Formation and structure of conjugated salen-cross-linked polymers and their application in asymmetric heterogeneous catalysis

Article abstract of DOI:10.1002/ejoc.200400451

A new type of cross-linked condensation polymer was obtained by the reaction of 1,3,5-tris[(5-tert-butyl-3-formyl-4-hydroxyphenyl)ethynyl]benzene (1) with ethylenediamine. Solid-state ¹³C{¹H} CP/MAS NMR analysis showed that the polym

Full text of DOI:10.1002/ejoc.200400451

FULL PAPER
Formation and Structure of Conjugated Salen-Cross-Linked Polymers and
Their Application in Asymmetric Heterogeneous Catalysis
Morten Nielsen,[a] Anne H. Thomsen,[a] Torben R. Jensen,[a] Hans J. Jakobsen,[b]
Jørgen Skibsted,[b] and Kurt V. Gothelf*[a]
Keywords: Polymers / Asymmetric heterogeneous catalysts / Nanostructures / Self-assembly
A new type of cross-linked condensation polymer was ob-
tained by the reaction of 1,3,5-tris[(5-tert-butyl-3-formyl-4-
hydroxyphenyl)ethynyl]benzene (1) with ethylenediamine.
Solid-state 13C{1H} CP/MAS NMR analysis showed that the
polymer was pure and highly cross-linked and powder syn-
chrotron X-ray diffraction indicated some degree of local
structural order in the polymer. A chiral manganese−salen-
diaminocyclohexane (3b) in the presence of Mn(OAc)2.
These rigid chiral metal−salen polymers can catalyze the
epoxidation of cis-2-methylstyrene with high conversion, di-
astereoselectivity and enantioselectivities of up to 67% ee.
The polymeric catalyst can be reused several times without
reduced activity or selectivity.
bridged polymer was obtained by the condensation of 1 with (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
(1R,2R)-1,2-diphenyl-1,2-diaminoethane (3a) or (1R,2R)-1,2-
Germany, 2005)
Introduction
manganese acetate led to the formation of chiral
manganeseϪsalen-linked polymers. The catalytic properties
of these chiral manganeseϪsalen-linked polymers in epox-
idation reactions have been explored.
Crystalline solid-state organic and metalϪorganic frame-
works are well known in supramolecular chemistry.[1,2] The
formation of covalently linked organic polymers that pos-
sess the same degree of structural order has, on the other
hand, rarely been reported. Highly conjugated all-carbon Results and Discussions
or carbon-rich 2D or 3D polymers are of immense interest
owing to their use as organic conductors.[3,4] Such materials
Salen-Linked Polymer
also have potential applications as catalysts. Heterogeneous
The metal-free polymer 2 was formed by condensation of
chiral metalϪorganic catalysts have received much attention
owing to the ease of recovery and the reuse of the often
expensive catalysts. Derivatives of otherwise homogeneous
catalysts can be immobilized on solid supports such as or-
ganic resins[5] or in dendrimers.[6] In some cases the catalyst
itself is part of the polymeric network, for example, as re-
ported by Pu[7] and Salvadori[8] and their co-workers. Pre-
viously, we reported the synthesis of new salen complexes,[9]
their immobilization on gold surfaces,[10] and their appli-
cation in DNA-directed coupling reactions.[11,12]
trisalicylaldehyde 1 with ethylenediamine (EDA). The trisal-
icylaldehyde 1 is a rigid compound in which the confor-
mational freedom is limited to rotation around the bonds
of the ethynyl and tert-butyl groups. It was obtained in a
few synthetic steps from 1,3,5-triethynylbenzene and 3-tert-
butyl-5-iodosalicylaldehyde.[13] Compound 1 was treated
with ethylenediamine in dichloromethane and salen poly-
mer 2 was formed (Scheme 1).
In this paper we report the formation and structure of a
new type of condensation polymer formed by the conden-
sation of a rigid trialdehyde with ethylenediamine. The con-
densation of this trialdehyde with chiral diamines and
[a]
Center for Catalysis and iNANO, Department of Chemistry,
˚
Arhus University,
˚
Langelandsgade 140, 8000 Arhus C, Denmark
Fax: (internat.) ϩ45-86196199
E-mail: kvg@chem.au.dk
[b]
Instrument Center for Solid-State NMR Spectroscopy,
˚
Department of Chemistry, Arhus University,
˚
Langelandsgade 140, 8000 Arhus C, Denmark
Scheme 1
342
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400451
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Conjugated Salen-Cross-Linked Polymers
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Polymer 2 was obtained as a yellow gel after filtration
and contains a substantial amount of solvent (DCM). On
drying under ambient conditions for a couple of hours the
solvent was evaporated and 2 formed a hard solid which
did not swell again on addition of DCM or other solvents.
