Synthesis and characterisation of Cu(II) complexes bearing N,N′-di(methoxybenzyl)-(R,R)-1,2-diaminocyclohexane as catalysts for the asymmetric nitroaldol reaction

Article abstract of DOI:10.1016/j.poly.2013.09.006

Enantiopure Cu(II) complexes with N,N′-di(p-methoxybenzyl)-(R,R)-1,2- diaminocyclohexane (L¹), N,N′-di(m-methoxybenzyl)-(R,R)-1,2- diaminocyclohexane (L²), and N,N′-di(m-methoxybenzyl)-(R,R)-1, 2-diaminocyclohexane (L³) li

Full text of DOI:10.1016/j.poly.2013.09.006

Polyhedron 67 (2014) 264–269
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Polyhedron
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Synthesis and characterisation of Cu(II) complexes bearing
0
N,N -di(methoxybenzyl)-(R,R)-1,2-diaminocyclohexane as
catalysts for the asymmetric nitroaldol reaction
⇑
Sang Eun Song, Quang Trung Nguyen, Jeong Jae Yu, Hong-In Lee, Jong Hwa Jeong
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Taegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
0
1
0
Article history:
Received 25 July 2013
Accepted 11 September 2013
Available online 21 September 2013
Enantiopure Cu(II) complexes with N,N -di(p-methoxybenzyl)-(R,R)-1,2-diaminocyclohexane (L ), N,N -
2
0
di(m-methoxybenzyl)-(R,R)-1,2-diaminocyclohexane (L ), and N,N -di(m-methoxybenzyl)-(R,R)-1,2-
diaminocyclohexane (L ) ligands were synthesised and characterised. The X-ray crystal structures of
Cu(II) complexes containing L and L were determined. The geometry around the Cu(II) centre for
2 2
L )CuCl was distorted square-planar. The crystal structure of (L )CuCl revealed a square-pyramidal
structure with a strong interaction between the Cu and O of the methoxy group. Their double catalytic
activation with diisopropylethylamine in the asymmetric nitroaldol reaction of benzaldehyde and nitro-
methane was examined. The reaction yield was moderate with an enantiomeric excess of up to 87%.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
3
1
3
1
3
(
Keywords:
Double catalysis
0
N,N -Di(methoxybenzyl)-(R,R)-1,2-
diaminocyclohexane
Enantiopure Cu(II) complex
X-ray structure
Asymmetric nitroaldol reaction
1
. Introduction
were not reported. Double catalytic activation is an important op-
tion for asymmetric syntheses. There have been few studies on the
Bifunctional catalysts that contain two reaction centres com-
prising a Lewis acid and a base are used in asymmetric synthesis
1–7]. The new concept of double catalytic activation was devel-
Cu(II) complexes of these enantiopure ligands. We reported the re-
sults of asymmetric nitroaldol reactions between benzaldehyde
and nitromethane when (N,N -dibenzyl-(R,R)-1,2-diaminocyclo-
0
[
oped recently [8]. Transition metal complexes acting as Lewis acids
are often not powerful enough to form bonds through the single
activation of nucleophiles; deprotonation with an amine base of
nucleophilic precursors is needed to activate the reaction. Some
double catalytic systems containing an enantiopure Lewis acid
complex and an amine base were successfully used for the asym-
2
hexane)CuCl and accompanying organic bases were used as dou-
ble catalytic systems [23]. It is likely that modification of the
0
aromatic rings of the N,N -dibenzyl-(R,R)-1,2-diaminocyclohexane
(L) ligand by different small substituents, thereby changing the
electronic and steric properties of the ligand, would alter the orbi-
tals at the metal centre and thus affect its electronic and geometric
properties. Small changes in the electron-donating or -withdraw-
ing ability of the ligand and the size of its substituents can have
a dramatic effect on the catalytic efficiency and enantioselectivity
in catalytic asymmetric syntheses [24]. The enantioselective pref-
erence depends on the position of the methoxy groups on the phe-
metric nitroaldol reaction. These systems include Cu(OTf)
bis(oxazoline)/Et [9], Cu(OTf) -bis(imidazoline)/Et [10],
Cu(OTf) -bis(thiazoline)/Et N [11], Zn(OTf) -aminoalcohol/(i-Pr)2-
EtN [12], CuCl N [13], Cu(OAc) -iminopyridine/
-(borabox)/Et N [15].
