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a
b
Table 1 Asymmetric epoxidation of alkenes with PhIO catalysed by 2–MCM-41(m). The results obtained by using free 2 (entries 10, 11, 17, 18) with p-
chlorostyrene and cis b-methylstyrene are also listed
Substrate
Entry
R1
R2
Content of 2/wt%c Yield (%)d
Ee (%)e
TON
T/°C
1
2
3
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
4-ClC
Ph
6
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
H
4
4
4
4
4
4
4
4
4
4
4
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.5
0.8
1.8
36
42
59
62
62
23
21
61
55
30
48
38
31
32
26
32
43
52
57 (R)
60 (R)
65 (R)
66 (R)
68 (R)
49 (R)
20 (R)
67 (R)
58 (R)
48 (R)
63 (R)
55 (R)
63 (R)
56 (R)
42 (1R,2S)
254
162
104
109
109
40
37
110
52
30
48
67
55
20
20
20
20
20
20
20
0
20
20
0
20
20
20
20
20
20
0
f
4
5
6
7
g
h
i
8
1.8
3.6
9
j
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
j
1.8
1.8
1.8
1.8
1.8
4-CF
4-FC
3
C
H
6
H
4
6
4
56
46
56
43
2
–CH CH
Ph
2
-1,2-C
6
4
H –
k
l
Me
Me
Me
73 (1R,2S)
j
j
k
l
Ph
Ph
54 (1R,2S)
m
l
62 (1R,2S)
52
a
All reactions were performed in toluene by using 20 mg of catalyst, 0.1 ml of substrate, and 16 mg of PhIO. This gives, for example, 0.56 mol% of 2 relative
b
c
to PhIO for the 1.8% content of 2. Reaction conditions: identical to those for 2–MCM-41(m) except 1 mol% of 2 (relative to PhIO) was used. The content
d
e
of 2 in 2–MCM-41(m) was determined on a POEMS ICP spectrometer. Determined by GC based on the PhI formed. Determined by GC with chiral column.
f
g
h
i
j
First, second, third, and fourth reuse of 2–MCM-41(m) containing 1.8% of 2. After these reuses, a total of 2–3% of Cr had leached into solution. Free
was used instead of 2–MCM-41(m). k The ratio of cis+trans = 66+31. cis Isomer. m The ratio of cis+trans = 91+9.
l
2
2
–MCM-41(m) up to four times. Notably, this catalyst could be
left open to the air for 3 h to allow full oxidation of chromium(ii) to
chromium(iii). After removal of the solvent, the residual solid was
reused at least two times with a total turnover number of 322
entries 3–5) without a decrease in enantioselectivity. In
recrystallised from CH
2
Cl
2
–hexane to give the desired product as a green
(
solid. Yield: 70%.
contrast, a recently reported Mn–MCM-41–salen catalyst
suffered from a loss of enantioselectivity of 40% ee when
reused for the first time.10 Extension of the above epoxidation
reaction to other substrates also gave moderate to relatively high
ees at room temperature (entries 12–16). The highest ee (73%)
was obtained for cis-b-methylstyrene (entry 16).
1
‡
1
Spectral data for 2: nmax(KBr)/cm2 ) 3443, 1603, 1507, 1430, 1302,
+
207, 1160, 1072, 820, 751, 715, 688, 539; m/z (FAB MS) 893 (M ), 858
+
([M 2 Cl] ) (Calc. for C34
H
18
2 2
N O Br
4 2
CrCl·2H O: C, 43.89; H, 2.37; N,
1
3
.01. Found: C, 43.71; H, 2.58; N, 3.21%). The H NMR spectrum of 2 is
featureless due to its paramagnetism.
§ Preparation of 2–MCM-41(m): A mixture of 2 and MCM-41(m) (100 mg)
in CH Cl (10 ml) was stirred at room temperature for 2 h. The solid product
2 2
Surprisingly, the heterogenised 2 exhibits significantly
higher enantioselectivity than free 2 under identical conditions
was then collected by filtration, thoroughly washed with CH
2 2
Cl , and dried
under vacuum.
