Aerobic enantioselective alkene epoxidation by a chiral trans-dioxo(D4-porphyrinato)ruthenium(VI) complex

Article abstract of DOI:10.1039/a802009d

A dioxoruthenium(VI) complex with a D₄-chiral porphyrin ligand has been prepared and characterized by spectroscopic methods and X-ray crystal analysis; the complex exhibits catalytic activity towards aerobic enantioselective epoxidation of proc

Full text of DOI:10.1039/a802009d

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Aerobic enantioselective alkene epoxidation by a chiral
trans-dioxo(D4-porphyrinato)ruthenium(vi) complex
Tat-Shing Lai,a Rui Zhang,a Kung-Kai Cheung,a Hoi-Lun Kwongb and Chi-Ming Che*a†
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
b Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, PR China
A dioxoruthenium(VI) complex with a D4-chiral porphyrin
ligand has been prepared and characterized by spectroscopic
methods and X-ray crystal analysis; the complex exhibits
catalytic activity towards aerobic enantioselective epoxida-
tion of prochiral alkenes with enantioselectivity up to 73% ee
at an oxygen pressure of 8 atm.
base catalytic system.13 The absolute configurations of the
major enantiomers of styrene oxide and cis-b-methylstyrene
oxide were determined to be (R) and (1R, 2S) respectively. The
epoxidation of cis-b-methylstyrene (entry 2) proceeded with
high retention of configuration. Epoxidation of trans-
b-methylstyrene gave only 20% ee and complete retention of
configuration (entry 3).
Among various terminal oxidants used in metalloporphyrin-
catalysed organic oxidations,1 dioxygen is the most appealing
since it is both economical and environmental friendly,2,3 but
reports on the use of dioxygen in enantioselective alkene
epoxidations are sparse. Chiral Mn(b-ketoiminato) complexes
were reported to catalyse asymmetric aerobic alkene epoxida-
tions but aldehyde is required as a sacrificial reducing agent.4
We herein report the first example of aerobic enantioselective
epoxidation of alkenes that does not rely on the use of a co-
reductant. The D4-porphyrin was synthesized according to the
literature procedure.5 The catalyst precursor is [RuII(por*)-
(CO)(EtOH)] 16,7 which has been characterized by X-ray
Interestingly, the dioxoruthenium complex catalysed en-
antioselective aerobic oxidation of alkenes with moderate to
good ee (Scheme 2). The results are listed in Table 1. Catalytic
oxygenation of styrene using air at 1 atm pressure in CH2Cl2
gave very low turnover. However, under 8 atm pressure of
oxygen, styrene oxide was obtained with 10 turnovers and 70%
ee (entry 1). With cis-b-methylstyrene as the substrate, cis-
b-methylstyrene oxide was observed with 20 turnovers and 69%
ee and the ee increased to 73% if toluene was used as the
solvent. The cis/trans ratio was 10 (entry 2), a value similar to
that obtained in the stoichiometric reaction. Prolonged reaction
time from one to two days did not increase the total turnovers.
At the end of the reaction, UV–VIS spectrophotometric analysis
of the reaction mixture revealed that the ruthenium–porphyr-
inato moiety remained intact (Soret band was at 411 nm). A
brown diamagnetic complex, which showed no n(CNO) IR
band, was isolated after silica gel-column chromatography
using ethyl acetate as the eluent. Its FABMS spectrum revealed
crystallography.7
Oxidation
of
complex
1
with
O
N
CO
Ru
N
N
N
N
N
N
Ru
N
O
C(84)
EtOH
C(78)
C(36)
1
2
N(4)
N(3)
m-chloroperoxybenzoic acid in CH2Cl2 afforded [RuVI-
(por*)O2] 2 in 60–70% yield after purification by column
chromatography.‡ Complex 2 is diamagnetic. Its FAB spectrum
shows peaks attributed to M+, [M+ 2 O] and [M+ 2 2O]. The
nas(RuO2) stretch and the oxidation marker band are positioned
at 822 and 1019 cm21 respectively. These spectroscopic data
agree with the trans-dioxoruthenium(vi) formulation.8,9 The
structure with a high Rw value was established by an X-ray
diffraction study (Fig. 1).§ The RuNO distances average 1.74 Å,
as expected for a RuNO bond.10 Unlike other reported
[RuVI(por)O2] (por = porphyrinato dianion) complexes,2,8,9
complex 2 is stable in solid state for months and in purified
MeCN or CH2Cl2 for more than 24 h. However in the presence
of pyrazole, it reacts with alkenes to give the corresponding
epoxides with moderate to good enantioselectivities and a
paramagnetic ruthenium product that was formulated to be
[RuIV(por*)(pz)2] [pz = pyrazolate, meff = 2.94 mB (solid
sample) at room temp., FABMS m/z = 1375] (Scheme 1). The
results are summarized in Table 1. For the five alkenes studied,
the ee ranged from 20 to 72%. The 65% ee of styrene oxide
(entry 1) is among the highest obtained with chiral porphyrin
catalysts.11,12 However, the ee obtained for cis-disubstituted
alkenes are not as high as those obtained with chiral Mn–Schiff
O(1)
O(2)
Ru(1)
N(2)
C(31)
N(1)
C(63)
C(68)
C(47)
C(52)
Fig. 1 Perspective drawing of 2. Selected bond lengths (Å) and angles (°):
Ru–O(1) 1.73(1), Ru–O(2) 1.75(1), Ru–N(1) 2.08(1), Ru–N(2) 2.069(10),
Ru–N(3) 2.05(1), Ru–N(4) 2.05(1); O(1)–Ru–O(2) 175.4(5), O(1)–Ru–
N(1) 91.1(5), N(1)–Ru–N(2) 90.3(4), N(1)–Ru–N(3) 178.3(5).
N
O
N
O
pyrazole
–H2O
RuIV
O
RuVI
RuIV
+
+
O
N
N
Scheme 1
Chem. Commun., 1998
1583

