Synthetic, spectral and catalytic activity studies of ruthenium bipyridine and terpyridine complexes: Implications in the mechanism of the ruthenium(pyridine-2,6-bisoxazoline)(pyridine-2,6-dicarboxylate)-catalyzed asymmetric epoxidation of olefins utilizing H2O2

Article abstract of DOI:10.1016/j.jorganchem.2005.12.069

Various Ru(L₁)(L₂) (1) complexes (L₁ = 2,2′-bipyridines, 2,2′:6′,2″-terpyridines, 6-(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl-2,2′-bipyridinyl or 2,2′-bipyridinyl-6-carboxylate; L₂ = pyridine-2,6-dicarboxylate, pyridine-2-carboxylate or 2,2′-bipyridinyl-6-carboxylate) have been synthesized (or in situ generated) and tested on epoxidation of olefins utilizing 30% aqueous H₂O₂. The complexes containing pyridine-2,6-dicarboxylate show extraordinarily high catalytic activity. Based on the stereoselective performance of chiral ruthenium complexes containing non-racemic 2,2′-bipyridines including 6-[(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl]-[2,2′]bipyridinyl new insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed enantioselective epoxidation are proposed. In addition, a simplified protocol for epoxidation of olefins using urea hydrogen peroxide complex as oxidizing agent has been developed.

Full text of DOI:10.1016/j.jorganchem.2005.12.069

Journal of Organometallic Chemistry 691 (2006) 4419–4433
www.elsevier.com/locate/jorganchem
Synthetic, spectral and catalytic activity studies of ruthenium bipyridine
and terpyridine complexes: Implications in the mechanism of the
ruthenium(pyridine-2,6-bisoxazoline)(pyridine-2,6-dicarboxylate)-
catalyzed asymmetric epoxidation of olefins utilizing H2O2
a
b
b
b
Man Kin Tse , Haijun Jiao , Gopinathan Anilkumar , Bianca Bitterlich ,
b
b,
*
Feyissa Gadissa Gelalcha , Matthias Beller
a
Center for Life Science Automation (Celisca), Friedrich-Barnewitz-Strasse, 4, 18119 Rostock, Germany
Leibniz-Institut fu¨r Organische Katalyse an der Universita¨t Rostock e.V., Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
b
Received 1 November 2005; received in revised form 9 December 2005; accepted 23 December 2005
Available online 7 March 2006
Abstract
Various Ru(L1)(L2) (1) complexes (L1 = 2,20-bipyridines, 2,20:60,200-terpyridines, 6-(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl-2,20-bipy-
ridinyl or 2,20-bipyridinyl-6-carboxylate; L2 = pyridine-2,6-dicarboxylate, pyridine-2-carboxylate or 2,20-bipyridinyl-6-carboxylate) have
been synthesized (or in situ generated) and tested on epoxidation of olefins utilizing 30% aqueous H2O2. The complexes containing pyr-
idine-2,6-dicarboxylate show extraordinarily high catalytic activity. Based on the stereoselective performance of chiral ruthenium com-
plexes containing non-racemic 2,20-bipyridines including 6-[(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl]-[2,20]bipyridinyl new insights on the
reaction intermediates and reaction pathway of the ruthenium-catalyzed enantioselective epoxidation are proposed. In addition, a sim-
plified protocol for epoxidation of olefins using urea hydrogen peroxide complex as oxidizing agent has been developed.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium; Olefin; Epoxidation; Homogeneous catalysis; Mechanism
1. Introduction
tions (50% atom efficiency), thus such processes produce
significant amount of waste from the co-reductant [3,4].
Apart from molecular oxygen, hydrogen peroxide (H2O2)
has been shown to be a useful oxidant with respect to the
criteria mentioned above [5,6]. Due to its handling and
price it is particularly useful for liquid-phase oxidations
for the synthesis of fine chemicals, pharmaceuticals, agro-
chemicals and electronic materials. Hence, developments
on new catalytic systems using H2O2 are an important
and challenging goal in oxidation chemistry [7,8].
With regard to enantioselective epoxidations of olefins,
titanium (Sharpless epoxidation) [9] and manganese (Jac-
obsen–Katsuki epoxidation) [10] based catalysts are still
in the state-of-the-art. Very recently Katsuki et al. demon-
strated that a titanium schiff base catalyst showed very
good activity and excellent ee in asymmetric epoxidation
utilizing aqueous H2O2 [11]. In the last decade significant
Oxidation reactions constitute one of the core technolo-
gies which convert simple bulk raw materials, such as alk-
anes and olefins, to value-added products in higher
oxidation states [1]. However, in the area of fine chemicals
and pharmaceuticals they are less established on industrial
scale compared to reductions and CC coupling reactions.
For a more general application of oxidation reactions,
the ‘‘ideal’’ oxidant should be in high atom-economy, envi-
ronmentally benign, readily available and safe as well [2].
Air (molecular oxygen) is undoubtedly the perfect oxidant
of choice for a number of oxidation reactions. However,
only one oxygen atom of O2 is productive for most oxida-
*
Corresponding author. Tel.: +49 381 12810; fax: +49 381 1281 5000.
E-mail address: matthias.beller@ifok-rostock.de (M. Beller).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.12.069

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M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
progress using organic catalysts based on chiral ketones
(Shi, Yang and other’s ketone catalysts) [12,13] has also
been reported. Besides, the polypeptide-catalyzed stereose-
lective epoxidation of enones under phase transfer condi-
tions seems to be an industrially applicable process [14].
Chiral Lewis acids [15] and chiral amines [16] catalyzed
epoxidation of a,b-unsaturated olefins also have been
demonstrated.
O
O
O
O
O
O
N
Ru
N
N
Ru
N
N
O
N
O
N
O
N
O
Ph
Ph
Ph
O
Ph
O
1
2
Generally, there are several problems associated with
the use of H2O2 in asymmetric epoxidations [8]. For
instance H2O2 usually decomposes to O2 in the presence
of transition metal catalysts. Over stoichiometric amount
of H2O2 is often employed to solve this problem. However,
the stability of the catalyst in high concentration of H2O2
as well as the selectivity, especially enantioselectivity, in
the presence of water obviously could be problematic. It
is also apparent that oxidative cleavage of the olefin com-
petes with the productive epoxidation. Therefore, even
after decades of extensive research efforts, the development
of general and catalytic asymmetric epoxidation methods
using H2O2 is still underway [16,11,17].
O
O
O
O
N
N
N
N
Ru
N
Ph
Ph
N
N
N
O
N
O
Ru
N
Ph
Ph
O
O
O
O
O
O
3
4
Fig. 1. Ruthenium catalysts for epoxidation of olefins with H2O2.
Our interest has been aroused by ruthenium-catalyzed
oxidation reactions with its wide range of applicability
and broad variation of ligand type especially in asymmetric
epoxidation of olefins [18]. In this regard, we chose Ru(pyr-
idine-2,6-bisoxazoline)(2,6-pyridinedicarboxylate) (1) as
our starting point as it contains two different meridional
ligands [19]. It is patently advantageous that by modifying
the chiral (pybox) and the achiral (pydic) ligands sepa-
rately, the reactivity and (enantio)selectivity should be pos-
sible to tune up easily. Applying this concept, we were able
to make 1 to become a more practical epoxidation catalyst
by adding a controlled amount of water to the reaction
mixture [20a]. This also led to the development of enantio-
selective epoxidation protocols applying alkyl peroxides
[20b] and hydrogen peroxide [20d,20e,20f]. Noteworthy,
highly productive and robust ruthenium catalysts with
turnover number (TON) up to 10000 have also been devel-
oped for non-asymmetric epoxidation with 30% aqueous
H2O2 [20c,20e]. Most recently, a ligand library of N,N,N-
tridentate pyridinebisimidazolines, so-called pybim ligands,
was also realized during the course of these studies [20f,21].
Fig. 1 shows four of more than 70 catalysts which we have
tested in the epoxidation of olefins with H2O2.
lectivity up to 99%; enantioselectivity up to 85%) [22].
With regard to the mechanism of this novel asymmetric
epoxidation, we have established the role of the protic sol-
vent, the effect of acid additives, and the relative rate of dif-
ferent catalysts. In addition, competitive reactions with
different olefins suggested an unsymmetric oxo-transfer
transition state [22b]. Modelling studies on the reactivity
of the core structure 10 at the B3LYP/LANL2DZ density
functional level of theory showed that an N-oxide coordi-
nating intermediate is likely to be the most thermodynam-
ically stable isomer of all possible intermediates. Scheme 1
shows a preliminary mechanistic picture of the reaction.
Initially, the carboxylate on 10 dissociates in a protic sol-
vent and makes the oxidation of the ruthenium center
much faster. Hence, the ruthenium(III) species D forms,
which is further oxidized to intermediates A. A3 is the most
stable one suggested by theoretical calculation. However,
from the evidence of kinetic studies, A1 is suspected to be
the active oxo-transfer intermediate [23]. After epoxidation
of the olefin D is regenerated (cycle A). Alternatively, inter-
mediates A can be further oxidized to complexes B which
transfer the oxygen atom to the olefin and regenerate inter-
mediates A (cycle B).
In this paper, we describe our work on the synthesis of
novel ruthenium(II) bipyridine and terpyridine complexes.
Spectral and catalytic activity studies using these complexes
shed light on the mechanism of the ruthenium-catalyzed
asymmetric epoxidation. Besides a simplified protocol for
the epoxidation of olefins utilizing urea hydrogen peroxide
complex has been developed.
Unfortunately, so far any isolation of ruthenium inter-
mediates in higher oxidation state was not successful.
Also initial attempts to synthesize the mono- or di-N-
oxide of 2,6-bis-[(4S)-4-phenyl-4,5-dihydrooxazol-2-yl]pyr-
idine (S,S-Ph2-pybox) by oxidation with m-CPBA [24] or
cyclization of (S)-2-hydroxy-amino-2-phenylethanol with
dimethyl pyridine-2,6-dicarboximidate [25] did not yield
the desired N-oxide ligands. Apparently S,S-Ph2-pybox
and its N-oxides are not stable in strong oxidizing and
acidic conditions.
2. Results and discussion
In our previous studies, we have shown that various
ruthenium complexes, such as 1 and 2 allow for the epox-
idation of aromatic olefins with high selectivity (chemose-
Therefore, we chose the more robust 2,20:60,200-terpyri-
dines as our model ligand system. Though the turnover

