Epoxidation activities of mononuclear ruthenium-oxo complexes with a square planar 6,6′-bis(benzoylamino)-2,2′-bipyridine and axial ligands

Article abstract of DOI:10.1016/S0040-4039(02)00041-2

Some ruthenium complexes, [Ru^II(babp)(dmso)(L)], with a square planar ligand, H₂BABP, and axial ligands, L (L=dmso, imidazole, or pyridine derivatives), have been prepared as catalysts for oxygen-transfer reactions. The catalytic act

Full text of DOI:10.1016/S0040-4039(02)00041-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1491–1494
Epoxidation activities of mononuclear ruthenium–oxo complexes
with a square planar 6,6%-bis(benzoylamino)-2,2%-bipyridine and
axial ligands
Koichiro Jitsukawa,* Hiroyoshi Shiozaki and Hideki Masuda
Department of Applied Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan
Received 28 November 2001; accepted 21 December 2001
Abstract—Some ruthenium complexes, [RuII(babp)(dmso)(L)], with a square planar ligand, H2BABP, and axial ligands, L
(L=dmso, imidazole, or pyridine derivatives), have been prepared as catalysts for oxygen-transfer reactions. The catalytic activity
and selectivity of the metal–oxo species derived from these complexes and PhIO have been affected by the axial ligands L, in
which active species has been interpreted in terms of contribution of both characters of Ru(V)ꢀO and Ru(IV)ꢁO·. © 2002 Elsevier
Science Ltd. All rights reserved.
In various oxidation processes using metal porphyrins
and oxygen donors, some high valent metal–oxo species
are considered as the active intermediates.1 The high
valent iron–oxo intermediates in the enzyme reactions
catalyzed by hemeproteins play an important role for
oxidative transformation of bio-related compounds.2
However, such FeꢀO species have not been fully char-
acterized because of their labilities. Therefore, the
RuꢀO species, which is the oxo complexes of the stable
homologous element and is often utilized in the place of
iron, have been investigated by a combination with
porphyrin or polypyridine ligands.3 Ru(IV, V, or VI)–
oxo species, generated through the reaction of low
valent Ru(II or III) complexes and various oxidants,
are widely utilized as the catalysts of oxygen-transfer
reactions,4 in which iodosobenzene (PhIO) is sometimes
employed as an efficient two-electron oxidant for such
ruthenium-catalyzed oxidations in order to avoid the
complicated oxidation pathway due to the peracid
itself.3,4 Recently, we have reported that ruthenium
complexes with tripodal tris(2-pyridylmethyl)amine
derivatives catalyze hydroxylation of adamantane and
epoxidation of cyclohexene with assistance of PhIO, in
which the oxidation activity has been affected by the
electronic and steric character of the oxo species.5 The
tripodal polypyridine derivatives, however, are not
sufficient ligands for understanding the character of the
RuꢀO species, because the bulky pivalamido groups
around metal center sterically suppress the reactions
and the a-hydrogen of the pyridine group is often
hydroxylated in the course of reaction.5,6 Hence, taking
account of the trans-influence that the catalytic activity
in cytochrome P-450 is affected by the axially-coordi-
nated proximal ligand in the position trans to metal–
oxo,7 the molecular design of a square-planar aromatic
ligand is required for controlling the oxidation activity
of the axial metal–oxo site by another axial ligand.
Here, a tetradentate square-planar ligand, 6,6%-bis(ben-
zoylamino)-2,2%-bipyridine
(H2BABP),
which
is
expected to resist the oxidative degradation, is
employed for this purpose.8 Some ruthenium complexes
with monodentate heterocyclic compounds as axial lig-
ands were prepared and their catalytic activity for
oxygen-transfer reaction were examined.
