Chiral metal?Organic framework as a platform for cooperative catalysis in asymmetric cyanosilylation of aldehydes

Article abstract of DOI:10.1021/acscatal.6b02359

In this work, we demonstrate cooperative asymmetric catalysis by a metal?organic framework (MOF) as exemplified in the context of catalyzing cyanation of aldehydes with a VO(salen)-MOF, which after oxidation affords remarkably increased stereoselectivity (up to >99% ee) compared to the homogeneous VO(salen) counterpart as a result of the pairs of VO(salen) units in close proximity within its open channels. The cooperative asymmetric catalysis has been evidenced by the significantly decreased stereoselectivity and activity when one VO(salen) in such pairs of VO(salen) units is replaced with one Cu(salen), which results in blocking the VO?VO synergistic pathway while prompting unimolecular activation of substrates. The heterogeneous nature of VO(salen)-MOF has been verified by the fact that it can be easily recycled and reused without significant loss of catalytic activity and enantioselectivity, and its practical utility as asymmetric cyanation catalysist has been illustrated in the gram-scale synthesis of the antiviral natural products (R)- and (S)-enantiomers of tembamide. Our work therefore advances chiral MOF as an attractive platform for cooperative asymmetric catalysis in a variety of syntheses.

Full text of DOI:10.1021/acscatal.6b02359

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Article
Chiral Metal-Organic Framework as a Platform for Cooperative
Catalysis in Asymmetric Cyanosilylation of Aldehydes
Chengfeng Zhu, Qingchun Xia, Xu Chen, Yan Liu, Xia Du, and Yong Cui
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02359 • Publication Date (Web): 02 Oct 2016
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Chiral MetalꢀOrganic Framework as a Platform for
Cooperative Catalysis in Asymmetric Cyanosilylation of
Aldehydes
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†
, §
† , §
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*,†
†
*,†,‡
Chengfeng Zhu, Qingchun Xia, Xu Chen, Yan Liu, Xia Du, and Yong Cui
†
School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites,
Shanghai Jiao Tong University, Shanghai 200240, China
‡
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
ABSTRACT: In this work we demonstrate for the first time the cooperative asymmetric catalysis by
metalꢀorganic framework (MOF) as exemplified in the context of catalyzing cyanation of aldehydes
with a VO(salen)ꢀMOF, which after oxidation affords remarkably increased stereoselectivity (up to >99%
ee) compared to the homogeneous VO(salen) counterpart as a result of the pairs of VO(salen) units in
close proximity within its open channels. The cooperative asymmetric catalysis has been evidenced by
the significantly decreased stereoselectivity and activity when one VO(salen) in such pairs of VO(salen)
units is replaced with one Cu(salen), which results in blocking the VOꢀVO synergistic pathway whilst
prompting unimolecular activation of substrates. The heterogeneous nature of VO(salen)ꢀMOF has been
verified by the fact that it can be easily recycled and reused without significant loss of catalytic activity
and enantioselectivity, and its practical utility as asymmetric cyanation catalysist has been illustrated in
the gram scale synthesis of the antiviral natural products (R)ꢀ and (S)ꢀenantiomers of tembamide. Our
work therefore advances chiral MOF as an attractive platform for cooperative asymmetric catalysis in a
variety of syntheses.
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KEYWORDS: metal-organic framework, porous material, chiral material, asymmetric catalysis,
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heterogeneoous catalysis.
INTRODUCTION
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Inspired by the intriguing mode of action of bimetallic enzymes and their impressive catalytic
1
performance, the synergistic cooperation of multiple metal centers has been intensively studied for
artificial catalyst systems, as they often show higher reactivity and selectivity than the analogous
2
mononuclear species. The proper arrangement of active centers in close proximity is the key to the
success for efficient catalysis. By employing metalꢀmetal, metalꢀorgano and organoꢀorgano catalysts,
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cooperative catalysis has become a powerful tool, especially in asymmetric synthesis, allowing access
3b,3c
to new chemical reactivity and high stereocontrol in diverse reactions.
However, it remains difficult
to create such a cooperative activation in a solid catalyst because of its inability to control the proper
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proximity and conformation of active centers.
Metalꢀorganic frameworks (MOFs) offer great potential in heterogeneous catalysis due to their large
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,6
surface areas, composition tunability and wellꢀdefined reaction environments. Among the asymmetric
MOF catalysts reported to date, the most efficient examples all contain privileged chiral ligands such as
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ꢀ10
BINOLꢀ and salenꢀbased derivatives.
