Synthesis and Characterization of Amidato Divalent Lanthanide Complexes and Their Use in Forming 2,4-Quinazolidinones from CO2 and 2-Aminobenzonitriles

Article abstract of DOI:10.1002/ejoc.201600291

Four amidato divalent lanthanide complexes, {LⁿLn[N(TMS)₂]THF}₂ [n = 1, Ln = Eu (1); n = 2, Ln = Eu (3), Yb (4); HL¹ = tBuC₆H₄CONHC₆H₃(iPr)₂; HL² = C₆H₅CONHC₆H₃(iPr)₂] and {L³Eu[N(TMS)₂]THF}{L³₂Eu(THF)₂} (2) [HL³ = ClC₆H₄CONHC₆H₃(iPr)₂], were synthesized and extensively characterized. This is the first time that the amidato lanthanide amides 1-4 were used to catalyze the reactions of CO₂ and 2-aminobenzonitriles to form quinazoline-2,4(1H,3H)-diones at atmospheric pressure. All the complexes efficiently catalyzed the transformation, with complex 3 showing the highest activity. This catalytic system gave good to excellent yields, and good functional group tolerance. Preliminary studies were conducted to investigate the reaction mechanism.

Full text of DOI:10.1002/ejoc.201600291

DOI: 10.1002/ejoc.201600291
Full Paper
Lanthanide Catalysis
Synthesis and Characterization of Amidato Divalent Lanthanide
Complexes and Their Use in Forming 2,4-Quinazolidinones from
CO and 2-Aminobenzonitriles
Qianyu Wang, Chengrong Lu, Bei Zhao,* and Yingming Yao*
2
[
a]
[a]
[a]
[a]
Abstract: Four amidato divalent lanthanide complexes, to form quinazoline-2,4(1H,3H)-diones at atmospheric pressure.
n
{
(
L Ln[N(TMS) ]THF} [n = 1, Ln = Eu (1); n = 2, Ln = Eu (3), Yb All the complexes efficiently catalyzed the transformation, with
2 2
1
2
4); HL = tBuC H CONHC H (iPr) ; HL = C H CONHC H (iPr) ] complex 3 showing the highest activity. This catalytic system
and {L Eu[N(TMS) ]THF}{L Eu(THF) } (2) [HL = ClC H CONH- gave good to excellent yields, and good functional group toler-
6
4
6
3
3
2
6
5
6
3
2
3
3
2
2
2
6 4
C H (iPr) ], were synthesized and extensively characterized. This ance. Preliminary studies were conducted to investigate the re-
6
3
2
is the first time that the amidato lanthanide amides 1–4 were action mechanism.
used to catalyze the reactions of CO and 2-aminobenzonitriles
2
Introduction
complexes have a broad range of applications in catalytic trans-
[
4]
formations. Four investigations of rare-earth metal amidates,
reported by Schafer and Zi, respectively, achieved good to high
activities in the ring-opening polymerization of lactones and
Transformations of CO have drawn considerable attention not
2
only because it is a major greenhouse gas but also because it
is abundant, nontoxic, nonflammable, easily available, low cost,
and renewable. It is an important C1 feedstock in synthetic
[
4a,4d]
lactides, amidation, and hydroamination.
Our group has
long been interested in the preparation of amidato rare-earth
metal amides and we have studied the use of such compounds
in several significant catalytic reactions such as carboxylation of
[
1]
chemistry. Quinazoline-2,4(1H,3H)-diones, which are pharma-
ceutical intermediates and biologically active additives, have a
[
2]
terminal alkynes with CO
, hydrophosphonylation of aldehydes
2
wide range of applications. In recent years, the synthesis of
quinazoline-2,4(1H,3H)-diones from CO2 and 2-aminobenzo-
nitriles has attracted considerable interest. Many catalysts have
been developed for this transformation, such as 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU),
as 1-butyl-3-methylimidazolium hydroxide and 1-butyl-3-meth-
and unactivated ketones, and addition of amines to carbodi-
[
4g–4i]
imides.
