Zinc(ii) ortho-hydroxyphenylhydrazo-β-diketonate complexes and their catalytic ability towards diastereoselective nitroaldol (Henry) reaction

Article abstract of DOI:10.1039/c0dt01457e

The zinc(ii) complexes with ortho-hydroxy substituted arylhydrazo-β- diketonates [Zn₂(CH₃OH)₂(μ-L ¹)₂] (5), [Zn{(CH₃)₂SO}(H ₂O)(L²)] (6), [Zn₂(H<

Full text of DOI:10.1039/c0dt01457e

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PAPER
Zinc(II) ortho-hydroxyphenylhydrazo-b-diketonate complexes and their
catalytic ability towards diastereoselective nitroaldol (Henry) reaction†
Maximilian N. Kopylovich,a Tatiana C. O. Mac Leod,a Kamran T. Mahmudov,a
M. Fa´tima C. Guedes da Silvaa,b and Armando J. L. Pombeiro*a
Received 23rd October 2010, Accepted 8th March 2011
DOI: 10.1039/c0dt01457e
The zinc(II) complexes with ortho-hydroxy substituted arylhydrazo-b-diketonates [Zn2(CH3OH)2(m-L1)2]
(5), [Zn{(CH3)2SO}(H2O)(L2)] (6), [Zn2(H2O)2(m-L3)2] (7) and [Zn(H2O)2(L4)]·H2O (8) were synthesized
by reaction of a zinc(II) salt with the appropriate hydrazo-b-diketone,
HO-2-C6H4-NHN C{C( O)CH3}2 (H2L1, 1), HO-2-O2N-4-C6H3-NHN C{C( O)CH3}2 (H2L2,
2), HO-2-C6H4-NHN CC( O)CH2C(CH3)2CH2C( O) (H2L3, 3) or
HO-2-O2N-4-C6H3-NHN CC( O)CH2C(CH3)2CH2C( O) (H2L4, 4). They were fully characterized,
namely by X-ray diffraction analysis that disclosed the formation of extensive H-bonds leading to 1D
chains (5 and 6), 2D layers (7) or 3D networks (8). The thermodynamic parameters of the Zn(II)
reaction with H2L2 in solution, as well as of the thermal decomposition of 1–8 were determined.
Complexes 5–8 act as diastereoselective catalysts for the nitroaldol (Henry) reaction. The threo/erythro
diastereoselectivity of the b-nitroalkanol products ranges from 8 : 1 to 1 : 10 with typical yields of
80–99%, depending on the catalyst and substrate used.
Introduction
Azoderivatives of b-diketones (ADB, Scheme 1) are versatile
compounds1 which can easily be prepared by diazotization with
subsequent azo-coupling from cheap starting materials (aromatic
amines and b-diketones), but their synthetic use in coordination
chemistry has been underestimated notwithstanding their poten-
Scheme 1 Schematic representations of the studied OHADB.
tially rich chelating ability. In comparison with some widely used
ligands, such as b-diketones or azocompounds, ADB have more
coordinative possibilities, expecting to be able to chelate both
“soft” and “hard” metal ions. The inclusion of the –OH group
at the ortho position of the aromatic ring of phenylhydrazo-b-
diketones creates a family of ADB (herein denoted by OHADB)
that bears one further chelating arm and facilitates the complex
formation2 in comparison with analogous unsubstituted ADB3 or
related pentane-2,4-diones.4 However, only several coordination
compounds with such ligands are known, most of them being
copper complexes.2,5 To our knowledge, no examples of OHADB
complexes with zinc(II) have been reported and hence the prepa-
ration of the first ones is a main aim of the current study.
Even more underexplored is the application of OHADB com-
plexes in catalysis (the use of copper(II)-OHADB complexes in
some oxidation reactions has only been recently reported).2 Hence,
the exploration of the potential of the novel synthesized Zn(II)-
OHADB complexes as catalysts constitutes another important
task. In particular, we have selected the nitroaldol or Henry
reaction6,7 which consists of reacting an aldehyde with nitroalkane
to give b-nitroalkanols (threo and erythro) (Scheme 2). It is one
of the important carbon–carbon bond formation reactions and is
known6c–e to be catalyzed by some zinc(II) complexes with N,O- or
O,O-ligands.
aCentro de Qu´ımica Estrutural, Complexo I, Instituto Superior Te´cnico, TU
Lisbon, Av. Rovisco Pais, 1049–001, Lisbon, Portugal. E-mail: pombeiro@
ist.utl.pt
bUniversidade Luso´fona de Humanidades e Tecnologias, ULHT Lisbon, Av.
Campo Grande n◦ 376, 1749-024, Lisbon, Portugal
† Electronic supplementary information (ESI) available. CCDC reference
numbers 797547–797550. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01457e
Scheme 2 Formation of nitroaldols by the Henry reaction.
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The b-nitroalkanols obtained from this reaction are valuable
starting materials for further synthesis of e.g. 1,2-amino alco-
hols or b-hydroxy acids.6l Usually this reaction is performed
with homogeneous basic catalysts, such as alkali metal hy-
droxides, alkoxides or amines, with a rather good efficiency.6a,b
However, the threo/erythro stereocontrol in the Henry reaction
remains a challenging field, and efforts have been focused on
the development of catalytic diastereo- or enantio-selective pro-
cesses. In particular, it was found that heterobimetallic com-
plexes with lanthanide BINOL (BINOL = 2,2¢-dihydroxy-1,1¢-
binaphthyl) systems,6f a rhodium complex in the presence of
a silyl ketene acetal,6g amberlyst A-21,6h benzyltrimethylammo-
nium hydroxide,6i proazaphosphatranes,6j Mg–Al hydrotalcite,6k,l
guanidines,6m cinchona alkaloid,6n Cu-bis(oxazoline)6o or Co-salen
(salen = N,N¢-bis(salicylidine)ethylenediamine) complexes6p,q are
appropriate enantio-differentiating catalysts for the above trans-
formation. In addition, zinc(II) containing compounds, e.g. Zn(II)
dinuclear complexes6c,d or a mixture of Zn(OTf)2 (OTf = triflate)
and (+)-N-methylephedrine,6e also appear to catalyse such a
reaction. However, yields and diastereoselectivities for most of
the reported systems are rather moderate, and/or expensive
components (e.g. metals such as Nb, La or Rd, chiral ligands
or ionic liquids) are used. Therefore, the introduction of a cheap
catalyst, containing e.g. zinc (a “green” metal), is of interest in this
field.
