.4 Hz, 2H), 7.17 (d, J = 8.3 Hz, 2H), 2.29 (s, 3H). 13C NMR (125 MHz,
DMSO-d6) : 161.4, 147.6, 145.1, 143.9, 143.2, 135.6, 133.2, 129.04, 120.5, 20.5. HRMS m/z calcd for C12H11N3O
[M + H]+ 214.0902, found 214.0913.
N-(4-Methoxyphenyl)pyrazine-2-carboxamide (36). White microcrystals (81%); m.p. 147–149 C
(Lit. m.p. 149–150 ◦C [73]). 1H NMR (500 MHz, DMSO-d6)
: 10.62 (s, 1H), 9.28 (d, J = 1.4 Hz, 1H), 8.92
(d, J = 2.5 Hz, 1H), 8.80 (dd, J = 2.5, 1.4 Hz, 1H), 7.80 (d, J = 9.0 Hz, 2H), 6.95 (d, J = 9.0 Hz, 2H), 3.75
δ
◦
δ
(s, 3H). 13C NMR (125 MHz, DMSO-d6)
δ: 161.2, 155.9, 147.5, 145.2, 143.9, 143.2, 131.2, 122.0, 113.8, 55.2.
HRMS m/z calcd for C12H11N3O2 [M + H]+ 230.0851, found 230.0843.
N-(4-Fluorophenyl)pyrazine-2-carboxamide (37). White microcrystals (98%); m.p. 155–157 ◦C (Lit. m.p.
154–155 ◦C [72]). 1H NMR (500 MHz, DMSO-d6)
δ
: 10.82 (s, 1H), 9.30 (d, J = 1.4 Hz, 1H), 8.97 (d, J = 2.5 Hz,
1H), 8.81 (dd, J = 2.5, 1.4 Hz, 1H), 8.14–7.69 (m, 2H), 7.24–7.19 (m, 2H). 13C NMR (125 MHz, DMSO-d6)
δ:
161.6, 159.5, 157.6, 147.7, 144.9, 144.0, 143.2, 134.6, 122.5, 115.3, 115.2. HRMS m/z calcd for C11H8FN3O
[M + H]+ 218.0651, found 218.0664.
◦
2,2-Dichloro-N-(p-tolyl) acetamide (39). White microcrystals (85%); m.p. 157–159 C (Lit. m.p.
◦
1
159–160 C [74]). H NMR (500 MHz, DMSO-d6) δ: 10.56 (s, 1H), 7.48 (d, J = 8.4 Hz, 2H), 7.17
(d, J = 8.4 Hz, 2H), 6.58 (s, 1H), 2.27 (s, 3H). 13C NMR (125 MHz, DMSO-d6)
δ: 161.5, 135.0, 133.8, 129.4,
129.4, 119.8, 119.8, 67.3, 20.5. HRMS m/z calcd for C9H9Cl2NO [M + H]+ 218.0061, found 218.0057.
2,2-Dichloro-N-(4-fluorophenyl) acetamide (40). White microcrystals (80%); m.p. 130–132 ◦C (Lit. m.p.
1
134–135 ◦C [74]). H NMR (500 MHz, DMSO-d6)
δ
: 10.71 (s, 1H), 7.71–7.55 (m, 2H), 7.24–7.19 (m, 2H),
δ: 161.7, 159.7, 133.9, 121.8, 121.7, 115.8, 115.6, 67.3.
6.59 (s, 1H). 13C NMR (126 MHz, DMSO-d6)
HRMS m/z calcd for C8H6Cl2FNO [M + H]+ 221.9810, found 221.9813.
2,2-Dichloro-N-(4-chlorophenyl) acetamide (41). White microcrystals (80%); m.p. 143–145 ◦C (Lit. m.p.
◦
1
141–142 C [74]). H NMR (500 MHz, DMSO-d6)
δ
: 10.79 (s, 1H), 7.63 (d, J = 8.8 Hz, 2H), 7.43
(d, J = 8.8 Hz, 2H), 6.59 (s, 1H). 13C NMR (125 MHz, DMSO-d6)
δ
: 161.8, 136.5, 128.9, 128.3, 128.3, 121.4,
121.4, 67.2. HRMS m/z calcd for C8H6Cl3NO [M + H]+ 237.9515, found 237.9550.