The salen-forming reaction is reversible in excess ethylened-
iamine and in the absence of metals.[9,14] In a test reaction
we observed that the polymer dissolves in a DCM solution
containing 0.15 m EDA. The product obtained by the dis-
solution of 2 in the presence of EDA was the tris-EDA-
1
monoimine of 1, as verified by H NMR spectroscopy and
mass spectrometry. Equilibrium conditions favor the forma-
tion of the thermodynamically most stable and structurally
most ordered polymer. Compound 1 is a rigid planar build-
ing block and the salen complexes that connect the mono- Figure 2. Powder synchrotron X-ray diffraction[16] of polymer 2
˚
loaded in a quartz capillary (0.7 mm), λ ϭ 1.0829 A
mers may adopt a coplanar structure. In the ideal case poly-
mer 2 would adopt a planar hexagonal structure, as de-
picted in Figure 1 (a), but we believe that hexagons only eton. Since the 13C MAS NMR spectrum was acquired by
appear locally in the polymer. To analyze the structure of using cross-polarization, the relative intensities of the indi-
polymer 2, solid-state 13C{1H} cross-polarization (CP) vidual resonances might not be quantitatively correct.
magic-angle spinning (MAS) NMR spectroscopy and pow- However, a single-pulse 13C MAS NMR spectrum of the
der synchrotron X-ray diffraction (Figure 1 and Figure 2) salen polymer confirmed that resonances from all of the
were employed. The 13C{1H} CP/MAS NMR spectrum of 13C sites in the polymer sample are observed in the 13C{1H}
the salen polymer 2 (see d in Figure 1) shows a number of CP/MAS NMR spectrum.
close and rather broad resonances, typical of a polymer
The 13C resonances were assigned to the 14 distinct car-
with mobile side-chains bonded to a nonflexible backbone bon atoms in the asymmetric unit (Figure 1, a) by em-
that consists of a quaternary and protonated carbon skel- ploying additional information obtained from 13C{1H}
Figure 1. (a) Fragment of an ideal hexagonal structure of salen polymer 2 and assignment of the carbon atoms in the asymmetric unit;
(b)Ϫ(d) 13C{1H} MAS NMR spectra (7.05 T) of 2 recorded with a spinning speed of 9.0 kHz; (b) spin-echo MAS spectrum obtained
without decoupling during the echo period (2τ ϭ 7.14 ms), a 15 s repetition delay, and 5200 scans; (c) cross-polarization-depolarization
(CPD) MAS spectrum recorded with a CP contact time of 5.0 ms, a CPD depolarization time of 80 µs, a repetition delay of 5 s, and
1024 scans; (d) standard CP/MAS NMR spectrum obtained with a CP contact time of 5.0 ms, a repetition delay of 5 s, and 2048 scans;
the asterisks indicate spinning sidebands while the numbers give the assignment of the resonances to the individual carbon atoms in the
asymmetric unit shown in part (a)
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K. V. Gothelf et al.
FULL PAPER
cross-polarization-depolarization (CPD)[15] experiments
combined with 13C chemical shiftϪstructure relationships
established by liquid-state NMR spectroscopy. In the
13C{1H} CPD/MAS NMR spectrum (Figure 1, c) recorded
with a short depolarization time (τCPD ϭ 80 µs), the reson-
ances from the CH and CH2 groups vanish completely,
which allows the resonances at δ ϭ 59.6 and 166.2 ppm to
be assigned to the C1 and C2 atoms of the polymer (Fig-
ure 1, a). The unsubstituted aromatic carbon atoms (C4, C6,
and C14) are observed as partly overlapping resonances in
the range of 133Ϫ137 ppm. The complete assignment of the
resonances in the 13C{1H} CP/MAS NMR spectrum to the
carbon atoms in the asymmetric unit of polymer 2 is shown
above in the spectrum in Figure 1, d. Most importantly, the
absence of a resonance from an aldehyde group (δiso
ഠ
185Ϫ210 ppm) demonstrates that residues of monomer 1
used in the synthesis are not present in polymer 2. The clear
distinction between CH, CH2 and C, CH3 groups in the
13C{1H} CPD/MAS experiment indicates that a rigid poly-
mer framework structure exists with rotating tBu groups.