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4421
Scheme 1. Proposed mechanism for the Ru(pybox)(pydic)-catalyzed epoxidation of olefins.
number (TON) of the ruthenium terpyridine complex 3 is
around 40 times larger than that of 1 (TON of 1 and 3
are ꢀ200 and ꢀ8000, respectively) [20d,20e], competition
reactions showed that the relative rate of 3 is only 1.8–
2.4 times faster than that of 1 [22b]. This shows good agree-
ment with our hypothesis that the high productivity of 3 is
due to its robustness and not electronic or steric reasons.
Owing to better solubility, 4,40,400-tri-tert-butyl-2,20:60,20-
terpyridine 5a was subjected to selective oxidation with
m-CPBA to yield 6 and 7 in 53% and 81%, respectively
(Scheme 2) [26].
electrospray ionization mass spectrometry (ESI-MS) after
removal of solvent. Moreover, unidentified products were
observed in the ESI-MS when 7 was used in the complexa-
tion reaction. Direct oxidation of 3a with m-CPBA in
CH2Cl2 turned the solution from violet to brown instanta-
neously. However, after removal of solvent, only a purple
solid with molecular mass as 3a was observed. Hence, titra-
tion of 3a with m-CPBA monitored by UV–Vis spectros-
copy in CH2Cl2 at 25 ꢁC was then performed. As shown
in Fig. 2 the absorption maxima at 526, 329, 321 nm were
gradually decreased and the shoulder at 337 nm was
increased when portions of m-CPBA solution was added
(see Section 4). Isosbestic points were observed at 354
and 327 nm. This phenomenon indicates that a single
Next, 6 and 7 were reacted with [Ru(p-cymene)Cl2]2 as
illustrated in Scheme 3. In case of 6 only metal species with
a molecular weight similar to 3a have been observed in the

4422
M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
tBu
tBu
N
tBu
tBu
tBu
tBu
1 equiv. m-CPBA
CH2Cl2, r.t.
N
N+
N
N
N
N
N
O-
5a
6 53%
tBu
tBu
tBu
tBu
tBu
tBu
3.8 equiv. m-CPBA
CH2Cl2, r.t.
N
N
N+
N+
O- -O
5a
7 81%
Scheme 2. Synthesis of terpyridine N-oxides.
t Bu
3a or Intermediate D
+
t Bu
t Bu
[Ru(p-cymene)Cl2]2,
N
Na2pydic, 65 ˚C, Ar,
MeOH or tert-AmOH
N+
N
O-
Intermediate A (trace)
6
tBu
N
t Bu
tBu
[Ru(p-cymene)Cl2]2,
unidentified products
Na2pydic, 65 ˚C, Ar,
MeOH or tert-AmOH
N+
N+
O- -O
7
t Bu
t Bu
t Bu
N
Ru
N
3a or Intermediate D
N
O
N
O
m-CPBA
CH2Cl2
+
Intermediate A (trace)
O
O
3a
Scheme 3. Attempts to synthesize catalytic intermediates A.
product was formed preferentially [27]. The ratio of 3a to
m-CPBA determined by the titration was approximately
1:1 in three different concentrations of 3a.
The ESI-MS of the 3a:m-CPBA 1:1 solution showed
only the molecular ion peak similar to 3a. No epoxide
was observed even in the presence of a 10-fold excess of sty-
rene. Reacting an excess of m-CPBA with 3a in CH2Cl2 or
CH3CN gave only a weak molecular ion peak of
[(3a + O + H)+] and high intensity of [(3a)+] in the ESI-
MS. Noteworthy, the molecular ion peak of [(3a + 2O)+]
has never been observed. In addition, there was no obser-
vable change in the UV–Vis spectrum when benzoic acid
was added to the solution of 3a. Hence, the UV–Vis
absorption spectrum shift is not due to the formation of
intermediate A nor protonation. Instead, we propose that
3a is easily oxidized by m-CPBA to intermediate D. This
is in agreement with the previous observation that 3 can
be oxidized to a paramagnetic Ru(III) state in solution
[28]. However, we could not exclude that intermediate A
is generated in low concentration or it decomposed back

