Synthesis, structure and catalytic applications of amidoterephthalate copper complexes in the diastereoselective Henry reaction in aqueous medium

Article abstract of DOI:10.1039/c4nj00878b

Terephthalic acid derivatives bearing amide side functional groups, viz. 2-propionamidoterephthalic acid (H₂L1) and 2-acetamidoterephthalic acid (H₂L2), have been structurally characterized using X-ray crystallography. While H_2<

Full text of DOI:10.1039/c4nj00878b

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Synthesis, structure and catalytic applications of
amidoterephthalate copper complexes in the
diastereoselective Henry reaction in aqueous
medium†
Cite this: New J. Chem., 2014,
38, 4837
Anirban Karmakar,* Susanta Hazra, M. Fatima C. Guedes da Silva* and
´
Armando J. L. Pombeiro*
Terephthalic acid derivatives bearing amide side functional groups, viz. 2-propionamidoterephthalic acid
(H2L1) and 2-acetamidoterephthalic acid (H2L2), have been structurally characterized using X-ray crystallo-
graphy. While H2L1 is hydrogen bonded with DMF molecules forming a 1D hydrogen bonded network, H2L2
constructs a 2D hydrogen bonded network. They have been used to synthesize two new isostructural
mononuclear copper complexes, [Cu(L1)(H2O)4] (1) and [Cu(L2)(H2O)4] (2), also characterized using X-ray
crystallography. In these structures the hydrogen bonded assemblies of carboxylate and coordinated
water molecules form a distorted cubane type of hydrogen bonded network. Complexes 1 and 2 act as
heterogeneous catalysts for the diastereoselective nitroaldol (Henry) reaction in aqueous medium, 1
providing high yields (up to 77%) and good diastereoselectivities under ambient conditions. These
catalysts can be recycled without significant loss of activity, and the catalytic process is efficient, simple,
easy to work-up and operates under ‘green’ conditions.
Received (in Porto Alegre, Brazil)
29th May 2014,
Accepted 14th July 2014
DOI: 10.1039/c4nj00878b
www.rsc.org/njc
The Henry reaction, also known as the nitro-aldol reaction,
is used to synthesize b-nitro alcohols from a nitroalkane and
Introduction
Copper(II) carboxylate complexes provide an interesting area of an aldehyde or a ketone via carbon–carbon bond formation.15
research as a result of their structural features and potential The obtained b-nitroalkanols are valuable starting materials
applications in various fields, namely those concerning their for further synthesis of e.g. 1,2-amino alcohols or b-hydroxy
biological activity,1 catalytic applications,2 anti-inflammatory3 acids.16 This reaction is usually catalyzed by homogeneous
and anti-fungal activities,4 capacity to act as enzyme models5 catalysts, such as alkali (or other s-block) metal hydroxides,
and molecular magnetism.6 They show a wide variety of coordina- alkoxides or amines, as well as complexes of d-block metal
tion schemes, from monomeric to polymeric and heterometallic.7 ions.17,18 There are some reports on Cu-MOF18n and Cu-
The structural diversity is accounted for by the variable donating complexes18o–r which are catalytically active for the Henry
ability of the oxygen atoms of the carboxylate moiety and by the reaction. However, only a few heterogeneous catalysts, such
wide nature of the attached ligands. In particular, the development as Amberlyst A-21,19 nickel hydroxyapatite nanocomposites,20
of porous copper metal organic frameworks (MOFs) by using mesoporous silica materials,21 polyethylene glycol-supported
multidentate aromatic carboxylate ligands8 has allowed us to copper catalysts,22 are found in the literature. Even though
envision a wide range of applications namely in nonlinear optics,9 high yields were achieved with reactions performed using homo-
gas storage,10 catalysis11 and host–guest induced separation.12
geneous catalysis, the achievement of a high stereoselectivity
The design and synthesis of a suitable catalyst for potentially is still challenging, and only a few examples are known23 using
applicable organic reactions have attracted much attention, heterogeneous catalysts.
and a number of them can be efficiently catalyzed by copper
carboxylate complexes.13,14
Some iron, copper18j,l and zinc18b,l containing complexes
have been reported from our group, which actively catalyze the
nitroaldol reaction, but most of them are also homogenous
catalysts, and now we wish to extend our study to hetero-
geneous systems.