Results and discussion
Synthesis and characterization of zinc(II) complexes with OHADB
The synthesis and characterization of the OHADB
compounds
3-(2-hydroxyphenylhydrazo)pentane-2,4-dione
(H2L1, 1), 3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione
(H2L2, 2), 5,5-dimethyl-2-(2-hydroxyphenylhydrazo)cyclohexane-
1,3-dione (H2L3, 3) and 5,5-dimethyl-2-(2-hydroxy-4-nitro-
phenylhydrazo)cyclohexane-1,3-dione (H2L4, 4) (Scheme 1) were
reported earlier by us,2 and hence will not be discussed (See
Electronic Supplementary Information†).
Although ZnII interacts with OHADB in solution,3 simple
solvent evaporation from the reaction mixture containing zinc
nitrate and 1–4 gives a mixture of the starting materials, in
contrast to the reactions of OHADB with copper(II) salts that
lead to complexes that can be easily isolated from the reaction
solution.2,5a The studied OHADB are known to have a difficult-
to-destroy intramolecular resonance assisted N–H ◊ ◊ ◊ O hydrogen
bond (RAHB) linking one of the carbonyl groups to the NH-
moiety of the hydrazo unit and further to the hydroxyl group of the
aromatic part of the molecule,2 thus hampering the coordination
to metal ions (Scheme 3a). Apparently, upon concentration of the
solution, the RAHB (destroyed by interaction with the solvent
molecules) regains its structure (Scheme 3b). Clearly, to shift the
overall process to the complex formation, one should remove the
proton by addition of some base but this can lead to hydrolysis
of ZnII with its precipitation as a hydroxide. Hence, firstly one
should quantitatively deprotonate the ligand and then add a ZnII
salt. Thus, the following procedure was used for the synthesis
of the complexes: dissolution of the free ligand in a methanol–
water (9 : 1 v/v) solution containing two equivalents of sodium
(or potassium) hydroxide (Scheme 3c), followed by addition (after
Thus, the main aims of the current work are as follows: (i) to
synthesize the first complexes of OHADB with Zn(II) and study
their properties including structural effects of substituents, and
some physico-chemical parameters of complex formation and
thermal decomposition; (ii) to study the catalytic activity and
diastereoselectivity of the synthesized zinc(II) complexes in the
nitroaldol reaction of nitroethane with various aldehydes.
Scheme 3 Rupture of hydrogen bonds in OHADB and coordination to ZnII.
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5 min) of zinc(II) chloride and concentration of the reaction
mixture (Scheme 3d).
General description of the X-ray crystal structures. Crystals
suitable for X-ray diffraction analysis were obtained upon recrys-
tallization from methanol for [Zn2(CH3OH)2(m-L1)2] (5), dimethyl-
sulfoxide for [Zn{(CH3)2SO}(H2O)(L2)] (6), and water–acetone
(1 : 9, v/v) for [Zn2(H2O)2(m-L3)2] (7) and [Zn(H2O)2(L4)]·H2O
(8) (the schematic representations of their formulae are shown
in Scheme 4). The ellipsoid plots of complexes 5–8 are depicted
in Fig. 1, and representations of their polymeric networks are
depicted in Figures S1 and S2 (Supporting Information).† Selected
bonding parameters, H-bond distances and crystallographic data
are given in Tables S1, S2 and experimental section, respectively.†
The crystal structures of these complexes reveal 1D chains (5 and
6), 2D layers (7) or 3D networks (8). The geometries around the
zinc ions in these complexes can be considered as distorted trigonal
bipyramid with the axial sites occupied by the oxygen atoms of the
chelating ligand. The disparity of the carbonyl moieties, one being
coordinated to the metal and the other free, was confirmed and,
accordingly, the C O bond distances are longer in the former
case (Table S1†).
Scheme 4 Schematic representations of 5–8.
(7) crystallize as binuclear species, the former with two methanol
and the latter with two water ligands (Scheme 4, Fig. 1, Tables S1
and S2†). In both structures, only half of the molecule is in the
asymmetric unit, the metal is in a general position and an inversion
centre is in the centre of a Zn2(m-O)2 core. 1 and 3 act as dianionic
N,O,O-tridentate ligands by means of the ortho-O atom (O1, upon
deprotonation of the ortho-OH group) which bridges two zinc
atoms, one of the nitrogen atoms (N1) of the hydrazone unit and
Description of X-ray crystal structures of compounds 5 and 7.
The complexes [Zn2(CH3OH)2(m-L1)2] (5) and [Zn2(H2O)2(m-L3)2]
Fig. 1 Ellipsoid plots with atom labelling scheme (ellipsoids are drawn at the 50% probability level) for complexes 5–8. For 7, the diagram shows the
structure with disorder model (types A and B atoms). H atoms and crystallization H2O molecule in 8 are omitted for clarity. Symmetry operators for
generating equivalent atoms: 1 - x, 1 - y, 1 - z (5) and 1 - x, 1 - y, -z (7).
