Stoichiornetric and catalytic oxidations of alkanes and alcohols mediated by highly oxidizing ruthenium-oxo complexes bearing 6,6'-dichloro-2,2'-bipyridine

Article abstract of DOI:10.1021/jo0010126

The ruthenium(II) complex cis-[Ru(6,6'-Cl₂bpy)₂(OH₂)₂](CF₃SO₃)₂ (1) is a robust catalyst for C-H bond oxidations of hydrocarbons, including linear alkanes, using tert-butyl hydroperoxide (TBHP) as terminal oxidant. Alcohols can be oxidized by the '1 + TBHP' protocol to the corresponding aldehydes/ketones with high product yields at ambient temperature. Oxidation of 1 with Ce(IV) in aqueous solution affords cis-[Ru(VI)(6,6'-Cl₂bpy)₂O₂]²⁺, which is isolated as a green/yellow perchlorate salt (2). Complex 2 is a powerful stoichiometric oxidant for cycloalkane oxidations under mild conditions. Oxidation of cis-decalin is highly stereoretentive; cis-decalinol is obtained in high yield, and formation of trans-decalinol is not observed. Mechanistic studies showing a large primary kinetic isotope effect suggest a hydrogen-atom abstraction pathway. The relative reactivities of cycloalkanes toward oxidation by 2 have been examined through competitive experiments, and comparisons with Gif-type processes are presented.

Full text of DOI:10.1021/jo0010126

7996
J . Org. Chem. 2000, 65, 7996-8000
Stoich iom etr ic a n d Ca ta lytic Oxid a tion s of Alk a n es a n d Alcoh ols
Med ia ted by High ly Oxid izin g Ru th en iu m -Oxo Com p lexes
Bea r in g 6,6′-Dich lor o-2,2′-bip yr id in e
Chi-Ming Che,*,† Kar-Wai Cheng,† Michael C. W. Chan,† Tai-Chu Lau,‡ and Chi-Keung Mak‡
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China, and
Department of Biology and Chemistry, City University of Hong Kong, Kowloon Tong,
Hong Kong SAR, China
cmche@hku.hk
Received J uly 6, 2000
The ruthenium(II) complex cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1) is a robust catalyst for C-H
bond oxidations of hydrocarbons, including linear alkanes, using tert-butyl hydroperoxide (TBHP)
as terminal oxidant. Alcohols can be oxidized by the “1 + TBHP” protocol to the corresponding
aldehydes/ketones with high product yields at ambient temperature. Oxidation of 1 with CeIV in
aqueous solution affords cis-[RuVI(6,6′-Cl2bpy)2O2]2+, which is isolated as a green/yellow perchlorate
salt (2). Complex 2 is a powerful stoichiometric oxidant for cycloalkane oxidations under mild
conditions. Oxidation of cis-decalin is highly stereoretentive; cis-decalinol is obtained in high yield,
and formation of trans-decalinol is not observed. Mechanistic studies showing a large primary kinetic
isotope effect suggest a hydrogen-atom abstraction pathway. The relative reactivities of cycloalkanes
toward oxidation by 2 have been examined through competitive experiments, and comparisons
with Gif-type processes are presented.
In tr od u ction
In our endeavor to develop new metal catalysts for
hydrocarbon functionalization, we deemed that mecha-
nistic information on the reactivities of highly oxidizing
MdO complexes toward different alkanes were impor-
tant. We have previously described the preparation of
high-valent ruthenium-oxo complexes supported by the
6,6′-dichloro-2,2′-bipyridine (6,6′-Cl2bpy) ligand.9,10 We
now report in detail on the reactivity of a cis-dioxo-
ruthenium(VI) complex containing the 6,6′-Cl2bpy ligand
toward stoichiometric alkane oxidations. In particular,
the observed relative reactivities with various cyclo-
alkanes are compared with the results previously dis-
closed for Gif oxidations. The catalytic activities of the
Ru(II) derivative cis-[Ru(6,6′-Cl2bpy)2(OH2)2]2+ in organic
oxidations with tert-butyl hydroperoxide (TBHP) as
terminal oxidant are also presented.11
Highly oxidizing metal-oxo (MdO) complexes are
usually invoked as reactive intermediates in biological
oxidations1 and in Gif chemistry,2 but literature examples
of isolable MdO complexes with well-defined structural
and redox properties and the ability to oxidize saturated
hydrocarbons at room temperature are sparse.3-7 Re-
garding the ongoing controversy surrounding the reaction
mechanism of Gif-type oxidations, pathways involving
[FeVdO] species and oxygenated free radicals have both
been proposed.2,8 It is difficult to differentiate these two
mechanistic possibilities, partly because there is a dearth
of reports describing the relative reactivities of discrete
metal-oxo complexes toward different saturated C-H
bonds.
