The thermal amidation of carboxylic acids revisited

Article abstract of DOI:10.1055/s-0028-1083277

Factors affecting the thermal condensation of carboxylic acids with amines have been investigated, and an effective protocol for this waste-minimized, environmentally benign transformation has been identified. Fourteen examples demonstrate the applicability of this procedure to aliphatic, aromatic and heteroaromatic carboxylic acids and primary and secondary aliphatic as well as aromatic amines. The approach leads to the corresponding amides in good yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083277

160
PAPER
The Thermal Amidation of Carboxylic Acids Revisited
The
Thermal
A
m
u
idation of
C
k
a
rboxylic
A
a
c
ids
R
evisi
s
ted J. Gooßen,* Dominik M. Ohlmann, Paul P. Lange
Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße 54, 67663 Kaiserslautern, Germany
Fax +49(631)3921; E-mail: goossen@chemie.uni-kl.de
Received 12 November 2008
of primary ammonium salts of short-chain aliphatic car-
boxylic acids12 or of b-phenethylamines with phenylace-
tic acid13 requires a reaction temperature of 180 °C.
Benzoic acid and aniline form benzanilide at 220 °C,14
Abstract: Factors affecting the thermal condensation of carboxylic
acids with amines have been investigated, and an effective protocol
for this waste-minimized, environmentally benign transformation
has been identified. Fourteen examples demonstrate the applicabil-
ity of this procedure to aliphatic, aromatic and heteroaromatic car- and oleic acid starts reacting with primary amines at tem-
peratures in excess of 230 °C,15 all in the absence of sol-
boxylic acids and primary and secondary aliphatic as well as
aromatic amines. The approach leads to the corresponding amides
vent.
in good yields.
The mechanism of this condensation is assumed to in-
volve an equilibrium of free carboxylic acid, amine, and
Key words: amides, amines, condensation, molecular sieves
ammonium salt, from which the product amide forms via
the reversal of an amide hydrolysis, driven by the removal
The synthesis of amides from carboxylic acids is certainly
of water from the mixture (Scheme 1).16
one of the most commonly used transformations in organ-
ic chemistry.1 A wealth of methods have been developed
O–
O
O
for this, which usually involve various coupling reagents.2
These reagents activate the carboxylic acid for nucleo-
philic substitution by derivatization into acid chlorides,
anhydrides, activated amides, or esters, thus replacing the
free hydroxy group by a leaving group. Driven in particu-
lar by the requirements of automated peptide synthesis,
these methods have become highly efficient. However,
they suffer from the need for a separate activation step and
from low atom economy. Alternatively, carboxylic acids
can be condensed with amines in the presence of stoichi-
ometric activators such as arylboronic acids,3 bis[bis(tri-
H2N R2
R1
+NH2
R2
+ OH– + H+
+
OH
R1
OH
R1
NH
R2
Scheme 1 Postulated mechanism for the direct amidation of car-
boxylic acids
In an ongoing synthetic project, we required a simple,
scalable, economically and ecologically viable process for
the amidation of a fatty acid. After obtaining disappoint-
ing results with various processes, we finally came across
a publication by Cossy et al., who used molecular sieves
for the condensation of carboxylic acids with amines to
give the amide products.17 We considered this to be a very
appealing approach although the reported examples in-
cluded only a very limited range of mainly short-chain
aliphatic carboxylic acids in combination with primary
amines. Thus, we decided to take a new look at this pro-
cess with the goal of making it generally applicable to a
broad range of carboxylic acid and amine substrates.
methylsilyl)amino]tin(II),4
titanium
tetrachloride,5
trimethylaluminium,6 Lawesson’s reagent,7 dimethyl
phosphoryl chloride,8 tetrazoles,9 benzoxazoles,10 and ox-
alates.11 Unfortunately, each of these reagents has their
own disadvantage, being unstable, toxic, expensive or
commercially unavailable, and requires the removal of
by-products.
