A chiral Cu-salan catalyst with a rotatable aromatic π-wall: Molecular recognition-oriented asymmetric henry transformation of aromatic aldehydes

Article abstract of DOI:10.1002/asia.201200388

An aromatic π-wall as a reaction activator and inactivator: A novel salan ligand was prepared from Ph-BINMOL. The derived Cu complex was capable to distinguish benzaldehyde and halide-substituted benzaldehydes from methyl-, methoxy-, hydroxy-, and nitro-s

Full text of DOI:10.1002/asia.201200388

DOI: 10.1002/asia.201200388
A Chiral Cu-Salan Catalyst with a Rotatable Aromatic p-Wall: Molecular
Recognition-Oriented Asymmetric Henry Transformation of Aromatic
Aldehydes
[
a]
[a]
[a]
[a]
[a]
Fei Li, Zhan-Jiang Zheng, Jun-Yan Shang, Ke-Zhi Jiang, Guo-Qiao Lai,
[
a]
[a, b]
Jian-Xiong Jiang, and Li-Wen Xu*
The study of molecular recognition is central to supra-
molecular chemistry and organic chemistry. The rapidly
growing field of molecular recognition contributes greatly to
research and development in areas ranging from organic
synthesis to chiral separation, from organometallic chemis-
try to catalysis, and from biomimetic chemistry to materi-
of a borate Lewis acid effectively distinguishing between ar-
omatic and aliphatic aldehydes in a catalytic manner, in
which a p pocket within a cage-shaped borate catalyst was
created by introducing various aromatic substituents at ap-
[12]
propriate positions.
Such a cage-shaped borate complex
allows a selective and competitive reaction through the aro-
matic–aromatic interaction between aromatic aldehydes and
adjustable aryl-substituted borates. To the best of our
knowledge, a chiral, molecular recognition-oriented enantio-
selective transformation of aromatic aldehydes with the aid
of noncovalent interactions involving aromatic p–p stacking
interaction of metal complexes has not yet been reported.
Herein, we report the synthesis of a novel chiral salan
ligand with large p-wall of aromatic rings and the corre-
sponding salan–Cu complex that selectively recognizes
simple aromatic aldehydes and can be applied to a highly
enantioselective and substrate-selective Henry reaction.
Very recently, we prepared simple and novel enantiomeri-
[
1,2]
als.
So far, many excellent examples of molecular recog-
nition based on hydrogen bonding, metal-coordination
bonding, and other noncovalent interactions have been re-
[
3–8]
ported in discrete molecular architectures,
such as molec-
[3]
[4]
[5]
[6]
[7]
ular tweezers, clips, capsules, cages, boxes, and
tubes. In addition, these artificially designed organic mole-
cules and metal complexes have been used as selective rec-
[8]
[9]
[10]
ognition and reaction centers for neutral molecules and
ionic species. Especially for metal complexes, reaction con-
trol by noncovalent interactions involving aromatic p–p
stacking has been shown to be a challenge in supramolec-
ular catalysis in terms of electronic or steric factors. The
structure and conformation of aromatic ligands in metal
complexes not only influences the outcome of metal-cata-
lyzed reactions but also becomes important for recognition
processes involved in chemical sensors. However, to date,
only few metal complexes bearing an aromatic group or a p
pocket have been reported with regard to the discrimination
and selective activation of similarly sized aromatic alde-
hydes with different simple substitutents in organic transfor-
cally pure BINOL-derived diols, 1,1’-binaphthalene-2-a-aryl-
3
methanol-2’-ol (Ar-BINMOLs), with C -axial and sp
2
carbon-central chirality through an asymmetric [1,2]-Wittig
rearrangement-based axial-to-central chiral transfer of mon-
[13]
oalkylated BINOLs. The Ar-BINMOLs have rigid struc-
tures, exhibit different characters as compared to BINOL,
and could act as a supramolecules due to hydrogen bonding
and aromatic p–p/CÀH···p interactions. Furthermore, they
[9–11]
mation.