2
-
3
N
2
3
N
2
3
2
2
-(ꢀ)sparteine/Et
3
2
(
i-Pr)
2
EtN [14], and Cu(OTf)
2
3
0
N,N -diaryl-(R,R)-1,2-diaminocyclohexanes have been used as
nyl rings; thus, we continue to develop CuCl
2
complexes bearing
0
ligands in a variety of asymmetric synthesis reactions. These in-
clude enantioselective reductions [16,17], allylation [18], Michael
additions [19], aldehyde crotylation [20], and aza-Henry reactions
N,N -di(methoxybenzyl)-(R,R)-1,2-diaminocyclohexane ligands as
double catalysts with an organic base, diisopropylamine, for asym-
metric nitroaldol reactions.
[
2
21]. Recently, these ligands were used with Cu(OAc) as bifunc-
tional catalysts for the asymmetric Henry (nitroaldol) reaction;
good yields and a high enantiomeric excess (ee) of up to 94% were
reported [22]. However, the X-ray structures of the complexes
2
. Experimental
2.1. General considerations and materials
⇑
Corresponding author. Tel.: +82 53 950 6343; fax: +82 53 950 6330.
E-mail address: jeongjh@knu.ac.kr (J.H. Jeong).
All reagents were of the reagent grade and solvents were used
as received from commercial supplies. (R,R)-1,2-diaminonium
0
277-5387/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.09.006

S.E. Song et al. / Polyhedron 67 (2014) 264–269
265
Table 1
Crystal data and structure refinement for (L )CuCl
Table 3
1
3
2
ꢁ3MeOH and (L )CuCl
2
.
The results of asymmetric nitroaldol reaction of nitromethane and benzaldehyde
using Cu(II) complexes of L , L and L3 as enantiopure catalysts.
1
2
Empirical formula
C
22
H30Cl
2
CuN
2
22 2 2 2
C H30Cl CuN O
O
2
ꢁ3MeOH
Entry
Catalyst (10%)
Base (%)
Yield (%)a
Ee (%)b
1
Formula weight
Crystal system
Space group
585.05
orthorhombic
P2
488.92
monoclinic
P2
1
2
3
4
5
6
7
8
9
(L )CuCl
2
2
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
(i-Pr)
2
2
2
2
2
2
2
2
2
Net (10)
Net (20)
Net (10)
Net (10)
Net (20)
Net (10)
Net (10)
Net (20)
Net (10)
63
46
90
58
47
91
26
20
43
83
87(S)
64(S)
82(S)
85(S)
61(S)
84(S)
62(S)
54(S)
63(S)
63(S)c
1
(L )CuCl
1
1
2
1
2
1
1
(L )Cu(OOCCH
3
3
)
2
2
2
(L )CuCl
2
Unit cell dimensions
2
(L )CuCl
2
a (Å)
b (Å)
c (Å)
b (°)
9.4484(5)
10.2217(5)
30.926(2)
13.133(1)
9.2030(8)
19.856(2)
107.832(7)
2284.5(3)
4
2
(L )Cu(OOCCH
)
3
(L )CuCl
2
3
(L )CuCl
2
3
3
(L )Cu(OOCCH
L )Cu(OOCCH
3
3
)
)
2
V (Å )
2986.8(3)
4
1.301
0.944
3
(
2
Diamine (2)
Z
D
calc (Mg/m3)
1.422
1.210
a
Conversion determined by 1H NMR spectroscopic analysis.
