Green synthesis of benzamides in solvent- and activation-free conditions

Article abstract of DOI:10.1080/00397911.2014.898072

Herein, we describe a clean and ecocompatible pathway for both N-benzoylation and N-acetylation of anilines, amines, diamines, and aminoalcohols using three enol esters with good yields. We have improved the use of vinyl benzoate for the direct introduction of a benzamido-moiety under solvent- and activation-free conditions. The recovered amides are easily isolated by crystallization. Copyright

Full text of DOI:10.1080/00397911.2014.898072

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Green Synthesis of Benzamides in
Solvent- and Activation-Free Conditions
Affef Alalla a , Mounia Merabet-Khelassi a , Louisa Aribi-Zouioueche a
& Olivier Riant b
a Ecocompatible Asymmetric Catalysis Laboratory (LCAE). Badji
Mokhtar Annaba University , Annaba , Algeria
b Institute of Condensed Matter and Nanosciences Molecules, Solids
and Reactivity (IMCN/MOST) , Université Catholique de Louvain ,
Louvain La Neuve , Belgium
Accepted author version posted online: 06 May 2014.Published
online: 13 Jun 2014.
To cite this article: Affef Alalla , Mounia Merabet-Khelassi , Louisa Aribi-Zouioueche & Olivier
Riant (2014) Green Synthesis of Benzamides in Solvent- and Activation-Free Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:16, 2364-2376, DOI: 10.1080/00397911.2014.898072
To link to this article: http://dx.doi.org/10.1080/00397911.2014.898072
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Synthetic Communications1, 44: 2364–2376, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.898072
GREEN SYNTHESIS OF BENZAMIDES IN SOLVENT-
AND ACTIVATION-FREE CONDITIONS
Affef Alalla,1 Mounia Merabet-Khelassi,1
Louisa Aribi-Zouioueche,1 and Olivier Riant2
1Ecocompatible Asymmetric Catalysis Laboratory (LCAE). Badji Mokhtar
Annaba University, Annaba, Algeria
2Institute of Condensed Matter and Nanosciences Molecules, Solids and
´
Reactivity (IMCN=MOST), Universite Catholique de Louvain,
Louvain La Neuve, Belgium
GRAPHICAL ABSTRACT
Abstract Herein, we describe a clean and ecocompatible pathway for both N-benzoylation
and N-acetylation of anilines, amines, diamines, and aminoalcohols using three enol esters
with good yields. We have improved the use of vinyl benzoate for the direct introduction of a
benzamido-moiety under solvent- and activation-free conditions. The recovered amides are
easily isolated by crystallization.
Keywords Acetylation; anilines; benzoylation; enol esters; green chemistry
INTRODUCTION
The development of new methodologies for the synthesis of bioactive
molecules is growing rapidly.[1] The amide bond is found in many key building
blocks involved in the synthesis of a variety of molecules[2] for therapeutic, cosmetic,
Received December 20, 2013.
Address correspondence to Louisa Aribi-Zouioueche, Ecocompatible Asymmetric Catalysis
Laboratory (LCAE), Badji Mokhtar Annaba-University, B.P 12, 23000 Annaba, Algeria. E-mail:
louisa.zouioueche@univ-annaba.org
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2364

