Aqueous phase C-H bond oxidation reaction of arylalkanes catalyzed by a water-soluble cationic Ru(III) complex [(pymox-Me2) 2RuCl2]+BF4-

Article abstract of DOI:10.1021/ol900097y

The cationic complex [(pymox-Me₂)RuCl₂] ^-BF₄^- was found to be a highly effective catalyst for the C-H bond oxidation reaction of arylalkanes in water. For example, the treatment of ethylbenzene (1.0 mmol) with t-BuOOH (3.0 mmol) and 1.0 mol % of the Ru catalyst in water (3 mL) cleanly produced PhCOCH₃ at room temperature. Both a large kinetic isotope effect (k_H/k _D) 14) and a relatively large Hammett value (ρ) -1.1) suggest a solvent-caged oxygen rebounding mechanism via a Ru(IV)-oxo intermediate species.

Full text of DOI:10.1021/ol900097y

ORGANIC
LETTERS
2
009
Vol. 11, No. 7
567-1569
Aqueous Phase C-H Bond Oxidation
Reaction of Arylalkanes Catalyzed by a
Water-Soluble Cationic Ru(III) Complex
1
+
-
[
(pymox-Me ) RuCl ] BF
2
2
2
4
Chae S. Yi,* Ki-Hyeok Kwon, and Do W. Lee
Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201-1881
Chae.yi@marquette.edu
Received January 29, 2009
ABSTRACT
+
4-
The cationic complex [(pymox-Me
in water. For example, the treatment of ethylbenzene (1.0 mmol) with t-BuOOH (3.0 mmol) and 1.0 mol % of the Ru catalyst in water (3 mL)
cleanly produced PhCOCH at room temperature. Both a large kinetic isotope effect (k /k ) 14) and a relatively large Hammett value (G )
1.1) suggest a solvent-caged oxygen rebounding mechanism via a Ru(IV)-oxo intermediate species.
2 2
)RuCl ] BF was found to be a highly effective catalyst for the C-H bond oxidation reaction of arylalkanes
3
H
D
-
2 2
of arylalkanes mediated by CAN/[Ru(tpa)(H O)
]+ system
in aqueous media. Li and co-workers devised a number of
Aqueous phase homogeneous catalysis has emerged as an
important tool for attaining new “green” chemical technology
in both industrial and fine chemical processes. Particular
5
1
oxidative coupling reactions involving C-H bond activation
6
attention has been centered on the development of water-
soluble metal catalysts for the C-H bond oxidation reactions,
and in this regard, late transition metal complexes with
nitrogen ligands have been shown to be effective for
mediating catalytic oxidation of saturated hydrocarbons in
in water. Surface-modified heterogeneous ruthenium-hy-
droxo catalysts have also been found to mediate selective
7
oxidation of benzylamines to arylamides in water. Despite
these recent advances, only a few well-defined synthetic
metal catalysts have been shown to mediate aerobic C-H
bond oxidation reactions in the aqueous phase, and consider-
able controversies still persist on the issues of reaction
mechanisms and the nature of reactive species.
2
protic media. A number of chemoselective allylic and
propargylic C-H bond oxidation and oxidative coupling
reactions of amines have recently been achieved by using
3
4
water-soluble dirhodium and ruthenium catalysts, respec-
As part of an ongoing effort to develop ruthenium-
8
tively. Fukuzumi reported an efficient C-H bond oxidation
catalyzed C-H bond activation reactions, we initially
screened several chelating nitrogen ligands to synthesize
(
1) Recent reviews and monographs: (a) Lindstr o¨ m, U. M. Chem. ReV.
2
002, 102, 2751–2772. (b) Aqueous-Phase Organometallic Catalysis;
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VCH: Weinheim, 2005; Vol. 1. (d) Li, C.-J. Chem. ReV. 2005, 105, 3095–
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(5) Hirai, Y.; Kojima, T.; Mizutani, Y.; Shiota, Y.; Yoshizawa, K.;
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(6) (a) Li, C.-J. Acc. Chem. Res. 2002, 35, 533–538. (b) Herrer ´ı as, C. I.;
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2008, 47, 9249–9251.
3
165.
(
2) (a) Sen, A.; Lin, M. Chem. Commun. 1992, 508–510. (b) Barton,
D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504–512. (c) Periana, R. A.;
Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998,
2
80, 560–564.
10.1021/ol900097y CCC: $40.75
Published on Web 02/26/2009
 2009 American Chemical Society