4.2. General Methods for Preparation of 2-Substituted Benzimidazoles
In a typical procedure, a mixture of o-phenylenediamine (1 equiv.) and acylbenzotriazole (1 equiv.)
was subjected to microwave irradiation (20 W, 50 ◦C) in water (3 mL) for 1 h. After completion of the
reaction, aqueous 4N HCl was added and the precipitates were filtered, followed by washing with
water. The isolated products were recrystallized in ethanol to get the desired benzimidazoles in pure
form. Benzotriazoles could be recovered from the aqueous layer by pH-controlled acidification.
2-Phenyl-1H-benzo[d]imidazole (52). White microcrystals (82%); m.p. 295–296 ◦C (lit. m.p. 295 ◦C [75]).
1H NMR (500 MHz, DMSO-d6)
δ8.32–8.30 (m, 2H), 7.85–7.82 (m, 2H), 7.75–7.70 (m, 3H), 7.54–7.52 (m, 2H);
13C NMR (125 MHz, DMSO-d6)
δ151.2, 143.8, 134.9, 130.2, 129.5, 128.9, 126.4, 122.5, 121.6, 118.9, 111.3.
HRMS m/z calcd for C13H10N2 [M + H]+ 195.0844, found 195.0856.
2-(4-Fluorophenyl)-1H-benzo[d]imidazole (53). White microcrystals (90%); m.p. 249–251 ◦C (lit. m.p.
248 ◦C [75]). 1H NMR (500 MHz, DMSO-d6)
δ
8.24–8.21 (m, 2H), 7.60–7.59 (m, 2H), 7.22–7.17 (m, 4H);
150.2, 147.7, 135.1, 134.5, 129.1, 129.0, 128.2, 122.8, 121.9, 116.9, 111.4.
HRMS m/z calcd for C13H9FN2 [M + H]+ 213.0750, found 312.0746.
13C NMR (125 MHz, DMSO-d6)
δ

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2-(4-Chlorophenyl)-1H-benzo[d]imidazole (54). White crystalline solid (88%); m.p. 299–301 ◦C (lit. m.p.
1
302 ◦C [75]). H NMR (500 MHz, DMSO-d6)
δ
12.72 (s, 1H, D2O exchangable, > NH), 8.11–7.93 (m, 2H),
7.46–7.43 (m, 4H), 7.16–7.14 (m, 2H); 13C NMR (125 MHz, DMSO-d6)
δ 150.1, 146.9, 135.0, 134.6, 129.1,
128.7, 128.6, 127.8, 121.8, 122.0, 117.2. HRMS m/z calcd for C13H9ClN2 [M + H]+ 229.0454, found
229.0458.
◦
4-(1H-Benzo[d]imidazol-2-yl)phenol (55). Yellow microcrystals (86%); m.p. 280–282 C (lit. m.p.
280 ◦C [75]). H NMR (500 MHz, DMSO-d6)
δ
10.03 (s, 1H, D2O exchangable, > NH), 7.98 (d, J = 8.1 Hz
,
1
2H), 7.55–7.48 (m, 2H), 7.20–7.14 (m, 2H), 6.91(d, J = 8.1 Hz, 2H); 13C NMR (125 MHz, DMSO-d6)
δ
165.0, 157.1, 144.2, 133.3, 127.5, 125.9, 123.7, 121.4, 120.1, 115.5. HRMS m/z calcd for C13H10N2O
[M + H]+ 211.0793, found 211.0792.
2-(4-Nitrophenyl)-1H-benzo[d]imidazole (56). Light yellow microcrystals (94%); m.p. 318–320 ◦C
(lit. m.p. 317 ◦C [75]). H NMR (500 MHz, DMSO-d6)
δ 13.05 (br s, 1H, D2O exchangable, > NH), 8.00
1
(
dd, J = 15.4, 7.8 Hz, 2H), 7.87 (td, J = 7.8, 1.2 Hz, 1H), 7.76 (td J = 7.8, 1.4 Hz, 1H), 7.65–7.42 (m, 2H),
7.25 (d, J = 5.5 Hz, 2H); 13C NMR (125 MHz, DMSO-d6)
δ 149.0, 147.6, 143.3, 134.6, 132.5, 130.7, 123.9,
123.1, 121.9, 119.2, 111.5. HRMS m/z calcd for C13H9N3O2 [M + H]+ 240.0695, found 240.0677.
