Catalytic asymmetric heterogeneous aziridination of styrene using Cu 2+ -exchanged zeolite Y: Effect of the counter-cation on enantioselectivity and on the reaction profile

Article abstract of DOI:10.1039/b410932p

Effective catalysts have Cu²⁺ -exchange levels of ca. 40-60% of the maximum concentration for electroneutrality and, consequently, Cu-zeolite Y catalysts contain an additional counter-cation (H⁺, Li^-, Na⁺, K

Full text of DOI:10.1039/b410932p

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P A P E R
Catalytic asymmetric heterogeneous aziridination of styrene using
Cu2+-exchanged zeolite Y: effect of the counter-cation on
enantioselectivity and on the reaction profilew
John Gullick,a Darragh Ryan,a Paul McMorn,a Donald Bethell,b Frank King,c
Frederick Hancockc and Graham Hutchings*a
a School of Chemistry, Cardiff University, P. O. Box 912, Cardiff, CF10 3TB UK.
E-mail: hutch@cf.ac.uk
b Department of Chemistry, University of Liverpool, Liverpool, L69 3BX UK
c Johnson Matthey Catalysts, P. O. Box 1, Billingham, Teeside, TS23 1LB UK
Received (in Montpellier, France) 14th July 2004, Accepted 17th September 2004
First published as an Advance Article on the web 15th November 2004
Effective catalysts have Cu2+-exchange levels of ca. 40–60% of the maximum concentration for
electroneutrality and, consequently, Cu-zeolite Y catalysts contain an additional counter-cation (H+, Li+,
Na+, K+, Rb+, Cs+) whose effect is explored. With Li+, Na+, K+ counter-cations, the zeolite structure is
not markedly affected but, with Rb+ and Cs+, there is some loss of crystallinity. Replacement of H+ by
group I cations does not markedly influence the overall ee observed for aziridine indicating that the presence
of protons in the Cu-HY catalysts are not detrimental to the reaction. Catalysts containing group I cations
typically give decreased leaching of Cu2+ during the reaction. At low nitrene donor to styrene molar ratios
(1 : 1), replacement of H+ by group I cations leads to a small enhancement in the ee of aziridine, although
the yield of aziridine formed is decreased under all reaction condition. At higher molar ratios of nitrene
donor to styrene, the ee is suppressed, particularly with Rb and Cs. The effect of reaction time on aziridine
yield reveals a reaction profile in which the reaction initially proceeds rapidly, then slows down prior to
accelerating again in the latter part of the reaction. This effect is accentuated by increasing the size of the
counter-cation. This reaction profile is also observed for the homogeneously catalysed pathway and,
consequently, it cannot be due solely to a confinement effect within the zeolite pores. Over addition of
reaction by-products (NsNH2, PhI) accentuates the shape of this reaction profile and the effect is discussed
in terms of the interactions of such molecules at the active site.
Diels Alder reactions8 and carbonyl- and imino-ene reactions.9
Introduction
To date, we have found that the optimum catalysts are
prepared by exchanging 40–60% of protons present within
the intracrystalline space of zeolite HY with Cu2+. This means
that the catalytically active Cu2+ cations also have protons
present within the microporous structure. In this study, we
investigate the effect of introducing alternative counter-cations
from group I (Li+, Na+, K+, Rb+, Cs+) to replace protons
and, in particular, we investigate their effect on the formation
of aziridine in the early stages of the reaction. Recently, we
have shown that the ee increases with conversion in the
aziridination reaction10 and this study also extends our inves-
tigation on the reaction profile of aziridination of styrene.
The synthesis of pure enantiomers using catalytic processes
continues to receive considerable research attention. The sig-
nificant success achieved with homogeneous catalysts has been
recognised with the recent award of the Nobel prize in chem-
istry to Sharpless, Noyori and Knowles.1 Recently, attention
has started to be focussed on the immobilisation of homo-
geneous catalysts,2 particularly using chiral bis(oxazoline)
3
ligands.z
This is particularly important as many homo-
geneous catalysts utilise relatively expensive chiral ligands
and their recovery and re-use are of immense importance.