water-soluble ruthenium catalysts. Thus, the treatment of
(COD)RuCl with 1.2 equivalents of 4,4-dimethyl-2-(2-
pyridyl)oxazoline (pymox-Me ) ligand in 1,2-dichloroethane
at 50 °C produced an orange-yellow colored complex
pymox-Me )Ru(COD)Cl (1), which was isolated in 65%
bond oxidation reaction in aqueous solution, even though
both 2 and 3 are soluble in water. Thus, the treatment of
ethylbenzene (1.0 mmol) with t-BuOOH (3 mmol, 70 wt
% in aqueous solution) in the presence of 1 mol % of 3
[
2 x
]
2
(
2
2
3
in water (3 mL) cleanly produced PhCOCH in >95%
yield after recrystallization in n-hexanes/CH
Cl
2 2
(Scheme 1).
conversion within 16 h at room temperature (eq 1). Salient
features of the catalyst 3 are that it retains significant
activity after repeated runs (61% yield after third run),
and it can be readily separated from the reaction mixture
by simple extraction.
Scheme 1
The scope of the oxidation reaction was surveyed by using
3
as the catalyst (Table 1). In general, the C-H bond
a
Table 1. Aqueous Phase C-H Bond Oxidation of Arylalkanes
The treatment of 1 (0.4 mmol) with pymox-Me
in 1,2-dichloroethane at 100 °C led to the isolation of a deep
blue-purple colored complex (pymox-Me RuCl (2) in 55%
yield. Alternatively, the complex 2 could be directly produced
from the treatment of [(COD)RuCl with excess amount
of pymox-Me in 1,2-dichloroethane at 100 °C (65% yield).
The subsequent treatment of 2 with NaBF and t-BuOOH
in CH Cl led to the cationic Ru(III) complex [(pymox-
Me RuCl
2
(1.9 mmol)
)
2 2
2
2 x
]
2
4
2
2
+
-
2
)
2
2 4
] BF (3) in 73% isolated yield. The structure
of these ruthenium complexes was completely established
by both spectroscopic and X-ray crystallographic methods.
The molecular structure of both 2 and 3 showed an octahedral
geometry with cis coordination of the chloride and anti
pyridine ligands. The average Ru-Cl bond distance of the
cationic Ru(III) complex 3 (2.33 Å) was found to be
considerably shorter than the neutral complex 2 (2.41 Å).
M
The magnetic moment of 3 (µeff ) 1.55 B ) as determined
by using the Evans NMR method was also consistent with
a paramagnetic Ru(III) complex.
9
a
Reaction conditions: substrate (1.0 mmol), t-BuOOH (3.0 mmol, 70
b
wt % in water), 3 (1.0 mol %), H
2
O (3 mL), 20-22 °C. Isolated product
c
yields. The GC product yields are listed in parenthesis. Less than 5% of
d
benzaldehyde derivative is formed. Five percent of 1,3-indandione is
formed. Substrate was dissolved in 1 mL of CH Cl . Products were not
2 2
e
f
isolated due to low conversion and difficulty in separation.
In a strikingly different reactivity pattern, only complex
was found to exhibit high catalytic activity for the C-H
oxidation of benzylic compounds occurred smoothly at room
temperature to give the ketone products. The formation of
C-C bond cleavage product for isobutylbenzene is remi-
niscent of the oxidation reaction promoted by transition metal
complexes (entry 5), where benzyloxy radical species has
3
(
8) (a) Yi, C. S.; Yun, S. Y. J. Am. Chem. Soc. 2005, 127, 17000–
7006. (b) Yi, C. S.; Zhang, J. Chem. Commun. 2008, 2349–2351.
9) See the Supporting Information for experimental details.
1
(
1568
Org. Lett., Vol. 11, No. 7, 2009