In recent years, water has been recognized as an attractive
medium for many organic reactions,24 since it is safe, economical,
non-toxic and environmentally benign, compared to typical
´
´
Centro de Quımica Estrutural, Instituto Superior Tecnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail: anirbanchem@gmail.com,
fatima.guedes@tecnico.ulisboa.pt, pombeiro@tecnico.ulisboa.pt
† Electronic supplementary information (ESI) available. CCDC 1005775–1005778.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4nj00878b
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structures were not obtained. The reaction of H2L1 or H2L2
with copper(II) chloride dihydrate under hydrothermal reaction
conditions in dimethylformamide and NH4OH aqueous solution
leads to the formation of [Cu(L1)(H2O)4] (L1 = 2-propionamido-
terephthalate) (1) or [Cu(L2)(H2O)4] (L2 = 2-acetamidotere-
phthalate) (2), respectively.
In the IR spectra of 1 and 2, the characteristic strong
bands of the coordinated carboxylate groups appear at 1588–
1562 cmÀ1 for the asymmetric stretching and 1352–1347 cmÀ1
for the symmetric one. C–O stretching of coordinated carboxy-
late group is observed between 1255 and 1260 cmÀ1. A strong band
between 1649 and 1670 cmÀ1 is due to n(CO) of uncoordinated
carboxylate groups,28 whereas n(OH) of water molecules is in
the 3332–3315 cmÀ1 region. The bands at 1670–1631 cmÀ1 and
1429–1418 cmÀ1 are attributed to n(CQC) of the aromatic rings.
Complexes 1 and 2 were also characterized by single crystal
X-ray diffraction (see below) and elemental analysis (see Experi-
mental section). The single crystal X-ray diffraction structures of
the ligand precursors H2L1 and H2L2 were also obtained (ESI†).
Thermal gravimetric analysis was also performed on com-
plexes 1 and 2 which show similar decompositions (Fig. S1, ESI†).
Complex 1 shows a weight loss of 9.7% between 94 and 154 1C,
corresponding to the loss of two molecules of water (calcd: 9.7%),
whereas this process occurs in the 86–146 1C range for complex 2.
Upon further heating, both complexes lose the other two coordi-
nated water molecules, in the temperature range of 189–254 1C
for 1 and of 181–249 1C for 2. Above this temperature, further
decomposition occurs towards the final CuO.
Scheme 1 H2L1 and H2L2 ligand precursors.
organic solvents.25 However, only scant reports are available where
the Henry reaction has been performed in aqueous medium.26
The development of new, efficient and selective heterogeneous
catalysts that can be used in water has been a challenging task,
and the introduction of a cheap heterogeneous catalyst would be
a significant addition in this field. Thus, the two main objectives
of the current work are as follows: (i) to synthesize Cu(II)-complexes
by using dicarboxylate linkers (under hydrothermal conditions);
(ii) to apply the synthesized complexes as heterogeneous catalysts
in the nitroaldol combination of nitroethane with various
aldehydes in aqueous medium.
Herein we describe the structural behaviour of two dicarboxylic
acid ligand precursors, 2-propionamidoterephthalic acid (H2L1)
and 2-acetamidoterephthalic acid (H2L2) (Scheme 1), along with
their copper(II) complexes prepared under hydrothermal con-
ditions. The structural features of the two new complexes
[Cu(L1)(H2O)4] (1) and [Cu(L2)(H2O)4] (2) were obtained by
single crystal X-ray diffraction analysis, and the compounds
were also subjected to thermogravimetric analyses.
Crystal structure analysis
The catalytic performances of these frameworks, as hetero-
geneous catalysts, in terms of activity, heterogeneity, and recycl-
ability, were successfully tested towards the nitroaldol (Henry)
reaction of nitroethane with various aldehydes in aqueous medium.
