Mechanisms of hydrogen-, oxygen-, and electron-transfer reactions of cumylperoxyl radical

Article abstract of DOI:10.1021/ja035156o

Rates of hydrogen-transfer reactions from a series of para-substituted N,N-dimethylanilines to cumylperoxyl radical and oxygen-transfer reactions from cumylperoxyl radical to a series of sulfides and phosphines have been determined in propionitrile (EtCN) and pentane at low temperatures by use of ESR. The observed rate constants exhibit first-order and second-order dependence with respect to concentrations of N,N-dimethylanilines. This indicates that the hydrogen- and oxygen-transfer reactions proceed via 1:1 charge-transfer (CT) complexes formed between the substrates and cumylperoxyl radical. The primary kinetic isotope effects are determined by comparing the rates of N,N-dimethylanilines and the corresponding N,N-bis(trideuteriomethyl)anilines. The isotope effect profiles are quite different from those reported for the P-450 model oxidation of the same series of substrates. Rates of electron-transfer reactions from ferrocene derivatives to cumylperoxyl radical have also been determined by use of ESR. The catalytic effects of Sc(OTf)₃ (OTf = triflate) on the electron-transfer reactions are compared with those of Sc(OTf)₃ on the hydrogen- and oxygen-transfer reactions. Such comparison provides strong evidence that the hydrogen- and oxygen- transfer reactions of cumylperoxyl radical proceed via a one-step hydrogen atom and oxygen atom transfer rather than via an electron transfer from substrates to cumylperoxyl radical.

Full text of DOI:10.1021/ja035156o

Published on Web 07/02/2003
Mechanisms of Hydrogen-, Oxygen-, and Electron-Transfer
Reactions of Cumylperoxyl Radical
Shunichi Fukuzumi,*,† Kanji Shimoosako,† Tomoyoshi Suenobu,† and
Yoshihito Watanabe‡
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation (JST),
Suita, Osaka 565-0871, Japan, and Department of Chemistry, Graduate School of Science,
Nagoya UniVersity, Chikusa-ku, Nagoya 464-8602, Japan
Received March 14, 2003; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: Rates of hydrogen-transfer reactions from a series of para-substituted N,N-dimethylanilines to
cumylperoxyl radical and oxygen-transfer reactions from cumylperoxyl radical to a series of sulfides and
phosphines have been determined in propionitrile (EtCN) and pentane at low temperatures by use of ESR.
The observed rate constants exhibit first-order and second-order dependence with respect to concentrations
of N,N-dimethylanilines. This indicates that the hydrogen- and oxygen-transfer reactions proceed via 1:1
charge-transfer (CT) complexes formed between the substrates and cumylperoxyl radical. The primary
kinetic isotope effects are determined by comparing the rates of N,N-dimethylanilines and the corresponding
N,N-bis(trideuteriomethyl)anilines. The isotope effect profiles are quite different from those reported for
the P-450 model oxidation of the same series of substrates. Rates of electron-transfer reactions from
ferrocene derivatives to cumylperoxyl radical have also been determined by use of ESR. The catalytic
effects of Sc(OTf)3 (OTf ) triflate) on the electron-transfer reactions are compared with those of Sc(OTf)3
on the hydrogen- and oxygen-transfer reactions. Such comparison provides strong evidence that the
hydrogen- and oxygen- transfer reactions of cumylperoxyl radical proceed via a one-step hydrogen atom
and oxygen atom transfer rather than via an electron transfer from substrates to cumylperoxyl radical.
Introduction
possibilities in the mechanisms of the hydrogen-transfer reac-
tions, i.e., a one-step hydrogen atom transfer or electron transfer
Hydrogen transfer is a common mechanism of organic free
radical chemistry, playing an important role in the oxidation
and halogenation of hydrocarbons1 and also in enzymatic
oxidation.2 The hydrogen-transfer reactions from amines to
alkoxyl3 and peroxyl radicals4 have attracted special attention
in relation to the enzymatic mechanism for the oxidative
dealkylation of amines by cytochrome P-450.5-11 There are two
followed by proton transfer.5-12 The mechanism of oxygen-
transfer reactions from the active oxygen intermediates of P-450
to versatile organic compounds such as sulfides and phosphines
has also been studied extensively.11,13,14 In the case of oxygen-
transfer reactions as well, a mechanistic dichotomy whether the
oxygen transfer occurs via a one-step direct oxygen atom transfer
or via electron transfer has been discussed.14 In this context,
we have previously reported that the effects of metal ions
provide a reliable criterion for distinguishing between the one-
step hydrogen-transfer and electron-transfer mechanisms in
† Osaka University.
‡ Nagoya University.
(1) (a) Free Radicals; Kochi, J. K. Ed.; Wiley: New York, 1973. (b) Perkins,
M. J. Free Radical Chemistry; Ellis Horwood: New York, 1994.
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(b) Jordan, A.; Reichard, P. Annu. ReV. Biochem. 1998, 67, 71. (c)
Whittaker, J. W. AdV. Protein Chem. 2002, 60, 1. (d) Itoh, S.; Taki, M.;
Fukuzumi, S. Coord. Chem. ReV. 2000, 198, 3. (e) Chaudhuri, P.;
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also two mechanisms: hydrogen atom transfer and proton-coupled electron
transfer. The precise definition of proton-coupled electron transfer has yet
to be widely accepted, but this term is often applied to mechanisms in
which the proton and electron are transferred between different sets of
orbitals. See: (a) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem.
1998, 49, 337. (b) Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31,
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2002, 124, 11142.
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9
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J. AM. CHEM. SOC. 2003, 125, 9074-9082
10.1021/ja035156o CCC: $25.00 © 2003 American Chemical Society

Transfer Reactions of Cumylperoxyl Radical
A R T I C L E S
temperatures. The g value was calibrated by using an Mn2+ marker.
