Chiral microporous Ti(salan)-based metal-organic frameworks for asymmetric sulfoxidation

Article abstract of DOI:10.1039/c3cc43225d

Chiral porous metal-metallosalan frameworks are constructed from an unsymmetrical chiral pyridinecarboxylate ligand derived from Ti(salan) and are shown to be heterogeneous catalysts for asymmetric oxidation of thioethers to sulfoxides.

Full text of DOI:10.1039/c3cc43225d

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Chiral microporous Ti(salan)-based metal–organic
frameworks for asymmetric sulfoxidation†
Cite this: Chem. Commun., 2013,
49, 7120
Chengfeng Zhu, Xu Chen, Zhiwei Yang, Xia Du, Yan Liu* and Yong Cui*
Received 1st May 2013,
Accepted 17th June 2013
DOI: 10.1039/c3cc43225d
www.rsc.org/chemcomm
Chiral porous metal–metallosalan frameworks are constructed from
an unsymmetrical chiral pyridinecarboxylate ligand derived from
Ti(salan) and are shown to be heterogeneous catalysts for asymmetric
oxidation of thioethers to sulfoxides.
Metal–organic frameworks (MOFs) are crystalline porous hybrid
solids composed of organic struts and inorganic nodes and have
shown promise for a wide range of applications in diverse areas.1
Unlike traditional inorganic materials, MOFs feature structural
diversity and amenability to be designed with desired functionalities
at the molecular level.2 In particular, MOFs offer great potential in
heterogeneous catalysis because of their advantages over other
immobilized catalyst systems such as crystalline structures, with
high catalyst loadings, more uniform, isolated and accessible active
sites and enhanced catalytic activity in some cases by providing
confined spaces for reactants.3 Incorporating chiral molecular
Scheme 1 Synthesis of H5L, metalloligand TiL(OBu)2, 1 and 2.
catalysts into MOFs means they can be used for asymmetric MeOH or EtOH at 100 1C afforded single-crystals of
catalysis,4 but there is still difficulty in making chiral MOF catalysts Cd3(m3-OH)Br[(TiLOMe)2O]2Á3DMFÁH2O (1) or Zn3(m3-OH)(OH)-
with stereoselectivity rivaling their homogeneous counterparts.2b,5
[(TiLOEt)2O]2Á3DMF (2) in good yields (Scheme 1). The products
Chiral salen ligands and their metal complexes have been were stable in air and insoluble in water and common organic
widely applied in asymmetric synthesis,3b,c,6 but their reduced solvents, and were formulated on the basis of elemental analysis,
forms of salans that have increased ligand flexibility and IR and thermogravimetric analysis (TGA).
stronger nitrogen donors are less studied.7 Herein we report
(R)-1 crystallizes in a chiral orthorhombic P21212 space group.‡
the rational synthesis of two chiral porous MOFs built from a The basic building unit in 1 is a Ti2L2(m2-O) dimer built of TiL
pyridinecarboxylate bridging ligand of Ti(salan) and show that monomers. The dimetallic core consists of two six-coordinate octa-
they can be used as heterogeneous catalysts for the asymmetric hedral Ti centers enclosed in the N2O2 pockets, binding one MeOÀ
oxidation of thioethers with aqueous H2O2 as an oxidant.
anion and linked by one m2-O atom. In the dimer, the two pyridyl
The unsymmetrical difunctionalized chiral salan ligand H5L groups of TiL associate in parallel pointing towards the same
was prepared in four steps in an overall 72% yield from 3-tert- direction, whereas the two carboxyl groups orient towards two
butyl-5-(4-pyridyl)salicylaldehyde and 3-tert-butyl-5-formyl-4-hydroxy- different directions (Fig. 1a). The Ti2L2(m2-O) dimer acts as a tetra-
benzoic acid. Metalation of H5L with Ti(OBu)4 (1 : 1 molar ratio) dentate ligand and binds to three Cd3O clusters using its two pyridyl
in THF gave TiL(OBu)2. Heating a mixture of TiL(OBu)2 and groups and two carboxylate groups in a bidentate fashion. Of the two
CdBr2Á4H2O or Zn(OAc)2Á2H2O (2 : 1 molar ratio) in DMF and independent Cd ions in [Cd3(m3-OH)Br(O2C)4], Cd1 is octahedrally
coordinated by one m3-OHÀ anion, one BrÀ anion, two carboxylate
School of Chemistry and Chemical Technology and State Key Laboratory of Metal
oxygen atoms and two pyridyl groups of four Ti2L2(m2-O) dimers, and
the Cd2 is octahedrally coordinated by one m3-OHÀ, one pyridyl
Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail: yongcui@sjtu.edu.cn, yanliu.sh@gmail.com; Fax: +86 21-5474-1297
group and four carboxylate oxygen atoms of three Ti2L2(m2-O).