* Corresponding author. Fax: +852 2857 1586.
† The University of Hong Kong.
‡ City University of Hong Kong.
Resu lts a n d Discu ssion
(1) Cytochrome P-450: Structure, Mechanism and Biochemistry, 2nd
ed.; Oritz de Montellano, P. R., Ed.; Plenum Press: New York, 1995.
(2) (a) Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504.
(b) Barton, D. H. R. Chem. Soc. Rev. 1996, 25, 237. (c) Barton, D. H.
R. Tetrahedron 1998, 54, 5805.
(3) (a) Griffith, W. P. Chem. Soc. Rev. 1992, 21, 179. (b) Tenaglia,
A.; Terranova, E.; Waegell, B. J . Org. Chem. 1992, 57, 5523. (c) Bailey,
A. J .; Griffith, W. P.; Savage, P. D. J . Chem. Soc., Dalton Trans. 1995,
3537.
P r ep a r a tion a n d Ch a r a cter iza tion of Ru th en iu m
Com p lexes. Treatment of cis-[RuII(6,6′-Cl2bpy)2Cl2]‚
2H2O with AgCF3SO3 in H2O yielded cis-[RuII(6,6′-Cl2-
bpy)2(OH2)2](CF3SO3)2 (1) as a deep red solid in 70% yield.
At ambient temperature, complex 1 is stable in solution
and as a solid for over 5 days. The UV-vis spectrum of
1 in H2O displays an intense absorption band at 494 nm,
(4) Goldstein, A. S.; Beer, R. H.; Drago, R. S. J . Am. Chem. Soc.
1994, 116, 2424.
(5) (a) Mayer, J . M. Acc. Chem. Res. 1998, 31, 441. (b) Cook, G. K.;
Mayer, J . M. J . Am. Chem. Soc. 1995, 117, 7139.
(6) (a) Che, C. M.; Tang, W. T.; Wong, W. T.; Lai, T. F. J . Am. Chem.
Soc. 1989, 111, 9048. (b) Che, C. M.; Yam, V. W. W.; Mak, T. C. W. J .
Am. Chem. Soc. 1990, 112, 2284. (c) Ho, C.; Lau, T. C.; Che, C. M. J .
Chem. Soc., Dalton Trans. 1991, 1259.
(7) (a) Dovletoglou, A.; Adeyemi, S. A.; Lynn, M. H.; Hodgson, D.
J .; Meyer, T. J . J . Am. Chem. Soc. 1990, 112, 8989. (b) Lau, T. C.;
Mak, C. K. J . Chem. Soc., Chem. Commun. 1995, 943. (c) Groves, J .
T.; Bonchio, M.; Carofiglio, T.; Shalyaev, K. J . Am. Chem. Soc. 1996,
118, 8961. (d) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879.
(8) (a) Perkins, M. J . Chem. Soc. Rev. 1996, 25, 229. (b) Newcomb,
M.; Simakov, P. A. Tetrahedron Lett. 1998, 39, 965.
(9) (a) Che, C. M.; Leung, W. H. J . Chem. Soc., Chem. Commun.
1987, 1376. (b) Che, C. M.; Lee, W. O. J . Chem. Soc., Chem. Commun.
1988, 1406. (c) Lau, T. C.; Che, C. M.; Lee, W. O.; Poon, C. K. J . Chem.
Soc., Chem. Commun. 1988, 1406.
(10) Che, C. M.; Ho, C.; Lau, T. C. J . Chem. Soc., Dalton Trans.
1991, 1901.
(11) Examples of metal complex/TBHP systems: (a) Conte, V.; Di
Furia, F.; Modena, G. In Organic Peroxides; Ando, W., Ed.; J ohn Wiley
and Sons: New York, 1992; p 559. (b) J ohnson, R. A.; Sharpless, K. B.
In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York,
1993; p 103. (c) Kojima, T.; Matsuo, H.; Matsuda, Y. Inorg. Chim. Acta
2000, 300-302, 661 and references therein.