The ruthenium complexes here employed were prepared
and characterized as follows: [RuII(babp)(dmso)2] (1),
was obtained as an orange crystal from the reaction of
cis-[RuCl2(dmso)4]9 and H2BABP in the presence of
NaH in DMF solution. Addition of equimolar amount
of heterocyclic compounds (imidazole (Im), pyridine
(Py), 4-phenylpyridine (Phpy), and 4-methylpyridine
(Mepy)) to the DMSO solution of 1 gave the corre-
sponding complexes, [RuII(babp)(dmso)(im)] (2),
[RuII(babp)(dmso)(py)] (3), [RuII(babp)(dmso)(phpy)]
(4), and [RuII(babp)(dmso)(mepy)] (5), respectively.
Characterization of these complexes were performed by
1H NMR spectroscopy and the elemental analyses.10
The crystal structure of 5, which was fortunately
obtained as a single crystal suitable for X-ray analysis,
Keywords: ruthenium and compounds; catalysts; oxygenation.
* Corresponding author. E-mail: jitkk@ach.nitech.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00041-2

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K. Jitsukawa et al. / Tetrahedron Letters 43 (2002) 1491–1494
revealed that the ruthenium center was 4-coordinated
with the deprotonated BABP as a dianion in a square
planar geometry and with monodentate 4-
methylpyridine and dimethyl sulfoxide at the two axial
positions, as shown in Fig. 1. The redox potentials
(Ru(II/III)) of these complexes, which were measured in
CH3CN solution by cyclic voltammetry, indicated the
ligand effect as shown in Fig. 2. The ruthenium com-
plex with imidazole ligand (2) showed lower redox
potential value in comparison with that with two dmso
molecules (1), while those with the pyridine derivatives
(3–5) showed higher values. This finding indicates that
the complex 2 stabilizes the higher oxidation state and
the latter complexes 3–5 do the lower oxidation state.
Ar atmosphere in the presence of PhIO
(Ru:olefin:PhIO=1:100:100). Epoxidation of cis-stil-
bene by 1 or 2 gave the corresponding epoxide accom-
panied by small amount of trans-epoxide and much
amount of CꢀC bond cleavage product, benzaldehyde,
as listed in Table 1. Such a reactivity has been also
reported by Katsuki et al.; Ru(III)–salen complexes
gave epoxide in poor yield in comparison with those by
the corresponding Mn(III) complexes.11 In the cases of
3 and 4 containing pyridine derivative as an axial
ligand, significant decrease in the yield of stilbene oxide
and increase in that of benzaldehyde were demon-
strated. These ruthenium complexes showed similar
reactivity for the oxidation of trans-stilbene as well as
the cases of cis-stilbene. These findings are comparable
to the previous report that the epoxidation of trans-stil-
bene by the combined system of RuCl3, NaIO4, and
chiral bis(dihydrooxazolylphenyl)oxalamide ligand in
CH2Cl2 solution gave epoxide in a 31% yield accompa-
nied with significant amount of benzaldehyde.12
Oxidation of olefins catalyzed by the ruthenium com-
plexes 1–4 has been carried out at 40°C for 4 h under
Epoxidation activity for cis-cyclooctene has been exam-
ined under the same conditions as above. The yields of
cyclooctene oxide decreased in the order of 2 (30%)>1
(24%)>3 (13%)>4 (4.2%), which was the same as the
case of stilbene oxidation. Such a decrease in the yield
significantly correlates to the order of redox potentials
of the complexes, implying that the catalytic activity of
the Ru complexes here employed depends on their axial
ligands. Generally, epoxidation of cis-cyclooctene cata-
lyzed by ruthenium complexes did not proceed quanti-
tatively; for example, cyclooctene oxide was obtained in
a 19% yield when the other mononuclear ruthenium-
(III) complex with Schiff base ligand was used.13 Oxida-
tion of cyclohexene catalyzed by 1–4 has also been
carried out and the results are summarized in Table 2.
It is well known that oxidation of cyclohexene initiated
by the metal–oxo species under anaerobic condition
gives cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclo-
hexen-1-one, where the first compound is obtained
through epoxidation and the latter two are derived
from allylic oxidation. The ratio of the epoxidation
product yield to the allylic oxidation ones was almost
constant during the initial stage of the reaction cata-
lyzed by these ruthenium complexes. Notably, catalytic
activity of 2 for epoxidation was superior to those of 3
and 4 as well as indicated in the case of other olefins.
