Cobalt(II)-Salen-Linked Complementary Double-Stranded Helical Catalysts for Asymmetric Nitro-Aldol Reaction

Article abstract of DOI:10.1021/acscatal.6b01627

Double-helical, bimetallic chiral Co(II)-salen complexes stabilized by chiral amidinium-carboxylate salt bridges efficiently catalyzed the asymmetric nitro-aldol (Henry) reaction, producing products with up to an 89% enantiomeric excess (ee); the reactivi

Full text of DOI:10.1021/acscatal.6b01627

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Letter
Cobalt(II)-Salen-Linked Complementary Double-Stranded
Helical Catalysts for Asymmetric Nitro-Aldol Reaction
Daisuke Taura, Shogo Hioki, Junki Tanabe, Naoki Ousaka, and Eiji Yashima
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01627 • Publication Date (Web): 14 Jun 2016
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ACS Catalysis
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Cobalt(II)-Salen-Linked Complementary Double-Stranded Helical
Catalysts for Asymmetric Nitro-Aldol Reaction
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Daisuke Taura, Shogo Hioki, Junki Tanabe, Naoki Ousaka, and Eiji Yashima*
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nago-
ya 464-8603, Japan
KEYWORDS: double-helix, salt bridge, Co-salen, supramolecular catalysis, nitro-aldol reaction
ABSTRACT: Double-helical, bimetallic chiral Co(II)-salen complexes stabilized by chiral amidinium–carboxylate salt bridges
efficiently catalyzed the asymmetric nitro-aldol (Henry) reaction, producing products with up to an 89% enantiomeric excess (ee);
the reactivity and enantioselectivity were higher than those catalyzed by the corresponding single strands. The key role of the chiral
double-helical framework for the supramolecular bimetallic catalysis has been revealed by a double-helical catalyst carrying achiral
Co(II)-salen units that promoted the Henry reaction, yielding the product with a 50-45% ee, while the corresponding single strands
showed a poor or no enantioselectivity.
The helix is inherently chiral and is one of the key structural
motifs for biological systems in which single-stranded helical
peptides and double-helical nucleic acids with a one-handed
helicity play a central role in their vital functions. In the organ-
ic, polymer, and supramolecular chemistry fields, a vast num-
ber of helical molecules, oligomers (foldamers), and polymers
has been synthesized, primary to mimicking biological helices,
and also to develop chiral materials with specific functionali-
ties that involve chiral recognition and sensing, asymmetric
catalysis, and chiral optoelectronic properties.1 However, most
of synthetic helical systems are composed of single-stranded
helices, while those of the double helices with defined struc-
tures remain limited to peptide nucleic acids (PNA),2
helicates,3 and aromatic oligoamides.4 Moreover, synthetic
double helices showing an asymmetric catalytic activity are
quite rare,5 regardless of the recent finding that the double-
helical DNA catalyzes a variety of reactions in a highly enan-
tioselective fashion when complexed with specific metal-
coordinated ligands,6 which indicate the potential capability of
double helices as a promising chiral framework for supramo-
lecular asymmetric catalysis.7
It was recently demonstrated by Hong and co-workers8 that
an optically-active Co(II)-salen complex bearing self-
complementary 2-pyridone and 2-amidopyridine units on each
side of the complex self-assembled to form a dinuclear zipper-
like homo-duplex through relatively weak interstrand hydro-
gen-bonding. The bimetallic Co(II)-salen duplex9 efficiently
catalyzed the asymmetric nitro-aldol (Henry) reaction10 with a
remarkable enhancement of the catalytic activity as well as the
enantioselectivity compared to those catalyzed by the mono-
meric Co(II)-salen complex. We anticipated that if such a self-
assembled dinuclear complex could take a one-handed double-
helical structure, the resulting supramolecular bimetallic dou-
ble-helical catalyst would enhance the enantioselectivity and
catalytic activity due to its intriguing synergistic effect of the
helical chirality and cooperative effect of the two metal units
arranging in close proximity to one another in a chiral fashion
(Figure 1).
