Potassium tert-Butoxide-Mediated Amine Acyl Exchange Reaction of N,N-Disubstituted Formamides with Aromatic Carbonyl Derivatives via Sequential C-N Bond Cleavage/Formation: An Approach to Aromatic Amides

Article abstract of DOI:10.1002/adsc.201500551

A novel potassium tert-butoxide-mediated amine acyl exchange of N,N-disubstituted formamides with aromatic carbonyl derivatives in a sequential C-N bond cleavage/formation process leading to aromatic amides is described. This methodology tolerates a wide range of aromatic carbonyl compounds, including aromatic aldehydes, acyl chlorides, unactivated esters, and acid anhydrides. The usage of inexpensive and readily available reagents, broad substrate scope, and the simple, mild (50°C) and transition metal-free conditions make this protocol very practical. In addition, a plausible reaction mechanism is proposed on the basis of experimental observations.

Full text of DOI:10.1002/adsc.201500551

FULL PAPERS
DOI: 10.1002/adsc.201500551
Potassium tert-Butoxide-Mediated Amine Acyl Exchange
Reaction of N,N-Disubstituted Formamides with Aromatic
À
Carbonyl Derivatives via Sequential C N Bond
Cleavage/Formation: an Approach to Aromatic Amides
a
a
a,b
a,
Ming-Zhong Zhang, Qing-Hu Guo, Wen-Bing Sheng, and Can-Cheng Guo *
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, Advanced Catalytic Engineering Research Center of the
Ministry of Education, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s
Republic of China
E-mail: ccguo@hnu.edu.cn
b
College of Pharmacy, Hunan University of Chinese Medicine, Changsha 410208, People’s Republic of China
Received: June 7, 2015; Revised: July 22, 2015; Published online: September 11, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500551.
Abstract: A novel potassium tert-butoxide-mediated strate scope, and the simple, mild (508C) and transi-
amine acyl exchange of N,N-disubstituted forma- tion metal-free conditions make this protocol very
mides with aromatic carbonyl derivatives in a sequen- practical. In addition, a plausible reaction mecha-
tial CÀN bond cleavage/formation process leading to nism is proposed on the basis of experimental obser-
aromatic amides is described. This methodology tol- vations.
erates a wide range of aromatic carbonyl compounds,
including aromatic aldehydes, acyl chlorides, unacti-
vated esters, and acid anhydrides. The usage of inex- Keywords: aerobic oxidation; amides; nucleophilic
pensive and readily available reagents, broad sub- addition; synthetic methods
Introduction
these amine acyl exchange reactions remained limited
to carboxylic anhydrides, acyl chlorides, and carboxyl-
The formation of the amide bond is an important pro- ic acids. Herein, we disclose the discovery of the
[
1]
cess in organic transformations. Amine acyl ex- formal amine acyl exchange of N,N-disubstituted for-
change is a rather challenging version of this process, mamides with aromatic carbonyl derivatives (includ-
where an amide CÀN bond from one of the reacting ing aldehydes, acyl chlorides, unactivated esters, and
partners is selectively cleaved, accompanied by for- acid anhydrides) toward aromatic amides mediated
mation of a more stable amide CÀN bond. Compared by KO-t-Bu via sequential CÀN bond cleavage and
to traditional amide bond forming reactions, amine formation under transition metal-free conditions
acyl exchange has potential applications on the cut- (Scheme 1).
ting edge of organic chemistry, biomaterials science,
and bio-medical engineering. Despite the impor- molecules, natural products, pharmaceuticals, poly-
tance of research in the field of amine acyl exchange, mers, and raw materials such as lubricants, detergents,
however, generally applicable and efficient ap- and engineering plastics. Moreover, primary amides
proaches are quite rare. Following the discovery of can be easily applied in various organic transforma-
chemoselective approach for the conversion of secon- tions, including the formation of primary amines, ni-
Amides are widely prevalent in biologically active
[
2]
[6]
[7]
dary amides to perfluorinated amides using perfluori- triles, and heterocycles. Because of the prominent
[
3]
nated anhydrides in 2010 by Rota, Flitsch described role of amides in biology, medicine, materials science,
a protease-catalyzed amine acyl exchange of amides and synthetic chemistry, numerous methodologies
with carboxylic acids in which an aqueous buffer is have been developed for their synthesis over the past
[4]
used for the in situ generation of free amines. Until few decades. Traditionally, amides are mainly pre-
[5]
now, the progress in this field is still very limited:
pared by coupling reactions between carboxylic acids
[8]
almost all of the carbonyl derivatives (acyl donors) of and amines as well as between aryl halides and N,N-
Adv. Synth. Catal. 2015, 357, 2855 – 2861
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Ming-Zhong Zhang et al.
