Enantioselective Diels-Alder reaction in the confined space of homochiral metal-organic frameworks

Article abstract of DOI:10.1039/c6ra23740a

A novel homochiral porous metal-organic framework (MOF) has been synthesized using (R)-2,2′-dihydroxy-1,1′-binaphthyl-4,4′-dibenzoic acid as the chiral ligand. This MOF acts as an effective heterogeneous catalyst for the enantioselective Diels-Alder react

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ARTICLE TYPE
Enantioselective Diels–Alder reaction in confined space of homochiral
metal–organic frameworks
a
a
a
b
b
Koichi Tanaka* , Shohei Nagase , Taku Anami , Michał Wierzbicki and Zofia UrbanczykꢀLipkowska
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
CO
2
H
2
CO H
A novel homochiral porous metal–organic framework
MOF) has been synthesized using (R)ꢀ2,2′ꢀdihydroxyꢀ1,1′ꢀ
(
2
CO H
binaphthylꢀ4,4′ꢀdibenzoic acid as the chiral ligand. This
MOF acts as an effective heterogeneous catalyst for the
enantioselective Diels–Alder reaction between isoprene
and Nꢀethyl maleimide.
HO2
C
C
OH
OH
OH
OH
OH
OH
OH
OH
(
R)-
(R)-
(R)-
(R)-
1
1
2
2
3
3
4
4
0
5
0
5
0
5
0
5
HO
2
2
CO H
1
2
3
4
Porous metal–organic frameworks (MOFs) with high surface
areas and thermal stabilities have attracted much attention
CO
2
H
2
CO H
1
recently owing to their versatile applications in storage,
O
O
a: R = Me
b: R = Et
a: R = Me
b: R = Et
c: R = -Pr
d: R = Ph
e: R = Cyclohexyl
2
3
4
separation, sensing, and catalysis. In particular, chiral
MOFs, which are assembled using chiral organic ligands and
metal ions, are of interest for applications in enantioselective
separation and catalysis that are important for the
pharmaceutical industry. Some homochiral MOFs and their
applications in enantioselective reactions and separation have
N
R
N R
c: R =
d: R = Ph
e: R = Cyclohexyl
n
-Pr
n
O
O
5
6
7
5
5
6
0
5
0
Chiral organic ligand 4 was synthesized by Suzuki crossꢀ
coupling of 4ꢀmethoxycarbonyl phenylboronic acid and (R)ꢀ
4,4′ꢀdibromoꢀ2,2′ꢀdiacetylꢀ1,1′ꢀbinaphthyl (prepared from (R)ꢀ
,4′ꢀdibromoꢀ2,2′ꢀdihydroxyꢀ1,1′ꢀbinaphthyl),
hydrolysis and acidification. (R)ꢀMOFꢀ4 was obtained as
5
been described in the literature, but there has been limited
1
3
4
followed by
success until now. We have previously reported the synthesis
of homochiral (R)ꢀMOFꢀ1 using chiral ligand 1, as well as its
6
7
green prisms after a solvothermal reaction of chiral organic
linker 4 and Cu(NO ) 3H O in a mixed solvent (DMF–H O)
3 2 2 2
application in the asymmetric aminolysis and alcoholysis of
8
epoxides, and in the asymmetric sulfoxidation of sulfides
at 55 °C for 4 days. The structure was characterized by IR
spectroscopy, thermogravimetric analysis, and single crystal
Xꢀray diffraction.The structure was characterized by IR
spectroscopy (Figure S1), thermogravimetric analysis (Figure
S2), solid CD spectroscopy (Figure S3), powder (Figure S4)
and single crystal Xꢀray diffraction. Xꢀray diffraction analysis
reveals that (R)ꢀMOFꢀ4 crystallizes in the trigonal space
using aqueous H O . We have also reported the efficient
2
2
HPLC enantioseparation of several racemates using
homochiral (R)ꢀMOFꢀ2 (prepared from chiral ligand 2) as the
9
chiral stationary phase.
The [4+2] cycloaddition reaction between a diene and a
dienophile, known as the Diels–Alder reaction, is one of the
most powerful methods for C–C bond construction in
1
0
65 group R32 (hexagonal axes) † Asymmetric unit of (R)ꢀMOFꢀ4
.
synthetic organic chemistry.