10.1021/jo0010126 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/24/2000

Oxidations of Alkanes by Ruthenium-Oxo Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7997
Ta ble 1. Stoich iom etr ic Oxid a tion of Or ga n ic Su bstr a tes
(100 m g) by cis-[Ru VI(6,6′-Cl2bp y)2O2](ClO4)2 (50 m g, 0.06
m m ol) in Aceton itr ile (3 m L) a t Room Tem p er a tu r e
u n d er a N2 Atm osp h er e; Rea ction Tim e 0.5 h
which can be assigned to the dπ(Ru) f π*(6,6′-Cl2bpy)
MLCT transition. The electrochemistry of 1 in aqueous
solution has been studied previously.12 The cyclic volta-
mmogram in pH 1.0 (0.1 M CF3CO2H) displays two
quasireversible couples at 0.93 and 1.17 V vs SCE, which
were assigned as Ru(IV)/Ru(II) and Ru(VI)/Ru(IV), re-
spectively. The cis configuration and a distorted octahe-
dral environment around the ruthenium atom in 1 were
confirmed by X-ray crystallography (see Supporting
Information). The average Ru-O distance of 2.128 Å is
comparable to that in [RuII(H2O)6](C7H7SO3)2 (2.122(16)
Å).13 The mean Ru-N distance of 2.055 Å is partially
shorter than the analogous distances of 2.076(2) and
2.090(3) Å in cis-[RuII(6,6′-Cl2bpy)2(CH3CN)2](ClO4)2;14
CH3CN is a stronger π-acid ligand than H2O, and thus
the bis(acetonitrile) complex presumably engages in
weaker Ru f diimine back-bonding.
entry
substrates
product(s)
yield (%)a
1
2
3
norbornylene
cyclohexene
styrene
exo-2,3-epoxynorbornane 78
2-cyclohexen-1-one
styrene oxide
60
3
benzaldehyde
cis-stilbene oxide
benzaldehyde
diphenylacetaldehyde
trans-stilbene oxide
benzaldehyde
cyclopentanone
cyclohexanone
70
7
4
5
cis-stilbene
16
21
50
18
46
trans-stilbene
6
7
8
cyclopentane
cyclohexane
cyclohexanec
b
50 (kH/kD ) 6.5)
cyclohexanone
cyclohexyl chloride
cyclooctanone
50
0.5
68
75
9
cyclooctane
10 cyclodecane
cyclodecanone
11 methylcyclohexane 1-methylcyclohexan-1-ol 69
Oxidation of 1 by Ce(IV) in aqueous NaClO4 solution
at 0 °C gave cis-[RuVI(6,6′-Cl2bpy)2O2](ClO4)2 (2) as a
green/yellow solid in 60% yield. We have previously
reported the synthesis of 2 from cis-[RuII(6,6′-Cl2bpy)2-
(OH2)2](ClO4)2,12 so only a brief description of its proper-
ties is given here. Complex 2 is stable for 1-2 h in the
solid state at 0 °C; complete conversion to [RuII(6,6′-Cl2-
bpy)2(CH3CN)2]2+ readily occurs in acetonitrile within 1
h at room temperature. As expected from its formulation,
the cyclic voltammogram of 2 measured in 0.1 M CF3-
CO2H is identical to that for 1. Complex 2 exhibits two
infrared bands at 846 and 860 cm-1 with different
intensities, and these are tentatively assigned to the
asymmetric and symmetric RudO stretching frequencies
respectively in a cis geometry. Similar IR spectral data
have been reported for related cis-dioxoruthenium(VI)
complexes, namely cis-[RuVI(2,2,2-tet-Me6)2O2](ClO4)2 (859
and 874 cm-1, 2,2,2-tet-Me6 ) N,N,N′,N′-3,6-hexamethyl-
3,6-diazaoctane-1,8-diamine)15 and cis-[RuVI(Me3tacn)-
O2(CF3CO2)]ClO4 (842 and 856 cm-1, Me3tacn ) 1,4,7-
trimethyl-1,4,7-triazacyclononane),16 which have been
characterized by X-ray crystallography. The UV-vis
spectrum of 2 in distilled water is featureless in the
visible region, but a rapid and quantitative conversion
to cis-[RuII(6,6′-Cl2bpy)2(OH2)2]2+ is observed upon addi-
tion of an organic reductant such as benzyl alcohol.