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4423
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
300
350
400
450
500
550
600
650
700
750
800
wavelength (nm)
Fig. 2. UV–Vis titration of 3a with m-CPBA.
Table 1
to 3a or intermediate D. In fact ruthenium (IV) oxo por-
phyrin has been reported to undergo disproportionation
to give a ruthenium (II) porphyrin and a ruthenium (VI)
dioxo porphyrin complex [29].
B3LYP/LANL2DZ total electronic energies (Etot, au) and total Gibbs free
energies (Gtot, au, at 298 K) and number of imaginary frequencies
(NImag)
Etot
Gtot
NImag
Thus, we computed the possible key intermediates aim-
ing to have further insight about the decomposition path-
ways of the oxidized complex 3 (Scheme 4 and Table 1).
All calculations were carried out by using the GAUSSIAN 03
program [30]. Structures were optimized at the B3LYP
[31] density functional level of theory with the LANL2DZ
basis set [32], and the nature of the optimized structures
on the potential energy surface (PES) was characterized
by the calculated number of imaginary frequency (NImag)
at the same level of theory (B3LYP/LANL2DZ), i.e.; mini-
mum structures without (NImag = 0) [33]. The correspond-
ing frequency calculations provide also the thermal
corrections to Gibbs free energies at 298 K. The energies
for discussion and interpretation are the Gibbs free energies
(DG = DH ꢁ TDS). At B3LYP/LANL2DZ, the parent
complex 3 has C2 symmetry, and the mono- and di-oxidized
species 3(O) and 3(O)2 are C1 and C2 symmetrical, respec-
tively. All three structures are energy minimums on the PES.
From the Gibbs free energy, both the bimolecular
decomposition of mono-oxo complex 3(O) to 3 and O2
and self decomposition of di-oxo ruthenium complex
3(O)2 to 3 and O2 are highly thermodynamically favourable
3/C2
3(O)/C1
3(O)2/C2
ꢁ1460.51505
ꢁ1535.65806
ꢁ1610.79774
ꢁ1460.23975
ꢁ1535.38146
ꢁ1610.51774
0
0
0
2½3ðOÞꢂ ! 2½3ꢂ þ O2 DG ¼ ꢁ30:0 kcal molꢁ1
½3ðOÞꢂ ! 2½3ꢂ þ O2 DG ¼ ꢁ33:6 kcal molꢁ1
ð1Þ
ð2Þ
ð3Þ
2½3ðOÞꢂ ! ½3ꢂ þ ½3ðOÞ ꢂ DG ¼ þ3:4 kcal molꢁ1
2
(about ꢁ30 kcal molꢁ1). Disproportionation of 3(O) to 3
and 3(O)2 is also not likely (+3.4 kcal molꢁ1). The calcula-
tions suggest that the ruthenium oxo-transfer species is not
3(O)2 due to insufficient stability, though the rate of this
decomposition is not so clear. Moreover, since the decom-
position of 3(O) is bimolecular, when the concentration of
3(O) is low (as in the catalytic reaction conditions: H2O2
is dosed to the catalytic reaction slowly in 12 h and pre-cat-
alyst is only in catalytic amount), the olefin acts as a good
trap and epoxide forms. The bimolecular decomposition
of two 3(O) to molecular oxygen also explain the observa-
tion in the ESI-MS [very low intensity of 3(O)] and the
N
N
N+
O
N+
O
N+
O
N
N
O
N
O
N
O
O
Ru
Ru
N
Ru
N
O
O
O
N
O
O
O
O
O
3
3(O)
3(O)2
Scheme 4. Modeling of the decomposition reactions of 3, 3(O) and 3(O)2.

4424
M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
negative results in the attempts of isolating 3(O) since con-
centrating of the solution of 3(O) favours the bimolecular
decomposition of 3(O) to 3 and O2 (infra supra).
Direct cyclization of unsymmetrical 6-[(4S)-4-phenyl-
4,5-dihydro-oxazol-2-yl]-[2,20]bipyridinyl (10) with (S)-2-
amino-2-phenylethanol and 2,20-bipyridine-6-carbonitrile
in refluxing anhydrous chlorobenzene using anhydrous
ZnCl2 as the catalyst showed inferior result and gave
only a trace of the desired product. To our delight, 10
was obtained in good yield when methyl 2,20-bipyri-
dine-6-carboxyimidate was refluxed with (S)-2-amino-2-
phenylethanol in CH2Cl2 for 48 h. Chiral non-racemic
2,20-bipyridine (11a) was reported to induce good enanti-
oselectivity in epoxidation of styrene with PhI(OAc)2,
though with low reactivity [36]. Therefore, we were inter-
ested to test this type of ligands in our reaction condi-
tions [20d]. Hence, 11a and 11b were synthesized by
Kro¨hnke condensation [37]. Subsequently, 11c and 11d
were obtained in good yields using the Suzuki–Miyaura
cross coupling reaction [37b]. Other commercially avail-
able ligands and co-ligands used in this study are also
shown in Scheme 5.
In order to have a better understanding of the reactivity
and transfer of chirality, we used the fragment-based
approach to address the essential part of the catalysts.
These C2 symmetric catalysts contain two meridional
ligands. Each of them contains four pyridine nitrogen
and two carboxylate oxygen atoms. We decided to synthe-
size and test new catalysts with similar coordinating groups
according to their possibility of modification, the availabil-
ity of starting materials, and the ease of synthesis. Hence,
different 2,20-bipyridines functionalized in the 6-position
were synthesized (Scheme 5). 2,20-Bipyridine N-oxide was
treated with TMSCN and benzoyl chloride at 0 ꢁC to room
temperature for 18 h to yield 2,20-bipyridine-6-carbonitrile
in 90% [34]. Hydrolysis of 2,20-bipyridine-6-carbonitrile
with HCl gave 2,20-bipyridine-6-carboxylic acid in good
yield (77%) [35].
a
b
N+
O-
N
CN
N
CO2H
N
N
N
9
c
OMe
NH
O
N
d
N
N
N
N
Ph
10
I-
e
N+
N
N
N
O
O
N
11a
I-
f
g
N+
Br
N
Br
N
R
N
N
11b
11c R = Ph
11d R = 1-Np
O
O
N
N
CO2H
N
N
HO2C
N
HO2C
N
N
N
Ph
Ph
Scheme 5. Synthesis of ligands and commercially available ligands in this study. Reagents and conditions: (a) TMSCN, PhCOCl, CH2Cl2, 0 ꢁC to r.t.,
18 h, 90%; (b) 37% HCl, reflux, 2 h, 77%; (c) Na, anhy. MeOH, Ar, r.t., 4 days, 99%; (d) (S)-2-amino-2-phenylethanol, anhy. CH2Cl2, Ar, reflux, 48 h,
92%; (e) (1R)-(ꢁ)-myrtenal, NH4OAc, formamide, 75 ꢁC, 6 h, 53%; (f) (1R)-(ꢁ)-myrtenal, NH4OAc, HOAc, reflux, 6 h, 42%; (g) 0.2 mol% Pd(PPh3)4,
RB(OH)2, toluene/H2O, K2CO3, 120 ꢁC, 13 h, 11c: 99%, 11d: 84%.

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4425
MeOH, 65 ˚C, Ar, 1 h
Ru(L1)(L2)
59-99%
L1
L2
[Ru(p-cymene)Cl2]2
+
+
NaOH
O
N
N
N
N
N
N
N
O
O
N
O
Ru
Ru
Ru
N
O
O
Cl
N
N
O
N
O
O
O
3 87%a
12 66%
13 84%
O
O
O
N
O
O
O
N
N
N
N
N
N
N
N
Ru
Ru
Ru
Ph
Ph
Ph
Ph
Ph
O
O
O
O
Cl
O
N
N
N
O
O
O
O
14 79%b
15 59%
16 77%
Scheme 6. Synthesis of ruthenium catalysts: aRef. [20e]; bRef. [22a].
O
R
*
Catalyst
R
30% H2O2
+
*
tert-AmOH, r.t.
12 h addition H2O2
R = H, Ph
Scheme 7. Ruthenium-catalyzed epoxidation of styrene and trans-stilbene.
Next, six different ruthenium complexes have been syn-
thesized in moderate to good yields by heating [Ru(p-cym-
ene)Cl2]2 with the corresponding ligand in MeOH at 65 ꢁC
under Ar in the presence of NaOH (1 equiv. per carboxylic
acid) for 1 h (see Section 4) (Scheme 6). All pre-catalysts were
well characterized and subjected to two prototypical epoxi-
dation reactions. The results are summarized in Scheme 7
and Table 2. In case of complexes with 2,20-bipyridines
Table 2
a
Ruthenium-catalyzed epoxidation of styrene and trans-stilbene with H2O2
O
Entry
Catalyst
O
*
*
Ph
Ph
Ph
*
Yield (%)
Ee (%)
Yield (%)
Ee (%)
1
2
3
4
5
6
7
8
9
0.5 mol% 3a
0.5 mol% 12b
0.5 mol% 13
5 mol% 14c
5 mol% 15
71
0
9
70
68
18
67
40
29
77
–
–
–
96
0
26
100
>99
14
84
28
19
69
–
–
–
+31d
ꢁ1
n.d.f
+4
ꢁ2
ꢁ4
0
ꢁ67e
0
5 mol% 16
n.d.
0
0
2.5 mol% [Ru(p-cymene)Cl2]2, 5 mol% 11a, 5 mol% H2pydic, 12 mol% Et3N
2.5 mol% [Ru(p-cymene)Cl2]2, 5 mol% 11b, 5 mol% H2pydic, 12 mol% Et3N
2.5 mol% [Ru(p-cymene)Cl2]2, 5 mol% 11c, 5 mol% H2pydic, 12 mol% Et3N
2.5 mol% [Ru(p-cymene)Cl2]2, 5 mol% 11d, 5 mol% H2pydic, 12 mol% Et3N
ꢁ3
10
a
0
Ref. [20e].
b
c
d
e
f
See Section 4.
Ref. [22b].
‘‘+’’ sign means (R)-(+)-styrene oxide is the major enantiomer.
‘‘ꢁ’’ sign means (S,S)-(ꢁ)-trans-stilbene oxide is the major enantiomer.
Not determined.