The enantiomeric excesses were determined by HPLC analysis with OD-H
Absorption coefficient
b
ꢀ
1
(
mm )
column.
F(000)
h (°)
Index ranges
1236
1020
c
The result reported in the literature [31].
1.32–25.47
ꢀ11 6 h 6 11,
1.63–25.47
ꢀ15 6 h 6 15,
ꢀ11 k 6 11,
ꢀ21 6 l 6 21
9502
ꢀ
ꢀ
12 k 6 12,
37 6 l 6 37
cyclohexane mono-(L)-(+)-tartrate salt (1) (99%), p-anisaldehyde
(98%), m-anisaldehyde (98%) and Copper(II) chloride dihydrate
Reflections collected
6542
Independent reflections
5548 (0.0114)
8351 (0.0163)
(
6
99%) were obtained from Aldrich chemical company. Silica gel
0 (230–400 mesh) from Merck, ethanol (99.8%) and methanol
int
(R )
Reflections observed
>2d)
4741
6625
0.993
1
3
(
99.9%) from Carlo Erba. The ligand, L and L were prepared
(
Data completeness
Refinement method
0.999
full-matrix least-
squares on F
according to the reported methods [25,26].
1
13
full-matrix least-
squares on F
8351/1/527
1.029
H NMR and C NMR spectra were recorded on a Bruker ad-
2
2
vance digital 400 (400 Hz)-NMR Spectrometer at ambient temper-
ature and chemical shifts (d) were referenced to tetramethylsilane
as internal standard and reported in ppm. Data were reported as
s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet,
br = broad. FT-IR spectra were measured on Jasco FT/IR-620 spec-
trophotometer. Band positions in IR spectra were reported as wave
Data/restraints/
parameters
5548/0/324
1.044
Goodness-of-fit (GOF)
on F2
Final R indices [I > 2d(I)]
R
1
= 0.0272
wR = 0.0708
= 0.0397
wR = 0.0740
R
1
= 0.0410
wR = 0.0996
= 0.0614
wR = 0.1051
2
2
R indices (all data)
R
1
R
1
ꢀ
1
number (m) in cm and band intensity in semiquantitative terms
2
2
Absolute structure
parameter
Largest difference in
ꢀ0.001(10)
ꢀ0.009(11)
(w = weak, m = medium, s = strong, v = variable, br = broad,
sh = sharp). Elemental analyses were determined on EA 1108-Ele-
mental Analyzer at the Chemical Analysis Laboratory of the Center
of Scientific Instruments of Kyungpook National University. Enan-
tiomeric excesses (ee) were determined by HPLC with a chiralcel
OD-H column using HPLC grade isopropanol (IPA) and n-hexane
0.285 and ꢀ0.286
0.965 and ꢀ1.019
3
peak and hole (e/Å )
(n-hex) were used as eluting solvents.
Table 2
1
3
2
Bond lengths [Å] and angles [°] for (L )CuCl
2
ꢁ3MeOH and (L )CuCl
2
.
2.2. Synthesis of L
L1)CuCl
ꢁ3MeOH
(L )CuCl
3
(
2
2
2
Ligand, L , was prepared following the synthetic procedure sim-
ilar to the one of L [25] to give a light yellow oil (78%). Anal. Calc.