Double-Helical Framework for Asymmetric Catalysis
R = 1-octynyl
R
R
X
X
Ph
Ph
OH
O
N
N
O
OH
H
N
N
H
Ph
Ph
TMS
TMS
TMS TMS
Carboxylic acid dimer
Amidine dimer
Complementary double-helix formation
R
MeO
X =
CHO
N
N
N
Co
or
O
O
–
+
O
O
TMS
TMS
+
N
H
tBu
tBu
tBu
TMS
TMS
Ph
N
H
X
X
Ph
CH3NO2
H
N
H
N
Ph
Ph
–
O
O
N
Co
MeO OH
O
O
tBu
NO2
R
*
Enhanced or induced enantioselectivity
Figure 1. Complementary double-helix formation between chiral
and/or achiral Co(II)-salen-linked carboxylic acid dimers and
amidine dimers stabilized by chiral amidinium–carboxylate salt
bridges that enhance or induce the enantioselectivity during the
asymmetric Henry reaction due to their one-handed double-helical
framework.
We now describe a proof-of-concept example of a double-
helical framework for supramolecular bimetallic catalysis and
demonstrate control over the reaction yield and enantioselec-
tivity that are significantly enhanced upon the one-handed
double-helix formation of chiral and/or achiral complementary
single strands carrying catalytically-active chiral and even
achiral metal species.
Our design strategy for constructing double-helical bimetallic
catalysts is based on our modular strategy developed for con-
structing m-terphenyl-based complementary double helices sta-
bilized by chiral amidinium–carboxylate salt bridges5a,11 that
have a high stability with association constants >107 M-1 in
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CHCl3 and a tolerance to various functional groups and
firmed by electron-spray ionization mass spectrometry (ESI-
metals,11b,d which provides a key advantage such that a variety of
linkers including metals can be introduced to the linkages while
maintaining the double-helical structures with a controlled hand-
edness biased by the chiral amidine residues.11
MS) (Figure S1). A double-helix formation was further inves-
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tigated in detail by measuring the 1D and 2D H NMR spec-
troscopies of the chiral Ni(II)-salen-linked 2a·2c complex in
CDCl . The salt-bridged N-H protons showed two sets of non-
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equivalent signals around δ = 13.3 ppm (Figure S2) and sever-
al NOE cross-peaks were clearly observed between the dimer
strands in the 2D NOESY spectrum (Figures S4 and S5),
which is indicative of the double-helix formation stabilized by
salt bridges as observed for other analogous complementary
double helices.11d An excess single-handed double-helical
structure was supported by the circular dichroism (CD) spec-
Chart 1. Structures of Chiral Amidine and Carboxylic
Acid Dimer Strands Connected through Chiral or Achiral
Salen and Metal-Salen Linkers
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trum of the 2a·2c complex in CDCl (Figure 2a); an intense
3
split-type Cotton effect appeared, especially in the absorption
region of the conjugated linkages (ca. 300–420 nm), whereas
the single strands 2a and 2c exhibited weak CD signals in the
same region, suggesting that the 2a·2c likely adopts a right-
handed double-helical structure whose helical sense is con-
trolled by the chiral amidine residues with an (R)-
configuration.11
Other duplexes composed of chiral and/or achiral Co(II)-
and/or Ni(II)-salen-linked amidine and carboxylic acid dimers
also exhibited more intense Cotton effects than those of the