† Electronic supplementary information (ESI) available: Details of the synthetic
Interestingly, six adjacent [Cd3OBr(O2C)4] clusters that are related
by C2 symmetry and linked by eight Ti2L2(m2-O) units encapsulate a
procedures and spectral data. CCDC 936226 and 936227. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3cc43225d
c
7120 Chem. Commun., 2013, 49, 7120--7122
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TGA revealed that the included guest molecules could be readily
released in the temperature range from 80 to 200 1C and the
frameworks are stable up to B315 1C. The phase purity of them
was supported by the powder X-ray diffraction (XRD) patterns of
the bulk samples, which are consistent with the calculated
patterns. The N2 sorption measurements of their apohost samples
at 77 K showed only surface adsorption (Fig. S13, ESI†), although
the theoretical values are expected to be 1650.87 m2 gÀ1 9
.
The presence of chiral TiL units in 1 prompted us to explore its
utilization as a heterogeneous catalyst for oxidation of thioethers to
sulfoxides.10 Optically active sulfoxides are important chiral auxiliaries
as well as valuable intermediates in organic synthesis for many
pharmaceuticals and biologically active compounds.11 The oxidation
reactions were conducted in acetone at 0 1C using 30% aqueous H2O2
(1.15 equiv.) as the oxidant. 1.12 mol% loading of (R)-1 afforded
54–90% conversions and 23–62% ee of the produced sulfoxides with
77–99% chemoselectivities after 16 h for the sulfides bearing both
electron-rich and electron-poor species (Table 1). However, introduc-
tion of a strong electron-withdrawing –NO2 group into the phenyl ring
of the substrate led to slightly lower conversion of 60% (Table 1,
entry 7). Compared with PhSMe, the sulfoxidation of methyl p-tolyl
sulfide gave comparable conversion and chemoselectivity, but the
product showed higher enantioselectivity. When the methyl group in
PhSMe was substituted with isopropyl, the oxidation reaction still gave
a conversion of 84%, but for a bulkier group like benzyl, the conversion
was decreased to 54%, probably due to the slow mass-diffusion of the
bulky substrate in the porous media (Table 1, entries 10 and 11).
Under otherwise identical conditions, we have also examined the
catalytic activities of MOF 2 for oxidation of sulfides, but the products
showed lower enantioselectivities and chemoselectivities than those
obtained using (R)-1. For example, oxidation of thioanisole and
methyl p-tolyl sulfide gave sulfoxides with 20 and 41% ee and 82
and 83% chemoselectivities, respectively. (S)-1 exhibits catalytic per-
formances closer to those of (R)-1, as evidenced by the oxidation of
thioanisole and methyl p-tolyl sulfide (Table 1, entries 2 and 5).
Fig. 1 Structure of 1 showing (a) the coordination environment of the Ti2L2(m2-O)
building unit (H atoms are omitted for clarity), atoms labelled gray, carbon; green,
titanium; red, oxygen; blue, nitrogen; (b) the cavity encapsulated by six trinuclear
[Cd3(m3-OH)Br(O2C)4] clusters (the cavity is shown as a colored sphere and the
Cd atoms are shown as green polyhedra) and space-filling representations of
(c) a 1D channel of 0.78 Â 0.54 nm along the b direction.
Table 1 Catalytic oxidation of sulfides by 1a
distorted octahedral cage. The cage has an irregular open cavity with
a maximum inner width of B1.7 nm (considering van der Waals
radii) (Fig. 1b and Fig. S6, ESI†). The irregular apertures on the faces
have a diagonal distance of B0.78 Â 0.54 nm or B0.65 Â 0.41 nm
(Fig. 1c). Sharing of the narrow windows with neighboring cages leads
to multidirectional zig-zag channels in the framework that are
occupied by DMF, water and MeOH guest molecules. 1 is a rare
example of MOFs decorated with chiral NH functionalities,1d which
are accessible to guest molecules and are beneficial for enantio-
selective processes.
2 is isostructural to 1 and adopts a similar porous 3D network
structure that is built from (TiL)2O(OEt)2 and [Zn3O(OH)(O2C)4]
units. The framework has open channels of B0.78 Â 0.54 nm
along the b-axis and B0.65 Â 0.41 nm along the c-axis. Calculations
using PLATON show that both 1 and 2 have about B45.4% of the
total volume available for guest inclusion.8
Entry Cat.b R1
R2
Conv.d (%) eee (%) Select.f (%)
1
R-1
S-1
R-TiL C6H4
R-1
S-1
C6H4
C6H4
Me
Me
Me
89
90
94
90
89
93
60
88
68
23
24
8
84
85
64
87
86
59
77
81
88
99
93
58
70
68
2
3c
4
p-MeC6H4 Me
p-MeC6H4 Me
46
43
12
50
52
62
50
48
10
—
—
5
6c
7
R-TiL p-MeC6H4 Me
R-1
R-1
R-1
R-1
R-1
p-NO2C6H4 Me
p-OMeC6H4 Me
2-Naphthyl Me
C6H4
C6H4
8
9
10
11
12c
13
14c
Isopropyl 84
Benzyl
Benzyl
54
87
R-TiL C6H4
R-1
R-TiL 2-Naphthyl Benzyl
2-Naphthyl Benzyl
o5
88
a
b
For reaction details see experimental section. 1.12 mol% loading of
1 based on sulfide. c Reaction time: 6 h; catalysis loading: 4.5 mol% (based
on the same TiL loading as that of the heterogenous system). d Calculated as
([sulfone] + [sulfoxide])/([sulfide] + [sulfoxide] + [sulfone]) Â 100. e ee deter-
Solid-state circular dichroism (CD) spectra of 1 and 2 made from
R and S enantiomers of the H5L ligand are mirror images of each
other, which indicate their enantiomeric nature (Fig. S9, ESI†). mined by HPLC. f Calculated as [sulfoxide]/([sulfoxide] + [sulfone]) Â 100.
