Chromium-catalyzed ligand-free amidation of esters with anilines

Article abstract of DOI:10.1246/BCSJ.20200277

Amides are important structural motifs in pharmaceutical and agrochemical chemistry because of the intriguing biological active properties. We report here the amidation of commercially available esters with anilines that was promoted by low-cost and air-stable chromium(III) pre-catalyst combined with magnesium, providing access to amides. This reaction occurs without the use of external ligands in a simple operation. Mechanistic studies indicate that a reactive aminated Cr species responsible for the amidation can be considered, which may be formed by reaction of low-valent Cr with aniline followed by reduction with hydrogen evolution.

Full text of DOI:10.1246/BCSJ.20200277

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Chromium-Catalyzed Ligand-Free Amidation of Esters with Anilines
Changpeng Chen, Liang Ling, Meiming Luo, and Xiaoming Zeng*
Advance Publication on the web November 26, 2020
doi:10.1246/bcsj.20200277
© 2020 The Chemical Society of Japan
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Publication manuscripts.

Chromium-Catalyzed Ligand-Free Amidation of Esters with Anilines
Changpeng Chen, Liang Ling, Meiming Luo and Xiaoming Zeng*
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064,
China
E-mail: zengxiaoming@scu.edu.cn
Xiaoming Zeng
Xiaoming Zeng received his B.Sc. degree at Sichuan Normal University, and his Ph.D. degree at Sichuan University in
2009. He spent two years as a visiting scholar in Professor Guy Bertrand’s laboratory at the University of California at
Riverside (2007–2009). As a JSPS postdoctoral fellow, he pursued synthetic chemistry with Professor Eiichi Nakamura
at University of Tokyo. He started his independent research group at Xi’an Jiaotong University as a Principal Investigator
in 2012. He moved to Sichuan University in 2017, and recently obtained awards including Thieme Chemistry Journal
Award, ACP Lectureship Awards (Japan & Singapore).
Abstract
Amides are important structural motifs in pharmaceutical
reported by Garg and Newman.25–27 In the pioneering
contributions, ancillary NHC ligands are commonly used for
enhancing the reactivity of metals of Pd and Ni in catalytic
cleavage of acyl C–O bonds for amidation with anilines. We are
wondering whether it’s possible to perform the reaction in the
absence of expensive external ligands by earth-abundant first-
row metal catalysis. This may provide the opportunity to
simplify the operation and decrease the cost for amide synthesis.
Herein, we report an advance in the catalytic synthesis of amides
by the coupling between anilines and esters using cost-effective
chromium catalysis (Scheme 1, bottom).28,29 It presents a rarely
example of catalytic amidation that can occur without the use of
external ligands in a simple operation.30
and agrochemical chemistry because of the intriguing biological
active properties. We report here the amidation of commercially
available esters with anilines that was promoted by low-cost and
air-stable chromium(III) pre-catalyst combined with magnesium,
providing access of amides. This reaction occurs without the use
of external ligands in a simple operation. Mechanistic studies
indicate that a reactive aminated Cr species in responsibility of
the amidation can be considered, which may be formed by
reaction of low-valent Cr with aniline followed by reduction
with hydrogen evolution.
Keywords: chromium, amidation, C–O activation
NHC-Pd- and Ni-catalyzed amidation of esters with anilines
O
O
cat. [M]
+
NH2
1. Introduction
N
H
O
M = NHC-Pd
NHC-Ni
The (O)C–N bonds in amides are essential building blocks
for the discovery of pharmaceuticals, agrochemicals, and
appealing chemicals.1–3 Owing to the interesting biologically
properties of amides, the amidation that is able to form the amide
bonds ranks among the frequently used transformations in the
development of medicinal and agrochemical ingredients.4,5 The
importance of amides in pharmaceutical and chemical industrials
spurs the use of non-precious metal catalysts in developing cost-
effective strategies for accessing amides.
Szostak, Garg, Newman et al.