corresponding single strands in the linker chromophore re-
gions, and the CD spectral patterns and signs of these duplexes
were almost comparable to each other (Figures 2 and S7),
indicating that these dimers most likely adopt a double-helical
structure with the same helical sense when hybridized with
their complementary strands, although their CD intensities
were more or less different from each other due to the differ-
ence in their linker salen chirality and metal species (Ni or
Co).13
We then synthesized a series of (R)-1-phenylethyl amidine
(a) and its complementary carboxylic acid (c) dimers with an
m-terphenyl backbone joined by chiral Ni(II)- (L2; 2a, 2c),
chiral Co(II)- (L3; 3a, 3c), and achiral Co(II)-salen (L5; 5a,
5c) linkers, respectively (Chart 1). These dimer strands were
prepared from the corresponding metal-free dimers (L1 and
L4; 1a, 1c, 4a, 4c) by metalation with nickel acetate or cobalt
acetate.12
Interestingly, the 5a·5c duplex joined by achiral Co(II)-salen
The duplex formations of a series of the complementary di-
mer strands prepared by mixing a 1:1 molar ratio were con-
Figure 2. (a-c) CD and absorption spectra of 2a, 2c, and 2a·2c (a), 3a, 3c, and 3a·3c (b), and 5a, 5c, and 5a·5c (c) in CDCl3 at 25 °C (0.50
mM). (d and e) The energy-minimized complementary double-helical structures of chiral Ni(II)-salen-linked 2a·2c (d) and achiral Ni(II)-
salen-linked 5a’·5c’ (e) obtained by the semiempirical molecular orbital calculations (see Figure S6) which indicate that the two metal-
salens with anti-geometry are arranged in a left-handed twist manner with a Ni–Ni distance of 4.90 and 5.01 Å for 2a·2c and 5a’·5c’, re-
spectively, and each salen unit possesses one of the enantiomerically-twisted conformations induced by the chirality of the salen (2a·2c)
and a one-handed double-helical framework of 5a’·5c’.
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ACS Catalysis
Table 1. Asymmetric Henry Reaction of o-
linkers showed a relatively intense Cotton effect in the linkage
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chromophore region (Figure 2c), resulting from the amidine chi-
rality of 5a that induces a preferred-handed double-helical struc-
ture when complexed with the totally achiral complementary 5c,
thus further inducing one of the enantiomeric twisted confor-
mations of both achiral Co(II)-salen units (Figure 2e and see
below). The double helices (2a·2c, 3a·3c, and 5a·5c) were stable
in CDCl and showed negligible changes in their CD spectra
within the temperature range from 50 to −10 °C (Figure S8). The
association constants for the double-helix formations between 2a
Methoxybenzaldehyde Catalyzed by a Series of Co(II)-
Salen Dimers and Complementary Double Helicesa
MeO
MeO OH
Co-salen
CHO
NO2
DIPEA (2 mol%)
CH3NO2
10 equiv.
+
CH2Cl2, –30 °C
7
8
catalyst
(Co(II)
-salen)
Co(II)-
salen
(mol%)
conversionb
(%)
yieldb
(%)
eec
(%)
3
time
(h)
entry
9
1
2d
3e
4
3a
3c
2
2
2
4
4
4
4
4
2
2
4
4
2
2
90
90
90
90
90
90
90
90
90
90
115
235
90
90
86
96
98
93
99
92
95
96
96
94
11
23
94
98
67
12
1
55
74
55
89
60
40
66
74
5
and 2c, and 3a and 3c at 25 °C in CDCl were estimated to be ca.
3
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8.2 × 1011 and > 1.8 x 109 M-1 based on the competition 1H NMR
titration (Figures S9 and S10)11d and CD dilution experiments
(Figure S11), respectively.