Stoich iom etr ic Oxidation s by cis-[Ru (6,6′-Cl2bpy)2-
O2](ClO4)2 (2). Complex 2 has a E° value of 1.17 V vs
SCE, and is one of the most oxidizing ruthenium-oxo
complexes to be isolated in solid form.17 A freshly
prepared sample of 2 is a powerful oxidant for various
organic substrates including saturated hydrocarbons. The
results of stoichiometric oxidations in acetonitrile at 298
K are summarized in Table 1. Control experiments in
the absence of the ruthenium complex showed that
oxidation of the organic substrates did not occur. Oxida-
tion of cyclohexene occurred exclusively at the allylic
C-H bond (entry 2), a feature which is typical for related
oxoruthenium complexes including cis-[RuVI(2,2,2-tet-
12 adamantane
13 n-hexane
14 toluene
adamantan-1-ol
-
75
-
benzaldehyde
sec-phenethyl alcohol
acetophenone
2-phenyl-2-propanol
acetophenone
cis-decalinol
cyclohexanone
benzaldehyde
benzoic acid
52
17
38
32
36
77
78
73
5
15 ethyl benzene
16 cumene
17 cis-decalin
18 cyclohexanol
19 benzyl alcohol
20 1-hexanol
21 dimethyl sulfide
hexanal
dimethyl sulfoxide
dimethyl sulfone
71
68
9
a
Product yield is based on the amount of ruthenium complex
b
used. Calculated from the competitive oxidation of cyclohexane
and cyclohexane-d12.
c CH3CN:CCl4 ) 9:1.
lective, so that both epoxidation and oxidative cleavage
of CdC bonds were observed. The oxidations of trans-
and cis-stilbene (entries 4 and 5) afforded the corre-
sponding stilbene oxide with high stereospecificity and
moderate yield, although oxidative cleavage was found
to be a significant pathway. The stereospecificities of
these transformations are dependent upon the integrity
of complex 2; for example, reaction of cis-stilbene with
an aged sample of 2 gave a mixture of cis- and trans-
stilbene oxides.
An important finding in this work is the ability of 2 to
oxidize unactivated saturated C-H bonds. The oxidation
of cyclohexane was completed within minutes, but linear
alkane substrates such as n-hexane did not yield any
detectable oxidized products. We note that the ruthenium
product after the oxidation was cis-[RuII(6,6′-Cl2bpy)2-
(CH3CN)2]2+, as indicated by the appearance of its MLCT
bands at λmax 301 and 445 nm in the UV-vis spectrum.
Cyclohexane was oxidized to cyclohexanone exclusively
(entry 7), with a primary kinetic isotope effect of 6.5. It
is apparent that the reaction involves formation of the
cyclohexyl radical, since ca. 1% cyclohexyl chloride was
detected when the oxidation was performed in the
presence of CCl4 (entry 8). Product yields from the
oxidations of relatively inert substrates were compara-
tively low (e.g., entries 6, 7, and 14), presumably because
reduction of 2 by acetonitrile became significant (see
above). Oxidation of methylcyclohexane (entry 11) and
adamantane (entry 12) occurred at the tertiary C-H
bond exclusively with no detectable products from sec-
Me6)O2]2+ 18
The oxidations of aryl alkenes were nonse-
.
(12) Wong, K. Y.; Lee, W. O.; Che, C. M.; Anson, F. C. J . Electroanal.
Chem. 1991, 319, 207.
(13) Bernhard, P.; Bu¨rgi, H.-B.; Hauser, J .; Lehmann, H.; Ludi, A.
Inorg. Chem. 1982, 21, 3936.
(14) Lee, W. O. Ph.D. Thesis, The University of Hong Kong, 1989.
(15) Li, C. K.; Che, C. M.; Tong, W. F.; Tang, W. T.; Wong, K. Y.;
Lai, T. F. J . Chem. Soc., Dalton Trans. 1992, 2109.
(16) Cheng, W. C.; Yu, W. Y.; Cheung, K. K.; Che, C. M. J . Chem.
Soc., Chem. Commun. 1994, 1063.
(18) Cheng, W. C.; Yu, W. Y.; Li, C. K.; Che, C. M. J . Org. Chem.
1995, 60, 6840.
(17) Che, C. M.; Yam, V. W. W. Adv. Inorg. Chem. 1992, 39, 233.

7998 J . Org. Chem., Vol. 65, No. 23, 2000
Che et al.
