Stereospecific and regioselective catalytic epoxidation of alkenes by a
novel ruthenium(II) complex under aerobic conditions
Venkitasamy Kesavan and Srinivasan Chandrasekaran*
Department of Organic Chemistry, Indian Institute of Science, Bangalore-560 012, India
Epoxidation of alkenes by molecular oxygen is effected in
high yields by catalysis of RuCl2(biox)2 using isobutyralde-
hyde as the co-reductant: the reaction is stereospecific and
regioselective.
As can be gauged from Table 1 this catalytic epoxidation
with 1 is highly stereospecific. Thus, trans-stilbene 2 and cis-
stilbene 4 under the reaction conditions afford the trans-stilbene
oxide 3 and cis-stilbene oxide 5 respectively in high yields.
Styrene epoxide 11 which is highly unstable under conditions of
peracid epoxidation is quite stable under the present reaction
conditions. Another salient feature of the present methodology
is the high regioselectivity. Thus in the oxidation of 4-vinyl-
cyclohexene 16 and limonene 18 catalysed by 1, the mono-
epoxides 17 and 19 respectively were the exclusive products
isolated in very good yields. A more dramatic example of stereo-
selectivity is illustrated in the epoxidation of cholest-5-ene 20.
Under conditions of catalytic epoxidation the 5β,6β-epoxide
21A is obtained in excess of 94% selectivity and in excellent
yields.
There has been considerable interest in recent years in both
the homogeneous transition metal-catalysed epoxidation of
alkenes1 and in the use of oxoruthenium complexes as catalysts
for organic oxidations.2 With regard to aerobic oxidation
catalysed by transition metal complexes several reactions
involving the combined use of molecular oxygen with reducing
agents have been reviewed.3 The combination of three com-
ponents, that is, a transition metal, organic ligands and a
reductant is considered to create an effective oxygenation sys-
tem for such aerobic reactions.
In 1985, Groves and Quinn reported a successful aerobic
epoxidation of olefins with a ruthenium–porphyrin catalyst.4
All the other epoxidations catalysed by ruthenium complexes
involved the use of NaIO4 or PhIO as the terminal oxidants
under biphasic conditions.5 Nishiyama et al.6 used dichloro-
bis(oxazolinyl)bipyridylruthenium() complex for the oxid-
ation of alkenes in the presence of PhIO and found oxidative
cleavage as the major pathway, while Barf et al. reported the
epoxidation of alkenes with a combination of (bipyridyl)RuCl2-
(DMSO)4 and tert-butylhydroperoxide.7
Thus we have developed a non-porphyrin ruthenium() sys-
tem which catalyses the epoxidation of alkenes under aerobic
conditions with great efficiency. The reaction is stereospecific
and regioselective and this kind of high stereospecificity and
regioselectivity has not been reported with other catalytic
epoxidation methodologies.5–7 Since C2-symmetric bisoxazoles
can be easily derived from optically active amino alcohols, chiral
ruthenium complexes are readily accessible and studies of
catalytic asymmetric epoxidation of unfunctionalized alkenes
with these systems are under progress.
C2-symmetric 4,4Ј,5,5Ј-tetrahydro-2,2Ј-bisoxazoles have been
used as ligands in catalytic asymmetric transfer hydrogenation
of ketones.8 Prior to achieving enantioselectivity in epoxidation
with ruthenium complexes it is essential to find conditions in
favour of epoxidation rather than oxidative cleavage. Drago has
demonstrated that in general trans-dioxoruthenium complexes
catalysed oxidation of alkenes to yield epoxides while the
cis-isomers mediate oxidative cleavage.9 The aim of the
present work is to use bisoxazoles as bidentate ligands so that
they form a square planar structure around the ruthenium core,
analogous to ruthenium–porphyrin systems.4 Accordingly,
ruthenium–bisoxazole complex 1 was synthesized10 and our
successful results of an oxo-transfer reaction with 1 are
presented in this communication.
Experimental
Synthesis of complex 1
trans-Tetrakis(acetonitrile)dichlororuthenium()11 (0.336 g, 1
mmol) was refluxed with 4,4Ј,5,5Ј-tetrahydro-2,2Ј-bisoxazole
(0.308 g, 2.2 mmol) for 6 h in ethanol (10 ml). The solvent was
removed under reduced pressure to give a red coloured solid.
The solid was recrystallized from EtOH–Et2O (1:3) and stored
in a desiccator; mp 280 ЊC (uncorrected); νmax(thin film)/cmϪ1
2950, 1610; δH(90 MHz, D2O) 3.4 (m, 8H), 3.8 (m, 8H); δC(22.5
MHz, D2O) 53.61, 68.87, 155.26 (Calc. for C12H16Cl2-
N4O4Ru: C, 31.86; H, 3.56; N, 12.38. Found: C, 31.81; H, 3.48;
N, 12.40%).
Cl
Typical experimental procedure for epoxidation
O
O
O
O
N
N
N
N
Ruthenium complex 1 (2.5 mol%) was added to the alkene (1
mmol) dissolved in dichloromethane (4 ml). To this homo-
geneous solution NaHCO3 (1.5 equiv.) and isobutyraldehyde
(1.5 equiv.) were added. The mixture was stirred under an
atmosphere of oxygen at 25 ЊC and the reaction was monitored
by TLC. Once the reaction was over, the reaction mixture was
diluted with CH2Cl2 and filtered through a pad of Celite and
silica gel. Removal of solvent yielded the crude product which
was purified by flash chromatography over neutral alumina or
distillation under reduced pressure.
Ru
Cl
1
In the preliminary investigations of oxidation of alkenes with
1, PhIO, NaIO4, urea–H2O2, NaOCl and TBHP were used as
terminal oxidants. In all cases the reaction was incomplete
(30–50% conversion) and the product was always a mixture of
epoxides and cleavage products. However, when the oxidation
of alkenes was carried out with 1 (2.5 mol%) in CH2Cl2 (25 ЊC,
6–12 h) in the presence of molecular oxygen as the oxidant
and isobutyraldehyde as the co-reductant excellent yields of
epoxides 2–21 were obtained. The results are summarized in
Table 1.
Acknowledgements
The authors thank the Department of Science and Technology,
New Delhi for financial support of this investigation. One of
the authors, V. K. thanks UGC for a research fellowship.
J. Chem. Soc., Perkin Trans. 1, 1997
3115
Kesavan, Venkitasamy