Bond lengths
Cu1–N1
Cu1–N2
Cu1–Cl1
Cu1–Cl2
Bond lengths
Cu1–N1
Cu1–N2
1
2.019(2)
2.015(2)
2.005(3)
Cu2–N3
Cu2–N4
Cu2–Cl3
Cu2–Cl4
2.017(4)
2.038(3)
2.262(1)
2.249(1)
2.516(3)
for C22
H
30
1
N
2
O
2
: C, 74.54; H, 8.53; N, 7.90. Found: C, 74.48; H, 8.60;
): d = 7.20(2H, m, HPh); 6.88(4H,
); 3.74(6H, s, OCH );
); 2.24(2H, m, HCHX); 2.45(2H, m, HCHX); 1.91(s,
H, s, HN); 1.71(2H, m, HCHX); 1.23(2H, m, HCHX); 1.02(2H, m,
2.055(3)
2.271(1)
2.266(1)
2.399(3)
2.2490(7) Cu1–Cl1
2.2383(7) Cu1–Cl2
Cu1ꢁ ꢁ ꢁO2
N, 7.85. H NMR (150.85 Hz, CDCl
m, HPh); 6.74(2H, m, HPh); 3.87(2H, d, CH
.62(2H, d, CH
2
3
2
3
Cu2ꢁ ꢁ ꢁO4
3
2
Bond angles
N2–Cu1–
N1
N2–Cu1–
Cl2
N1–Cu1–
Cl2
N2–Cu1–
Cl1
N1–Cu1–
Cl1
Cl2–Cu1–
Cl1
Bond angles
N1–Cu–N2
84.86(7)
84.13(13) N3–Cu2–
N4
169.7(1)
83.8(1)
1
3
HCHX). C NMR (400 Hz, CDCl
3
) d = 160.16 (2C, CPh), 143.28 (2C,
159.09(6) N1–Cu1–
Cl2
N3–Cu2–
Cl4
N4–Cu2–
Cl4
N3–Cu2–
Cl3
N4–Cu2–
Cl3
173.47(9)
89.7(1)
C
(
(
2
Ph), 129.80 (2C, CPh), 120.78 (2C, CPh), 113.76 (2C, CPh), 112.84
2C, CPh), 61.29 (2C, CMe), 55.52 (2C, C
2C, CHX), 25.43 (2C, CHX). FT-IR (KBr):
N
), 51.27 (2C, CHX), 31.93
= 3293 (w, br), 2926(m),
92.63(5)
N2–Cu1–
Cl2
N1–Cu–Cl1 88.47(9)
92.2(1)
m
846(m), 1595 (s), 1450(s), 1357(m), 1298(m), 1258(s), 1152(s),
1107(m), 1041(s), 985(w), 860(m), 774(s), 690(s), 630(w).
93.43(6)
91.94(9)
166.2(1)
94.55(5)
82.7(1)
159.29(5) N2–Cu–Cl1 172.1(1)
1
2
3
96.01(3)
Cl2–Cu1–
Cl1
N1–
94.62(5)
87.6(1)
93.3(1)
Cl4–Cu2–
Cl3
N3–
2.3. Synthesis of (L )CuCl
2
, (L )CuCl
2
and (L )CuCl
2
1
1
To prepare (L )CuCl
2
, a solution of L (1.43 g, 4.03 mmol) in CH2-
Cu1ꢁ ꢁ ꢁO2
Cu2ꢁ ꢁ ꢁO4
Cl
.01 mmol) in CH
ambient temperature. The solvent was removed to give green solid
1.86 g, 95%). Anal. Calc. for C22 CuN : C, 54.04; H, 6.18; N,
.73. Found: C, 54.10; H, 6.20; N, 5.68. FT-IR (KBr): = 3195(w),
933(w), 2841(w), 2358(w), 1695(w), 1607(m), 1510(s),
2
(5 ml) was added dropwise to suspension of CuCl
2
ꢁ2H
2
O (0.69 g,
N2–
N4–
105.4(1)
98.43(8)
87.01(9)
4
2
Cl (10 ml). The mixture was stirred overnight at
2
Cu1ꢁ ꢁ ꢁO2
Cl2–
Cu2ꢁ ꢁ ꢁO4
102.34(8) Cl4–
Cu1ꢁ ꢁ ꢁO2
Cu2ꢁ ꢁ ꢁO4
Cl3–
Cu2ꢁ ꢁ ꢁO4
(
5
2
H30Cl
2
2 2
O
Cl1–
89.15(8)
m
Cu1ꢁ ꢁ ꢁO2

2
66
S.E. Song et al. / Polyhedron 67 (2014) 264–269
were applied based on
w scans [27]. The crystal structures were
determined by direct methods and refined by full-matrix least-
squares using SHELXS-97 and SHELXL-97 program packages [28]. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were constrained by using riding models. Crystal data and
1
3
structure refinement for (L )CuCl
2
and (L )CuCl
2
were presented
in Table 1.