Ta ble 2. Com p etitive Oxid a tion of Or ga n ic Su bstr a tes (1 m m ol) by cis-[Ru VI(6,6′-Cl2bp y)2O2](ClO4)2 (50 m g, 0.06 m m ol)
in Aceton itr ile (3 m L) a t Room Tem p er a tu r e u n d er a N2 Atm osp h er e; Rea ction Tim e 0.5 h
entry
1
substrates
products (yield/%)a
relative reactivity per H atom
cyclohexane and cyclopentane
cyclohexanone (20)
cyclopentanone (8)
cyclohexanone (13)
cyclooctanone (27)
cyclohexanone (2)
cyclodecanone (14)
cyclohexanone (4)
2-phenyl-2-propanol (52)
acetophenone (22)
cyclohexanone (1)
adamantan-1-ol (56)
cyclohexanone (9)
cyclooctanone (52)
1
0.48
1
1.6
1
4.2
1
144
2
3
4
cyclohexane and cyclooctane
cyclohexane and cyclodecane
cyclohexane and cumene
5
6
cyclohexane and adamantane
cyclohexanol and cyclooctanol
1
84
1
5.8
a
Product yield is based on the amount of ruthenium complex used.
Ta ble 3. Oxid a tion of Alk a n es (1.5 m m ol) w ith TBHP
(3.2 m m ol) Ca ta lyzed by cis-[Ru II(6,6′-Cl2Bp y)2(OH2)2]-
(CF 3SO3)2 (0.001 m m ol) in Aceton e (3 m L) a t Room
Tem p er a tu r e; Rea ction Tim e 24 h , u n less
Oth er w ise Sta ted
ondary C-H bond oxidation. Because the oxidation of cis-
decalin to cis-decalinol is highly stereoretentive (entry
17), we suggest that the alkane oxidations proceed via a
H-atom abstraction pathway with rapid radical recom-
bination. Dimethyl sulfide was oxidized to a mixture of
dimethyl sulfoxide and dimethyl sulfone (entry 21). A
related report using cis-[RuVI(Me3tacn)O2(CF3CO2)]+ as
oxidant showed that dimethyl sulfide is oxidized to
dimethyl sulfone only.16
entry
1
substrates
product(s)
yield (%)a
cyclohexane
cyclohexanol
33
cyclohexanone
52 (kH/kD ) 4.8)
2
3
cyclohexaneb
-
-
28
41
11
-
13
19
49
8
cyclohexanec cyclohexanol
cyclohexanone
In competitive experiments of cyclohexane/cycloalkanes
in acetonitrile at 298 K using complex 2 as a stoichio-
metric oxidant (Table 2, entries 1-3), the relative reac-
tivities per H atom (normalized) follow the order: cyclo-
decane (4.2) > cyclooctane (1.6) > cyclohexane (1) >
cyclopentane (0.48). The results of analogous competitive
experiments between cyclohexane/cumene, cyclohexane/
adamantane, and cyclohexanol/cyclooctanol are also listed
in Table 2. The employment of such competitive experi-
ments to distinguish between radical and Gif chemistry
has been proposed.19 Hence, it is pertinent to compare
the reactivities of complex 2 with the oxidation chemistry
of Gif reactions. On the basis of the C5/C6 ratio (0.48 for
2 in CH3CN, 0.60-0.85 for Gif in pyridine20) and the
kinetic isotope effect for cyclohexane oxidation (6.5 for
2, 2.1-2.3 for Gif20), it is apparent that 2 and Gif systems
behave differently toward alkane oxidations. Further-
more, the relative order of reactivities for cycloalkane
oxidations by complex 2 contrasts with that in Fe(III)-
catalyzed Gif oxidations, whereby the relative reaction
rates per hydrogen for cyclooctane is smaller than for
cyclohexane.19b The substantial increase in the oxidation
reactivity of 2 from cyclohexane to adamantane is
consistent with the stability of the alkyl radicals gener-
ated through H-atom abstraction, i.e., tertiary C-H
bonds are weaker than secondary.
cyclohexyl chloride
4
5
cyclohexaned
-
cyclohexanee cyclohexanol
cyclohexanone
acetophenone
cyclooctanol
6
7f
cyclooctane
cyclooctanone
adamantane adamantan-1-ol
77
62
adamantan-2-ol and -2-one 25
8
cumene
2-phenyl-2-propanol
acetophenone
61
29
71
18
2
9g
10f
ethylbenzene acetophenone
sec-phenethyl alcohol
n-hexan-2-o1
n-hexane
n-hexan-2-one
n-hexan-3-ol
n-hexan-3-one
cis-decalinol
12
3
12
15
62
11
cis-decalin
trans-decalinol
a
b
Product yield is based on conversion of alkane. In the
presence of 5-fold excess of ionol. c Acetone:CCl4 ) 9:1. Acetoni-
d
trile was used as solvent. e CHP was used as terminal oxidant.