Immobilisation can avoid the necessity of ligand recovery
and hence will improve asymmetric catalysts potential in
commercial processes applications.
We have previously shown that Cu2+ immobilised within
microporous and mesoporous materials prepared by ion-ex-
change and modified by chiral bis(oxazoline) is effective in the
asymmetric aziridination of alkenes.4–7 In particular, higher
enantioselection can be observed with these immobilised cata-
lysts when compared with non-immobilised catalysts investi-
gated under identical conditions.6 More recently, we have
extended the use of these immobilised catalysts to asymmetric
Results and discussion
Catalyst characterisation
A series of Cu2+-exchanged zeolite Y catalysts were prepared,
containing ca. 4.0% Cu2+ from zeolite Y that had been
previously ion-exchanged with group I cations (see Table 1).
Ion-exchange is known to affect the stability of the zeolite
framework and, consequently, to investigate this, the catalysts
were characterised using powder X-ray diffraction and 27Al
MAS NMR spectroscopy (see electronic supplementary infor-
mation, ESI) following calcination at 550 1C for 3 h. The X-ray
diffraction patterns of Cu-LiY, Cu-NaY, Cu-KY are very
similar to Cu-HY but there is some loss of crystallinity for
the zeolites containing the larger group I cations, Cu-RbY and
w Electronic supplementary information (ESI) available: powder X-ray
diffraction patterns, 27Al MAS NMR spectra and thermogravimetric
analyses of Cu-Y catalysts with group I counter-cations Li+, Na+
K
,
+, Rb+, and Cs+. See http://www.rsc.org/suppdata/nj/b4/b410932p/
z The IUPAC name for 1,3-oxazoline is 4,5-dihydro-1,3-oxazole.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
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Table 1 Effect of group I cations on the heterogeneously catalysed aziridination of styrene using copper-exchanged zeolite Ya
PhI NNs : styrene mol ratio
Q
1.1
1.3
1.5
b
Catalyst
[Cu]/wt %
Yield
ee
Yield
ee
Yield
ee
% Cu leached
Cu-HY
Cu-LiY
Cu-NaY
Cu-KY
Cu-RbY
Cu-CsY
a
3.7
3.5
4.0
3.6
4.0
3.8
85
85
68
76
74
66
b
81
92
88
91
89
90
80
80
72
75
75
60
92
87
91
93
90
90
85
66
67
56
75
60
91
77
85
72
74
77
1.9
1.6
1.5
1.6
1.6
1.3
Reaction conditions: CH3CN, 25 1C, 4 h % leached over 4 h reaction period from fresh catalyst using PhI NNs : styrene mol ratio = 1.5 as this
Q
leads to the highest rate of Cu leaching
Cu-CsY. 27Al MAS NMR confirmed an increase in intensity of
the signal associated with octahedrally coordinated extra-
framework Al for the Cu-RbY and Cu-CsY due to de-alumi-
nation of the tetrahedrally coordinated framework alumina.
However, de-alumination is not extensive and it is considered
that the zeolite framework remains largely intact in these
comparative experiments. Thermal gravimetric analysis
(TGA, see ESI) of the samples showed a single feature at ca.
150 1C associated with loss of water from the catalysts. In
general, group I cation-exchanged Cu-Y zeolites exhibited 9%
weight loss, which is attributed to the additional water asso-
ciated with the group I counter-cation. All catalysts were
calcined (550 1C, 3 h) immediately prior to use and, conse-
quently, the materials were essentially anhydrous at the begin-
ning of the reaction.
the effect may be due to the diffusion limitations introduced,
perhaps, as a result of the larger size of the group I cations
compared to proton.
