Kinetics and mechanism of the oxygenation of potassium flavonolate. Evidence for an electron transfer mechanism

Article abstract of DOI:10.1021/jo991926w

The oxygenation of the potassium salt of flavonol (flaH) in absolute DMF leads to potassium O-benzoylsalicylate and carbon monoxide in 95% yield at 40 °C. Kinetic measurements resulted in the rate law -d[flaK]/dt = k₂[flaK][O₂]. The rate constant, activation enthalpy, and entropy at 313.16 K are as follows: k₂/M^-1 s^-1 = (3.28 ± 0.10) × 10^-1 ΔH^?/kJ mol^-1 = 29 ± 2, ΔS^?/J mol^-1 K^-1 = -161 ± 6. The reaction fits a Hammett linear free energy relationship for 4′-substituted flavonols, and electron-releasing groups make the oxygenation reaction faster. The anodic oxidation wave potentials E^a of the 4′-substituted flavonolates correlate well with reaction rates. At more negative E^a values faster reaction rates were observed. EPR spectrum of the reaction mixture (g = 2.0038, dH = 1.8 G, a_H = 0.9 G) showed the presence of flavonoxyl radical as a result of a SET from the flavonolate to dioxygen.

Full text of DOI:10.1021/jo991926w

J . Org. Chem. 2000, 65, 3449-3452
3449
Kin etics a n d Mech a n ism of th e Oxygen a tion of P ota ssiu m
F la von ola te. Evid en ce for a n Electr on Tr a n sfer Mech a n ism
†
‡
,†,‡
L a´ szl o´ Barh a´ cs, J o´ zsef Kaizer, and G a´ bor Speier*
Department of Organic Chemistry, University of Veszpr e´ m, 8201 Veszpr e´ m, Hungary, and Research
Group for Petrochemistry of the Hungarian Academy of Sciences, 8201 Veszpr e´ m, Hungary
Received December 16, 1999
The oxygenation of the potassium salt of flavonol (flaH) in absolute DMF leads to potassium
O-benzoylsalicylate and carbon monoxide in 95% yield at 40 °C. Kinetic measurements resulted in
the rate law -d[flaK]/dt ) k
2
-
[flaK][O
2
]. The rate constant, activation enthalpy, and entropy at
1
-1
-1
q
-1
q
-1
-1
3
13.16 K are as follows: k
2
/M
s
) (3.28 ( 0.10) × 10 , ∆H /kJ mol ) 29 ( 2, ∆S /J mol
K
)
-161 ( 6. The reaction fits a Hammett linear free energy relationship for 4′-substituted flavonols,
and electron-releasing groups make the oxygenation reaction faster. The anodic oxidation wave
a
potentials E of the 4′-substituted flavonolates correlate well with reaction rates. At more negative
a
E values faster reaction rates were observed. EPR spectrum of the reaction mixture (g ) 2.0038,
dH ) 1.8 G, a
flavonolate to dioxygen.
H
) 0.9 G) showed the presence of flavonoxyl radical as a result of a SET from the
In tr od u ction
Sch em e 1
Biological oxygenation of natural substances by oxy-
genases are an important part of metabolic processes.1-8
The structure and mechanism of many of these enzymes
are still obscure, and only model reactions serve with
some guidance so far as their chemistry is concerned.9,10
The oxygenation of quercetin (1a ) and 3-hydroxyflavones
(flavonols, flaH, 1b) catalyzed by quercetin 2,3-dioxyge-
nase leads to oxidative cleavage of the heterocyclic ring
2
0-30
metalloenzyme, metal complexes with copper
and
to give the corresponding depsides (2) and carbon mon-
cobalt3
1-34
have been used in model reactions. Autoxida-
tion reactions of 3-hydroxyflavones in protic and apro-
tic solvents also resulted in enzyme-like products, and
some efforts have also been made to elucidate the
mechanism of the reaction. However, no kinetic studies
oxide (Scheme 1).1
1-19
Since it is a copper-containing
3
5
3
6
*
To whom correspondence should be addressed. Tel.: +36 88 422
0
22/4657. Fax: +36 88 427 492. E-mail: speier@almos.vein.hu.
†
University of Veszpr e´ m.
Research Group for Petrochemistry of the Hungarian Academy of
‡
Sciences.
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1
0.1021/jo991926w CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/29/2000

3
450 J . Org. Chem., Vol. 65, No. 11, 2000
Barh a´ cs et al.
were carried out. Attempts to detect or isolate intermedi-
ates of the oxygenation reaction have failed. We report
here kinetic studies on the oxygenation of potassium
flavonolates in DMF and EPR investigations on possible
intermediates during the reaction.
