Stoichiometric enantioselective alkene epoxidation with a chiral dioxoruthenium(VI) D4-porphyrinato complex

Article abstract of DOI:10.1039/a802587h

A dioxoruthenium(VI) complex containing a D₄-porphyrinato ligand por* {H₂por* = 5,10,15,20-tetrakis-[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8- dimethanoanthracen-9-yl]porphyrin} has been prepared by oxidation of its ruthenium(II) carbonyl precursor with m-chloroperoxybenzoic acid and characterised by spectroscopic methods. The [Ru^VI(por*)O₂] complex undergoes enantioselective epoxidation of alkenes and the highest enantiomeric excess (ee) attainable is 77%. In the presence of pyrazole the complex transforms to [Ru^IV(por*)(pz)₂] when reacting with alkenes. The kinetics of the epoxidation of para-substituted styrenes has been studied. The experimental rate law is - d[Ru^VI]/dt = k₂[Ru^VI][alkene]. The second order rate constants k₂ at 25°C fall in a narrow range, 2.1 × 10^-3-9.7 × 10^-3 dm³ mol^-1 s^-1. Comparison of the Hammett plot (log k_rel vs. σ*) with those for achiral analogues [Ru^VI(tpp)O₂] (H₂tpp = 5,10,15,20-tetraphenylporphyrin) and [Ru^VI(oep)O₂] (H₂oep = 2,3,7,8,12,13,17,18-octaethylporphyrin) suggests the formation of a radical intermediate for the alkene epoxidations. Both [Ru^II(por*)(CO)(EtOH)] and [Ru^VI(por*)O₂] were examined for enantioselective catalysis. Enantioselectivities of the stoichiometric and catalytic reactions showed good correlation. There is no solvent dependence on enantioselectivity when changing the solvent from dichloromethane to benzene.

Full text of DOI:10.1039/a802587h

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Stoichiometric enantioselective alkene epoxidation with a chiral
dioxoruthenium(VI) D4-porphyrinato complex
Tat-Shing Lai,a Hoi-Lun Kwong,b Rui Zhanga and Chi-Ming Che*a
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong,
P. R. China
b Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue,
Kowloon Tong, Hong Kong, P. R. China
Received 6th April 1998, Accepted 29th July 1998
A dioxoruthenium() complex containing a D4-porphyrinato ligand por* {H2por* = 5,10,15,20-tetrakis-
[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracen-9-yl]porphyrin} has been prepared
by oxidation of its ruthenium() carbonyl precursor with m-chloroperoxybenzoic acid and characterised by
spectroscopic methods. The [RuVI(por*)O2] complex undergoes enantioselective epoxidation of alkenes and
the highest enantiomeric excess (ee) attainable is 77%. In the presence of pyrazole the complex transforms to
[RuIV(por*)(pz)2] when reacting with alkenes. The kinetics of the epoxidation of para-substituted styrenes has
been studied. The experimental rate law is Ϫd[RuVI]/dt = k2[RuVI][alkene]. The second order rate constants k2 at
25 ЊC fall in a narrow range, 2.1 × 10Ϫ3–9.7 × 10Ϫ3 dm3 molϪ1 sϪ1. Comparison of the Hammett plot (log krel vs. σ )
with those for achiral analogues [RuVI(tpp)O2] (H2tpp = 5,10,15,20-tetraphenylporphyrin) and [RuVI(oep)O2]
(H2oep = 2,3,7,8,12,13,17,18-octaethylporphyrin) suggests the formation of a radical intermediate for the alkene
epoxidations. Both [RuII(por*)(CO)(EtOH)] and [RuVI(por*)O2] were examined for enantioselective catalysis.
Enantioselectivities of the stoichiometric and catalytic reactions showed good correlation. There is no solvent
dependence on enantioselectivity when changing the solvent from dichloromethane to benzene.