3c
3a·3c
3a·2c
2a·3c
3a·5c
5a·3c
5a
91
27
20
70
87
38
16
6
5
With this structural information in hand, the catalytic activ-
ity and enantioselectivity of the chiral and/or achiral bimetallic
Co/Co- and Co/Ni-salen-linked complementary double helices
were investigated regarding the asymmetric Henry reaction of
4-nitrobenzaldehyde (7) with nitromethane in CH2Cl2 at
−30 °C (Table 1).14 For comparison, the corresponding Co(II)-
salen-linked dimers along with the model Co(II)-salen com-
plex 6Co were also used as the catalysts. Interestingly, the chi-
ral bimetallic Co(II)-salen-linked double-helix 3a·3c efficient-
ly catalyzed the Henry reaction to produce the nitromethyl
adduct (8) in 91% yield with the excellent enantioselectivity of
89% ee ((S)-rich) (entry 4),15 whereas the corresponding ami-
dine dimer catalyst 3a afforded the product in good yield
(67%), but with a modest enantioselectivity (55% ee) being
comparable to that catalyzed by 6Co (56% ee) (entries 1 and
13).
6
7
8
9
10
11f
12f
13
14e
5c
0
5a·5c
5a·5c
6Co
50
45
56
53
13
17
1
6Co
a
The reaction was carried out in CH2Cl2 or THF at –30 °C. [catalyst] =
24.8 mM. Conversion and yield were estimated by 1H NMR. c Ee values
b
d
e
((S)-rich) were determined by chiral HPLC. Dispersed in CH2Cl2. In
THF. f [5a·5c] = 1.46 mM.
the two achiral Ni-salen units of 5a’·5c’ adopt one of the en-
antiomerically-twisted dynamic conformations20 resulting
from the right-handed double-helical framework generated by
the chirality of the amidine residues of 5a’, which further in-
duces a left-handed helical arrangement of the two salen units
(Figures 2e and S6d); the chiral conformations of the two
salen units of 5a’·5c’ and their helical arrangement are similar
to those of 2a·2c (Figure 2d). Therefore, the analogous achiral
Co/Co-bimetallic salen catalyst 5a·5c enantioselectively pro-
duced the (S)-rich 8 as did 3a·3c.
The carboxylic acid dimer 3c produced 8 with a relatively
higher ee (74%) than 3a and model 6Co, although the product
yield was low (12%)16 (entry 2) because 3c was not soluble in
CH2Cl2 and used in the dispersed state.17
In order to obtain more convincing evidence for the cooper-
ative catalytic and enantioselective activities assisted by the
Co/Co-bimetallic salen catalysts during the Henry reaction,10
one of the chiral Co(II)-salen linkers of 3a·3c was replaced by
the catalytically-inactive chiral Ni(II)-salen linker. The Co/Ni-
bimetallic catalysts (3a·2c and 2a·3c) afforded the product 8
in much lower yields (27 and 20%) accompanied by a de-
crease in the enantioselectivity (60 and 40% ee, respectively)
(entries 5 and 6), demonstrating the important role of the chi-
ral Co/Co-bimetallic salen-linked double-helical nature of
3a·3c on the reaction yield and enantioselectivity for the
present Henry reaction.
The chiral and achiral Co/Co-bimetallic salen-linked dou-
ble-helical catalysts (3a·5c and 5a·3c) also gave 8 in good
yields (70 and 87%) with slightly lower enantioselectivities
(66 and 74% ee, respectively) (entries 7 and 8) than 3a·3c.
Based on these results, we concluded that the bimetallic
Co/Co-salen catalysts mainly contribute to the catalytic effi-
ciency to produce the desired product mostly independent of
the chirality, while the enantioselectivity is determined by
both the one-handed double-helical framework induced by the
chiral amidine residues and the chirality of the salen-linkers.
Thus, the fully double-helical bimetallic chiral Co/Co-salen
catalyst (3a·3c) showed a high efficiency and enantioselectiv-
ity in the present Henry reaction.