Incorporation of molecular catalysts in the MOFs allows for the
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possibility of spatial control of the different groups. They can be either separated by far away from
each other to activate substrates in a unimolecular fashion or intimately mixed with suitable orientations
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to achieve the synergistic activation catalysis. Nonetheless, MOFs that take advantage of cooperative
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effects to improve catalytic activities and/or selectivities have been rarely reported.
It’s been known that VO(salen) complexes are asymmetric catalysts for cyanation of aldehydes to
1
2
produce cyanohydrins that follow either a monoꢀ and bimetallic coꢀactivation mechanism. Albeit the
bimetallic pathway gives intrinsically faster catalysts with higher stereoselectivity, it remains a
challenge to control the reaction mechanism particularly in homogeneous systems. Nevertheless, such
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challenge can be addressed via the construction of two chiral porous MOFs based upon VO(salen) and
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Cu(salen) complexes as reported herein. The channel surfaces of the constructed two chiral porous
MOFs are decorated with pairs of VO(salen) or VO(salen) and Cu(salen) units in close proximity that
are capable of asymmetrically catalyzing cyanation of aldehydes via a bimetallic or monometallic
activation pathway. Our work thus represents the very first example of cooperative asymmetric catalysis
by MOF.
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RESULTS AND DISCUSSION
Scheme 1. Synthesis of MOFs 1 and 2.
Synthesis and characterization. As shown in Scheme 1, oliveꢀgreen single crystals of
[
Cd (VOL) ]·5H O (1) and [Cd (VOL)(CuL)]·4H O (2) were obtained by heating Cd(NO ) ·6H O and
2 2 2 2 2 3 2 2
VO(H L) (a 1:1 molar ratio) or [VO(H L)]/[Cu(H L)(H O)] (1:0.5:0.5 molar ratio) in DMF and EtOH
2
2
2
2
at 100 °C, respectively. The products were stable in air and insoluble in water and common organic
solvents and were formulated based on elemental analysis, IR spectroscopy and thermogravimetric
analysis (TGA). The 1:1 molar ratio of V to Cu in 2 was confirmed by ICPꢀOES (Inductively coupled
plasma optical emission spectrometry). The phase purity of the bulk samples was verified by the
consistence between their observed and simulated powder Xꢀray diffraction (PXRD) patterns.
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1
adopts a 3D chiral porous framework and crystallizes in the chiral orthorhombic space group
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P222 . The basic building block is infinite CdꢀOꢀC chains linked together by VOL ligands (Figure 1a).
1
Each V center adopts a squareꢀpyramidal geometry with the equatorial plane occupied by the N O
2
2
donors of one L ligand and the apical position by one double bond oxygen atom. The V=O bond lengths
1
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of 1.382(8) and 1.421(11) Å are typical Vꢀoxido species. Of the four independent sixꢀcoordinated Cd
ions, two (Cd1 and Cd4) are coordinated by two water molecules and four oxygen atoms from two
bidentate and two tridentate carboxylate groups, another two (Cd2 and Cd3) are coordinated by six
oxygen atoms from two bidentate and four tridentate carboxylate groups. The infinite CdꢀOꢀC rods are
thus linked by VOL ligands to give a 3D framework with 1D quadrilateral channels of 1.26 nm along an
edge and 2.72 nm along the diagonal (Figure 1c). The channel surfaces are uniformly lined with chiral
VOL units with coordinatively unsaturated V ions that are accessible to guest molecules.
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(a)
(b)
7.87 Å
7.35 Å
(c)
(d)
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Figure 1. Xꢀray structure of (R)ꢀ1 and (R)ꢀ2. Cdꢀ carboxylate chains linked by VOL ligands in 1 (a) and a mixed
ligands of VOL and CuL in 2 (b). View of 3D porous structures of 1 (c) and 2 (d) along the cꢀaxis (the Cd atoms are
shown in polyhedra). Cd yellow, V green, Cu purple, O red, C gray, N blue.
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2 is isostructural to 1 and has an almost identical porous 3D structure with open channels of 1.2 × 0.8
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nm along the aꢀaxis. In contrast to 1, the infinite CdꢀOꢀC chains in 2 are linked together by an equal
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mixture of VOL and CuL(H O) ligands (Figure 1b and 1d). PLATON calculations show that both 1 and
2
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2
have about 56% of void spaces available for guest inclusion.