ؒ
Functionalization of hydrocarbons via epoxidation with tran-
sition metal catalysts has continued to receive great attention.1,2
In particular, there has been considerable interest in the oxid-
ation chemistry of metalloporphyrins owing to their biological
relevance to enzymatic reactions mediated by cytochrome
P-450. Owing to the periodic relationship of ruthenium to iron,
ruthenium porphyrins have been of recent interest. They have
been shown to be active catalysts for epoxidation,3 aziridin-
ation4 and cyclopropanation5 of alkenes and hydroxylation
of alkanes.6 More importantly, reactive oxo,7 imido8 and
carbene5a,b ruthenium porphyrinato complexes have been
isolated and/or spectroscopically characterized. This renders
direct measurement of rates of group and atom transfer reac-
tions feasible. Although oxometal species are tacitly agreed to
be the active intermediates in many discussions of asymmetric
alkene epoxidations,1b their characterization is elusive. Indeed,
in some of the reported metalloporphyrin catalysed organic
oxidations, more than one reactive intermediate has been
suggested.
Interpretation of these kinetic data provides a new picture for
understanding the mechanism of asymmetric epoxidation by
oxometal species. In this work the results of catalytic epoxid-
ation of alkenes with PhIO as a terminal oxidant will be
presented and compared with the stoichiometric ones.10
We anticipated that isolation of highly reactive and chiral
oxometal porphyrin complexes and examination of their reac-
tivities towards asymmetric alkene epoxidations should partly
provide a clue to the factors affecting enantioselectivity. In our
laboratory, a chiral monooxoruthenium() complex supported
by a C2-chiral meridional ligand has been found to epoxidize
styrene-type substrates stoichiometrically with enantioselectiv-
ity up to 57% enantiomeric excess (e.e.).9 Here, we report a
detailed study of enantioselective epoxidation with a chiral
dioxoruthenium() complex, [RuVI(por*)O2], containing a D4-
porphyrin ligand, H2por*{5,10,15,20-tetrakis[(1S,4R,5R,8S)-
1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracen-9-yl]-
porphyrin}. Its use in stoichiometric asymmetric epoxidations
and the kinetics of its reactions with para-substituted styrenes
are also presented. The Hammett plots of the achiral [Ru-
(tpp)O2] (H2tpp = 5,10,15,20-tetraphenylporphyrin) and [Ru-
Experimental
Preparation of compounds
Ruthenium dodecacarbonyl [Ru3(CO)12] was purchased from
Strem, meta-chloroperoxybenzoic acid (Merck, 85%) was used
as received and H2por* was prepared according to the literature
method.11 All solvents for the syntheses were of analytical
grade.
[RuII(por*)(CO)(EtOH)]. A mixture of H2por* (100 mg) and
[Ru3(CO)12] (120 mg) in decalin (50 cm3) was refluxed under an
inert atmosphere for 36 h. After cooling, the orange solution
was chromatographed on a silica gel column. The product was
(oep)O2]
(H2oep = 2,3,7,8,12,13,17,18-octaethylporphyrin)
complexes are compared with that for this chiral analogue.
J. Chem. Soc., Dalton Trans., 1998, 3559–3564
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eluted by dichloromethane as an orange band. Removal of
the solvent and recrystallization of the crude product using
CH2Cl2–EtOH gave a red crystalline solid. Yield 80%. 1H NMR
(300 MHz, CDCl3): δ Ϫ2.56 (s, 2 H), 1.05 (m, 8 H), 1.32 (m, 24
H), 1.85 (m, 8 H) 2.00 (d, 8 H, J = 8.0 Hz), 2.77 (s, 8 H), 3.56
(s, 8 H), 7.36 (s, 4 H) and 8.72 (s, 8 H). IR (KBr): 2969, 1942
and 1918 cmϪ1. UV: 414 and 529 nm. FAB MS: m/z 1270 (Mϩ)
and 1242 (Mϩ Ϫ CO).
were measured by monitoring the decrease in absorbance of a
1,2-dichloroethane solution of the complex in the presence of
2% pyrazole at 424 nm. The reactions were carried out with
[alkene] ӷ [RuVI] (at more than 1000 fold). Plots of ln |A∞ Ϫ At|
vs. time were linear over at least three half-lives. The pseudo-
first order rate constants (kobs) were determined on the basis of
least squares fits using eqn. (1) where A∞ and At are the absorb-
ln|A∞ Ϫ At | = Ϫkobst Ϫ ln|A∞ Ϫ A0 |
(1)
[RuVI(por*)O2]. A dichloromethane solution of [RuII(por*)-
(CO)(EtOH)] (50 mg) was added to a well stirred solution of
m-chloroperoxybenzoic acid in dichloromethane (100 mg in 15
cm3). After 3 min the solution was chromatographed on a short
alumina column. The product was eluted by dichloromethane.