This is further supported by the 13C{1H} spin-echo MAS
1
NMR spectrum (Figure 1, b) obtained without H decoup-
ling during the full echo period (2τ ϭ 7.14 ms) in which
signals are suppressed for all carbon resonances except for
the aliphatic C9 and C10 atoms. This is caused by dipolar
dephasing for all rigid carbon atoms during the quite long
spin-echo period. Finally, this spectrum demonstrates that
the tBu groups exhibit a high degree of mobility (i.e., ro-
tation around the C7ϪC9 axis).
Powder synchrotron X-ray diffraction was also per-
formed on polymer 2. The polymer displayed diffraction
which indicates some degree of local order in the polymer.
However, the broadness of the diffraction peaks makes it
impossible to unequivocally index the peaks. The structural
Scheme 2
served in the absence of Mn(OAc)2 when using the chiral
diamines 3a or 3b.
The polymers 4a and 4b precipitate as hard solids and do
not swell in organic solvents. In contrast to polymer 2
formed from 1 and ethylenediamine in the absence of a me-
tal, the metal-containing polymers 4a and 4b exhibit no
powder X-ray diffraction pattern. Apparently, the polymer
formed in the presence of Mn(OAc)2 lacks the order ob-
served in the pure organic polymer 2.
˚
correlation length was estimated to be approximately 30 A
from the full width at half maximum of the observed dif-
fraction peaks. This low structural correlation length sug-
gests that ordered local domains are very small.
Chiral Metal؊Salen Polymers
Salen ligands have been used with tremendous success
with a wide range of metals in the preparation of stable
homogeneous metal؊ligand complexes.[17] In particular
Asymmetric Epoxidation with Mn؊Salen Polymers
Polymers 4a,b were used as chiral catalysts in the asym-
chiral manganeseϪsalen complexes have proven successful metric epoxidation of alkenes (Scheme 3). The test reaction
in the Jacobsen asymmetric epoxidation.[14] Thus, with the of choice was the epoxidation of cis-2-methylstyrene with
achiral polymer described above at hand, the next target m-CPBA as the oxygen source and N-methylmorpholine N-
was to explore the applicability of chiral derivatives of this oxide (NMO) as the co-oxidant at 0 °C. The reactions have
new type of polymer as a ligand for chiral catalysts. The been performed on a 0.1 mmol scale with 2 equiv. of m-
manganeseϪsalen-linked polymers 4a,b were obtained by CPBA and 5 equiv. of NMO in DCM. MnϪsalen polymers
the condensation of 1 with the amines 3a,b in the presence 4a,b were crushed in a mortar and a catalyst loading of
of Mn(OAc)2 (Scheme 2). Trialdehyde 1 and Mn(OAc)2 15 mol % was employed. The actual loading is presumably
were mixed and stirred in CH2Cl2. The diamine of choice considerably lower because of the inaccessibility of catalytic
was added and the reaction mixture was stirred for 48 hours sites in the core of the polymeric material. After 2 h the
and the polymer was filtered off, washed, and dried under reaction mixture was diluted with diethyl ether to lower the
argon. Pentane (1 mL) was then added to facilitate an im- density of the solvent and the reaction was stopped by pre-
mediate precipitation of the polymer as a black solid. In cipitating the catalyst by centrifugation and decantation of
1
contrast to the application of EDA, no precipitate was ob- the solution. Analysis of the crude product by H NMR
344
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Conjugated Salen-Cross-Linked Polymers
FULL PAPER
spectroscopy revealed a conversion of 73% (Table 1, entry ment containing the solid catalyst 4a showed 73% conver-
1). A cis/trans ratio of 12:1 was obtained which indicates sion after 2 h.
that the reaction proceeds with high diastereoselectivity.
