Asymmetric alcoholytic kinetic resolution of styrene oxide catalysed by chiral metal-organic framework crystals

Article abstract of DOI:10.1039/c0nj00038h

The methanolytic kinetic resolution of styrene oxide catalyzed by chiral metal-organic framework crystals afforded both 2-methoxy-2-phenylethanol and unreacted styrene oxide in good enantiomeric excesses.

Full text of DOI:10.1039/c0nj00038h

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Asymmetric alcoholytic kinetic resolution of styrene oxide catalysed
by chiral metal–organic framework crystalsw
Koichi Tanaka* and Ken-ichi Otani
Received (in Montpellier, France) 19th January 2010, Accepted 10th March 2010
DOI: 10.1039/c0nj00038h
The methanolytic kinetic resolution of styrene oxide catalyzed
by chiral metal–organic framework crystals afforded both
2-methoxy-2-phenylethanol and unreacted styrene oxide in good
enantiomeric excesses.
the presence of (R)-3 (20 mg, 0.046 mmol based on the formula
unit) was stirred at 25 1C for 24 h, 2-methoxy-2-phenylethanol
(S)-2 of 81% ee and unreacted (S)-1 of 5% ee were obtained in
5% and 83% yields, respectively (Table 1, entry 1). No reaction
occurred without the catalyst under these conditions. When the
reactions were carried out at higher temperatures (40 and 60 1C),
the ee of unreacted 1 became higher and the ee of product 2 became
lower, and the highest ee (98%) of recovered 1 was obtained in
the reaction performed at 60 1C (Table 1, entry 2 and 3). Next,
we performed the reactions using (R)- and (S)-1 as substrate,
(S)-2 of >99% ee and (R)-2 of >99% ee were obtained in 48%
and 5% yields, respectively (Table 1, entries 4 and 5). This means
styrene oxide (R)-1 reacts with MeOH about two times faster
than (S)-1 in the presence of (R)-3.
The field of metal–organic frameworks (MOFs) has grown
explosively in recent years1 and numerous studies have been
reported owing to their potential applications in gas storage,2
separation,3 luminescent materials4 and heterogeneous
catalysis.5 While a large number of MOF are being discovered
so far, only a few examples of chiral MOF for enantioselective
separations or heterogeneous asymmetric catalysis have been
investigated.6 Recently, we have reported the synthesis of a
novel chiral porous metal–organic framework (R)-3 and its
application to the asymmetric catalyst for asymmetric ring
opening reaction of epoxide with amine under heterogeneous
conditions.7 Here we wish to report the alcoholytic kinetic
resolution of styrene oxide 1 in the presence of chiral metal–
organic framework crystals (R)-3.
It is interesting to note that the reaction is very sensitive to
the structure of alcohol. In the case of the reactions between
styrene oxide 1 with more bulky alcohols such as EtOH,
i-PrOH and t-BuOH under the same conditions, the conversion
as well as the enantioselectivity dropped dramatically (Table 1,
entries 6–8). This may be due to the hindered diffusion of
bulky alcohols inside the pores of the MOF catalyst.
To confirm the heterogeneity of this reaction, we filtered the
catalyst after the reaction of entry 1. The reaction was
continued with the filtrate for another 12 h, but no further
conversion was observed. The reusability of (R)-3 was also
investigated in the reaction. The crystals of (R)-3 were almost
recovered by simple filtration and reused in the next cycles of
the reaction without appreciable loss of both reactivity and
enantioselectivity.
To further study the kinetic resolution of styrene oxide 1 in
the presence of (R)-3, the relationship between the conversion
of the reaction and the ee of that of the recovered 1 as well as
that of the product 2 has been determined (Fig. 1). The results
show that the ee of unreacted 1 increase with conversion, while
the ee of product 2 decreases with conversion. After 50%
conversion of 1, the ee of recovered 1 increase markedly and
the highest ee was obtained after 90% conversion. In parallel,
the ee of the product 2 decrease from about 60% to 10%
during this conversion.
The chiral MOF (R)-3 was prepared according to our