Very recently and during the preparation of this
have been used as supramolecular auxiliaries/catalysts to
mediate the asymmetric Michael reaction of anthrone to
trans-b-nitrostyrene and as chiral diol ligands to promote
the asymmetric 1,2-addition of diethylzinc to aldehydes in
manuscript, Baba and coworkers reported the first example
[
a] F. Li, Z.-J. Zheng, J.-Y. Shang, K.-Z. Jiang, G.-Q. Lai, J.-X. Jiang,
Prof. Dr. L.-W. Xu
excellent
yields
and
enantioselectivities
(up
to
[
14]
Key Laboratory of Organosilicon Chemistry and Material Technology
of Ministry of Education
Hangzhou Normal University
Hangzhou 310012 (P. R. China)
Fax : (+86)571-28865135
>99.9%ee). Thus, we had high expectations regarding the
creation of a novel metal complex with a large aromatic p-
wall and accessible cavity based on the structural modifica-
tion of BINOL and corresponding Ar-BINMOLs. To con-
struct copper complexes with a more flexible aromatic
group and large aromatic p-wall, we designed and prepared
a salan ligand from Ar-BINMOL in high yield (Scheme 1,
see also the Supporting Information).
E-mail: licpxulw@yahoo.com
liwenxu@hznu.edu.cn
[
b] Prof. Dr. L.-W. Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
The Henry reaction and its asymmetric version are impor-
tant and fundamental carbon-carbon bond-forming reactions
Lanzhou, 730000 (P. R. China)
[15]
in organic chemistry, constituting essentially a CÀC cou-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200388.
pling reaction between an aromatic or aliphatic carbonyl
Chem. Asian J. 2012, 00, 0 – 0
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COMMUNICATION
and enantioselectivity be-
tween aromatic aldehydes
and aliphatic aldehydes were
[16]
observed.
By contrast, we
have found that substituted
benzaldehydes have either
a low or a high reactivity in
Henry reactions performed
in the presence of ligand L6.
As
shown
in
Table 2,
methyl-, methoxy-, hydroxy-,
and nitro-substituted benzal-
dehydes resulted in poor or
no conversion, whereas halo-
gen-substituted
benzalde-
hydes were effective sub-
strates in this Henry reaction
(
90–99% yield), although
the enantioselectivity varied
between 58% and 85%ee.
These results indicated that
benzaldehydes
methyl-, methoxy-, hydroxy-,
and nitro-groups at any posi-
bearing
Scheme 1. Synthesis of salen ligands L5 and L6 from Ph-BINMOL.
tion are unsuitable sub-
compound (aldehyde or ketone) and a nitroalkane. Accord-
ingly, we chose herein this type of reaction as a model to in-
vestigate the molecular interaction between aromatic alde-
hydes and Cu-salan (L6) complexes.
strates. In addition, the Henry reaction of cyclohexanecar-
baldehyde was very sluggish, and other linear aliphatic alde-
hydes, such as 1-decanal or 3-phenylpropanal, exhibited
After preparing the salan ligand L6 through six
Table 1. Asymmetric salen-Cu or salan-Cu catalyzed Henry transformation of benzal-
dehyde.
steps in a high combined yield, we initially exam-
ined its potential as a copper ligand for the stereo-
selective Henry reaction of benzaldehyde with ni-
tromethane. For comparison, reactions with vari-
ous chiral salen ligands based on cyclohexane-1,2-
diamine, copper salts, and solvents were carried
out under standard reaction conditions (Table 1).
These ligands proved to be different with regard
to reactivity and enantioselectivity. Differing from
ligands L1–L5 (see Schemes 1 and 2), which gave
-nitro-1-phenylethanol with low to moderate
enantioselectivities (5–52%ee), the ligand L6 af-
forded the desired product in excellent yield
95%) and enantioselectivity (91%ee) in the pres-
ence of copper acetate in EtOH. Interestingly,
with other commercial available copper salts, such
[
c]
[b]
Entry
Copper salts
Ligand
Solvent
T [8C]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
11
1
1
1
1
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
CuCl
CuBr
CuI
CuCl
CuBr
A
T
N
T
E
N
N
2
2
2
2
2
L1
L2
L3
L4
L5
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
L6
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
iPrOH
20
20
20
20
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
0
90
<5
80
90
60
12
n.d.