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M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
11a–d and 2,6-pyridinedicarboxylic acid we used an in situ
catalyst generation protocol [20a], due to the easy hydrolysis
of the resulting complexes [36].
NMR spectrum of the isolated product showed diamag-
netic behaviour and was not explainable. A careful analysis
of the ultra high resolution mass spectrum showed
three major peaks at m/z = 636.05765 (C30H22N4O6Ru+),
650.03980 (C30H20N4O7Ru+), and 652.05383 (C30H22N4-
O7Ru+). We believe that 14 is oxidized to a Ru(III) species
together with two Ru complexes with one oxygen atom
more, in which one of them has an aromatized oxazoline
ring. Catalytic tests with the chiral catalyst 15 demon-
strated that even one aromatized oxazoline ring on the cat-
alyst may lead to a total loss of enantioselectivity. In
agreement with this observation, the suspected Ru(S,S-
Ph2-pybox)(pydic)(O) mixture gave a significantly lower
enantioselectivity compared with the original catalyst 14.
The testing of in situ generated catalysts with chiral bipyri-
dines 11a–d resulted in 29–77% of trans-stilbene oxide,
however none of the catalysts gave any reasonable ee (0–
4%) (Scheme 8).
All catalytic experiments were run with 30% aqueous
H2O2 at room temperature. Complex 3 has been shown
to be a general and efficient catalyst because of its robust-
ness [20e]. Surprisingly complexes 12 and 13, which contain
the same aromatic terpyridine unit as 3, showed a drastic
decrease of activity. Simply changing 2,6-pyridinedicarb-
oxylate (pydic) to 2-pyridinecarboxylate, the resulting
ruthenium complex 12 showed no reactivity at all. Even
13 containing four pyridine nitrogen and two carboxylate
groups similar to 3 gave only poor yield for the epoxides
[38]. Clearly, the chelating sub-unit ‘‘O–N–O’’ of pydic,
which stabilizes the Ru(III) oxidation state by donating
electron density to the metal center, plays a dominating
role on the catalytic reactivity.
The importance of this ‘‘O–N–O’’ sub-unit is also
reflected by the reduction and oxidation potentials of dif-
ferent ruthenium complexes (Table 3). It is with good
agreement that the higher the oxidation potential of
Ru2+/Ru3+, the lower the reactivity in the epoxidation
reaction. On the other hand, the higher the E1/2 (reduction
potential) the higher the stability of the ligands towards
reduction. Hence, 3 showed the highest reduction potential
on terpyridine. Moreover, no reduction potential of pydic
was observed [39]. As both terpyridine and ‘‘pydic’’ are
more stable, the whole catalytic system becomes more
robust.
Finally, we were interested in improving our general
epoxidation protocol. In our original procedure, 30% aque-
ous H2O2 was delivered to the olefin in tert-amyl alcohol in
the presence of 0.5 mol% of 3 for 12 h by a syringe pump to
prevent unproductive decomposition of H2O2.
For a laboratory procedure, it is advantageous to apply
a solid oxidizing agent which can be dosed to the reaction
mixture without any additional equipment. In case of a
lower solubility of the solid oxidant in tert-amyl alcohol,
unwanted decomposition of H2O2 to O2 should be mini-
mized. Hence, we tested common solid oxidants for the
epoxidation of trans-stilbene (Table 4). To our delight, urea
hydrogen peroxide complex deserves our hypothesis and
gave an excellent yield of epoxide (>99%) in our model
reaction in only 1 h. Next, we tested this simplified protocol
and the results are shown in Table 5. UPH oxidized aro-
matic olefins in the presence of 0.5 mol% 3 with moderate
to very good selectivity. Compared to the original proce-
Attempts to isolate catalysis intermediates from epoxida-
tion reactions in the presence of Ru(S,S-Ph2-pybox)(pydic)
(14) resulted in the isolation of suspected Ru(S,S-Ph2-
1
pybox)(pydic)(O) complexes in low yield (<10%). The H
Table 3
Redox potentials of various ruthenium terpyridine and pydic complexes
Complex
E1/2 (oxidation) [V] E1/2 (reduction) [V]
Ref.
Table 4
1
2
Screening of various solid oxidant for epoxidation of trans-stilbene
Ru(tpy)2+
+1.31a
ꢁ1.24 ꢁ1.49
ꢁ1.39 ꢁ2.12
ꢁ1.36 ꢁ1.63
[38]
[39]
[38]
Time Conv. (%) Yield (%) Selec. (%)a
(h)
[Ru(tpy)(pic)][PF6] +0.88b
Entry Oxidant
Ru(tpy)(bpyCO2)
Ru(tpy)(pydic) (3) +0.60
Ru(bpyCO2)2 (13)
KRu(pydic)2
+0.90
ꢁ1.53 not observed [39]
1
2
3
4
5
30% H2O2
UHPb
12
1
100
100
0
9
14
96
>99
0
4
0
96
>99
0
44
0
+0.52
+0.21c
ꢁ1.49 ꢁ1.79
n.d. n.d.
[38]
[40]
Na2CO3 Æ 1.5H2O2 16
Oxoneꢂc
Ca(OCl)2
16
16
n.d., Not determined.
a
Reduction potential vs. SCE.
Initial reported reduction potentials were referenced to Fc/Fc+ which
b
a
b
c
Selectivity towards epoxide.
Urea hydrogen peroxide complex.
2KHSO4 Æ KHSO4 Æ K2SO4.
is taken as +0.48 V here.
c
Determined by potentiometric titration at pH 7.
O
Ru(tpy)(pydic) 3
Oxidant
+
tert-AmOH, r.t.
Scheme 8. Screening of various solid oxidant for epoxidation of trans-stilbene.