(
entry 3 vs. 10 or 16 vs. 17). For example, with cis-b-
methylstyrene as substrate, the ee increased by as much as ca.
0% upon immobilisation of 2 onto MCM-41(m) (entries 16
1
D. Brunel, N. Bellocq, P. Sutra, A. Cauvel, M. Laspéras, P. Moreau,
F. D. Renzo, A. Galarneau and F. Fajula, Coord. Chem. Rev., 1998,
78–180, 1085.
2
and 17). As the support MCM-41(m) showed negligible
catalytic activity toward these oxidation reactions, the increase
in chiral recognition could arise from the enhanced stability of
the chromium complex upon immobilisation. Alternatively, it
could also result from the unique spatial environment consti-
tuted by both the chiral binaphthyl Schiff base ligand and the
surface of the support. To make clear which environment, inside
or outside the channels, is responsible for the enhancement of
chiral induction, we immobilised 2 on another modified MCM-
1
2 J. Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem., Int. Ed.,
1999, 38, 56.
3 A. Choplin and F. Quignard, Coord. Chem. Rev., 1998, 178–180,
1
679.
D. R. Leanord and J. R. Lindsay Smith, J. Chem. Soc., Perkin Trans 2,
991, 25.
4
5
6
1
I. F. J. Vankelecom, D. Tas, R. F. Parton, V. V. de Vyver and P. A.
Jacobs, Angew. Chem., Int. Ed. Engl., 1996, 35, 1346.
R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New
York, 1994, ch. 8, p. 346.
4
1 designated as MCM-41(m-in), whose external surface was
passivated with Ph SiCl , thus placing the same surface-bound
2
2
7 B. B. De, B. B. Lohray, S. Sivaram and P. K. Dhal, Tetrahedron:
Asymmetry, 1995, 6, 2105.
8 F. Minutolo, D. Pini, A. Petri and P. Salvadori, Tetrahedron:
Asymmetry, 1996, 7, 2293.
2
tether containing the terminal NH groups as in MCM-41(m)
inside the channels only.14 Under the same conditions with p-
chlorostyrene as substrate, both 2–MCM-41(m-in) and
9
M. J. Sabater, A. Corma, A. Domenech, V. Fornés and H. García, Chem.
Commun., 1997, 1285.
2
–MCM-41(m), having similar content of 2, afforded the
corresponding epoxide in almost the same yield and ee. This
shows that the environment inside the channels of the support
leads to the enhancement in the chiral induction.
1
0 P. Piaggio, P. McMorn, C. Langham, D. Bethell, P. C. Bulman-Page,
F. E. Hancock and G. J. Hutchings, New J. Chem., 1998, 1167.
1 S. B. Ogunwumi and T. Bein, Chem. Commun., 1997, 901.
2 M.-C. Cheng, M. C.-W. Chan, S.-M. Peng, K.-K. Cheung and C.-M.
Che, J. Chem. Soc., Dalton Trans., 1997, 3479.
1
1
We acknowledge support from The University of Hong Kong
and the Hong Kong Research Grants Council.
1
3 C.-J. Liu, W.-Y. Yu, S.-G. Li and C.-M. Che, J. Org. Chem., 1998, 63,
7
364,
1
4 D. S. Shephard, W. Zhou, T. Maschmeyer, J. M. Matters, C. L. Roper,
S. Parsons, B. F. G. Johnson and M. J. Duer, Angew. Chem., Int. Ed.,
Notes and references
†
Preparation of 2: A mixture of anhydrous chromium(ii) chloride (1 mmol)
1
998, 37, 2719.
and 1 (1 mmol) in THF (20 ml) was stirred under argon at room temperature
for 3 h, leading to a color change from brown to green. The mixture was then
Communication 9/05313A
1790
Chem. Commun., 1999, 1789–1790