Single-crystal X-ray diffraction studies reveal that [Cu(L1)(H2O)4] (1)
crystallizes in the triclinic P1 space group and that the asymmetric
%
unit contains one Cu2+ ion coordinated to one L12À ligand and four
water molecules (Fig. 1). The Cu1 centre presents a square pyr-
amidal geometry (t = 0.19) with the coordination sphere formed by
one carboxylate oxygen atom of a L12À unit [Cu1–O1, 2.001(2) Å],
which occupies one of the equatorial sites, and four O-atoms from
Results and discussion
Syntheses and characterization
Two dicarboxylic acid pro-ligands, 2-propionamidoterephthalic water molecules [Cu1–O12, 1.996(3) Å; Cu1–O13, 2.417(3) Å;
acid (H2L1) and 2-acetamidoterephthalic acid (H2L2), were Cu1–O14, 2.007(3) Å; and Cu1–O15, 2.028(4) Å]. As expected and
synthesized from 2-aminoterephthalic acid and recrystallized due to Jahn–Teller distortion29 the axial Cu–O bond distance is
from DMF and methanol, respectively. The synthetic procedure slightly larger [Cu1–O13, 2.417(3) Å] than the equatorial ones
and preliminary characterization of these compounds has involving the water molecules. Selected bond distances and
already been reported in our previous paper,27a but the crystal angles of complex 1 are listed in Table S3 (ESI†).
Fig. 1 (A) Coordination scheme in complex 1. (B) Schematic representation of complex 1.
4838 | New J. Chem., 2014, 38, 4837--4846
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Fig. 2 (A) Hydrogen bonded networks of complex 1 viewed parallel to the crystallographic bc-plane. Formation of the hydrogen bonded distorted close
cubane. (B) Hydrogen bonded cubane-type assembly constructed by carboxylate and coordinated water molecules.
The intermolecular organization in the crystal is characterized by
hydrogen bond interactions involving the carboxylate groups as
acceptors and the coordinated water molecules as donors expand-
ing the structure to the third dimension. As in the organic pre-
cursor, the amide group is intramolecularly hydrogen bonded with
carboxylate oxygen (dD–A 2.673(4) Å; o D–HÁÁÁA 1391). In 1, very
interesting hydrogen contacts involving non-coordinated carboxy-
late oxygen O3 and O4 (as acceptors) and the O13 and O14 hydrogen
atoms of water molecules (as bifurcated donors) are observed,
which form a close distorted cubane (Fig. 2). This distorted
cubane is stabilized by extensive hydrogen bond interactions
such as O13–H13AÁ Á ÁO3 [dD–A 2.918(4) Å, oD–HÁ Á ÁA 177(4)1],
O13–H13BÁ Á ÁO4 [dD–A 2.773(4) Å, oD–HÁ Á ÁA 166(4)1], O14–
H14AÁÁÁO4 [dD–A 3.021(5) Å, oD–HÁÁÁA 166(4)1], O14–H14BÁÁÁO3
[dD–A 3.027(4) Å, oD–HÁÁÁA 171(4)1]. The O12 and O15 water
molecules also participate in H-bond contacts, the former linking
the O5-amide oxygen and the O2-carboxylate oxygen as acceptor
atoms, and the latter connecting the O3-carboxylate oxygen and the
O13-water oxygen as acceptor atoms. These interactions lead to the
formation of infinite 3-D networks (Fig. 2A). Relevant hydrogen
bond distances and angles are listed in Table S2 (ESI†).
An interesting feature in 1 is that the –NH and the –CH3
groups of the L12À amido moiety are oriented in a syn con-
formation with respect to the CO–CH2 bond, contrasting with
H2L1 (see above) where such groups are in relative anti position
(Fig. 3A). The syn conformation for such functional group is
Fig. 3 (A) Structural conformation of ligand in complex 1 (syn conformation)
very rare because of steric interaction. In our case the syn
and H2L1 (anti conformation). (B) Weak interactions present in complex 1.
conformation occurs due to weak C–HÁ Á ÁO and C–HÁ Á Áp inter-
actions, the former resulting from the –CH3 group interaction
with a carboxylate oxygen via C11–H11CÁ Á ÁO3 [dD–A 3.527 Å, and that its asymmetric unit contains one Cu2+ ion, one L22À
oD–HÁ Á ÁA 1561] and the latter resulting from the –CH2 group ligand, and four coordinated water molecules (Fig. 4). Simi-
contact with the phenyl group of a vicinal molecule [dD–p 3.451 Å], larly to complex 1, the square pyramidal (t = 0.2) coordination
which may have influenced the larger twisted nature of side sphere around the copper cation is made of an O-atom from
functional arm of the ligand (Fig. 3B). In complex 1, the N–C(O)– the monodentate L22À ligand and four O-atoms from water
C–C torsion angle is 3.781 while in H2L1 is À177.021 which molecules the one in the axial position presenting a higher
indicates the different orientations of the side functional arms Cu–O bond distance resulting from the Jahn–Teller distortion
in these compounds.
effect.