Cr-catalyzed, ligand-free amidation
O
CrCl3
(10 mol%)
O
Ar
+
NH2
Ar
N
H
O
Mg (2 equiv)
THF, 80 o
C
• Non-precious Cr catalysis
• Without external ligands
• C–O bond activation
Given the low-cost of commercially available anilines, the
use of these widely accessible motifs as precursors to react with
bulk chemicals provides a particularly promising route to access
to amides by the formation of (O)C–N bonds.6–8 The amidation
by catalytic cleavage of unactivated C–O bonds of esters remains
a challenging because of the unfavorable thermodynamic
contribution of the process. As a result, early studies in this arena
usually focused on disclosing nucleophilic substitution of esters
with electron-rich aliphatic amines by Lewis acid catalysis.9–12
Recent years has witnessed breakthroughs by catalytic cleavage
of unactivated C–O bonds with reactive metals for cross-
coupling.13–22 Using N-heterocyclic carbene (NHC)-supported
palladium complexes, Szostak and co-workers disclosed
interesting catalytic strategy in achieving the coupling of esters
with anilines (Scheme 1, top).23,24 The discovery of robust nickel
catalysts for the amidation between esters and anilines was
Scheme 1. Catalytic synthesis of amides using readily available
anilines and esters by the cleavage of unactivated C–O bonds.
2. Results and Discussion
Our interest in the development of cost-effective strategy
to form amide bonds stems from the consideration of its potential
utility in accessing valuable motifs.31–34 To avoid the synthesis
and purification of reactive metal catalysts, we commenced our
studies by using commercially available metal salts as pre-
catalysts for the amidation of ethyl benzoate (1a) with aniline
(2a). With our endeavor in the exploration of Cr-catalyzed cross-
coupling reactions,35 we speculated that a reactive low-valent Cr,
being formed in situ by reduction with Mg,36,37 may possess the
ability in the cleavage of C–O bonds of esters and facilitate the
formation of the amide C–N bonds.
Based on this consideration, low-cost and environmentally

benign CrCl2 salt was chose as pre-catalyst by the treatment with
magnesium in the amidation (Table 1).38–42 Gratifyingly, the
formation of N-phenylbenzamide (3a) in 90% yield was
cyclopentenyl, tetrahydro-2H-pyranyl and cyclohexanyl
carboxamides can be efficiently prepared (3v–3z). Interestingly,
bulky tert-butyl and adamantly-substituted ethoxylcarbonyls
undergo the reaction smoothly to give the related amides 3aa and
3ab. Furthermore, this Cr-catalyzed strategy can be extended to
the synthesis of 1,3-diphenylurea motif by catalytic cleavage of
C–O bond (3ac).
o
observed when performing the reaction in THF at 80 C (entry
2). It was proved that the amidation cannot occur without Cr salt.
The air-stable CrCl3 salt displays similar reactivity to provide 3a
(entry 3).43,44 However, the amidation using other Cr complexes
such as Cr(OAc)3, Cr(acac)3 and Cr(CO)6 did not proceed, and
the color of the solutions was not changed to black (entries 4–6).
We proposed that the ligands of these complexes may largely
affect the conversion, and reactive Cr species may be not formed
in these reactions. Other first-row metal salts of FeCl2 and NiCl2
showed no efficiency in the amidation (entries 7 and 8).45 It was
noteworthy that the reaction with CuCl occur smoothly, albeit
with affording 3a in low yield (entry 9). Other reductants such
as manganese and zinc cannot promote the Cr-catalyzed reaction
(entry 11). While decreasing the amount of CrCl3 to 5 mol% led
to a low conversion.