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dant. Furthermore, from a synthetic perspective, the
simple, mild (508C), and transition metal-free condi-
tions make this protocol very practical.
Results and Discussion
Initially, the amine acyl exchange reaction of N,N-di-
methylformamide (1a) with benzaldehyde (2a) was
chosen as the model to optimize the reaction parame-
ters, including the base and the solvent. As shown in
Scheme 1. Amine acyl exchange of N,N-disubstituted forma-
mides with aromatic carbonyl derivatives leading to aromat- Table 1, this reaction proceeded by a sequential two-
ic amides.
step operation. For the first step, 1a was subjected to
a base in an anhydrous solvent at room temperature
Nevertheless, most of for 15 min. In the second step, a mixture of 2a and an-
[
9]
disubstituted formamides.
these methods rely on prior activation of carboxylic hydrous solvent was slowly added dropwise after the
acids or amines using hazardous reagents; besides, the solution was heated to 508C. The amounts of 1a and
presence of halide anions makes these reactions envi- base (KO-t-Bu) were tested using anhydrous THF as
ronmentally unfavorable. Alternatively, transition the solvent in the first step, and a combination of
[
10]
metal-catalyzed CÀC bond cleavage and oxidative 3.0 equiv. of 1a and KO-t-Bu (relative to the amount
[
11]
coupling reactions have emerged as powerful ap- of 2a) gave a positive effect (entries 1 and 2). The
proaches for the synthesis of amides owing to their yield of isolated N,N-dimethylbenzamide (3a) was
potential advantages, such as high efficiency and se- dramatically improved to 70% after prolonging the
lectivity. Other elegant methods include the Stauding- reaction time to 12 h (entry 3). Encouraged by these
[
12]
[13]
er reaction, the Schmidt reaction, the Beckmann results, various anhydrous solvents, such as THF/t-
[
14]
[15]
rearrangement, transamidation of carboxamides,
carbonylation of alkenes or alkynes,
thioacids or nitriles, and oxidative amidation of al-
dehydes. Despite these significant advances, there is Table 1. Optimization of the reaction conditions.
still an urgent need for chemists to find more efficient
and greener methodologies for accessing amides both
from academic and industrial viewpoints.
BuOH, toluene/t-BuOH, toluene, and DMSO were
[
16]
amidation of
[
17]
[
18]
[a]
N,N-Disubstituted formamides are easy-to-handle,
inexpensive, and commercially available, rendering
them ideal as amination, acylation, and formylation
[
19,20]
[b]
Entry
Base (equiv.)
Solvent
Yield [%]
reagents in organic reactions.
Under this concept,
developed a metal-free cross-coupling
reaction between aldehydes and N,N-disubstituted
[
21]
Wan et al.
[c]
1
2
KO-t-Bu (1.5)
KO-t-Bu (3.0)
KO-t-Bu (3.0)
KO-t-Bu (3.0)
KO-t-Bu (3.0)
KO-t-Bu (3.0)
KO-t-Bu (3.0)
KOH (3.0)
NaOH (3.0)
MeONa (3.0)
EtONa (3.0)
NaH (3.0)
THF
THF
THF
THF/t-BuOH
toluene/t-BuOH
toluene
DMSO
THF
THF
THF
THF
THF
43
56
70
30
52
49
0
<10
0
trace
32
20
[d]
formamides to amides by using a Bu NI/TBHP cata-
4
3
[
[
e]
f]
lytic oxidation system. Subsequently, the groups of Li
and Wang independently achieved the three-compo-
nent reaction of phenylacetonitriles, sulfur and N,N-
dimethylformamide and the oxidative coupling reac-
tion of benzylamines with N,N-disubstituted forma-
mides for the preparation of N,N-dimethylthio-
4
5
6
7
8
9
1
1
1
0
1
2
[
22]
[23]
amides
and amides,
respectively. Inspired by
these exciting advances and our interest in developing
a novel and general approach toward amides, we
were especially interested in amine acyl exchange re-
action. To the best of our knowledge, this work dem-
onstrates the first construction of aromatic amides
from N,N-disubstituted formamides and aromatic car-
bonyl derivatives in an amine acyl exchange fashion
mediated by KO-t-Bu. The current methodology is
characterized by its good functional group compatibil-
ity, broad substrate scope, and the use of inexpensive
and easily accessible reagents and air as the green oxi-
[
a]
Reaction conditions: For the first step, 1a (3.0 equiv.),
base (x equiv.), anhydrous solvent (2.0 mL), room tem-
perature, 15 min. For the second step, 2a (1.0 mmol), an-
hydrous solvent (1.0 mL), 508C, 12 h, Schlenk tube under
air.