The effect of group I counter-cations on catalyst stability
during the reaction was also investigated and Cu2+ leaching,
observed during the reaction, was determined (Table 1). We
have previously made a detailed study of Cu2+ leaching in this
reaction system7 and we found that, although up to ca. 7% of
the Cu2+ initially present in Cu-HY can leach under standard
reaction conditions (PhI NNs : styrene molar ratio = 1.5, 1,
Q
25 1C, CH3CN), nevertheless, the reaction was due to the
heterogeneously catalysed process. Indeed, the leached Cu2+
was ineffective as a catalyst since the homogeneously catalysed
aziridination reaction is poisoned preferentially by the presence
of PhI, a by-product issued from the decomposition of the
nitrene donor. Interestingly, in the present study, the leaching
of Cu2+ was decreased by ca. 15–20% in group I ion-
exchanged Cu-Y zeolites. Again, the two possible explanations
stated before may apply here. Confinement effects have been
observed to play a role in microporous and mesoporous
heterogeneous asymmetric catalysts. In the present case, the
larger cations may restrict access to Cu2+ by N-containing
Effect of group I counter-cation on the aziridination of styrene
Aziridination of styrene using [N-(p-nitrophenylsulfonyl)imi-
no]phenyliodinaney (PhI NNs) as the nitrene donor with
Q
Cu-exchanged zeolites modified by (S,S)-2-20-isopropylidene-bis
(4-phenyl-2-oxazoline), 1, was investigated and the results,
after a reaction time of 4 h, are shown in Table 1. When a
10% molar excess of nitrene donor relative to styrene was used,
the ee was enhanced slightly for group I ion-exchanged zeolites,
but, this was associated with lower yields of aziridine.
materials, that is, the excess PhI NNs and CH CN solvents
Q
that potentially lead to leaching. The enhancement in ee at low
3
molar excesses of PhI NNs and decreased Cu2+ leaching
Q
indicate that group I ion-exchanged zeolites may be preferable
catalysts to the standard Cu2+-exchanged zeolite Y used in
previous studies in terms of the enhanced ee outweighing the
slight decrease in aziridine yield.
Effect of reaction time on aziridine yield with group I
ion-exchanged zeolite Y
This effect is considered to be due to the stability of the nitrene
donor under the reaction conditions employed. At low PhI
The effect of the reaction time on the conversion of styrene and
the yield of aziridine was investigated for the group I ion-
Q
NNs concentration, the presence of protons in Cu-HY can lead
exchanged zeolite as a function of the PhI NNs : styrene
Q
to enhanced decomposition of the nitrene donor to form
sulfonamide. At higher PhI NNs concentration, it is possible
molar ratio (Figs. 1–3). In general, increasing the PhI
Q
NNs : styrene molar ratio enhances the yield in the early stages
of the reaction and this is particularly marked for the Cu-CsY
catalyst. With a 10% molar excess of nitrene donor, the rate of
styrene conversion and the yield of aziridine in the early
reaction stages (0–100 min) are suppressed with the increasing
size of group I cations (Fig. 1). These results are a further
evidence that the counter-cation present in the zeolite intra-
crystalline space can play a significant role in the course of the
reaction. Overall, replacement of protons with small group I
cations can be beneficial at low molar excesses of nitrene donor
and may, therefore, be useful in other catalytic applications.
However, larger cations are deleterious as they lead to lower
rates of reaction, probably due to steric effects.
Q
that the small amount of residual water associated with group I
cations or introduced with the solvent leads to enhanced
formation of PhIO which subsequently reacts with styrene to
form benzaldehyde, thereby decreasing the aziridine yield.11
However, the key result is that the protons present within the
Cu-HY zeolite, which has been extensively studied to date,2,4–7
do not markedly influence the function of the aziridine. Indeed,
their replacement does not affect the reaction markedly indi-
cating that the protons are not central to the reaction mechan-
ism. There are two possible explanations for this observation.