The solution obtained was evaporated to dryness by rotatory
evaporation. A dark purple residue (35 mg) was obtained. Yield
70%. IR (KBr): 2960vs, 2867s, 1684m, 1559s, 1292vs, 1106m,
1076m, 1019m, 965w, 948w, 822s, 797s, 754m and 705s cmϪ1.
1H NMR (300 MHz, CDCl3): δ 1.12 (m, 8 H), 1.35 (m, 24 H),
1.88 (m, 8 H), 2.05 (m, 8 H), 2.88 (s, 8 H), 3.62 (s, 8 H), 7.44 (s,
4 H) and 8.96 (s, 8 H). 13C NMR (300 MHz, CDCl3): δ 27.183,
27.565, 42.417, 44.403, 49.339, 113.988, 118.923, 127.908,
130.780, 141.533, 144.272 and 148.062. UV/VIS: 424 nm (log ε
5.38). FAB MS: m/z 1274 (Mϩ), 1258 (Mϩ Ϫ O) and 1242
(Mϩ Ϫ 2O).
ance at the completion of reaction and at time t respectively; A∞
readings were obtained after at least four half-lives. Second
order rate constants (k2) were determined from plots of kobs vs.
[alkene].
Oxidation of alkenes by PhIO catalysed by [RuVI(por*)O2] or
[RuII(por*)(CO)(EtOH)]
A mixture of substrate (100 mg), PhIO (50 mg) and catalyst (2
mg) in dichloromethane (4 cm3) was stirred with strict exclusion
of air. The reaction was completed when all PhIO solid dis-
solved. The GC analysis of the reaction mixture was carried out
with halogenated aromatics such as 1-bromo-4-chlorobenzene
as internal standard. The yields were calculated using PhIO as
the limiting reactant.
Instrumentation
Results and discussion
Ultraviolet-visible spectra were recorded on a HPUV 8452
spectrophotometer, infrared spectra as KBr discs on a Bio-Rad
FTIR spectrophotometer. The GLC analyses were performed
on a HP GC instrument equipped with a flame ionization
detector. The NMR spectra were recorded on a DPX300 spec-
trophotometer. The chiral columns for separation of enanti-
omers were J & W cyclodex-B (30 m) and G-TA (30 m). The
enantiomeric excesses of 3-nitrostyrene oxide and 4-methyl-
styrene oxide were determined by H NMR in the presence of
the chiral shift reagent {[Eu(hfc)3] = tris[3-(heptafluoropropyl-
hydroxymethylene)--camphorato]europium()}
The temperatures of kinetic measurements were stabilized with
Synthesis of [RuVI(por*)O2]
Preparation of dioxoruthenium() porphyrin through oxid-
ation of its ruthenium() carbonyl precursor by PhIO or m-
chloroperoxybenzoic acid in CH2Cl2 or CH2Cl2–alcohol has
previously been reported.3b The literature method works for a
variety of porphyrin ligands. For the non-bulky octaethyl-
porphyrin alcohol is needed to suppress the µ-oxo dimer form-
ation.3c The H2por* ligand used in this work is bulky and hence
dimerization via Ru–O–Ru formation is not favored. Therefore
only dichloromethane was used as the solvent for the synthesis.
The [RuVI(por*)O2] complex was obtained in a high yield by
treating m-chloroperoxybenzoic acid with [RuII(por*)(CO)-
(EtOH)]; the structures for both [RuVI(por*)O2]12 and [RuII-
(por*)(CO)(EtOH)]6 have been determined by X-ray crystal-
lography. The [RuVI(por*)O2] complex is air stable, diamagnetic
and shows no manifest spectroscopic changes when dissolved
in purified CH2Cl2 for hours at room temperature. It can be
stored as a solid at Ϫ20 ЊC for months. It was characterized by
1H, 13C NMR, IR and UV/VIS spectroscopy. The νasym
(O᎐Ru᎐O) occurs at 822 cmϪ1 which falls into the range
1
(Aldrich).
a thermostat ( 1.0 ЊC).