A direct conversion of carboxylic acids involving nucleo-
philic substitution of the hydroxy group by an amine is
very desirable but intrinsically difficult: Under acidic con-
ditions, the hydroxy group of a carboxylic acid is proto-
nated and easily substituted via addition–elimination
reactions allowing, for example, direct esterifications. In
contrast, the acid is deprotonated by basic amines to form
a resonance-stabilized carboxylate, which impedes nu-
cleophilic attack at the carbonyl carbon.
We chose the amidation of undecenoic acid with benzyl-
amine as our model reaction in order to optimize the me-
diator and the reaction conditions (Table 1).
The original procedure using activated 4 Å molecular
sieves gave an encouraging 59% yield of N-benzyl-10-
undecenamide after pretreating the molecular sieves by
microwave heating (entry 1). Other dehydrating agents,
Lewis or Brønsted acids led to decreased yields (entries
2–4). When attempting to further increase yields by vary-
ing the pore size (3 Å, entry 5), the pretreatment (drying,
grinding) or the amount of the molecular sieves (entries
6–8), we were surprised to see only marginal differences.
A control experiment performed in the absence of drying
agents or mediators led to the surprising discovery that al-
most the same yields could be achieved when the reaction
A dehydration of the resulting ammonium carboxylates
usually requires such drastic conditions that this approach
can be applied only to particularly reactive and otherwise
unfunctionalized derivatives. For example, the pyrolysis
SYNTHESIS 2009, No. 1, pp 0160–0164
0
2
.
0
1
.2
0
0
9
Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1083277; Art ID: C06008SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
The Thermal Amidation of Carboxylic Acids Revisited
161
Table 1 Optimization of Reaction Conditionsa
Aliphatic carboxylic acids reacted smoothly with primary
and secondary aliphatic as well as aromatic amines. Even
morpholine, which had previously been reported to be
completely unreactive,17 gave a good yield when convert-
ed just below its boiling point (120 °C). The protocol is
particularly advantageous for the synthesis of fatty acid
amides, which are of considerable commercial interest as
antifriction lubricants and chemical additives, as well as
nonstick and protective coatings.19 The presence of alco-
holic or phenolic OH groups is tolerated; in all cases, the
amides rather than the esters were formed exclusively.
The reaction of benzoic acid with a secondary amine rep-
resents the current performance limit of the transforma-
tion: in this case only traces of product were detected.
O
O
MS
H
N
H
Ph
N
H
Ph
+
OH
8
8
1a
2a
3aa
Entry
MS size
(Å)b
MS
Additive
Temp
Yield
(%)c
(mg)
100
100
100
100
100
200
400
60
(°C)
120
120
120
120
120
120
120
120
120
160
160
1
2
4
4
4
4
3
4
4
4
–
4
–
–
59
39
14
45
48
58
52
49
49
89
95
MgSO4
3
Yb(OTf)3
4
H2SO4
The workup of the reaction mixtures is simple, and con-
sists of dilution with methanol and filtration. After remov-
al of the volatile components, the products are in many
cases obtained in high purity. Otherwise, they could easily
be purified by recrystallization or column chromatogra-
phy.
5
–
–
–
–
–
–
–
6
7
8
9
–
Table 2 Substrate Scope of the Direct Amidationa
10
11
100
–
O
O
3 Å MS
neat
H
R3
R3
N
R2
+
R1
N
R1
OH
– H2O
R2
a Reaction conditions: 10-undecenoic acid (1a; 2 mmol), benzylamine
(2a; 2 mmol), additive (0.2 equiv), 2 h.
1
2
3
b Activated molecular sieves (MS).
Product
Time (h) Temp (°C) Yield (%)b
c Determined by GC using n-tetradecane as internal standard.
O
N
H
2
2
7
160
160
160
92
99
85
8
water was simply allowed to evaporate (entry 9). After in-
creasing the temperature to 160 °C, quantitative conver-
sion was achieved after two hours both with and without
molecular sieves present (entries 10 and 11).