Solidꢀstate circular dichroism (CD) spectra of 1 and 2 made from R and S enantiomers of H L are
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mirror images of each other, suggesting their enantiomeric nature. XPS (Xꢀray Photoelectron
Spectroscopy) spectra of 1 and 2 show the appearance of V 2p3/2 line at ~516.1 eV, indicative of +4
oxidation state for the V ions. TGA (thermal gravimetric analysis) reveals that the guest molecules
o
could be removed in the temperature range from 100 to 220 C and the frameworks are stable up to
o
~
370 C. PXRD indicated that their framework and crystallinity remain intact upon removal of the guest
molecules. Their permanent porosities were confirmed by N adsorption isotherms at 77K and liquidꢀ
2
phase adsorptions. Both 1 and 2 exhibited a TypeꢀI sorption behavior, with BET surface areas of 628
2
and 690 m /g, respectively (Figure 2). They could adsorb ~1.4 methyl orange (MO, ~1.47 nm × 0.53 nm
×
0.53 nm in size) per formula unit and the inclusion samples had almost the same PXRD pattern as the
pristine solids.
a)
(
(
b)
Figure 2. N sorption isotherms in 1 (a) and 2 (b) at 77 K.
2
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Asymmetric catalysis. We evacuated the catalytic activities of 1 (10 mol% loading) in the context
of cyanation reactions between aromatic aldehydes and TMSCN but with low yields were observed.
This may be attributed to that weak Lewis acidity of V(IV) does not favor its strong binding to the
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substrate thus not facilitating the reaction to occur. Treatments of 1 or 2 with (NH ) Ce(NO )
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3 6
V
afforded the oxidated V OꢀMOFs 1a and 2a, accompanying with a slightly color change from dark
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green to green. The individual crystals remained transparent but with apparent fracturing, preventing us
to get their singleꢀcrystal structures. The oxidation of V(IV) to V(V) was confirmed by the V 2p3/2 line
appeared at ~518.0 eV in the XPS spectra. PXRD analysis showed that the oxidized samples 1a and 2a
2
remained high crystallinity. They displayed decreased BET surface areas of 543 and 557 m /g,
respectively, as compared with the parent MOFs and still can adsorb ~1.3 MO per formula unit in
solution, indicative of their structural integrity and porosity.
After optimization of reaction conditions, 1a was found to be an active catalyst for cyanation of
o
aldehydes with TMSCN at ꢀ78 C. Typically, 0.5 mol% loading of (R)ꢀ1a with regard to the aldehydes
afforded cyanohydrin trimethylsilyl ethers in 96ꢀ99% conversions and 82ꢀ99% ee in 24h (Table 1).
Cyanation of benzaldehyde by (S)ꢀ1a gave the R enantiomer over the S enantiomer (87% ee), indicating
that the chiral nature of the product was controlled by the handedness of the catalyst. The catalytic
reactions proceeded in excellent conversions and good to excellent enantioselectivity with
benzaldehydes bearing an electronꢀdonating group, but the introduction of an electronꢀwithdrawing ꢀBr
group into the phenyl ring of the substrate led to slightly decreased enantioselection (82% ee, entry 5).
Heterocyclic compounds 2ꢀfurylaldehyde and 3ꢀpyridylaldehyde gave ~99% and 98% conversions with
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3% and 82% ee, respectively. For the vinylꢀtype aromatic aldehyde cinnamaldehyde, 95% ee and 97%
conversion were obtained. 1ꢀ and 2ꢀnaphthaldehydes were readily converted to the products with 96%
and >99% ee, respectively.
When a more sterically demanding aldehyde, 3ꢀGOC H CHO, was subjected to the cyanation
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condition, less than 5% conversion was observed after 24 h, which was much lower than ~62%
V
conversion obtained with V O(Me L). This result showed that this bulky substrate may not access the
2
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catalytic sites located within the pore walls due to the size exclusion, which conversely supports that the
cyanosilylation is indeed occurring within the pores for other smaller substrates.
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a
Table 1. Cyanation of Aldehydes Catalyzed by the MOFs.