2-(4-Methylphenyl)-1H-benzo[d]imidazole (57). White microcrystals (92%); m.p. 274–276 ◦C (lit. m.p.
275 ◦C [75]). H NMR (500 MHz, DMSO-d6)
δ
12.59 (s, 1H, D2O exchangable, > NH), 8.08 (d, J = 8.1 Hz
,
1
2H), 7.58–7.44 (m, 2H), 7.35 (d, J = 7.8 Hz, 2H), 7.12–7.11 (m, 2H), 2.35 (s, 3H); 13C NMR (125 MHz,
DMSO-d6)
δ
151.4, 139.4, 129.4, 127.4, 126.4, 121.9, 116.3, 21.0. HRMS m/z calcd for C14H12N2 [M + H]+
209.1000, found 209.1018.
2-(4-Trifluoromethylphenyl)-1H-benzo[d]imidazole (58). White microcrystals (83%); m.p. 264–265 ◦C
(lit. m.p. 263 ◦C [75]); 1H NMR (500 MHz, DMSO-d6)
δ
12.79 (s, 1H, D2O exchangable, > NH), 8.34
(d, J = 8.8 Hz, 2H), 7.79–7.68 (m, 2H), 7.61 (d, J = 8.8 Hz, 2H), 7.04–6.96 (m, 2H); 13C NMR (125 MHz,
DMSO-d6)
δ 160.3, 151.3, 138.2, 127.7, 126.1, 124.5, 122.4, 121.4, 115.1. HRMS m/z calcd for C14H9F3N2
[M + H]+ 263.0718, found 263.0722.
2-(4-Methoxyphenyl)-1H-benzo[d]imidazole (59). White microcrystals (94%); m.p. 227–229 ◦C
◦
1
(lit. m.p. 228 C [75]); H NMR (500 MHz, DMSO-d6)
8.17 (d, J = 8.8 Hz, 2H), 7.59–7.57 (m, 2H), 7.20–7.17 (m, 2H), 7.04 (d, J = 8.8 Hz, 2H), 3.86 (s, 3H); 13
NMR (125 MHz, DMSO-d6) 160.3, 151.3, 127.7, 130.1, 122.4, 121.4, 116.5, 114.3, 54.8. HRMS m/z calcd
δ 12.74 (s, 1H, D2O exchangable, > NH),
C
δ
for C14H12N2O [M + H]+ 225.0950, found 225.0976.
2-(3,4,5-Trimethoxyphenyl)-1H-benzo[d]imidazole (60). Light yellow microcrystals (92%); m.p. 259–260 ◦C
(lit. m.p. 259 ◦C [75]). H NMR (500 MHz, DMSO-d6)
δ7.78 (d, J = 7.8 Hz, 1H), 7.65 (d, J = 7.6 Hz, 1H),
1
7.45 (t, J = 7.2 Hz, 1H), 7.30 (t, J = 7.2 Hz, 1H), 7.01 (s, 2H), 4.04 (s, 6H), 3.88 (s, 3H); 13C NMR (125 MHz,
DMSO-d6) δ155.1, 153.1, 142.2, 138.4, 124.7, 123.2, 121.9, 108.8, 60.1, 57.4. HRMS m/z calcd for C16H16N2O3
[M + H]+ 285.1161, found 285.1168.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/11/2501/s1,
general experimental data and NMR spectra.
Author Contributions: Conceptualization, S.S.P. and M.E.; Investigation, T.S.I., I.A.S., S.S.P. and M.E.; Supervision,
T.S.I., S.S.P., A.M.M.A.-M. and Z.K.M.A.-S.; Writing—Original draft, T.S.I. and S.S.P.; Writing—Review & editing,
T.S.I., I.A.S., S.S.P., A.M.M.A.-M., Z.K.M.A.-S., N.A.A. and H.Z.A.; Funding acquisition, T.S.I., S.S.P., N.A.A and
H.Z.A. All authors have read and agreed to the published version of the manuscript.
Funding: This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah,
under grant No.(RG-8-166-41). The authors, therefore, gratefully acknowledge DSR technical and financial support.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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