5
-52
38
91
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
84
A
H
U
G
E
N
G
ACHTUNGTRENNUNG
A
H
U
T
E
N
G
A
H
U
G
R
N
U
G
A
T
N
T
E
U
G
2
95
A
H
U
G
R
N
U
G
3
CN)
4
PF
6
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
60
50
65
85
<10
<10
A
T
N
R
N
U
G
2
A
H
U
G
E
U
G
3 2
COO)
2
2
3
4
5
2
(
2
CuI
CuF2
2
16
1
1
2
2
22
23
7
8
0
1
Cu
Cu
Cu
Cu
Cu
Cu
A
T
N
T
E
U
2
2
2
2
2
2
as CuBr , CuCl, Cu CAHTUNGTERNN(UGN OTf) , Cu ACHTUNGTENRNUN(G CH CN) PF , and
2 3 4 6
2
A
H
U
T
E
U
G
CH
2
Cl
2
71
90
91
91
Cu ACHTUNGTRENNUNG( OOCCF ) , almost no conversion of aldehydes
3 2
and poor chemical yield of the desired product
were detected, thus suggesting that the copper
A
T
N
T
E
U
G
MeOH
THF
EtOH
THF
A
H
U
G
R
N
U
G
A
H
U
G
E
N
N
complex derived from Cu
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
0
94
2
site-specifically activated benzaldehyde in ethanol.
When previously reported chiral copper com-
plex catalysts controlled by various salen/salan/ni-
trogen-containing ligands were used in Henry re-
actions with various aldehydes, no differences in
reactivity and no obvious differences in reactivity
[a] All reactions were performed on a 1 mmol scale with 10 mol% of Cu salt and
10 mol% of ligand (L1–L6) at a 0.5m concentration using 10 equiv of nitromethane in
solvent. Reactions were run at room temperature in a screw-capped vial for 24 h.
[
b] Enantiomeric excess was determined by HPLC using Chiracel OD-H columns, and
the absolute configuration of the major isomer was determined to be (R) by compari-
[16]
son with literature data except that of entry 4.
yield.
n.d., not determined. [c] Isolated
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Molecular Recognition-Oriented Asymmetric Henry Transformation
poor reactivity with almost no conversion after 24 hours.
This pronounced effect also indicated that a suitably linked
benzyl p-wall on L6 might be the decisive factor for the
preferential activation and enantiocontrol in the Henry ad-
dition. We believed that a small and suitable cavity in L6-Cu
might exist if the protic polar solvent (EtOH) induces the
rotation of the hydrophobic aromatic p-wall to form a cage-
shaped copper complex.
To examine the hypothesis that the complex L6-Cu could
discriminate between aliphatic and aromatic aldehydes, we
carried out a competitive reaction between benzaldehyde
and cyclohexanecarbaldehyde, both of which are similar in
size and in their steric demands in the reaction with nitro-
methane. As shown in Scheme 3, the L6-Cu complex cata-
Scheme 2. Copper-catalyzed Henry reaction of benzaldehyde with nitro-
methane in the presence of various ligands.
Scheme 3. Competitive Henry reaction of benzaldehyde and cyclohexa-
necarbaldehyde catalyzed by salan(L6)-Cu.
Table 2. Scope of the asymmetric Henry reaction: Variation of alde-
hydes.
lyzed the reaction of benzaldehyde more effectively than
that of cyclohexanecarbaldehyde. Moreover, in the presence
of both substrates, the enantiomeric excess of products 9a
and 9u was decreased to 17%ee and 38%ee, respectively, as
compared to 91%ee and 48%ee obtained with each sub-
strate alone. These results suggest that the benzyl aromatic
ring in the salan-Cu complex is crucial for the noncovalent
activation of benzaldehyde due to the selective aggregation
of p–p stacking interactions between the catalyst and sub-
strate.