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d
For reaction details see Experimental section. Catalyst loading based on aldehyde. Calculated by H NMR. ee
e
values were determined by HPLC (letters in brackets specify the preferable isomer). Catalyzed by 1 mol% (R)ꢀ
f
VO(Me L) and reaction time of 12 h. See Figure S1 for the structure of 3ꢀGOC H CHO.
2
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The supernatant from cyanation of benzaldehyde after filtration did not give any additional
cyanohydrin product. Upon completion of the reaction, (R)ꢀ1a could be recovered in quantitative yield
and used repetitively several times, as shown in Table 2. The initial TOF of the catalyst was gradually
decreased upon recycling, which is typical of supported catalysts, but the recycled MOF catalyst still
afforded 91% conversion of benzaldehyde and 84% ee of the product after ten cycles. These results
indicate that the MOF catalyst had good efficiency and recyclability in enantioselective cyanation of
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aldehydes. PXRD analysis indicated that 1a remained its crystallinity after ten cycles, although a
structural distortion occurred. Moreover, the solid was still able to adsorb ~1.3 MO per formula unit in
solution, further suggesting the maintenance of its structural integrity. The XPS measurement showed
that V in the recovered MOF retained +5 oxidation state, as evidenced by the V 2p3/2 line at ~518.0 eV.
ICPꢀOES analysis of the product solution indicated negligible loss of the metal ions (~0.0002% for Cd
and ~0.0035% for V from the structure per cycle).
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Table 2. Recycling Studies of 1a in cyanation of benzaldehyde
b
ꢀ1 d
cycle
Conv. (%)
Initial TOF (h )
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h
12 h
90
88
88
82
82
81
83
80
78
24 h
c
c
c
c
c
c
c
c
c
c
1
2
44
46
46
43
42
44
42
42
41
38
98 (85)
97 (87)
98 (84)
94 (82)
94 (85)
92 (87)
93 (85)
91 (85)
92 (85)
23.1
24.5
21.7
19.4
19.4
19.2
19.0
19.0
18.6
18.1
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10
78
91 (84)
a
b
1
d
For reaction details see Experimental section. Calculated by H NMR. ee values after 24 h that were determined by
HPLC.
V
V
Using 1.0 mol% V O(Me L) (the same loading of V O(salen) as the heterogeneous reaction),
2
cyanation of benzaldehyde and 4ꢀmethoxybenzaldehyde, 4ꢀbromobenzaldehyde and 2ꢀfurylꢀaldehyde
after 12 h afforded 99% conversion with 78% ee of the product and 97% conversion with 81% ee, 99%
conversion with 73% ee and 99% conversion with 79% ee, respectively. The frameworkꢀconfined
catalyst thus gave about 8ꢀ15% higher ee values than its homogeneous counterpart, although it needed
longer reaction time for the diffusion of reactants into the pores.
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V
Table 3. Cyanation of Benzaldehyde Catalyzed by (R)-1a and (R)-V O(Me L) at Different
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a
Catalyst/Substrate Ratios.
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c
For reaction details see Experimental section. The molar ratio of VO(salen) to benzaldehyde. Conversions were
1
d
calculated by H NMR. ee values were determined by HPLC (letters in brackets specify the preferable
e
isomer). Obtained in 12h.
The difference between the heterogeneous and homogeneous catalysts in catalytic performances
became even more significant as the C/S ratio (the molar ratio of VO(salen) to substrate) decreased. As
shown in Table 3, at a C/S of 1:1000, 1a is still capable of catalyzing cyanation of benzaldehyde
V
affording 78 % conversion with 82% ee of the product in 24 h, and V O(Me L) gave 74% conversion
2
with 70% ee. When the C/S ratio was decreased from 1:1000 to 1:5000, 1a is able to afford 42 %
V
conversion with 78% ee whereas V O(Me L) only gives 27% conversion with 51% ee. Even at a C/S
2
ratio as low as 1:10000, 1a can catalyze the reaction giving 31% conversion with 68% ee, whilst
V
V O(Me L) only gave <5% conversion. These results highlight that the VO(salen) catalyst confined in
2
the MOFs exhibits much higher activity than the homogeneous counterpart, which indicates that the
V
V OꢀMOF catalyzed reactions could go through a different pathway from that of homogeneous
reactions.
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A careful examination of the crystal structure of 1 revealed the channel surfaces are uniformly lined
with twisted VO(salen) units with open V sites pointing to the open channels. Importantly, the adjacent
VꢀV distances with a faceꢀtoꢀface orientation for both active sites in the channel surfaces are 7.87 Å.