First, the counter-cation replacing the proton will have a
different electrostatic affinity towards the zeolite12 and this
may be crucial in retaining the copper within the zeolite, this
effect binge further influenced by the size of the cation. Second,
An interesting observation with these reaction time studies
concern the nature of the reaction profile for both styrene
conversion and aziridine yield. This is also apparent for Cu-
HY catalyst in data we have previously published6 but we have
y The IUPAC name for iodinane is l3-iodane.
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Fig. 1 Effect of group I cations on the aziridination of styrene using Cu-exchanged zeolite Y, bis(oxazoline) 1, CH CN, 25 1C, PhI NNs : styrene
Q
3
mol ratio = 1.1. (E) Aziridine yield; (’) styrene conversion. (a) Cu-LiY; (b) Cu-NaY; (c) Cu-KY; (d) Cu-RbY; (e) Cu-CsY.
not, to date, commented on this effect. The aziridination
reaction is characterised by an initial rapid conversion of
styrene and formation of aziridine; however, after the initial
period, the rates of styrene conversion and aziridine formation
slow down before subsequently accelerating again as the reac-
tion proceeds. It is apparent that there is a marked difference in
the yield of aziridine and styrene conversion, and plots of
styrene conversion minus aziridine yield are useful in quantify-
ing this effect since it varies with reaction time and clearly
shows the key characteristic of the reaction profile (Fig. 4).6 It
is observed that, the larger the size of the counter-cation, the
more apparent the effect. In the heterogeneously catalysed
reaction, the active site within the supercage of the zeolite is
highly confined. As noted previously,6 this leads to an en-
hancement in the ee observed with these catalysts. It is also
apparent that the active site can also interact with reaction by-
products (e.g. PhI, NsNH2), as well as the resulting aziridine
and the starting nitrene donor. In our preceding communica-
tion,10 we showed that aziridine could interact with NsNH2
and the nitrene donor and these interactions played a role in
the observed enhancement in ee with styrene conversion. This
(PhI NNs : styrene, 1 : 1.5). At these levels, the addition of
Q
NsNH2 and PhI significantly affected the rate of aziridine
formation. When a lower PhI NNs : styrene molar ratio is
Q
used, the reaction profile of aziridine formation with reaction
time is particularly marked for the heterogeneous Cu-HY
catalyst (Fig. 5). It is still apparent, although not as marked,
in the homogeneously catalysed reaction with Cu(OTf)2
(Fig. 6). This is an important observation since, although it
is apparent that the steric confinement of the active site within
the zeolite pore accentuates this effect, it demonstrates that
confinement is not the sole cause of this interesting phenom-
enon concerning the reaction profile of aziridine formation. In
addition, in the homogeneously catalysed reaction, there is no
counter-cation in the system (i.e. H+, Li+, Na+, K+, Rb+,
Cs+), and no solid/liquid interface. Hence, we consider that it
is easier to study the origin of the reaction profile in the
homogeneously catalysed reaction system. The initial decrease
in rate following the initial rapid reaction is observed at ca. 20–
40% conversion and, at this time, the reaction mixture com-
prises styrene, PhI NNs, PhI, and aziridine, with a smaller
Q
amount of NsNH2. To indicate the possible effect of low levels
of NsNH2 and PhI on the formation of aziridine, experiments
were conducted in which 2 mol% of these compounds were
added at the start of the reaction for the homogeneously
catalysed reaction (Fig. 7). Addition of NsNH2 exaggerates
the shape of the reaction profile, while addition of PhI at this
low level does not have such a marked effect.
observation shows that the coordination sphere of Cu2+
,
although highly confined within the zeolite, changes in its
nature during the reaction. We believe that this also manifests
itself in the observation of the S-shaped reaction profile (Figs.
1–3) and that the size of the cation would, therefore, play a
significant role by enhancing steric confinement of the active
centre.