The mechanism of the asymmetric Jacobsen epoxidation
The enantioselectivity for the cis isomer was shown to be has been the subject of some debate.[18,19] It has been ar-
57% ee by GC analysis, which is approximately 30% lower gued that in the key intermediate of the reaction, one of the
than that obtained in reactions of the corresponding homo- salicylimine groups of the salen ligand changes confor-
geneous catalyst.[9,14]
mation.[18,19a] It is most unlikely that the manganeseϪsalen
complexes in the highly cross-linked polymers 4a and 4b
can undergo the structural changes required for such a con-
formal change. The fact that the reaction still proceeds, al-
though with lower selectivities than in the homogeneous re-
action, indicates that the structural change described above
is not a prerequisite for the epoxidation reaction.
Scheme 3
In summary, we have synthesized a new type of highly
cross-linked condensation polymer from a rigid trialdehyde
and ethylenediamine. The solid-state 13C MAS NMR
experiments demonstrate that this new type of salen-linked
polymer is highly cross-linked while powder synchrotron X-
ray diffraction data indicate that the polymer may have lo-
cally ordered domains. The polymer has few open ends, as
demonstrated by the absence of signals from terminal am-
ino and salicyl moieties in the NMR spectra. This polymer
network may have unique properties with respect to in-
clusion of guest molecules and metal ions in its structure
The reaction was also performed without grinding the
polymeric catalyst 4a in a mortar, which presumably would
lead to a significantly lower number of catalytic sites. In this
case the conversion as well as the enantioselectivity dropped
dramatically, probably because the catalyzed reaction path
was slowed significantly, while the noncatalyzed reaction
path continued at the same rate.
The solid polymer 4a or 4b left in the test-tube after fil-
tration and decanting of the solution could be used again
in repeated epoxidation reactions. The results of six or four
epoxidation reactions using the same batch of polymer 4a
or 4b are collected in Table 1. As the data in Table 1 clearly
indicate, the yields and selectivities are consistent during the
catalyst-recycling experiments. Because there is no filtration
in this procedure and since the polymeric catalyst is kept in
the same tube during the repeated experiments there is no
loss of material, which in part explains the results obtained.
The fact that the levels of reactivity and selectivity of the
polymers were maintained in the catalyst-recycling experi-
ments also shows that the polymers have a high stability.
Furthermore, the catalysts 3a or 3b used in the experiments
described above could be stored for three months and re-
used without a decrease in the reactivity and selectivity of
the reaction. To test for any leaking of catalytically active
species from the polymer into solution a series of experi-
ments were performed. In the main experiment the poly-
meric catalyst was filtered off after 30% conversion, as ob-
served by GC. No further conversion was observed after
and as
a
conducting material. The two chiral
manganeseϪsalen linked polymers 4a and 4b are amorph-
ous, however they catalyze the asymmetric epoxidation of
an alkene to give up to 67% ee. The catalysts can be reco-
vered by centrifugation and used repeatedly in up to six
consecutive reactions with no significant drop in selectivity
or reactivity. In future investigations on this new type of
condensation polymer we will optimize and further study
the properties of this new material.
Experimental Section
General Conditions: Standard Schlenk and vacuum line techniques
were employed using argon as the inert atmosphere for all manipu-
lations of air- or moisture-sensitive compounds. Yields refer to
chromatographically isolated and spectroscopically homogeneous
materials, unless otherwise stated. Commercially available starting
removal of the solid catalyst, whereas the control experi- materials were used without further purification. Solvents were
Table 1. Consecutive use of 3a or 3b as catalysts in the asymmetric epoxidation of cis-2-methylstyrene (reagents: cis-2-methylstyrene, m-
CPBA and N-methylmorpholine N-oxide in CH2Cl2 at 0 °C)
Entry
Catalyst/cycle
Conversion [%]
cis/trans ratio
ee (cis) [%]
ee (trans) [%]
1
2
3
4
5
6
7
8
9
10
3a/1
3a/2
3a/3
3a/4
3a/5
3a/6
3b/1
3b/2
3b/3
3b/4
73
51
52
67
78
84
56
59
64
83
12.0
10.1
10.1
14.9
17.0
14.4
6.3
7.8
8.4
8.0
57
49
53
62
67
64
46
51
56
60
20
12
12
23
23
21
34
43
44
48
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K. V. Gothelf et al.