Moreover, amidato divalent europium amides
show high catalytic activities in the direct addition of terminal
[
4f]
[
3a,3b]
[3c]
alkynes to aromatic nitriles. These results encouraged us to
expand the catalytic applications of amidato divalent lanthan-
ide amides. Two new amidato divalent europium amides were
synthesized and characterized, and their catalytic activities in
Cs CO , ionic liquids (such
2 3
[
3d,3e]
[3f]
ylimidazolium acetate),
MgO–ZrO2,
1,1,3,3-tetramethyl-
[
3g]
[3h]
guanidine, N-methyltetrahydropyrimidine, and TBA [WO ]
2
4
[
3i,3j]
the reaction between CO and 2-aminobenzonitriles were stud-
2
(
TBA = tetra-n-butylammonium).
However, despite many
ied. These amidato divalent lanthanide complexes are highly
efficient catalysts for the formation of quinazoline-2,4(1H,3H)-di-
ones at atmospheric pressure.
studies, these reactions still have major limitations such as the
need for relatively high pressures (1.3–10 MPa), the use of
equivalent amounts of base, and narrow substrate scope. The
design of highly effective catalysts for quinazoline-2,4(1H,3H)-
dione synthesis under mild conditions and with wide substrate
scope is therefore still needed.
Results and Discussion
Studies have shown that as one type of non-cyclopentadi- Syntheses and Characterization of Amidato Divalent
enyl ancillary ligands, steric and electronic properties of amid- Lanthanide Amides
ate ligands are easily tunable, and many amidato lanthanide
1
Two divalent europium complexes, {L Eu[N(SiMe ) ]THF} (1)
3
2
2
3
3
1
and
{L Eu[N(SiMe ) ]THF}{L Eu(THF) }
(2)
[HL
=
3
2
2
2
3
tBuC H CONHC H (iPr) , HL = ClC H CONHC H (iPr) ], were
6
4
6
3
2
6
4
6
3
2
[
a] Key Laboratory of Organic Synthesis of Jiangsu Province,
College of Chemistry, Chemical Engineering and Materials Science,
Dushu Lake Campus, Soochow University,
Suzhou 215123, P. R. China
synthesized by simple metathesis reactions of the proligand HL
with appropriate lanthanide precursors Ln[N(SiMe ) ] in THF at
3
2 2
6
0 °C (Scheme 1). Two known amidato divalent lanthanide am-
E-mail: zhaobei@suda.edu.cn
ides, {L Ln[N(SiMe ) ]THF} [Ln = Eu (3), Yb (4), HL2
2
=
3
2
2
yaoym@suda.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600291.
C H CONHC H (iPr) ], were prepared as described in our previ-
6
5
6
4f]
3
2
[
ous work.
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completely different from that of complex 1 (Figure 2). One
metal is seven-coordinate with two oxygen atoms and two
nitrogen atoms from two amidate ligands, one oxygen atom
from the other amidate ligand, and two oxygen atoms from
THF solvation molecules. The second europium atom is six-co-
ordinate with one oxygen atom and one nitrogen atom from
an amidate ligand, one nitrogen atom from an –N(SiMe3)2
group, two oxygen atoms from the other two amidate ligands,
and one oxygen atom from the coordinating THF molecule. The
2
three amidato groups all adopt the κ -(N,O):μ-O bonding mode
to coordinate to the europium atom, and the bridging oxygen
2
atom in the κ -(N,O):μ-O bound amidato group coordinates to
the other europium atoms in the cell to connect the two subun-
its.
Scheme 1. Synthesis of amidato divalent lanthanide amides 1–4.