Intermolecular cyanide transfer is only possible if two adjacent VO(salen) units adopt a faceꢀtoꢀface
orientation related to their open sites, thus allowing the activated aldehyde and cyanide to be located
close to each other. Therefore, the appropriate proximity and spatial orientation of adjacent VO(salen)
units with a distance of 7.87Å is anticipated to facilitate that the cyanide activated by one VO(salen)
species could attack the aldehyde activated by another VO(salen). As a result, the main cycle of the
cyanation catalysis may thus be switched from the two parallel catalytic cycles to the bimetallic cycle,
thereby leading to significant increase in the reaction rate and stereoseletivity. Whereas in homogenous
systems, the possibility for VO(salen) species to meet each other decreases when at much lower C/S
ratios, thus unfavorable for the bimetallic activation process thereby resulting in dramatic drop in
catalytic activitys. In contrast, even at lower C/S, the MOF catalyst is still capable of providing the
bimetallic cooperative activation, thus to afford much better substrate conversions and enantioselectivity.
In order to prove for the presence of the synergistic dual activation catalysis, we examined the
catalytic activity of 2a that contains the close proximity of the Cu(salen) and VO(salen) facing each
other, precluding the possibility of a cooperative VOꢀVO mechanism of nucleophile and electrophile
activation. As shown in Table 1, 1.0 mol% Rꢀ2a (the same loading of VO(salen) as Rꢀ1a) catalyzed
cyanation of benzaldehyde, 3ꢀmethoxybenzaldehyde, 4ꢀmethoxybenzaldehyde, 1ꢀnaphthaldehyde and 2ꢀ
naphthaldehyde to afford 49% conversion with 76% ee of the product and 47% conversion with 64% ee,
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5% conversion with 74% ee, 51% conversion with 69% ee and 49% conversion with 77% ee,
respectively. Obviously, both conversions and ee values are significantly lower (about 41ꢀ51% and 10ꢀ
1%, respectively) than those observed for 1a. Several tests also demonstrated the heterogeneous nature
of 2a in cyanation reactions.
3
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(a)
(b)
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Figure 3. Proposed bimolecular (a) and unimolecular (b) activation pathways for cyanation of aldehyde by 1a and 2a,
respectively.
When Cu(Me L)(H O) was used as the catalyst, only a trace of products was detected in cyanation of
2
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2b,18
benzaldehyde, 4ꢀmethoxybenzaldehyde or 2ꢀnaphthaldehyde.
It suggests that the VO(salen)
moieties, instead of Cu(salen) and Cd centers in 1a and 2a, are the active sites for the cyanation. In
contrast to 1a with fixed pairs of VOL to generate cooperative effects, substituting one CuL for one
VOL in each pair of VO(salen) may block the VOꢀVO synergistic pathway and prompt the
unimolecular activation of substrates (Figure 3), leading to decreased stereoselectivity and activity.
Synergistic catalysis by the VOꢀCu pair in 2a might also be involved in the reaction, but should not be
the dominated pathway, because such cooperativity usually improves the performance of a catalyst.
V
Indeed, cyanation of benzaldehyde with 2a gave much lower reactivity than V O(Me L), at different
2
C/S ratios. Meanwhile, both the reactivity and stereoselectivity are significantly lower than those
obtained with 1a. In particular, at a C/S ratio of 1:10000, 2a almost can not catalyze the reaction, but 1a
is able to afford 31% conversion with 68% ee of the product over 24 h, further suggesting that cyanation
reactions catalyzed by 2a mainly follow a unimolecular activation way. A more inꢀdepth investigation
of the mechanistic aspect of this catalytic system is still in progress.
The silylcyanation of aldehydes and ketones catalyzed by asymmetric homogeneous catalysts to
produce enantiopure cyanohydrins, which are versatile synthetic intermediates for pharmaceuticals, has
1
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received much attention. The present enantioselectivities for cyanation of aldehydes are comparable
with those of reported highꢀperforming homogeneous VO(salen) catalysts and higher than those of other
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heterogeneous VO(salen) catalysts immobilized on diverse supports such as mesoporous silica, carbon
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nanotubes, and activated carbon. Note that cyanation of aldehydes catalyzed by MOFs, including two
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d,10f
asymmetric examples with moderate to high enantioselectivtiy has also been documented.