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dried according to standard procedures. The products were purified
by flash chromatography (FC) using Merck silica gel 60 (230Ϫ400
mesh). The liquid-state 1H and 13C NMR spectra were recorded
at 400 and 100 MHz, respectively, on a Varian Mercury (9.4 T)
spectrometer with CDCl3 as the solvent. The chemical shifts are
reported in ppm downfield from TMS (δ ϭ 0.00 ppm) in the 1H
NMR spectra and relative to the central CDCl3 resonance (δ ϭ
77.00 ppm) in the 13C NMR spectra. Mass spectra and high-resolu-
tion mass spectra were obtained on an LC-TOF spectrometer (Mi-
cromass).
tion mixture at 0 °C for 2 h, diethyl ether (1 mL) was added, the
mixture was centrifuged, and the polymeric material isolated by
decanting off the liquid phase. NaOH (0.5 mL, 1 m) was added to
the liquid phase, the organic layer was separated, washed with brine
(1 mL) and water (1 mL), and dried over MgSO4. The enantiomeric
excess and diastereomeric ratio were determined by chiral GC-MS
by using an Astec B-DM column. A 1 mm solution was injected
into the GC apparatus and an initial column temperature of 70 °C
was maintained for 5 minutes, after which it was increased at a rate
of 5 °C·minϪ1 for 15 min. The retention times were tR (cis-epox-
ide) ϭ 11.8 (major) and 12.5 min (minor) and tR (trans-epoxide)
11.3 and 11.4 min. After decanting off the liquid phase the
MnϪsalen polymer could be reused in another experiment without
further purification.
Materials: Triethylamine was distilled from CaH2 under argon
prior to use. Bis(triphenylphosphine)palladium(ii) chloride was
purchased from Aldrich. 3-tert-Butyl-5-iodosalicylaldehyde[9] and
1,3,5-triethynylbenzene[20] were synthesized according to literature
procedures.
Solid-State 13C{1H} MAS NMR Experiments: Solid-sate 13C{1H}
MAS NMR experiments were performed on a Varian INOVA-300
(7.05 T) spectrometer using a home-built CP/MAS NMR probe for
5 mm o.d. zirconia (PSZ) rotors. The 13C{1H} CP and CPD/MAS
experiments were performed with a repetition delay of 5 s, a CP
contact time of 5.0 ms, rf field strengths of γCB1C/2π ϭ γHB2H/2π
~ 40 kHz for the HartmannϪHahn match during the CP and CPD
contact periods, and γHB2H/2π ϭ 100 kHz for the 90° 1H pulse and
1,3,5-Tris[(5-tert-butyl-3-formyl-4-hydroxyphenyl)ethynyl]benzene
(1): Bis(triphenylphosphane)palladium(ii) chloride (288 mg,
0.41 mmol) and copper iodide (156 mg, 0.82 mmol) were stirred in
a Schlenk flask under vacuum for 30 min. 5-Iodo-3-tert-butylsal-
icylaldehyde (2.60 g, 8.55 mmol), 1,3,5-triethynylbenzene (306 mg,
2.04 mmol), and NEt3 (30 mL) were added and the reaction mix-
ture was stirred under argon at 50 °C for 21 h. The reaction mixture
was poured into 10% aqueous NH4Cl (40 mL) and extracted with
CH2Cl2 (3 ϫ 20 mL). The combined organic fractions were washed
with water, dried over MgSO4, and concentrated. The crude prod-
uct was purified by flash chromatography on silica gel (CH2Cl2/
1
the H decoupling rf field strength. The 13C{1H} spin-echo MAS
experiment was performed with a repetition delay of 15 s, a rf field
strength of γCB1C/2π ϭ 60 kHz for the 90° and 180° 13C pulses,
and a 1H decoupling rf field strength of γHB2H/2π ϭ 100 kHz. The
chemical shifts in the 13C NMR spectra are referenced to TMS and
an external sample of hexamethylbenzene was used as a secondary
reference (δiso ϭ 17.20 ppm for the aliphatic carbon atoms). In the
13C{1H} CP/MAS NMR spectrum (Figure 1, d) of polymer 2, the
13C resonances are observed at 29.6 (C10), 35.2 (C9), 59.6 (C1), 89.3
(C11, C12), 113.7 (C5), 119.0 (C13), 124.3 (C3), 133.4 (C4, C6), 136.8
(C7, C14), 160.8 (C8), and 166.2 ppm (C2), with the assignment to
the carbon atoms in the asymmetric unit of polymer 2 (see a in
Figure 1) given in parentheses.