Suitable single crystals of complexes 1 and 2 were obtained
from a mixed THF/hexane solvent. The solid-state structures
were determined by using X-ray diffraction. The europium
amide 1 has a centrosymmetric binuclear structure, and com-
plex 2 is binuclear with three ligands in one unit cell. The crystal
structure of complex 1 and selected bond lengths and angles
are given in Figure 1. Each europium center in complex 1 is
2
five-coordinate, and the amidato group adopts κ -(N,O):μ-O
2
bonding. The bridging oxygen atom in the κ -(N,O):μ-O bound
amidato group coordinates to the second europium atom in
the cell to connect the two subunits; this is the same as the
[
4f]
previously reported structure of complex 3. When the pro-
3
ligand was changed to HL , the crystal structure of complex 2
was also a dinuclear molecule, although the solid structure was
Figure 2. Structure of amidato divalent europium amide 2. Hydrogen atoms
in the unit cell are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: Eu(1)–O(1) 2.640(6), Eu(1)–O(2)2.607(5), Eu(1)–O(5) 2.465(6), Eu(1)–
N(1) 2.645(7), Eu(1)–N(2) 2.663(7), O(1)–C(1) 1.308(10), N(1)–C(1) 1.282(10),
Eu(2)–O(1) 2.475(6), Eu(2)–O(2) 2.481(6), Eu(2)–O(5) 2.670(5), Eu(2)–N(3)
2
.682(7), Eu(2)–N(4) 2.512(7), O(5)–C(47) 1.324(9), N(3)–C(47) 1.349(10); N1–
Eu1–O1 50.00(19), N2–Eu1–O2 50.30(19), O1–Eu1–O2 73.25(18), O1–Eu1–O5
1.62(19), O2–Eu1–O5 75.54(18), O5–Eu1–N1 93.7(2), O5–Eu1–N2 122.26(19),
7
Eu1–O1–C1 95.0(5), Eu1–N1–C1 95.4(5), O1–C1–N1 119.2(8) 50.01(18), N3–
Eu2–O5 50.01(18), N4–Eu2–O5 179.7(2), N3–Eu2–N4 130.2(2), Eu1–O1–Eu2
9
1.86(19), Eu1–O2–Eu2 92.5(2), Eu1–O5–Eu2 91.37(18), Eu2–N3–C47 96.3(5),
Eu2–O5–C47 97.5(5), N3–C47–O5 115.7(8).
Catalytic Activities of Complexes 1–4 in Formation of 2,4-
Quinazolidinones from CO and 2-Aminobenzonitriles
2
Lanthanide amides readily undergo amine exchange reactions
when they are treated with amines or substrates containing
Figure 1. Structure of amidato divalent europium amide 1. Hydrogen atoms
in the unit cell are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: Eu(1)–O(1) 2.414(2), Eu(1)–O(1A) 2.556(2), Eu(1)–N(1A) 2.682(3),
Eu(1)–N(2) 2.467(3), O(1)–C(1) 1.296(4), N(1)–C(1) 1.298(4); N(2)–Eu(1)–O(1)
active hydrogen, and this process is widely believed to launch
[4f–4h,5]
catalytic reactions involving amines.
It was therefore hy-
1
06.66(10), N(2)–Eu(1)–O(1A) 127.00(10), O(1A)–Eu(1)–O(1) 71.06(8), N(2)–
Eu(1)–N(1A) 119.99(10), O(1)–Eu(1)–N(1A) 119.16(8), O(1A)–Eu(1)–N(1A)
0.43(8), C(1)–O(1)–Eu(1A) 97.3(2), C(1)–O(1)–Eu(1) 151.9(2), Eu(1A)–O(1)–
Eu(1) 108.94(8), C(1)–N(1)–Eu(1A) 91.4(2), N(1)–C(1)–O(1) 118.9(3).
pothesized that the reaction of 2-aminobenzonitrile with CO2
would be activated by using amidato divalent lanthanide am-
ides. The activity of complex 3 was first tested in the model
5
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reaction of 2-aminobenzonitrile 5a and CO . The expected reac-
The catalytic system was further assessed by using a range
2
tion proceeded at 100 °C at ambient pressure, but the yield of of substituted 2-aminobenzonitriles in the preparation of quin-
quinazoline-2,4(1H,3H)-dione (6a) was low (Table 1, entry 1). azoline-2,4(1H,3H)-diones; the results are listed in Table 2. The
Bases such as DBU, triethylenediamine (1,4-diazabicyclo- reactions of CO and 2-aminobenzonitriles bearing an electron-
2
[
2.2.2]octane; DABCO), and Cs CO promoted the formation of donating group such as a methoxy or methyl group proceeded
2 3
quinazoline-2,4(1H,3H)-dione from 2-aminobenzonitrile and smoothly with yields of up to 99 % (entries 2 and 3). The yields
[
3a,3c]
CO at high pressure.