previously reported method7 and dried in vacuum at 80 1C
for 3 h prior to the reaction. For the alcoholysis reaction,
styrene oxide and the catalyst (R)-3 were stirred in alcohol
under the conditions shown in Table 1. The products were
identified by GC-MS and the yield was determined by 1H-NMR
analysis. The enantiomeric excess was determined by HPLC
using Chiralpak IA and AS (Daicel). In a typical experiment, a
solution of styrene oxide rac-1 (0.5 mmol) in MeOH (0.5 mL) in
The mechanism of the kinetic resolution of styrene oxide
rac-1 catalysed by (R)-3 could be inferred from the methanolysis
of styrene oxide since the methoxy group was incorporated at
a-carbon to afford the product 2 exclusively. The reaction may
proceed as follows. Firstly, (R)-2 coordinates to Lewis acidic
Cu site of (R)-3 to form an adduct predominantly due to the
steric reason. Next, methanol would attack the a-carbon atom
of (R)-1 from the backside position to give (S)-2 with inversion
of stereochemistry (Scheme 1).
Department of Chemistry and Materials Engineering,
Faculty of Chemistry, Materials and Bioengineering,
Kansai University, Suita, Osaka 564-8680, Japan.
E-mail: ktanaka@ipcku.kansai-u.ac.jp; Fax: +81-06-6368-0861;
Tel: +81-06-6368-0861
w This article is part of a themed issue on Coordination polymers:
structure and function.
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Table 1 Kinetic resolution of styrene oxide in the presence of (R)-3a
Unreacted 1
Product 2
Yield (%)
eeb (%)
Yield (%)
eeb (%)
Entry
Epoxide
Alcohol
Temp/1C
1
2
3
4
5
6
7
8
rac-1
rac-1
rac-1
(R)-1
(S)-1
rac-1
rac-1
rac-1
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
25
40
60
40
40
40
40
40
83
64
29
50
92
81
84
86
5 (S)
5
20
66
48
5
3
2
0
81 (S)
62 (S)
37 (S)
>99 (S)
>99 (R)
12c
19 (S)
98 (S)
>99 (R)
>99 (S)
0.4c
i-PrOH
t-BuOH
0.3c
4c
0.3c
—
a
b
Reactions were carried out for 24 h. Determined by HPLC. Determined by GC.
c
In summary, we have developed the first example of chiral
MOF catalysed methanolytic kinetic resolution of styrene
oxide. This raises the interesting possibility for applying this
methodology to the catalytic asymmetric synthesis of various
compounds. Further investigation of the scope and limitations
of this reaction is now underway.
Experimental
Representative kinetic resolution of rac-1: (R)-3 is desolvated at
80 1C for 3 h prior to reaction. The resultant solid catalyst is
suspended in a MeOH (0.5 mL) solution of styrene oxide
(0.06 g, 0.5 mmol) and stirred for 24 h. Then, the solid catalyst
was collected by filtration, washed with MeOH. All MeOH
portions are combined and evaporated under reduced
pressure, and the yield is determined by NMR. 1H NMR
(400 MHz, CDCl3, d ppm): 7.25–7.39 (m, Ph, 5H), 4.31 (dd,
J = 4.4 Hz, 1H), 3.56–3.72 (m, CH2OH, 2H), 3.30 (s, OMe,
3H), 2.74 (br s, OH, 1H). The optical purity of 2-methoxy-2-
phenylethanol 2 was determined by HPLC using Chiralpak IA
(Daicel) column (hexane/2-PrOH: 97/3, flow rate 0.3 ml
minÀ1, tr 37 min (S), 45 min (R)). The optical purity of styrene
oxide 1 was determined by HPLC using Chiralpak AS (Daicel)
Fig. 1 Plot of enantiomeric excess of product 2 (K) and recovered 1
(J) as a function of conversion in methanolytic kinetic resolution of
rac-1 at 60 1C.
Scheme 1 A plausible mechanism of kinetic ring opening reaction.
column (hexane/2-PrOH: 99/1, flow rate 0.3 ml minÀ1
,
tr 26 min (R), 29 min (S)).
The excellent catalytic properties of (R)-3 in the methanolysis
of styrene oxide may be due to the following reasons. The
evacuated dense MOF is transformed to 2D sheets with open
structure in the presence of MeOH7 (Scheme 2), in which the
substrate is accessible to the Cu active site through diffusion.
The transformation is accompanied by pronounced color
changes from black to green in MeOH. However, no color
changes were observed in bulkier alcohols i-PrOH and
t-BuOH. The adsorption experiment by mixing the evacuated
MOF and styrene oxide for 24 h at room temperature resulted
in no detectable inclusion of styrene oxide in the MOF
crystals.
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