Upon cutoff of the light, the decay of the ESR intensity was recorded
with time. The decay rate was accelerated by the presence of DMA
(1.0 × 10-2 M). Rates of hydrogen transfer from para-substituted N,N-
dimethylanilines (DMAs) to PhCMe2OO• were monitored by measuring
the decay of ESR signal of PhCMe2OO• in the presence of various
concentrations of DMAs in EtCN and pentane at 193 K. Pseudo-first-
order rate constants were determined by a least-squares curve fit using
a Macintosh personal computer. The first-order plots of ln(I - I∞) versus
time (I and I∞ are the ESR intensity at time t and the final intensity,
respectively) were linear for three or more half-lives with the correlation
coefficient F > 0.99. In each case, it was confirmed that the rate
constants determined from at least five independent measurements
agreed within an experimental error of (5%. The primary kinetic
isotope effects were determined by determining the rates of hydrogen
transfer from DMA or DMA-(CD3)2 to PhCMe2OO• at various
temperatures. Rates of electron transfer from ferrocene derivatives to
PhCMe2OO• were also monitored by measuring the decay of ESR signal
of PhCMe2OO• in the presence of various concentrations of ferrocene
derivatives in EtCN at 193 K. The effects of metal ions on the electron-
transfer reactions were examined by determining the decay rates of
PhCMe2OO• in the presence of ferrocene derivatives and metal ions.
Reaction Procedure. Typically, triphenylphosphine (Ph3P) (7.0 ×
10-2 M) was added to the 1-cm UV cell that contained an O2-saturated
CD3CN solution of di-tert-butyl peroxide (1.0 × 10-1 M) and cumene
(1.0 × 10-1 M) at room temperature and was photoirradiated for 15
min. After photoirradiation, the product was identified as triphenylphos-
hydrogen-transfer reactions of radical species.15 However, there
has been no report on the effects of metal ions on the electron-
transfer reactions of alkoxyl and peroxyl radicals. Direct
measurements of the rates of alkoxyl radicals by ESR have not
been possible because of the extremely short lifetimes of the
radicals.16
We report herein comprehensive studies on determination of
the absolute rates of hydrogen-transfer reactions from a series
of para-substituted N,N-dimethylanilines to cumylperoxyl radi-
cal and those of oxygen transfer from cumylperoxyl radical to
sulfides and phosphines by use of ESR at low temperatures.
Cumylperoxyl radical, while much less reactive than alkoxyl
radicals, is known to follow the same pattern of relative
reactivity with a variety of substrates.17,18 The detailed kinetic
study provides valuable insight into the hydrogen- and oxygen-
transfer mechanisms. Electron-transfer rates from ferrocene
derivatives to cumylperoxyl radical have also been determined
for the first time at low temperatures by ESR. The resulting
data are evaluated in light of the Marcus theory of electron
transfer19 to determine the reorganization energy of electron
transfer of cumylperoxyl radical. The effects of metal ions on
the electron-, hydrogen-, and oxygen-transfer reactions of
cumylperoxyl radical are compared in order to distinguish
between a one-step hydrogen atom or oxygen atom transfer
mechanism and an electron-transfer mechanism.
1
phine oxide (Ph3PdO) by comparing the H NMR spectra with those
of authentic samples. The 1H NMR measurements were performed with
Japan Electron Optics JNM-GSX-400 (400 MHz) NMR spectrometers.
1H NMR (CD3CN, 298 K) δ (Me4Si, ppm): Ph3PdO δ 7.5-7.7 (m,
15H); MePhSdO δ 2.7 (s, 3H), 7.5-7.7 (m, 5H).
Experimental Section
Materials. Di-tert-butyl peroxide was purchased from Nacalai
Tesque Co., Ltd. and purified by chromatography through alumina,
which removes traces of the hydroperoxide. Cumene was purchased
from Tokyo Kasei Co., Ltd. para-Substituted N,N-dimethylanilines
(DMAs) [N,N-dimethylaniline, DMA; p-methoxy-N,N-dimethylaniline,
MeO-DMA; p-methyl-N,N-dimethylaniline, Me-DMA; p-bromo-N,N-
dimethylaniline, Br-DMA; p-cyano-N,N-dimethylaniline, CN-DMA]