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one oxygen atom from a carbonyl group. The pentacoordinated
zinc(II) ions pertain to three different metallacycles: one endo ring
of the Zn2(m-O)2 core, which is the central planar ring of the
molecule, and two fused six- and five-membered exo metallacycles.
The O–Zn–O angle in the central core is larger in 5 than in 7
(81.84(8)◦ and 77.78(9)◦, respectively), while the Zn–O–Zn angle
is smaller in 5 than in 7 (98.16(8)◦ and 102.22(9)◦, respectively).
The intramolecular Zn–Zn distance in 5 is considerably shorter
with L2 is less stable in solution than that with pentane-2,4-dione
(log b = 17.6).4a The negative values of DG◦ (-56.3 kJ mol-1) and
DH◦ (-31.8 kJ mol-1) (Table S4†) indicate that the complexation
is spontaneous and is mainly contributed by the enthalpy factor.
The positive entropy change (82.1 J mol-1 K-1) conceivably relates
to the abstraction of ZnII-coordinated water molecules during the
reaction, i.e. with the increase of the number of free molecules in
the system. By comparing the results obtained herein with those of
previous works,5 we can conclude that the studied metal ions form
complexes with L2 in ethanol–water solution with the following
˚
˚
than in 7 (3.0955(5) A and 3.1551(5) A, respectively), possibly due
to the bulkiness of the b-diketone fragment of 3 in comparison
with 1. Moreover, the structure twisting in 5 is more evident than
in 7; while in 5 the angle between the plane defined by the Zn(m-
O)2Zn core and that defined by the other coordinating atoms is of
49.69◦. In 7, that angle is only of 24.26◦. Cooperative hydrogen
bond networks are present in these complexes (Fig. S1†). In 5,
they involve infinite 1D chains of the zinc dimers connected by
intermolecular O4–H4 ◊ ◊ ◊ O2 interactions [Table S2†; d (D ◊ ◊ ◊ A)
general order of complex stability: FeIII > CuII > UO2 > NiII >
+2
CoII > MnII > ZnII > CdII.
The thermal decomposition of 1–4 and their zinc(II) complexes
was investigated by DTG-DTA thermogravimetric analysis (Fig.
S3, Table S5†). Upon heating of 1–4, no thermal decomposition
occurs until 194 ◦C, then a sharp mass loss appears at ca. 194–
320 ◦C. All the complexes decompose in several stages (Fig. S3†),
the first of them (in the ca. 90–250 ◦C range) being apparently
connected to elimination of the coordinated or hydrated solvent
molecules.
2.629(4) A; ∠(D–H ◊ ◊ ◊ A) 178(5) ◦], supported by less intense
˚
˚
intermolecular C3–H3A ◊ ◊ ◊ O1 contacts [d (D ◊ ◊ ◊ A) 3.313(4) A;
∠(D–H ◊ ◊ ◊ A) 149◦]. Other selected relevant interchain contacts
are established between one hydrogen from the methyl ester
moieties and the six-membered metallacycles of neighbouring
Catalytic activity of the complexes 5–8 in the Henry reaction
◦
˚
chains [d (H3B ◊ ◊ ◊ centroid) 2.88 A; ∠(C3–H3B ◊ ◊ ◊ centroid) 117 ].
In 7, however, the hydrogen bond network leads to 2D layers,
since each coordinated water molecule is doubly bound to the
free carbonyl of two vicinal neighbouring dimers. The shortest
In this second part of the work, we have tested the catalytic activity
of the ZnII complexes 5–8 for the Henry reaction (Scheme 2). The
optimization of the reaction conditions (temperature, reaction
time, amount of catalyst, solvent) was carried out in a model
nitroethane–benzaldehyde system with complex 6 as the catalyst
precursor (Scheme 2, Table 1). The products were mixtures of
b-nitroalkanol diastereoisomers (threo and erythro forms, with
predominance of the former) according to 1H NMR analysis
(see Experimental), and high yields up to 99% (based on the
aldehyde) were achieved at room temperature. Nitromethane is
also a substrate, undergoing a high conversion into the nitroaldol
(99% yield after 5 h reaction catalyzed by 6; entry 34, Table 1).
Control reactions were carried out in the absence of catalyst 6
but in the presence of ZnCl2, Zn(NO3)2 or free ligand 2 (entries
23–32, Table 1). No nitroaldol reaction between benzaldehyde
and nitroethane takes place in the absence of the metal complex,
even in the presence of zinc chloride or zinc nitrate (entries
32, 23, 24, respectively). The use of free ligand 2 (metal-free
conditions) leads, after an extended reaction time of 48 h, to a
yield of 60% of the b-nitroalkanol (entry 31), which, however, is
substantially lower than those obtained in the presence of our
ZnII complexes, in a shorter time (e.g. 98% for 6, 24 h—entry 6
and Fig. S4†). The systems exhibit (Table 1) diastereoselectivity
towards the threo isomer, typically leading to threo/erythro molar
ratios in the 80 : 20–70 : 30 range using nitroethane as substrate.
This selectivity is usually higher than those reported for most
of other systems (Table 2).6,7 The stereochemical control of the
two newly generated carbon centres is difficult, in particular due
to the easy epimerization of the nitro-substituent on the carbon
chain,7u,v but some enantio-differentiating catalysts can overcome
this difficulty.7w,x Our catalysts have the advantages of being rather
cheap and easy-to-prepare. In our systems, the stereoselectivity
decreases significantly after ca. 24 h (entries 1–7, Table 1) whereas
the yield does not increase appreciably after this time (entries 6
and 7, Table 1; Fig. S4†), which, thus, was chosen as the typical
one for most of the experiments, namely those discussed below.
˚
intermolecular Zn ◊ ◊ ◊ Zn separation is of 6.8604(6) A in 5 and
˚
6.6632(7) A in 7.