In 2, the non-coordinated carboxylate oxygen atoms O3 and
Single-crystal X-ray diffraction studies reveal that
%
[Cu(L2)(H2O)4] (2) crystallizes in the triclinic P1 space group O4 are involved in hydrogen bond interactions with the O13 and
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Fig. 4 (A) Coordination scheme in complex 2. (B) Schematic representation of complex 2.
typical reaction, a mixture of aldehyde (0.50 mmol), nitroethane
(2.50 mmol), 3 mol% of a Cu-complex catalyst and water con-
tained in a capped glass vessel was stirred at 70 1C for 30 h. After
a specific time, the mixture was centrifuged to remove the solid
catalyst. The filtrate was extracted with dichloromethane. The
organic extracts were collected over anhydrous sodium sulfate;
subsequent evaporation of the solvent gave the crude product,
which was a mixture of the b-nitroalkanol diastereoisomers (syn
and anti forms, with the predominance of the former), which
1
were analyzed using H NMR.
By using benzaldehyde as a test compound, we found that 1
led to a higher conversion than 2 after the same reaction time
and at the same temperature. Consequently, the optimization
of the reaction conditions (temperature, reaction time, amount
of catalyst, solvent) was carried out in a model nitroethane–
benzaldehyde system with 1 as the catalyst precursor (Scheme 2
under typical reaction conditions; Table 1).
When 3 mol% of solid complex 1 is used as the catalyst at
70 1C, a conversion of 77% (syn/anti: 82: 18) of benzaldehyde into
b-nitroalkanol is achieved (entry 5, Table 1) after 30 h. Upon
extending the reaction time to 48 h, the reaction yield decreases
slightly (68%, syn/anti: 81 : 19). With complex 2 a conversion of
70% (syn/anti: 81: 19) was obtained (entry 17, Table 1). The plot
of yield versus time for the Henry reaction of benzaldehyde and
nitroethane with complex 1 is presented in Fig. 6 (first cycle).
Blank tests were carried out with benzaldehyde in the
absence of a catalyst, at 70 1C in water. This test reaction gave
11% conversion of aldehyde into b-nitroalkanol, after a reaction
time of 30 h. We have also checked the reactivity of ligands
H2L1, H2L2 and CuCl2Á2H2O in water medium and the obtained
reaction yields were much lower, i.e., ca. 10% for the ligands,
and 21% for the copper salt (entries 18–21, Table 1).
Fig. 5 (A) Hydrogen bonded networks of complex 2. (B) Node-and-linker-
type descriptions of the 3D hydrogen bonded frameworks in 1 and 2.
O14 water molecules, also forming a distorted close cubane-type
assembly. The hydrogen bond distances and angles involved in
such association are listed in Table S2 (ESI†) together with the
hydrogen contacts pertaining to the other two water molecules.
Additionally, there are C–HÁ Á Áp contact interactions involving
the methyl groups of L2À and the phenyl group of a vicinal
complex molecule, which help to stabilize the hydrogen bonded
3D structure of 2 (Fig. 5A). These interactions present a mini-
mum value of 2.623 Å, and are considered as strong.
We have also carried out topological analysis of the hydro-
gen bonded networks of 1 and 2.30 Both structures have similar
types of 3D hydrogen bonded networks in the lattice and can be
represented as a 6-connected uninodal net (Fig. 5B) with
topological type sxd; 6/3/o1.