were also commercially available and purified by the standard
procedure.20 Deuterated compounds [N,N-bis(trideuteriomethyl)aniline,
DMA-(CD3)2; p-methyl-N,N-bis(trideuteriomethyl)aniline, Me-DMA-
(CD3)2; p-methoxy-N,N-bis(trideuteriomethyl)aniline, MeO-DMA-
(CD3)2] were prepared according to the literature.21 para-Substituted
thioanisole derivatives [thioanisole, TA; p-methoxythioanisole, MeO-
TA; p-methylthioanisole, Me-TA; p-chlorothioanisole, Cl-TA], dialkyl
sulfides, diphenyl sulfide, and phosphines were purchased from Aldrich
Co., Ltd. or Tokyo Kasei Co., Ltd. Ferrocene derivatives were purchased
from Aldrich Co., Ltd. Pentane and propionitrile (EtCN) used as
solvents were purified and dried by the standard procedure.20
Electrochemical Measurements. Cyclic voltammetry (CV) mea-
surements of DMAs, sulfides, and phosphines were performed on a
BAS 100B electrochemical analyzer in deaerated MeCN containing
-
0.10 M n-Bu4N+PF6 (TBAPF6) as a supporting electrolyte at 298 K.
The CV of PhCMe2OO• was measured in deaerated EtCN containing
0.10 M TBAPF6 as a supporting electrolyte, PhCMe2OOH (2.0 mM),
and n-Bu4NOH (2.0 mM) as a base at 198 K. The second harmonic ac
voltammetry (SHACV)22 measurements of sulfides and phosphines were
also performed on a BAS 100B electrochemical analyzer in deaerated
MeCN containing 0.10 M TBAPF6 as a supporting electrolyte at 298
K. The platinum working electrode was polished with BAS polishing
alumina suspension and rinsed with acetone before use. The counter
electrode was a platinum wire. The measured potentials were recorded
with respect to an Ag/AgNO3 (0.01 M) reference electrode. The E0
ox
values (vs Ag/Ag+) are converted to those vs SCE by adding 0.29 V.23
Theoretical Calculations. The theoretical studies were performed
using the PM3 molecular orbital method.24 The calculations were
performed by using the MOL-MOLIS program ver. 2.8 by Daikin
Industries, Ltd. Final geometries and energetics were obtained by
optimizing the total molecular energy with respect to all structural
variables. The geometries of the radicals were optimized using the
unrestricted Hartree-Fock (UHF) formalism. The ∆Hf values of the
radicals were calculated with the UHF-optimized structures using the
half-electron (HE) method with the restricted Hartree-Fock (RHF)
formalism.25 Density-functional theory (DFT) calculations were per-
Kinetic Measurements. Kinetic measurements were performed on
a JEOL X-band ESR spectrometer (JES-ME-LX) at low temperatures
(193-233 K). Typically, photoirradiation of an oxygen-saturated
propionitrile solution containing di-tert-butyl peroxide (1.0 M) and
cumene (1.0 M) with a 1000-W mercury lamp resulted in formation of
cumylperoxyl radical (g ) 2.0156), which could be detected at low
(15) (a) Fukuzumi, S.; Tokuda, Y.; Chiba, Y.; Greci, L.; Carloni, P.; Damiani,
E. J. Chem. Soc., Chem. Commun. 1993, 1575. (b) Nakanishi, I.; Miyazaki,
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S.; Fukuhara, K. J. Phys. Chem. A 2002, 106, 11123.
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Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. Soc. 1978, 100, 296.
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Butterworth-Heinemann: Oxford, 1998.
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7111.
(22) The SHACV method provides a superior approach for directly evaluating
the one-electron redox potentials in the presence of a follow-up chemical
reaction, relative to the better-known dc and fundamental harmonic ac
methods: (a) Bond, A. M.; Smith, D. E. Anal. Chem. 1974, 46, 1946. (b)
Arnett, E. M.; Amarnath, K.; Harvey, N. G.; Cheng, J.-P. J. Am. Chem.
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Systems; Marcel Dekker: New York, 1970.
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1985; p 97.
9
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A R T I C L E S
Fukuzumi et al.
formed on a COMPAQ DS20E computer. Geometry optimizations were
carried out using the B3LYP functional and 6-31G* basis set26,27 as
implemented in the Gaussian 98 program.28 The geometries of the
radicals were optimized using the unrestricted Hartree-Fock (UHF)
formalism. At the stationary point a vibrational frequency analysis was
performed to calculate the contribution of the zero-point vibrational
energy (ZPE). The ZPEs from these frequency calculations were scaled
by a factor of 0.980 to correct for the anharmonicity in the vibrational
potential energy surface, the limited basis set, and the missing
correlation effects.29,30 The enthalpy changes of the hydrogen-transfer
reactions (∆∆Hf) were calculated from the difference in the ZPE
corrected total energies of each reactants and products according to
the literature.31
Results and Discussion
Hydrogen Transfer from DMAs to Cumylperoxyl Radical.
Direct measurements of the rates of hydrogen transfer from a
series of para-substituted N,N-dimethylanilines (DMAs) to
cumylperoxyl radical were performed in propionitrile (EtCN)
at various temperatures by means of ESR. The photoirradiation
of an oxygen-saturated EtCN solution containing di-tert-
butylperoxide (ButOOBut) and cumene with a 1000-W mercury
lamp results in formation of cumylperoxyl radical, which was
readily detected by ESR as shown in Figure 1. The cumylper-
oxyl radical is formed via a radical chain process shown in
Scheme 1.32,33 The photoirradiation of ButOOBut results in the
homolytic cleavage of the O-O bond to produce ButO•,34,35
which abstracts a hydrogen from cumene to give cumyl radical,
followed by the facile addition of oxygen to cumyl radical. The
cumylperoxyl radical can also abstract a hydrogen atom from
cumene in the propagation step to yield cumene hydroperoxide,
accompanied by regeneration of cumyl radical (Scheme 1).36
In the termination step, cumylperoxyl radicals decay by a
Figure 1. (a) ESR signal observed in photolysis of an O2-saturated EtCN
solution of di-tert-butyl peroxide (1.0 M) and cumene (1.0 M) with a
1000-W mercury lamp at 193 K. (b) The decay profile of the ESR intensity
upon cutting off the light in O2-saturated EtCN at 193 K. Inset: second-
order plot.
Scheme 1
(26) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.;
Parr, R. G. Phys. ReV. B 1988, 37, 785.
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A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
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P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
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(32) (a) Sheldon, R. A. In The ActiVation of Dioxygen and Homogeneous
Catalytic Oxidation; Barton, D. H. R., Martell, A. E., Sawyer, D. T., Eds.;
Plenum: New York and London, 1933; pp 9-30. (b) Parshall, G. W.; Ittel,
S. D. Homogeneous Catalysis, 2nd ed.; Wiley: New York, 1992; Chapter
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bimolecular reaction to yield the corresponding peroxide and
oxygen (Scheme 1).4 When the light is cut off, the ESR signal
intensity decays obeying second-order kinetics due to the
bimolecular reaction in Scheme 1 as shown in Figure 1 (see
the inset for the second-order plot for the decay of the ESR
intensity of cumylperoxyl radicals).4
In the presence of DMA, the decay rate of cumylperoxyl
radical after cutting off the light becomes much faster than that
in the absence of DMA as shown in Figure 2. The decay rate
in the presence of DMA (1.0 × 10-2 M) obeys pseudo-first-
order kinetics rather than second-order kinetics (see the inset
of Figure 2 for the first-order plot). Thus, this decay process is
(36) (a) Fukuzumi, S.; Ono, Y. J. Chem. Soc., Perkin Trans. 2 1977, 622. (b)
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Transfer Reactions of Cumylperoxyl Radical
A R T I C L E S
Figure 2. The decay profile of the ESR signal intensity due to PhCMe2-
OO• in the presence of DMA (1.0 × 10-2 M) in O2-saturated EtCN at 193
K. Inset: pseudo-first-order plot.
Figure 4. Arrhenius plots of kobs for hydrogen transfer from DMA and
DMA-(CD3)2 to PhCMe2OO• in O2-saturated EtCN.
