Copper-catalyzed oxidative coupling of carboxylic acids with N,N-dialkylformamides: An approach to the synthesis of amides

Article abstract of DOI:10.1002/ejoc.201201544

A new synthetic approach for amide bond formation through the oxidative coupling of N,N-dialkylformamides with carboxylic acids was achieved by using a copper catalyst. Furthermore, this method was applied in the coupling of chiral amino acids in which the stereochemistry was retained in the resulting amide products. A new synthetic approach to amide bond formation through the oxidative coupling of N,N-dialkylformamides with carboxylic acids was achieved by using a copper catalyst and aqueous tert-butyl hydroperoxide (TBHP) as a sacrificial oxidant. Furthermore, this method was applied in the coupling of chiral amino acids in which the stereochemistry was retained in the resulting amide products. Copyright

Full text of DOI:10.1002/ejoc.201201544

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201544
Copper-Catalyzed Oxidative Coupling of Carboxylic Acids with
N,N-Dialkylformamides: An Approach to the Synthesis of Amides
P. Santhosh Kumar,[a] G. Sathish Kumar,[a] R. Arun Kumar,[a] N. Veera Reddy,[a] and
K. Rajender Reddy*[a]
Dedicated to Professor Gautam R. Desiraju on the occasion of his 60th birthday
Keywords: Synthetic methods / Cross-coupling / Amides / Carboxylic acids / Copper
A new synthetic approach for amide bond formation through
the oxidative coupling of N,N-dialkylformamides with
carboxylic acids was achieved by using a copper catalyst.
Furthermore, this method was applied in the coupling of chi-
ral amino acids in which the stereochemistry was retained in
the resulting amide products.
Introduction
Amide linkages (–CONH–) are widely found in nature
and play a significant role in biological systems and in the
fields of pharmaceuticals, natural products, and polymers.[1]
Although several attractive approaches involving a non-
carboxylic acid route have been reported over the last few
decades,[2] conventional methods involving the reaction be-
tween carboxylic acids or their activated analogues with
amines dominate synthetic amide chemistry (Scheme 1,
method A).[3] Looking at the importance of these amide
units in pharmaceutical products, therapeutic peptides, and
in the preparation of modified peptides and proteins, the
construction of amide bonds from carboxylic acids, particu-
larly amino acid analogues, by a simple and convenient ap-
proach is highly desirable but challenging.
In recent years, cross dehydrogenative couplings or oxi-
dative cross-couplings have become a powerful synthetic
strategy in organic chemistry.[4] Generally, these reactions
are accomplished by direct coupling of two functional
groups by using transition-metal catalysts with different ox-
idants such as organic peroxides, H2O2, oxygen, and so on.
Looking at synthetic potential and green chemistry aspects,
several research groups have focused on the synthesis of the
amide functionality by an oxidative coupling approach.
Among these, reactions with aldehydes or alcohols with
Scheme 1. Different approaches to the formation of amides.
amines under oxidative conditions are the most widely
studied (Scheme 1, method B).[5]
Traditionally, formamides are known to be useful as sol-
vents and also as a source for CO, Me2N, Me2NCO, and
oxygen in metal-catalyzed organic reactions.[6] Moreover,
the coupling chemistry of formamides with aryl halides
through aminocarboxylation is well known.[7] In contrast,
the coupling chemistry of formamides with other functional
groups under oxidative conditions is less explored. Because
of their easy synthesis and availability, in recent years sev-
eral research groups have put their efforts into exploring
the synthetic utility of these formamides in useful organic
transformations.[6a,8] Similarly, few reports have appeared
on amidation reactions by using formamide as the amide
and amine source under oxidative conditions.[9,10] For ex-
[a] Inorganic and Physical Chemistry Division, CSIR-Indian
Institute of Chemical Technology (CSIR-IICT)
Tarnaka, Hyderabad 500607, India
Fax: +91-40-27160921
E-mail: rajender@iict.res.in
rajenderkallu@yahoo.com
Homepage: http://www.iictindia.org/new/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201544.