3aa
3ba
O
Intrigued by this finding, we searched for literature evi-
dence that unmediated thermal amidations can be per-
formed even at such relatively low temperatures. Indeed,
Jursic et al. reported thermal amidations to proceed at
160–180 °C and successfully applied this method to the
synthesis of a small selection of amides.18 Jursic’s find-
ings confirm that the molecular sieves do not mediate the
actual amidation step but merely trap the released reaction
water. We still found the use of pelleted 3 Å molecular
sieves to be beneficial from a practical point of view, as
this way closed vessels can be used, and these prevent the
evaporation of volatile amines. By trapping the reaction
water, the molecular sieves help to reduce pressure build-
up at temperatures in excess of 100 °C.
N
7
7
H
O
N
H
3ca
3da
3ea
O
24
24
7
160
160
160
96
75
85
N
H
O
In order to investigate the scope of such thermal amida-
tion reactions in more detail, we tested the scope of our
optimized protocol by applying it to various combinations
of amines and carboxylic acids (Table 2). We were
pleased to find that the new protocol allows extending the
method from aliphatic and olefinic carboxylic acids to the
less reactive aromatic and even heteroaromatic carboxylic
acids. Their reaction with benzylamine gave the corre-
sponding amides in mostly high yields.
N
H
O
N
H
S
3fa
Synthesis 2009, No. 1, 160–164 © Thieme Stuttgart · New York

162
L. J. Gooßen et al.
PAPER
Table 2 Substrate Scope of the Direct Amidationa (continued)
All solvents and reagents were purified by standard techniques.
Crude products were purified by column chromatography on silica
gel or by recrystallization from EtOH or CHCl3/hexane. IR spectra
O
O
3 Å MS
neat
H
R3
R3
N
R2
1
were recorded on a Perkin–Elmer FT IR spectrometer. H and 13C
+
R1
N
R1
OH
– H2O
NMR spectra were recorded at r.t. in CDCl3 or CD3OD solvent on
Bruker DPX 400 or Bruker Avance 600 NMR spectrometers.
Chemical shifts (d) are reported in ppm relative to solvent signals
(d = 7.25 and 77.0 ppm for CDCl3 or 3.30/4.78 and 49.0 ppm for
CD3OD). Coupling constants (J) are quoted in Hz. Mass spectra
were obtained on a Varian Saturn 2100 T mass spectrometer oper-
ating at 70 eV using EI ionization. Melting points were determined
on a Büchi 520 melting point apparatus and are uncorrected.
R2
1
2
3
Product
Time (h) Temp (°C) Yield (%)b
O
O
N
H
24
7
160
160
160
75
55
96
N
3 Å molecular sieves (MS) were pre-treated in a household micro-
wave for 5 min at 700 W in an open beaker. It is advisable to start
with pre-dried sieves and to continually monitor the microwave pro-
cess because tightly packed, wet sieves may dramatically overheat,
leading to a meltdown of the material. In a flame-dried vessel, they
were additionally heated in vacuo with a heat gun and cooled to r.t.
before use.
3ga
N
H
O
3ha
O
N-Benzyloleamide (3ba);21 Typical Amidation Procedure
A mixture of oleic acid (1b; 5.65 g, 20.0 mmol) and N-benzylamine
(2a; 2.14 g, 20.0 mmol) was placed in a flame-dried 20 mL head-
space vial, highly activated MS (3 Å, 1.80 g) were added in one por-
tion and the mixture was heated at 160 °C. After completion of the
reaction (2 h, monitored by TLC), the mixture was cooled, diluted
with MeOH (150 mL), filtered through a thin pad of Celite, washed
with MeOH (150 mL) and concentrated in vacuo to afford the
amide 3ba.
N
12
7
7
Me
3bb
O
O
OH
12
24
160
160
92
75
N
H
7
7
7
3bc
Yield: 7.41 g (99%); white solid; mp 58–59 °C; Rf = 0.8 (silica gel;
EtOAc–hexane, 1:1).