Stoichiometric oxidation of alkenes by [RuVI(por*)O2] and
isolation of [RuIV(por*)(pz)2]
Alkene (0.2 g) and pyrazole (0.05 g) were dissolved in dichloro-
methane (5 cm3). The [RuVI(por*)O2] complex (50 mg) was
added with stirring and the resulting solution stirred for 12 h.
Organic products were obtained through column chrom-
atography with Et2O–light petroleum (1:10) as eluent and
[RuIV(por*)(pz)2] was eluted with dichloromethane. The organic
᎐
᎐
reported for other achiral trans-dioxoruthenium() porphyrins
and the oxidation marker of [RuVI(por*)O2] at 1019 cmϪ1 is in
accordance with the ruthenium() formation.13 Comparing
1
products were analysed by H NMR and/or GC. The complex
[RuIV(por*)(pz)2] was characterized by UV/VIS, IR, MS and
magnetic moment measurements. Yield 72%. IR (KBr): 2957vs,
2865vs, 1639m, 1518w, 1469m, 1445m, 1293s, 1193m, 1106s,
1064m, 1009s, 948m, 862w, 796s, 755m and 710m cmϪ1. FAB
MS: m/z 1376 (Mϩ) and 1242 (Mϩ Ϫ 2pz). UV/VIS (CH2Cl2):
λmax 414 and 512 nm. µeff(solid sample, r.t.): 2.9 µB.
1
the H NMR spectra of [RuII(por*)(CO)(EtOH)] with [RuVI-
(por*)O2], the double doublet of the pyrrolic protons of the
former converge to a singlet signal in the latter, as expected for
the change of symmetry of the molecule from C4 to D4.
Alternatively, [RuVI(por*)O2] was generated in situ. A mixture
of PhIO (0.1 g) and [RuII(por*)(CO)(EtOH)] (0.1 g) in CH2Cl2
was stirred for 10 min. The complete conversion of [RuII-
(por*)(CO)(EtOH)] into [RuVI(por*)O2] was confirmed by elec-
tronic absorption spectroscopy. The solution was then injected
into a solution of alkene containing 3 equivalents of pyrazole.
The yield of organic product was calculated on the assumption
of a RuVI → RuIV transformation.
Stoichiometric oxidation
Oxygen atom transfer from [RuVI(por*)O2] to alkenes occurs
readily at room temperature with the organic epoxides being the
major products. Results of stoichiometric alkene epoxidations
are summarized in Table 1. The reactions were carried out in
two solvents, CH2Cl2 and benzene, with higher product yields
found in the latter solvent. Lowering the temperature was found
to lower the epoxide yield. The enantiomeric excess (e.e.) of the
epoxides ranged from 40 to 77% with the highest value for 1,2-
dihydronaphthalene oxide obtained in the reaction of [RuVI-
(por*)O2] with 1,2-dihydronaphthalene in dichloromethane at
Ϫ15 ЊC. To our knowledge, this is the highest e.e. attainable
in stoichiometric alkene epoxidation using well characterized
Kinetic measurements
Dichloromethane was distilled over CaH2. Alkenes were from
commercial sources and purified by distillation or chrom-
atography. The rates of reduction of [RuVI(por*)O2] by alkenes
3560
J. Chem. Soc., Dalton Trans., 1998, 3559–3564

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Table 1 Stoichiometric epoxidation of alkenes by [RuVI(por*)O2].
Entry
1
Substrate
Product
Solvent
CH2Cl2
Temperature
% Yield
% e.e.
a
r.t.
r.t.
43
61
59 (R)
65 (R)
a
C6H6
2
CH2Cl2
C6H6
r.t.
r.t.
32
71
50 (R)
45 (R
a
3
4
CH2Cl2
C6H6
r.t.
r.t.
35
41
41
40
CH2Cl2
CH2Cl2
C6H6 a
C6H6
r.t.
Ϫ15 ЊC
r.t.
72 (cis/trans = 9.9)
68 (cis/trans = 11)
70 (cis/trans = 8.3)
64 (cis/trans = 11)
67 (1R,2S)
70 (1R,2S)
67 (1R,2S)
72 (1R,2S)
r.t.
5
CH2Cl2
CH2Cl2
C6H6
r.t.
Ϫ15 ЊC
r.t.