We have also tested the effects of temperature, catalyst
amount and solvents. The increase of the amount of catalyst
1 from 1.0 to 3.0 mol% enhances the product yield from 47 to
Catalytic activity of the Cu complexes in the Henry reaction
We have tested the catalytic activity the copper compounds 1
and 2 as solid heterogeneous catalysts in the nitroaldol (or
Henry) reaction of nitroethane with various aldehydes. In a
Scheme 2 Nitroaldol (Henry) reaction.
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Table 1 Optimization of the parameters of the Henry nitroaldol reaction between benzaldehyde and nitroethane with 1 as the catalysta
Paper
Amount of
catalyst (mol%)
Selectivityc
(syn/anti)
Entry
Catalyst
Time (h)
T (1C)
Solvent
Yieldb (%)
TONd
TOFe (hÀ1
)
1
2
3
4
5
6
1
1
1
1
1
1
2
6
8
20
30
48
3.0
3.0
3.0
3.0
3.0
3.0
70
70
70
70
70
70
Water
Water
Water
Water
Water
Water
46
62
66
70
77
68
78 : 22
78 : 22
79 : 21
80 : 20
82 : 18
81 : 19
15.3
20.6
22.0
23.3
25.6
22.6
7.6
3.4
2.7
1.2
0.8
0.5
7
8
9
1
1
1
30
30
30
1.0
5.0
7.0
70
70
70
Water
Water
Water
47
68
71
78 : 22
78 : 22
79 : 21
47
13.6
10.1
1.5
0.4
0.3
10
11
12
1
1
1
30
30
30
3.0
3.0
3.0
70
70
70
CH3CN
THF
MeOH
No reaction
45
58
—
78 : 22
75 : 25
0
15.0
19.3
0
0.5
0.6
13
14
15
16
1
1
1
1
30
30
30
30
3.0
3.0
3.0
3.0
RT (15 1C)
30
50
100
Water
Water
Water
Water
6
24
61
65
83 : 17
77 : 23
78 : 22
79 : 21
2.0
8.0
20.3
21.6
0.06
0.26
0.67
0.7
17
2
30
3.0
70
Water
70
81 : 19
23.3
1.2
18
19
20
21
Blank
30
30
30
30
—
70
70
70
70
Water
Water
Water
Water
11
21
9
79 : 21
82 : 18
78 : 22
76 : 24
—
7.0
—
—
—
0.20
—
CuCl2Á2H2O
3.0
3.0
3.0
H2L1
H2L2
12
—
a
b
Reaction conditions: 3.0 mol% of catalyst 1, benzaldehyde (0.50 mmol), nitroethane (2.5 mmol) and water (2 mL). Number of moles of b-
nitroalkanol per mole of aldehyde  100. Calculated using 1H NMR. Number of moles of b-nitroalkanol per mole of the catalyst. TON per
c
d
e
hour.
and THF, a yield of 58% or 45%, respectively, was obtained
(Table 1, entries 12 and 11). The syn/anti selectivity was also lower
for these solvents than in the case of water.
Ranging the temperature from 15 to 70 1C improved
the yield of b-nitroalkanol from 6 to 77% but a further tem-
perature increase had a negative effect (entries 13–16, Table 1).
The systems exhibit diastereoselectivity towards the syn isomer,
typically leading to syn/anti molar ratios in the 82 : 18 to 75 : 25
range using the nitroethane as substrate. The selectivity in syn
and anti products is an important issue in the Henry reaction
and efforts have been focused on the development of catalytic
diastereo- or enantio-selective processes.31
We also examined the catalytic activity of 1 with different
types of substituted aromatic and aliphatic aldehydes in the reaction
with nitroethane. The results are summarized in Table 2. p-Nitro-
benzaldehyde produced the maximum yield (94%), while the lowest
yield (35%) was obtained for p-methoxybenzaldehyde, but no clear
Fig. 6 Plots of b-nitroalkanol yield vs. time for the Henry reaction of
correlation with the electronic properties of the substituent was
benzaldehyde and nitroethane in the presence of compound 1 as the
observed (e.g. for p-chlorobenzaldehyde, the yield was only 42%).
catalyst.
The yields for aliphatic aldehydes are good (82–85%) and even
higher than that for benzaldehyde.
77%, but further increase in the amount of catalyst leads to a
slight decrease of the reaction yield (entries 7–9, Table 1).