A key role of the chiral double-helical framework for su-
pramolecular bimetallic catalysis has been further unambigu-
ously revealed by the achiral Co/Co-bimetallic salen-linked
double-helical catalyst 5a·5c that promoted the Henry reaction
with an encouraging enantioselectivity, yielding the product 8
with a 50-45% ee (entries 11 and 12), while the corresponding
single strands 5a and 5c showed a poor (5% ee) and no enanti-
oselectivity, respectively (entries 9 and 10), although the yield
was low (6-13%) due to the poor solubility of the catalyst
5a·5c in CH2Cl2 (1.46 mM). The energy-minimized structures
of the right-handed double helices joined by the chiral (2a·2c)
and achiral (5a’·5c’) Ni(II)-salen linkers (Figures 2d, e and
S6) suggest that the two chiral amidinium–carboxylate salt
bridges enabled the two strands to be held together, thus inter-
twining into the right-handed double-helical structure11a,d and
We believe that the present bimetallic salen-based chiral
catalysts possessing a double-helical framework with a con-
trolled handedness assisted by the complementary chiral salt
bridges could be further applied to developing unique double-
helical catalysts, to which the desired metal species can be
introduced using the metal-free amidine and carboxylic acid
dimers (1a/1c and 4a/4c) by metalation. Work along this line
is now in progress in our laboratory.
ASSOCIATED CONTENT
Supporting Information
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Page 4 of 5
(9) For reviews on metal salen-based supramolecules: (a) Wezenberg,
The Supporting Information is available free of charge on the
ACS Publications website.
S. J.; Kleij, A. W. Angew. Chem., Int. Ed. 2008, 47, 2354-2364. (b) Akine,
S.; Nabeshima, T. Dalton Trans. 2009, 10395-10408. (c) Wiester, M. J.;
Ulmann, P. A.; Mirkin, C. A. Angew. Chem., Int. Ed. 2011, 50, 114-137.
(10) For a review on the asymmetric Henry reaction: Palomo, C.;
Oiarbide, M.; Mielgo, A. Angew. Chem., Int. Ed. 2004, 43, 5442-5444.
(11) (a) Tanaka, Y.; Katagiri, H.; Furusho, Y.; Yashima, E. Angew.
Chem., Int. Ed. 2005, 44, 3867-3870. (b) Ikeda, M.; Tanaka, Y.; Ha-
segawa, T.; Furusho, Y.; Yashima, E. J. Am. Chem. Soc. 2006, 128,
6806-6807. (c) Furusho, Y.; Yashima, E. Macromol. Rapid Commun.
2011, 32, 136-146. (d) Yamada, H.; Wu, Z.-Q.; Furusho, Y.; Yashima,
E. J. Am. Chem. Soc. 2012, 134, 9506-9520.
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Experimental procedures, characterizations of dimer strands, and
additional spectroscopic data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: yashima@apchem.nagoya-u.ac.jp
9
Notes
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(12) See the Supporting Information.
The authors declare no competing financial interest.
(13) The chiral Co(II)-salen-linked 3a·3c showed a relatively weak
CD in the longer wavelength region when compared to that of the
Ni(II)-salen-linked 2a·2c (Figure 2). The model Co(II)-salen complex
ACKNOWLEDGMENT
6
Co also exhibited a weaker CD than that of 6Ni (Figure S16). Therefore,
This work was supported in part by JSPS KAKENHI (Grant-in-
Aid for Scientific Research (S), no. 25220804 (E.Y.) and Grant-
in-Aid for Young Scientists (B) and 26810048 (D.T.)). J.T. ex-
presses his thanks for the JSPS Research Fellowship for Young
Scientists (no. 8886).
the difference in the CD intensities observed for 2a·2c and 3a·3c may
be originated from the metal species coordinating to the salen unit.
14) Chiral metal-salen complexes developed by Jacobsen et al. ef-
ficiently catalyze the enantioselective epoxide hydrolysis, in which
two metal centers are involved in the transition state. (a) Tokunaga,
M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277,
936-938. (b) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421-431.