In a final set of experiments, the homogeneously catalysed
reaction was carried out at 10 1C and 0 1C to investigate
whether the effect could be more readily studied at lower
temperature. Compared with yields of Z 80% in aziridine
for the reaction at 25 1C for 3 h, the yields were 65% and
15% at 10 1C and 0 1C, respectively. Reaction at 10 1C
proceeded slowly. However, for reactions at 0 1C, only the
Effect of by-products on reaction profile
In our previous study,6 we investigated the effect of adding
significant amounts (10 mol%) of NsNH2 and PhI in the
formation of aziridine using PhI NNs as the nitrene donor
Q
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Fig. 2 Effect of group I cations on the aziridination of styrene using Cu-exchanged zeolite Y, bis(oxazoline) 1, CH CN, 25 1C, PhI NNs : styrene
Q
3
mol ratio = 13. (E) Aziridine yield; (’) styrene conversion. (a) Cu-LiY; (b) Cu-NaY; (c) Cu-KY; (d) Cu-RbY; (e) Cu-CsY.
initial rapid formation of aziridine was observed and no
subsequent aziridine formation was noted during the 3 h
reaction (Fig. 8). This indicates that there may be a two-stage
process. First, there is a rapid, initial reaction leading to the
formation of aziridine and by-products. The product and by-
products subsequently interact with the copper-active centre
and modify the course of the reaction. During this second
period, the ee is enhanced6,10 due to the complex interactions
occurring at the active centre. This is a central feature for both
homogeneous and heterogeneous catalysis reactions.
particular, the presence of by-products together with aziridine
can influence the process as shown in the present study when
the reaction profile is shown to be very sensitive to the
introduction of the sulfonamide by-product.
Given this complexity, we wish to consider the nature of the
reaction pathway that would lead to the generation of a
reaction profile in which an initial rapid reaction is followed
by a period of relatively slow reaction, before once again
observing a relative reaction profile. This type of behaviour
is not immediately apparent for the operation of two pathways
involving different nitrene intermediate, but is more likely to
result from competitive interaction at the active centre of the
many competing reaction components. Leaving aside the
complicating role exerted by the sulfonamide by-product and
of the iodobenzene produced in the reaction, a possible inter-
pretation of the initial phase is that it arises from a simple
competitive formation of enantiomeric aziridines by reaction
of the alkene with the bis(oxazoline) complexed copper(II)-
nitrene, the product aziridines then acting as competitive
inhibitors of the catalytic copper centres. The rate of conver-
sion of alkene then approaches zero when all the copper is
complexed with aziridine. At this stage a second route, slow up
to this point begins to become apparent and its form suggests
the occurrence of a co-operative effect, which could be auto-
catalysis. Our interpretation is speculative at this time since our
attempts to simulate the kinetics are not yet complete, but we
can make some preliminary remarks.
Comments on the origin of the reaction profile
The shape of the conversion versus time curves indicates a two-
stage process for the consumption of alkene: (i) an initial
phase, the extent of which appears to be determined roughly
by the initial ratio of catalyst to alkene and which is complete
after about one hour under the conditions used here; and (ii) a
second phase, which starts slowly but then accelerates before
slowing down as the reaction reaches completion, resulting in
the characteristic reaction profiles observed for both hetero-
geneous and homogeneous catalysts.
The mechanism of aziridination catalysed by copper cations
is known to be a very complex process, but it has yet to be fully
elucidated. Previous studies13,14 have suggested the possible co-
existence of two mechanisms, involving singlet and triplet
nitrene intermediates. Clearly these two processes could lie at
the heart of the observed reaction profile, particularly as it is
still apparent in the homogeneously catalysed process. How-
ever, it is clear that many factors affect the rate of the
aziridination reaction and the extent of enantioselectivity. In
Our recent observation of the enhancement of enantioselec-
tivity with conversion in the aziridination of styrene using
copper bis(oxazoline) complexes was interpreted in terms of
interconversion of (R) and (S) aziridines by reaction with the
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Fig. 3 Effect of group I cations on the aziridination of styrene using Cu-exchanged zeolite Y, bis(oxazoline) 1, CH CN, 25 1C, PhI NNs : styrene
Q
3
mol ratio = 1.5. (E) Aziridine yield; (’) styrene conversion. (a) Cu-LiY; (b) Cu-NaY; (c) Cu-KY; (d) Cu-RbY; (e) Cu-CsY.
copper nitrene species perhaps by way of an intermediate ring-
opened diamine.