In particular, asymmetric
consists of diꢀcopper(II) cations, two anionic ligands 4, three
molecules of N-methylformamide (including two disordered
between two positions with ca. 0.64:0.36 occupancy) and one
molecule of methanol. As expected, metalꢀorganic framework
is organized around diꢀcopper(II) ions as quasi squareꢀplanar
network (Figure 1). It contains one well localized DMF
molecule that is Hꢀbonded to one of the hydroxy groups of the
ligand and two disordered DMF molecules that are
coordinated in axial positions of both Cu(II) atoms. In
catalytic variants of this reaction have received much
attention, owing to their ability to rapidly provide
1
1
enantioenriched carbocycles from simple substrates. Chiral
MOFs are promising candidates for these asymmetric Diels–
Alder reactions because they can encapsulate the reactants and
organize them in a confined chiral space that contains Lewis
acidic metal sites. However, there has been only two reports
on MOFꢀcatalyzed heterogeneous Diels–Alder reactions;
7
7
8
0
5
0
1
2
however, these were not enantioselective. Herein, we report
the synthesis of novel homochiral (R)ꢀMOFꢀ4 and its
successful application as a heterogeneous catalyst in the
enantioselective Diels–Alder reaction between isoprene and
Nꢀethyl maleimide.
contrast
to
previously
reported
relatively
porous
1
2b
interpenetrating homochiral (R)ꢀMOFꢀ3, in the present case
ca 88 % of crystal volume is occupied by already localized
R)ꢀMOFꢀ4 and DMF molecules (Figure 2). As seen in Figure
, crystals contain discrete voids of approximately 4.6 Å
radius that are not arranged in tunnels pointing accros the
(
1
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whole crystals. However, when all solvent molecules present
in crystal are removed another channel like of a 2.4 Å radius
are formed around catalytic centres that may afford crystal
penetration by relatively small linear organic molecules
(Figure 1, bottom part). In this case about 50.1 % of the
crystal volume is accessible. Therefore, one might assume that
enantioselection may take place inside of an internal empty
larger chiral cavity generated by evacuation of included DMF
solvents. Thus the reaction of relatively samll reactant may
occur in the cavity.
Table 1 Effect of temperature on the asymmetric DielsꢀAlder reactions of
isoprene and Nꢀethyl maleimide
O
2
3
3
5
0
5
O
(
R)-MOF-4
+
N
Et
N
Et
5
20℃, 24 h, solvent
5
O
6b
7b
O
a
Entry
Solvent
Yield (%)
Ee (%)
1
2
3
4
5
6
7
8
MeOH
EtOH
i-PrOH
AcOEt
43
66
87
49
85
77
59
80
0
21
33
71
13
21
63
31
10
CHCl
Toluene
3
Cyclohexane
n-Hexane
a
HPLC column: Chiralcel OBꢀH, Eluent: Hexane/i-PrOH = 90/10, Flow
40
rate: 0.2mL/min, Detection: UV 220 nm
Crystalline evacuated (R)ꢀMOFꢀ4 exhibited excellent
catalytic activity in the asymmetric Diels–Alder reaction
between isoprene and Nꢀethyl maleimide when used as a
heterogeneous catalyst. When isoprene
5 and Nꢀethyl
4
5
5
5
0
5
maleimide 6b were stirred in various solvents in the presence
of evacuated (R)ꢀMOFꢀ4 at 20 °C for 24 h, optically active
adduct 7b was formed in the yields shown in Table 1. When
the reaction was carried out in AcOEt or cyclohexane,
relatively higher enantioselectivities (71% ee and 63% ee,
respectively) were obtained (Table 1, entries 4 and 7). In
contrast, reactions in EtOH, i-PrOH, CHCl , toluene, and n-
3
hexane resulted in lower enantioselectivities (Table 1, entries
2
, 3, 5–8), and only the racemic product was isolated when the
reaction was performed in MeOH (Table 1, entry 1).
Table 2 Effect of temperature on the asymmetric DielsꢀAlder reactions of
isoprene and Nꢀethyl maleimide
O
O
(
R)-MOF-4
AcOEt
+
N
Et
N
Et
Figure 1. 2D coordination polyhedron of (R)ꢀMOFꢀ4 showing
6
6
0
5
15
coordination of diꢀcopper(II) ion by a chiral ligand 4 and presence of
discrete void spaces in case when DMF/MeOH molecules localized in Xꢀ
ray studies are present (upper Figure) and when they were all removed
5
O
6b
7b O
a
Entry
Temp (°C)
Time (h)
Yield (%)
Ee (%)
(bottom Figure).