Description of X-ray crystal structures of compounds 6 and
8. The monomeric complexes [Zn{(CH3)2SO}(H2O)(L2)] (6)
and [Zn(H2O)2(L4)]·H2O (8) are formed by coordination of the
dianions (L2)2- and (L4)2-, respectively (Scheme 4, Fig. 1). In the
structure of 6, the neighbouring zinc units are positioned in relative
˚
proximity to each other (Zn ◊ ◊ ◊ Zn separation of 5.060 A) as a
result of their linkage via strong H-bonds (Fig. S2 and Table S2†)
between one of the hydrogen water molecules of one unit and
the coordinated phenolate oxygen of a vicinal one [d(D ◊ ◊ ◊ A)
˚
of 2.542(6) A], and are further assembled via slightly weaker
hydrogen bonds involving the other hydrogen of the coordinated
water molecule and the uncoordinated carbonyl oxygen moiety
˚
[d(D ◊ ◊ ◊ A) of 2.687(8) A]. These interactions give rise to the
generation of infinite 1D chains. The units of 8, however, display
multiple H-bonds also involving the crystallization water molecule
(Fig. S2†) thus leading to the formation of a 3D network. As a
result of such contacts, the Zn ◊ ◊ ◊ Zn distance in 8 is the shortest
˚
one found in this work [4.835(3) A]. It is also noteworthy to
mention that both 6 and 8 have two labile ligands (water and
dimethylsulfoxide) which can be of significance for their catalytic
application (see below).
Complex formation in solution and thermal decomposition. The
constant of complex formation in solution is an important
parameter for various applications,3 and we have evaluated it for
the reaction of ZnII with 2 by pH-metric titration of a mixture
of zinc(II) nitrate and 2, in aqueous ethanol solution, using
the Martell–Chaberek method8a and well known relationships
for thermodynamic functions8b (details are given in Supporting
Information, Tables S4 and S5†). The stability constant is found to
be log k = 9.86 0.04 (Table S3†), showing that the zinc(II) complex
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Table 1 Optimization of the parameters of the Henry nitroaldol reaction between benzaldehyde and nitroethane with 6 as the catalyst precursora
Amount of
catalyst (mol%)
Selectivity
Entry
Catalyst
Time (h)
T (◦C)
Solvent
Yield (%)b
threo/erythroc
TOFd
1
2
3
4
5
6
7
6
6
6
6
6
6
6
0.5
1
3
3.01
3.01
3.01
3.01
3.01
3.01
3.01
20
20
20
20
20
20
20
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
32
43
48
55
68
98
98
78 : 22
78 : 22
78 : 22
77 : 23
76 : 24
76 : 24
70 : 30
22
14
5.4
3.6
2.3
1.4
0.7
5
10
24
48
8
9
10
11
12
6
6
6
6
6
24
24
24
24
24
0.03
0.27
0.82
1.37
5.48
20
20
20
20
20
MeOH
MeOH
MeOH
MeOH
MeOH
24
35
90
94
97
67 : 33
72 : 28
72 : 28
73 : 27
76 : 24
37
5.3
4.6
2.9
0.7
13e
14
15
16
17
6
6
6
6
6
24
24
24
24
24
3.01
3.01
3.01
3.01
3.01
20
20
20
20
20
—
THF
DMF
CH3CN
MeOH+H2O
(1.5/0.1 v/v)
MeOH+H2O
(1.0/0.5 v/v)
82
13
99
82
73
71 : 29
71 : 29
80 : 20
74 : 26
71 : 29
1.1
0.2
1.4
1.1
1.0
18
6
24
3.01
20
71
71 : 29
1.0
19
20
21
22
6
5
5
5
0.5
48
48
0.5
1
3
5
10
24
48
48
5
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
3.01
—
20
50
80
80
20
20
20
20
20
20
20
20
20
20
20
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
55
74
82
55
—
—
11
16
18
29
35
57
60
—
99
72 : 28
72 : 28
70 : 30
58 : 42
—
3.6
4.9
5.5
37
—
—
7.0
5.2
1.9
1.9
1.2
0.8
0.4
—
6
6
6
23f
24f
25f
26f
27f
28f
29f
30f
31f
33f
34g
ZnCl2
Zn(NO3)2
—
2
77 : 23
76 : 24
77 : 23
77 : 23
76 : 24
76 : 24
77 : 23
—
2
2
2
2
2
2
Blank
6
3.01
—
6.6
a Reaction conditions: 0.03–5.48 mol% (0.27–54.8 mmol) of catalyst precursor (typically 3.01 mol%), methanol (2 mL), nitroethane (4 mmol) and aldehyde
(1 mmol). b Determined by 1H NMR analysis (see Experimental). c Calculated by 1H NMR. d TOF (h-1) was estimated as moles of products (threo- and
erythro-b-nitroalkanol)/mol of catalyst per hour. e Methanol (solvent)-free conditions, using nitroethane as solvent (2 mL). f For comparison purposes.
g Nitromethane was used as substrate.
The rise of the catalyst amount greatly enhances the product
yield from 24 to 97% for the respective amounts of 0.03 and 5.48
mol% of catalyst 6, but with a decrease of the overall TOF from
37 to 0.7 (entries 8–12, Table 1). In general, 6 is highly effective
even at low catalyst loadings, e.g. for 8.2 mmol i.e. 0.82 mol % vs.
substrate loading, a TOF of 4.6 h-1 with an yield of 90% were
reached (entry 10), what is an excellent result if compared with
other zinc catalysts described in the literature, where 5–20 mol
% loadings were commonly used to reach yields and selectivities
comparable with ours (Table 2).6c,7b,c
Further, we examined the effect of solvents (entries 6 and 13–
18, Table 1). THF falls below all the others, giving only 13% total
yield which conceivably relates to the low solubility of 6 in this
solvent. In all the other tested solvents, the yields are comparable
and almost quantitative for MeOH and DMF (entries 6 and
15). If the nitroethane reagent was used as solvent (solvent-free
conditions, entry 13) a lower but still considerable yield (82%) was
achieved. The use of mixed methanol–water solvents disfavour the
reaction (entries 17, 18), which relates with other data on nitroaldol
reactions.7d The polar solvents DMF or MeOH facilitate the
formation of threo-diastereomers (entries 6 and 15). On the other
hand, increasing the temperature in the 20–80 ◦C range improves
the yield of b-nitroalkanol from 55 to 82% for 5 h reaction time
(entries 19–21).