[
a]
[b]
[c]
Entry
Aldehydes
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2-methylbenzaldehyde
3-methylbenzaldehyde
4-methylbenzaldehyde
2-methoxybenzaldehyde
3-methoxybenzaldehyde
4-methoxybenzaldehyde
2-hydroxybenzaldehyde
3-hydroxybenzaldehyde
4-hydroxybenzaldehyde
2-nitrobenzaldehyde
3-nitrobenzaldehyde
4-nitrobenzaldehyde
2-fluorobenzaldehyde
3-fluorobenzaldehyde
4-fluorobenzaldehyde
2-chlorobenzaldehyde
3-chlorobenzaldehyde
2-bromobenzaldehyde
4-bromobenzaldehyde
cyclohexanecarbaldehyde
1-decanal
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
99
92
98
92
90
90
91
38
<5
<5
9b: n.d.
9c: n.d.
9d: n.d.
9e: n.d.
9 f: n.d.
9g: n.d.
9h: n.d.
9i: n.d.
9j: n.d.
9k: n.d.
9l: n.d.
9m: n.d.
9n: 59
9o: 85
9p: 79
9q: 58
9r: 81
9s: 76
9t: 83
9u: 48
9v: n.d.
9w: n.d.
Aware of the above-mentioned aromatic p-wall/door-
opened Henry reactions, we envisioned that the interactions
between Cu-salan and an aromatic aldehyde might be deter-
mined by UV/Vis spectrocopy in the absence of nitrome-
thane under the reaction conditions. Thus, to gain more in-
sight into the interaction of aldehydes with salan(L6)-Cu,
concentration-dependent UV/Vis spectroscopic studies were
performed. For the mixture of salan(L6)-Cu and benzalde-
0
1
2
3
4
5
6
7
8
9
0
1
2
À5
hyde, at relatively low concentrations (3–5ꢁ10 m, sub-
strate/catalyst=6:1–10:1), the UV/Vis spectra (Figure 1a)
displayed a peak at around 234 nm in the largest absorption
band. Upon increasing the concentration of the mixture, the
spectra became broader and the absorption maximum shift-
ed slightly to 243 nm, thus reflecting the electronic coupling
and p–p stacking interaction between salan(L6)-Cu and
benzaldehyde. In addition, a nonlinear relation of the in-
crease in absorbance at 234 nm with increasing concentra-
tion was observed (inset in Figure 1a), suggesting the in-
volvement of hydrogen bonding and strong p–p stacking in-
teractions between the salan ligand and benzaldehyde.
Moreover, when the concentration of benzaldehyde ranged
3-phenylpropanal
[
a] All reactions were performed on a 1 mmol scale with 10 mol% of Cu-
(OAc) and 10 mol% of ligand (L6) at a 0.5m concentration using
0 equiv of nitromethane in ethanol. Reactions were run at room temper-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
1
ature in a screw-capped vial for 24 h. [b] Isolated yield, and almost no
conversion was determined for the aldehydes of entries 1–12, 21, and 22
(
<5% yield). [c] Enantiomeric excess was determined by HPLC. n.d.,
not determined.
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Li-Wen Xu et al.
COMMUNICATION
of salan(L6)-Cu and 4-methylbenzaldehyde displayed a dif-
ferent behavior and obeyed well the Lambert–Beer law
(
Figure 1c), thereby indicating that the molecular interac-
tion between salan(L6)-Cu and 4-methylbenzaldehyde is
weak. Although both 4-methylbenzaldehyde and benzalde-
hyde have an aromatic p-conjugated ring, the difference in
their steric demand might account for the selective recogni-
tion.
Fluorescence spectroscopy is a convenient means to moni-
tor changes in the molecular recognition or noncovalent in-
teractions and therefore we employed this method next. As
shown in Figures S4–S8 in the Supporting Information,
when the concentration of benzaldehyde was gradually de-
creased, the fluorescence emission intensity of the solution
increased. For the 5:1 and 5:2 mixtures of salan-Cu to ben-
zaldehyde at low concentrations, the fluorescence spectra
display a maximum emission band at 290–340 nm (Figure S6
in the Supporting Information). Interestingly, a different
maximum emission band at 340–390 nm appeared at lower
ratios (ꢀ5:3) of mixtures and its intensity decreased strongly
with increasing concentration of benzaldehyde. These spec-
tral features are highly indicative of specific noncovalent in-
teractions and possibly an aggregation between benzalde-
hyde and salan(L6)-Cu.