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4427
Table 5
ing epoxides. Functional groups like alcohol and halogens
are tolerant in the reaction system. Clearly, slow dosage of
the oxidant is no longer needed for this method.
Scope and limitations of Ru(tpy)(pydic) 3-catalyzed epoxidation of olefin
with UHP
Entry
1
Olefin
Time
(h)
Conv.
(%)
Yield
(%)a
Selec.
(%)b
3. Conclusion
3
41
39
95
Ph
In summary, mechanistic studies of ruthenium-cata-
lyzed epoxidations of olefins were performed experimen-
tally and theoretically with high level density functional
theory calculations. Mono-N-oxide A3 and active oxo-
transfer catalyst A1 are possible intermediates in the reac-
tion pathway. A fragment-based catalyst design showed
that the 2,6-pyridinedicarboxylic acid ligand is essential
for the reactivity. Moreover, a C2 symmetric chiral ligand
provides the necessary chiral induction. Hence, catalyst 15
gave no ee for the epoxidation of trans-stilbene and sty-
rene. This suggests that a partial ligand oxidation of the
catalyst is possibly one of the non-productive asymmetric
epoxidation pathways. Moreover, a general simplified and
more active ruthenium-catalyzed epoxidation procedure
of olefins utilizing urea hydrogen peroxide complex has
been developed. Further development towards asymmetric
epoxidation reactions are under investigation in our
laboratory.
2
3
81
74
91
H3C
3
4
3
3
49
40
39
28
80
71
F
Cl
5
6
3
3
100
100
>99
95
>99
95
Ph
MeO
Ph
7
3
100
92c
92
OPh
OH
4. Experimental
8
9
3
3
100
65
63
59
63
91
Ph
Ph
4.1. General
Unless specified, all chemicals are commercially avail-
able and used as received. Ru(tpy)(pydic) (3) [19],
Ru(tBu3-tpy)(pydic) (3a) [20e], bpyCO2H (9) [32], 2,20-
bipyridines (11a–d) [37], Ru(bpyCO2)2 (13) [38], and
Ru(S,S-Ph2-pybox)(pydic) (14) [19], are synthesized
according to the literature procedures. Qualitative analysis
of reaction products was done on a gas chromatograph HP
5890 with mass selective detector HP 5989A (Hewlett-
Packard) and a capillary column of type HP 5 was used
for separation. For quantitative analysis of reaction mix-
ture, the measurement was performed on a HP 6890 gas
chromatograph (Hewlett-Packard) with flame ionization
detector. The separation is obtained on a capillary column
of type HP 5 (5% phenylmethylsiloxane, length 30 m, inner
diameter 250 lm, film thickness 0.25) with argon as flowing
gas. Enantiomeric excess of epoxides were determined with
HP 1090 liquid chromatography (Hewlett-Packard)
equipped with a DAD detector. UV–Vis spectroscopic
measurements were performed with a Shimadzu UV-1601
spectrophotometer. Nuclear magnetic resonance spectra
were recorded on Bruker ARX300 or ARX400 spectrome-
ters. All NMR spectra were taken in pure deuteriated sol-
vents, such as CDCl3, CD2Cl2, etc. Chemical shifts (d) are
given in ppm and refer to residual solvent as internal stan-
dard. Signal multiplicity and coupling constants (J in Hz)
are shown in the parentheses. For multiplicity the follow-
ing abbreviations are used: s, singlet; d, duplet; dd, duplet
of duplet; t, triplet and m, multiplet.
10
11
12
3
3
3
67
100
99
51
83
95
76
83
96
Ph
Ph
Ph
Ph
13
3
99
92
93
14
15
3
3
100
53
96d
48
96
91
Ph
Ph
Ph
a
GC-yields.
Chemoselectivity towards epoxide.
Isolated yield.
b
c
d
NMR yield.
dure the reaction time is shortened. Mono-, 1,1-di, trans-
and cis-1,2-di-, tri- and even tetra-substituted olefins all
have good to excellent selectivity towards the correspond-

4428
M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4.2. Ligand synthesis
0 ꢁC and TMSCN (1.0 ml, 8.71 mmol) was added. Benzoyl
chloride (404 ll, 3.48 mmol) was added dropwise. The reac-
tion mixture was stirred overnight. Then 10% Na2CO3
(10 ml) was added and the mixture was separated and the
aqueous layer was extracted with CH2Cl2 (5 ml · 3). The
combined organic layer was dried over MgSO4, filtered
and evaporated under reduced pressure. The crude product
was chromatographed on silica gel (70–230 mesh) using
CH2Cl2 to CH2Cl2:ethyl acetate (10:1) as the gradient
eluent. An off-white solid was obtained after removal of
solvent (285 mg, 90%). 2,20-bipyridine-6-carbonitrile:
Rf = 0.68 (CH2Cl2:ethyl acetate 6:1); 1H NMR
(400.1 MHz, CDCl3): d = 7.37 (ddd, J = 7.5, 4.8, 1.2 Hz,
1H), 7.68 (dd, J = 7.6, 0.9 Hz, 1H), 7.85 (unresolved ddd,
1H), 7.93 (unresolved dd, 1H), 8.45 (d, J = 7.9 Hz, 1H),
8.66–8.68 (m, 2H); 13C NMR (100.6 MHz, CDCl3):
d = 117.33, 121.65, 124.27, 124.80, 128.17, 133.18, 137.41,
137.92, 149.09, 153.82, 157.44; MS (EI, 70 eV) m/z 181 (M+).
4.2.1. Synthesis of 4,40,400-tri-tert-butyl-2,20:60,200-terpyridine
1-N-oxide (6)
To the solution of 4,40,400-tri-tert-butyl-2,20:60,200-terpyri-
dine (5a) (300 mg, 0.75 mmol) in CH2Cl2 (7.5 ml), m-CPBA
(77%, 167 mg, 0.75 mmol) in CH2Cl2 (7.5 ml) was added
dropwise. The reaction mixture was stirred at r.t. for
15 h. It was then diluted with CH2Cl2 (10 ml) and washed
with NaHCO3 (10 ml · 3). The organic layer was dried
over MgSO4, filtered and evaporated under reduced pres-
sure. The crude product was chromatographed on silica
gel (70–230 mesh) using ethyl acetate:Et3N:MeOH
100:1:0 to 100:1:1 as the gradient eluent. An off-white solid
was obtained after removal of solvent (165 mg, 53%). 6:
1
Rf = 0.60 (ethyl acetate:Et3N:MeOH 100:1:10); H NMR
(400.1 MHz, CDCl3): d = 1.37 (s, 9H), 1.38 (s, 9H), 1.42
(s, 9H), 7.25 (dd, J = 6.9, 3.0 Hz, 1H), 7.35 (dd, J = 5.4,
2.0 Hz, 1H), 8.23 (d, J = 6.9 Hz, 1H), 8.47 (d, J = 3.0 Hz,
1H), 8.53 (d, J = 2.0 Hz, 1H), 8.57 (d, J = 1.7 Hz, 1H),
8.62 (d, J = 5.4 Hz, 1H), 9.17 (d, J = 1.7 Hz, 1H); 13C
NMR (100.6 MHz, CDCl3): d = 30.46, 30.67, 34.65,
35.08, 35.44, 116.29, 118.51, 118.95, 121.89, 122.37,
123.12, 124.86, 140.06, 146.35, 148.18, 148.93, 150.12,
154.52, 155.50, 161.82; IR (KBr) 1588, 1546, 1480, 1377,
1261, 1192, 1067, 895, 831; 728, 609; EI-MS m/z 417
(M+); FAB-MS m/z 418 (M + H+); HRMS Calc. for
C27H36ON3: 418.28583. Found: 418.28564.
4.2.3.2. Synthesis of methyl 2,20-bipyridine-6-carboxyimi-
date. 2,20-Bipyridine-6-carbonitrile (1.0 g, 5.5 mmol) was
dissolved in anhydrous MeOH with gentle heating under
Ar. After it was cooled to r.t., Na (13 mg, 0.58 mmol) was
added. The reaction mixture was stirred for four days at
r.t. HOAc (33 ll, 0.58 mmol) was then added. The solvent
was removed under reduced pressure to give an off-white
solid (1.17 g, 99%). Methyl 2,20-bipyridine-6-carboxyimi-
1
date: m.p. 69.4–69.7 ꢁC; H NMR (300.1 MHz, d7-DMF):
d = 3.99 (s, 3H), 7.52 (ddd, J = 7.5, 4.8, 1.3 Hz, 1H), 7.91
(dd, J = 7.6, 1.0 Hz, 1H), 8.02 (ddd, J = 7.7, 7.7, 1.9 Hz,
1H), 8.13–8.19 (unresolved dd, 1H), 8.59 (dd, J = 7.9,
1.1 Hz, 1H), 8.67–8.70 (unresolved ddd 1H), 8.74 (ddd,
J = 4.8, 1.8, 0.9 Hz, 1H); 13C NMR (75.5 MHz, d7-DMF):
d = 53.94, 121.62, 121.67, 123.06, 124.31, 125.29, 138.03,
139.74, 147.37, 150.10, 155.51, 156.24, 166.61; MS (EI,
70eV) m/z 213 (M+); IR (KBr, cmꢁ1): 3290, 3050, 3002,
2951, 1976, 1654, 1185, 1099, 784, 748, 709; HRMS Calc.
for C19H15N3O m/z: 213.0897. Found: 213.08844.
4.2.2. Synthesis of 4,40,400-tri-tert-butyl-2,20:60,200-terpyridine
1,100-di-N-oxide (7)
To the solution of 4,40,400-tri-tert-butyl-2,20:60,200-terpyri-
dine (5a) (100 mg, 0.25 mmol) in CH2Cl2 (2.5 ml), m-CPBA
(77%, 210 mg, 0.94 mmol) in CH2Cl2 (2.5 ml) was added in
one portion. The reaction mixture was stirred at r.t. for
15 h. It was then diluted with CH2Cl2 (15 ml) and washed
with NaHCO3 (10 ml · 3). The organic layer was dried over
MgSO4, filtered and evaporated under reduced pressure.
The crude product was chromatographed on silica gel
(70–230 mesh) using ethyl acetate:Et3N:MeOH 100:1:0 to
100:1:3 as the gradient eluent. A white solid was obtained
after removal of solvent (88 mg, 81%). 7: Rf = 0.23 (ethyl
acetate:Et3N:MeOH 100:1:10); 1H NMR (400.1 MHz,
CDCl3): d = 1.34 (s, 18H), 1.39 (s, 9H), 7.29 (dd, J = 6.9,
3.0 Hz, 2H), 8.18 (d, J = 3.0 Hz, 2H), 8.29 (d, J = 6.9 Hz,
2H), 8.96 (s, 2H); 13C NMR (100.6 MHz, CDCl3):
d = 30.44, 30.56, 34.76, 35.38, 122.62, 123.71, 125.02,
139.94, 146.48, 149.23, 151.57, 160.87; IR (KBr) 1590,
1547, 1476, 1383, 1255, 1193, 887, 827; 709, 617, 604; EI-
MS m/z 433 (M+); FAB-MS m/z 434 (M + H+); HRMS
Calc. for C27H35O2N3: 433.27292. Found: 433.27271.
4.2.3.3. Synthesis of 6-[(4S)-4-phenyl-4,5-dihydro-oxazol-
2-yl]-[2,20]bipyridinyl (10). Methyl 2,20-bipyridine-6-
carboxyimidate (400 mg, 1.88 mmol) and (S)-2-amino-2-
phenylethanol (257 mg, 1.88 mmol) were dissolved in
anhydrous CH2Cl2 (8 ml) under Ar in a pressure tube.
The mixture was then heated at 60 ꢁC for three days. After
removal of solvent, the crude product was chromato-
graphed on silica gel (70–230 mesh) using CH2Cl2:Et3N:-
MeOH 100:1:0 to 100:1:2 as the gradient eluent. A pale
yellow solid was obtained after removal of solvent
(519 mg, 92%). Analytically pure product was obtained
by slow evaporation of the etherate solution of 10 to yield
an off-white product (310 mg, 55%). 10: Rf = 0.22 (CH2Cl2:
1
4.2.3. Synthesis of 6-[(4S)-4-phenyl-4,5-dihydro-oxazol-2-
yl]-[2,20]bipyridinyl (10)
4.2.3.1. Synthesis of 2,20-bipyridine-6-carbonitrile. 2,20-
Bipyridine N-oxide (300 mg, 1.74 mmol) was dissolved in
anhydrous CH2Cl2 (5 ml) under Ar. It was then cooled to
Et3N: MeOH = 100:1:5); m.p. 108.0–111.8 ꢁC; H NMR
(300.1 MHz, CDCl3): d = 4.37–4.42 (unresolved dd, 1H),
4.91 (dd, J = 10.2, 8.6 Hz, 1H), 5.46 (dd, J = 10.2,
8.5 Hz, 1H), 7.25–7.37 (m, 6H), 7.79 (ddd, J = 7.8, 7.8,
1.8 Hz, 1H), 7.88–7.93 (unresolved dd, 1H), 8.17 (dd,