In order to examine the stability of 1 in the Henry reaction, it
was recycled in three consecutive experiments, and it was
To select the most suitable solvent, experiments with various observed that its activity remained almost the same (Fig. 6).
solvents (CH3CN, THF and MeOH, apart from H2O) have been FT-IR and powder X-ray diffraction of catalyst 1 taken before
carried out with complex 1 and the results (Table 1, entries 10–12) and after the reaction indicated that the structure of the solid
indicate that H2O (conversion of 77%) is the best polar solvent, was retained (Fig. 7). Additionally, the filtrated solution, after
whereas the worst one is CH3CN (0% conversion). In methanol the separation of the catalyst, was evaporated to dryness and
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Table
2
Henry reaction of various aldehydes and nitroethane with
catalyst 1a
Yieldb
(%)
Selectivityc
(syn/anti)
TOFe
Aldehyde
TONd
31.3
(hÀ1
)
94
35
42
40
80 : 20
80 : 20
82 : 18
82 : 18
1.0
11.6
14
0.4
0.46
0.44
13.3
61
79 : 21
20.3
0.67
CH3CHO
CH3CH2CHO
85
82
79 : 21
75 : 25
28.3
27.3
0.9
0.9
Scheme 3 Proposed catalytic cycle for the formation of the b-nitroalkanol
in the Henry reaction catalyzed by 1.
a
Reaction conditions: 3.0 mol% of catalyst 1, aldehyde (0.50 mmol),
b
nitroethane (2.5 mmol) and water (2 mL). Number of moles of
b-nitroalkanol per mole of aldehyde  100. Calculated using 1H NMR.
c
our complexes are found to be new, effective, recyclable and
environmentally ‘green’ catalysts for the Henry reaction.
d
e
Number of moles of b-nitroalkanol per mole of the catalyst. TON per
hour.
A possible catalytic cycle for the Henry reaction catalyzed by 1 is
presented in Scheme 3, on the basis of reported proposals.18a,27 The
activation of both the aldehyde and nitroethane by the metal centre
(with deprotonation of the former) is followed by the formation of
the C–C bond upon electrophilic addition leading to the formation
of b-nitroalkanol. The proton abstraction from the nitroalkane
ligand and the protonation of the C–C coupled species can be
assisted by the L1 ligand (with a carboxylate and an amide group)
and also by water (amphoteric behaviour), thus possibly accounting
for the good activity of our catalyst in the presence of water.
the amount of copper determined, being only 0.01% of the
amount used in the reaction, thus ruling out any significant
leaching of the catalyst.
In comparison with other reported heterogeneous cata-
lysts18p,20,32 for the Henry reaction, our catalysts have the
advantages of being rather cheap and easy-to-prepare. More-
over, in most of the reported cases, organic solvents have been
used for this reaction, and only very few examples are known
where water has been applied.26 In our system, interestingly, the
reaction yield and the selectivity are higher in aqueous medium
than in an organic solvent. The use of an aqueous medium has
Conclusion
many advantages owing to the unique properties of water, such The hydrogen bonding and coordination ability of two amido-
as non-toxic, safe and environmentally benign. In that context, terephthalate ligands, 2-propionamidoterephthalate (L12À) and
Fig. 7 (A) FT-IR spectra of compound 1 before (black line) and after (pink line) the Henry reaction. (B) PXRD diffractograms of 1 before (pink line) and
after (green line) the Henry reaction.
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2-acetamidoterephthalate (L22À), bearing different flexible side temperature for 1 h. The precipitate was dissolved upon addi-
functional groups, are presented in this paper. The protonated tion of 0.50 mL of 28% aqueous solution of NH4OH, and the
form H2L1 establishes a 1D hydrogen bonded network with two resulting dark blue solution was sealed in an 8.0 mL glass
DMF molecules, whereas H2L2 is hydrogen bonded with one vessel and heated at 80 1C for 48 h. Subsequent gradual cooling
methanol molecule in a 2D layer structure. Two new isostructural to room temperature (0.2 1C minÀ1) afforded block-like deep
copper(II) complexes (1 and 2) with such ligands were synthesized. blue crystals. Yield: 83% (based on Cu). Anal. calcd for
Their X-ray structure analysis revealed that they are mononuclear
C11H17CuNO9 (M = 370.80): C, 35.63; H, 4.62; N, 3.78. Found:
having extensive hydrogen bonded with coordinated water mole- C, 35.00; H, 5.06; N, 4.11. FT-IR (KBr, cmÀ1): 3315 (bs), 3232
cules. The hydrogen bonded assembly of carboxylate and coordi- (bs), 3281 (bs), 1670 (s), 1631 (s), 1588 (s), 1562 (s), 1523 (m),
nated water molecules form a distorted cubane type network.