1218
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Copper-Catalyzed Oxidative Coupling of Carboxylic Acids
ample, direct amidation of azoles with formamides through Table 1. Optimization of the reaction conditions.[a]
metal-free C–H bond activation was achieved by using tert-
butyl perbenzoate.[9] More recently, molecular iodine and
tetra-butylammonium iodide based catalysts were employed
for the direct amidation of alcohols or aldehydes with di-
alkyl formamides by using tert-butyl hydroperoxide (TBHP)
as the oxidant (Scheme 1, method C).[10]
Entry
Catalyst
Oxidant
Isolated yield [%][b]
In our previous work, we used potassium iodide as a
catalyst for various organic transformations,[11a–11d] includ-
ing amide bond formation by using aldehydes and amines
under oxidative conditions.[11e] During these investigations,
we anticipated the possible formation of the corresponding
acids as one of the intermediates. However, no amide prod-
ucts were observed when the acids were treated with amines
under those conditions. A similar observation was also
made by Wan and co-workers during the synthesis of
amides with aldehydes and formamides catalyzed by tetra-
butylammonium iodide (Bu4NI) by using TBHP as the oxi-
dant.[10a] Very recently, we showed the use of copper cata-
lysts in oxidative couplings,[12] and in one of the examples
enol and phenol carbamates were synthesized from for-
mamides under oxidative conditions.[12b] In continuation,
looking at the synthetic usefulness of amides, we explored
the possible coupling of acids with formamides by using
copper catalysts (Scheme 1, method D).
1
2
3
4
5
6
7
8
Cu(OAc)2·H2O
Cu(OAc)
Cu(OAc)2
CuO
CuBr
CuI
CuCl
Cu(OH)2
CuCl2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
–
TBHP
UHP
TBP
O2
m-CPBA
H2O2
58
33
42
35
44
21
48
43
63
15
52
78
–
9
10
11
12
13
14
15
16
17
18
19
20
CuBr2
CuSO4·5H2O
Cu(ClO4)2·6H2O
–
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
–
75[c]
12
18
–
20
22
[a] Reaction conditions: acid 1a (1 equiv.), catalyst (10 mol-%),
DMF (2a; 2 mL, 27 equiv.), oxidant (1.5 equiv.), 100 °C, 15 h.
[b] Yield based on isolated yield of the product. [c] 5–6 m TBHP in
decane was used.
Results and Discussion
Our initial experiment with p-methylbenzoic acid and an Table 2. Both electron-rich and electron-deficient aromatic
excess amount of DMF by using Cu(OAc)2·H2O (10 mol- acids could be smoothly transformed into the desired prod-
%) as the catalyst and TBHP (70 wt.-% aqueous solution) ucts (Table 2, 3a–j). It was generally observed that electron-
as the oxidant at 100 °C provided 58% of coupled product deficient aromatic acids were more reactive and provided
3a (Table 1, entry 1). Further optimizations were carried the products in shorter reaction times. A similar observa-
out with this substrate, and the results are summarized in tion was also made for diethylformamide as one of the reac-
Table 1. Experiments with a series of copper salts clearly tion partners (Table 2, 3k–m). It is interesting to note that
demonstrate the generality of this reaction for both CuI and this catalytic system also works well for aliphatic acids. The
CuII salts (Table 1, entries 1–12). However, we found a rea- reactions of heptanoic acid, decanoic acid, and dodecanoic
sonable influence of the counterion on the yields of the acid with DMF provided corresponding amides 3n, 3o, and
products. Among the different copper salts, Cu(ClO4)2· 3p in good yields.