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4429
J = 7.7, 1.1 Hz, 1H), 8.52 (ddd, J = 7.9, 2.5, 1.1 Hz, 2H),
4.3.3. Synthesis of ruthenium {6-[(4S)-4-phenyl-4,5-
dihydro-oxazol-2-yl]-[2,20]bipyridinyl}
(2,6-pyridinedicarboxylate) complex (15)
8.66 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H); 13C NMR (75.5 MHz,
CDCl3): d = 70.31, 75.36, 121.64, 123.11, 124.04, 124.31,
126.81, 127.71, 128.76, 136.97, 137.55, 141.84, 146.13,
149.09, 155.30, 156.22, 164.04; MS (EI, 70 eV) m/z 301
(M+); [a]D = ꢁ115.1ꢁ (c = 0.50, CHCl3); IR (KBr, cmꢁ1):
3069, 3032, 2972, 2897, 1643, 1263, 1114, 787, 749, 700.
Elementary analysis Calc. for C19H15N3O: C, 75.73; H,
5.02; N, 13.94. Found: C, 75.69; H, 4.62; N, 14.03%.
[Ru(p-cymene)Cl2]2 (102 mg, 0.17 mmol) and 10
(100 mg, 0.33 mmol) were dissolved in MeOH (2 ml) at
r.t. under Ar to form a deep violet solution. Disodium
2,6-pyridinedicarboxylate (70 mg, 0.33 mmol) was dis-
solved in H2O (1 ml) and MeOH (1 ml) was then added.
This solution was purged with Ar for ꢀ15 min and then
added dropwise to the reaction mixture via a cannular.
The whole reaction mixture was heated at 65 ꢁC for 1 h.
It turned to deep red in ꢀ5 min at 65 ꢁC. After the reaction
mixture was cooled to r.t., CH2Cl2 (50 ml) and H2O (25 ml)
were added. The organic layer was separated and the aque-
ous layer was extracted with CH2Cl2 (5 ml · 3). The com-
bined organic layer was dried over MgSO4, filtered and
evaporated under reduced pressure. It was then chromato-
graphed on silica gel (70–230 mesh) using CH2Cl2:MeOH
100:3 to 100:7 as the gradient eluent. After removal of sol-
vent, a purple solid was obtained (111 mg, 59%). The prod-
uct can be further purified by recrystallization in CH2Cl2/
hexane to give a deep purple solid. Rf = 0.09 (CH2Cl2/
4.3. Synthesis of ruthenium complexes
4.3.1. Synthesis of chloro-ruthenium (2,20:60,200-terpyridine)
(pyridine-2-carboxylate) complex (12)
[Ru(p-cymene)Cl2]2 (131 mg, 0.21 mmol) and 2,20:60,200-
terpyridine (100 mg, 0.43 mmol) were dissolved in MeOH
(7 ml) at r.t. under Ar to form a deep violet solution. Sodium
2-pyridinecarboxylate (62 mg, 0.43 mmol) was dissolved in
H2O (3 ml) and MeOH (4 ml) was added. This solution
was purged with Ar for ꢀ15 min and then added dropwise
to the reaction mixture via a cannular. The whole reaction
mixture was heated at 70 ꢁC for 1 h. It turned to deep purple
in ꢀ15 min at 70 ꢁC. After the reaction mixture was cooled to
r.t., CH2Cl2 (25 ml) and H2O (25 ml) were added. The
organic layer was separated and the aqueous layer was
extracted with CH2Cl2 (5 ml · 3). The combined organic
layer was dried over MgSO4, filtered and evaporated under
reduced pressure. It was then chromatographed on silica
gel (70–230 mesh) using CH2Cl2:MeOH 100:1 to 100:5 as
the gradient eluent. After removal of solvent, a purple solid
was obtained (140 mg, 66%). The product can be further
purified by recrystallization in CH2Cl2/hexane to give a deep
purple solid. The product was not soluble enough to give sat-
isfactory 1H and 13C spectra. 12:Rf = 0.15 (CH2Cl2/MeOH
100:5); UV–Vis (CH2Cl2, kmax/nm, loge) 326 (4.40), 402
(3.94), 551 (3.82). HRMS Calc. for (C21H15ClN4O296Ru)
m/z: 485.99611. Found: 485.99540. Elementary analysis
Calc. C21H15ClN4O2RuÅCH2Cl2 (%) C, 45.81; H, 2.97; N,
9.71. Found: C, 45.87; H, 3.21; N, 10.17.
1
MeOH 100:5); H NMR (300 MHz, CDCl3, ppm) d 4.63
(dd, J = 10.6, 8.9 Hz, 1H), 4.81–4.88 (unresolved dd, 1H),
5.17–5.23 (unresolved dd, 1H), 6.78 (d, J = 7.2 Hz, 2H),
7.07–7.22 (m, 4H), 7.35 (d, J = 5.3 Hz, 1H), 7.55–7.68
(m, 3H), 7.81 (t, J = 7.7 Hz, 1H), 8.04 (d, J = 7.7 Hz,
1H), 8.09–8.22 (m, 3H); 13C NMR (75.5 MHz, CDCl3,
ppm) d 67.91, 78.18, 121.12, 121.90, 123.76, 126.10,
126.71, 126.90, 127.25, 127.98, 128.59, 129.18, 134.01,
134.81, 136.42, 146.70, 149.40, 150.23, 151.09, 158.32,
167.67, 171.40, 171.92; UV–Vis (CH2Cl2,kmax/nm,loge)
399 (3.94), 514 (4.10). HRMS Calc. for (C26H18N4-
O5102Ru + H+) m/z: 569.03989. Found: 569.03990. Ele-
mentary analysis Calc. C26H18N4O5Ru Æ H2O: C, 53.33;
H, 3.44; N, 9.57. Found: C, 52.89; H, 3.00; N, 9.35%.
4.3.4. Synthesis of chloro-ruthenium {bis[(4S)-4-phenyl-4,5-
dihydro-oxazol-2-yl]-pyridine}(2,6-pyridinedicarboxylate)
complex (16)
4.3.2. Synthesis of ruthenium bis-(2,20-bipyridine-6-
[Ru(p-cymene)Cl2]2 (83 mg, 0.14 mmol) and bis[(4S)-4-
phenyl-4,5-dihydro-oxazol-2-yl]-pyridine (100 mg, 0.27
mmol) were dissolved in MeOH (2 ml) at r.t. under Ar to
form a deep red solution. Disodium 2,6-pyridinedicarboxy-
late (70 mg, 0.33 mmol) was dissolved in H2O (1 ml) and
MeOH (1 ml) was then added. This solution was purged
with Ar for ꢀ15 min and then added dropwise to the reac-
tion mixture via a cannular. The whole reaction mixture
was heated at 65 ꢁC for 1 h. It turned first to orange then
brownish orange in color at 65 ꢁC. After the reaction mix-
ture was cooled to r.t., CH2Cl2 (20 ml) and H2O (20 ml)
were added. The organic layer was separated and the
aqueous layer was extracted with CH2Cl2 (5 ml · 3). The
combined organic layer was dried over MgSO4, filtered
and evaporated under reduced pressure. It was then chro-
matographed on silica gel (70–230 mesh) using
CH2Cl2:MeOH 100:1 to 100:5 as the gradient eluent. After
carboxylate) complex (13) [38]
[Ru(p-cymene)Cl2]2 (46 mg, 0.075 mmol) and 2,20-bipyri-
dine-6-carboxylic acid (60 mg, 0.30 mmol) were dissolved in
anhydrous MeOH (10 ml) under Ar at r.t. and Et3N (42 ll,
0.30 mmol) was then added. The reaction mixture was
heated at 65 ꢁC for ꢀ13 h and turned from pale orange to
deep orange in color. After the reaction mixture was cooled
to r.t., a purple solid precipitated with a clear orange solu-
tion. The purple solid was filtered, washed with MeOH and
dried under high vacuum (63 mg, 84%). 13: Rf = 0.57
(CH2Cl2/MeOH 10:1); 1H NMR (300 MHz, d6-DMSO,
ppm) d 7.10 (d, J = 5.1 Hz, 1H), 7.19–7.21 (m, 1H), 7.77–
7.82 (m, 1H), 8.10–8.14 (m, 2H), 8.69 (d, J = 7.2 Hz, 1H),
8.91–8.97 (m, 1H); UV–Vis (EtOH:MeOH = 4:1, kmax
/
nm, loge) 298 (4.69), 369 (3.89), 511 (4.11). HRMS Calc.
for (C22H14N4O496Ru) m/z: 499.00868. Found: 499.008554.