1429 (w), 1370 (s), 1347 (s), 1302 (m), 1270 (m), 1250 (m), 1035
These complexes effectively catalyze the Henry reaction of (w), 971 (w), 893 (w), 811 (m), 779 (s), 712 (w), 595 (w), 552 (w).
nitroethane with various aldehydes in aqueous medium produ-
Synthesis of [Cu(L2)(H2O)4] (2)
cing the corresponding b-nitroalkanols in high yields. The use of
water instead of an organic solvent brings a number of benefits A mixture of CuCl2Á2H2O (10 mg, 0.060 mmol) and H2L2
because it is safe, economical, environmentally benign and non- (15 mg, 0.067 mmol) was dissolved in 2.0 mL of DMF. A green
toxic. We have also proved the stability and recyclability of the precipitate was obtained when the mixture was stirred at room
catalysts. Thus, these copper(II) carboxylate complexes can be temperature for 1 h. The precipitate was dissolved upon addi-
considered as ‘green’ catalysts for the preparation of simple nitro tion of 0.50 mL of 28% aqueous solution of NH4OH, the
alcohols. The syn/anti ratio of the nitroaldol products depends resulting dark blue solution was sealed in an 8.0 mL glass
on various factors such as the amount of catalyst, the electro- vessel and heated at 80 1C for 48 h. Subsequent gradual cooling
philicity of the substrates and the reaction conditions.
to room temperature (0.2 1C minÀ1) afforded block-like deep
The above observations provide further evidence that simple blue crystals. Anal. calcd for C10H15CuNO9 (M = 356.77): C,
Cu(II) complexes can be utilized as effective heterogeneous 33.66; H, 4.24; N, 3.93. Found: C, 34.71; H, 4.21; N, 3.76. FT-IR
catalysts in the important types of reactions of this study. (KBr, cmÀ1) 3332 (bs), 3162 (bs), 1649 (s), 1562 (s), 1510 (s),
Further explorations of the uses of this catalyst family in other 1418 (s), 1352 (s), 1298 (s), 1260 (m), 1239 (m), 1016 (w),
organic transformations, as well as mechanistic investigations, 914 (m), 784 (s), 544 (w).
are ongoing.
Procedure for the nitroaldol (Henry) reaction catalyzed by
Cu-complexes
In a typical reaction, a mixture of an aldehyde (0.50 mmol),
nitroethane (2.50 mmol) and Cu-complex catalyst (5.50 mg for 1
and 5.30 mg of 2, 3 mol%) was placed in a capped glass vessel,
and then 2.0 mL water were added into it. The mixture
Experimental
The synthetic work was performed in air and at room tempera-
ture. All the chemicals were obtained from commercial sources
and used as received. The infrared spectra (4000–400 cmÀ1
)
was heated at 70 1C for 30 h, and subsequently quenched by
centrifugation and filtration at room temperature. The filtrate
was extracted with dichloromethane. The organic extracts were
collected over anhydrous sodium sulfate; subsequent evapora-
tion of the solvent gave the crude product. The product was
dissolved in DMSO-d6 and analyzed using 1H NMR. The yield of
the b-nitroalkanol product (relative to the aldehyde) was estab-
lished typically by taking into consideration the relative
amounts of these compounds, as given by 1H NMR and as
previously reported.27a,33 The ratio between the syn and anti
were recorded on a Bruker Vertex 70 instrument in KBr pellets;
abbreviations: s = strong, m = medium, w = weak, bs = broad
and strong, mb = medium and broad. Carbon, hydrogen, and
nitrogen elemental analyses were carried out by the Microanaly-
´
tical Service of the Instituto Superior Tecnico. Thermogravimetric
analysis were carried out using a Perkin-Elmer Instrument system
(STA6000) at a heating rate of 10 1C minÀ1 in a dinitrogen
atmosphere, in the temperature range of room temperature to
ca. 700 1C. Powder X-ray diffraction (PXRD) was conducted on a
D8 Advance Bruker AXS (Bragg Brentano geometry) theta–2theta
diffractometer, with copper radiation (Cu Ka, l = 1.5406 Å) and a
secondary monochromator, operated at 40 kV and 40 mA. Flat
plate configuration was used and the typical data collection range
was between 51 and 401.