(15) The 3a·3c complex exhibited as an intense CD in a mixture of
CH2Cl2 and CH3NO2 (3/2, v/v) in the presence of 1 equiv. of diiso-
propylethylamine (DIPEA) as that in CHCl3 at 25 °C even at a very
low concentration (0.50 mM) (Figure S12), indicating that the double-
helical structure was retained during the Henry reaction. In addition,
the amidinium–carboxylate salt bridges are stable in the presence of a
Co(II)-salen complex (Figure S13).
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(16) Conversion of 7 was quite high (entry 2), despite the low yield
of 8. This is probably due to side reactions during the Henry reaction.
See: Luzzio, F. A. Tetrahedron 2001, 57, 915-945.
(17) We first anticipated that the observed enhancement of the enanti-
oselectivity for 3c could be ascribed to its self-associated homo-double
helix formation with an excess handedness as observed for an analogous
carboxylic acid dimer (CC) (Figure S15b).18 However, the enantioselectivi-
ty of 6Co increased from 56 to 77% ee (10% yield) in the presence of the
carboxylic acid monomer (C) being comparable to that catalyzed by 3c
(74% ee), suggesting the complex formation of the Co(II)-salen unit with
the carboxy groups of 3c19 which may contribute to enhancing the enanti-
oselectivity during the Henry reaction, although such a homo-double helix-
like aggregate formation of 3c could not be completely excluded because
the CD spectral pattern and intensity of 3c were significantly different from
those of 6Co and its complex with C (Figure S17). Diffusion-ordered 1H
NMR spectroscopy (DOSY) measurements for 2c support such an aggre-
gate formation (Figure S15a). The enantioselectivity of 3c decreased (55%
ee) when the reaction was carried out in THF (entry 3), which will hamper
the complex formation between the Co(II)-salen unit and the carboxy
groups as well as the self-associated duplex formation of 3c, leading to
dissociation into single strands, therefore, its CD signal significantly de-
creased in THF (Figure S14).
(18) Makiguchi, W.; Kobayashi, S.; Furusho, Y.; Yashima, E. An-
gew. Chem., Int. Ed. 2013, 52, 5275-5279.
(19) Vinck, E.; Carter, E.; Murphy, D. M.; Van Doorslaer, S. Inorg.
Chem. 2012, 51, 8014-8024.
(20) Achiral metal-salen-based asymmetric catalysts have been de-
veloped by Katsuki, List, and Takata et al., in which one of the enan-
tiomeric conformations was induced by chiral additives or chiral sub-
stituents. (a) Hashihayata, T.; Ito, Y.; Katsuki, T. Synlett 1996, 1079-
1081. (b) Furusho, Y.; Maeda, T.; Takeuchi, T.; Makino, N.; Takata,
T. Chem. Lett. 2001, 1020-1021. (c) Merten, C.; Pollok, C. H.; Liao,
S.; List, B. Angew. Chem., Int. Ed. 2015, 54, 8841-8845.
(3) For reviews: (a) Lehn, J.-M. Supramolecular Chemistry: Con-
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C.; Bernardinelli, G.; Hopfgartner, G. Chem. Rev. 1997, 97, 2005-
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L.; Léger, J.-M.; Huc, I. Angew. Chem., Int. Ed. 2006, 45, 4625-4628. (c)
Guichard, G.; Huc, I. Chem. Commun. 2011, 47, 5933-5941. (d) Gan, Q.;
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(6) For reviews: (a) Boersma, A. J.; Megens, R. P.; Feringa, B. L.;
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Page 5 of 5
ACS Catalysis
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Table of Contents
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5
Double-Helical Framework for Asymmetric Catalysis
R
MeO
CHO
chiral
6
7
8
9
N
N
N
N
Co
or
O
O
–
+
O
O
TMS
TMS
+
N
TMS
TMS
Ph
N
H
tBu
tBu
X
X
Ph
CH3NO2
H
N
H
N
H
X =
Ph
Ph
–
O
achiral
O
Co
MeO OH
O
O
tBu
tBu
NO2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R
*
Enhanced or induced enantioselectivity
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