The work
is expected to inspire new types of enantioselective heterogeneous cooperative catalysts based on
crystalline frameworks.
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The utility of the present cyanation procedure for practical synthesis was demonstrated in the gram
scale synthesis of (R)ꢀ and (S)ꢀenantiomers of tembamide, which have different biological activities,
displaying good hypoglycemic activity and the antiꢀHIV activity, respectively. With 0.5% mol loading
of 1a as the catalyst, the 4ꢀmethoxybenzaldehyde was converted to cyanohydrin trimethylsilyl ether and
then cyanohydrin acetate in 90% yield with 94% ee (Scheme 2). The reduction of cyanohydrin acetate
with LiAlH afforded the intermediate amino alcohol, which was acylated with benzoyl chloride gave
4
(R)ꢀ or (S)ꢀ tembamide in 80% twoꢀstep yield with 94% ee.
Scheme 2. Synthesis of (R) and (S)-tembamides.
CONCLUSIONS
In summary, we reported two chiral porous MOFs of VO(salen) and Cu(salen) complexes and, after
oxidation, the VOꢀMOF displayed enhanced stereoselectivity and comparable activity in cyanation of
aldehydes when compared to its homogeneous counterpart via an efficient VOꢀVO cooperative
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ACS Catalysis
activation pathway. Such cooperativity was evidenced by the control experiments on the VO/CuꢀMOF,
which gave much more decreased activity and steroselectivity through a monometallic activation
pathway. Our work therefore advances chiral MOF as a new platform for cooperative asymmetric
catalysis in a variety of syntheses, and also highlights the unique role of MOF catalysts in the study of
reaction pathways/mechanisms and in organic synthesis by exploiting novel and distinct reaction
pathways.
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EXPERIMENTAL SECTION
Materials and general procedures. All reagents and solvents used in these studies are commercially
available and used without further purification. Elemental analyses were performed with an EA1110
CHNSꢀ0 CE elemental analyzer. The IR (KBr pellet) spectra were recorded (400ꢀ4000 cmꢀ1 region) on
a Nicolet Magna 750 FTꢀIR spectrometer. The CD spectra were recorded on a Jꢀ800 spectropolarimeter
(Jasco, Japan). All UV/Vis absorption spectra were recorded on a Lambda 20 UV/Vis Spectrometer
(Perkin Elmer, Inc., USA). Thermogravimetric analyses (TGA) were carried out in an air atmosphere
o
with a heating rate of 10 C/min on a STA449C integration thermal analyzer. Powder Xꢀray diffraction
(
PXRD) data were collected on a DMAX2500 diffractometer using CuꢀKα radiation. The calculated
PXRD patterns were produced using the SHELXTLꢀXPOW program and single crystal reflection data.
H and 13C NMR experiments were carried out on a MERCURYplus 400 spectrometer operating at
1
resonance frequencies of 100.63 MHz. Electrospray ionization mass spectra (ESꢀMS) were recorded on
a Finnigan LCQ mass spectrometer using DMSO as mobile phase. Analytical high performance liquid
chromatography (HPLC) was performed on a YLꢀ9100 HPLC with UV detection. Analytical
CHIRALCEL ODꢀH and ASꢀH columns (4.6 mm × 25 cm) from Daicel were used.
Synthesis of MOFs 1 and 2. A mixture of Cd(NO ) ·6H O (3.5 mg, 0.01 mmol) and (1R, 2R)ꢀ
3
2
2
[
[
VO(H L)] (7.4 mg, 0.01 mmol) or (1R, 2R)ꢀ[VO(H L)] (3.7 mg, 0.005 mmol)/(1R, 2R)ꢀ
2 2
Cu(H L)(H O)] (3.8 mg, 0.005 mmol), DMF (1.0 mL) and EtOH (0.05 mL) in a capped vial was
2
2
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heated at 100 °C for 12 hours. Olive green blockꢀlike crystals were filtered, washed with EtOH, and
dried at room temperature. Yield: (R)ꢀ1, ~6.0 mg (~60.0 %); (R)ꢀ2, ~5.8 mg (~58.0 %). The products
can be best formulated as [Cd (VOL) ]·5H O (1) and [Cd (VOL)(CuL)]·4H O (2) on the basis of
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microanalysis, singleꢀcrystal diffraction and TGA. Elemental analysis of 1: Calcd for
C H Cd N O V : C, 56.35; H, 5.29; N, 3.13;; N, 3.91. Found: C, 55.97; H, 5.24; N, 3.11. IR (KBr
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84 94
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2
pellet, ν/cmꢀ1): 3428 (m), 2942 (m), 2865 (m), 1663 (s), 1600 (s), 1537 (s), 1394 (s), 1347 (m), 1314
m),1269 (m), 1256 (m),1198 (w), 1180 (w), 1172 (m), 1101 (w), 1032 (w), 1015 (w), 984 (m), 929 (w),
896 (w), 859 (w), 816 (w),787 (m), 772 (m), 728 (w), 710 (w), 665 (w), 618 (w), 562 (w), 517 (w), 490
w), 451 (w). Elemental analysis of 2: Calcd for C H Cd CuN O V: C, 56.46; H, 5.30; N, 3.14;.