Resu lts a n d Discu ssion
Oxygen a tion of P ota ssiu m F la von ola te a n d EP R
Stu d ies. The potassium salts of flavonol and 4′-substi-
tuted flavonols (3b-f) were prepared in THF by reacting
metallic potassium with the flavonols (1). The salts were
characterized by elemental analysis, IR spectra, and UV-
vis spectroscopy. In the salts, the CdO stretching fre-
3
7
quencies shift to lower wavenumbers by around 50
F igu r e 1. EPR spectrum of a DMF solution of flaK after
exposure to dioxygen (the two intense signals are from the
Mn(II) standard).
cm , and the π - π* transition of the heterocyclic ring38
undergoes bathochromic shifts up to 125 nm. The data
are listed in the Experimental Section. Bulk oxygenation
of the potassium salt of flavonol (3b) in DMF at 50 °C
resulted in the formation of the potassium salt of the
depside O-benzoylsalicylate (4b) in 76% yield. The forma-
tion of 4b from 3b requires dioxygen, which was mea-
sured by volumetry (no apparent dioxygen uptake was
observed, because the absorption of oxygen and the
liberation of carbon monoxide compensate each other).
The GC analysis of the gas phase showed 0.87 mmol of
CO (87%) evolved/1 mmol 3b.
-1
F igu r e 2. Time course of the oxygenation of potassium
flavonolate.
In DMF, the intense red color of the solution of 3b
turns to intense green on coming into contact with
dioxygen. The UV-vis spectrum of the solution changes
and a new intense band arises at 676 nm. This absorption
substrate flaK is one. This could be also confirmed by
the straight line obtained by plotting the reaction rate
against the initial flaK concentration (Supporting Infor-
mation, SFigure 1). The first-order dependence of the
reaction rate on the dioxygen concentration was estab-
lished by plotting the reaction rate against the O
concentration (Supporting Information, SFigure 2). On
that basis, the rate equation is as follows (2):
3
9-42
has been assigned to the flavonoxy radical
formed
in a single electron transfer from the flavonolate anion
to dioxygen with the concomitant formation of superoxide
anion.43 The presence of the flavonoxy radical could also
be confirmed by EPR spectroscopy. A weak EPR signal
appeared in the solution of the flavonolate exposing to
dioxygen at g ) 2.0038, line broadness dH ) 1.8 G, and
2
reaction rate ) k [flaK] [O ]
(2)
2
2
coupling parameter of a
H
) 0.9 G (Figure 1).
The rate constant at 313.16 K was found to be k
2
)
-
1
-1 -1
Kin etic Mea su r em en ts. The kinetic measurements
on the oxygenation of potassium flavonolate (flaK) were
followed by UV-vis spectroscopy at 465.5 nm at constant
(3.28 ( 0.10) × 10
M
s
(Supporting Information,
STable 1). From the temperature dependence of the
reaction rates (Supporting Information, SFigure 3), the
4
4
q
-1
q
dioxygen pressure under pseudo-first-order conditions.
A typical concentration time profile can be seen in Figure
, indicating that the reaction order with respect to the
activation parameters ∆H ) 29 ( 2 kJ mol , ∆S )
1
-
-1
-161 ( 6 J mol
K
could be calculated. Addition of
2
tetrabutylammonium perchlorate to flavonol (1b) in an
equimolar amount accelerated the reaction rate by one-
-
1
-1 -1
(37) Bellamy, L. M. Ultrarot Spektrum und Chemisce Konstitution;
third (k
2
) 5.18 ( 0.16) × 10
M
s ) due to onium
Dr. Dietrich Steinkopf Verlag: Darmstadt, 1966; p 112.
45
catalysis of better ion separation.
(
38) J urd, L.; Geisssman, T. X. J . Org. Chem. 1956, 21, 1395.
The influence of the 4′-substituted groups on the
reaction rate of the oxygenation showed a linear Ham-
mett plot with a reaction constant of F ) -0.73 (Figure
(39) Itoh, S.; Taki, M.; Takayama, S.; Nagatomo, S.; Kitagawa, T.;
Sakurada, N.; Arakawa, R.; Fukuzimi, S. Angew. Chem., Int. Ed. Engl.
1
999, 38, 2774.
(40) White, L. S.; Que, L., J r. J . Mol. Catal. 1985, 33, 139.
3
; Supporting Information, STable 2), indicating that
(41) White, L. S.; Hellman, E. J .; Que, L., J r. J . Org. Chem. 1982,
4
7, 3766.
electron-releasing groups result in enhanced reactions
rates.