of quinazoline-2,4(1H,3H)-diones were lower when 2-aminoben-
2
zonitriles bearing a strong electron-withdrawing group (CF or
3
2
Table 1. Screening of optimum conditions for the reaction of CO and 2-
aminobenzonitrile catalyzed by complex 3.
NO ) were used as the starting materials (entries 4 and 5). The
2
[
a]
situation is complicated when a halogen atom is present on the
phenyl ring of 2-aminobenzonitrile. The yields of quinazoline-
2
,4-(1H,3H)-diones decreased from 97 % to 44 % in the order
fluorine, chlorine, bromine, and iodine, when the halogen atom
was located at the 4-position (entries 6–9). The reaction of 2-
aminobenzonitrile with a chlorine atom located at the 5-posi-
tion produced the desired product in good yield (62 %; en-
try 10). However, the yields dropped to low to moderate when
the halogen atom was at either the 3- or 6-position (entries 11–
Entry
Catalyst
Additive Loading [mol-%] T [°C] Yield [%][b]
1
2
3
4
5
6
7
8
9
3
–
3
3
3
4
1
2
3
–
–
5
100
100
100
100
100
100
100
100
100
100
12
45
91
28
35
61
83
72
48
42
DBU
DBU
DABCO
1
3), probably because of increased steric hindrance. It is worth
5
20
5
noting that the reaction of heteroaromatic aminonitrile 5n and
CsCO
3
CO underwent the transformation in moderate yield; only one
2
DBU
DBU
DBU
DBU
DBU
5
5
5
2.5
5
similar example with poor yield has previously been re-
[
3d]
ported
(entry 14). The product from 4,5-dimethoxy-N-ethyl-
2
-aminobenzonitrile 5p, a secondary aromatic amine, was not
10
Eu[N(SiMe
3
) ]
2 2
observed in the presence of complex 3 and DBU in a 1:1 molar
ratio (entry 15).
[
[
a] Reaction conditions: 2-aminobenzonitrile (1.0 mmol), CO
b] Isolated yield.
2
(balloon).
The mechanism of the catalytic transformation was investi-
gated by performing the stoichiometric reaction of complex 3
with 1 equiv. 5a. Compound A was isolated in good yield
Inspired by these results, we used bases as additives to pro-
[
6]
mote the transformation. The addition of 5 mol-% DBU had a
significant effect on the reaction, and the yield of 6a from the
reaction catalyzed by 3 increased dramatically from 12 to 91 %.
The same reaction without catalyst 3 gave only 45 % quin-
azoline-2,4(1H,3H)-dione in the presence of 5 mol-% DBU
(
(
84 %). The peak ascribed to the coordinated N(SiMe ) group
3 2
[
4g]
1
δ = 0.20 ppm)
in complex 3 was not observed in the H
NMR spectrum of A, an amino H assignable to 2-aminobenzo-
nitrile (δ =9.73 ppm) was observed instead. Elemental analyses
showed that complex A consists of an amidato group, a 2-
aminobenzonitrile, and a europium atom. One more informa-
tive structure determination that could be obtained from crys-
talline A came from the MS analysis, which showed fragment
peaks of the amidate ligand and of the substrate (see the Sup-
porting Information, Figure S3). Unfortunately, no crystals suit-
able for single-crystal X-ray diffraction were obtained, despite
(
Table 1, entries 1–3). Other additives, namely DABCO and
Cs CO , increased the yields slightly (entries 4 and 5). The diva-
2
3
lent ytterbium amide 4 was also tested; however, the 61 % yield
of 6a was clearly lower than that achieved by using complex 3
entries 3 and 6). The effects of the configurations of complexes
, 2, and 3 on the activities were also investigated. The activity
of complex 1 was similar to that of complex 3, but although
the central metal was the same, the activity of complex 2 was
lower than that of complex 3, probably because of the more
crowded structure of complex 2 (entries 3, 7, and 8). Complex
(
1
many efforts. Complex A catalyzed the reaction of CO and 2-
2
aminobenzonitrile in the presence of 5 mol-% DBU to give 6a
in 85 % yield, which is close to the value obtained by using
complex 3 as the catalyst. It is therefore reasonable to propose
that the aminolysis of complex 3 readily occurred in the initiat-
ing stage to afford A as the active catalyst.