Figure 3. Plots of (a) kobs versus [DMA] and (b) kobs versus [DMA-(CD3)2]
for hydrogen transfer from DMA and DMA-(CD3)2 to PhCMe2OO• in O2-
saturated EtCN at 193 K.
ascribed to the hydrogen transfer from DMA to cumylperoxyl
Figure 5. Plots of (a) kobs/[DMA] versus [DMA] and (b) kobs/[DMA-(CD3)2]
versus [DMA-(CD3)2] for hydrogen transfer from DMA or DMA-(CD3)2
to PhCMe2OO• in O2-saturated EtCN at 193 K.
radical (Scheme 1). The pseudo-first-order rate constants (kobs
)
increase with an increase in [DMA] to exhibit first-order
dependence on [DMA] at low concentrations, changing to
second-order dependence at high concentrations as shown in
Figure 3a. Such a first-order and second-order dependence of
The first-order and second-order dependence of kobs on
[DMA] can be separated by a linear correlation between
k
obs on substrate concentration is often observed in a variety of
k
obs/[DMA] and [DMA] as given by eq 1. The linear plot of
electrophilic reactions such as bromination of electron donors
(olefins, alkylbenzenes and organometals), which proceeds via
charge-transfer complexes formed between electron donors and
electrophiles.37 The mechanistic insight of the first-order and
second-order dependence of kobs on [DMA] will be discussed
in detail later.
A similar dependence of kobs on the concentration of a
hydrogen donor is obtained when DMA is replaced by N,N-
bis(trideuteriomethyl)aniline (DMA-(CD3)2) as shown in Figure
3b. The kobs value of DMA-(CD3)2 is much smaller than that of
DMA, indicating that there is a large primary kinetic isotope
effect (kH/kD). The kH/kD value increases with increasing the
DMA or DMA-(CD3)2 concentration. Arrhenius plots of the kobs
values for DMA and DMA-(CD3)2 at a constant DMA or DMA-
(CD3)2 concentration (5.0 × 10-3 M) are shown in Figure 4,
which indicates that the apparent kH/kD value decreases signifi-
cantly with increasing temperature (e.g., kH/kD ) 4.2 at 193 K
and kH/kD ) 1.7 at 233 K).
kobs
) k1 + k2[DMA]
(1)
[DMA]
k
obs/[DMA] versus [DMA] is shown in Figure 5a together with
the corresponding plot for DMA-(CD3)2 (Figure 5b). From the
slopes and intercepts are obtained the k1 and k2 values. The
primary kinetic isotope effect for the first-order dependence
(k1H/k1 ) 2.8) is much smaller than that for the second-order
D
D
dependence (k2H/k2 ) 15). This is the reason the apparent
kobsH/kobsD values vary depending on the DMA and DMA-(CD3)2
concentrations and temperature.38 Similar linear plots were
obtained for hydrogen-transfer reactions from a series of DMAs
and the deuterated compounds to cumylperoxyl radical in EtCN
and pentane at 193 K (see Supporting Information S1). The k1
and k2 values thus determined are summarized in Table 1
(38) The different primary kinetic isotope effects (k1H/k1D ) 2.8 and k2H/k2
)
D
15) in Figure 5 indicate involvement of different species in the hydrogen-
transfer reaction. The large k2H/k2 value indicates the contribution of
D
D
(37) Fukuzumi, S.; Kochi, J. K. Int. J. Chem. Kinet. 1983, 15, 249 and references
tunneling, but the reason of the difference from the k1H/k1 value has yet
therein.
to be clarified.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9077

A R T I C L E S
Fukuzumi et al.
Table 1. Oxidation Potentials and Rate Constants (k1, k2) for
Hydrogen Transfer from para-Substituted DMA to PhCMe2OO• in
O2-Saturated EtCN and Pentane at 193 K
0
E ox
k1b
k2b
(M-2 s-1
)
p-substituted DMA
(vs SCE,a V)
(M-1 s-1
)
MeO-DMA
MeO-DMA-(CD3)2
Me-DMA
Me-DMA-(CD3)2
DMA
DMA-(CD3)2
Br-DMA
CN-DMA
0.52
0.52
0.69
0.69
0.76
0.76
0.92
1.15
4.9 (4.3)c
4.4 (1.2)c
3.2 (3.2)c
2.4 (2.7)c
6.7 (5.1)c
2.4 (0.7)c
1.9
1.2 × 103 (1.2 × 103)c
1.2 × 103 (1.1 × 103)c
1.4 × 103 (2.0 × 103)c
1.0 × 103 (1.9 × 102)c
9.6 × 102 (5.0 × 102)c
6.4 × 10 (8.0 × 10)c
6.0 × 10
5.0 × 10-1 9.0
a Determined in MeCN. b Determined in EtCN. c Values in parentheses
are determined in pentane.
Figure 7. Plots of log k1 and log k2 versus ∆∆Hf for hydrogen transfer
from DMAs to PhCMe2OO• in O2-saturated EtCN and pentane at 193 K.
The ∆∆Hf value of DMA is taken as zero.
DMAs•+.40 Plots of log k1 and log k2 versus ∆∆Hf are shown
in Figure 7, where linear correlations are obtained, supporting
the one-step hydrogen atom transfer mechanism. Similar results
are obtained from the DFT calculations (see Supporting
Information, S2).41
Oxygen Transfer from Cumylperoxyl Radical to Tri-
phenylphosphines and Sulfides. The decay rate of cumyl-
peroxyl radical after cutting off the light is accelerated by the
presence of triphenylphosphine as a result of an oxygen transfer
from cumylperoxyl radical to triphenylphosphine. The oxygen-
ated product was identified as triphenylphosphine oxide after
the photoirradiation of the ButOOBut-cumene-Ph3P system (see
Experimental Section). The pseudo-first-order rate constants for
oxygen-transfer reactions from cumylperoxyl radicals to a series
of para-substituted triphenylphosphines in EtCN at 193 K
increase linearly with an increase in the concentrations of
triphenylphosphines in propionitrile to give only the observed
second-order rate constant (k1) (see Supporting Information, S3).
