C. Abebrese et al. / Journal of Inorganic Biochemistry 105 (2011) 1555–1561
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2.3. Catalytic competitive oxidations
changes in sulfoxide yields. The absorption spectra of the final
ruthenium(IV) complexes also remained same to those without
pyrazole. This may be taken as evidence that the large excess sulfide
substrate could act as competent ligand to the putative RuIV-oxo
product, thereby preventing the reaction pathway away from a bis
(pyrazolato)ruthenium(IV) porphyrin complex, i.e. [RuIV(Por)(pz)2].
A CHCl3 solution containing equal amounts of two substrates, e.g.
thioanisole (0.5 mmol) and substituted thioanisole (0.5 mmol),
ruthenium(II) porphyrin catalyst (1 μmol) and an internal standard
of 1,2,4-trichlorobenzene (0.5 mmol) was prepared (final volu-
me=5 mL). The standards were shown to be stable to the oxidation
conditions in control reactions. PhIO or TBHP (0.1 mmol) was added,
and the mixture was stirred under an inert atmosphere at ambient
temperature (ca. 23 °C) until the reaction was complete. The amounts
of substrates before and after the reactions were determined by GC
(FID, DB-5). Relative rate ratios for oxidations were determined based
on the following equation.
3.2. Kinetic studies
In kinetic studies, solutions containing the trans-dioxoruthenium
(VI) oxidant were mixed with solutions containing large excesses of
sulfide substrate, and pseudo-first-order rate constants for decay of the
ruthenium-oxo species were measured spectroscopically. The dioxo 2
decayed rapidly in the presence of the thioanisoles, reacting as fast as 30
seconds. Therefore, a stopped-flow mixing unit was employed. For all
oxo species, we monitored decay of the Soret-band λmax at 422 nm (2a),
418 nm (2b) and 412 nm (2c). Figs. 2 and 3 show typical kinetic results
for reactions of TMP oxo 2a and TPFPP oxo 2c, respectively. As shown in
Figs. 2 and 3, the conversion of RuVI to RuIV is manifested by the decay of
the Soret bands at λmax 422 nm (2a), 412 nm (2c) with concomitant
developmentof a new absorption bandatλmax 406 nm(3a) and 400 nm
(3c) with. Isosbestic points are located at λ=412, 436, 506, 533 nm for
2a and 406, 430, 518, 540 nm for 2c, showing no intermediates
accumulated during the oxidation processes. In the presence of Hpz, the
kinetic profile for the RuVI to RuIV transformation was not changed and
therefore, in this work all of the kinetic studies were conducted in
chloroform without any additives under the condition that
krel = kY = kH = logðYf = YiÞ= logðHf = HiÞ
where Yf and Yi are the final and initial quantities of the substituted
thioanisole; Hf and Hi are the final and initial quantities of thioanisole.
The values reported in Table 3 are the averages of 2–3 runs with a
minor error (b5%). Note that all catalytic reactions gave a minor
amount (b10%) of sulfone products from the over-oxidation of
sulfoxides.
3. Results and discussions
3.1. Stoichiometric sulfoxidations by [RuVI(Por)O2]
We studied the sulfoxidation reactions with three porphyrin
systems that differ in electronic and steric effects of their aryl
substituents, i.e. TMP (a), TPP (b) and TPFPP (c). The synthesis of the
trans-dioxoruthenium(VI) porphyrins was effectively performed
based on known procedures [9,10]. The oxidation of the sterically
encumbered carbonyl ruthenium complex, 1a and 1c, by m-CPBA
yielded the corresponding trans-dioxoruthenium complex, 2a and 2c,
respectively. The dioxoruthenium(VI) complexes are sufficiently
stable for chromatographic purification on an alumina column using
CH2Cl2 as the eluant and the spectroscopically pure complexes [RuVI
(Por)O2] (2a, 2c) are obtained in up to 90% yield. An ethanolic solution
of m-CPBA was used for the preparation of the non-sterically hindered
[RuVI(TPP)O2] (2b), because its coordinating ability to ruthenium
metal can avoid the undesired dimerization process [10]. Fig. 1 shows
UV–vis spectra of the precursors (1) and ruthenium(VI) porphyrin (2)
formed from reactions of the precursors with mCPBA. In each case, the
formed species 2 display a stronger red-shifted Soret and a blue-
shifted weaker Q bands that are characteristic for the corresponding
trans-dioxoruthenium(VI) porphyrins [9,10]. Complexes 2 were also
characterized by well-resolved 1H NMR consistent with a diamagnetic
dx2y electronic ground state. IR spectra of 2 show an absorptions at ca.