2.5. Catalytic reactions
Reactions were carried out at ꢀ20 °C. At first 10 mol% of the
Scheme 1. Syntheses of enantiopure diamine ligands: (i) 2 equiv. Na
CH Cl , 24 h; (ii) NaBH , MeOH, 4 h.
2 3 2
CO , H O,
2
2
4
copper complexes with chloro ligands were dissolved in 25 ml
methanol. Then 10 mol% or 20 mol% of diisopropylethylamine
was added after adding 1.1 ml nitromethane (20 mmol.) and benz-
1
9
447(m), 1304(w), 1246(s), 1176(m), 1102(w), 1026(s), 975(m),
2
aldehyde (10 mmol). Herein, i-Pr NEt was chosen as co-catalyst
20(m), 841(s), 814(s), 754(m), 705(w), 655(w), 612(w).
because its good activity was proven in the literature [14,29].
And 10 mol% of the copper complexes with acetate ligands ob-
tained in situ from reaction of the dichloro-complexes with
Ag(OAc) used as catalysts at the same catalytic reactions above
using 10 mol% of diisopropylethylamine. Reactions were moni-
tored by TLC. After stirring for 48 h, reactions were quenched with
2
was synthesized using L2 by an analogous procedure
(
L )CuCl
2
1
of (L )CuCl
Cl CuN : C, 54.04; H, 6.18; N, 5.73. Found: C, 54.08; H, 6.21; N,
.78. FT-IR (KBr): = 3164(w), 2930(w), 2836(w), 2365(w),
2 30-
to afford blue solid (1.90 g, 97%). Anal. Calc. for C22H
2
2 2
O
5
1
1
7
m
697(w), 1590(s), 1546(w), 1487(m), 1443(s), 1313(w), 1259(s),
157(m), 1095(w), 1035(s), 985(m), 932(m), 870(s), 784(s),
36(s), 692(s), 654(w), 616(w).
1
ml of 1 N HCl solution and then evaporated. The products were
extracted by CH Cl
(3 times ꢂ 20 ml) and dried over anhydrous
MgSO , then filtered and the solvents were removed under reduced
pressure. The crude products were purified by column chromatog-
raphy (30% Et O/hexane) to give a yellowish oil of 1-phenyl-2-
2
2
3
3
(
L )CuCl
2
was synthesized with L by an analogous procedure of
to afford blue solid (1.86 g, 95%). Anal. Calc. for C22
: C, 54.04; H, 6.18; N, 5.73. Found: C, 54.00; H, 6.21; N, 5.78.
FT-IR (KBr): = 3182(w), 2934(w), 2845(w), 2358(w), 1697(w),
593(w), 1497(m), 1456(m), 1294(w), 1244(s), 1171(w),
4
1
(
L )CuCl
2
H30Cl2-
CuN
2 2
O
2
m
1
nitroethanol. H NMR (400 Hz, CDCl
3
): d 7.30 (5H, m, Ph), 5.38
), 2.89 (1H, br,
1
1
7
(
1H, dd, CH), 4.51(1H, dd, CH ), 4.41 (1H, dd, CH
2
2
112(m), 1056(m), 1020(s), 983(w), 924(m), 869(w), 822(w),
63(s), 723(w), 633(w), 583(w).
s, OH). Enantiomeric excess (ee) was determined using HPLC
on Chiracel OD-H column (hexanes:isopropanol = 95:5; flow
r
rate = 1.5 mL/min; k = 215 nm); R enantiomer t = 18.4 min, S enan-
2.4. X-ray crystallography
tiomer t = 22.3 min and shown in Table 3.