CrCl3
(10 mol%)
O
O
Ph
+
Ph NH2
Et
N
H
O
Mg (2 equiv)
THF, 80 oC, 12 h
2a
1
3
O
O
O
O
Me
Ph
Ph
Ph
MeO
Ph
N
H
N
H
N
H
N
H
Me
3b (92%)
3a (90%)
3d (82%)
3c (95%)
O
O
O
O
Ph
F3C
Ph Cl
Ph
Ph
N
H
N
H
N
H
N
H
Cl
F
3e (71%)
3f (93%)
3g (80%)
3h (66%)
Me
O
Me
O
O
OMe O
Table 1. Optimizing Reaction Conditions
Ph
Ph
Ph
Ph
N
H
N
H
N
H
N
H
Me
Me
3k (30%)
O
O
metal salt
(10 mol%)
3l (76%)
3i (87%)
3j (73%)
Ph
+
Ph NH2
OEt
N
H
O
O
O
O
O
reductant (2 equiv)
THF, 80 oC, 12 h
Ph
Ph
Ph
Ph
Ph
N
H
N
H
N
N
H
N
H
O
H
3a
O
1a
2a
O
S
3p (49%)
3m (80%)
3n (53%)
3o (51%)
3q (72%)
Entry
1
2
Metal salt
none
CrCl2
Reductant
Mg
Yield (3a)
ndb
O
O
O
Me
Ph
O
Ph
Ph
N
H
N
H
N
H
Ph
N
Mg
90%
S
H
90% (74%)c
(99%)d
ndb
3t (67%)
3u (88%)
3r (50%)
3s (41%)
3
CrCl3
Mg
O
O
O
O
O
Ph
N
H
Ph
Ph
Ph
Ph
N
H
N
H
4
5
6
7
8
Cr(OAc)3
Cr(acac)3
Cr(CO)6
FeCl2
Mg
Mg
Mg
Mg
Mg
N
N
H
H
O
ndb
3z (40%)
3y (83%)
3v (82%)
3x (78%)
3w (85%)
ndb
Me
O
ndb
O
O
O
O
Ph
Ph
N
H
N
H
ndb
Ph
Ph
N
H
NiCl2
N
H
N
H
Cl
9
10
11
CuCl
CrCl3
CrCl3
Mg
none
Mn or Zn
27%
3ac (43%)
3aa (61%)
3ab (87%)
3ad (81%)
ndb
Scheme 2. Chromium-catalyzed amidation of esters with aniline.
Reaction Conditions: 1 (0.8 mmol), aniline (0.4 mmol), CrCl3
(0.04 mmol), Mg (0.8 mmol), THF, 80 oC, 12 h. Isolated yields
are given.
ndb
aReaction conditions: 1a (0.8 mmol), 2a (0.4 mmol), metal salt (0.04 or 0.02 mmol),
Mg (0.8 mmol), THF, 80 oC, 12 h. Isolated yields are given. bNot detected. cThe
related conversion of 1a. dConversion of 2a.
The scope of anilines was next probed by the treatment
with ethyl benzoate. As shown in Scheme 3, mono-substituted
anilines containing methyl, tert-butyl, methoxy, fluoride,
chloride and trifluoromethyl groups reacted effectively to form
the related amides 3ae–am in preparatively useful yields. The
steric hindrance caused by the ortho substituent did not impact
on the transformation (3an and ao). This strategy can be used to
prepare N-(2,6-difluorophenyl)-benzamide 3ap. The reaction
with naphthalen-2-amine occurred in affording the amide 3aq in
85% yield. In addition to primary anilines, secondary anilines
are suitable precursors, thus offering a route to the synthesis of
tertiary amide derivative (3ar). On the other hand, unactivated
secondary aliphatic amines reacted with ethyl benzoate smoothly,
providing access to the amide products 3as and 3at.