(42) Muto, S.; Bruce, T. C. J . Am. Chem. Soc. 1980, 102, 4472.
43) Winterbourn, C. C.; Hawkins, R. S.; Brian, M.; Carrel, R. W.
(
J . Lab. Clin. Med. 1975, 85, 337.
44) Espenson, J . H. Chemical Kinetics and Reaction Mechanism,
nd ed.; McGraw-Hill: New York, 1995.
(
(45) Starks, C. M.; Liotta, C. Phase Transfer Catalysis; Academic
Press: New York, 1978; p 29.
2

Oxygenation of Potassium Flavonolate
J . Org. Chem., Vol. 65, No. 11, 2000 3451
Sch em e 2
F igu r e 3. Hammett plot of the oxygenation of potassium
flavonolate.
ions,50 and flavonolates are more negative than that of
O . The stability of the intermediate radicals depends on
2
their delocalization of the unpaired electron. Due to that,
the phenoxy radicals exhibit higher stability, while the
stability of the flavonoxy radicals is low. The stability
depends also on the solvents, and DMF was found to be
the most appropriate one. The weak signal shows only a
poor hyperfine structure due to either one or three
equivalent hydrogen atoms. From the data, it can be
5
1
concluded that it is not to assign to superoxide, and it
is not a carbon-centered radical52 (the g value is higher
than 2.003). We assume a structure where the unpaired
electron resides mostly on the oxygen and to a less extent
on the carbon atoms.
F igu r e 4. Reaction rate vs anodic oxidation potentials profile
of various 4′-substituted potassium flavonolates.
The second-order rate expression suggests that the
rate-determining step is the reaction between the fla-
vonolate ion and dioxygen. In the first step, the flavonoxy
radical and superoxide ion are formed. The reaction is
entropy driven, and the Hammett plot shows that a
higher electron density on the flavonolate ion leads to
faster reaction rates. This is also valid for the redox
properties of the flavonolates; electron-releasing groups
in the 4′-position increase the negative electrode potential
of the flavonolates, which may be correlated with their
participating HOMO orbitals in a similar manner as
The electrochemistry of the potassium flavonolates
(3b-f) in DMF showed irreversible anodic oxidation
waves (Supporting Information, SFigures 4-8). The data
shown in SFigures 4-8 (Supporting Information) and
compiled in STable 3 (Supporting Information) make
clear that flavonolates with electron-releasing groups
have a more negative anodic electron potential. The
irreversibility of the redox peaks is due to the relatively
unstable flavonoxy radical.33 The linear correlation be-
tween the anodic oxidation potential and the reaction rate
supports that a higher electron density on the flavonolate
makes the electron transfer easier (Figure 4). The kinetic
and the EPR data on the oxygenation reactions of the
potassium salts of flavonolates show similar features as
those of phenolates46 and carbanions. Since the reducing
power of these anions is high and a direct electron-
transfer results in radicals, from which the phenoxy
radicals show a relative good stability while those formed
from carbanions and flavonolates are less so. Despite the
fact that the first electron reduction of dioxygen to
54
found by the catechols. The linear correlation between
the anodic wave of the CV spectra and the reaction rates
of the various 4′-substituted flavonolates are a good proof
for that. All the consecutive reactions of the flavonoxy
radical and superoxide ion are shown in Scheme 2. In a
radical-radical coupling reaction, the flavonoxy radical
and the superoxide ion react with the deprotonated
peroxidic species 7. This undergoes then an intramolecu-
lar nucleophilic attack at the 4-CdO position to give the
47
(49) Mihailovic, M. Lj.; Cekovic, Z. In The Chemistry of the Hydroxyl
superoxide ion is unfavorable (E° ) -0.62 V vs NHE in
Group; Patai, S., Ed.; Wiley: New York, 1971; Part 1, pp 505-592.
(50) Eberson, L. Electron-Transfer Reactions in Organic Chemistry;
Springer-Verlag: Berlin, 1987; p 39.
DMF),48 the reduction potentials of phenolates,49 carban-
(
51) Astruc, D.; Hamon, J .-R.; Roman, E.; Michaud, P. J . Am. Chem.
(
46) March, J . Advanced Organic Chemistry, 4th ed.; Wiley-Inter-
science: New York, 1992; p 1160.
47) Bordwell, F. G.; Bausch, M. J . J . Am. Chem. Soc. 1986, 108,
985.
48) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New
York, 1991; p 34.
Soc. 1981, 103, 7502.