r
An Enraf-Nonius CAD-4 diffractometer equipped with a graph-
ite monochromated Mo K
a
radiation (k = 0.71073 Å) and a photo-
3. Results and discussion
multiplier detector was used measuring diffraction intensities. A
single crystals suitable for X-ray analysis crystallized in methanol
were sealed in a thin walled glass capillary and mounted on an
Enraf-Nonius CAD-4 diffractometer. Unit cell dimensions with esti-
mated standard deviations were determined by least-squares
using 25 well-centered reflections in the range of 10–13°. Total
3.1. Synthesis
1
2
3
The known ligands L [25], L , and L [26] were prepared by the
condensation of an aldehyde with (R,R)-1,2-diaminocyclohexane
generated from the hydrolysis of (R,R)-1,2-diaminocyclohexane
reflections were collected using the
x
/2h scan mode. The intensity
2 3
mono-(+)-tartrate salt in Na CO solution to form the imine inter-
data were corrected for Lorentz-polarization effect. Data were
carried out empirical absorption corrections with ABSCALC program
mediate. These intermediates were reduced easily with NaBH
afford the desired enantiopure ligands (Scheme 1).
4
to
1
Fig. 1. The molecular structure of (L )CuCl
2
with the numbering scheme at 30% probability level.

S.E. Song et al. / Polyhedron 67 (2014) 264–269
267
3
Fig. 2. The molecular structure of (L )CuCl
2
with the numbering scheme at 30% probability level.
1
The obtained enantiopure diamine ligands were reacted with
P2
P2
1
2
1
2
1
for (L )CuCl
2
ꢁ3MeOH and monoclinic with the space group
3
1
CuCl
2
in a 1:1 molar ratio to afford the Cu(II) complexes. The com-
1
for (L )CuCl
2
. The central Cu ion of (L )CuCl
2
ꢁ3MeOH was coor-
plexes formed after stirring for a few hours. The products were
characterised by infrared (IR) spectroscopy and elemental analysis.
Differences in the IR spectra of the ligands and the complexes were
observed, especially in the N–H region, which is sensitive to the
coordination between ligand nitrogens and metals. The IR spectra
of the ligands showed the characteristic broad absorption bands
dinated by two nitrogen atoms of the ligand and two chlorine
atoms in a distorted square-planar geometry (Fig. 1), which was
similar to the (L)CuCl
around the Cu ion of (L )CuCl was square-pyramidal, with two
2
nitrogen atoms of the ligand, two chlorine atoms, and a weakly-
bound methoxy oxygen atom coordinated to the Cu ion (Fig. 2).
The Cu1–N1 and Cu1–N2 bond lengths in (L )CuCl
2.019(2) and 2.015(2) ÅA , respectively, which are close to those in
2
complex [23]. However, the geometry
3
ꢀ1
1
for N–H at around 3300 cm , while the spectra of the Cu(II) com-
plexes showed the characteristic broad N–H absorption bands at
2
were
0
0
ꢀ1
1
2
3
3
195, 3164, and 3182 cm for (L )CuCl
2
, (L )CuCl
2
, and (L )CuCl
2
(L)CuCl
in the complex were 2.2490(7) and 2.2383(7) ÅA , respectively,
which are similar to the Cu–Cl bond lengths in (L)CuCl , i.e.,
2.2448(7) AÅ . The N2–Cu1–Cl2 and N1–Cu1–Cl1 bond angles were
159.09(6)° and 159.29(5)°, which are similar to those of (L)CuCl
2
0
, i.e., 2.017(2) ÅA . The Cu1–Cl1 and Cu1–Cl2 bond lengths
respectively. The isolated Cu(II) complexes were soluble in polar
organic solvents such as acetonitrile, methanol, and dichlorometh-
ane (See Table 2).