[b]
[c]
Isolated yields based on 2a.
1
a (1.5 equiv.) and 508C, 3 h for the second step.
[
[
[
d]
e]
f]
508C, 3 h for the second step.
THF/t-BuOH (v/v, 1:1).
Toluene/t-BuOH (v/v, 1:1).
2
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Potassium tert-Butoxide-Mediated Amine Acyl Exchange Reaction
Table 2. Amine acyl exchange: scope of the aromatic alde- also productive reaction partners in this transforma-
[
a,b]
hydes.
tion (3f, 3g). To our delight, electron-donating sub-
stituents, such as Me, MeO and NMe groups on the
2
aryl ring were tolerated well, affording the corre-
sponding products in good to high yields (65–82%,
3
h–3k). Interestingly, naphthaldehydes also under-
went amine acyl exchange with 1a to give the corre-
sponding amides 3l and 3m in 66% and 52% yields,
respectively. As expected, 3-phenylpropanal, an ali-
phatic aldehyde, gave a complex mixture and the cor-
responding product was not obtained.
Subsequently, the scope of formamides was investi-
gated under the standard reaction conditions
(
Table 3). Screening revealed that a range of forma-
mides, including acyclic and cyclic formamides, were
suitable for the amine acyl exchange reaction. For ex-
ample, the monoalkyl-substituted formamides which
are usually sensitive to strong oxidation conditions, as
exemplified by N-methylformamide, could also serve
as suitable reaction partners in this protocol and gave
the desired product in 39% yield (4a). An acyclic
N,N-disubstituted formamide, with the example of
N,N-diethylformamide, was also compatible with the
reaction conditions, exhibiting relatively high reactivi-
ty (75%, 4b). Interestingly, sterically bulky acyclic for-
mamides such as N-methyl-N-phenylformamide react-
ed with 2a to afford the desired product in a relatively
lower yield (25%, 4c), whereas N,N-diphenylform-
amide gave no reactivity (0%, 4d). It was suspected
[
a]
Reaction conditions: For the first step, 1a (3.0 equiv.),
KO-t-Bu (3.0 equiv.), anhydrous THF (2.0 mL), room
temperature, 15 min. For the second step, 2 (1.0 mmol),
anhydrous THF (1.0 mL), 508C, 12 h, Schlenk tube
under air.
Table 3. Amine acyl exchange: scope of the N,N-disubstitut-
[
a,b]
[
[
b]
c]
ed formamides.
Isolated yields based on 2.
N.O. =not obtained.
tested: but they were less efficient than THF (entry 3
vs. entries 4–7). Screening a range of bases revealed
that KO-t-Bu performed best among other bases in-
cluding KOH, NaOH, MeONa, EtONa and NaH (en-
tries 8–12). In addition, when the amine acyl exchange
of 1a with 2a was performed under an N atmosphere,
2
only 42% of 3a was isolated, highlighting the impor-
tance of air for this transformation. Moreover, when
reactions were performed in new, acid-washed
Schlenk tubes, similar yields were also obtained,
thereby excluding the influence of traces of transition
metals (from the old flasks) on this conversion.