In pentane, however, the pseudo-first-order rate constants
increase with an increase in the concentrations of triphenylphos-
phines to exhibit first-order dependence at low concentrations,
changing to second-order dependence at high concentrations as
observed in hydrogen-transfer reactions from DMAs to cumyl-
peroxyl radicals (S4). The reason of such a difference in the
kinetic order depending on solvents will be discussed in relation
with the mechanism later. The k1 and k2 values for the oxygen-
transfer reactions are summarized in Table 2 together with the
one-electron oxidation potentials of triphenylphosphines (E0ox).
The dependence of log k1 in EtCN on E0ox and that of log k1
Figure 6. Plots of log k1 and log k2 versus E0 of DMAs for hydrogen
ox
transfer from DMAs to PhCMe2OO• in O2-saturated EtCN and pentane at
193 K.
together with the one-electron oxidation potentials of DMAs
(E0ox), which were determined by the cyclic voltammetry
measurements (see Experimental Section).39
The dependence of log k1 and log k2 on E0 is shown in
ox
Figure 6. The k1 and k2 values increase with decreasing E0
ox
but they decrease slightly through the maximum values at the
lower E0 values. Such a bell-shaped type dependence of k1
ox
and k2 on E0 is inconsistent with an electron-transfer mech-
ox
anism, since the rate of electron transfer would increase with
decreasing the E0 value as the electron transfer becomes
ox
thermodynamically more favorable. The similar k1 and k2 values
in a polar solvent (EtCN) and a nonpolar solvent (pentane) in
Table 1 suggest that the hydrogen transfer from DMAs to
cumylperoxyl radical proceeds via a one-step hydrogen atom
transfer rather than sequential transfer of electron and proton.
If this is the case, the k1 and k2 values may be correlated with
the enthalpy change of the hydrogen-transfer reactions. The
difference in heat of formation (∆∆Hf) between DMAs and the
hydrogen-abstracted radicals DMA(-H)s is calculated by using
the PM3 semiempirical MO method (see Experimental Section).
The substituent effects of electron-donating or electron-
withdrawing groups of DMAs on the ∆∆Hf values are rather
small because these effects on the electron-transfer oxidation
of DMAs and those on the deprotonation of DMAs•+ are
opposite, e.g., the electron-donating group favors the electron-
transfer process but disfavors the deprotonation process of
and log k2 in pentane on E0 are shown in Figure 8. In each
ox
case the log k1 and log k2 values are rather constant irrespective
of E0 and the k1 values in a nonpolar solvent (pentane) are
ox
much larger than those in a polar solvent (EtCN). These trends
are inconsistent with an electron-transfer mechanism, indicating
that the oxygen transfer proceeds via a direct transfer of oxygen
atom from cumylperoxyl radical to triphenylphosphines as
shown in Scheme 2.
(40) Such opposite effects may result in the bell-shaped nearly constant
dependence of kobs on E0ox in Figure 6. Similarly, the C-H bond dissociation
energies of para-substituted toluene derivatives are reported to be hardly
affected by the substitution, although the ionization potentials are sensitive
to the ring substituents; see ref 31.
(41) Because the difference in ∆∆Hf values is small (<2 kcal mol-1), the
correlations in the plots of log k1 and log k2 versus ∆∆Hf are qualitative
rather than quantitative.
(39) The E0 values were determined as the reversible oxidation potentials by
ox
the CV and SHACV.
9
9078 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Transfer Reactions of Cumylperoxyl Radical
A R T I C L E S
Table 2. Oxidation Potentials (E0ox) and Rate Constants (k1, k2)
for Oxygen Transfer from PhCMe2OO• to Phosphines in
O2-Saturated EtCN and Pentane at 193 K
Table 3. One-Electron Oxidation Potentials and Rate Constants
(k1, k2) for Oxygen Transfer from PhCMe2OO• to Sulfides in
O2-Saturated Pentane at 193 K
0
0
E ox
k1b
(M-1 s-1
k2b
(M-2 s-1
E ox
k1b
(M-1 s-1
k2b
(M-2 s-1
)
phosphine
(vs SCE,a V)
)
)
sulfide
(vs SCE,a V)
)
(p-MeOC6H4)3P
(p-MeC6H4)3P
(p-MeC6H4)Ph2P
Ph3P
(p-FC6H4)3P
(p-ClC6H4)3P
EtPh2P
Et2PhP
Me2PhP
MePh2P
n-Pr3P
0.80
0.97
1.02
1.06
1.17
1.28
1.23
1.27
1.28
1.30
1.56
c (1.3)d
c
MeO-TA
Me-TA
TA
Cl-TA
Me2S
Ph2S
Et2S
n-Pr2S
t-Bu2S
n-Bu2S
1.13
1.24
1.34
1.37
1.37
1.43
1.57
1.58
1.59
1.63
1.4
1.3 × 102
9.0 × 10-2
5.3
2.0 × 102 (1.8)d
1.2 × 102 (2.1)d
1.0 × 102 (2.9)d
8.4 × 10 (2.1)d
c (2.0)d
5.0 × 105
3.7 × 105
1.9 × 105
1.3 × 105
c
1.6 × 10-1
3.5 × 10-1
1.7
1.9
5.5 × 10-1
7.3 × 10-1
3.9
5.0
8.4 × 10
6.1 × 102
2.3 × 10
7.0 × 10-1
1.4 × 102
8.3 × 10
7.0 × 104
2.0 × 104
1.7 × 105
1.1 × 105
1.0 × 104
6.1 × 10
2.1
5.2 × 10
1.1 × 10-1
8.0 × 10
2.7
1.6 × 10
a Determined in MeCN. b Determined in pentane at 193 K.
a Determined in MeCN. b Determined in pentane. c Not determined.
d Values in parentheses are determined in EtCN.
Figure 9. Plots of log k1 and log k2 versus E0 of sulfides for oxygen
ox
transfer from PhCMe2OO• to sulfides [MeO-TA 1, Me-TA 2, TA 3, Cl-TA
4, Me2S 5, Ph2S 6, Et2S 7, n-Pr2S 8, t-Bu2S 9, and n-Bu2S 10] in O2-
saturated pentane at 193 K.