32
30
41
71
77
72
a In the presence of 20 mg pyrazole.
chiral oxometal complexes.9 The enantioselectivity shows little
solvent dependence while lowering the reaction temperature
tends to give higher e.e. values. In general, addition of pyrazole
to the reaction mixture has little effect on the enantioselectivity
except for the epoxidation of styrene to styrene oxide. In the
epoxidation of para-substituted styrenes the side products were
benzaldehydes and arylacetaldehydes. Changing the substrate
from styrene to p-chlorostyrene lowered the e.e. from 65 to 45%.
The absolute configurations of styrene and chlorostyrene
epoxides were determined to be (R). In the reaction with cis-β-
methylstyrene, cis-β-methylstyrene oxide was obtained in a high
yield and with good % e.e. Besides, the epoxidation was highly
stereospecific with a stereoretention of more than 90%. The
absolute configuration of cis-β-methylstyrene epoxide was
determined to be (1R,2S), thus, the configuration of the car-
bons bearing the phenyl group are the same for both styrene
and cis-β-methylstyrene.
Fig. 1 Spectral trace of the reaction of styrene (0.20 mol dmϪ3) with
[RuVI(por*)O2] in the presence of pyrazole (3.0%) in 1,2-dichloroethane
at 298 K; scan interval 90 s.
Kinetics of the reduction of [RuVI(por*)O2] by alkenes
phyrin) with alkenes in the presence of pyrazole was also
found to give [RuIV(dpp)(pz)2], the structure of which was
determined by X-ray crystallography.14 The observation of
isosbestic spectral traces indicates no accumulation of inter-
mediate in the conversion from [RuVI(por*)O2] to [RuIV(por*)-
(pz)2]. We propose that [RuIV(por*)O] was the immediate
product of the oxygen atom transfer from [RuVI(por*)O2] to
When [RuVI(por*)O2] reacted with styrene in dichloromethane
in the absence of pyrazole the spectral trace did not exhibit
isosbestic points. This can be attributed to the accumulation
and/or disproportionation of the [RuIV(por*)O] intermediate
formed during the oxidation (Scheme 1). Indeed, Groves and
Scheme 1
co-workers13 had reported that [RuIV(tmp)O] (H2tpm = 5,10,
15,20-tetramesitylporphyrin) is unstable and undergoes dis-
proportionation in solution at room temperature. However, in
the presence of pyrazole, isosbestic spectral changes for the
reaction of [RuVI(por*)O2] with styrene in dichloromethane
were found (Fig. 1). The final ruthenium product was
[RuIV(por*)(pz)2] when the reaction was carried out in the
presence of 3 equivalents of pyrazole. As expected for para-
magnetic ruthenium() complexes, the µeff of [RuIV(por*)(pz)2]
was found to be 2.9 µB. Similar reaction of [RuVI(dpp)O2]
(H2dpp = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylpor-
Scheme 2
alkene, which rapidly reacted with pyrazole to give [RuIV(por*)-
(pz)2] (Scheme 2). The kobs values, however, are independent of
the pyrazole concentrations. Under the condition [alkene] ӷ
[RuVI] and [alkene] р 2 mol dmϪ3, pseudo-first order decay of
J. Chem. Soc., Dalton Trans., 1998, 3559–3564
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Table 2 Second order rate constants for the oxidation of alkenes by
[RuVI(por*)O2] in 1,2-dichloroethane at 298 K.
k2/dm3
Entry
1
Alkene
molϪ1 sϪ1
log krel
σϩ
TE
0
0.00219
0
0
2
3
0.00262
0.00459
0.0779
0.3214
Ϫ0.0073
0.16
0.23
0.114
4
5
0.00361
0.00965
0.00720
0.000340
0.00173
0.00289
0.00560
0.2171
0.6441
0.5169
Ϫ0.311
Ϫ0.778
0.42
1.08
6
7
8
9
10
11
12
0.0012
0.0016
Fig. 2 Hammett plots for the oxidation of para-substituted styrenes
by (a) [RuVI(por*)O2], (b) [RuVI(tpp)O2] and (c) [RuVI(oep)O2].
krel = k2(para-substituted styrene)/k2(styrene) and k2 is the rate of
the achiral dioxoruthenium() porphyrin [RuVI(tpp)O2] exhib-
oxidation of various alkenes by the dioxo complex.
