Reverse reactivity in hydroxylation of adamantane and epoxidation of cyclohexene catalyzed by the mononuclear ruthenium-oxo complexes with 6-substituted tripodal polypyridine ligands

Article abstract of DOI:10.1016/S0040-4039(01)00379-3

The electronic character of the ruthenium complexes with tripodal polypyridine ligands, which is controlled by the substituted groups at pyridine 6-position, gives rise to differences in the reactivity for the ruthenium catalyzed hydroxylation of adamantane and epoxidation of cyclohexene with PhIO as an oxidant; Ru complexes containing electron-withdrawing groups (1, 3, and 5) promote the epoxidation, while those containing electron-donating groups (2, 4, and 6) promote the hydroxylation.

Full text of DOI:10.1016/S0040-4039(01)00379-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3467–3469
Reverse reactivity in hydroxylation of adamantane and
epoxidation of cyclohexene catalyzed by the mononuclear
ruthenium-oxo complexes with 6-substituted tripodal polypyridine
ligands
Koichiro Jitsukawa,* Yoshiyuki Oka, Hisahiko Einaga and Hideki Masuda
Department of Applied Chemistry, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan
Received 11 January 2001; revised 26 February 2001; accepted 2 March 2001
Abstract—The electronic character of the ruthenium complexes with tripodal polypyridine ligands, which is controlled by the
substituted groups at pyridine 6-position, gives rise to differences in the reactivity for the ruthenium catalyzed hydroxylation of
adamantane and epoxidation of cyclohexene with PhIO as an oxidant; Ru complexes containing electron-withdrawing groups (1,
3, and 5) promote the epoxidation, while those containing electron-donating groups (2, 4, and 6) promote the hydroxylation.
© 2001 Elsevier Science Ltd. All rights reserved.
High valent metal-oxo species have been considered as
active intermediates in various oxidation processes cata-
lyzed by transition metal complexes.1 Ruthenium-oxo
species generated through the reactions of low valent
ruthenium complexes and various oxidants are widely
utilized as catalysts of oxygen transfer reactions.2 In
previous work, tetradentate tripodal polypyridine lig-
ands were used to prepare the ruthenium complexes.3
We have also investigated the stability/reactivity of
superoxide and peroxide species combined with the
transition metal complexes with tripodal polypyridine
ligands, in which hydrogen bonds, steric effects and
electronic effects have been shown to be crucial fac-
tors.4 Recent reports have described that ruthenium
complexes with the tris(2-pyridylmethyl)amine (TPA)
ligand catalyzed the hydroxylation and epoxidation of
organic compounds in the presence of MCPBA (m-
chloroperbenzoic acid) through the putative RuꢀO spe-
cies.5 Evidence for the high valent Ru species on the
above reactions has not yet been confirmed. Further
complications are caused by the fact that MCPBA itself
is capable of the hydroxylation of adamantane and
promotion of the Baeyer–Villiger reaction of ketones.
Here, using PhIO (iodosobenzene), an efficient 2-elec-
tron oxidant for ruthenium catalyzed oxidations,2,6 we
have investigated the catalytic reactivity of ruthenium
complexes with tripodal ligands for hydroxylation of
adamantane and epoxidation of cyclohexene.
The tripodal polypyridine ligands, tppa, bppa, mppa,
tnpa, bnpa and mnpa were synthesized according to the
methods previously reported.4 Tppa, bppa and mppa
have electron-withdrawing character, while tnpa, bnpa
and mnpa are electron-donating. The ruthenium com-
R2
R1=R2=R3=tBuCONH
=tBuCH2NH
R1=R2=tBuCONH, R3=H : 3
=tBuCH2NH, =H : 4
: 1
: 2
R1
N
OH
O
R3
Ru
N
N
N
R1=tBuCONH, R2=R3=H : 5
=tBuCH2NH,
=H : 6
PhIO, 40°C, Ar
Keywords: ruthenium and compounds; hydroxylation; epoxidation.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00379-3