been implicated for the C-C bond cleavage reactions of
-0.4 to -0.6), but somewhat lower than the ones catalyzed
by (PPh RuCl /t-BuOOH and cytochrome P-450 and their
synthetic model systems (F ) -1.3 to -1.6). A relatively
high -F value suggested of a substantial charge transfer from
a metal-oxo species to the substrate during the C-H bond
cleavage step.
1
0
alkylbenzenes. The oxidation of tertiary benzylic C-H
bond is favored over the primary ones to give the alcohol
product (entry 6). The dehydrogenation product was favored
over the oxidation product for the 9,10-anthracene case (entry
3
)
3
2
1
2
10). The oxidation of cyclic alkanes was found to be sluggish,
giving only modest conversions under the similar reaction
conditions (entry 12, 13).
(3) The initially inactive 2 became an active catalyst upon
4
addition of NaBF for the oxidation reaction. This fact and
We performed the following experiments to gain further
mechanistic insights on the oxidation reaction. (1) A very
a relatively low Ru(II)/Ru(III) redox potential (E ) +0.22
o
V) clearly indicate that the cationic Ru(III) complex is the
1
3
large kinetic isotope effect of k
/k
H D
) 14 was obtained from
catalytically active species for the oxidation reaction. The
observation of a strong metal-to-ligand charge transfer band
at 360 nm (dp-π*) from the reaction mixture of 2 with
the pseudo-first order plots of the oxidation reaction of
-
2
ethylbenzene vs ethylbenzene-d10 at 20 °C (kobs ) 2.1 × 10
-1
-3 -1
h
and 1.5 × 10 h , respectively) (Figure S1, Supporting
t-BuOOH and NaBF also supports the formation of a Ru(III)
4
9
Information). Such a large deuterium isotope effect has been
rarely observed in C-H bond oxidation reactions mediated
by synthetic metal catalysts, but more commonly observed
in enzyme-catalyzed oxidation reactions where quantum
mechanical tunneling effect has been ascribed to effect the
species (Figure S2, Supporting Information). These data are
most consistent with a “solvent-caged” oxygen rebound
mechanism of the rate-limiting C-H oxidation step from a
1
2,13
Ru(IV)-oxo species.
The fact that a radical scavenger
TEMPO (10 mol %) did not significantly affected the rate
of the oxidation reaction also supports the notion of a solvent-
caged mechanism.
1
1
rate-limiting C-H activation step.
2) The Hammett correlation of para-substituted ethyl-
benzene substrates p-X-C CH CH (X ) OMe, CH , H,
F, Cl) led to F ) -1.1 (Figure 1). The observed F value is
(
6
H
4
2
3
3
In summary, the cationic Ru(III) complex 3 was found to
be a highly effective catalyst for the benzylic C-H bond
oxidation reaction in water. While high valent metal-oxo
species have been invoked in both nonheme and Gif-type
2
b,14
oxidations,
catalytic C-H bond oxidation reactions
mediated by well-defined Ru(III) complexes have been rarely
13
reported. Efforts are currently underway to extend the scope
of the oxidation reaction as well as to establish the nature
of reactive species.
Acknowledgment. Financial support from the National
Institute of Health, General Medical Sciences (R15 GM55987)
is gratefully acknowledged). We also thank Dr. Sergey
Lindeman (Marquette University) for X-ray crystallographic
determination of the ruthenium complexes.
Figure 1
.
Hammett plot for the oxidation reaction of F-X-
(X ) OMe, CH , H, F, Cl) in water.
C
6
H
4
CH CH
2
3
3
Supporting Information Available: Experimental pro-
cedures and crystallographic data of 1, 2, and 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
substantially higher than the oxidation reactions catalyzed
by free radical species such as t-BuO• and t-BuOO• (F )
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