alkenes in epoxidation.15 To our knowledge, this is an
unprecedented example that the utilization of axial ligand can
lead to a dramatic increase in regioselectivity for metal-
loporphyrin catalyzed alkene epoxidation although the mecha-
nistic aspects have not been amply understood.
Table 1 The effect of various organic bases in regioselective epoxidation of
limonene by catalyst 2 using Clorox as oxidanta
Turnover
number
% of the less
substituted epoxideb
Entry
Axial ligand
We acknowledge support from Research Grant Council of
Hong Kong (HKUST 6182/99P & AoE/P-10/01).
1
2
3
4
5
6
7
8
9
Chloride
259
521
501
560
532
593
216
461
371
33
57
57
65
66
64
54
42
61
4-Cyanopyridine
4-Phenylpyridine
DMAPc
4-tert-Butylpyridine
Pyridine
Imidazole
Morpholine
Methylmorpholine
Notes and references
† Crystallographic data for (1): C96H66F4N4·2CH2Cl2, M = 1350.52,
crystal dimension = 0.40 mm 3 0.20 mm 3 0.20 mm, monoclinic, space
group P21/n, a = 11.0987(10), b = 24.357(2), c = 14.4675(12) Å, a = 90,
b = 98.770(2), g = 90°, U = 3865.3(6) Å3, T = 100 K, Z = 2, Dc = 1.307
a Reaction condition: all reactions were carried out in CH2Cl2 at room
temperature, with a catalyst+axial ligand+oxidant+alkenes molar ratio of
1+30+800+2000 and stirred for 24 h; b Determined by GC-MS with HP-5
capillary column. c DMAP = 4-N,N-dimethylaminopyridine.
Mg m23, Mo-Ka (l = 0.71073 Å), absorption coefficient = 0.216 mm21
,
F(000) = 1580, max and min. transmission 0.9581 and 0.9187, reflections
collected = 21372, independent reflections = 7867 [R(int) = 0.0397], total
parameters = 518, R1 = 0.0650 and wR2 = 0.1487 for I > 2s(I), GOF =
1.018, the final difference map give maxima and minima 0.693 and 20.577
e Å23 respectively. CCDC 196833. See http://www.rsc.org/suppdata/cc/b2/
b210645k/ for crystallographic data in CIF or other electronic format.
provement by axial ligation is the presence of structural
modulation that the binding of the organic bases in the highly
crowded environment would restrict the rotational freedom of
the terphenyl rings against the porphyrin plane leading to a
comparatively more rigid pocket on the other side for efficient
molecular regionalization results. Also, the increase in re-
gioselectivity may be due to a subtle alteration of the pocket
shape caused by conformational changes such as ruffling of the
porphyrin plane upon coordination. One of the complications
could be the involvement of N-oxides as axial ligand because
the amines are prone to oxidation although it is regarded as a
minor problem in alkene epoxidation.13 Another important
consideration is the involvement of different reactive species
generated depending on axial ligands.14 By spectrophotometric
titration, the binding constant Keq for DMAP to coordinate to 2
was determined to be 7.48 3 103 dm3 mol21 (298.0 K) that is
comparable to that for the reaction between Zn(TPP) and
pyridine. Thus, no extra difficulty is encountered for the axial
coordination in the steric environment. DMAP was used in the
following experiments. The same scenario repeated for
1-methyl-1,2,4,5-cyclohexadiene, 1,2,5,6-undecadiene and
7,7-dimethyl-1,2,5,6-octadiene. Fig. 2 provides a pictorial
comparison for the dienes among MCPBA, 2–Clorox and
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a
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Fig. 2 Percentage of less substituted epoxides formed by epoxidation of
dienes using MCPBA, 2–Clorox and 2– Clorox–DMAP.
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