Time-resolved kinetic study of the electron-transfer reactions between ring-substituted cumyloxyl radicals and alkylferrocenes. Evidence for an inner-sphere mechanism

Article abstract of DOI:10.1021/jo102420p

A time-resolved kinetic study of the reactions of ring-substituted cumyloxyl radicals (4-X-CumO^.: X = OMe, t-Bu, Me, Cl, CF₃) with methylferrocenes (Me_NFc: n = 2, 8, 10) has been carried out in acetonitrile solution. Evide

Full text of DOI:10.1021/jo102420p

ARTICLE
pubs.acs.org/joc
Time-Resolved Kinetic Study of the Electron-Transfer Reactions
between Ring-Substituted Cumyloxyl Radicals and Alkylferrocenes.
Evidence for an Inner-Sphere Mechanism
Massimo Bietti,*,† Gino A. DiLabio,*,‡ Osvaldo Lanzalunga,*,§ and Michela Salamone†
†Dipartimento di Scienze e Tecnologie Chimiche, Universitꢀa “Tor Vergata”, Via della Ricerca Scientifica, 1 I-00133 Rome, Italy
‡National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive, Edmonton, AB,
Canada T6G 2M9
§Dipartimento di Chimica, Sapienza Universitꢀa di Roma and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione
Meccanismi di Reazione, c/o Dipartimento di Chimica, Sapienza Universitꢀa di Roma, P.le A. Moro, 5 I-00185 Rome, Italy
S Supporting Information
b
ABSTRACT: A time-resolved kinetic study of the reactions of
ring-substituted cumyloxyl radicals (4-X-CumO•: X = OMe,
t-Bu, Me, Cl, CF3) with methylferrocenes (MenFc: n = 2, 8, 10)
has been carried out in acetonitrile solution. Evidence for an
electron transfer (ET) process has been obtained for all radicals
and an increase in reactivity has been observed on decreasing
the oxidation potential of the ferrocene donor and on going
from electron-releasing to electron-withdrawing ring substituents. Computations predict the formation of strongly bound π-stacked
4-X-CumO•/DcMFc complexes, characterized by intracomplex π-π distances around 4 Å. These findings point toward a
(nonbonded) inner-sphere ET mechanism for the reactions of the 4-X-CumO•/MenFc couples.
’ INTRODUCTION
In order to probe the role of electronic effects on these ET
processes and to obtain additional mechanistic information, we
carried out a time-resolved kinetic study on the reactions of ring-
substituted cumyloxyl radicals (4-X-CumO•: X = OMe, t-Bu, Me,
Cl, CF3) with a series of electron-rich methyl ferrocenes (MenFc),
namely: 1,10-dimethylferrocene (DMFc), 1,10,2,20,3,30,4,40-octa-
methylferrocene (OMFc), and decamethylferrocene (DcMFc),
the structures for which are displayed in Chart 1. Computational
modeling provides additional insights into the nature of the
encounter complexes formed during the ET reactions.
The electron-transfer (ET) reactivity of organic free radicals
has attracted considerable interest in recent years, with notable
examples that include the study of the ET properties of
peroxyl,1-6 phenoxyl,7-10 and N-oxyl radicals.11-13 However,
limited information is available on alkoxyl radicals,14-16 one of
the main classes of oxygen centered radicals. In this context, we
have recently shown that the tert-butoxyl (t-BuO•), cumyloxyl
(CumO•) and benzyloxyl (BnO•) radicals react with alkyl
ferrocenes via an ET mechanism.17 The results of our study
led us to suggest that the ET reactions of BnO• and CumO• with
ferrocene donors may not be described in terms of a straightfor-
ward outer-sphere ET mechanism. Computational modeling
predicted the formation of π-stacked encounter complexes
between the radicals and ferrocene donors. In these complexes,
the alkoxyl radical aromatic ring can act as an electron relay
shuttling the electron from the ferrocene to the formal oxygen
atom radical center. This hypothesis was supported by the
observation that, with t-BuO•, where this interaction is not
possible, the ET rate constants for reaction with the ferrocene
donors are at least 2 orders of magnitude lower than those
measured for the corresponding reactions of BnO• and CumO•.
This large difference in reactivity is present despite the
relatively small differences in reduction potential between
these three radicals, viz., E°RO•/RO- = -0.30, - 0.19, and -
0.10 V/SCE in acetonitrile for t-BuO•,16c BnO•,17 and
CumO•,16a respectively.
’ RESULTS AND DISCUSSION
Ring-substituted cumyloxyl radicals (4-X-CumO•) were gen-
erated by 266 nm laser flash photolysis (LFP) of nitrogen
saturated MeCN solutions (T = 25 °C) containing the parent
tert-butyl 4-X-cumyl peroxides (eq 1).18
As described previously, in MeCN solution ring-substituted
cumyloxyl radicals are characterized by a broad absorption band
Received: December 7, 2010
Published: February 24, 2011
r
2011 American Chemical Society
1789
dx.doi.org/10.1021/jo102420p J. Org. Chem. 2011, 76, 1789–1794
|