1
isomers was also determined by H NMR spectroscopy. In the
1H NMR spectra, the values of vicinal coupling constants
(for the b-nitroalkanol products) between the a-N–C–H and
the a-O–C–H protons identify the isomers, being J = 7–9 or
3.2–4 Hz for the syn or anti isomers, respectively.33
The 1H-NMR spectra and the calculation of the yield and
selectivity for compound 1 in the Henry reaction are presented
in Fig. S2 (ESI†).
Synthesis and characterization of the pro-ligands 2-propion-
amidoterephthalic acid (H2L1) and 2-acetamidoterephthalic
acid (H2L2) are reported in our previous paper.27a
In order to perform the recycling experiment, we first
washed the used catalyst (separated by centrifugation and
Synthesis of [Cu(L1)(H2O)4] (1)
A mixture of CuCl2Á2H2O (10 mg, 0.060 mmol) and H2L1 filtration of the supernatant solution) with methanol and dried
(15 mg, 0.063 mmol) was dissolved in 2.0 mL of DMF. A green it in air. It was then reused for the nitroaldol reaction as
precipitate was obtained when the mixture was stirred at room described above.
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Paper
NJC
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X-ray quality single crystals of the compounds were immersed
in cryo-oil, mounted in a nylon loop and measured at 150 K (1)
or at room temperature (2, H2L1 and H2L2). Intensity data
were collected using a Bruker AXS-KAPPA APEX II or a Bruker
APEX-II PHOTON 100 diffractometer with graphite monochro-
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phi and omega scans of 0.51 per frame and a full sphere of data
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SMART34 software and refined using Bruker SAINT34a on all
the observed reflections. Absorption corrections were applied
using SADABS.34a Structures were solved by direct methods
using the SHELXS-97 package34b and refined using SHELXL-
97.34b Calculations were performed using the WinGX System–
Version 1.80.03.34c The hydrogen atoms attached to carbon
atoms and to the nitrogen atoms were inserted at geometrically
calculated positions and included in the refinement using the
riding-model approximation; Uiso(H) were defined as 1.2Ueq of
the parent nitrogen atoms or the carbon atoms for phenyl and
methylene residues, and 1.5Ueq of the parent carbon atoms for
the methyl groups. The hydrogen atoms of coordinated water
molecules were located from the final difference Fourier map
and the isotropic thermal parameters were set to 1.5 times the
average thermal parameters of the belonging oxygen atoms.
Least square refinements with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic ones
for the remaining atoms were employed. Crystallographic data
are summarized in Table S1 (ESI†) and selected bond distances
and angles are presented in Table S2 (ESI†). CCDC 1005775–
1005778 for 1, 2, H2L1 and H2L2 respectively.
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Acknowledgements
˜
ˆ
This work has been supported by the Fundaçao para a Ciencia e
a Tecnologia (FCT), Portugal (projects PTDC/QUI-QUI/102150/
2008 and PEst-OE/QUI/UI0100/2013). Author A. Karmakar and
S. Hazra express their gratitude to the FCT for post-doctoral
fellowships (Ref. No. SFRH/BPD/76192/2011 and SFRH/BPD/
78264/2011). The authors acknowledge the Portuguese NMR
Network (IST-UL Centre) for access to the NMR facility, and the
IST Node of the Portuguese Network of mass-spectrometry
˜
(Dr Conceiçao Oliveira) for the ESI-MS measurements.
´ˇ
ˇ
M. F. C. G. da Silva, L. Dlhan, R. Boca and A. J. L. Pombeiro,
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