f Substrate ) 1 mmol and TBHP ) 3.2 mmol. Reaction time )
g
48 h.
stoichiometric oxidations by 2 (preliminary results have
been communicated9c). In control experiments, no oxi-
dized product was detected in the absence of either the
ruthenium catalyst or TBHP. Examination of the reaction
medium by UV-vis spectroscopy after alkane oxidation
indicated that complex 1 remains intact. Cyclohexane
was oxidized to a mixture of cyclohexanol and cyclohex-
anone (ratio 1:1.6; entry 1), but the latter was formed
exclusively in the stoichiometric oxidation. We note that
other ruthenium-oxo complexes have been reported to
oxidize cyclohexane to give cyclohexanone exclusively.17
In the presence of 5-fold excess 2,6-di-tert-butyl-4-meth-
ylphenol (ionol), no oxidized product was detected (entry
2). When the reaction was carried out in an acetone/CCl4
(9:1 v/v) mixture, formation of the oxygenated product-
(s) was suppressed and cyclohexyl chloride was afforded
(entry 3). Oxidation of cyclohexane was completely
inhibited if CH3CN was used as solvent instead of acetone
(entry 4); presumably acetonitrile prevents the oxidation
Catalytic Oxidation s by cis-[Ru (6,6′-Cl2bpy)2(OH2)2]-
(CF 3SO3)2 (1) a n d TBHP . Efforts have been made to
harness this class of complexes for catalytic oxidations
through the use of a terminal oxidant, namely tert-butyl
hydroperoxide (TBHP), and complex 1 was employed as
the catalyst since it is relatively stable and robust. We
have indeed observed catalytic alkane oxidations based
on the “1 + TBHP” system (Table 3), and product
analyses have revealed notable differences compared to
(19) For example, see: (a) Barton, D. H. R.; Csuhai, E.; Doller, D.;
Ozbalik, N.; Senglet, N. Tetrahedron Lett. 1990, 31, 3097. (b) Barton,
D. H. R.; Launay, F.; Le Gloahec, V. N.; Li, T.; Smith, F. Tetrahedron
Lett. 1997, 38, 8491.
(20) Barton, D. H. R.; Doller, D.; Geletii, Y. V. Tetrahedron Lett.
1991, 32, 3811.

Oxidations of Alkanes by Ruthenium-Oxo Complexes
J . Org. Chem., Vol. 65, No. 23, 2000 7999
Ta ble 4. Oxid a tion of Alcoh ols (1 m m ol) Ca ta lyzed by
cis-[Ru II(6,6′-Cl2Bp y)2(OH2)2](CF 3SO3)2 (0.001 m m ol) w ith
TBHP in Aceton e (3 m L) a t Room Tem p er a tu r e
reaction by binding to the Ru(II) center. Cyclooctane was
oxidized to yield a mixture of cyclooctanol and cyclooc-
tanone in a ratio of 1:10 (entry 6). Oxidation of adaman-
tane occurred preferentially at the tertiary carbon atoms
to yield adamantan-1-ol (entry 7), but unlike the sto-
ichiometric oxidation, significant amounts of products
arising from secondary C-H bond oxidation were found.
The smaller kinetic isotope effect (kH/kD ) 4.8) and the
tertiary to secondary C-H bond relative reactivities (ktert
/
ksec ) 7.4; statistical factor corrected) in the catalytic
cyclohexane and adamantane oxidations, respectively,
contrast sharply with the stoichiometric oxidation results.
Significantly, Drago and co-workers have reported very
similar data for the catalytic [Ru(dmp)2(OH2)2](PF6)2 +
H2O2 system (kH/kD ) 4.0 for cyclohexane; ktert/ksec ) 7.1
for adamantane; dmp ) 2,9-dimethyl-1,10-phenanthro-
line), for which a free-radical mechanism was proposed.4
Furthermore, the “1 + TBHP” protocol is capable of
oxidizing linear chain alkanes such as n-hexane (entry
10). Thus, the active intermediate in the catalytic oxida-
tions is unlikely to be a ruthenium-oxo derivative, or at
least solely this species. Indeed, our results from the
cyclohexane oxidations suggest that the active species has
substantial radical character: (a) addition of ionol sup-
pressed the catalytic oxidation (entry 2); (b) a substantial
amount of cyclohexyl chloride was produced in the
presence of CCl4 (entry 3); (c) when the terminal oxidant
was changed from TBHP to cumene hydroperoxide
(CHP), the product profile is dominated by acetophenone,
which is generated by â-scission of cumyloxy radicals
(entry 5). We also note that stirring a mixture of TBHP
(0.3 mL) and 1 (2 mg) in dichloromethane (3 mL) at room
temperature for 20 h resulted in the formation of acetone,
albeit in very low yield (<1% based on TBHP).