IR (KBr): 3298, 1640, 1553 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.22–7.33 (m, 5 H), 6.32 (s, 1 H),
5.35 (s, 2 H), 4.38 (d, J = 5.7 Hz, 2 H), 2.18 (t, J = 7.6 Hz, 2 H),
2.02 (m, 4 H), 1.62 (m, 2 H), 1.29 (m, 20 H), 0.89 (t, J = 6.4 Hz,
3 H).
13C NMR (100 MHz, CDCl3): d = 172.9, 138.5, 129.9, 129.6, 128.5,
127.6, 127.2, 43.4, 36.6, 31.8, 29.7, 29.6, 29.4, 29.2, 29.0, 27.1,
25.7, 22.5, 13.9.
N
H
OH
7
3bd
O
7
120
73
N
O
3ce
MS (EI, 70 eV): m/z (%) = 371 (48) [M+], 216 (50), 162 (75), 149
(100), 91 (86).
O
O
24
24
160
160
49
N
H
N-Benzyl-10-undecenamide (3aa)22
Synthesized from 1a (369 mg, 2.00 mmol) and N-benzylamine (2a;
214 mg, 2.00 mmol) following the typical procedure (160 °C, 2 h)
and purified by recrystallization (hexane).
3ef
N
traces
Yield: 503 mg (92%); white solid.
Me
1H NMR (400 MHz, CD3OD): d = 7.21–7.32 (m, 5 H), 5.74–5.85
(m, 1 H), 4.90–5.01 (m, 2 H), 4.34 (s, 2 H), 2.22 (t, J = 7.5 Hz, 2 H),
2.03 (q, J = 6.9 Hz, 2 H), 1.57–1.66 (m, 2 H), 1.33–1.41 (m, 2 H),
1.30 (s, 8 H).
3eb
a Reaction conditions: carboxylic acid 1 (2 mmol), amine 2 (2 mmol),
activated MS (3 Å, 100 mg).
b Isolated yields.
N-Benzyl-N-methyloleamide (3bb)23
Synthesized from 1b (1.41 g, 5.00 mmol) and N-methylbenzyl-
amine (2b; 606 mg, 5.00 mmol) following the typical procedure
(160 °C, 12 h) and purified by column chromatography (Et2O–hex-
ane, 1:3). Spectroscopic data of this amide were obtained as a mix-
ture of two rotational isomers consistent with literature.
Overall, the results show that the direct condensation of
carboxylic acids and amines to give the corresponding
amides is of higher synthetic applicability than commonly
believed. This environmentally friendly method can be
applied to the synthesis of a wide range of amides that are
currently produced via thionyl chloride based protocols.20
Yield: 1.89 g (96%); light-yellow liquid.
1H NMR (400 MHz, CDCl3): d = 7.27–7.35 (m, 2 H), 7.21–7.25 (m,
2 H), 7.15 (d, J = 7.5 Hz, 1 H), 5.34 (d, J = 4.8 Hz, 2 H), 4.58 (s,
1 H), 4.52 (s, 1 H), 2.88–2.94 (m, 3 H), 2.36 (t, J = 7.5 Hz, 2 H),
Synthesis 2009, No. 1, 160–164 © Thieme Stuttgart · New York

PAPER
The Thermal Amidation of Carboxylic Acids Revisited
163
2.01 (s, 4 H), 1.68 (d, J = 6.8 Hz, 2 H), 1.33 (s, 8 H), 1.27 (s, 13 H),
0.88 (t, J = 6.3 Hz, 3 H).
Yield: 553 mg (85%); white solid; mp 92–93 °C; Rf = 0.1 (Et2O–
hexane, 1:1).
1H NMR (600 MHz, CDCl3): d = 7.86–7.88 (m, 1 H), 7.37–7.40 (m,
1 H), 7.27–7.34 (m, 6 H), 6.45 (s, 1 H), 4.58 (d, J = 5.9 Hz, 2 H).
13C NMR (150 MHz, CDCl3): d = 162.9, 138.1, 137.2, 128.7, 128.4,
127.9, 127.5, 126.5, 126.0, 43.8.
MS (EI, 70 eV): m/z (%) = 217 (99) [M+], 184 (100), 156 (10), 111
(98), 83 (20), 77 (27), 51 (16).
N-(2-Hydroxyethyl)oleamide (3bc)24
Synthesized from 1b (1.41 g, 5.00 mmol) and 2-aminoethanol (2c;
305 mg, 5.00 mmol) following the typical procedure (160 °C,
12 h).