With the optimized protocol in hand, the scope of
the reaction with a variety of aldehydes was investi-
gated (Table 2). We found that aldehydes with both
electron-deficient and electron-rich substituents on
the aryl ring can be smoothly converted into the de-
sired products. The arylaldehydes with halo-substitu-
ents (F, Cl, and Br) were tolerated under the opti-
mized conditions leading to the corresponding amides
in moderate to good yields (51–67%, 3b–3e). Impor-
[a]
Reaction conditions: For the first step, 1 (3.0 equiv.), KO-
t-Bu (3.0 equiv.), anhydrous THF (2.0 mL), room temper-
ature, 15 min. For the second step, 2a (1.0 mmol), anhy-
drous THF (1.0 mL), 508C, 12 h, Schlenk tube under air.
Isolated yields based on 2a.
[
b]
tantly, p-NO and p-CN substituted aldehydes were
2
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Table 4. Amine acyl exchange: scope of the aromatic car- benzoic anhydride (5i) and 4-methylbenzoic anhy-
[a]
boxylic acid derivatives.
dride (5j) were also compatible substrates for this
transformation, and all reactions provided the desired
amides in good yields (entries 9 and 10).
To understand the mechanism of the current proto-
col, some information has been gathered. First, the re-
action proceeded well in the presence of 2.0 equiv. of
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, a radi-
cal-inhibiting reagent), which may exclude a radical
process in this transformation [Scheme 2, Eq. (a)].
1
3
Second, a C-isotope labeling experiment suggested
that the carbonyl group of 3a comes from 2a rather
than 1a [Scheme 2, Eq. (b)]. It is well established that
1
a decarbonylates under heating to produce dimethyl-
amine and carbon monoxide in the presence of
[24]
base. Hence we envisioned that our reaction would
involve free dimethylamine that is generated in situ
from 1a. To further confirm this, commercially avail-
[
[
a]
b]
Reaction conditions: For the first step, 1 (3.0 equiv.), KO-
t-Bu (3.0 equiv.), anhydrous THF (2.0 mL), room temper-
ature, 15 min. For the second step, 5 (1.0 mmol), anhy-
drous THF (1.0 mL), 508C, 12 h, Schlenk tube under air.
Isolated yields based on 5.
that the decreased reaction efficiency could be due to
its increased steric hindrance. Gratifyingly, a series of
cyclic formamides readily undergo the amine acyl ex-
change with 2a, delivering the corresponding products
in good to high yields (64–80%, 4e–4h). In addition,
other amides were also investigated (for details, see
the Supporting Information). It was shown that this
KO-t-Bu-mediated protocol can tolerate a range of
formamides and represents a practical and low-cost
approach for the preparation of aromatic amides.
Encouraged by the aforementioned amine acyl ex-
change reaction of N,N-disubstituted formamides with
aromatic aldehydes, we next turned our attention
toward expanding the substrate scope (acyl donors) to
aromatic carboxylic acid derivatives. Much to our sur-
prise, a range of aromatic carboxylic acid derivatives,
including acyl chlorides, unactivated esters, and acid
anhydrides, were compatible reaction partners in this
KO-t-Bu-mediated protocol (Table 4). Amine acyl ex-
change of 1a with benzoyl chlorides 5a and 5b gave
the corresponding products 3a and 3h in 84% and
88% yield, respectively (entries 1 and 2). Gratifyingly,
a variety of unactivated esters, including methyl ben-
zoate (5c), ethyl benzoate (5d), tert-butyl benzoate
(
(
5e), benzyl benzoate (5f), methyl 4-methoxybenzoate
5g) and methyl 4-chlorobenzoate (5h) were well tol-
erated in these amine acyl exchange reactions, deliv-
ering the corresponding amide products in 55–83%
yields (entries 3–8). Similarly, acid anhydrides such as Scheme 2. Control experiments.
2
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Potassium tert-Butoxide-Mediated Amine Acyl Exchange Reaction
able piperidine (6) was employed to react with 2a formamide 1a and benzaldehyde 2a as the model). In-
under the optimized reaction conditions, and a moder- itially, dimethylamine A is generated from thermal
ate yield (57%) of 4e was still obtained [Scheme 2, decarbonylation of 1a with the release of carbon mon-
[24]
Eq. (c)]. As a result, this protocol is likely to proceed oxide in the presence of base. Subsequently, nucleo-
through the slow generation of free amine from the philic addition of A to 2a would lead to an unstable
[18,25]
N,N-disubstituted formamide and thus minimizes the tetrahedral intermediate (hemiaminal) B,
which
undesired side reactions. According to the literature, might undergo deprotonation to form a tetrahedral
[26]
the reaction of an amine with an aldehyde resulted in intermediate C.