(52) Gerson, F. High Resolution E.S.R. Spectroscopy; J ohn Wiley &
Sons Ltd: London, 1970.
(53) Peover, M. E. J . Chem. Soc. 1962, 4540.
(54) Clennan, E. L.; Foote, C. S. In Organic Peroxides; Ando, W.,
Ed.; Wiley: Chichester, 1992; p 225.
(
1
(

3
452 J . Org. Chem., Vol. 65, No. 11, 2000
Barh a´ cs et al.
°C; IR (KBr) 1593, 1567 cm-1; UV-vis (DMF) (λmax, log ꢀ) 464.5
endoperoxide 8, which after fast decomposition and
release of carbon monoxide results in O-benzoylsalicylate
(
8
4.003), 292.5 (3.845), 265.0 (3.952). Anal. Calcd for C15H -
ClO
3
K: C, 57.97; H, 2.59. Found: C, 57.32; H, 2.28. 4′-CN-
(4). All attempts to isolate or detect the endoperoxide 8
fla K (3f) (1.13 g, 75%): mp 218-22 °C; IR (KBr) 2225, 1596,
failed, suggesting that this type of endoperoxides (8) is
very unstable if compared to other endoperoxides54 where
the elimination of CO is not easily feasible. In the case
of 8, the homolytic cleavage of the O-O bond and
simultaneous elimination of CO seems to be very facile.
Until now, no endoperoxides of this general stucture (9)
could be isolated or characterized.
-
1
1
(
573 cm ; UV-vis (DMF) (λmax, log ꢀ) 486.5 (3.811), 296.0
3.636), 267 (3.834). Anal. Calcd for C16 NO K: C, 63.77; H,
H
8
3
2
.68; N, 4.65. Found: C, 63.32; H, 2.78; N, 4.25.
Oxygen a tion of P ota ssiu m F la von ola te (3b). FlaK
(0.476 g, 2 mmol) in DMF (10 mL) was treated with dioxygen
(0.1 MPa) at 50 °C for 8 h, yielding a green solution. The
mixture was poured into ice-cooled dilute hydrochloric acid and
extracted with ether. Unchanged 1b was precipitated (0.11 g,
2
3%). After removal of 1b, the extract was dried (Na
2 4
SO ) and
evaporated to give 2b as crystals (0.37 g, 76%): mp 131-33
-
1
°
C; IR (Nujol) 1739, 1700 cm ; The GC-MS analysis of 2b,
after treatment with etheral diazomethane, shows the pres-
ence of O-benzoylsalicylic acid methyl ester: GC-MS m/z 256
+
(
M , 3), 225 (1), 105 (100). The GC analysis of the gas phase
From the results obtained, the conclusion may be
drawn that the oxygenolysis of the flavonolate ion in
shows the presence of 1.73 mmol of CO (87%).
The other 4′-substituted flavonolate salts of potassium were
reacted in an identical manner. 2c (0.49 g, 90%): mp 141-
aprotic solvents has a single electron transfer (SET)
-
1
mechanism55 and it obeys the second-order rate equation
143 °C; IR (Nujol) 3216, 2835, 1740, 1695 cm
; GC-MS m/z
+
2
°
86 (M , 4), 255 (1), 135 (100). 2d (0.44 g, 85%): mp 134-137
followed by a fast radical-radical coupling reaction of the
flavonoxy radical and superoxide ion to give a deproto-
nated hydroperoxide species, which after an intra-
-
1
+
C; IR (Nujol) 2864, 1737, 1684 cm ; GC-MS m/z 270 (M ,
5
(
), 239 (2), 119 (100). 2e (0.33 g, 60%): mp 123-125 °C; IR
-1
+
Nujol) 1743, 1687 cm ; GC-MS m/z 290 (M , 8), 259 (4), 139
molecular A
N
reaction on 4CdO leads to an unstable
(100). 2f (0.16 g, 30%): mp 118-122 °C; IR (Nujol) 2227, 1720,
-
1
+
endoperoxide of the structure 8, and its decomposition
results then in the O-benzoylsalicylate and carbon mon-
oxide. Further work is in progress to isolate endoperox-
ides of the type 9 for studies of their thermal decompo-
sition in order to understand the chemistry of this
unusual reaction.
1707 cm ; GC-MS m/z 281 (M , 11), 208 (6), 139 (100).