4430
M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
removal of solvent, a purple solid was obtained (131 mg,
77%). The product can be further purified by recrystalliza-
tion in CH2Cl2/hexane. Rf = 0.20 (CH2Cl2/MeOH 100:5);
1H NMR (300 MHz, CDCl3, ppm) d 4.49–4.69 (m, 3H),
4.93–5.00 (unresolved dd, 1H), 5.09–5.26 (m, 2H), 6.75
(d, J = 7.0 Hz, 2H), 6.89–7.16 (m, 9H), 7.38–7.39 (m,
2H), 7.58 (t, J = 7.8 Hz, 1H), 7.75–7.80 (m, 2H), 8.96 (d,
J = 5.3 Hz, 1H); 13C NMR (75.5 MHz, CDCl3, ppm) d
68.35, 68.69, 78.22, 78.43, 123.23, 123.25, 124.99, 125.21,
127.01, 127.32, 127.99, 128.13, 128.32, 128.68, 134.24,
136.27, 136.80, 150.12, 150.16, 150.52, 150.80, 167.21,
167.64, 172.48; MS (EI 70 eV) m/z 627 (M+). UV–Vis
(CH2Cl2, kmax/nm, loge) 378 (3.45), 506 (4.11). Elementary
analysis Calc. C29H23ClN4O4RuÅH2O: C, 53.91; H, 3.90; N,
8.67. Found: C, 54.24; H, 4.21; N, 8.52%.
(0.025 mmol) and trans-stilbene (90.1 mg, 0.50 mmol) were
stirred with gentle heating in tert-amyl alcohol (9 ml) until
all trans-stilbene was dissolved. After the reaction mixture
was cooled to r.t., dodecane (GC internal standard, 100 ll)
was added. To this reaction mixture, a solution of 30%
hydrogen peroxide (170 ll, 1.5 mmol) in tert-amyl alcohol
(830 ll) was added over a period of 12 h by a syringe pump.
After the addition, aliquots were taken from the reaction
mixture and subjected to GC analysis for determination
of yield and conversion data. The reaction mixture was
then quenched with Na2SO3solution (ꢀ10 ml) and
extracted with dichloromethane (10 ml · 2) and washed
with water (ꢀ20 ml). The combined organic layer was dried
over MgSO4 and evaporated to give the crude epoxide. It
was then dissolved in n-hexane for HPLC measurement.
4.4. UV–Vis spectroscopic titration
4.5.1.3. General procedure for in situ generation of catalyst
in asymmetric epoxidation with hydrogen peroxide. In a
25 ml Schlenk tube, [Ru(p-cymene)Cl2]2 (7.7 mg,
0.013 mmol) and the chiral ligand (0.025 mmol) were dis-
solved in tert-amyl alcohol (2 ml) under Ar. To this solution,
H2pydic (4.2 mg, 0.025 mmol) and Et3N (8.4 ll,
0.060 mmol) in tert-amyl alcohol (2 ml) were added drop-
wise via a cannular. The reaction mixture was heated at
65 ꢁC for 1 h. trans-Stilbene (90.1 mg, 0.50 mmol) and
tert-amyl alcohol (5 ml) were added and the whole reaction
mixture was heated until all trans-stilbene was dissolved.
After the reaction mixture was cooled to r.t., dodecane
(GC internal standard, 100 ll) was added. To this mixture,
a solution of 30% hydrogen peroxide (170 ll, 1.5 mmol) in
tert-amyl alcohol (830 ll) was added over a period of 12 h
by a syringe pump. After the addition, aliquots were taken
from the reaction mixture and subjected to GC analysis
for determination of yield and conversion data. The reaction
mixture was then quenched with Na2SO3 solution (ꢀ10 ml),
extracted with dichloromethane (10 ml · 2) and washed
with water (ꢀ20 ml). The combined organic layer was dried
over MgSO4 and evaporated to give the crude epoxide. It
was then dissolved in n-hexane for HPLC measurement.
Ru(tBu3-tpy)(pydic) 3a (2.70 mg, 4.04 · 10ꢁ3 mmol) was
dissolved in CH2Cl2 (100.00 ml) in a volumetric flask. m-
CPBA was purified according to literature before use
[41]. m-CPBA (10.51 mg, 6.09 · 10ꢁ3 mmol) was dissolved
in CH2Cl2 (10.00 ml) in a volumetric flask. A solution of
3a (3.00 ml) was transferred to a Teflon stoppered 1.0 cm
quartz cell. Then the initial UV–Vis spectrum was
recorded. Next m-CPBA solution (2.0 ll) was added to
the solution of 3a and the reaction was monitored by
UV–Vis spectroscopy. When there was no further change
indicated by the absorption spectrum, further portions of
m-CPBA solution (2.0 ll each) were given. The ratio
between 3a and m-CPBA was determined at different wave
lengths (at least 5) other than the isosbestic points.
4.5. Catalytic reactions
All olefins and epoxides are known compounds. The
conversions and yields were determined by comparing the
authentic samples with an internal standard on GC-FID.
The identities of the products were further confirmed by
GC–MS.
4.5.1.4. General procedure for screening of solid
oxidants. In a 25 ml Schlenk tube, Ru(tpy)(pydic) (3)
(0.0025 mmol) and trans-stilbene (90.1 mg, 0.50 mmol) were
stirred with gentle heating in tert-amyl alcohol (10 ml) until
all trans-stilbene was dissolved. After the reaction mixture
was cooled to r.t., dodecane (GC internal standard, 100 ll)
was added. To this reaction mixture, the solid oxidant
(1.5 mmol) was added. After the addition, aliquots were
taken from the reaction mixture and subjected to GC analy-
sis for determination of yield and conversion data.
4.5.1. Screening of different oxidants
4.5.1.1. General procedure for non-asymmetric epoxidation
with hydrogen peroxide. In a 25 ml Schlenk tube, the cat-
alyst (0.0025 mmol) and trans-stilbene (90.1 mg,
0.50 mmol) were stirred with gentle heating in tert-amyl
alcohol (9 ml) until all trans-stilbene was dissolved. After
the reaction mixture was cooled to r.t., dodecane (GC inter-
nal standard, 100 ll) was added. To this reaction mixture, a
solution of 30% hydrogen peroxide (170 ll, 1.5 mmol) in
tert-amyl alcohol (830 ll) was added over a period of 12 h
by a syringe pump. After the addition, aliquots were taken
from the reaction mixture and subjected to GC analysis for
determination of yield and conversion data.
4.5.1.5. General procedure for epoxidation with urea hydro-
gen peroxide complex for solid olefins. In a 25 ml Schlenk
tube, Ru(tpy)(pydic)
3
(0.0025 mmol) and olefin
(0.50 mmol) were stirred with gentle heating in tert-amyl
alcohol (10 ml) until it was dissolved. After the reaction
mixture was cooled to r.t., dodecane (GC internal stan-
4.5.1.2. General procedure for asymmetric epoxidation with
hydrogen peroxide. In a 25 ml Schlenk tube, the catalyst