On the other hand, allylic oxidation products were
formed in almost equal amounts for 1–4. In the oxida-
tion of cis-cyclooctene without active allylic methylene
protons,14 only the electrophilicity of the catalytic spe-
cies might control the reactivity, while in the case of
cyclohexene, the substrate has alternative reaction
points for making an estimate of the reactivity of the
metal–oxo species. We consider that oxidation path-
ways catalyzed by the ruthenium–oxo species for epoxi-
dation and allylic oxidation might proceed through
different intermediates to each other. Probably, epoxi-
dation favors higher oxidation state of the metal–oxo
species.
Figure 1. ORTEP drawing of [Ru(babp)(dmso)(mepy)] com-
,
plex (5). Typical bond lengths (A) and angles (°): RuꢁN(3)
2.132(8), RuꢁS(1) 2.239(3), RuꢁN(1a) 1.987(9), RuꢁN(1b)
1.992(9),
S(1)ꢁRuꢁN(3)
N(1a)ꢁRuꢁO(1a) 92.8(3), N(1a)ꢁRuꢁO(1b) 173.2(3).
RuꢁO(1a)
2.082(7),
RuꢁO(1b)
2.071(7);
81.3(4),
174.9(2),
N(1a)ꢁRuꢁN(1b)
L
Ph
Ph
N
O
O
Ru
N
N
N
[Ru(babp)(dmso)(L)] complexes
dmso
mepy
528
im
460
phpy
568
py
531
dmso
521
L =
550
500
450
Redox Potential / mV (vs. S.C.E.)
Figure 2. Redox potential values of ruthenium complexes,
[Ru(babp)(dmso)(L)].
According to the Meyer’s discussion that Ru(IV)ꢀO
species prefers attack on the CꢁH bond to that on the

K. Jitsukawa et al. / Tetrahedron Letters 43 (2002) 1491–1494
1493
Table 1. Oxidation of cis- and trans-stilbenes catalyzed by Ru–babp complexesa
Complexes
Yield (%)b
CHO
O
O
cis-Stilbene
2.3
2.2
1.3
1.4
[Ru(babp)(dmso)(im)] (2)
[Ru(babp)(dmso)2] (1)
[Ru(babp)(dmso)(py)] (3)
[Ru(babp)(dmso)(phpy)] (4)
28
20
7.8
6.6
32
48
50
53
trans-Stilbene
[Ru(babp)(dmso)(im)] (2)
[Ru(babp)(dmso)2] (1)
[Ru(babp)(dmso)(py)] (3)
[Ru(babp)(dmso)(phpy)] (4)
Not detected
Not detected
Not detected
Not detected
28
19
14
12
28
34
44
45
a Catalyst, 1 mM; substrate 100 mM; PhIO 100 mM; 1,2-dichloroethane, 40°C, 4 h.
b GC yield based on the substrate.
olefinic double bond,15 the complex 2, exhibiting epoxi-
dation activity rather than allylic oxidation one, might
proceed through the reaction intermediate higher than
the Ru(IV) state.4 Fortunately, the formation of high
valent RuꢀO species has been demonstrated by posi-
tive-ion ESI mass spectroscopy: The reaction of 1 and
10 equiv. of PhIO in dichloromethane showed promi-
nent peak clusters at m/z 494.0 and 510.2, correspond-
ing to the [RuIII(babp)]+ and [RuV(babp)(O)]+ ions,
respectively (Fig. 3).16 The latter was not detected with
addition of olefins. These results indicate that the cata-
lytic species of the ruthenium complexes might be a
mononuclear Ru(V)ꢀO generated through the two-elec-
tron oxidation process by PhIO, e.g. Ru(III)/Ru(V)
redox couple, where the Ru(III) species could be gener-
ated through comproportionation between the starting
Ru(II) complex and the Ru(IV) species oxidized by
PhIO. The redox potentials of 1–4 (Fig. 2) reveal that
the complex 2 could stabilize the higher oxidation state
compared with 1, 3 and 4, so that the electrophilicity of
the metal–oxo species in 2 is superior to those of 1, 3
and 4. On account for the accompanying reactions,
allylic oxidation and CꢀC bond cleavage reaction, radi-
cal character is presumed as a reaction intermediate.