(
(
84
94
2
4
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Found: C, 55.91; H, 5.27; N, 3.11. ICPꢀOES showed a 1:1 molar ratio of V to Cu. IR (KBr pellet, ν/cmꢀ
1): 3430(s),2935(s), 2860(w), 1660(m), 1602(s), 1532(s), 1416(s), 1347(w), 1316(w), 1256(w),
1167(m), 1100(w), 1032(w), 1014(w), 983(m), 895(w), 858(m), 815(w), 787(m), 771(m), 727(w),
7
10(w), 663(w), 517(m), 489(w).
Single-Crystal X-ray Diffraction. Singleꢀcrystal XRD data for compounds 1 and 2 was collected
on a Bruker SMART Apex II CCDꢀbased Xꢀray diffractometer with CuꢀKα radiation (λ = 1.54178 Å) at
173 K. We have collected about several datasets for 1 using CuꢀKα radiation. Among the several
datasets for 1, the best dataset was used for structure solution and refinement. The empirical absorption
correction was applied by using the SADABS program (G. M. Sheldrick, SADABS, program for
empirical absorption correction of area detector data; University of Göttingen, Göttingen, Germany,
1
996). The structure was solved using direct method, and refined by fullꢀmatrix leastꢀsquares on F (G.
2
M. Sheldrick, SHELXTL97, program for crystal structure refinement, University of Göttingen,
Germany, 1997). In the compound, the all nonꢀH atoms were subjected to anisotropic refinement by fullꢀ
matri program. Contributions to scattering due to these highly disordered solvent molecules were
removed using the SQUEEZE routine of PLATON. The structures were then refined again using the
data generated. Crystal data and details of the data collection are given in Table S1, while the selected
bond distances and angles are presented in Tables S2 and S3.
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ASSOCIATED CONTENT
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Supporting Information.
Details on experimental procedures, Xꢀray crystallographic data, supporting figures, reaction procedures
and spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*
*
yongcui@sjtu.edu.cn
liuy@sjtu.edu.cn
Author Contributions
§
C.Z , and Q.X contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the National Science Foundation of China (Grants 21371119, 21431004,
2
2
1401128, 21522104, and 21620102001), the National Key Basic Research Program of China (Grants
014CB932102, 2012CB8217, and 2016YFA0203400), and the Shanghai “Eastern Scholar”Program.
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(14) CCDC 1055182 (1) and 1055183 (2) contain the supplementary crystallographic data. These data
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18) In the presence of Ph PO, the Cu(salen) complexes could catalyze cyanation of aldehydes.
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Activity & selectivity
Activity & selectivity
Two chiral porous isostructural metalꢀorganic frameworks (MOFs) are constructed by utilizing
dicarboxylꢀfunctionalized ligands of VO(salen) or a mixed VO/Cu(salen). After oxidation, the
V
V OꢀMOF is a highly efficient and recyclable heterogeneous catalyst for cyanation of aldehydes
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with up to >99% ee and >99% conversion, whereas the V O/CuꢀMOF is less reactive and
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V
V
selective. Compared with the homogeneous V O(salen) catalyst, the V Oꢀ and V O/CuꢀMOFs
displayer markedly increased and decreased catalytic performance, respectively, owing to that
the frameworks fix pairs of VO(salen) units or VO/Cu(salen) into close proximity that facilitate
activation of substrates via a bimolecular or unimolecular pathway. The practical utility of this
cyanation was demonstrated in the gram scale synthesis of the antiviral natural products (R)ꢀ and
(
S)ꢀtembamide.
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