The reactions of a variety of para- or ortho-substituted aromatic
and aliphatic aldehydes with nitroethane (Table 3; 6 was used as
the catalyst precursor) were also studied and shown to provide
the respective b-nitroalkanols with yields ranging from 28 to 99%
and with selectivities up to ca. 90% for either the threo or the
erythro isomer. The nature of substrates greatly influences the
yields and selectivities. Thus, for aryl aldehydes, those bearing
electron-withdrawing groups exhibit higher reactivities compared
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Table 2 Comparison of activities of catalysts towards the Henry reaction using aldehyde and nitroethane or nitromethane
Catalysta
Solvent
Aldehyde
Yield (%)
Selectivity threo/erythro
Reference
6
MeOH
THF
THF
—
CH2Cl2
—
THF
THF
THF
THF
CH3OH
CH3CN
THF
THF
THF
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
2-methylpropanal
p-(NO2)(C6H4CHO)
p-(NO2)(C6H4CHO)
p-(NO2)(C6H4CHO)
98
48
85
24
92
14
62
80
90
75
40
90
94
85
95
76 : 24
72 : 28
92 : 8
60 : 40
60 : 40
50 : 50
40 : 60
55 : 45
ndn
This work
6h
La/Na/amideb
Nd/Na/amidec
6h
6i
[{Rh(C5Me5)Cl}2(m-Cl)2]d
Copper(II) Schiff basee
Guanidine-ionic liquidf
Mg:Al hydrotalcite
Zn Brucineg
7p
7o
6n
7o
7p
7d
7r
7s
7t
Zn-Fam Catalysth
Phenoxide substituted Cu complexesi
Zn-ethylenediaminej
Zn-guanidinesk
ndn
ndn
ndn
Dentrimerl
46 : 54
43 : 57
44 : 56
7t
Triethylamine
DIPEAm
7t
a Composition is given in Table S7, see Supporting information.† b Lanthanum (La(OiPr)3)/sodium source/amide ligand heterobimetallic catalyst.
c Neodymium (Nd(OiPr)3)/sodium source/amide ligand heterobimetallic catalyst. d Rh complex in the presence of a silyl ketene acetal. e Chiral
binuclear Cu(II) Schiff base complexes. f 1,1,3,3-tetramethyl guanidine (TMG)-based ionic liquid. g Brucine-derived aminoalcohol. h Ferrocenyl-substituted
aziridinylmethanol (Fam) was used as a catalyst with Zn. i Phenoxide substituted Cu complexes. j Mono ethylenediamine Zn complex. k Zn(II) with chiral
guanidine ligands. l Tertiary amines dendritically encapsulated within a flexible peptidic shell. m N,N-diisopropyl ethylamine. n nd: not determined, because
nitromethane was used as substrate.
to those having electron-donating moieties (entries 6 and 7 vs.
2–5, Table 3), which relates with the increased electrophilicity of
the substrate with the former substituents. In the case of 2,4,6-
trimethylbenzaldehyde, no selectivity and a low yield are observed,
probably due to steric hindrance, whereas 2-methylbenzaldehyde
shows a high selectivity (75% for threo isomer) with a good yield
(85%) (entry 5, Table 3).
The linear aliphatic aldehydes (entries 10–14, Table 3) lead
to higher selectivities for the erythro diasteroisomer than the
aromatic ones, and the highest yield (99%), within the aliphatic
aldehydes, is observed for the shortest ones (entries 10, 11).
Furthermore, the diastereoselectivity towards the threo isomer and
the yield are markedly improved in PhCH CHCHO relative to
PhCH2CH2CHO (entries 8 and 9), the conjugation of the aldehyde
group with the aromatic phenyl ring in the former possibly playing
a role.
For comparison, all the synthesized Zn-OHADB complexes
were tested as catalysts in the Henry addition of nitroethane and
benzaldehyde (Table 4). All the catalysts preferentially produce
the threo isomer, with variable degrees of diastereoselectivity, and
high yields of the b-nitroalkanol mixture. The activities of the
monomeric complexes 6 and 8 are similar (product yields up to
97%, Table 4, entries 2 and 4) and higher than those of the dinuclear
ones 5 and 7 (entries 1 and 3, Table 4). The lowest activity of
catalyst 7 may be accounted for by its lower solubility, since the
related complex 5, with a higher solubility, leads to an higher yield
of b-nitroaldol. The higher erythro-selectivity of 7 in comparison
with 5 is in accord with an earlier study where the selectivity could
be promoted by steric hindrance within the bicyclic transition state
using heterobimetallic complexes with a rare earth metal.7e,f Under
solvent-free conditions, the decrease in the b-nitroalkanol yield
compared to what achieved in methanol points out the possible
role of the protic solvent to facilitate the proton transfer in the
nitroaldol reaction (see below).
For example, Cu-catalyzed reactions are known to proceed with
involvement of monometallic forms of the active species,7g–m
whereas Zn-catalyzed reactions can involve either mononuclear6s
or di- or polynuclear6c,e complexes. However, considering the
higher efficiency of the mononuclear zinc complexes, in our case
the reaction is supposed to proceed with the participation of
such a type of compound. This is corroborated by the ESI-
MS+ detection, in the reaction medium, of monomeric species
formed upon dissociation of the Zn-OHADB dimers 5 and 7.