NsNH2 and PhI, with the active copper site during the course
of the reaction. In our preceding communication,10 we showed
that this significantly affected the ee. Other contributing factors
may arise from product inhibition of the catalytic centres as the
reaction proceeds. The nature of the active site is, therefore,
more complex than a bis(oxazoline)-modified copper cation
Scheme 1 [in which S represents the S-enantiomer of the
bis(oxazoline) used] incorporates this process. Attempts to
simulate the scheme, led us to a two phase conversion versus
time curve, but did not reproduce the plateau that is observed
after an hour, as well as the sigmoidal shape thereafter. We
ascribe this to the absence in Scheme 1 of an autocatalytic
process. We are currently carrying out further studies in this
area to attempt to eludicate the mechanism further.
interacting with the PhI NNs nitrene donor, since the pro-
Q
ducts and by-products also play a role in generating the
enantioselective active site.
Experimental
Conclusions
Methods
In the heterogeneously catalysed aziridination of styrene with
Q
PhI NNs using Cu2+-exchanged zeolites modified by bis(ox-
azoline), the counter-cation can be an important component of
1H NMR spectra were obtained using a Bruker Avance
400 MHz DPX spectrometer, equipped with Silicon Graphics
workstation. The chemical shifts of 1H NMR spectra are
recorded in CDCl3 and d6-DMSO. Spectra were recorded on
the d scale and signals quoted in the form: chemical shift in
ppm (number. of protons, multiplicity, assignment).
27Al magic-angle spinning (MAS) NMR spectra were re-
corded at 9.4 T using a Chemagnetics CMX Infinity 400
spectrometer with zirconia rotors 4 mm in diameter spun at 6
KHz. The spectra were measured at a frequency of 104.3 MHz
with a 1.0 s pulse delay. All spectra were referenced to an
external kaolin [Al(H2O)63+] standard.
the system at low PhI NNs : styrene molar ratios. Replace-
Q
ment of H+ by Li+, Na+, K+, Rb+, Cs+ does not have a
marked effect in the reaction but at low nitrene donor to
styrene molar ratios replacement of H+ by group I metal
cations can lead to a slight enhancement in ee with a loss of
yield due to increased formation of NsNH2 as a nitrogen-
containing by-product. The formation of aziridine with reac-
tion time demonstrates a complex profile which, although more
pronounced in the heterogeneously catalysed reaction, parti-
cularly when large group I cations are present, is still observed
in the homogeneously catalysed reaction using Cu(OTf)2 as
catalyst. This effect is considered to be caused in part by the
interaction of the product, aziridine, and the by-products,
Powder X-ray diffraction patterns were obtained using an
Enraf Nonius FR 590 instrument with a monochromatic
CuKa radiation. The X-ray generator was set at 1.2 kW (30 mA
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Fig. 4 Plots of styrene conversion and aziridine yield to demonstrate the nature of the reaction profile. (a) Cu-LiY; (b) Cu-NaY; (c) Cu-KY;
(d) Cu-RbY; (e) Cu-CsY.
and 40 Kv). Each sample was measured from 2y = 4.4 to
124.6 for 30 min. The diffraction pattern was collected using a
semi spherical position sensitive detector (Inel PSD120). The
ionisation gas in the detector was a mixture of 15% ethane in
argon. To permit the phase identification of the unknown
samples, the pattern collected was corrected against a standard
Si pattern, followed by background subtraction and removal of
Ka2 peaks to provide the d-spacings. Phase identification was
carried out by comparison with data from the IZA website
(http://www.iza-structure.org/).
