Efficient copper(II)-catalyzed transamidation of non-activated primary carboxamides and ureas with amines

Article abstract of DOI:10.1002/anie.201108599

Amid(e) them all: Primary carboxamides and ureas react with aromatic and aliphatic amines in the presence of a copper catalyst to give a wide range of functionalized amides (see scheme). Copyright

Full text of DOI:10.1002/anie.201108599

Angewandte
Chemie
DOI: 10.1002/anie.201108599
Synthetic Methods
Efficient Copper(II)-Catalyzed Transamidation of Non-Activated
Primary Carboxamides and Ureas with Amines**
Min Zhang, Sebastian Imm, Sebastian Bꢀhn, Lorenz Neubert, Helfried Neumann, and
Matthias Beller*
Carboxamides represent an important class of compounds
found in numerous bioactive products,[1] such as top-sold
medicines (i.e., Lipitor, Lidoderm, Vyvanse),[2] as well as in
biological and synthetic polymers (i.e., proteins and nylons).[3]
In addition, amides serve as versatile building blocks for the
preparation of pharmaceuticals, agrochemicals, polymers,
etc.[4] In general, substituted amides are prepared by sub-
stitution reactions of carboxylic acid derivatives with
amines[14] [Scheme 1, Eq. (1)], or by the coupling of aryl/
alkyl halides with primary amides[15] [Scheme 1, Eq. (2)].
Alternatively, well-established name reactions (i.e., Ritter,[5]
Schmidt,[6] Beckmann,[7] Ugi,[8] Wolff,[9] etc.) and more
recently established approaches[10–15] are applied for their
synthesis.
activation reagents (2–3 equivalents), or the reactions were
reversible and gave mixtures of amides.
During our recent work on the catalytic amination of a-
hydroxy amides,[19,20] serendipitously we observed the forma-
tion of amides.[21] While selective amination of the alcohol
occurred in the presence of [Ru3(CO)12]/bidentate phosphine
as the catalyst, the use of copper(II) complexes resulted in
unexpected transamidation products (Scheme 2).
Scheme 2. Selective transamidation versus alcohol amination in a-
hydroxycarboxamides.
To the best of our knowledge, copper-catalyzed trans-
amidations have not yet been reported. Considering the
economic attractiveness and excellent functional group
tolerance of copper in homogenous catalysis, we became
interested in developing a general transamidation method-
ology of nonactivated primary carboxamides with amines.
Herein, we describe our results for the first time.
Scheme 1. General approaches for the synthesis of amides.
In spite of these significant developments, the so-called
transamidation process in which amides are reacted with
a second, but different amine represents an interesting
unconventional route for the functionalization of a given
carboxamide.[16] Recent elegant examples of transamidation
reactions from the groups of Stahl[17] and Myers[18] showed the
possibility of preparing secondary or tertiary amides under
mild reaction conditions. Unfortunately, the existing methods
to accomplish the desired amide exchange process involve
either the use of an excess of expensive and waste-generating
Our initial studies focused on developing a more efficient
catalytic system for transamidations and we used the reaction
of 1a with 2a as a model system (for structures see Table 1).
At the start of our work the effect of representative
precatalysts, as well as different temperatures and solvents
were explored (see Table S1 in the Supporting Information).
An optimal yield of 3a was obtained at 1408C by using
10 mol% of Cu(OAc)2 as a catalyst and t-amyl alcohol as the
solvent.
With an optimized catalytic system in hand, we then
examined the generality of this copper-catalyzed transamida-
tion protocol. As shown in Table 1, all the reactions pro-
ceeded smoothly and afforded the desired products in
reasonable to excellent yields upon isolation. Electron-with-
drawing and electron-donating substituents on the aryl ring of
the anilines were tolerated with only little influence on the
reactivity (Table 1, entries 1–3, 7–9, 11–14, and 16). We
observed that alkyl amines (Table 1, entries 4–6, 17, and 18)
as well as the hydrazine 2g (Table 1, entry 10) exhibited
excellent reactivity and yielded full conversion of benchmark
substrates in only 5 hours. Notably, the transamidation
reactions of a-hydroxy amides with aryl amines gave the
corresponding products in good yields within 10 hours (70–
82%: Table 1, entries 7–14). The resulting products are
synthetically interesting building blocks for the preparation
[*] Dr. M. Zhang, S. Imm, S. Bꢀhn, L. Neubert, Dr. H. Neumann,
Prof. Dr. M. Beller
Leibniz-Institut fꢁr Katalyse an der Universitꢀt Rostock e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
Dr. M. Zhang
School of Chemical and Materials Engineering
Jiangnan University, 1800 Lihu Road, Wuxi, 214122 (P. R. China)
[**] This work has been supported by Evonik and the Deutsche
Forschungsgemeinschaft (Leibniz Prize). Z.M. thanks the Alexander
von Humboldt Foundation for a grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108599.