Through
a
well-established
a
hemiaminal intermediate that is oxidized to Cannizzaro-fashion hydride ion transfer (from C to
[
18]
[27a]
amide. Accordingly, we conducted a series of verifi- 2a),
this unstable species C would convert to the
cation experiments in which KO-t-Bu was found to corresponding 3a along with the formation of potassi-
[27b]
play a crucial role for the formation of the final prod- um alkoxide D.
Since the reaction provided a good
uct, implying that an aerobic hemiaminal oxidation yield (70%) of product 3a, an argument could be
involvement in the reaction process seems unlikely made that alkoxide D could be further oxidized to 2a
[
Scheme 2, Eq. (d)].
by air (O ) under the assistance of base, thus being in-
2
To further probe the mechanism, we also tried to volved in the next reaction cycle. The validity of this
capture more information by GC-MS analysis. More proposal can be supported by the reaction results of
importantly, after the reaction, 6% of benzyl alcohol 1a with 7 [Scheme 2, Eq. (e)]. To the best of our
(
7) was detected by GC and GC-MS (for details, see knowledge, this base-facilitated aerobic alcohol oxida-
[28a]
the Supporting Information). It was found that a 31% tion process had also been proposed by Xu et al.
yield of 3a was also obtained when 7 was subjected to and Zhao et al.
[28b]
in their transition metal-free N-al-
the standard reaction conditions. From this surprising kylation reactions.
result, anaerobic (N ) and base-free control experi-
2
ments were performed: no traces of 3a were detected,
implying that a base assisted oxidation process from Conclusions
an alcohol to an aldehyde is likely involved in the re-
action mechanism [Scheme 2, Eq. (e)].
In conclusion, a novel KO-t-Bu-mediated amine acyl
On the basis of these preliminary results and previ- exchange of N,N-disubstituted formamides with aro-
ous related reports, a tentative mechanism for this matic carbonyl derivatives has been developed. Such
KO-t-Bu-mediated amine acyl exchange is proposed a simple and green protocol, which utilizes transition
and shown in Figure 1 (by employing N,N-dimethyl- metal-free and mild (508C) reaction conditions, cheap
and readily available reagents, and air as the environ-
mentally benign oxidant, provides a general and prac-
tical approach to aromatic amides. This base-mediat-
ed protocol is achieved via a sequential CÀN bond
cleavage and formation process. Preliminary mecha-
nistic studies suggest that KO-t-Bu plays multiple
roles in this transformation, which is mechanistically
distinct from the traditional approaches. Furthermore,
in the presence of KO-t-Bu, a series of aromatic car-
boxylic acid derivatives reacted satisfactorily to deliv-
er the corresponding amide products, which renders it
a powerful selectable approach for accessing aromatic
amides. Most importantly, this new and expedient
protocol can also serve as a useful complement to the
classical amine acyl exchange reaction and should po-
tentially be of wide interest and broad utility in many
fields. Further studies to clearly understand the reac-
tion mechanism and relevant reactions are currently
underway.
Figure 1. Possible mechanism.
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Experimental Section
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General Procedure for Potassium tert-Butoxide-
Mediated Amine Acyl Exchange Reaction of N,N-
Disubstituted Formamides with Aromatic Carbonyl
Derivatives
[
50, 12595; b) Y. Kuninobu, T. Uesugi, A. Kawata, K.
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A 50-mL Schlenk tube was charged with 1 (3.0 mmol), KO-
t-Bu (3.0 mmol), and anhydrous THF (2.0 mL). The solution
was stirred at room temperature for 15 min and was then
heated to 508C, a mixture of 2 (1.0 mmol) and anhydrous
THF (1.0 mL) was slowly added dropwise, after which the
solution was further stirred at 508C under an air atmosphere
for 12 h. After being cooled to room temperature, 10 mL of
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H O were added and the mixture was extracted with CH Cl
2
2
2
(3”15 mL). The combined organic layers were dried over
287, 2007.
Na SO , filtered, and concentrated under vacuum. The
2
4
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Acknowledgements
Financial support from the National Science Foundation of
China (No. 21372068) is gratefully acknowledged.
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