1
2
3
4
5
6
0
0
0
24
48
72
24
24
24
33
81
70
49
83
61
73
75
71
71
59
55
20
40
60
a
70
HPLC column: Chiralcel OBꢀH, Eluent: Hexane/i-PrOH = 90/10, Flow
rate: 0.2mL/min, Detection: UV 220 nm
Next, we examined the effect of reaction temperature (Table
2
), and the best result (81% yield, 75% ee) was obtained when
75
the reaction was carried out at 0 °C for 48 h (Table 2, entry 2).
The enantioselectivity of the reaction decreased at elevated
temperatures (entries 4–6).
To explore this catalytic activity further, we used a variety of
Nꢀsubstituted maleimide derivatives (Table 3) as dienophiles.
Nꢀmethyl maleimide 6a showed lower reactivity and
enantioselectivity than 6b (Table 3, entry 1). Sterically bulky
Nꢀsubstituted maleimides 6c–6e resulted in poor catalytic
efficiency in terms of both product yield and
80
20
Figure 2. Interwoven network of (R)ꢀMOFꢀ4 showing viewed down
triple symmetry.
2
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DOI: 10.1039/C6RA23740A
enantioselectivity. This may be due to the weaker
encapsulation of sterically bulky dienophiles in the chiral
channels of (R)ꢀMOFꢀ4.
Table 5 Control experiment of DielsꢀAlder reactions of isoprene and Nꢀ
ethyl maleimide
60
O
O
Cat.
Table 3 Asymmetric DielsꢀAlder reactions of isoprene and Nꢀsubstituted
maleimide
+
N
Et
N
Et
5
0
℃, 48 h, AcOEt
O
O
5
6b
7b
O
O
6
5
(R)-MOF-4
a
Entry
Cat.
none
Yield (%)
Ee (%)
+
N
R
6
N R
1
2
3
4
5
10
25
21
0
0
0
0
0
℃, 48 h, solvent
Cu(OAc)
(R)ꢀBINOL
2
5
O
7
O
1
1
0
5
a
70
Cu(OAc) +(R)ꢀBINOL 10
Entry
R
Solvent
Yield (%)
Ee (%)
2
(R)ꢀMOFꢀ4
81
75
1
2
3
4
5
Me
Et
n-Pr
Ph
AcOEt
AcOEt
AcOEt
AcOEt
37
81
11
17
18
37
75
5
7
0
a
HPLC column: Chiralcel OBꢀH, Eluent: Hexane/i-PrOH = 90/10, Flow
rate: 0.2mL/min, Detection: UV 220 nm
Cyclohexyl AcOEt
a
75 In summary, the synthesis and structural elucidation of a
novel extended homochiral MOF, (R)ꢀMOFꢀ4 with a nonꢀ
interpenetrating framework, were successfully achieved. With
the free void space of homochiral (R)ꢀMOFꢀ4, it is reasonable
to assume that the reactions occur inside its chiral pores.
HPLC column: Chiralcel OBꢀH, Eluent: Hexane/i-PrOH = 90/10, Flow
rate: 0.2mL/min, Detection: UV 220 nm
2
0
The catalytic performances of (R)ꢀMOFꢀ1, (R)ꢀMOFꢀ2, and
(
R)ꢀMOFꢀ3 in the asymmetric Diels–Alder reaction between
isoprene and Nꢀethyl maleimide were compared with that of
R)ꢀMOFꢀ4. (R)ꢀMOFꢀ1 and (R)ꢀMOFꢀ2 afforded the
8
8
9
0
5
0
Obtained (R)ꢀMOFꢀ4 exhibited excellent catalytic activity as
a heterogeneous catalyst in the asymmetric Diels–Alder
reaction between isoprene and Nꢀethyl maleimide. Balky
reactants have been shown to have lower reactivity and
enantioselectivity due to their weaker interaction with the
chiral MOF. Further study of the mechanism and scope of this
reaction is now underway.
This study was supported by a GrantꢀinꢀAid for Scientific
Research (C) (No.26410126) from The Ministry of Education,
Culture, Sports, Science and Technology (MEXT). We thank
the PETRA III synchrotron facility in Hamburg for the
allocation of synchrotron radiation beamtime.