1
pentane, 1:1 Ǟ 3:2) to yield 1 as white crystals (841 mg, 61%): H
NMR (CDCl3, 400 MHz): δ ϭ 1.44 (s, 27 H), 7.61 (s, 3 H), 7.64
(s, 3 H), 7.66 (s, 3 H), 9.87 (s, 3 H), 11.97 (s, 3 H) ppm. 13C NMR
(CDCl3, 100 MHz): δ ϭ 29.3, 35.2, 87.0, 89.9, 114.0, 120.7, 124.2,
134.0, 135.5, 137.3, 139.3, 161.8, 196.9 ppm.
Polymer 2: Compound 1 (200 mg, 0.29 mmol) was dissolved in
CH2Cl2 (20 mL) in a Schlenk flask. Ethylenediamine (36 mg,
0.435 mmol) was added and the reaction mixture was stirred at
room temperature for 19 hours. The resulting yellow polymeric ma-
terial was filtered off, washed with CH2Cl2 (2 ϫ 5 mL), and dried
under argon to yield 2 as a hard yellow polymer (209 mg). The
polymer was characterized by solid-state 13C NMR spectroscopy;
no signals due to an aldehyde group were observed.
Powder Synchrotron X-ray Diffraction: Powder synchrotron X-ray
diffraction data were collected at beamline I711 at the Max II
synchrotron in Lund, Sweden, as described in detail elsewhere.[16a]
A monochromatic beam was selected by using a Si(111) mono-
chromator single crystal and the wavelength was refined by using
˚
a Si sample as standard, λ ϭ 1.08286(1) A. Powder diffraction data
Polymer 4a: Trialdehyde 1 (100 mg, 0.15 mmol) was dissolved in
CH2Cl2 (20 mL) in a Schlenk flask. (1R,2R)-1,2-Diphenyl-1,2-di-
aminoethane (3a) (47 mg, 0.22 mmol) and manganese acetate tetra-
hydrate (55 mg, 0.22 mmol) were added and the reaction mixture
was stirred at room temperature for 48 hours. The resulting black
polymeric material was filtered off, washed with CH2Cl2 (2 ϫ
5 mL), and dried under argon to yield 4a as a black polymer
(200 mg). The polymer was used without further purification.
were obtained in transmission geometry mode by using a Huber
G670 imaging-plate Guinier powder diffraction camera.[16b] Data
were collected from a sample of the polymer mounted in a quartz
capillary (0.7 mm) at room temperature with an exposure time of
300 s. The background was corrected by subtracting the scattered
intensity from an empty quartz capillary measured with the same
exposure time.
Polymer 4b: Trialdehyde 1 (100 mg, 0.15 mmol) was dissolved in
CH2Cl2 (20 mL) in a Schlenk flask. (1R,2R)-1,2-Diaminocyclohex-
ane (3b) (25 mg, 0.22 mmol) and manganese acetate tetrahydrate
(55 mg, 0.22 mmol) were added and the reaction mixture was
stirred at room temperature for 48 hours. The resulting black poly-
meric material was filtered off, washed with CH2Cl2 (2 ϫ 5 mL),
and dried under argon to yield 4b as a black polymer (201 mg).
The polymer was used without further purification.
Acknowledgments
This study was supported and funded in part by the Danish Techni-
cal Research Council, the Danish National Research Foundation
and Carlsbergfondet. T. R. J. is grateful to the SNF (The Danish
Natural Science Research Council) for a Steno stipend. MAXlab
and Dr. Y. Cerenius are thanked for beam time and their invaluable
support. The use of the facilities at the Instrument Centre for Solid-
˚
State NMR Spectroscopy, University of Arhus, is acknowledged.
Asymmetric Epoxidation using 4a or 4b as the Catalyst: NMO
(58 mg, 0.5 mmol) and (Z)-2-methylstyrene (12 µL, 0.1 mmol) was
added to a suspension of polymer 4a (11 mg) or 4b (10 mg) in
CH2Cl2 (3 mL). The suspension was cooled to 0 °C and m-CPBA
(58 mg, 57Ϫ86% w/w, 0.2 mmol) was added. After stirring the reac-
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