Considering the simple operation using air-stable CrCl3 as
pre-catalyst, the scope of amide motifs that can be formed by this
strategy was examined. The incorporation of both electron-
donating and electron-withdrawing substituents into the arenes
of ethyl benzoates did not greatly influence the reaction,
resulting in the formation of the amides 3b-3h in moderate to
excellent yields (Scheme 2). The functional groups of
trifluoromethyl, fluoride and chloride in arenes can be retained
after the reaction. The steric hindrance caused by an ortho-
methyl or methoxy group in ethyl benzoate did not hamper the
amidation to form 3i and j in good yields. In contrast, a relatively
low conversion was obtained when using more bulky ethyl 2,4,6-
trimethylbenzoate in the reaction (3k). In addition to benzoates,
naphthoates are suitable precursors to form the amides of 3l and
3m. Heteroaryl esters containing furanyl, benzofuranyl and
thienyl motifs can be used to couple with aniline, providing
access to heterocycle-bearing amide derivatives 3n–r. a,b-
Unsaturated N-phenylcinnamamide can be derived from related
precursor with keeping the olefin group intact (3s). Besides ethyl
aryl esters, the amides with aliphatic esters coupled with aniline
smoothly, giving the amides 3t and 3u in 67% and 88% yields,
respectively. By this strategy, diversely substituted amides such
as
N-phenyl-substituted
cyclopropanyl,
cyclobutanyl,

eqs 1 and 2). The stoichiometric reaction was next carried out by
treatment of 1a and 2a with low-valent Cr that was formed by
reduction CrCl3 with Mg after filtration (eq 3). However, the
desired amide 3a was not formed, indicating that Mg is required
for the reaction in addition to the reduction of CrCl3.
O
O
CrCl3 (10 mol%)
OEt
N
N
+
H
O
Mg (2 equiv)
THF, 80 oC, 12 h
1a
2
3
OMe
Me
O
O
O
N
Me
N
N
H
N
CrCl3 (10 mol%)
H
H
H
3ag (55%)
3a
(eq 1)
+
2a
1a
1a
3ae (87%)
3af (60%)
Mg (2 equiv)
TEMPO (2 equiv)
THF, 80 oC, 12 h
3ah (60%)
(88%)
F
Cl
Cl
O
O
O
O
CrCl3 (10 mol%)
N
H
N
N
H
N
H
+
2a
3a
(eq 2)
H
F
Mg (2 equiv)
1,1-Diphenylethylene (4 equiv)
THF, 80 oC, 12 h
(80%)
3ak (88%)
3al (90%)
3ai (70%)
3aj (62%)
F
O
O
O
O
Mg
(4 equiv)
1a (0.4 mol)
2a (0.2 mmol)
N
H
N
H
N
H
N
H
CF3
Me
OMe
F
CrCl3
(eq 3)
3a
THF, 80 oC, 6 h
then filtration
80 oC, 12 h
O
3ap (64%)
3am (71%)
3ao (81%)
3an (84%)
(0.2 mmol)
(not detected)
O
N
O
N
O
O
Ph
N
H
O
O
CrCl3
(10 mol%)
Ph
N
N
H
O
Me
Ph
4 (16%)
1a
+
2a
3a (90%)
+
(eq 4)
3at (27%)
3as (30%)
3aq (85%)
3ar (41%)
Mg (2 equiv)
THF, 80 oC, 12 h
1a
(26%
recovery)
Ph
Ph
N
H2
H
Scheme 3. Cr-catalyzed synthesis of amides by the variation of
(22%)
(≈5%)
(24%)
aniline and amine precursors.
CrCl3 (10 mol%)
Mg(NHPh)2
+
(eq 5)
H2
Ph NH2
Mg (2 equiv)
THF, 80 oC, 12 h
2a
∼ 10% yield
Of note is that benzocyclic esters can be used as precursors
in the amidation, thereby providing a method in the synthesis of
hydroxyl-tethered amide compounds 3au and 3av (Scheme 4).
The effect of leaving groups in esters was studied, and the results
show that the replacement of ethyl group in ester 1a by methyl,
tert-butyl and phenyl did not affect the amidation. Moreover, the
Cr-catalyzed cross-coupling of ester with aniline is scalable and
can be performed on gram scale without loss of efficiency.
Without CrCl3
Not detected
OMe
(eq 6)
CrCl3
(X mol%)
O
NH2
1a (2 equiv)
N
H
Mg (4 equiv)
THF, 80 oC, 6 h
then filtration
80 oC, 12 h
MeO
3ah
2ah
Recoverya
X
3ah (Yield)a
1a
2ah
30
50
21%
48%
94%
70%
23%
12%
Catalytic synthesis of hydroxyl-tethered aromatic amides
aYields are determined by GC analysis using n-tridecane as internal standard.