The data suggest that the same Zn-OHADB Lewis acid centre
activates both nitroethane (increasing its acidity) and aldehyde
(increasing its electrophilic character), as shown in the proposed
Scheme 5. In fact, both intermediates with benzaldehyde (a) and
benzaldehyde + nitroethane (b or derived c or d) were detected.
The OHADB ligand can function as a Brønsted base, promoting
the deprotonation of the acidic nitroethane with formation of
the reactive nitronate species (c) (step 2) which adds to the ligated
benzaldehyde via a nucleophilic intramolecular attack with forma-
tion of a C–C bond to give an intermediate with a b-nitroalkanol
(d) (step 3).7b,x The deprotonation of nitroalkane and the C–C
bond formation are crucial, and the Lewis acid and Brønsted
base interactions of the zinc centre with the substrates allow
a high efficiency of the overall process.7b Subsequently, another
benzaldehyde coordinates, leading (step 4) to the formation of the
intermediate (e) (which was also detected by ESI-MS+). Release
of the b-nitroalkanol (step 5) would complete the catalytic cycle.
The linear plot of ln[benzaldehyde] vs. time (Table S6, Figure
S5†) indicates a first-order dependence and thus supports the
proposed mechanism. The reaction was also monitored by UV-
vis spectroscopy (Figure S6 and S7†) but the results were not
conclusive.
Conclusions
A noteworthy aspect of the Henry reaction is the possible
involvement of metallic complexes with different nuclearities.
We have achieved simple and effective syntheses of mono- and
dinuclear 5-coordinate zinc(II) complexes with OHADB ligands,
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Table 3 Henry reaction of various aldehydes and nitroethane with
catalyst 6a
Table 4 Catalytic activity of various Zn(II)-OHADB complexes in the
Henry reactiona
Selectivity
Selectivity
Entry
1
Substrate
Yield (%)b
98
threo/erythroc
Entry
Catalyst
Solvent
Yield (%)b
threo/erythroc TOFd
70 : 30
1
2
3
5
6
7
8
5
6
7
8
MeOH
MeOH
MeOH
MeOH
—
—
—
81
98
71
99
97
82
10
83
75 : 25
70 : 30
55 : 45
75 : 25
75 : 25
71 : 29
54 : 46
67 : 33
1.9
1.4
1.8
1.6
2.3
1.1
0.3
1.3
4
5e
6e
7e
8e
2
34
50 : 50
—
a Reaction conditions: catalyst precursor: 5 (1.75 mol%), 6 (3.00 mol%),
7 (1.61 mol%), 8 (2.60 mol%), methanol (2 mL), nitroethane (4 mmol)
and benzaldehyde (1 mmol), reaction time: 24 h. b Determined by 1H
NMR, based on the starting benzaldehyde. c Calculated by 1H NMR (see
Experimental). d TOF (h-1) was estimated as moles of products (threo- and
erythro-b-nitroalkanol)/mol of catalyst per hour. e Methanol (solvent)-free
conditions, using nitroethane as solvent (2 mL).
3
4
5
32
28
85
75 : 25
77 : 23
75 : 25
6
7
8
9
96
89
89
66
63 : 37
79 : 21
89 : 11
10 : 90
Scheme 5 Proposed catalytic cycle for the Henry reaction catalyzed by
2
3
the complexes 5–8. [Zn] = Zn(L) (L = h or h -L); a, b (or c or d) and e were
detected by ESI-MS(+) [m/z = 437 (and 455 for the corresponding complex
with ligated H2O), 513 and 619, respectively, for the case of catalyst 6].
10
11
12
13
14
CH3CH2CHO
99
99
92
86
83
9 : 91
CH3(CH2)2CHO
CH3(CH2)3CHO
CH3(CH2)4CHO
CH3(CH2)5CHO
34 : 66
25 : 75
14 : 86
10 : 90
sponding b-nitroalkanols in high yields and diastereoselectivities.
The threo/erythro diastereoselectivity of the nitroaldol products
can be controlled over a considerably wide range, depending on the
catalyst, the substrates and experimental conditions. Comparison
of the activities of the Zn-OHADB complexes with those of other
catalysts indicates that the former have a promising potential
towards the Henry diastereoselective transformation, being, to
our knowledge, among the most active and diasteroselective
ones so far reported for the nitroaldol reaction under mild
conditions.
a Reaction conditions: 3.01 mol% of catalyst 6, methanol (2 mL),
nitroethane (4 mmol) and aldehyde (1 mmol), reaction time: 24 h.
b Determined by 1H NMR analysis (see Experimental). c Calculated by
1H NMR.
which appear to be the first reported examples of Zn-OHADB
complexes. Their structure and nuclearity are dependent on
substituents in the b-diketone and aromatic parts of the OHADB
ligands. The complexation process in solution is spontaneous,
exothermic and entropically favourable, but in the solid state
the formation of strong ligand hydrogen bonds leads to ex-
tensive 1D chains (5 and 6), 2D layers (7) or 3D networks
(8).