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2365
and food-processing uses. In organic synthesis, the creation of an amide bond is a
necessary step to protect a primary amine in mono-, bi-, or=and poly-functional
molecules. The acetamido- or benzamido-moieties are especially important for
protecting free amino-groups.[3] In fact, the study of N- and O-acetylation of amines,
alcohols, amino acids, and amino alcohols is still drawing the attention of chemists[4]
and the development of new ecocompatible ways to synthesize amides remains
challenging.[5] Compared to the N-benzoylation,[6] the N-acetylation of amines has
been well studied.[7] It is usually performed using acetyl chloride,[8] anhydrides,[9]
carboxylic acids,[10] or esters.[11] However, the first one is routinely realized with ben-
zoyl chloride despite its greater toxicity and requires a base such as pyridine, triethy-
lamine, or sodium hydroxide as catalyst.
The literature shows that in order to perform the acetylation of amines,
activation such as heating, microwave irradiation, or a catalyst is required. Although
being environmentally friendly, enol esters as acetyl donors are scarcely described as
amidation agents. Enol esters are largely used in enzymatic transesterification, allow-
ing the kinetic resolution of racemic benzylic alcohols.[12] The first catalyst- and
base-free synthesis of amides using enol amino esters was described in 1991 by
Kabouche et al.[11a] This seminal work was later developed by Chen et al., who
described the selective acetylation of amines and diols using vinyl carboxylates as leav-
ing groups.[11b] Ahmed and Lier synthesized acetanilides with good yields by employ-
ing isopropenyl acetate at 85 to 90 ꢀC with iodine as catalyst.[11c] The acetylation of
primary amines with vinyl acetate under microwave irradiation has been described
by Ferroud et al.[11d] More recently, Pelagalli et al.[13] disclosed that the acetylation
of primary and secondary amines with isopropenyl acetate can be realized without
solvent, although heating at 60 ꢀC was required to achieve the desired reaction.
The protection of aromatic amines, especially anilines, is very important for the
production of colorants, pesticides, herbicides, and drugs, although difficult to
perform.[14] This work describes the new synthesis of benzamides by vinyl benzoates
and N-acetylation of amines, especially the aromatic one using enol esters as acyl
donors. Three enol esters are compared and the scope of this reaction has been extended
to aliphatic, cyclic, aromatic and benzylic primary amines, diamines, and amino alcohols.
ACETAMIDE SYNTHESIS
First, to determine the optimal conditions, we have studied the N-acetylation
of benzylic, aliphatic, and aromatic primary amines (1–4) by vinyl acetate (VA)
and isopropenyl acetate (IA) (Scheme 1). These amines have been selected for their
potential industrial and therapeutic interest.[15] The reaction was performed with
1 mmol of amine and 1 to 3 equivalents of the acetylating agent, without solvent,
and under magnetic stirring. The reaction was monitored by thin-layer chromato-
graphy (TLC).
After complete consumption of the starting amine the excess of the acetylating
agent was removed by evaporation and the crude reaction mixture was analyzed by
1H NMR. This analysis shows complete conversion of the starting material and
quantitative formation of a single amide for each substrate depicted in Table 1.
The desired amides were isolated as pure products after purification as shown by
1H and 13C NMR analysis. The results are presented in Table 1.

2366
A. ALALLA ET AL.
Scheme 1. Acetamide synthesis.
Table 1. Acetylation of various amines with vinyl acetate and isopropenyl acetate
Entry
Substratea=product
AA Amount of AA (mmol) Time (h)b Yield (%)c
1
2
VA
3
1
3
2
1
3
1
3
2
1
3
1
3
2
1
3
1
3
2
1
2
4
85
61
92
90
87
89
75
90
89
83
70
72
86
85
62
70
61
69
59
57
3
IA
3
4
12
21
1
5
6
VA
IA
7
21
2
8
9
5
10
11
12
13
14
15
16
17
18
19
20
5
VA
IA
2
3
1 min
1
1
VA
IA
24
48
24
24
48
a1 mmol of amine, the appropriate amount of enol ester, without solvent, under magnetic stirring at
room temperature.
bAfter total conversion.
cIsolated yields of pure acetamides after purification by simple workup or by crystallization.

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2367
The concentration of the acetyl donor has a significant effect on the N-acety-
lation of primary amines (1–4). Increasing the amount of the acetylating agent from
1 to 3 equivalents decreases the reaction time and improves the isolated yields of the
amides (1–4). Indeed, only 1 equivalent of the acetyl donor led to a slower reaction.
However, using 3 equivalents led to a much faster reaction (3 h vs. 21 h, entries 3 and
5; 1 min vs. 1 h, entries 13 and 15). As can be seen in Table 1, the results strongly
depend on the nature of the amine. Using aliphatic amines 2 and 3 led to short reac-
tion times whatever acetylating agent was employed (entries 6–15). With benzylic
amine 1 reaction times of 2 h using VA and 3 h using IA were observed. The reaction
was much slower using aromatic amine 4 (entries 16–20). We were pleased to find
that the analgesic acetanilide 4a has been obtained within 70% yield (entry 16) under
milder conditions than previously described, without heating and in absence of
catalyst.[13]
It was thus found that isopropenyl acetate afforded much shorter reaction
times than vinyl acetate and allows isolation of the acetamides in good yields. There-
fore, isopropenyl acetate can be considered as a better acetyl donor than vinyl acet-
ate. The best results were obtained using 3 mmol of IA. It can be easily removed from
the reaction mixture and no secondary reaction has been observed. The whole
process seems competitive for the preparation of amides.
SCOPE OF THE AMIDATION OF PRIMARY AMINES USING IA
The scope of the reaction was determined using various primary amines.
Aromatic, benzylic, cyclic, and heterocyclic amines and amino alcohol were
employed (5–23) to study electronic or steric effects (Scheme 2). The reactions were
performed under the optimal conditions previously determined, with (1:3) amine=
isopropenyl acetate ratio and at room temperature.
All the amides were obtained after evaporation of residual IA and acetone
by-product. The results are presented in Table 2. The structures of all products were
1
determined by H and 13C NMR analyses.
The results mentioned in Tables 1 and 2 show that isopropenyl acetate is an
efficient acetyl donor toward the N-amidation of most amines. Benzylic amides were
obtained quantitatively (Table 1, entry 3; Table 2, entries 1–9) within 3 h or less.
Increase of the acetylation time rate of amine 6 compared to 5 is due to the presence
of two activating methoxy groups (Table 2, entries 1 and 2). Total conversions of
aliphatic and benzylic amines were observed and the acetamides were isolated in
good yields. The reaction times depend on the structure of the amine. While the
acetamide 3a was obtained instantaneously (Table 1, entry 13), the acetamide 2a
Scheme 2. Amidation of primary amines using IA.

2368
A. ALALLA ET AL.
Table 2. Acetylation of primary amines by isopropenyl acetate
Entry
1
Substratea=product
Time (h)b
Yieldc (%)
100
3
2
20 min
100
3
4
2
2
100
100
5
6
1.5
3
100
85
7
8
9
3
3
2
70
100
100
(Continued )

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2369
Table 2. Continued
Entry
10
Substratea=product
Time (h)b
Yieldc (%)
3
100
11
24
94
12
13
24
96
73
28
14
3
90
15
NR
100
16
17
24
24
60
(Continued )

2370
A. ALALLA ET AL.
Table 2. Continued
Entry
Substratea=product
Time (h)b
40 min
Yieldc (%)
18
19
100
1
100
a1 mmol of amine, 3 mmol of isopropenyl acetate, without solvent, under magnetic stirring at room
temperature.
bAfter total conversion.
cIsolated yields of pure acetamides after purification by simple workup or by crystallization.
was formed within 2 h (Table 1, entry 8) and the cyclic acetamide 15a within 24 h
(Table 2, entry 11).
Aromatic amides were obtained in good yields under the same conditions in
some cases. Both reaction time and isolated yields were influenced by the substitu-
tions on the aromatic ring. Aniline 4 (Table 1, entry 18) and para-toluidine 16
(Table 2, entry 12) were acetylated within 24 h in 69% and 73% yields respectively.
The presence of ÀCF3 group led to a decrease in yield of 17a and an increase in reac-
tion time (4 days, Table 2, entry 13). On the other hand, the presence of an acetyl
substituent in meta-position the aromatic ring (Table 2, entry 14) afforded the
corresponding acetamide 18a in good yield. Furthermore, the aminothiophene 10
and aminofurane 11 have been acylated in 3 h and corresponding amides were
obtained in 70% and 85% yields respectively (entries 6 and 7). The acetylation of
aminopyridine 19 did not proceed (entry 15). We have also tried the acetylation of
an alkylaminopyridines without any results.
The study of N-acetylation was extended to bifunctional molecules of thera-
peutic interest and studied under the conditions determined previously. The primary
amine function of amino alcohols 20 and 21 was selectively N-acetylated (entries 16
and 17). The isolated yields depend on the steric effects. The substrate 22 substituted
by morpholine group is protected quantitatively within 40 min (entry 18). While the
bulky substrate 23 with a furoyl substituent possessing secondary amine moiety was
N-acetylated (entry 19) and amide 23a was obtained quantitatively in short time.
BENZAMIDES SYNTHESIS USING VINYL BENZOATE (VB)
Because N-benzoylation is an important reaction in organic synthesis, it was
decided to study the protection of whole substrates (1–25) with vinyl benzoate
(VB) under conditions previously determined (Scheme 3). The reaction was
performed using 3 equivalents of the benzoylating agent and monitored by thin-layer

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2371
Scheme 3. Benzoylation of primary amines.
chromatography (TLC). After consumption of the starting amine, the corresponding
benzamides were isolated by crystallization or by flash chromatography. The results
are summarized in Table 3.
Table 3. Benzamide synthesis using vinyl benzoate
Entry
Substratea=benzamide
Time (h)b
Yieldc (%)
1
4
90
2
3
4
3
87
82
70
24
24
5
6
24
24
68
55
(Continued )

2372
A. ALALLA ET AL.
Table 3. Continued
Entry
Substratea=benzamide
Time (h)b
Yieldc (%)
7
24
60
8
24
94
9
24
24
48
50
10
11
24
69
12
13
24
24
69
55
(Continued )

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2373
Table 3. Continued
Entry
Substratea=benzamide
Time (h)b
Yieldc (%)
14
48
53
15
16
24
24
63
83
17
40 min
100
18
24
90
19
24
73
a1 mmol of amine, 3 mmol of vinyl benzoate, without solvent, under magnetic stirring at room temperature.
bAfter total conversion.
cIsolated yields of pure benzamides after purification by crystallization.
As shown in Table 3, vinyl benzoate acts as an efficient benzoylating agent.
Reaction times were varied from 40 min to 48 h. Isolated yields from 48% to
100%, depending on the steric and the electronic properties of amines, were

2374
A. ALALLA ET AL.
obtained. Most of the substrates reacted smoothly (entries 1–12), whereas aromatic
amines, especially anilines 4, 16, 17, and 18 did not react to form the desired benza-
mides. It was unexpected, considering that the acetylation of such amines was poss-
ible by using IA (Table 1, entry 18; and Table 2, entries 12–14). On the other hand,
the heterocyclic substrates such as the alkylaminopyridines 24 and 25 afforded the
corresponding benzamides 24b and 25b with 90% and 73% yields, respectively
(entries 18 and 19), while their acetylation could not be realized. These reversed reac-
tivities compared to the acetylation might be explained by the bulkiness and the elec-
tronic effects of both the benzoylating agent and the amines. The amino alcohols 20
and 21 were selectively N-benzoylated (entries 15 and 16).
Vinyl benzoate afforded the desired products in lower yields and longer reac-
tion times than isopropenyl acetate. It might be explained by steric effects, because
vinyl benzoate is more hindered than IA.
CONCLUSION
In summary, to perform the protection of several primary amines, anilines, and
bifunctional molecules, three enol esters were compared. Vinyl acetate, isopropenyl
acetate, and vinyl benzoate were employed to synthesize various amides. Both the
N-benzoylations and the N-acetylations were performed with satisfactory isolated
yields and reaction times. Soft reaction conditions were employed, at room tempera-
ture, without solvent and without any catalyst. The direct introduction of a
benzamido-moiety realized under solvent-free conditions and without activation is
especially important for protecting amino groups. The difference of reactivity among
BV, VA, and IA showed their complementarity, allowing the protection of a large
broad of amines and of some bifunctional molecules. This methodology respects
the principles of green chemistry and can therefore be applied toward the clean
synthesis of polyfunctional molecules.
EXPERIMENTAL
All starting materials and reagents used in this study were obtained commer-
cially from Aldrich and Acros and were used without purification. All reactions were
monitored by thin-layer chromatography (TLC) carried out on 0.25-mm Merck
silica-gel plates (60F-254) using ultraviolet light (254 nm) as the visualizing agent
and ninhydrine solution and heat as developing agents. NMR spectra were recorded
1
on Brucker spectrometers (300 MHz for H, 75 MHz for 13C). Chemical shifts are
reported in d ppm from tetramethylsilane (TMS) with the solvent resonance as the
internal standard for 1H NMR and chloroform-d (d 77.0 ppm) for 13C NMR. Coup-
ling constants (J) are given in hertz. The following abbreviations classify the multi-
plicity: s, singulet; d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad signal.
Preparation of the Acetamides
In a round-bottomed flask, a mixture of a suitable amine (1 eq.) and the appro-
priate enol ester (isopropenyl acetate or vinyl acetate) (3 eq.) was stirred at room
temperature for the indicated time. The evolution of the reaction was monitored

EFFICIENT N-BENZOYLATION UNDER GREEN CONDITIONS
2375
by TLC. After complete consumption of the amine, the crude reaction mixture was
concentrated in vacuo and the amide was directly obtained pure. If it was not the
case, a simple acidic workup was required to purify the obtained acetamide.
Preparation of the Benzamides
In a round-bottomed flask, a mixture of a suitable amine (1 eq.) and 3 eq. of
vinyl benzoate were stirred at room temperature for 24 h. The evolution of the reac-
tion was monitored by TLC. After complete consumption of the amine, the crude
reaction mixture was concentrated in vacuo and purified by flash chromatography
on silica gel (CH2Cl2 as eluent) or by crystallization using hexane as the crystalliza-
tion solvent to gave the pure benzamides.
ACKNOWLEDGMENT
We thank M. Lo¨ıc Cornelissen for his contribution for the preparation of this
article.
FUNDING
MESRS (FNR 2000 and PNR) and FNRS are gratefully acknowledged for
financial support of this work.
SUPPORTING INFORMATION
All experimental details, NMR characterization, and spectra can be accessed
on the publisher’s website.
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