Facile amidation of esters with aromatic amines promoted by lanthanide tris (amide) complexes

Article abstract of DOI:10.1002/aoc.5517

The development of catalysts capable of catalyzing amidation of esters with amines to construct amides under mild conditions is of great importance. Compared to aliphatic amines, the direct catalytic amidation of esters with less nucleophilic aromatic amines is rather difficult. Employing simple lanthanide tris (amide) complexes Ln[N (SiMe₃)₂]₃(μ-Cl)Li (THF)₃ as the catalysts, it was found a broad range of aromatic amines and esters were efficiently converted into various amides in good yields under mild conditions. A plausible mechanism for this transformation was experimentally supported as starting from an amide exchange reaction between the lanthanide tris (amide) complex and the substrate amine.

Full text of DOI:10.1002/aoc.5517

Received: 25 October 2019
DOI: 10.1002/aoc.5517
Revised: 2 January 2020
Accepted: 3 January 2020
F U L L P A P E R
Facile amidation of esters with aromatic amines promoted
by lanthanide tris (amide) complexes
Zhao Li1,2 | Chenjun Guo1 | Jue Chen3 | Yingming Yao2 | Yunjie Luo1
1School of Material Science and Chemical Engineering, Ningbo University, Ningbo, 315211, P. R. China
2College of Chemistry, Chemical Engineering & Materials Science, Soochow University, Suzhou, 215123, P. R. China
3School of Biological and Chemical Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo, 315100, P. R. China
Correspondence
The development of catalysts capable of catalyzing amidation of esters with
amines to construct amides under mild conditions is of great importance. Com-
pared to aliphatic amines, the direct catalytic amidation of esters with less
nucleophilic aromatic amines is rather difficult. Employing simple lanthanide
tris (amide) complexes Ln[N (SiMe3)2]3(μ-Cl)Li (THF)3 as the catalysts, it was
found a broad range of aromatic amines and esters were efficiently converted
into various amides in good yields under mild conditions. A plausible mecha-
nism for this transformation was experimentally supported as starting from an
amide exchange reaction between the lanthanide tris (amide) complex and the
substrate amine.
Yingming Yao, College of Chemistry,
Chemical Engineering & Materials
Science, Soochow University, Suzhou
215123, P. R. China.
Email: yaoym@suda.edu.cn
Yunjie Luo, School of Material Science
and Chemical Engineering, Ningbo
University, Ningbo 315211, P. R. China.
Email: luoyunjie@nbu.edu.cn
Funding information
K. C. Wong Magna Fund in Ningbo
University; the National Natural Science
Foundation of China, Grant/Award
Number: 21572205, 21971130, 21871198
and 21674070; the Natural Science
Foundation of Ningbo Municipal, Grant/
Award Number: 2019A610030,
K E Y W O R D S
amidation, aromatic amine, ester, lanthanide amide complex
2019A610129; the Natural Science
Foundation of Zhejiang Province, Grant/
Award Number: LY19B040002; Natural
Science Foundation of Ningbo Municipal,
Grant/Award Numbers: 2019A610129,
2019A610030; Natural Science Foundation
of Zhejiang Province, Grant/Award
Number: LY19B040002; National Natural
Science Foundation of China, Grant/
Award Numbers: 21674070, 21871198,
21572205, 21971130
1 | INTRODUCTION
scaffolds to stabilize metal complexes.[2] Therefore, the
investigation of -CO-N- amide bond formation to con-
Amides play a vital role in many organic molecules,
such as natural products, peptides, and pharmaceutical
agents.[1] They are also ready to serve as ligand
struct amides has attracted increasing interest in the
chemical community.[3] Condensation of carboxylic
acids and amines in the presence of stoichiometric
amounts of condensing regents is one of the conven-
tional methods to synthesize amides, which always
produces stoichiometric amounts of waste byproduct.
Zhao Li and Chenjun Guo contributed equally to this work.
Appl Organometal Chem. 2020;e5517.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5517