Figure 8. Plots of log k1 and log k2 versus E0ox of phosphines for oxygen
transfer from PhCMe2OO• to para-substituted R3P’s [(p-MeOC6H4)3P 1,
(p-MeC6H4)3P 2, (p-MeC6H4)Ph2P 3, Ph3P 4, (p-FC6H4)3P 5, (p-ClC6H4)3P
6, EtPh2P 7, Et2PhP 8, Me2PhP 9, MePh2P 10, and n-Pr3P 11] in O2-saturated
EtCN (b) and pentane (O, 0) at 193 K.
Scheme 2
The rates of oxygen-transfer reactions of cumylperoxyl radical
were also determined for the reactions with a variety of sulfides.
The oxygen-transfer rates in EtCN were too slow to be
determined accurately, and therefore they were determined only
in pentane. The pseudo-first-order rate constants for the decay
rates of cumylperoxyl radical in the presence of sulfides also
exhibited first-order dependence with respect to the sulfide
concentrations at low concentrations, changing to second-order
dependence at high concentrations (S5). The k1 and k2 values
for the oxygen-transfer reactions are summarized in Table 3
together with the one-electron oxidation potentials of sulfides
(E0ox) (see Experimental Section).
Figure 10. Plot of log k1 versus ∆∆Hf for oxygen transfer from PhCMe2-
OO• to sulfides [MeO-TA 1, Me-TA 2, TA 3, Cl-TA 4, Me2S 5, Ph2S 6,
Et2S 7, n-Pr2S 8, t-Bu2S 9, and n-Bu2S 10] in O2-saturated pentane at 193
K. The ∆∆Hf value of TA is taken as zero.
oxygen transfer reactions (∆∆Hf) as shown in Figure 10, where
the ∆∆Hf values are calculated as the difference in heat of
formation between sulfides and the corresponding sulfoxides
by using the PM3 semiempirical MO method (see Experimental
Section). This indicates that oxygen-transfer reactions from
cumylperoxyl radicals to sulfides proceed via a direct oxygen
atom transfer rather than via electron transfer as shown in
Scheme 3. The substituent effects of electron-donating or
electron-withdrawing groups of thioanisole on the ∆∆Hf values
in Figure 10 are small as the case of DMAs in Figure 7 (vide
The dependence of log k1 and log k2 for oxygen-transfer
reactions from cumylperoxyl radical to sulfides on E0ox is shown
in Figure 9, where log k1 and log k2 exhibit rather constant but
somewhat scattered dependence on E0ox. In contrast to such
scattered dependence of log k1 and log k2 on E0 in Figure 9,
ox
they are much better correlated with the enthalpy change of the
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9079

A R T I C L E S
Fukuzumi et al.
Table 4. Free Energy Changes (∆G0et), Rate Constants (ket), and
Reorganization Energies (λex) for Electron Transfer from Ferrocene
Derivatives to PhCMe2OO• in O2-Saturated EtCN at 193 K
0
a
b
∆G et
ket
(M-1 s-1
λex
(kcal mol-1
)
ferrocene derivative
(eV)
)
Fe(C5Me5)2
-0.73
0.40
0.39
0.34
0.28
9.1 × 10
3.2
3.0
189
184
186
c
Fe(C5H4Bun)2
Fe(C5H4Me)2
Fe(C5H5)(C5H4Bun)
Fe(C5H5)2
0.8
2.0 × 10-2
185
a Determined in MeCN. b Determined in EtCN. c Not determined because
no λex′ value of the ferrocene derivative is obtained.
cumylperoxyl radical is energetically feasible. In fact, the decay
rate of cumylperoxyl radical was significantly enhanced by the
presence of ferrocene derivatives. The decay rate in the presence
of ferrocene derivatives also obeyed pseudo-first-order kinetics
and the pseudo-first-order rate constants increase linearly with
increasing the concentrations of ferrocene derivatives (S6). The
observed second-order rate constants of the electron-transfer
reactions (ket) are summarized in Table 4 together with the free
Figure 11. Cyclic voltammogram of PhCMe2OO- formed in the reaction
of PhCMe2OOH (2.0 mM) with n-Bu4NOH (2.0 mM) in deaerated EtCN
containing 0.1 M TBAPF6. Working electrode, Pt; counter electrode, Pt;
reference electrode, Ag/AgNO3; scan rate, 500 mV s-1
.
Scheme 3
energy change of the electron transfer (∆G0 ), which is obtained
et
from the E0 values of ferrocene derivatives47 and the E0
ox
red
value of cumylperoxyl radical. The ket value increases with
decreasing the ∆G0et value, as expected for the electron transfer
from ferrocene derivatives to cumylperoxyl radical.
The self-exchange rate constants (kex) of PhCMe2OO•/
PhCMe2OO- can be determined from the electron-transfer
rate constant (ket) and the self-exchange rate constants (kex′)
of ferrocene derivative/ferricenium derivative48 by using the
Marcus eqs 2 and 3:19
supra). In the case of thioanisoles (TAs) as well, the substituent
effects on the electron-transfer oxidation of TAs and those on
the O- affinity of TAs•+ are opposite, e.g., the electron donating
group favors the electron-transfer process but disfavors the O-
binding of TAs•+.
Because some substrates have relatively low one-electron
oxidation potentials, it may be possible that an electron-transfer
step plays an important role in some hydrogen- and oxygen-
transfer reactions examined in this study. Thus, the possible
contribution of an electron-transfer step can be further examined
by comparing the rate constants determined for electron-transfer
reactions from ferrocene derivatives to cumylperoxyl radical
and the effects of metal ions on the electron-transfer rates with
those for the hydrogen-transfer reactions from DMAs to
cumylperoxyl radical (vide infra).