Based on the experimental results and spectroscopic anal-
yses, we therefore propose that a supramolecular encapsula-
tion within the salan-Cu complex leads to a possible pseudo
cage-shaped binaphthyl framework with rotatable benzyl
arms. As shown schematically in Figure 2, the C -axial bi-
2
naphthyl framework with rotatable benzyl arms can effec-
tively induce the Henry addition of nitromethane to aromat-
ic aldehydes within the salan-Cu complex. The interaction
between the aromatic p-wall and aromatic aldehydes causes
their differentiation over aliphatic aldehydes. In addition, it
can also discriminate different aromatic aldehydes if these
bear bulky and branched groups on aromatic rings. Con-
versely, the outcomes of enantioselective transformations of
aromatic or aliphatic aldehydes provide direct evidence of
a selective molecular interaction between aldehydes and the
copper-salan complex.
In conclusion, a novel salan-Cu complex has been synthe-
sized from BINOL, and its activity in the Henry reaction of
aldehydes has been investigated. The unique salan ligand
bearing a large aromatic p-wall led to an interesting and re-
markable discrimination of different aldehydes in this type
of reaction, which we reasoned is due to noncovalent inter-
actions involving aromatic p–p stacking interactions be-
tween the copper ligand and aldehydes. In addition, the Cu
complex could discriminate benzaldehyde and halide-con-
taining aromatic aldehydes from methyl-, methoxy-, hy-
droxy-, and nitro-substituted aromatic aldehydes as well as
aliphatic aldehydes. Our study is the first example of a mo-
lecular recognition-oriented asymmetric Henry transforma-
tion, which is reminiscent of enzyme-controlled, substrate-
selective reactions. Further efforts will be devoted to apply
the salan-Cu complex to asymmetric catalysis and selective
molecular recognition of functional organic molecules.
Figure 1. Concentration dependence of the UV/Vis absorption of
À6
salan(L6)-Cu (5ꢁ10 m in EtOH) in the presence of a) benzaldehyde
À5
À5
(
3.0–7.5ꢁ10 m) or b) 4-methylbenzaldehyde (3.0–7.5ꢁ10 m). c) The ab-
sorbance of salan(L6)-Cu in the absence of aldehyde.
À6
À5
from 1ꢁ10 to 1ꢁ10 m, the UV/Vis spectra also exhibit
an unusual and non-linear absorption maximum (Figure S1
in the Supporting Information). By comparison, the mixture
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Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

Molecular Recognition-Oriented Asymmetric Henry Transformation
Acknowledgements
This project was supported by the Na-
tional Natural Science Foundation of
China (Nos. 20973051 and 21173064),
Program for Excellent Young Teachers
in Hangzhou Normal University
(
HNUEYT, JTAS 2011-01-014).
Keywords: copper
·
Henry
reaction
catalysis
·
homogeneous
molecular
·
recognition · p interactions
Figure 2. Schematic representation and concept of a pseudo cage-shaped chiral salan-metal catalyst having an
+
aromatic p-wall as a recognition site (MM -optimized geometry for salan(L6)-Cu).
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Revised: May 17, 2012
Published online: && &&, 0000
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

COMMUNICATION
An aromatic p-wall as a reaction acti-
vator and inactivator: A novel salan
ligand was prepared from Ph-
Molecular Recognition
Fei Li, Zhan-Jiang Zheng,
BINMOL. The derived Cu complex
was capable to distinguish benzalde-
hyde and halide-substituted benzalde-
hydes from methyl-, methoxy-, hy-
droxy-, and nitro-substituted benzalde-
hydes as well as aliphatic aldehydes.
The observed molecular recognition
was examined by concentration-depen-
dent UV/Vis and fluorescence spec-
troscopy.
Jun-Yan Shang, Ke-Zhi Jiang,
Guo-Qiao Lai, Jian-Xiong Jiang,
Li-Wen Xu*
&&&& — &&&&
A Chiral Cu-Salan Catalyst with
a Rotatable Aromatic p-Wall: Molec-
ular Recognition-Oriented Asym-
metric Henry Transformation of
Aromatic Aldehydes
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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