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LI ET AL.
In view of environmentally benign development, many
efforts have been made to establish efficient means to
access amides by a catalytic process. Among, direct
amidation of esters with amines is considered to be
an alternative approach to obtain amides as only a
small amount of catalyst is needed. Moreover,
this approach is superior to conventional methods with
respect to that the sole byproduct is an alcohol.
Some catalytic systems have been developed to
promote this transformation, including inorganic bases
(such as LiOH,[4] K3PO4,[5] NaOMe[6]), organic
catalysts (such as BEMP,[7] 1,5,7-triazabicyclo[4.4.0]dec-
5-ene (TBD),[8] DBU,[9] 1,2,4-triazole,[10] NHCs,[11]),
enzyme,[12] and metal complexes (such as Pd (IPr)
(allyl)Cl,[13] Ni (cod)2,[14] La (OTf)3,[15] Ru-PNN,[16]
2 | EXPERIMENTAL
2.1 | General information
All manipulations were performed in a purified argon
atmosphere using standard Schlenk techniques. The sol-
vents were degassed and distilled from sodium benzophe-
none ketyl under argon prior to use. Ln[N (SiMe3)2]3(μ-
Cl)Li (THF)3 (Ln = La, Nd, Sm, Yb, Y) were prepared
according to the literature.[19] Solid esters and amines
were recrystallized before use, liquid esters and amines
1
were distilled before use. H and 13C NMR spectra were
recorded on Bruker AV-400 apparatus. HRMS was deter-
mined on Bruker ESI-TOF (in the supplementary
material).
AlMe3,[17]
Zr
complexes,[18]
Ln2Na8(OCH2CF3)14
[19]
(THF)6,
CaI2[20]). Nevertheless, further improvement
is still required to expand the substrate scope and to
realize the catalytic reaction under mild conditions.
For example, in the reported successful cases the
amines used are mostly restricted to aliphatic amines,
whilst less nucleophilic aromatic amines are poorly
exploited.[13,14,17,19] Furthermore, the amidation of
esters with aromatic amines is usually performed at a
2.2 | A general procedure for the direct
amidation reaction
A
20 ml Schlenk flask was charged with
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (131.9 mg, 0.3 mmol),
ester (0.5 mmol), amine (0.75 mmol), and THF (2.5 ml).
ꢀ
The reaction mixture was stirred at 30 C for 20 min.
high temperature and needs
a
long reaction
After the reaction was completed, the reaction mixture
was hydrolyzed with water (3.0 ml), extracted by ethyl
acetate (5.0 ml), dried over anhydrous Na2SO4, and fil-
tered. After the solvent was removed under vacuum, the
crude product was purified by column chromatography
(eluent: ethyl acetate/petroleum ether = 1/5) to give the
final outcome.
time.[13,14,17,19] Therefore, the development of catalysts
capable of promoting the amidation of esters with aro-
matic amines under mild conditions is of great
importance.
Lanthanide tris (amide) complexes Ln[N (SiMe3)2]3
(μ-Cl)Li (THF)3 are easily available, and have been
widely used as starting materials to prepare lanthanide
derivatives.[21,22] Besides, these simple lanthanide tris
(amide) complexes are catalytic active for various
organic transformations, such as hydroelementation,[23]
hydrophosphonylation,[24] amidation of aldehydes with
amines,[25] addition of alkynes to nitriles,[26] aza-Henry
reaction,[27] guanylation of amines,[28] cross-diinsertion
of C ≡ N/C ≡ C into an N-H bond,[29] cycloa-
midination of aminoalkenes with nitriles,[30] C-C bond
cleavage reaction,[31] addition of alcohols to
carbodiimides,[32] guanylation/cyclization of amino acid
esters and carbodiimides,[33] alkene hydroamination,[34]
tandem insertion of secondary amines with
2-alkynylbenzonitriles,[35] addition of thiols to
isocyanates,[36] the Tishchenko reaction,[37] and dehy-
drocoupling of alanes with amines,[38] etc.. In contrast,
the application of lanthanide tris (amide) complexes in
catalyzing amidation reaction between esters and
amines has not been reported to our knowledge.
Herein we report that Ln[N (SiMe3)2]3(μ-Cl)Li (THF)3
could efficiently mediate the amidation of esters with
aromatic amines under mild conditions.
3 | RESULTS AND DISCUSSION
The reaction between methyl benzoate and N-
methylaniline was selected to obtain the optimal condi-
tions. Treatment of methyl benzoate with 1.5 equivalents
of N-methylaniline in the presence of 10 mol% LaCl3 in
THF at room temperature gave only a trace amount of
aimed product after 60 min (Table 1, entry 1). The utiliza-
tion of 10 mol% lanthanum tris (amide) complex
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 increased the yield to
30% (Table 1, entry 2). The yields got improved as the cat-
alyst loading increased, and a good yield of 90% was
achieved when the catalyst loading was 30% (Table 1,
entries 2–4). Assessment of lanthanide tris (amide) com-
plexes with different metal sizes showed that the catalytic
activity was strongly dependent on metal size, and the
activity tendency was in well line with the increase of
metal sizes (La > Nd > Sm > Y > Yb)[39] (Table 1, entries
4–8). Other reaction conditions such as solvent, reaction
time, and temperature were also screened. As a result,