Experimental section
General comments
The Zn-OHADB complexes are effective catalysts for the
addition of nitroethane to various aldehydes producing the corre-
All the synthetic work was performed in air and at room
temperature. All the chemicals were obtained from commercial
5358 | Dalton Trans., 2011, 40, 5352–5361
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sources and used as received. The infrared spectra (4000–400 cm-1)
were recorded on a BIO-RAD FTS 3000 MX instrument in KBr
1H NMR (300.13 MHz, CD3OD), d: 2.21 (s, 3H, CH3), 2.31 (s,
1
3H, CH3), 7.21–7.72 (3H, Ar–H). 13C{ H} NMR (75.468 MHz,
1
pellets. The H and 13C NMR spectra were recorded at ambient
CD3OD), d: 25.23 (CH3), 30.85 (CH3), 38.11 (2CH3), 109.91
temperature on a Bruker Avance II + 300 (UltraShieldTM Magnet)
spectrometer operating at 300.130 and 75.468 MHz for proton
and carbon-13, respectively. The chemical shifts are reported in
ppm using tetramethylsilane as the internal reference. Carbon,
hydrogen, and nitrogen elemental analyses were carried out by
the Microanalytical Service of the Instituto Superior Te´cnico.
Electrospray mass spectra (ESI-MS) were run with an ion-trap
instrument (Varian 500-MS LC Ion Trap Mass Spectrometer)
equipped with an electrospray ion source. For electrospray ion-
ization, the drying gas and flow rate were optimized according to
the particular sample with 35 p.s.i. nebulizer pressure. Scanning
was performed from m/z 100 to 1200 in methanol solution. The
compounds were observed in the positive mode (capillary voltage =
80–105 V). Thermal properties were analyzed with a Perk◦in-Elmer
Instrument system (STA6000) at a heating rate of 10 C min-1
under a dinitrogen atmosphere.
The acidity of the solutions was measured using an I-130
potentiometer with an ESL-43-07 glass electrode and an EVL-
1M3.1 silver–silver chloride electrode. To maintain the required
pH, a commercial volumetric concentrate of HCl (pH 1–2) and
an ammonium acetate buffer solution (pH 3–10) were used. In
aqueous solutions of the salts, the concentrations of metal ions
were determined by atomic absorption spectroscopy.9a The pH-
metric titration of mixtures of reagent solutions with ZnCl2 were
carried out in aqueous-ethanol solution with consideration of the
Bates correction9b at temperatures of 298 0.5, 308 0.5, and 318
0.5 K. A constant temperature was maintained within 0.5 K
by using an ultrathermostat (Neslab 2 RTE 220).
(CAr–H), 114.22 (CAr–H), 115.71 (CAr–H), 134.28 (C N), 135.12
(CAr–NO ), 139.01 (CAr–NZn–N), 144.01 (CAr–OZn), 189.10 and 197.33
2
(C O).
[Zn2(H2O)2(m-L3)2] 7: Orange powder soluble in acetone,
methanol, ethanol, acetonitrile, dimethylformamide and DMSO.
Anal. Calcd for C28H32N4O8Zn2 (M = 683.4): C, 49.21; H, 4.72; N,
8.20. Found: C, 49.09; H, 4.73; N, 8.10. IR, cm-1: 1592 n(C O),
1
1567 n(C O), 1503 n(C N). MS (ESI): m/z: 684 [M+H]+. H
NMR (300.13 MHz, DMSO-d6), d: 1.04 (s, 6H, CH3), 2.36 (s,
1
2H, CH2), 2.49 (s, 2H, CH2), 6.46–7.51 (4H, Ar–H). 13C{ H}
NMR (75.468 MHz, DMSO-d6), d: 20.10 (CH3), 30.70(CH3),
51.28 (CH2), 51.81 (CH2), 38.9 (Cipso), 114.50 and 119.74 (CAr–H),
129.89 (CAr–NZn–N), 130.43 (C N), 140.11 (CAr–OZn), 185.31 and
192.89 (C O).
[Zn(H2O)2(L4)]·H2O 8: Red powder soluble in acetone,
methanol, ethanol, acetonitrile, dimethylformamide and DMSO.
Anal. Calcd for C14H19N3O8Zn (M = 422.7): C, 39.78; H, 4.53; N,
9.94. Found: C, 39.45; H, 4.47; N, 9.87. IR, cm-1: 1655 n(C O),
1637 n(C O), 1604 n(C N). MS (ESI): m/z: 406 [M–H2O +
H]+. 1H NMR (300.13 MHz, DMSO-d6), d: 1.03 (s, 6H, CH3), 2.40
1
(s, 2H, CH2), 2.49 (s, 2H, CH2), 7.25–7.60 (3H, Ar–H). 13C{ H}
NMR (75.468 MHz, DMSO-d6), d: 28.16 (CH3), 30.69 (CH3),
51.60 (CH2), 51.92 (CH2), 38.77 (Cipso), 108.32, 112,92 and 114.30
(CAr–H), 131.82 (C N), 146.24 (CAr–NO ), 147.63 (CAr–NZn–N), 162.57
2
(CAr–OZn), 187.73 and 193.37 (C O).
X-ray structure determinations
The X-ray quality single crystals of complexes 5–8 were immersed
in cryo-oil, mounted in a Nylon loop and measured at a
temperature of 150 K or 298 K (6), (Table 5). Intensity data were
collected using a Bruker AXS-KAPPA APEX II diffractometer
with graphite monochromated Mo-Ka (l 0.71073) radiation.
Data were collected using omega scans of 0.5◦ per frame and full
sphere of data were obtained. Cell parameters were retrieved using
Bruker SMART software and refined using Bruker SAINT10a on
all the observed reflections. Absorption corrections were applied
using SADABS.10a Structures were solved by direct methods by
using the SHELXS-97 package10b and refined with SHELXL-
97.10c Calculations were performed using the WinGX System-
Version 1.80.03.10d All hydrogens were inserted in calculated
positions. Least square refinements with anisotropic thermal
motion parameters for all the non-hydrogen atoms and isotropic
for the remaining atoms were employed. There was some disorder
in the structure of 7 with carbon atoms C13, C14, C17 and C18
having inadequate thermal parameters. The occupancies of the
disordered carbon atoms (in Fig. 1 notations A and B represent
the disordered positions of those atoms) were determined by using
the ‘‘free variable’’ (FVAR) option in SHELXL software,10c which
means that occupancy was allowed to vary during the refinement.