3
was therefore chosen as the ideal catalyst for further studies.
The use of a smaller amount of DBU led to a decreased yield
of the product, possibly because it lowered the capacity of the
system to fix CO2 (entry 9). The europium precursor
The catalytic cycle in Scheme 2 is proposed, based on experi-
[
3a]
Eu[N(SiMe ) ] was also used in the reaction between 2-amino- mental findings and on previous reports.
Aminolysis of the
3
2 2
benzonitrile and CO in the presence of 5 mol-% DBU as the amidato divalent europium amide 3 produces complex A; a CO
2
2
additive; however, the yield of 6a was only 42 % (entry 10). molecule, which has already been activated by DBU, is then
Other reaction conditions, namely solvent, catalyst loading, inserted into the Ln–N bond of A to give intermediate B. Inter-
temperature, and reaction time, were also screened (Table S1, mediate C is formed by cyclization, hydrogen migration, acyloxy
entries 1–9). A combination of 5 mol-% amidato divalent euro- bond cleavage, and nitrogen nucleophilic attack in turn. The
pium amide 3 and 5 mol-% DBU in dimethyl sulfoxide (DMSO) extraction of a hydrogen atom from another 2-aminobenzo-
was identified as the best system for catalyzing the reaction of nitrile molecule gives the product quinazoline-2,4(1H,3H)-dione,
2
-aminobenzonitrile with CO at 100 °C and ambient pressure and complex A is regenerated. This mechanism suggests that
2
for 24 h (entry 3).
the hydrogen atom in the amino group in 2-aminobenzonitrile
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Table 2. Synthesis of quinazoline-2,4(1H,3H)-diones.[a]
is important, and explains why no reaction occurred with sec-
ondary amines (Table 2, entry 15).
Scheme 2. Possible mechanism for formation of quinazoline-2,4(1H,3H)-
diones catalyzed by amidato lanthanide amides.
Conclusions
Two amidato divalent europium amides, {[tBuC H CONHC H -
6
4
6 3
(
[
iPr) ]Eu[N(SiMe ) ]THF} (1) and {[ClC H CONHC H (iPr) ]Eu-
2 3 2 2 6 4 6 3 2
N(SiMe ) ]THF}{[ClC H CONHC H (iPr) ] Eu(THF) } (2), were
3
2
6
4
6
3
2 2
2
prepared in good yields. Single-crystal X-ray diffraction showed
that the structures of complexes 1 and 2 are completely differ-
ent. The catalytic activities of 1 and 2 and of the two known
amidato divalent lanthanide amides 3 and 4 in the formation
of quinazoline-2,4(1H,3H)-diones from CO and 2-aminobenzo-
2
nitriles were investigated. All the amides showed moderate to
high activities in the reaction, especially catalyst 3, which gave
the best performance. A possible mechanism based on experi-
mental evidence is proposed.
Experimental Section
General: All manipulations and reactions were conducted under a
purified argon or CO atmosphere by using standard Schlenk tech-
2
niques. Solvents were distilled from sodium benzophenone ketyl
under argon prior to use unless otherwise noted. Lanthanide
amides Ln[N(SiMe ) ] (THF) (Ln = Yb, Eu) and two known amidate
3
2 2
2
2
divalent lanthanide amides {L Eu[N(SiMe ) ](THF)} [Ln = Eu (3),
3
2
2
2
Yb(4), HL = C H CONHC H (iPr) ] were prepared by followed the
6
5
6
3
2
[
4f]
procedures in previous work.