0
2
1
3
Suitable single crystals of (L )CuCl
2
ꢁ3MeOH and (L )CuCl
2
for X-
2
,
3
ray diffraction analyses were obtained from methanol solutions.
The crystal systems were orthorhombic with the space group
i.e., 155.82(6)°. The X-ray crystal structure of (L )CuCl
independent molecules per asymmetric unit. The bond lengths
2
had two
1
Fig. 3. The hydrogen bonds within the crystal structure of (L )CuCl
2
ꢁ3MeOH. Methoxyphenyl groups were omitted for clarity. Cl ꢁ ꢁ ꢁH–O1Sꢁ ꢁ ꢁH–N1(x ꢀ 1/2, ꢀy + 1/2, ꢀz),
2
Cl1ꢁ ꢁ ꢁH–O2Sꢁ ꢁ ꢁH–O3Sꢁ ꢁ ꢁH–N2(x + 2, y ꢀ 1/2, z ꢀ 1/2), N–Hꢁ ꢁ ꢁO1S–H(x ꢀ 1/2, ꢀy + 1/2, ꢀz)ꢁ ꢁ ꢁCl2(x ꢀ 1/2, ꢀy + 1/2, ꢀz) and N2–Hꢁ ꢁ ꢁO2S–H(ꢀx + 2, y + 1/2, ꢀz + 1/2)ꢁ ꢁ ꢁO2S–
H(ꢀx + 2, y + 1/2, ꢀz + 1/2)ꢁ ꢁ ꢁCl1(ꢀx + 2, y + 1/2, ꢀz + 1/2).

2
68
S.E. Song et al. / Polyhedron 67 (2014) 264–269
Scheme 2. Mechanism proposed for the double catalysts.
3
1
and angles for (L )CuCl
2
were similar to those of (L )CuCl
2
except
by Skarzewski [31], and resulted in a very slow reaction. The meth-
oxy oxygen atom of L can partially coordinate with transition
metals (Fig. 2) like a SALAN ligand [32] and thereby hinder ap-
proach by reactants.
The nitroaldol reaction with 20 mol% (i-Pr) NEt and 10 mol%
2
enantiopure Cu(II) complexes was carried out (Table 3, entries 2,
5, 8). The reaction yields and ee values were lower than with
10 mol% (i-Pr) Net. The high amine concentration might promote
2
the reaction between the complexes and the amine to form inac-
tive agents and thereby decrease the activity and enantioselectivity
of the Cu(II) complexes.
Based on the experimental observations, X-ray structural data,
and previously reported steric considerations, especially the path-
way proposed by Jørgensen [33] for nitroaldol reactions between
3
for the short Cuꢁ ꢁ ꢁO contact of the methoxy group. The short
0
Cu1ꢁ ꢁ ꢁO2 and Cu2ꢁ ꢁ ꢁO4 distances were 2.399(3) and 2.516(3) ÅA ,
respectively, which indicates very strong interactions. Therefore,
the space around the Cu ion is not crowded. Coordination of the
1
3
substituted ligands L and L with Cu(II) also generated two chiral
centres, noted as R and R , which were derived from the (R,R)-1,2-
diaminocyclohexane fragment. The X-ray structures showed that
the two stereogenic nitrogen atoms were both R
but one stereogenic nitrogen atom was R and the other was R
2
. The hydrogen atoms of the chiral carbon and nitrogen
atoms were in a head-to-tail conformation in (L )CuCl
head-to-head conformation in (L )CuCl
tant five-membered chelate ring made up of N–C–C–N atoms and
C
C
1
N
in (L )CuCl
2
,
N
S
3
in (L )CuCl
1
2
and in a
2
. Furthermore, the resul-
3
the Cu atom in each complex were in the k conformation. There
nitromethane and a-keto esters in the presence of enantiopure
were hydrogen bonds present within the crystal structure of
Cu(II)–Box complexes and triethylamine as the co-catalyst, we
propose that the Cu complexes play a double role as the reaction
promoter and the deactivator of the diisopropylethylamine
(Scheme 2). The enantiopure Lewis acid complex/amine ratio
greatly influenced the outcome of the reaction. When a high con-
centration of amine was used as the base, it reacted with the Cu(II)
complexes and resulted in a lower concentration of Lewis acid
Cu(II) complexes. This caused a significant reduction in the enanti-
oselectivity and yield. If only the amine base was used for the Henry
reaction, i.e., without the enantiopure metal complex, then a race-
mic nitroaldol adduct was formed in high yield [30]. The nitroaldol
0
1
(
L )CuCl
2
ꢁ3MeOH; the distances were 2.31, 2.68 ÅA for Cl
2
ꢁ ꢁ ꢁH–
0
O1Sꢁ ꢁ ꢁH–N1 and 2.35, 1.96, 2.17 ÅA for Cl1ꢁ ꢁ ꢁH–O2Sꢁ ꢁ ꢁH–O3Sꢁ ꢁ ꢁH–
N2 (Fig. 3).