In str u m en ta tion . Infrared spectra and electronic spectra
-1
were recorded as λmax (cm and nm, respectively). GC analyses
were performed with a flame-ionization and TCD detector with
a CP SIL 8CB and molecular sieve 5A column. GC-MS
measurements were recorded at 75 eV. CV measurements
were carried out with a Pt working electrode in DMF solution
-
3
4 4
at room temperature with ca. 10 M solutions, NBu ClO as
Exp er im en ta l Section
-1
supporting electrolyte, and scan rate 100 mV s . The potential
values are relative to the NHE using an Ag/AgCl reference
electrode. Microanalyses were carried out by Microanalytical
Service of the University of Veszpr e´ m.
Ma ter ia ls a n d Meth od s. All manipulations were per-
formed under a pure dinitrogen or argon atmosphere unless
otherwise stated using standard Schlenk-type inert gas tech-
5
6
Kin etic Mea su r em en ts. Reactions of flaK with O2 were
performed in DMF solutions. In a typical experiment, flaK was
dissolved under argon atmosphere in a thermostated reaction
vessel with an inlet for taking samples with a syringe and
connected to a mercury manometer to regulate constant
pressure. The solution was then heated to the appropriate
temperature. A sample was then taken by syringe, and the
initial concentration of flaK was determined by UV-vis
spectroscopy measuring the absorbance of the reaction mixture
at 465.5 nm (log ꢀ ) 4.186) [λmax of a typical band of flaK].
The argon was then replaced with dioxygen, and the consump-
tion of flaK was analyzed periodically (ca. every 5 min).
Experimental conditions are summarized in STable 1 (Sup-
porting Information). The temperature was determined with
an accuracy of (0.5 °C; the concentrations of KFla were
measured with a relative mean error of ca. (2%; the pressure
of dioxygen was determined with an accuracy of (0.5%. The
niques. Solvents used for the reactions were purified by
5
7
literature methods and stored under argon. 3-Hydroxyfla-
3
5
58
vone, 3-hydroxy-4′-methoxyflavone, 3-hydroxy-4′-methylfla-
5
8
58
vone, 3-hydroxy-4′-chloroflavone, and 3-hydroxy-4′-cyanofla-
5
8
vone were prepared according to literature methods. Diazo-
methane was freshly prepared according to the literature in
ether and immediately used for the methylation reactions.59
P ota ssiu m F la von ola te (fla K, 3b). Potassium (0.195 g, 5
mmol) and flavonol (1.119 g, 5 mmol) in anhydrous tetrahy-
drofuran (50 mL) were stirred under argon for 5 h until
dihydrogen evolution ceased. The orange-colored precipitate
was filtered, washed with ether, and dried in a vacuum to give
the potassium flavonolate (3b) (1.24, 90%): mp 229-32 °C;
-
1
IR (KBr) 1592, 1556 cm ; UV-vis (DMF) (λmax, log ꢀ) 465.5
4.186), 293.5 (2.995), 261.0 (3.061), 240.0 (3.111). Anal. Calcd
for C15 K: C, 65.45; H, 3.27. Found C, 65.78; H, 2.98.
(
9 3
H O
The other 4′-substituted flavonolate salts of potassium were
prepared in an identical manner. 4′-OMe-fla K (3c) (1.38 g,
6
0
2
O concentration was calculated from literature data taking
6
1
0%): mp 210-13 °C; IR (KBr) 3214, 2837, 1593, 1565 cm-1
into account the partial pressure of DMF and assuming the
validity of Dalton’s law.
9
;
UV-vis (DMF) (λmax, log ꢀ) 464.5 (4.082), 302.0 (3.964), 264.0
(
4.089), 240.0 (3.111). Anal. Calcd for C16
H, 3.62. Found: C, 63.12; H, 3.59. 4′-Me-fla K (3d ) (1.24 g,
5%): mp 208-210 °C; IR (KBr) 2866, 1593, 1567 cm-1; UV-
vis (DMF) (λmax, log ꢀ) 464.5 (4.085), 297.3 (3.951), 263.0
4.019). Anal. Calcd for C16 K: C, 66.19, H, 3.82. Found:
C, 65.83, H, 3.52. 4′Cl-fla K (3e) (1.24 g, 80%): mp 214-18
11 4
H O K: C, 62.73;
Ack n ow led gm en t. The authors thank Dr. Antal
Rockenbauer for help with EPR measurements and
Professor L a´ szl o´ Tomcs a´ nyi for CV facilities and as-
sistance. This work was supported by the Hungarian
Research Fund (OTKA nos. T16285 and T30400).
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Su p p or tin g In for m a tion Ava ila ble: Kinetic diagrams,
cyclic voltammograms, and tables of kinetic data and anodic
potentials for 3b-f. This material is available free of charge
via the Internet at http://pubs.acs.org.
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