Fig. 5 Effect of reaction time on the formation of aziridine and conversion of styrene, in the presence of excess by-products, under heterogeneous
catalysis. Reaction conditions: Cu-exchanged zeolite HY, bis(oxazoline) 1, CH CN, 25 1C. (a) 1 : 1.1 styrene : PhI NNs; (b) plot of (a) as styrene
Q
3
conversion ꢀ aziridine yield; (c) 1 : 1.3 styrene : PhI NNs. (d) plot of (c) as styrene conversion ꢀ aziridine yield. (B) Aziridine yield, (&) styrene
Q
conversion.
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Fig. 6 Effect of reaction time on the formation of aziridine and conversion of styrene in the presence of excess by-products, under homogeneous
catalysis. Reaction conditions: Cu(OTf) , bis(oxazoline) 1, CH CN, 25 1C. (a) 1 : 1.1 styrene : PhI NNs; (b) styrene conversion ꢀ aziridine yield
Q
2
3
plot from (a); (c) 1 : 1.3 styrene : PhI NNs ratio; (d) styrene conversion ꢀ aziridine yield plot from (c). (B) Aziridine yield, (&) styrene conversion.
Q
Thermal gravimetric analysis was carried out using a Perkin
Elmer TG7 instrument. Each sample (2–3 mg) was placed in
the platinum sample pan and heated from 40 1C to 900 1C at
25 1C minꢀ1 under a flow of N2 (20 ml minꢀ1).
Flash column chromatography was performed on Merck
Kieselgel 60 (230–400 mesh) and analytical TLC on silica gel
60 F-254 plates.
Atomic absorption spectroscopy was performed on a Per-
kin-Elmer 373 Atomic Absorption spectrometer using an air-
acetylene flame.
HPLC analysis was recorded using a Dynamax SD200 pump
equipped with Dynamax Al-3 autosampler, Dynamax injector
and UV absorbance detector. An Apex ODS 5m column was used
for analytical work. The eluent system was acetonitrile–water
Fig. 7 Effect of reaction time on the formation of aziridine and the conversion of styrene with Cu(OTf)2 as catalyst, bis(oxazoline) 1, CH3CN, 25
1C, and in the presence of 2 mol % NsNH (a, d), 2 mol % PhI (b, e) and 2 mol % NsNH + PhI (c, f). PhI NNs : styrene molar ratio is 1.1 for (a,
b, c) and 1.3 for (d, e, f) . (E) Aziridine yield; (’) styrene conversion.
Q
2
2
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Fig.
Cu(OTf)2 as catalyst, bis(oxazoline), CH CN, PhI NNs : styrene
8
Effect of temperature on aziridination of styrene using
Q
3
mol ratio = 1.1. (E) Aziridine yield at 10 1C; (’) styrene conversion
at 10 1C; (B) aziridine yield at 0 1C; (&) styrene conversion at 0 1C.
Scheme 1
85 : 15. Baseline separation was achieved for all reagents and
products. For chiral HPLC analysis, a 25 cm Chiralcel OJ
column was used. The eluent system was hexane–propan-2-ol
82 : 18. Baseline separation was achieved for both enantiomers.
Absolute configuration was confirmed by optical polarimetry
and comparison with the literature.6
acetate (50 ml) as eluent. Flash chromatography gave the
aziridine as a white solid. Experiments were carried out in
triplicate and reproducible results were obtained; the experi-
mental error associated with the styrene conversion and
aziridine yield are r ꢁ1%.