₁
ited a linear free-energy plot of log k2 vs. E (one electron
₂
potential of alkenes) with a slope of Ϫ1.1 VϪ1. Furthermore,
the oxidation of substituted styrenes by [RuVI(tpp)O2] and
[RuVI(oep)O2] exhibited U-shaped Hammett plots. These
observations were attributed to little degree of charge transfer
in the transition state and a mechanism involving a continuum
of transition states was suggested.
the absorbance at 424 nm (Soret band) was observed. The
observed first order rate constants, kobs, display a linear
dependence on [alkene] and the rate law (1) was established.
Ϫd[RuVI]/dt = k2[RuVI] [alkene]
(1)
The second order rate constants (k2) were determined by
measuring the slope of the plots of kobs vs. [alkene]. The kinetic
data are summarized in Table 2. The k2 for the oxidation of
styrene is 2.19 × 10Ϫ3 dm3 molϪ1 sϪ1. It is comparable to that by
[RuVI(oep)O2] (k2 = 1.55 × 10Ϫ3 dm3 molϪ1 sϪ1) and [RuVI-
(tpp)O2] (k2 = 4.30 × 10Ϫ3 dm3 molϪ1 sϪ1). With the exception of
p-bromo- and p-methoxy-styrenes, the k2 values for the oxid-
ation of para-substituted styrenes by [RuVI(por*)O2] fall into a
narrow range. Similar observations have previously been
reported for the two achiral [RuVI(oep)O2] and [RuVI(tpp)O2]
systems.7 Previous studies showed that oxidation of alkenes by
In this report, we adopt the (TE = total substituent effect
parameter) values developed by Wu et al.15 to construct the
Hammett plots. Hammett plots for three trans-dioxo-
ruthenium() porphyrin complexes are shown in Fig. 2(a)–(c).
The data for the [RuVI(tpp)O2] and [RuVI(oep)O2] complexes
were taken from the literature paper. For each case, reasonable
obedience to a linear correlation is obtained. The best linear fit
is found for the [RuVI(oep)O2] system probably due to minimum
steric complication. Thus, we propose that the rate-determining
step of the oxygen atom transfer involves a loosely bound
radical intermediate as depicted in Scheme 3. The involvement
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J. Chem. Soc., Dalton Trans., 1998, 3559–3564

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Table 3 Catalytic epoxidation of alkenes by PhIO with [RuII(por*)(CO)(EtOH)] (A) and [RuVI(por*)O2] (B) as catalyst.
Yield (%)
Entry
1
Substrate
Catalyst
Solvent
Arylacetaldehyde
Benzaldehyde
Epoxide
% e.e.
A
A
B
CH2Cl2
C6H6
CH2Cl2
5.5
3
4
21
42
23
71
57
52
55 (R)
63 (R)
51 (R)
2
3
A
A
B
CH2Cl2
C6H6
CH2Cl2
7
20
11
30
40
42
51
35
41
40
40
38
A
A
B
CH2Cl2
C6H6
CH2Cl2
Trace
Trace
32
38
51
66
41 (R)
51 (R)
4
5
A
A
CH2Cl2
C6H6
Trace
0
35
38
53
40
54
52
A
A
B
CH2Cl2
C6H6
CH2Cl2
—
—
—
12
19
21
59 (cis/trans = 9.9)
52 (cis/trans = 6.3)
53 (cis/trans = 11)
58 cis (1R,2S)
52 cis (1R,2S)
55 cis (1R,2S)
6
7
A
A
CH2Cl2
C6H6
—
—
—
—
62
46
30
62
A
A
B
CH2Cl2
C6H6
CH2Cl2
—
47
52
61
45
41
31
16 trans
17 trans
13 trans
8
A
B
CH2Cl2
CH2Cl2
—
—
—
—
55
61
8
7
were studied to see if this chiral oxidant is capable of differen-
tiating a pair of chiral substrates (Table 2, entries 11 and 12). In
this work the rate constant for the reduction of [RuVI(por*)O2]
by (Ϫ)-limonene was found to be higher than that by (ϩ)-
limonene by a factor of ≈1.3.