LANTHANIDE COMPLEXES CATALYZED AMIDATION OF ESTERS WITH ARYLAMINES
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TABLE 1 Optimization of reaction conditionsa
Entry
1
Catalyst (mol%)
Solvent
THF
Temperature (ꢀC)
Time (min)
Yield (%)b
LaCl3 (10)
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
40
50
60
60
60
60
60
60
60
60
60
60
60
60
60
15
20
30
45
120
60
60
trace
30
2
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (10)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (20)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
Yb[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
Sm[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
Nd[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
La[N (SiMe3)2]3(μ-Cl)Li (THF)3 (30)
THF
3
THF
55
4
THF
90
5
THF
trace
66
6
THF
7
THF
70
8
THF
77
9
Toluene
Ether
CH2Cl2
1,4-dioxane
CH3CN
THF
80
10
11
12
13
14
15
16
17
18
19
20
5
74
75
trace
86
THF
88
THF
90
THF
91
THF
90
THF
86
THF
88
aReaction conditions: methyl benzoate (0.5 mmol), N-methylaniline (0.75 mmol), and catalyst (amount required) were stirred in THF (2.5 ml) for 1 hr at 30 ꢀC.
bIsolated yield.
THF proved to be the optimal reaction medium (Table 1,
entries 4, 9–13), prolonging reaction time from 15 min to
2 h did not lead to significant improved yields (Table 1,
entries 4, 14–18), and reaction temperature had a hardly
effect on the reaction outcome (Table 1, entries 4, 19, 20).
Therefore, the reactions could be performed under mild
and the reactions should be performed at a high tempera-
ture and a long reaction time.[13,14,17,19] As shown in
Tableꢀ2, when the amidation reactions were carried out
at 30 C for 20 min in the presence of La[N (SiMe3)2]3(μ-
Cl)Li (THF)3, methyl benzoate exhibited an excellent
reactivity toward various aromatic amines, producing the
amides 3a-e in 60–91% yields. Rich-electronic density on
nitrogen atoms of aromatic amines was in favor of this
transformation as the yield of 3b (91%) was much higher
than 3a (60%). However, it seemed that the activity of
anilines bearing -H, electron withdrawing (-Cl) or donat-
ing (−Me) group (3a, 60%; 3f, 50%; 3 g, 46%) had no strik-
ing dependence on the electronic configuration of
nitrogen atoms in these aromatic amines. The reaction
with diphenylamine still afforded N,N-diphenyl-
benzamide 3d in a moderate yield of 65%, whilst the reac-
tion with the steric hindrance 2,6-diisopropylaniline gave
ꢀ
condition such as at ambient temperature (30 C) and a
short time (20 min). Compared to the low catalyst load-
ing of La (OTf)3 in such catalysis,[15] the need of a high
catalyst loading of La[N (SiMe3)2]3(μ-Cl)Li (THF)3 might
be ascribed to the detriment of the active intermediate[40]
or La[N (SiMe3)2]3(μ-Cl)Li (THF)3 by the alcohol formed
in the reaction as a byproduct (vide supra).
With the optimal reaction conditions in hand, the
scope of aromatic amines in the amidation reaction was
explored. In view of the limited reported examples, ami-
dation of esters with aromatic amines is not promising,

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LI ET AL.
TABLE 2 Scope of aminesa
aReaction conditions: methyl benzoate (0.5 mmol), amine (0.75 mmol), and lanthanum amide (0.15 mmol) were stirred in THF (2.5 ml) for 20 min at 30 ꢀ
unless otherwise noted; isolated yield.
bIsopropyl benzoate (0.5 mmol) instead of methyl benzoate.
C
only a trace amount of the desired product 3j. Notably,
the hetero-atom containing aromatic 2-aminopyridine
also exhibited a comparable activity to aniline (3 k in 55%
yield). In comparison, aliphatic amines showed a rela-
tively low activity toward this transformation (3 l, 3 m).
These results demonstrated that lanthanide tris (amide)
complexes were capable of mediating efficiently the ami-
dation of esters with a board range of aromatic amines
under mild conditions, and displayed a unique property
of lanthanide complexes in this catalytic process.
Since the amidation reaction with various amines was
applicable, the study was further extended to esters with
different steric and electronic properties. As shown in
Table 3, esters with R3 was aliphatic (4 k) showed much
lower activity than aromatic ones. Reactions between
N-methylaniline 2a and methyl benzoate bearing either
electron withdrawing (-F, -Cl, -I) or donating (−Me, -
OMe) substituents proceeded rapidly, and generated the
desired amides 4a-4e in good yields. The steric effect of
aniline was not an important factor for this reaction, as
o-, m-, and p-isomers gave similar yields (4e, 4f and 4 g).
The influence of the electron withdrawing on the reactiv-
ity was obvious (4a > 4b > 4c), which was in accordance
with the electron-withdrawing ability of substituents.
The good performance of methyl furan-2-carboxylate
(4j in 83% yield) and methyl isonicotinate (4i in 96%
yield) showed that the heteroatoms from the esters
did also not deactivate the catalyst activity, as
mentioned above. Methyl cinnamate could also gave the
product 4 hr in reasonable yield. In contrast, Boc-
protected esters showed hardly activity in this transfor-
mation (4o-p).
Influence of byproduct alcohols on this transforma-
tion was investigated base on esters such as isopropyl
benzoate, tert-butyl benzoate, n-butyl benzoate and ethyl
benzoate (Table 4). The preliminary results showed that