We investigated the competitive oxidation of cyclohex-
ane/cyclopentane using the “1 + TBHP” protocol and
found that the relative reactivity per H atom for cyclo-
pentane (0.38 based on cyclohexane) differed slightly from
the stoichiometric oxidation by 2 (0.48). For cyclohexane/
cumene, cumene was 19 times more reactive than cyclo-
hexane; this is in contrast to the stoichiometric oxidation
where cumene was 144 times more reactive. Similarly
in the competitive oxidation of cyclohexanol/cyclooctanol,
cyclooctanol was 1.4 times more reactive than cyclohex-
anol, compared to a reactivities ratio of 6 found in the
stoichiometric reactions. These observations further im-
ply that the active species in the stoichiometric and
catalytic oxidations are not identical.
Our investigation showed that complex 1 is also an
efficient catalyst for the oxidation of alcohols by TBHP
(Table 4). Treatment of benzyl alcohol (1 mmol) with
TBHP (3 mmol) in the presence of 1 in acetone (3 mL)
afforded a mixture of benzaldehyde and benzoic acid in
a ratio of 2:1 (87% conversion) after stirring for 24 h at
ambient temperature (entry 1). When 1.2 equiv of TBHP
was used, the ratio increased to 12:1 (47% conversion).
Changing the solvent to CH2Cl2 led to decreased catalytic
activity, and the reaction solution became yellow in color.
Aliphatic, cyclic, and allylic alcohols were efficiently
converted into the corresponding aldehydes and ketones
in high to quantitative yields in the presence of excess
TBHP (3-6 equiv). While oxidation of primary alcohols
(entry 1) afforded a mixture of the corresponding alde-
hyde and acid, secondary alcohols such as 2-hexanol
(entry 2) and 3-hexanol (entry 3) gave the analogous
ketones. Likewise, sec-phenethyl alcohol (entry 4) was
a
b
Product yield is based on conversion of alcohol. Catalyst:
substrate:TBHP ) 1:1000:3000. c Catalyst:substrate:TBHP
1:1000:6000.
)
oxidized to acetophenone. With cyclic alcohols such as
cyclohexanol (entry 5) and cyclooctanol (entry 6), the
corresponding ketones were obtained without the forma-
tion of ring-opened products. With allylic alcohol like
2-cyclohexen-1-ol (entry 7), alcohol oxidation prevailed
over alkene epoxidation to yield 2-cyclohexen-1-one as the
sole product. In the oxidation of trans-cinnamyl alcohol
(entry 8), the trans-aldehyde together with the trans-acid
and the cleavage product benzaldehyde were observed.
Con clu sion
We have illustrated that the robust bis(6,6′-dichloro-
2,2′-bipyridine)ruthenium fragment can mediate the sto-
ichiometric and catalytic oxidations of saturated C-H
bonds, although our results suggest that the mechanistic
pathways for the respective transformations are different.
The cis-[RuVI(6,6′-Cl2bpy)2O2](ClO4)2 complex (2) has a
high E° value and is in a select group of isolated metal-
oxo complexes that can oxidize saturated hydrocarbons

8000 J . Org. Chem., Vol. 65, No. 23, 2000
Che et al.
P r ep a r a t ion of cis-[R u VI(6,6′-Cl2b p y)2O2](ClO4)2 (2).12
Complex 1 (0.10 g, 0.11 mmol) was dissolved in a minimum
amount of warm water (50 °C). The resulting solution was
filtered and cooled in an ice bath. Upon addition of an aqueous
solution of (NH4)2[CeIV(NO3)6] (2 g in 5 mL H2O) containing
NaClO4 (0.5 g), the color of the solution changed from deep
red to greenish-yellow immediately. A greenish-yellow solid
slowly precipitated from the solution and this was collected,
washed with ice-cold water, and dried in vacuo. Yield ) 0.05
g, 60%. IR: 846, 860 cm-1 (RudO).
at ambient temperature; a hydrogen-atom abstraction
pathway is proposed for these oxidations. Highly oxidiz-
ing metal-oxo species have been assumed as reactive
intermediates in Gif oxidation chemistry, yet differences
are apparent between the reactivities of the cis-dioxoru-
thenium(VI) complex 2 and Gif-type systems toward
C-H bond oxidations, notably in the competitive oxida-
tions of cycloalkanes. The “1 + TBHP” protocol for
catalytic oxidations of hydrocarbons and alcohols has
been developed. Mechanistically, our results from this
system are fully consistent with a free-radical process.