We consider that this oxygen-transfer reaction cata-
lyzed by the ruthenium complex with a square planar
ligand might proceed through the reaction intermediate
having a combination of oxo and radical characters in
the transition state, although various mechanistic inves-
tigations have been reported on the ruthenium–oxo
complexes.3,15 The ruthenium complex 2 with imidazole
as an axial ligand exhibiting lower redox potential value
might induce the higher oxidation intermediate such as
Ru(V)ꢀO species, while the pyridine derivatives, 3 and
4, preferentially afford the lower one, Ru(IV)ꢁO·.
In summary, the oxygen-transfer reaction catalyzed by
the high valent ruthenium complexes with a square
planar geometry and PhIO have been examined. The
catalytic intermediate species might be demonstrated in
terms of contribution of both characters of Ru(V)ꢀO
and Ru(IV)ꢁO·, which are controlled by the axial lig-
and of the complexes.
Table 2. Oxidation of cyclohexene catalyzed by Ru–babp
complexesa
Complexes
Yield (%)b
Allylic oxidation
Epoxidation
OH
O
O
[Ru(babp)(dmso)(im)]
22
0.8
5.3
(2)
[Ru(babp)(dmso)2] (1)
[Ru(babp)(dmso)(py)]
(3)
19
10
0.4
0.5
5.2
6.8
[Ru(babp)(dmso)(phpy)] 7.5
0.7
7.9
(4)
a Catalyst,
1 mM; substrate 100 mM; PhIO 100 mM; 1,2-
dichloroethane, 40°C, 4 h.
b GC yield based on the substrate.

1494
K. Jitsukawa et al. / Tetrahedron Letters 43 (2002) 1491–1494
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Figure 3. (a) Positive-ion ESI mass spectrum of the intermedi-
ate species generated from the reaction of [RuII(babp)(dmso)2]
(1) and PhIO. (b) Simulation pattern for [RuIII(babp)]+. (c)
Simulation pattern for [RuV(babp)(O)]+.
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plexes. Anal. calcd for 1 (C28H28N4O4S2Ru·0.1H2O): C,
51.62; H, 4.36; N, 8.60. Found: C, 51.41; H, 4.47; N,
8.84. Anal. calcd for 2 (C29H26N6O3SRu·0.5H2O): C,
53.69; H, 4.20; N, 12.96. Found: C, 53.66; H, 4.09; N,
12.74. Anal. calcd for 3 (C31H27N5O3SRu·0.2DMSO): C,
56.60; H, 4.27; N, 10.51. Found: C, 56.84; H, 4.23; N,
10.24. Anal. calcd for 4 (C37H31N5O3SRu·0.4H2O): C,
60.54; H, 4.37; N, 9.54. Found: C, 60.28; H, 4.23; N,
9.25. Anal. calcd for 5 (C32H29N5O3SRu·0.5acetone): C,
58.00; H, 4.65; N, 10.09. Found: C, 57.84; H, 4.45; N,
9.99.
11. Nakata, K.; Takeda, T.; Mihara, J.; Irie, R.; Katsuki, T.
Chem. Eur. J. 2001, 7, 3776.
12. End, N.; Pfaltz, A. Chem. Commun. 1998, 589.
13. Upadhyay, M. J.; Bhattacharya, P. K.; Ganesphure, P.
A.; Satish, S. J. Mol. Catal. 1992, 73, 277.
Acknowledgements
This work was supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Science,
and Culture, Japan (11228203), to which our thanks are
due.
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