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Table 1: Copper-catalyzed transamidation of primary amides with amines.[a]
Entry
1
1
2
Yield [%][b]
Entry
10[f]
1
2
Yield [%][b]
PhNH2
PhNHNH2
2g
1a
1a
1a
2a
3a: 78 (69)[c]
1b
1c
1d
3j: 71
3k: 70
3l: 76
PhNH2
2a
2
11[f]
12[f]
13[f]
2b
3b: 92
PhNH2
2a
3
2c
3c: 81
4[d]
1a
1a
2d[e]
3d: 86
3e: 86
1d
1d
2b
2c
3m: 76
3n: 73
5[d]
14[f]
2e
6[d]
15
16
1a
1b
2 f
3 f: 90
3g: 82
1e
1 f
2 f
3o: 30
3p: 93
PhNH2
2a
PhNH2
2a
7[f]
8[f]
17[d]
1b
1b
2b
2c
3h: 71
3i: 78
1 f
2h
2i
3q: 53
3r: 83
9[f]
18[g]
1a
[a] Reaction conditions: Unless otherwise specified, all reactions were carried out under argon protection with 1 (1 mmol), 2 (1.3 mmol) and Cu(OAc)2
(0.1 mmol) in tert-amyl alcohol (2 mL) at 1408C for 16 h. [b] Yield of isoalted product. [c] Yield of product isolated from a 10 mmol scale experiment:
1a (10 mmol, 1.35 g), 2a (13 mmol, 1.21 g), Cu(OAc)2 (1 mmol, 0.18 g), tert-amyl alcohol (3 mL). [d] Reactions time: 5 h. [e] 2d (0.5 mmol).
[f] Reaction time: 10 h. [g] Other tested linear secondary amines (i.e. dibenzylamine) showed lower reactivity.
of a-amino acid derivatives through a direct alcohol amina-
tion strategy.[21] The modest reactivity of 3-methylbenzamide
(1e; Table 1, entry 15) is consistent with the known stabiliza-
tion of metal centers by arylamidate ligands, which should
decrease the catalyst activity.[22]
Interestingly, the reaction between the primary amide 1a
and diamine 2d gave the corresponding diamide 3d in
excellent yield (Table 1, entry 4). The amino-acid-derived
amide 2h can be employed to produce the dipeptide 3q in
reasonable yield (Table 1, entry 17). These results demon-
strate the potential of our transamidation protocol for
manufacturing polyamides or peptides by using appropriate
substrates.
In addition to transamidations of carboxamides, the
related reaction of ureas constitutes a demanding goal.
Hence, we started a set of investigations to also functionalize
urea derivatives in a selective manner. Gratifyingly, the
copper catalyst proved to be active for these transformations,
too. All the reactions proceeded efficiently and furnished all
types of substituted ureas in good to excellent yields
(Table 2). The reaction between inexpensive urea and aniline
resulted in diphenyl urea (5a) in 72% yield (Table 2, entry 1).
Diamines such as cyclohexane-1,2-diamine (2j) and benzene-
1,2-diamine (2k) were successfully converted into biologically
interesting cyclic ureas[23] in high yields (Table 2, entries 2 and
3). Interestingly, the reaction of 1-phenylurea (4b) and
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Table 2: Copper-catalyzed transamidation of ureas with amines.[a]
respectively, in good yields with retention of the chiral
configuration (Scheme 3). These results show that the cop-
per(II)-catalyzed transamidation protocol is also applicable
for the preparation of chiral amides from chiral substrates.
Entry
1[c]
4
2
Yield [%][b]
PhNH2
4a
2a
5a: 72
2
4a
2j
5b: 76
Scheme 3. Copper(II)-catalyzed synthesis of a chiral amides.