LANTHANIDE COMPLEXES CATALYZED AMIDATION OF ESTERS WITH ARYLAMINES
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TABLE 3 Scope of estersa
aReaction conditions: ester (0.5 mmol), amine (0.75 mmol), and lanthanum amide (0.15 mmol) were stirred in THF (2.5 ml) for 20 min at 30 ꢀC. Isolated yields.
TABLE 4 Scope of estersa
Entry
R4
Yield
99%
74%
68%
59%
1
2
3
4
isopropyl
tert-butyl
n-butyl
ethyl
aReaction conditions: ester (0.5 mmol), amine (0.75 mmol), and lanthanum amide (0.15 mmol) were stirred in THF (2.5 ml) for 20 min at 30 ꢀC. Isolated yields.

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LI ET AL.
FIGURE 1 1H NMR spectra (in C6D6) of
Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 (a), a mixture
of Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 and HNPh2
(b), methyl benzoate (c), and a mixture of
Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 and methyl
benzoate (d)
SCHEME 1 Synthesis of
complex 4
a strong acidity of alcohol would dramatically deactivate
the catalyst.
when Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 was treated with
one equivalent of methyl benzoate (Figure 1), suggesting
no nucleophilic attack took place between -N (SiMe3)2
group and carbonyl group. Based on these facts, a plausi-
ble mechanism of the amidation is proposed as follows
(Figure 3). Deprotonation of the substrate amine with the
-N (SiMe3)2 group of Ln[N (SiMe3)2]3(μ-Cl)Li (THF)3 gen-
erates an intermediate species A. Nucleophilic attack
between A and the ester produces an amide and a lantha-
nide methoxide complex. The latter abstracts the proton
of a second amine to regenerate A to continue a new cat-
alytic cycle. The byproduct alcohol (such as MeOH)
formed in the pathway step might react with A to gener-
ate some inert species and shut down the catalytic cycle
to some extent, which might be one of the main reasons
for a high loading of lanthanide amide complexes in this
catalytic reaction. This reaction pathway is quite different
to the most reported catalytic cycles, in which the process
begins from the activation of esters.[8–10]
To elucidate the reaction mechanism, 1H NMR moni-
toring reaction between Y[N (SiMe3)2]3(μ-Cl)Li (THF)3
and a stoichiometric amount of HNPh2 was performed,
which showed the presence of a singlet resonance δ at
0 ppm with the integration assignable for one equivalent
of free HN (SiMe3)2. The signals for -N (SiMe3)2 groups
bound to the yttrium center shifted upfield slightly
(Figure 1). Despite the poor quality of crystals of complex
4 obtained from the reaction of Y[N (SiMe3)2]3(μ-Cl)Li
(THF)3 with one equiv. of HNPh2 in THF (Scheme 1), the
rough molecular structure supported that one N (SiMe3)2
group in Y[N (SiMe3)2]3(μ-Cl)Li (THF)3 was replaced by
NPh2 group. The yttrium center is four-coordinated by
two N (SiMe3)2, one NPh2 and one THF molecule
(Figure 2), which is similar to an analogous structure of
Yb reported by Zhang and Wang et al (in the supplemen-
tary material).[41] Remarkably, no reaction occurred

LANTHANIDE COMPLEXES CATALYZED AMIDATION OF ESTERS WITH ARYLAMINES
7 of 8
FIGURE 2 Rough molecular structure
of complex 4
molecular skeletons under mild conditions. The interme-
diate separated from the reaction of the lanthanide cata-
lyst with diphenylamine sheds some light on this
nucleophilic substitution transformation, mostly starting
from an amide exchange reaction between the catalyst
and substrate amine. Further study is on-going in our
laboratory to improve catalytic performance.
ACKNOWLEDGEMENTS
We acknowledge financial support from the National
Natural Science Foundation of China (21971130,
21572205, 21871198 and 21674070), the Natural Science
Foundation of Zhejiang Province (LY19B040002), the
Natural Science Foundation of Ningbo Municipal
(2019A610030, 2019A610129), and K. C. Wong Magna
Fund in Ningbo University.
FIGURE 3 Plausible mechanism
ORCID
Yunjie Luo https://orcid.org/0000-0002-2480-1385
4 | CONCLUSIONS
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How to cite this article: Li Z, Guo C, Chen J,
Yao Y, Luo Y. Facile amidation of esters with
aromatic amines promoted by lanthanide tris
(amide) complexes. Appl Organometal Chem. 2020;
e5517. https://doi.org/10.1002/aoc.5517