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K. Jitsukawa et al. / Tetrahedron Letters 42 (2001) 3467–3469
plexes, [RuII(tppa)Cl](PF6) (1), [RuIII(tnpa)Cl2](PF6)
(2), [RuII(bppa)Cl](PF6) (3), [RuIII(bnpa)Cl2](PF6) (4),
[RuII(mppa)Cl](PF6) (5) and [RuIII(mnpa)Cl2](PF6) (6),
were prepared and crystallized according to the method
for [RuII(tpa)Cl]2(ClO4)2.7,8 Oxidation of adamantane
(68 mg, 0.5 mmol) in 1,2-dichloroethane solution (5
mL) was carried out at 40°C in the presence of PhIO
(110 mg, 0.5 mmol) and a catalytic amount of ruthe-
nium complex (0.005 mmol) under an Ar atmosphere.
1-Adamantanol was produced in moderate yield by 4,
but obtained in poor yield in the case of 3 as shown in
Fig. 1. The yields of 1-adamantanol, a major hydroxy-
lation product, were 3.9% in 1, 12.1% in 2, 4.9% in 3,
11.8% in 4, 3.5% in 5 and 12.0% in 6, respectively, at
the initial stage (4 h) of the reactions. The catalytic
activities of the ruthenium complexes containing elec-
tron-withdrawing 6-pivalamide groups (1, 3, and 5)9
were relatively low in comparison with those containing
electron-donating 6-neopentyl groups (2, 4, and 6). This
finding clearly indicates a correlation with the redox
properties of the complexes.10
Fig. 2. Formation rates for cyclohexene oxide in the oxida-
tion of cyclohexene catalyzed by the complexes 3 (ꢀ) and 4
(ꢁ).
The epoxidation activities catalyzed by these complexes
were also affected by the substituent groups of the
ligands. Complex 3 efficiently epoxidized cyclohexene
under the same conditions as the hydroxylation of
adamantane, as shown in Fig. 2. After 4 h, 46% of
cyclohexene oxide and small amounts of allylic oxida-
tion products, 2-cyclohexen-1-ol and 2-cyclohexen-1-
one, were obtained. On the other hand, the reaction
catalyzed by complex 4 gave 20% of cyclohexene oxide.
This is in contrast to the above hydroxylation, in which
the oxidation rate catalyzed by the complex 4 is faster
than that by 3. Reasons for such conflicting results
obtained for hydroxylation and epoxidation are not yet
clear. According to Meyer’s argument, in which the
Ru(IV)ꢀO species prefers an attack on the CꢁH bond
rather than the olefin double bond,11 the ruthenium
complexes 2, 4 and 6 have the Ru(IV)ꢀO character in
the presence of PhIO. Notably, the complexes 1, 3, and
5 exhibit epoxidation activity rather than hydroxyla-
tion, which suggests that the reaction intermediate is in
an oxidation state higher than Ru(IV).2 A positive
ESI-mass spectrum of the reaction intermediate gener-
ated from 3 and 10 equiv. of PhIO in dichloromethane
showed prominent peak clusters at m/z 624.2 and
640.2, corresponding to the [RuIII(bppa−)Cl]+ and
[RuV(bppa−)(O)Cl]+ ions, respectively (Fig. 3). These
results indicate that the catalytic species of the ruthe-
nium complexes containing electron-withdrawing
groups for epoxidation might be high valent mononu-
clear Ru(V)ꢀO intermediates generated through the two
electron process with PhIO, e.g. Ru(III)/Ru(V) redox
couple. The Ru(III) species from 3 could be generated
through disproportionation of the starting Ru(II) com-
plex and the Ru(IV) species oxidized by PhIO.
In conclusion, the catalytic activities of the ruthenium
complexes for the hydroxylation of alkanes and the
epoxidation of olefins in the presence of a 2-electron
Fig. 3. (a) Positive-ion ESI-mass spectrum of the intermediate
species generated from [RuII(bppa−)Cl](PF6) (3) and PhIO. (b)
Magnification of the parent peak clusters of the [Ru(bppa−)-
(O)Cl]+ region. (c) Simulation pattern for [Ru(bppa−)(O)Cl]+.
Fig. 1. Formation rates for 1-adamantanol in the oxidation
of adamantane catalyzed by the complexes 3 (") and 4 (ꢂ).

K. Jitsukawa et al. / Tetrahedron Letters 42 (2001) 3467–3469
3469
co-oxidant can be fine-tuned by the use of tripodal
polypyridine ligands containing different substituent
groups at the pyridine 6-position.
Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.;
Mukai, M.; Kitagawa, T.; Einaga, H. Angew. Chem., Int.
Ed. 1998, 37, 798; (e) Harata, M.; Hasegawa, K.; Jit-
sukawa, K.; Masuda, H.; Einaga, H. Bull. Chem. Soc.
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Acknowledgements
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121; (b) For Ru(II) complex: Yamaguchi, M.; Kousaka,
H.; Yamagishi, T. Chem. Lett. 1997, 769.
We gratefully acknowledge Mr. Y. Takahashi, JEOL
Co. Ltd., for measurement of ESI-mass spectra. This
work was supported by a Grant-in Aid for Scientific
Research from the Ministry of Education, Science,
Sports, and Culture, Japan (H.M.) and partly by the
Hibi Research Grant (K.J.).
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8. Abbreviations: tris(6-pivalamido-2-pyridylmethyl)amine,
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