2
ket
kex
)
(2)
(3)
kex′Ket f
(ln Ket)2
kexkex′
ln f )
4 ln
(
)
Z2
where Ket is the equilibrium constant for electron transfer, f is
a correction factor (generally close to 1), and Z is the collision
frequency, taken as 1.0 × 1011 M-1 s-1. The Ket values are
Electron Transfer from Ferrocene Derivatives to Cumyl-
peroxyl Radical. Alkylperoxyl radicals (ROO•) are regarded
as rather strong one-electron oxidants judging from the positive
obtained from the ∆G0 values by using eq 4. The kex values
et
one-electron reduction potentials (E0 vs NHE ) 0.6-1.2 V
of PhCMe2OO•/PhCMe2OO- are converted to the reorganization
energy (λex) for the self-exchange electron-transfer reaction
between PhCMe2OO• and PhCMe2OO- using eq 5.
red
depending on R).42,43 There have been reports on electron-
transfer reactions from a variety of electron donors to alkyl-
peroxyl radicals.44-46 The direct determination of the E0red value
of cumylperoxyl radical was performed by the cyclic voltam-
mogram of PhCMe2OO- produced by the reaction of PhCMe2-
OOH with Me4NOH (Figure 11).
-∆G0
RT
et
Ket ) exp
(4)
(5)
(
)
The E0 value is determined as 0.65 V versus SCE. Thus,
red
λex ) 4RT(ln Z - ln kex)
electron transfer from ferrocene (E0 vs SCE ) 0.37 V)47 to
ox
The λex values thus determined are also listed in Table 4. The
large λex value in Table 2 indicates that a significant outer- and
inner-reorganization is required for electron-transfer reactions
of cumylperoxyl radical as is reported in the case of electron-
transfer reactions of oxygen.49,50
(42) Jonsson, M. J. Phys. Chem. 1996, 100, 6814.
(43) Merenyi, G.; Lind, J.; Engman, L. J. Chem. Soc., Perkin Trans. 2 1994,
2551.
(44) (a) Alfassi, Z. B.; Mosseri, S.; Neta, P. J. Phys. Chem. 1989, 93, 1380. (b)
Khaikin, G. I.; Alfassi, Z. B.; Neta, P. J. Phys. Chem. 1995, 99, 16722.
(45) (a) Khaikin, G. I.; Alfassi, Z. B.; Huie, R. E.; Neta, P. J. Phys. Chem.
1996, 100, 7072. (b) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref.
Data 1990, 19, 413.
(46) Workentin, M. S.; Maran, F.; Wayner, D. D. M. J. Am. Chem. Soc. 1995,
117, 2120.
(47) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28, 2459.
(48) Yang, E. S.; Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1980, 84, 3094.
(49) Lind, J.; Shen, X.; Mere´nyi, G.; Jonsson, B. O¨ . J. Am. Chem. Soc. 1989,
111, 7654.
9
9080 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Transfer Reactions of Cumylperoxyl Radical
A R T I C L E S
Figure 12. Dependence of log ket and log k1 on ∆G0 for (a) electron
et
transfer from ferrocene derivatives to PhCMe2OO•, (b) hydrogen transfer
from DMAs to PhCMe2OO•, (c) oxygen transfer from PhCMe2OO• to
phosphines, and (d) oxygen transfer from PhCMe2OO• to sulfides in O2-
saturated EtCN and pentane at 193 K.
The dependence of log ket on ∆G0 is given by eq 6, which
et
2
∆G0
λ
4
et
1 +
( )(
)
λ
log ket ) log Z -
(6)
2.3RT
is derived from eqs 2-4. The reorganization energy of electron
transfer (λ) corresponds to the average of each component, λex
for the electron self-exchange of PhCMe2OO•/PhCMe2OO- and
λex′ of ferrocene derivative/ferricenium ion derivative. The fit
of the curves to the Marcus theory of adiabatic outer-sphere
electron transfer (eq 6) indicates that the rate variations at a
given ∆G0et value arise from the difference in the λ value given
in Figure 12 and not from the nonadiabaticity.51
Figure 13. (a) Plots of kobs versus [Mn+] (Mn+ ) Sc3+ and Y3+) for electron
transfer from Fe(C5H5)2 (2.0 × 10-3 M) to PhCMe2OO• in the presence of
Mn+ in O2-saturated EtCN at 193 K. (b) Plot of kobs versus [Sc3+] for
hydrogen transfer from DMA (1.0 × 10-2 M) to PhCMe2OO• in the presence
of Sc3+ in O2-saturated EtCN at 193 K.
The log k1 values for hydrogen- and oxygen-transfer reactions
of cumylperoxyl radical with DMAs and those for oxygen-
transfer reactions with triphenylphosphines and sulfides are
plotted in Figure 12 for comparison. The log k1 values in each
case are much larger than the corresponding ket values at the
Scheme 4
same ∆G0 values, indicating also that the hydrogen- and
et
oxygen-transfer reactions proceed via a one-step hydrogen atom
or an oxygen atom transfer rather than the rate-determining
electron transfer as shown in Schemes 1, 2, and 3, respectively.
The rate of electron transfer from ferrocene to cumylperoxyl
radical is enhanced significantly by the presence of metal ions
such as Sc(OTf)3 and Y(OTf)3. The ket values increase with
increasing the metal ion concentration as shown in Figure 13a,
whereas Mg2+ shows a slight acceleration effect on the rate of
electron transfer from ferrocene to cumylperoxyl radical (see
Supporting Information S7). The acceleration effect of metal
ions can be ascribed to the binding of the PhCMe2OO- to the
metal ions, which results in a decrease in the free energy of the
electron transfer as shown in Scheme 4.52,53
Sc(OTf)3 on the rate of hydrogen transfer from DMA to
cumylperoxyl radical in EtCN as shown in Figure 13b.53,54 This
indicates that there is no contribution of an electron-transfer
step from DMA to cumylperoxyl radical in the hydrogen-transfer
reaction.
Hydrogen- and Oxygen-Transfer Reactions via Charge-
Transfer Complexes. Although no complete transfer of an
electron is involved in the hydrogen-transfer reaction from
DMAs to cumylperoxyl radical (vide supra), the first-order and
second-order dependence of kobs with respect to the DMAs
concentrations suggests the involvement of charge-transfer
In contrast to the case of electron-transfer reactions of
cumylperoxyl radical (Figure 13a), there was little effect of
(53) (a) Fukuzumi, S. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 4, pp 3-67. (b) Fukuzumi, S. Bull. Chem.
Soc. Jpn. 1997, 70, 1. (c) Fukuzumi, S. Org. Biomol. Chem. 2003, 1, 609.