Yield: 1.57 g (92%); colorless solid.
1H NMR (400 MHz, CDCl3): d = 6.34 (s, 1 H), 5.31 (ddd, J = 5.5,
3.7, 3.4 Hz, 2 H), 3.56–3.68 (m, 3 H), 3.33–3.40 (m, 2 H), 2.11–
2.20 (m, 2 H), 1.92–2.02 (m, 4 H), 1.53–1.63 (m, 2 H), 1.25 (m,
20 H), 0.81–0.88 (m, 3 H).
Anal. Calcd for C12H11NOS: C, 66.33; H, 5.10; N, 6.45. Found: C,
66.31; H, 5.05; N, 6.66.
N-Benzylpicolinamide (3ga)29
N-(3-Hydroxyphenyl)oleamide (3bd)25
Synthesized from 1b (1.41 g, 5.00 mmol) and 3-aminophenol (2d;
546 mg, 5.00 mmol) following the typical procedure (160 °C, 24 h)
and purified by column chromatography (Et2O–hexane, 1:2).
Synthesized from picolinic acid (1g; 369 mg, 3.00 mmol) and N-
benzylamine (2a; 321 mg, 3.00 mmol) following the typical proce-
dure (160 °C, 24 h) and purified by column chromatography
(EtOAc–hexane, 1:1).
Yield: 477 mg (75%); white solid.
Yield: 1.41 g (75%); colorless solid; mp 93–94 °C.
1H NMR (600 MHz, CDCl3): d = 8.51 (d, J = 4.1 Hz, 1 H), 8.38 (s,
1 H), 8.23 (d, J = 7.9 Hz, 1 H), 7.83 (td, J = 7.7, 1.7 Hz, 1 H), 7.40
(ddd, J = 7.6, 4.7, 1.3 Hz, 1 H), 7.31–7.37 (m, 4 H), 7.27 (t,
J = 7.0 Hz, 1 H), 4.66 (d, J = 5.9 Hz, 2 H).
1H NMR (400 MHz, CDCl3): d = 7.95 (t, J = 2.0 Hz, 1 H), 7.25 (s,
1 H), 7.14 (t, J = 8.5 Hz, 1 H), 6.63–6.66 (m, 1 H), 6.49–6.52 (m,
1 H), 5.29–5.38 (m, 2 H), 2.37 (t, J = 7.5 Hz, 2 H), 1.96–2.05 (m,
5 H), 1.69–1.77 (m, 2 H), 1.24–1.35 (m, 20 H), 0.87 (t, J = 6.5 Hz,
3 H).
13C NMR (100 MHz, CDCl3): d = 172.6, 157.7, 138.6, 130.0, 129.7,
129.6, 111.9, 107.5, 38.1, 31.9, 29.8, 29.3, 29.2, 29.1, 27.2, 25.8,
22.7, 14.2.
N-Benzylfuran-3-carboxamide (3ha)30
Synthesized from furan-3-carboxylic acid (1h; 336 mg, 3.00 mmol)
and N-benzylamine (2a; 321 mg, 3.00 mmol) following the typical
procedure (160 °C, 24 h) and purified by column chromatography
(EtOAc–hexane, 3:1).
N-Benzyl-2-phenylacetamide (3ca)26
Yield: 331 mg (55%); light-yellow solid.
Synthesized from phenylacetic acid (1c; 408 mg, 3.00 mmol) and
N-benzylamine (2a; 321 mg, 3.00 mmol) following the typical pro-
cedure (160 °C, 7 h) and purified by column chromatography
(Et2O–hexane, 9:1).
1H NMR (600 MHz, CDCl3): d = 7.94 (s, 1 H), 7.41 (dt, J = 16.1,
1.7 Hz, 1 H), 7.27–7.33 (m, 5 H), 6.62 (d, J = 1.0 Hz, 1 H), 6.38 (s,
1 H), 4.55 (d, J = 5.9 Hz, 2 H).
Yield: 574 mg (85%); white solid.