Catalytic epoxidation using iodosylbenzene as oxidant
Both [RuII(por*)(CO)(EtOH)] and [RuVI(por*)O2] were exam-
ined as catalysts for enantioselective alkene epoxidation using
PhIO as a terminal oxidant. Both catalysts showed enantio-
selectivity for a wide range of alkene substrates. The best e.e.
values were observed with styrene (63% e.e.) (Table 3). In the
epoxidation of para-substituted styrenes, benzaldehydes and
arylacetaldehydes were found as side products. The epoxide
yields are in the range of 31–71%. Unlike the stoichiometric
reactions the yields are higher in CH2Cl2 than in benzene. The
absolute configuration of styrene and chlorostyrene oxides were
determined to be (R). The epoxidation of cis-β-methylstyrene
was highly stereospecific with up to 91% stereoretention in
dichloromethane. The absolute configuration of cis-β-methyl-
styrene oxide was determined to be (1R,2S). In the cases of
trans-β-methylstyrene and 1-phenylcyclohexene, low e.e. values,
17 and 8% respectively, were obtained. Unlike the results
obtained with the ruthenium() D2-porphyrin catalyst,17 there
is only a little difference in e.e. of the organic epoxides when
changing the solvent from CH2Cl2 to benzene.
Scheme 3
of this radical intermediate can be used to explain the produc-
tion of trans-β-methylstyrene oxide in the epoxidation of cis-β-
methylstyrene as bond rotation of this carbon center radical
can lead to loss of stereochemistry (Scheme 4). Its existence can
Comparison between stoichiometric and catalytic epoxidation by
[RuVI(por*)O2]
The comparison of the e.e. values of organic epoxides obtained
in stoichiometric and in catalytic alkene epoxidations is an
objective of this work. In the catalytic styrene epoxidation,
arylacetaldehyde and benzaldehyde were obtained as side
products. This is similar to the stoichiometric reaction. Of the
five alkenes (styrene, 4-chlorostyrene, 4-methylstyrene, cis-β-
methylstyrene and 1,2-dihydronaphthalene) studied in this
work similar e.e. values for both the catalytic and stoichiometric
Scheme 4
also explain the rearrangement product, 2-arylacetaldehyde, in
the epoxidation of styrene-type substrates through a hydrogen-
atom 1,2-shift process (Scheme 3). A radical mechanism has
recently been suggested to be involved in the Mn(salen) cata-
lysed epoxidation reaction.16
The oxidation of (ϩ) and (Ϫ)-limonenes by [RuVI(por*)O2]
J. Chem. Soc., Dalton Trans., 1998, 3559–3564
3563

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reactions were obtained. This strongly argues that the [RuVI-
(por*)O2] complex is the major oxidizing active intermediate in
the catalytic epoxidation.
2 J. R. Valbert, J. G. Zajacek and D. I. Orenbuch, in Encyclopedia of
Chemical Processing and Designing, ed. J. McKetta, Marcel Dekker,
New York, 1993, vol. 45, pp. 88–137.
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Conclusion
4 C.-M. Che and M.-C. Cheng, unpublished work.
In this work the first detailed study of a well defined chiral
trans-dioxoruthenium() porphyrin complex as a stoichio-
metric oxidant for epoxidation of alkenes is presented. Its use as
a catalyst for alkene epoxidation by PhIO has also been exam-
ined. The linearity of Hammett plots (log krel vs. TE) observed
for the oxidation of para-substituted styrenes by the [RuVI-
(por*)O2] complex supports a carbon center radical intermedi-
ate formed during the course of oxygen atom transfer reactions.
Theparallelenantioselectivitybetweenthecatalyticandstoichio-
metric epoxidations suggests that [RuVI(por*)O2] is the major
oxidizing intermediate in the catalytic alkene epoxidation by
PhIO.
5 (a) W. C. Lo, C. M. Che, K. F. Cheng and T. C. W. Mak, Chem.
Commun., 1997, 1205; (b) E. Galardon, P. Le Maux and G.
Simonneaux, Chem. Commun., 1997, 927.
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J. Chem. Soc., Chem. Commun., 1995, 2007.
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Acknowledgements
We acknowledge support from the Hong Kong Research Grants
Council, the University of Hong Kong and City University of
Hong Kong.
15 Y.-D. Wu, C.-L. Wong, K. W. K. Chan, G.-Z. Ji and X.-K. Jiang,
J. Org. Chem., 1996, 61, 747 and refs. therein.
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