A value of 0.41 as an average for FVAR was obtained and the
R-value improved in 10%. CCDC 797547–797550 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Preparation of Zn complexes and their characterization
To 50 mL of a methanol–water (9 : 1 v/v) solution of 1–4
(0.2 mmol), 0.4 mmol KOH were added and dissolved. After 5 min,
0.2 mmol ZnCl2 were added. The mixture was stirred under solvent
refluxing for 5 min and then left for slow solvent evaporation. The
solid (greenish–grey 5, dark red 6 and 8, or orange 7) was filtered
off after 5 d, washed with a small amount of methanol and dried in
air. Crystals suitable for X-ray diffraction analysis were obtained
upon recrystallization from methanol (5), dimethylsulfoxide (6)
and water–acetone (1 : 9 v/v) (7 and 8) (41, 47, 44 and 58% yields,
respectively, based on Zn).
[Zn2(CH3OH)2(m-L1)2] 5: Greenish–grey powder soluble in
acetone, methanol, ethanol, acetonitrile, dimethylformamide and
DMSO. Anal. Calcd for C24H28N4O8Zn2 (M = 631.24): C, 45.66;
H, 4.47; N, 8.87. Found: C, 45.15; H, 4.18; N, 8.80. IR, cm-1:
1629 n(C O), 1595 n(C O), 1521 n(C N). MS (ESI): m/z: 632
[M+H]+. 1H NMR (300.13 MHz, DMSO-d6), d: 2.16 (s, 3H, CH3),
1
2.37 (s, 3H, CH3), 6.00–7.63 (4H, Ar–H). 13C{ H} NMR (75.468
MHz, DMSO-d6), d: 27.96 (CH3), 30.72 (CH3), 113.89 (CAr–H),
114.73 (CAr–H), 115.61 (CAr–H), 119.28 (CAr–H), 127.07 (C N),
132.99 (CAr–NZn–N), 140.97 (CAr–OZn), 187.70 and 197.13 (C O).
[Zn{(CH3)2SO}(H2O)(L2)] 6: Red powder soluble in acetone,
methanol, ethanol, acetonitrile, dimethylformamide and DMSO.
Anal. Calcd for C13H17N3O7SZn (M = 424.74):C, 36.76;H, 4.03; N,
9.89. Found: C, 36.45; H, 3.98; N, 9.74. IR, cm-1: 1655 n(C O),
1628 n(C O), 1600 n(C N). MS (ESI): m/z: 426.0 [M+H]+.
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Table 5 Crystal data and structure refinement details for 5–8
5
6
7
8
Formula unit
C24H28Zn2N4O8
631.24
C13H17ZnN3O7S
424.73
C28H32Zn2N4O8
683.32
C14H19ZnN3O8
422.69
Monoclinic
Formula weight (g mol-1)
Crystal system
Triclinic
Triclinic
Triclinic
¯
¯
¯
Space group
P1
P1
P1
P 21/c
˚
a (A)
8.0218(4)
8.8006(3)
10.3280(5)
93.178(2)
103.423(3)
111.763(2)
1
8.782(6)
10.621(9)
10.703(9)
115.83(2)
106.16(2)
90.23(3)
2
6.8688(3)
9.8233(5)
11.9184(6)
106.825(2)
105.369(3)
95.625(2)
1
6.5844(11)
15.743(3)
16.287(3)
90.00
96.006(10)
90.00
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
Z
4
3
˚
Volume (A )
650.53(5)
1.611
1.899
6173
2320
0.0415
0.0337, 0.0734
0.969
854.0(12)
1.652
1.602
4652
2859
0.0813
0.0603, 0.0797
0.839
728.81(6)
1.557
1.701
4919
2601
0.0283
0.0359, 0.0819
1.049
1679.0(5)
1.672
1.513
8485
2936
0.1851
0.1032, 0.2737
1.086
Dc (g cm-3)
m (Mo-Ka) (mm-1)
Reflections collected
Unique reflections
Rint
Final R1a, wR2 (I ≥ 2s)
b
GOF on F2
2
a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = [R[w(Fo - Fc2)2]/R[w(Fo2)2]]1/2
Catalytic activity studies
express gratitude to the FCT for their post-doctoral fellowships.
The authors acknowledge the Portuguese NMR Network (IST-
UTL Centre) for access to the NMR facility, and IST Node of
the Portuguese Network of mass-spectrometry (Dr Conceic¸a˜o
Oliveira) for the ESI-MS measurements.
A typical reaction was carried out under air as follows: to 0.03–5.48
mol% (0.27–54.8 mmol) of catalyst precursor (typically 3.01 mol%)
contained in the reaction flask were added methanol (2 mL),
nitroethane (4 mmol) and aldehyde (1 mmol), in that order.
The reaction mixture was stirred for the required time at room
temperature and air atmospheric pressure. After evaporation of
the solvent, the residue was dissolved in DMSO-d6 and analyzed
by 1H NMR. The yield of b-nitroalkanol product (relatively to the
aldehyde) was established typically by taking into consideration
the relative amounts of these compounds, as given by 1H NMR and
previously reported.7 The adequacy of this procedure was verified
by repeating a number of the 1H NMR analyses in the presence of
1,2-dimethoxyethane as internal standard, added to the DMSO-d6
solution, which gave yields similar to those obtained by the above
method. Moreover, the internal standard method also confirmed
that no side products were formed. The ratio between the threo and
erythro isomers was also determined by 1H 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 threo or erythro isomers, respectively.6l,7a For monitoring the
reaction along the time, samples were withdrawn at specific time
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This work has been partially supported by the Foundation for Sci-
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5360 | Dalton Trans., 2011, 40, 5352–5361
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