Solid 2-aminobenzonitriles were
carefully degassed before use. Single-crystal X-ray diffraction data
1
were recorded with a Rigaku Mercury CCD X-ray diffractometer. H,
13
19
C, and F NMR spectra were obtained with a Bruker-400 spec-
trometer in [D ]DMSO or CDCl . Carbon, hydrogen, and nitrogen
analyses were performed by direct combustion with a Carlo–Erba
[
a] Reaction conditions: 2-aminobenzonitrile (1.0 mmol), CO
2
(balloon), cata-
6
3
lyst 3 (0.05 mmol), DMSO (2.0 mL). [b] Isolated yield.
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EA-1110 instrument. Metal analyses were carried out by complexo-
metric titration.
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1
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2
4
2
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3
2 2
2
1
hydrofuran solution of HL (10 mL, 2 mmol) was added dropwise.
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by dissolving in a minimum amount of hexane, with a few drops of
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3
2
2
2
tetrahydrofuran solution of Eu[N(TMS) ] (THF) (10 mL, 2 mmol), a
2
2
2
3
tetrahydrofuran solution of HL (10 mL, 3 mmol) was added drop-
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trated under vacuum to give a yellow solid. The product was recrys-
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Synthesis of Complex A: To a stirred toluene solution of
2
{
L Eu[N(SiMe ) ](THF)} (5 mL, 1 mmol), a toluene solution of 2-
3
2
2
NH C H CN (5 mL, 1 mmol) was added dropwise. The mixture was
2
6 4
stirred for 15 h at 25 °C and then concentrated under vacuum to
give a yellow solid. The product was recrystallized from a minimum
amount of toluene with a few drops of hexanes (84 % yield).
4
067; g) D. Nagai, T. Endo, J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
6
53–657; h) Y. P. Patil, P. J. Tambade, K. D. Parghi, R. V. Jayaram, B. M.
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diones from Reactions of CO2 and 2-Aminobenzonitriles: A
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1
5 mL Schlenk flask equipped with a magnetic stirring bar was
charged with complex 3 (66.6 mg, 0.05 mmol) and DBU (7.5 μL,
.05 mmol) in turn. 2-Aminobenzonitrile (1.0 mmol) and anhydrous
1
2
24, 6804; j) T. Kimura, H. Sunaba, K. Kamata, N. Mizuno, Inorg. Chem.
012, 51, 13001–13008; k) J. Ma, B. X. Han, J. L. Song, J. Y. Hu, W. J. Lu,
0
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DMSO (2 mL) were added by using a syringe shortly after purging
the Schlenk flask with a CO2 atmosphere three times. Thereafter,
carbon dioxide was introduced with a balloon at ambient pressure.
The reaction mixture was stirred at 100 °C for 24 h, then cooled to
room temperature. The reaction system was quenched with hydro-
chloric acid (2 mol/L, 10 mL), filtered, and the crude sample was
washed with water (3 × 10 mL) and toluene (3 × 10 mL) to remove
DBU and the proligand, respectively. After workup, the target quin-
6
1
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3
C NMR were collected in [D ]DMSO, which were all consistent
6
with the reported data.[
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The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (NSFC) (grant
numbers 21172165, 21132002, 21372172, and 21572151), PAPD,
the Major Research Project of the Natural Science Foundation
of the Jiangsu Higher Education Institutions (project
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[
Keywords: Homogeneous catalysis · Lanthanides · N,O
ligands · C1 building blocks · Carbon dioxide · Nitrogen
heterocycles
4
0, 9098–9100.
Received: March 11, 2016
Published Online: April 24, 2016
Eur. J. Org. Chem. 2016, 2555–2559
www.eurjoc.org
2559
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim