Benzoic acid-catalyzed transamidation reactions of carboxamides, phthalimide, ureas and thioamide with amines

Article abstract of DOI:10.1002/adsc.201400068

An efficient and simple method for the transamidation of carboxamides, phthalimide, ureas and thioamide with amines catalyzed by commercially available benzoic acid under metal-free conditions is described. Furthermore, to the best of our knowledge, this is the first report about the transamidation of an aromatic thioamide with amines.

Full text of DOI:10.1002/adsc.201400068

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DOI: 10.1002/adsc.201400068
Benzoic Acid-Catalyzed Transamidation Reactions of
Carboxamides, Phthalimide, Ureas and Thioamide with Amines
Ji-Wei Wu,a Ya-Dong Wu,a Jian-Jun Dai,a,* and Hua-Jian Xua,b,*
a
School of Chemical Engineering, School of Medical Engineering, Hefei University of Technology, Hefei 230009, Peopleꢀs
Republic of China
E-mail: daijj@hfut.edu.cn
b
Key Laboratory of Advanced Functional Materials and Devices, Anhui Province, Hefei 230009, Peopleꢀs Republic of
China
Fax : (+86)-551-6290-4405; phone: (+86)-551-6290-4353; e-mail: hjxu@hfut.edu.cn
Received: January 17, 2014; Revised: March 10, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400068.
Abstract: An efficient and simple method for the
transamidation of carboxamides, phthalimide, ureas
and thioamide with amines catalyzed by commer-
cially available benzoic acid under metal-free condi-
tions is described. Furthermore, to the best of our
knowledge, this is the first report about the trans-
amided, transamidation usually requires harsh condi-
tions, such as long reaction times, high temperatures
(>2508C) or stoichiometric amounts of activating
ACHTUNGTRENNUNGamidation of an aromatic thioamide with amines.
Keywords: benzoic acid; metal-free conditions; thi-
oamides; transamidation
The amide linkage is one of the most important func-
tional groups in nature because it constitutes the
backbone of all natural peptides and proteins.[1] Also,
it has been regarded as the most important linkage in
the fields of industrial and medicinal chemistry.[2] Ac-
cording to the authoritative statistics reports from the
leading pharmaceutical companies, nearly 25% of the
drug molecules contain an amide bond unit,[3] which
demonstrates its importance in organic synthetic
chemistry. Amides, as an important class of com-
pounds, have been widely found in bioactive prod-
ucts,[4] biological[5] and synthetic polymers.[6] Conse-
quently, many methods for the synthesis of amides
have been reported, such as the reactions of amines
with carboxylic acid derivatives,[7] esters,[8] aldehydes[9]
or alcohols,[10] rearrangement of aldoximes,[11] hydra-
tion of nitriles,[12] aminocarbonylation processes,[13] hy-
droamination of alkynes.[14] However, they always
suffer from some limitations, such as stoichiometric
amounts of activating agents, tedious work-up proce-
dures and low reactivity. Therefore, as one of most
straightforward and convenient methods, transamida-
tion has gained more and more attention in this field.
Scheme 1. Various methods for the transamidation of
Unfortunately, because of the high stability of amides with amines.
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Table 1. Optimization of the reaction conditions.[a]
agents to cleave the amide bond.[15] In the last two de-
cades, great efforts have been made in this area in
order to develop a simple method that allows the re-
actions to occur under relatively mild conditions. Ber-
trand,[16] Beller (Scheme 1, ii),[17] Myers,[18] Akaman-
chi,[19] Shimizu (Scheme 1, i),[20] Stahl,[21] and Wil-
liams[22] have reported a series of metal-catalyzed
methods of transamidation. Despite the advances ach-
ieved, all of these methods were catalyzed by transi-
tion or lanthanide metals, and these bring great diffi- Entry
Solvent
Time [h]
Yield [%][b]
culties for the separation and purification of the
target product, especially in the synthesis of pharma-
ceutical molecules. Furthermore, these methods also
3
1
2
H2O
20
20
20
20
20
20
20
20
20
20
20
20
10
8
13
16
10
49
28
23
57
84
60
66
98
63
97
98
83
84
87
t-BuOH
n-C6H11OH
butanone
CH3CN
DMSO
suffer from certain demerits, for example, the use of
expensive activation reagents and stoichiometric
amounts of catalysts. To solve these problems,
Allen,[23] Nguyen (Scheme 1, iii),[24] Adimurthy[25] and
Singh[26] have reported a series of metal-free catalyzed
methods for transamidation. However, these methods
suffer from the shortcoming of long reaction times
and limited substrate scope. The transamidation of
aliphatic thioamides with amines has been reported
by Schlatter.[27]
4
5
6
7
8
9
10
11
12
13
14
15
16[c]
17[d]
benzene
PhMe
o-xylene
m-xylene
p-xylene
mesitylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
Herein, we have developed an efficient and simple
method for the transamidation of carboxamides,
phthalimide, ureas and thioamide with amines cata-
lyzed by commercially available benzoic acid at
1308C under metal-free conditions. To the best of our
knowledge, the transamidation of aromatic thioa-
mides with amines has not yet been reported
(Scheme 1, iv).
6
8
8
[a]
Benzamide (1 mmol), benzylamine (lmmol), catalyst
(15 mol%), solvent (1 mL), in a sealed tube.
Determined by GC.
[b]
[c]
[d]
Temperature: 1208C.
In air.
Our initial studies were focused on screening for an
efficient catalyst for the reaction of benzamide with
benzylamine. Organic acids like aliphatic monocar- which indicated the substituent on the benzene ring
boxylic acids, aliphatic diacids, aromatic acids and showed no effect on this reaction. And also, on react-
heterocyclic carboxylic acids were investigated and ing aliphatic primary amines, good to excellent yields
we found that benzoic acid was the best catalyst.[28] To (80–92%) were obtained (Table 2, 1e–i). Cyclic secon-
further optimize the reaction conditions, different sol- dary amines could be also reacted effectively under
vents, times and temperatures were screened in the the optimized condition in good yields (Table 2, 1l).
presence of benzoic acid. Through a screening of the However, the present method was not so effective for
different solvents, we found that p-xylene was the aromatic amines, which only afforded moderate yields
best solvent for this reaction which can afford the de- of corresponding products (Table 2, 1m and 1n).
sired product with a 98% yield (Table 1, entries 1–12).
In previous works, the transamidation of heterocy-
Then the reaction time was screened and results clic amides and heterocyclic amines were reported
showed that the yield of the target product was not only rarely. However, in our reaction system, the het-
reduced even when the reaction time was decreased erocyclic amines could react well with benzamide to
to 8 h (compare entries 11 and 13 with entry 14). The give the corresponding transamidation products
yield of 1a declined significantly when the reaction (Table 3, 2a–d) in excellent yields (90–96%). What is
temperature was 1208C (Table 1, entry 16). The reac- more, heterocyclic amides could also react with the
tion was also carried out in air with a low yield of 1a benzylamine to form the target products (Table 3, 2e,
(Table 1, entry 17).
2n and 2o) in excellent yields (94–96%). Similarly, the
With the optimized conditions in hand, we then in- transamidation of nicotinamide with heterocyclic
vestigated the generality of this protocol. The trans- amines also formed the corresponding products
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Benzoic Acid-Catalyzed Transamidation Reactions
Table 2. The transamidation of benzene aromatic amides with amines.[a]
[a]
Isolated yields of 1 based on amide are shown.
Table 3. The transamidation of heterocyclic amides with amines.[a]
[a]
Isolated yields of 2 based on amide are shown.
It is noteworthy to emphasize that the transamida- form the corresponding products in moderate to ex-
tion of aliphatic amides with amines could be realized cellent yields (53–98%) (Table 4).
under milder conditions: at 808C for 1 h. Under these
Phthalimide, a common potential precursor of pes-
conditions, aliphatic amides including formamide, ticides, dyestuffs, medicinal and many other fine
acetamide and trifluoroacetamide reacted with a varie- chemicals, is widely used in organic synthesis. Thus
ty of benzylic, aromatic, and heterocyclic amines to we examined the benzoic acid-catalyzed transamida-
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Table 4. The transamidation of aliphatic amides with amines.[a]
[a]
Isolated yields of 3 based on amide are shown.
Table 5. The transamidation of phthalimide with amines.[a]
ylurea and aniline or 4-toluidine gave diphenyl or 1,3-
bis(p-tolyl)urea exclusively by replacing the ethyl
AHCTUNGTRENNUNG
group with the phenyl or tolyl group (Table 6, en-
tries 3 and 4, 5a and 5c). The reaction of 1-phenylurea
and aniline resulted in diphenylurea in 96% yield. In
contrast, the reaction of 1-phenylurea and 4-toluidine
gave 1-phenyl-3-(p-tolyl)urea as the only product
(Table 6, entries 5 and 6, 5a, and 5d).
Finally, our study was focused on the reaction of
thioamide with various amines. To the best of our
knowledge, there are no reports about the transami-
dation of aromatic thioamide with amines in previous
documents. Due to the high stability of the thioamide,
the reaction time was extended to 36 h (Table 7). It
was observed that moderate yields (50–61%) of the
corresponding products were obtained when the sub-
stituents on the phenyl ring are fluoro, chloro or me-
thoxy groups.
[a]
Isolated yields of 4 based on amide are shown.
We were excited to find that the current method
had a very good selectivity (Scheme 2). The transami-
tion of phthalimide with various primary amines dation of benzamide with 3-aminobenzylamine and
(Table 5). To our delight, phthalimide showed a high tryptamine gave, respectively, 7a and 7b as the only
activity in this reaction system. Good to excellent products. So the developed method has the potential
yields (80–98%) were obtained from the reaction of to be applied for the selective protection of the amino
phthalimide with primary amines such as benzyl- group.
A
ACHTUNGTRENNUNG
In order to elucidate the mechanism, some control
experiments were carried out and the results are pre-
To further extend the scope of the protocol, the sented in Scheme 3. First, we collected the gas of Eq.
benzoic acid-catalyzed transamidations of ureas were (1). Then, we observed a dark blue precipitate when
examined with various amines. The transamidation of the gas was passed into a saturated aqueous solution
ureas needed copper as catalyst in previous docu- of CuSO4 which identified that NH3 was formed in
ments.[17] Fortunately, all types of substituted ureas Eq. (1). Then the secondary and tertiary amides were
could be reacted with aromatic amines in good to ex- employed to react with benzylamine and only 1a was
À
cellent yields with our developed protocol (Table 6). obtained which indicated that the C N bond of the
In general, the transamidation of ureas with aniline amides was broken [Eqs. (2) and (3)]. The yield of
and benzene-1,2-diamine gave the corresponding the target product decreased obviously with methyl
target products in excellent yields (Table 6, entries 1 benzoate instead of benzoic acid as the catalyst [Eq.
and 2, 5a and 5b). Interestingly, the reaction of 1-eth- (4)]. According the results of Eq. (4) and Allenꢀs
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Table 6. The transamidation of ureas with amines.[a]
[a]
[b]
Isolated yields of 5 based on amide are shown.
Urea (1 mmol), amine (2 mmol).
report,[23] we speculated that the hydrogen atom of
À
the COOH group is important for efficient transami-
dations. Additionally, the conglomerate of benzyl-
AHCTUNGTRENNUNG
Table 7. The transamidation of thioamide with amines.[a]
AHCTUNGTRENNUNG
[Eqs. (5) and (6)]. So we speculated that 8a is the in-
termediate of the reaction.
Based on the above results and previous literature
reports,[23,24,26] a tentative mechanism was proposed as
shown in Scheme 4. Firstly, the reaction is started by
a proton transfer from benzoic acid to the amine to
generate ammonium salt A. Then amide attacks am-
monium salt A to form the target product, NH3 is re-
leased and simultaneously the benzoic acid is regener-
ated. Compared with the pathway I, another pathway
is possible, because under the aforementioned condi-
tions the reaction of 8a with benzamide gave only 1a
in 72% isolated yield which is much lower than the
real yield (92%). Simultaneously, Allen[23] and
Nguyen[24] have verified that the transamidation of
amides with amines could proceed smoothly under
the influence of hydrogen bonding. So, we think the
hydrogen bond of the benzoic acid to the amide
through a two-point contact could happen in this re-
[a]
Isolated yields of 6 based on amide are shown.
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Scheme 2. The experiments on selectivity.
Scheme 3. The control experiments.
action system. In pathway II, the reaction is started
by intermolecular hydrogen bonds between benzoic
acid and amide to generate the intermediate B. Then
amine attacks the intermediate B, generating the
target product and NH3.
In conclusion, we have developed an efficient and
simple method for the transamidation of carbox-
ACHTUNGTRENNUNGamides, phthalimide, ureas and thioamide with amines
catalyzed by commercially available benzoic acid
under metal-free conditions. Compared to known cat-
alysts, benzoic acid is inexpensive, effective, and read-
ily available. Due to the mild reaction conditions,
wide range of substrates, good to excellent yields and
an easy work-up procedure, the present protocol has
the potential to be widely applied.
Scheme 4. The possible reaction mechanism.
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General Procedure
Amide (1 mmol) and benzoic acid (18.3 mg, 0.15 mmol)
were added to a 25-mL Schlenk tube equipped with a mag-
neton. The Schlenk tube was purged with argon three times
and was charged with amine (1.0 mmol) and p-xylene
(1.0 mL) by syringe. The reaction mixture was stirred at
1308C for 8 h. After cooling to room temperature, the re-
sulting solution mixture was diluted by ethyl acetate
(10 mL). The reaction mixture was purified by column chro-
matography on silica gel to give the desired product.
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Acknowledgements
We gratefully acknowledge financial support from the Na-
tional Natural Science Foundation of China (No. 21272050,
21072040) and the Program for New Century Excellent Tal-
ents in University of the Chinese Ministry of Education
(NCET-11-0627).
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