c
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To study the confinement effect of the chiral MOF, the catalytic
In conclusion, we have synthesized two microporous 3D MOFs
performance of the monomer TiL(OBu)2 was also studied. utilizing a pyridine- and carboxylic acid-functionalized Ti(salan)
Thioanisole, methyl p-tolyl sulfide and phenyl benzyl sulfide were ligand. The frameworks were shown to be heterogeneous cata-
oxidized by aqueous H2O2 with 4.5 mol% TiL(OBu)2 as the catalyst lysts for asymmetric oxidation of thioethers to sulfoxides, with
(the same TiL loading as that of the heterogenous system). The improved enantioselectivity relative to the homogeneous catalyst.
conversions are obviously higher than those of the corresponding The ready tunability of such a modular approach based on chiral
heterogeneous systems after 6 h, which afforded 8, 12 and 10% ee for metallosalans promises to lead to a variety of framework materials
sulfoxides with 64, 59 and 58% chemoselectivities, respectively with novel enantioselective functions.
(Table 1, entries 3, 6 and 12). Therefore, the TiL-containing frame-
This work was supported by NSFC-21025103 and the Shanghai
work displayed obviously enhanced enantioselectivities and chemo- Science and Technology Committee (10DJ1400100 and 12XD1406300).
selectivities with respect to its homogeneous catalyst, although the
heterogeneous reaction needs a longer time for diffusion of species
into the porous structure. A careful examination of the crystal
structure of 1 reveals that the channel surfaces are uniformly lined
Notes and references
‡ Crystallographic data for 1: M = 3427.85, orthorhombic, space group
P21212, a = 34.4419(13) Å, b = 16.7622(8) Å, c = 18.4473(8) Å, V =
10650.0(8) Å3, Z = 2, Dc = 1.069 Mg mÀ3, F(000) = 3522, 11 722 unique
(Rint = 0.0894), R1 = 0.0794, wR2 = 0.1873 [I > 2s(I)], GOF = 1.074, Flack
parameter = 0.005(12). 2: M = 3246.95, orthorhombic, space group
P21212, a = 34.0597(5) Å, b = 16.9014(2) Å, c = 18.3315(3) Å, V =
10552.6(3) Å3, Z = 2, Dc = 1.022 Mg mÀ3, F(000) = 3392, 12 261 unique
(Rint = 0.0391), R1 = 0.0984, wR2 = 0.2573 [I > 2s(I)], GOF = 1.084, Flack
parameter = 0.031(13). CCDC 936226 and 936227.
with chiral Ti2L2(m2-O) units with Ti sites pointing to the open
channels, and the shortest Ti–Ti distance of adjacent surfaces in a
channel is 3.599 Å (Fig. S5, ESI†). The increased enantioselectivity and
chemoselectivities may arise from geometrical constraints imposed
by the unique chiral micro-environments around the site-isolated
Ti centers, which may effectively restrict molecular motion, leading to
efficient enantiodiscrimination.3c,12 The turnover frequency (TOF)
of (R)-1 and (R)-TiL is 1.24 and 3.50, 1.26 and 3.46, and 0.75 and
3.24 hÀ1, respectively, for oxidation of thioanisole, methyl p-tolyl
sulfide, and phenyl benzyl sulfide; the lower TOF of (R)-1 is probably
due to the diffusion of substrates in the porous structure, which
makes the access of the substrate to the active site slow.
To probe whether oxidation of the sulfides occurs inside the
chiral pores or on the surface of the solid framework catalyst,
a more sterically bulky substrate, benzyl 2-naphthyl sulfide, was
subjected to the oxidation conditions. Less than 5% conversion
was observed when catalyzed by (R)-1 after 16 h, which was
much lower than 88% conversion obtained using the homo-
geneous TiL catalyst within 6 h (Table 1, entries 13 and 14).
This result suggests that the bulky substrate cannot access the
catalytic Ti sites in the porous structure due to its large
diameter and so the oxidation reaction indeed occurs within
the chiral MOF. This point is also supported by the fact that
ground and unground particles of (R)-1 exhibited similar cata-
lytic performance in oxidation of methyl p-tolyl sulfide (91% vs.
90% and 44% vs. 46%, conversions and enantioselectivities
after 16 hours, respectively).
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7122 Chem. Commun., 2013, 49, 7120--7122
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