(d) Fukuzumi, S.; Itoh, S. In AdVances in Photochemistry; Neckers, D. C.,
Volman, D. H., von Bu¨nau, G., Eds.; Wiley: New York, 1998; Vol. 25,
pp 107-172. (e) Fukuzumi, S.; Okamoto, K.; Imahori, H. Angew. Chem.,
Int. Ed. 2002, 41, 620. (f) Fukuzumi, S.; Okamoto, K.; Yoshida, Y.; Imahori,
H.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2003, 125, 1007.
(50) (a) McDowell, M. S.; Espenson, J. H.; Bakacˇ, A. Inorg. Chem. 1984, 23,
2232. (b) Zahir, K.; Espenson, J. H.; Bakacˇ, A. J. Am. Chem. Soc. 1988,
110, 5059.
(51) For the different λex′ values, see ref 50.
(52) There was no effect of Sc(OTf)3 on the oxidation potential of ferrocene.
There was no effect of Sc(OTf)3 on the ESR spectrum of PhCMe2OO•,
-
either. Thus, the complexation of RO2 with Sc(OTf)3, which is coupled
with the electron transfer, is responsible for the acceleration effect of
Sc(OTf)3 on the electron transfer. For the detailed discussion on metal
ion-promoted electron-transfer reactions, see ref 53.
(54) In the presence of Sc(OTf)3, the rate constant decreases slightly with increase
in the Sc(OTf)3 concentration (Figure 13b), probably as a result of the
complex formation of Sc(OTf)3 with DMA.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9081

A R T I C L E S
Fukuzumi et al.
Scheme 5
the second-order dependence of kobs on substrate concentration
becomes more important as compared with the case in a polar
solvent (EtCN) as reported for brominations of electron donors
in nonpolar solvents.37 Although it is difficult to observe the
CT spectra of the DMA- and TA-cumylperoxyl radical com-
plexes because of the instability of the radical complexes, a
decrease in the g value of cumylperoxyl radical is observed with
increasing the DMA and TA concentrations (see Supporting
Information S8).59 This indicates the formation of the CT
complex of cumylperoxyl radical with DMA and TA.60
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Area (nos.
13440216 and 14045249) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We thank Dr.
Y. Goto for supplying us DMAs-(CD3)2.
complexes formed between DMAs and cumylperoxyl radical
as shown in Scheme 5. The formation of charge-transfer
complexes between DMAs and a variety of electron donors has
been well-documented in the literature.55,56 Since cumylperoxyl
radical is a strong electron acceptor judging from the highly
Supporting Information Available: Plots of kobs versus
concentrations of N,N-dimethylanilines for hydrogen transfer
from DMAs to PhCMe2OO• in EtCN (S1), plots of log k1 and
log k2 versus ∆∆Hf (S2), plots of kobs versus concentrations of
triphenylphosphine derivatives for oxygen transfer from PhCMe2-
OO• to triphenylphosphine derivatives in EtCN (S3), plots of
positive E0 value (0.65 V vs SCE) as compared to other
red
electron acceptors that are known to form the CT complexes
with DMAs, cumylperoxyl radical may form CT complexes with
DMAs. The first-order and second-order dependence of kobs on
[DMAs] may thereby be ascribed to the intracomplex hydrogen
transfer in the CT complex (first-order with respect to [DMAs])
and the bimolecular hydrogen transfer from DMA to the CT
complex (second-order with respect to [DMAs]), respectively.57
The bimolecular hydrogen transfer from DMA to the CT
complex may proceed via a 2:1 CT complex formed between
DMA and cumylperoxyl radical. In the case of oxygen-transfer
reactions from cumylperoxyl radical to triphenylphosphines and
sulfides as well, the observed first-order and second-order
dependence of kobs on the substrate concentration indicates that
CT complexes formed between cumylperoxyl radical and
substrates act as reactive intermediates for the one-step oxygen
atom transfer reactions. In nonpolar solvents such as pentane,
the CT complex may be more stabilized by forming a 2:1 CT
complex between substrates and cumylperoxyl radical,58 when
kobs versus concentrations of triphenylphosphine derivatives for
oxygen transfer from PhCMe2OO• to triphenylphosphine deriva-
tives in pentane (S4), plots of kobs versus concentrations of TAs
for oxygen transfer from PhCMe2OO• to TAs in pentane (S5),
plots of kobs versus concentrations of ferrocene derivatives for
electron transfer from ferrocene derivatives to PhCMe2OO• (S6),
plot of kobs versus [Mg2+] for electron transfer from Fe(C5H5)2
to PhCMe2OO• in the presence of Mg2+ (S7), and plot of g
value of PhCMe2OO• versus concentrations of DMA and TA
(S8). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA035156O
(58) The addition interaction of a substrate to a 1:1 CT complex in nonpolar
solvents may enhance the CT interaction as reported in ref 37.
(55) Mulliken, R. S.; Person, W. B. Molecular Complexes, a Lecture and Reprint
Volume; Wiley-Interscience: New York, 1969.
(59) The larger g value of cumylperoxyl radical than the free spin value (2.0023)
is ascribed to the spin-orbit coupling interaction on oxygen atom, which
has a large spin-orbit coupling constant. The charge-transfer interaction
of cumylperoxyl radical with electron donors results in a decrease in the
spin density on oxygen, which causes the decrease in the g value (S8).
(60) For the CT complex formation of cumylperoxyl radical with pyridine, see:
Fukuzumi, S.; Ono, Y. Bull. Chem. Soc. Jpn. 1977, 50, 2063.
(56) Zweig, A.; Lancaster, J. E.; Neglia, M. T.; Jura, W. H. J. Am. Chem. Soc.
1964, 86, 4130.
(57) According to Scheme 5, kobs ) k1[PhCMe2OO• DMA] + k2[PhCMe2OO•
DMA][DMA], where [PhCMe2OO• DMA] ) K[PhCMe2OO•][DMA] and
thus, kobs ) k1K[PhCMe2OO•][DMA] + k2K[PhCMe2OO•][DMA]2.
9
9082 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003