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Table 1 Enantioselective epoxidation of alkenes by [RuVI(por*)O2]
future catalysts for efficient aerobic enantioselective epoxida-
tion without the need of a sacrificial reducing agent.
Supports from the Hong Kong Research Grants Council, The
University of Hong Kong and City University of Hong Kong are
gratefully acknowledged.
Stoichiometrica
Catalytic aerobicb
Entry Substrate
Product
% yieldc
% eed turnover no.e % eed
O
61f 65 (R)f,g
22
10
70 (R)
1
Notes and References
benzaldehyde
5
1
†E-mail: cmche@hkucc.hku.hk
2-phenylacetaldehyde 13
‡ 1H NMR (300 MHz, CDCl3), d 0.8–2.4 (m), 2.58 (s, 8 H), 3.80 (s, 8 H),
7.44 (m, 4 H, ArH), 8.73 (s, 8 H, pyrrolic H). 13C NMR (300 MHz, CDCl3),
d 27.183, 27.565, 42.417, 44.403, 49.339, 113.988, 118.923, 127.908,
130.780, 141.533, 144.272, 148.062. FABMS+: m/z 1274 (M+, 35%), 1258
(M+ 2 O, 23%), 1242 (M+ 2 2O, 98%).
O
72 (1R, 2S)h 20 69 (1R, 2S)
21i 73 (1R, 2S)i
2
64
O
§ Crystallographic data for 2: [Ru(por*)O2]·3MeCN): C90H85N7O2Ru, M
= 1397.78, monoclinic, space group P21 (no. 4), a = 14.734(2), b =
18.447(3), c = 15.627(3) Å, b = 110.61(2)°, U = 3975(1) Å3, Z = 2, Dc
= 1.170 g cm23, F(000) = 1468, m = 2.48 cm21, crystal dimensions 0.10
3 0.05 3 0.30 mm. Diffraction data were collected at 28 °C on a MAR
diffractometer with a 300 mm image plate detector using graphite
monochromated Mo-Ka radiation (l = 0.710 73 Å). The images were
interpreted and intensities integrated using program DENZO. 3692 Unique
and independent reflections were obtained, 3048 with I > 3s(I) were used
in the structural analysis. These reflections were in the range h 0–13, k 0–17,
l 215 to 15 with 2qmax = 51.2°. The structure was solved by Patterson
methods and expanded by Fourier methods (PATTY) and refinement by
full-matrix least squares using the software package TeXsan on a Silicon
Graphics Indy computer. In the least-squares refinement, in view of the
large thermal parameters of the four chiral radical C16H17, only the Ru atom
was refined anisotropically and all the other 99 non-H atoms were refined
isotropically and 88 H atoms at calculated positions with thermal
parameters equal to 1.3 times that of the attached C atoms were not refined.
Convergence for 405 variable parameters by least-squares refinement on F
with w = 4Fo2/s2(Fo2), where s2(Fo2) = [s2(I) + (0.063Fo2)2] for 3048
reflections with I > 3s(I) was reached at R = 0.084 and wR = 0.117 with
a goodness-of-fit of 3.00, (D/s)max = 0.04 for atoms of the porphyrin
skeleton. The final difference Fourier map had maximum positive and
negative peaks of 1.15 and 0.51 e Å23 respectively. CCDC 182/892.
5
n.d.
2
n.d.
n.d.
3i
O
66f
20f
—
11
14
—
52 (R)
56
3
O
O
45 (R)j
71
4
71
61
Cl
Cl
5
a
Stoichiometric reactions were conducted at room temperature for 12 h;
oxidant 4 mg, substrate 200 mg, pyrazole 50 mg in 5 ml CH2Cl2 unless