M.K. Tse et al. / Journal of Organometallic Chemistry 691 (2006) 4419–4433
4431
(k) A.M. Khenkin, L.J.W. Shimon, R. Neumann, Inorg. Chem. 42
(2003) 3331;
(l) T. Nishimura, S. Uemura, Synlett (2004) 201.
dard, 100 ll) was added. Urea hydrogen peroxide complex
(47 mg, 0.50 mmol) was added in three portions at 0, 1, and
2 h as a total of (141 mg, 1.5 mmol). The reaction mixture
was stirred at r.t. for an additional hour. Aliquots were
taken from the reaction mixture and subjected to GC anal-
ysis for determination of yield and conversion data.
[4] For examples in which both oxygen atoms are used see: (a) J.T.
Groves, R. Quinn, J. Am. Chem. Soc. 107 (1985) 5790;
(b) I.R. Paeng, K. Nakamoto, J. Am. Chem. Soc. 112 (1990) 3289;
´
´
(c) I.E. Marko, P.R. Giles, M. Tsukazaki, I. Chelle-Regnant, C.J.
Urch, S.M. Brown, J. Am. Chem. Soc. 119 (1997) 12661;
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(e) K.S. Coleman, C.Y. Lorber, J.A. Osborn, Eur. J. Inorg. Chem.
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4.5.1.6. General procedure for epoxidation with urea hydro-
gen peroxide complex for liquid olefins. In a 25 ml Schlenk
tube, Ru(tpy)(pydic) (3) (0.0025 mmol) was stirred with
gentle heating in tert-amyl alcohol (10 ml). After the reac-
tion mixture was cooled to r.t., olefin (0.50 mmol) and
dodecane (GC internal standard, 100 ll) were added. Urea
hydrogen peroxide complex (47 mg, 0.50 mmol) was added
in three portions at 0, 1 and 2 h as a total of (141 mg,
1.5 mmol). The reaction mixture was stirred at r.t. for an
additional hour. Aliquots were taken from the reaction
mixture and subjected to GC analysis for determination
of yield and conversion data.
(f) J. Christoffers, J. Org. Chem. 64 (1999) 7668;
(g) C. Do¨bler, G. Mehltretter, M. Beller, Angew. Chem. 111 (1999)
3211;
Angew. Chem. Int. Ed. 38 (1999) 3026;
(h) C. Do¨bler, G. Mehltretter, U. Sundermeier, M. Beller, J. Am.
Chem. Soc. 122 (2000) 10289;
(i) G.M. Mehltretter, C. Do¨bler, U. Sundermeier, M. Beller,
Tetrahedron Lett. 41 (2000) 8083;
(j) C. Do¨bler, G.M. Mehltretter, U. Sundermeier, M. Beller, J.
Organomet. Chem. 621 (2001) 70;
(k) U. Sundermeier, C. Do¨bler, G.M. Mehltretter, W. Baumann, M.
Beller, Chirality 15 (2003) 127.
[5] (a) G. Strukul (Ed.), Catalytic Oxidations with Hydrogen Peroxide as
Oxidant, Kluwer Academic, Dordrecht, 1992;
Acknowledgements
(b) C.W. Jones, Applications of Hydrogen Peroxide and Derivatives,
Royal Society of Chemistry, Cambridge, 1999.
[6] (a) B. Elvers, S. Hawkins, M. Ravenscroft, G. Schulz (Eds.),
Ullmann’s Encyclopedia of Industrial Chemistry, fifth ed., vol. A13,
VCH, New York, 1989, p. 443;
This work has been financed by the State of Mecklen-
burg-Vorpommern, the Bundesministerium fur Bildung
¨
und Forschung (BMBF), and the Deutsche Forschungs-
gemeinshaft (SPP ‘‘Sekunda¨re Wechselwirkungen’’). We
thank Mrs. C. Mewes, Mrs. H. Baudisch, Mrs. A. Leh-
mann, and Mrs. S. Buchholz and Dr. C. Fischer (all IfOK)
for their excellent technical and analytical support. Mr. En-
rico Schmidt (Celisca) is acknowledged for ultra-high mass
spectrometric measurements.
(b) J.I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer Encyclo-
pedia of Chemical Technology, fourth ed., vol. 13, Wiley, New York,
1995, p. 961.
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(c) W.A. Hermann, R.M. Kratzer, H. Ding, W.R. Thiel, H. Glas, J.
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