6H2O provided relatively higher yields of the product
The substrate scope was further expanded with different
(Table 1, entry 12). The absence of product formation in para-substituted phenylacetic acid derivatives. As shown in
blank experiments (Table 1, entries 13 and 14) clearly dem- the Table 3, we observed the exclusive formation of simple
onstrates the importance of both the catalyst and the oxi- amide products 5a–c for Cl-, MeO-, and F-substituted
dant in this coupling reaction. The isolated yields of the phenylacetic acid derivatives. However, in case of more elec-
amide products were comparable for both a 70 wt.-% aque- tron-withdrawing p-nitrophenyl acetic acid as a substrate,
ous solution of TBHP and a solution of TBHP in decane, the formation of keto amide 6d and aldehyde 7d was ob-
and this establishes the fact that the presence of water in served as the major products.
the reaction medium does not influence the formation of
Encouraged by the results obtained with aromatic/benz-
the product. Screening of other oxidants such as urea–hy- ylic and aliphatic acids, we further explored the possibility
drogen peroxide (UHP), tert-butyl peroxide (TBP), molecu- of extending the reaction to more synthetically useful
lar oxygen, m-chloroperbenzoic acid (m-CPBA) and hydro- amino acid derivatives. Gratifyingly, the reaction worked
gen peroxide (H2O2) under the same reaction conditions well for various amine-protected enantiopure amino acid
(Table 1, entries 15–20) showed inferior results.
derivatives, and the amides were obtained in good yields.
The general applicability of this method was further The optical purities of the compounds were compared with
evaluated for structurally diverse acid derivatives under the reported values, which clearly indicated retention of the
optimized reaction conditions, and the results are shown in stereochemistry (Table 4).
Eur. J. Org. Chem. 2013, 1218–1222
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K. R. Reddy et al.
SHORT COMMUNICATION
Table 2. Reaction of carboxylic acids with N,N-dialkylform-
Table 4. Reaction of amine-protected amino acids with N,N-di-
amides.[a]
methylformamide.[a]
[a] Reaction conditions: Amino acid (1 equiv.), Cu(ClO4)2·6H2O
(10 mol-%), DMF (2a; 2 mL, 27 equiv.), TBHP (1.5 equiv.), 100 °C,
15 h. [b] Yield based on isolated yield of the product.
(TEMPO), which indicates that the reaction might be go
through a radical intermediate (Scheme 2, a). The absence
of an amide product with dimethylamine also eliminates the
possibility of direct coupling between the acyl and amine
radical intermediates (Scheme 2, b). Depending on the reac-
tants and the catalytic system, various modes for the forma-
tion of radicals have been proposed for amide synthesis un-
der oxidative conditions.[5b,9,10a,10b] For example, the forma-
tion of an amine radical was proposed via tert-butyl di-
methylcarbamoperoxoate in the direct synthesis of amides
from aldehydes and formamides by using Bu4NI as the cat-
alyst and TBHP as the oxidant.[10a] However, amide forma-
tion from the corresponding acids was not observed. Sim-
[a] Reaction conditions: acid (1 equiv.), Cu(ClO4)2·6H2O (10 mol-
%), formamide 2a (2 mL, 27 equiv.), TBHP (1.5 equiv.), 100 °C.
Numbers in parentheses are the isolated yields of the products.
Table 3. Reaction of phenylacetic acids with N,N-dimethylform-
amide.[a]
Entry
X
Product, yield [%]
1
2
3
4
Me
OMe
F
5a, 84
5b, 78
5c, 73
5d, 0
6a, 0
6b, 0
6c, 0
6d, 34
7a, Ͻ5
7b, Ͻ5
7c, Ͻ5
7d, 40
NO2
[a] Reaction conditions: phenylacetic acid (1 equiv.), Cu(ClO4)2·
6H2O (10 mol-%), DMF (2a; 2 mL, 27 equiv.), TBHP (1.5 equiv.),
100 °C, 6 h. Numbers in parentheses are the isolated yields of the
products.