4-(Phenylacetyl)morpholine (3ce)31
1H NMR (600 MHz, CDCl3): d = 7.31–7.34 (m, 2 H), 7.21–7.29 (m,
6 H), 7.14–7.17 (m, 2 H), 5.80 (s, 1 H), 4.38 (d, J = 5.9 Hz, 2 H),
3.59 (s, 2 H).
Synthesized from phenylacetic acid (1c; 408 mg, 3.00 mmol) and
morpholine (2e; 261 mg, 3.00 mmol) following the typical proce-
dure (120 °C, 7 h) and purified by column chromatography
(EtOAc–hexane, 3:1).
(E)-N-Benzyl Cinnamic Amide (3da)27
Synthesized from cinnamic acid (1d; 444 mg, 3.00 mmol) and N-
benzylamine (2a; 321 mg, 3.00 mmol) following the typical proce-
dure (160 °C, 24 h) and recrystallized (CHCl3–hexane).
Yield: 452 mg (73%); light-yellow solid.
1H NMR (600 MHz, CDCl3): d = 7.31 (t, J = 7.5 Hz, 2 H), 7.21–
7.25 (m, 3 H), 3.72 (s, 2 H), 3.63 (s, 4 H), 3.40–3.47 (m, 4 H).
Yield: 685 mg (96%); light-yellow solid.
N-Phenylbenzamide (3ef)32
1H NMR (600 MHz, CDCl3): d = 8.64 (t, J = 5.8 Hz, 1 H), 7.55–
7.61 (m, 2 H), 7.47 (d, J = 15.9 Hz, 1 H), 7.35–7.42 (m, 3 H), 7.28–
7.34 (m, 3 H), 7.24 (t, J = 7.2 Hz, 1 H), 6.70 (d, J = 15.9 Hz, 1 H),
4.40 (d, J = 5.9 Hz, 2 H), 2.49 (d, J = 3.3 Hz, 1 H).
Synthesized from benzoic acid (1e; 366 mg, 3.00 mmol) and aniline
(2f; 279 mg, 3.00 mmol) following the typical procedure (160 °C,
24 h) and purified by column chromatography (EtOAc–hexane,
1:2) and recrystallization (CHCl3–hexane).
N-Benzylbenzamide (3ea)28
Yield: 287 mg (49%); colorless solid.
1H NMR (600 MHz, CDCl3): d = 7.83–7.88 (m, 3 H), 7.64 (d,
J = 7.7 Hz, 2 H), 7.54 (t, J = 7.4 Hz, 1 H), 7.48 (t, J = 7.6 Hz, 2 H),
7.36 (t, J = 8.1 Hz, 2 H), 7.15 (t, J = 7.4 Hz, 1 H).
Synthesized from benzoic acid (1e; 374 mg, 3.00 mmol) and N-ben-
zylamine (2a; 321 mg, 3.00 mmol) following the typical procedure
(160 °C, 24 h) and purified by column chromatography (EtOAc–
hexane, 2:3).
Yield: 478 mg (75%); white solid.
Acknowledgment
1H NMR (600 MHz, CDCl3): d = 7.79 (d, J = 7.4 Hz, 2 H), 7.44–
7.50 (m, 1 H), 7.39–7.42 (m, 2 H), 7.32–7.35 (m, 4 H), 7.27–7.30
(m, 1 H), 6.60 (s, 1 H), 4.62 (d, J = 5.6 Hz, 2 H).
We gratefully acknowledge the financial support by Cognis GmbH
and NanoKat and thank Felix Rudolphi for helpful discussions and
technical assistance.
N-Benzylthiophene-3-carboxamide (3fa)
Synthesized from thiophene-3-carboxylic acid (1f; 384 mg,
3.00 mmol) and N-benzylamine (2a; 321 mg, 3.00 mmol) following
the typical procedure (160 °C, 7 h) and purified by column chroma-
tography (EtOAc–hexane, 2:3).
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Synthesis 2009, No. 1, 160–164 © Thieme Stuttgart · New York

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L. J. Gooßen et al.
PAPER
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