(
optically active products with 5% ee and 25% ee, respectively
(Table 4, entries 1 and 2), whereas interpenetrating (R)ꢀMOFꢀ
3
(
2
5
gave low product yields and did not show enantioselectivity
Table 4, entry 3).
Table 4 Asymmetric DielsꢀAlder reactions of isoprene and Nꢀethyl
maleimide catalyzed by various chiral MOFs
3
0
O
O
MOF
+
N
Et
N
Et
0
℃, 48 h, AcOEt
5
O
6b
7b
O
Notes and references
3
4
5
0
a
a
Entry
MOF
none
Yield (%)
Ee (%)
Department of Chemistry and Materials Engineering, Faculty of
1
2
3
4
5
10
12
15
11
81
0
5
25
0
75
Chemistry, Materials and Bioengineering, Kansai University, Suita,
95 Osaka 564-8680, Japan. Fax: +81-06-6368-0861; Tel: +81-06-6368-
(R)ꢀMOFꢀ1
(R)ꢀMOFꢀ2
(R)ꢀMOFꢀ3
(R)ꢀMOFꢀ4
0861; E-mail: ktanaka@kansai-u.ac.jp
b
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka
44/52, 01-224 Warszawa, Poland. Fax: +48 22 6326681; Tel: +48 22
3432207; E-mail: ocryst@icho.edu.pl
00 Electronic Supplementary Information (ESI) available: [synthesis of (R)ꢀ
MOFꢀ4, catalitic DielsꢀAlder reactions, IR, CD, TG spectra and HPLC
chromatographic data, XRD data]. See DOI: 10.1039/b000000x/
a
HPLC column: Chiralcel OBꢀH, Eluent: Hexane/i-PrOH = 90/10, Flow
rate: 0.2mL/min, Detection: UV 220 nm
1
1
1
†
Crystal data for (R)ꢀMOFꢀ4: C78
H
64Cu2No
3
O16, MW = 1426.40, a = b =
4
5
5
5
0
5
We also performed control experiments to study the
confinement effect of (R)ꢀMOFꢀ4 on the catalyst. When the
reaction was performed using Cu(OAc)2 as the catalyst,
cycloaddition product racꢀ7b was obtained in 25% yield
3
3
3.835(5), c = 39.859(8) Å, β = 120.0 , V = 39517(14) Å , F(000) =
ꢀ3
05
13302, dexp = 1.328(4) and dcalc = 1.079 Mg m , trigonal; space group
R32 (No. 155), Z = 18, ꢀ(MoKα) = 0.541 cm , λ = 0.6888 Å, T =
100(2) K, 137923 reflections measured, 17540 unique (Rint =
ꢀ1
0
1 2
.0292), final R = 0.0732, wR = 0.2230, for 16757 observed
(
Table 5, entry 2). When the reaction was performed using
reflections with I > 2σ(I); GOF = 1.041. CCDC: 1483410. These
data can be obtained free of charge from the Cambridge
Crystallographic
viawww.ccdc.cam.ac.uk/data_request/cif.
(R)ꢀ1,1′ꢀbiꢀ2ꢀnaphthol (BINOL), racꢀ7b was obtained in 21%
yield (Table 5, entry 3). With a combination of (R)ꢀBINOL
and Cu(OAc)2 as the catalyst, the reaction proceeded with
similar efficiency, affording racꢀ7b in 10% yield (Table 4,
entry 4), which is comparable to that for the reaction without
any catalyst (Table 4, entry 1). The catalyst was separated by 115
filtration and washed several times with methanol.The
recovered catalyst can be used for further catalytic cycles.
10
Data
Center
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DOI: 10.1039/C6RA23740A
Enantioselective Diels–Alder reaction in confined space of homochiral
metal–organic frameworks
Koichi Tanaka*, Shohei Nagase, Taku Anami, Michał Wierzbicki, Zofia Urbanczyk-Lipkowska
A novel homochiral porous metal–organic framework (MOF) has been synthesized using (R)-2,2′-dihydroxy-
1,1′-binaphthyl-4,4′-dibenzoic acid as chiral ligand, which acts as an effective heterogeneous catalyst for
asymmetric Diels–Alder reaction between isoprene and N-ethyl maleimide.
O
+
O
N
Et
N
Et
O
Yield: 85%, Ee: 75%
O