To gain insight into the mechanism of the reaction, a
series of experiments were carried out under the optimized
conditions (Scheme 2). No product was observed with
radical scavenger 2,2,6,6-tetramethylpiperidin-1-yloxyl Scheme 2. Investigation into the reaction mechanism.
1220 Eur. J. Org. Chem. 2013, 1218–1222
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Copper-Catalyzed Oxidative Coupling of Carboxylic Acids
ilar results were observed in our case when acids were procedure is quite distinct from the traditional approach.
treated with DMF by using iodine or iodide catalysts The present approach works very well for aliphatic and aro-
(Scheme 2, c).
matic acids with N,N-disubstituted formamides under mild
On the basis of our present experimental results and pre- reaction conditions. Moreover, we also showed the applica-
vious investigations by others, a possible mechanism is tion of this methodology for the synthesis of amides from
shown in Scheme 3. We anticipate that the amide product chiral amino acids with retention of the stereochemistry.
could be formed by the decarboxylation of carbamic anhy- Further studies on the scope of this reaction and mechan-
dride intermediate B (Scheme 3), which itself may be istic details are currently under investigation.
formed by the direct coupling of the acid with the for-
mamide (Scheme 3, route 1) or through the formation of
tert-butyl peroxybenzoate A (Scheme 3, route 2). Regarding
route 2, Rawlinson and co-workers showed that peroxyester
A reacts with dimethylformamide in the presence of copper
ions and subsequently transforms into the amide prod-
uct.[16] They proposed carbamic anhydride B as an interme-
diate, which decomposes into the amide with the loss of
CO2. Formation of the amide product with pure B in the
presence of additives such as tert-butyl alcohol, DMF, or
trace amounts of CuCl clearly establishes the intermediacy
of B. We also observed the formation of the amide product
with tert-butyl perbenzoate with DMF by using the copper
catalyst (Scheme 2, d).[17] Although a recent report suggests
the formation of peroxybenzoate A by the direct coupling
of the aldehyde with TBHP in the presence of a catalytic
amount of tetrabutylammonium iodide,[18] we did not ob-
serve A with any of the acids under our experimental condi-
tions. These results strongly suggest that, in the present
case, carbamic anhydride intermediate B may result from
the direct coupling of the acid with the formamide. The
retention of chirality reveals that the carbonyl transfer dur-
ing the decarboxylation could be from the formamide
group but not from the acid side. Previous work on 13C-
isotope labeling experiments in amide synthesis from alde-
hydes and formamides also supports the preferential loss
of the carbonyl group from the formamide rather than the
aldehyde.[10a]
Experimental Section
General Procedure for the Synthesis of Amides: A mixture of acid
1 (1.0 mmol) and Cu(ClO4)2·6H2O (10 mol-%) in N,N-dialkyl-
formamide 2 (2.0 mL) was placed in a two-necked reactor with a
condenser at room temperature. Then, TBHP (70 wt.-% in H2O,
1.5 equiv.) was added to the reaction mixture slowly over a 5 min
period, and the mixture was placed in an oil bath with magnetic
stirring at 100 °C in an open atmosphere. Completion of the reac-
tion was monitored by TLC. The reaction mixture was cooled to
room temperature. The reaction mixture was extracted with ethyl
acetate and washed with saturated NaHCO3 solution, water, and
brine solution. The organic layer was collected and dried with
Na2SO4. The solvent was removed under reduced pressure. The
residue was purified by column chromatography on silica gel (pe-
troleum ether/ethyl acetate) to afford the pure product.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, product characterization and copies
1
of H and 13C NMR spectra as well as and GC-MS data.
Acknowledgments
P. S. K. thanks the University Grants Commission (UGC) and
G. S. K., R. A. K., and N. V. R. thank the Council of Scientific and
Industrial Research (CSIR), New Delhi for their respective fellow-
ships.
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Received: November 17, 2012
Published Online: January 24, 2013
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Eur. J. Org. Chem. 2013, 1218–1222