Heterogeneous aziridine reaction catalysed by Cu-exchanged
zeolite Y
Materials
Styrene (99+%) and the bis(oxazolines) (98%) were obtained
from Aldrich and Fluka respectively. Acetonitrile (499%
purity) was obtained from Fisher scientific. Ultrastabilised
Styrene (0.101 g, 1.0 mmol), nitrene donor (1.5 mmol), Cu-HY
(0.3 g) were stirred together in dry acetonitrile (2.5 ml) at 25 1C.
If a chiral bis(oxazoline) (0.07 mmol, Aldrich, 98%) was
added, it was stirred with Cu-HY in acetonitrile prior to the
addition of styrene and nitrene donor. Reaction times varied
depending on the nitrene donor. The reaction mixture was
stirred in air at 25 1C until complete dissolution of the nitrene
donor. The reaction mixture was then filtered through a plug of
silica with ethyl acetate (50 ml) as eluent. Flash chromato-
graphy gave the aziridine as a white solid. Experiments were
carried out in triplicate and reproducible results were obtained;
the experimental error associated with the styrene conversion
and aziridine yield are r ꢁ1%.
+
NH4 Y zeolite (Union Carbide, LZY84, 5.0 g) was calcined
at 550 1C for 5 h, then stirred in 0.5 mol solution of copper(II)
acetate solution (100 ml) for 24 h at room temperature. The
mixture was then centrifuged and washed with distilled water.
This was repeated a twice. The Cu-HY zeolite was then dried at
100 1C for 24 h, then recalcined (550 1C) for 5 h. Cu content
3.7% by weight. Cu2+-exchanged zeolites containing group I
cations were prepared in a similar manner. Zeolite HY (4.0 g)
was refluxed with group I metal nitrate (0.1 mol lꢀ1, 100 ml) for
24 h and then recovered by hot filtration, washed with water
and the process repeated twice, each time using a fresh solution
of the metal nitrate. After the third treatment, the metal-
exchanged zeolite was dried before exchange with Cu2+. The
metal-exchanged zeolite was stirred with copper sulfate (0.1
mol lꢀ1, 100 ml) at 25 1C for 24 h. The zeolite was recovered by
filtration, dried and calcined as described above.
Determination of leached Cu2+ for the heterogeneously
catalysed reaction
Following the reaction described above, the mixture was
filtered through a celite plug to remove the zeolite catalyst.
The filtrate was then analysed for its Cu2+ content using
atomic absorption spectroscopy.
Preparation of PhI NNs
Q
Iodobenzene diacetate (3.22 g; 1.0 mmol) was added to a
stirred mixture of potassium hydroxide (1.4 g; 0.025 mmol) and
p-nitrobenzenesulfonamide (2.02 g; 1.0 mmol) in HPLC grade
methanol (40 ml), keeping the temperature at 0 1C during the
addition. The solution was stirred at room temperature and
over a 4 h period a cream precipitate formed. The precipitate
was then filtered, washed with distilled water and dried at room
temperature in a vacuum desiccator (3.23 g, 80.6%). dH (d6-
DMSO, 400 MHz) 8.2 (doublet, 2H), 7.95 (multiplet, 4H), 7.56
(multiplet, 1H), 7.4 (multiplet, 2H). Anal. calcd. C 35.69, H
2.27, N 6.93%; found C 35.56, H 2.29, N 6.62%.
Kinetic simulation
Simulations were carried out using the program ‘‘Kinetics’’
(Chemistry Courseware Consortium, University of Liverpool).
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Styrene (0.101 g, 1.0 mmol), nitrene donor (1.5 mmol), and
copper(II) triflate (0.15 mmol) were stirred in acetonitrile (2.5
ml) at 25 1C. If a chiral bis(oxazoline) (0.07 mmol, Aldrich,
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triflate in dry acetonitrile prior to the addition of styrene and
nitrene donor. Reaction times varied depending on the differ-
ent nitrene precursors. The reaction was stirred in air at 25 1C
until complete dissolution of the nitrene donor. The reaction
mixture was then filtered through a plug of silica with ethyl
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