820 cm− 1 which are assigned as the O=Ru=O stretches. The
oxidation state of the ruthenium(VI) center is also evidenced by the
oxidation marker band at ca.1018 cm−1[8].
[Ru]≪[sulfide]b1 mmol dm−3
.
Each of the reactions of 2 with the substrates studied in this work
appeared to show first-order behavior with respect to the substrate and
first-order to the oxidant. As shown in the in Figs. 2(B) and 3(C), the
observed pseudo-first-order rate constants varied linearly with the
concentration of substrate. The kinetic results are described by Eq. (1),
where kobs isthe observed rate constant, ko is a background rate constant
found in the absence of added substrate, kox is the second-order rate
constant for reaction with the substrate, and [Sub] is the concentration
of substrate. In a plot of kobs versus the concentration of substrate, the
slope is the second-order rate constant (k2). Second-order rate
constants for reactions of the trans-dioxoruthenium(VI) porphyrins
are listed in Table 1.
kobs = k0 + k2½Subꢀ
ð1Þ
It is noteworthy that the reactions we monitored were the initial
oxidation reactions of the substrates. Secondary reactions were not
important under our time scale (≤600 s) because the oxidation of
substrates by the putative ruthenium(IV)-oxo formed in the reactions
were several orders of magnitude smaller than those by dioxoruthe-
nium(VI) species [9,10]. If a secondary oxidation reaction was
tremendously fast, for example, comparably fast than the initial
oxidation reaction, such that it could compete with the initial
oxidation reaction, then we would have observed apparent deviation
from the first-order exponential decay of the oxo species. In fact, the
reactions displayed pseudo-first-order kinetic behavior for at least
four half-lives (see Fig. 3(B). That conclusion is consistent with the
involvement of ruthenium(IV)-oxo species in the oxygen atom
transfer reactions [13].
The second-order rate constants for reactions of trans-dioxoruthenium
(VI) complexes with a variety of organic sulfides were summarized in
Table 1. The rate constants for the sulfoxidation reactions demonstrate
both electronic and steric effects. With a given thioanisole substrate, the
TPFPP complex (k2=59.6 2.0 M−1s−1) reacted faster than the TPP
complex (k2=48.0 2.0 M−1s−1), which is consistent with the electro-
philic nature of metal-oxo species since the TPPFPP is apparently more
electron demanding. However, the TMP oxo complex reacted least rapidly
with unusually small rate constant (k2=8.0 0.4 M−1s−1). Since the
The [RuVI(Por)O2] complexes 2a-c are competent oxidants for
sulfide oxidations at ambient conditions. In a typical reaction,
thioanisole (2 mmol) in a degassed CDCl3 solution (2 mL) was treated
with 2 (20–30 mg) at room temperature and the mixture was stirred for
5 to10 min. Sulfoxide wasproduced inN90% yield for2a and 2c, and 78%
for 2b. The product yields were determined by 1H-NMR analysis directly
on the crude mixture withdiphenylmethane asinternal standard (ca. 5%
error of stated values). In all stoichiometric reactions, the over-oxidation
product sulfone was not detected (b5%). The current consensus
mechanism is that [RuVI(Por)O2] functions as a two-electron oxidant
in the oxygen atom transfer process with formation of a ruthenium(IV)-
oxo porphyrin complex (Scheme 1) [13]. The absorption spectrum of
the final complex product is consistent with that of the known
ruthenium(IV) porphyrin species [10]. Addition of 5% w/w pyrazole
(Hpz), a donor ligand, to the stoichiometric oxidations caused no