Finally, data from these studies indicate that the oxida-
tions of saturated hydrocarbons by complex 2, the “1 +
TBHP” protocol and Gif-type systems are not alike from
a mechanistic viewpoint.
Gen er a l P r oced u r e for Stoich iom etr ic Oxid a tion s by
Com p lex 2. A solution of the organic substrate (100 mg) and
complex 2 (50 mg, 0.06 mmol) in degassed acetonitrile (3 mL)
was stirred at room temperature under a nitrogen atmosphere
for 30 min. A control experiment without the ruthenium
complex was performed under identical conditions. The aliquot
was analyzed on a capillary gas chromatograph equipped with
a flame ionization detector. Products were quantified by the
internal standard method, and identifications were achieved
by comparing the retention times with authentic samples. For
the oxidation of cis- and trans-stilbenes, 1H NMR analysis was
used for the determination of stilbene oxides.
Exp er im en ta l Section
Gen er a l. 6,6′-Dichloro-2,2′-bipyridine21 and cis-[RuII(6,6′-
Cl2bpy)2Cl2]‚2H2O12 were prepared by literature procedures.
tert-Butyl hydroperoxide (80% in di-tert-butylperoxide) was
purchased from Merck and the concentration was standardized
by iodometric titration. All organic substrates were obtained
from Aldrich and purified by literature methods, and their
purity were verified by GC or 1H NMR analysis before use.
Solvents were of analytical grade and used as received. Silver
trifluoromethanesulfonate (99+ %, Aldrich) was dried in a
vacuum oven prior to use.
Gen er a l P r oced u r e for Ca ta lytic Oxid a tion s by Com -
p lex 1 a n d TBHP . To a solution of organic substrate (1.5
mmol) and TBHP (3.2 mmol) in acetone (3 mL) was added
complex 1 (0.001 mmol), and the mixture was stirred for 24 h
at room temperature. A control experiment without the
catalyst was performed under identical conditions. The aliquot
was analyzed as above.
P r ep a r a tion of cis-[Ru II(6,6′-Cl2bp y)2(OH2)2](CF 3SO3)2
(1). A mixture of cis-[RuII(6,6′-Cl2bpy)2Cl2]‚2H2O (0.50 g, 0.76
mmol) and Ag(CF3SO3) (0.59 g, 2.28 mmol) in water (40 mL)
was heated at 70 °C for 30 min. The red solution was then
filtered to remove AgCl. The red crystalline solid obtained by
slow removal of the solvent was collected, washed with diethyl
ether, and dried in vacuo. Yield ) 0.47 g, 70%. Anal. Calcd
for C22H16N4O8Cl4F6RuS2: C, 29.84; H, 1.82; N, 6.33. Found:
C, 29.45; H, 2.05; N, 5.95. 1H NMR (CD3CN): δ 7.60 (dd, 2H),
7.81 (dd, 2H), 8.09 (t, 2H), 8.18 (t, 2H), 8.36 (dd, 2H), 8.41
(dd, 2H). FAB-MS: m/z 587 [M]+ and 551 [M - 2(H2O)]+. UV
Ack n ow led gm en t. We are grateful for financial
support from The University of Hong Kong and the
Research Grants Council of the Hong Kong SAR, China
[HKU 7092/98P]. Dr. K. K. Cheung is thanked for the
structural determination of complex 1.
Su p p or tin g In for m a tion Ava ila ble: ORTEP plot and
listings of crystal data, atomic coordinates, calculated coordi-
nates, anisotropic displacement parameters, and bond lengths
and angles for 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
λ
max, nm (ꢀ, dm3 mol-1 cm-1) in H2O: 247 (14700), 306 (44100),
494 (9100).
(21) Ogawa, S.; Shiraishi, S. J . Chem. Soc., Perkin Trans. 1 1980,
2527.
J O0010126