otherwise stated. b Reactions were performed at room temp. at ca. 8 atm for
22–24 h; catalyst 4 mg and substrate 40 mg in 4 ml CH2Cl2 unless otherwise
stated. c Yield were calculated based on the amount of ruthenium complex.
Enantiomeric excesses were determined by chiral GC (J&W Scientific
Cyclodex B; length 30 m for entries 1, 2, 4 and 5, chiraldex G-TA, 30 m for
entry 3). The product yields for the calculation of turnover no. were
determined by GC with p-dichlorobenzene or p-bromochlorobenzene as
internal standards. f In benzene. g Absolute configuration was determined by
d
e
h
comparison with an authentic sample.
Absolute configuration was
determined by matching the order of elution of the two enantiomers on a
Cyclodex-B column. i In toluene. j Absolute configuration was determined
by comparing its 1H NMR spectrum in the presence of Eu(hfc)3 with that of
a sample of known enantiomeric composition, J. T. Groves and R. S. Myres,
J. Am. Chem. Soc., 1983, 105, 5791.
1 Metalloporphyrins in Catalytic Oxidations, ed. R. S. Sheldon, M.
Dekker, New York, 1994; B. Meunier, Chem. Rev., 1992, 92, 1411.
2 J. T. Groves and R. Quinn, J. Am. Chem. Soc., 1985, 107, 5790.
3 W. H. Leung, C. M. Che, C. H. Yeung and C. K. Poon, Polyhedron,
1993, 12, 2331.
4 T. Mukaiyama, T. Yamada, T. Nagata and K. Imagawa, Chem. Lett.,
1993, 327.
5 R. L. Halterman and S. T. Jan, J. Org. Chem., 1991, 56, 5253.
6 A. Berkessel and M. Frauenkron, J. Chem. Soc., Perkin Trans. 1, 1997,
2265.
O
8 atm O2, catalyst 2
*
Ar
*
R
Ar
R
CH2Cl2, room temp.
Scheme 2
7 W. C. Lo, C.-M. Che, K. F. Cheng and T. C. W. Mak, Chem. Commun.,
1997, 1205.
a [Ru(por*)(C9H10O)] ion peak [m/z = 1377 (20%)]. The
dioxoruthenium(vi) complex is likely to be the active inter-
mediate of the reaction, since with the exception of 1,2-dihy-
dronaphthalene, both the catalytic and stoichiometric reactions
produced similar ee for the same substrate, and thus the
mechanism of this reaction could be similar to that of the
[Ru(tmp)O2] (H2tmp = tetramesitylporphyrin) system reported
by Groves and Quinn.2 As the Ru–D4-porphyrin moiety has
survived the oxidation, we envisage that high catalytic turn-
overs can be achieved through optimizing the reaction condi-
tions. Thus this work has provided a starting point for designing
8 J. T. Groves and R. Quinn, Inorg. Chem., 1984, 23, 3844.
9 W. H. Leung and C.M. Che, J. Am. Chem. Soc., 1989, 111, 8812.
10 W. C. Cheng, W. Y. Yu, K. K. Cheung and C. M. Che, J. Chem. Soc.,
Dalton Trans., 1994, 57.
11 J. P. Collman, X. Zhang, V. J. Lee, E. S. Uffelman and J. I. Brauman,
Science, 1993, 261, 1404.
12 R. L. Halterman, S. T. Jan, H. L. Nimmons, D. J. Standlee and M. A.
Khan, Tetrahedron, 1997, 53, 11257.
13 E. N. Jacobsen, W. Zhang, A. R. Muci, J. R. Ecker and L. Deng, J. Am.
Chem. Soc., 1991, 113, 7063.
Received in Cambridge, UK, 12th March 1998; 8/02009D
1584
Chem. Commun., 1998