3
4
4a
4b
4b
2k
5c: 91
5d: 83
The reaction mechanism has not yet been elucidated, but
on the basis of the previously reported mechanistic studies of
amine exchange processes conducted with amidate complexes
by Stahl and co-workers,[17b,d] we assume the following
mechanism (Scheme 4): Initially, formation of an active
mono- or bisamidate copper complex B takes place by
2 f[c]
5[d]
2b
(5a’/5a’’
= 87:4)[e]
PhNH2
6
7
4c
4c
2a
5e: 72
2c
5 f: 70
[a] Reaction conditions: Unless otherwise specified, all reactions were carried
out under argon protection with 4 (1 mmol), 2 (1.3 mmol) and Cu(OAc)2
(0.1 mmol) in tert-amyl alcohol (2 mL) at 1408C for 8 h. [b] Yield of isolated
product. [c] 2 f (2.4 mmol). [d] Mol ratio: 4b/2b=1:10. [e] Ratio was
determined by GC and GC/MS analysis.
cyclohexylamine (2 f) gave 1,3-dicyclohexylurea (5d) exclu-
sively by replacing the aryl group with the cyclohexyl group
(Table 2, entry 4). The exchange of two anilines having similar
nucleophilicity (i.e., 2b and 2a) in ureas also took place and
gave mixed products (see Scheme S1 in the Supporting
Information). However, the reaction can be driven in the
direction of forming 1,3-bis(p-tolyl)urea (5a’) by using an
excess amount of amine 2b (Table 2, entry 5). In contrast,
reactions of 1-ethylurea (4c) with anilines 2a and 2c gave the
unsymmetrical ureas 5e (72%) and 5 f (70%), respectively,
with retention of the ethyl group (Table 2, entries 6 and 7).
These results indicate that this copper(II)-catalyzed trans-
amidation of ureas undergoes a nucleophilic substitution
mechanism, wherein the more nucleophilic amine substitutes
the weaker one on the respective urea derivative. It should be
noted that this urea transamidation protocol also provides
new pathways to construct substituted heterocycles and
peptides.
Scheme 4. Possible mechanism for the copper(II)-catalyzed transami-
dation of primary carboxamides.
substitution of the acetate ligand. Indeed, the expected
product 3’ (step S1) can be detected in the reaction mixture
by means of GC/MS. The resulting amido ligand in complex A
can abstract a proton from the coordinated amide ligand to
give the catalytically active species B (step S2). After
coordination of an amine and proton transfer, the intermedi-
ate C (step S3) is formed. Then, nucleophilic attack of the
amido ligand onto the carboxamide results in the formation of
the tetrahedral intermediate D (step S4), which can either
revert to C or isomerize to E through coordination of the
second nitrogen atom to the copper(II) center (step S5).
Cleavage of E (step S6) and exchange of neutral carboxamide
ligand gives G and the transamidation product (steps S6 and
S7). Finally, proton transfer from the amide to the amido
ligand (step S8) completes the amine exchange process along
Interestingly, the reaction of the primary amide 1a with
enantiopure amine (R)-(+)-2e’ or the reaction between
enantiopure amide (S)-(+)-1d’ and aniline 2b produced
enantiopure carboxamides (R)-(+)-3e’ and (S)-(À)-3m’,
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with regeneration of copper(II) catalyst B and liberation of
ammonia. Notably, the release of ammonia also shifts the
equilibrium to the desired product. As one of the referees
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In summary, we have developed a straightforward copper-
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Primary carboxamides and ureas can be selectively trans-
aminated in an efficient manner. Compared to previously
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À
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Experimental Section
Typical procedure for the preparation of 3a: Cu(OAc)2 (18 mg,
0.1 mmol), 2-phenylacetamide 1a (135 mg, 1 mmol), aniline 2a
(121 mg, 1.3 mmol), and tert-amyl alcohol (2 mL) were added
successively to a Schlenk tube (25 mL) equipped with a magnetic
stirrer bar. The Schlenk tube was then closed and the resulting
mixture was stirred at 1408C for 14 h under an argon atmosphere. The
accumulated ammonia gas in the pressure tube was then released
under argon protection, and the reaction mixture was stirred at 1408C
for another 2 h. After cooling down to room temperature, the
reaction solvent was removed under vacuum. The residue was directly
purified by flash chromatography on silica gel eluting with heptane/
ethanol (25:1) to give N,2-diphenylacetamide (3a) as a white solid
(165 mg, 78%).
À
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Received: December 6, 2011
Revised: January 23, 2012
Published online: March 7, 2012
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Keywords: amides · amines · copper · homogeneous catalysis ·
transamidation
.
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