O
O
Ph
CrCl3 (10 mol%)
N
H
O
+
(eq 7)
Mg(NHPh)2
(0.2 mmol)
+
1a
Ph NH2
3a
THF, 80 oC, 12 h
Mg (2 equiv)
THF, 80 oC, 12 h
then NH4Cl (aq)
OH
n
(81% yield
was based on 1a)
(0.4 mmol)
n
3au, n = 0, 36%
3av, n = 1, 35%
2a
1
Scheme 5. Preliminary mechanistic studies.
Studying the effect of leaving groups in esters on the amidation
O
O
CrCl3 (10 mol%)
Ph
+
Ph NH2
OR
N
It was noteworthy that the liberation of hydrogen in nearly
22% yield was observed by GC/MS analysis of the reaction
between 1a and 2a, in combination with ester-reduced products
of toluene and N-benzylaniline (eq 4).46 The reaction within 4 h
gives hydrogen in 43% yield, which can be consumed by the
reduction of ester under present conditions. In addition,
hydrogen was detected in the reaction of aniline with Mg and
CrCl3, and the amounts of hydrogen are almost equal to that of
CrCl3 (eq 5). These indicate that a stoichiometric magnesiation
relative to Cr can be considered for evolution of hydrogen, and
the aminated magnesium species such as Mg(NHPh)2 may be
formed in the reaction.47 It was noted that the reaction without
CrCl3 cannot lead to hydrogen. Interestingly, by the treatment of
1a with the filtrate of reaction between aniline 2ah, CrCl3 and
Mg, the amide product 3ah can be formed, and the yields are
usually lower than the amounts of CrCl3 salt (eq 6). Furthermore,
it was proved that Mg(NHPh)2 reacts with ester of 1a smoothly,
allowing for the formation the amide compound 3a in good yield
(eq 7).
H
Mg (2 equiv)
THF, 80 oC, 12 h
3a
1
2a
O
O
O
OtBu
OPh
OMe
1c (85%)a
1b (89%)a
a The yeild of 3a was given in parenthesis.
1d (83%)a
Gram-Scale Amidation
O
Ph
CrCl3 (10 mol%)
N
1a
+
2a
H
Mg (2 equiv)
THF, 80 oC, 12 h
3a
(92%, 1.81 g)
(22 mmol)
(11 mmol, 1.02 g)
Scheme 4. Catalytic synthesis of hydroxyl-tethered amides,
probing the effect of leaving groups on the amidation and gram-
scale reaction.
It was noted that the addition of commonly used radical
scavengers such as 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) and 1,1-diphenylethylene into the reaction system had
no effect on the amidation, indicating that a radical-involving
mechanism can be excluded in the transformation (Scheme 5,
To test the valent state of Cr by reduction of CrCl3 with Mg,
we conducted the stoichiometric reaction at 40 oC for overnight.
The oxidation states of Cr species in the resulting mixture was

analyzed by Cr 2p core-level X-ray photoelectron spectroscopy
(XPS), which were fitted with spin-orbital split 2p3/2 and 2p1/2
components (see Supporting Information). The related spectrum
shows two sets of XPS peaks, with the 2p1/2 component
appearing at 584.1 and the 2p3/2 component appearing at 573.9
eV of the binding energy, respectively. These data are nearly in
accordance with the signals for the 2p components (583.5 eV for
Cr 2p1/2, and 574.3 eV for Cr 2p3/2) of Cr(0). These results may
indicate that the formation of Cr(0) by reducing CrCl3 with Mg
can be considered.
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Figure 1. Proposed mechanism.
Based on previous results and our mechanistic studies, we
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CrCl3 with Mg,36,37 which may insertion into aniline in affording
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provides a cost-effective, scalable and operationally simple
strategy to the synthesis of biologically interesting amide motifs.
The evolution of hydrogen was detected, which may favor the
process of forming reactive Cr species for the transformation.
Further Studies on synthesis and characterization of reactive Cr
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Acknowledgement
We thank the National Natural Science Foundation of
China (Nos. 21572175, 21871186 and 21971168) and the
Fundamental Research Funds for the Central Universities
(20826041D4117) for financial support of this research.
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