3.1.1. Results of catalytic reactions
The asymmetric nitroaldol reaction of carbonyl compounds
using enantiopure transition metal catalysts is an important tool
for the preparation of intermediates in the synthesis of natural
products and pharmaceuticals [30]. The main goal of this study
was to evaluate enantiopure Cu(II) complexes as the chiral compo-
nents of double catalysts with diisopropylamine in the nitroaldol
reaction of benzaldehyde and nitromethane. When 10 mol% of
the Cu(II) complexes with chloro ligands bearing these enantiopure
substituted ligands was used with 10 mol% of diisopropylamine,
the product yields were in the range of 26–63% (Table 3, entries
2
reaction with 10 mol% (i-Pr) NEt and 10 mol% the Cu(II) complexes
with acetate ligands was also carried out (Table 3, entries 2, 5, 8).
The yields were higher and the ee values somewhat lower than
with the 10 mol% the Cu(II) complexes with chloro ligands.
1
, 4, 7), similar to the case of Cu(L)Cl
2
[23]. However, the enantiose-
4. Conclusions
lectivity was very different. Using the catalysts with the substitu-
ents in the para or meta position led to an increase in
enantioselectivity (ee = 87 or 85%) because of the bulkier methoxy
group (Table 3, entry 1, 4). Using the catalyst with the substituents
in the ortho position afforded 62% ee, comparable to those reported
Cu(II) complexes bearing the di(methoxybenzyl)-(R,R)-1,2-diami-
nocyclohexane ligand were synthesised and characterised. The
1
3
crystal structures of (L )CuCl
2
ꢁ3MeOH and (L )CuCl
2
were deter-
mined and revealed that the geometry around the central Cu ion

S.E. Song et al. / Polyhedron 67 (2014) 264–269
269
1
3
was distorted square-planar for (L )CuCl
2
. The geometry for (L )CuCl
2
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was distorted square-pyramidal because of the strong interaction of
methoxy O atoms with the Cu atom. The configuration of the two ste-
reogenic nitrogen atoms in (L )CuCl
S 2
and R in (L )CuCl . The k conformation was found in both complexes
for the five-membered chelate rings formed between the N–C–C–N
atoms and the Cu ions. The enantiopure complex catalysts with the
methoxy substituents at the para and meta positions of the phenyl
rings did not affect the yields, but ortho substitution led to much low-
had the highest ee (up to 87%), possibly caused by
reasons other than just the steric influence of the phenyl rings. In the
1
were both R
N
, but they were R
N
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er yields. (L )CuCl
2
[
[
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2 2
other cases, using (L )CuCl and (L)CuCl as catalysts afforded lower
1
ee values than with (L )CuCl
2
.
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[
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Acknowledgements
This Research was supported by Kyungpook National University
Research Fund, 2012.
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Appendix A. Supplementary data
[
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[
1
3
4
graphic data for (L )CuCl
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ꢁ3MeOH and (L )CuCl
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