The Journal of Organic Chemistry
ARTICLE
Chart 1
in the visible region of the spectrum whose position, as compared
to the unsubstituted cumyloxyl radical for which λmax (vis) =
485 nm,19 is red-shifted by electron-donating (ED) ring substituents
(X = OMe, Me) and blue-shifted by electron-withdrawing (EW)
ones (X = Cl, CF3).19-21 Along this line, the time-resolved ab-
sorption spectrum observed after 266 nm LFP of tert-butyl 4-tert-
butyl-cumyl peroxide, and displayed in the Supporting Informa-
tion as Figure S1, is characterized by a visible absorption band with
λmax = 510 nm that is assigned to 4-t-Bu-CumO•. Under these
conditions, the 4-X-CumO• radicals decay mainly by C-CH3
β-scission, with rate constants that are not influenced to a signi-
ficant extent by the nature of the ring substituent (kβ between
0.7 and 1.1 ꢀ 106 s-1).20 With 4-t-Bu-CumO•, the rate constant
for C-CH3 β-scission was measured following the decay of the
visible absorption band at 510 nm as kβ = 7.7 ꢀ 105 s-1
.
The reactions of the 4-X-CumO• radicals with the ferrocene
donors were studied in MeCN solution by LFP. Figure 1 shows
the time-resolved absorption spectra observed after 266 nm LFP
of a nitrogen-saturated MeCN solution (T = 25 °C) containing
tert-butyl 4-chlorocumyl peroxide and DcMFc. The spectrum
recorded 63 ns after the laser pulse (black circles) shows the
characteristic 4-Cl-CumO• band centered at 500 nm.20 The
decay of this band (inset b), which is accelerated by the presence
of DcMFc, is accompanied by a corresponding buildup of
absorption at 270, 320, and 700 nm (insets a and c). Two
isosbestic points can be identified at 410 and 580 nm. These
absorption bands are assigned to the decamethylferrocenium ion
(DcMFcþ) by comparison to literature data,22 and to the
spectrum observed previously after reaction between CumO•
and DcMFc.17
Figure 1. Time-resolved absorption spectra observed after 266 nm LFP
of a nitrogen-saturated MeCN solution (T = 25 °C) containing tert-butyl
4-chlorocumyl peroxide (5.0 mM) and decamethylferrocene (0.19 mM)
at 63 ns (black circles), 191 ns (white circles), 511 ns (gray circles), and
4.4 μs (black squares) after the 8 ns, 10 mJ laser pulse. Insets: (b) Decay
of the 4-chlorocumyloxyl radical monitored at 500 nm. (a and c)
Corresponding buildup of absorption at 320 (a) and 700 nm (c)
assigned to the formation of the decamethylferrocenium ion.
buildup of the pertinent ferrocenium ion bands was observed,
indicating that ET reactions occurred. The time-resolved spectra
observed after reaction of 4-Me-CumO• with OMFc are dis-
played in the Supporting Information (Figure S2).
Kinetic studies were carried out by LFP following the buildup
of the ferrocenium ions between 300 and 330 nm and with
4-MeO-CumO•, characterized by the most intense visible ab-
sorption band among the series of cumyloxyl radicals
investigated19a,20,21 also by following the decay of this band at
580 nm.
Plots of the observed rate constants (kobs) against [MenFc]
produced excellent linear fits. The second-order rate constants
for the one-electron oxidation of the ferrocene donors by 4-X-
CumO• (k2) were obtained from the slopes of these plots. All the
kinetic data are collected in Table 1. For comparison, also
included are the kinetic data obtained previously for CumO•
and BnO•,17 and the calculated solvent-phase (MeCN) electron
affinities (EA) of the radicals. The plots of kobs vs [MenFc] for
the reactions of the 4-X-CumO• radicals are displayed in the
Supporting Information (Figures S3-S15).
The formation of DcMFcþ clearly indicates the occurrence of
an ET reaction from DcMFc to 4-Cl-CumO• (eq 2), in accor-
dance with our previous observations of the analogous reaction
involving the unsubstituted cumyloxyl radical.17
The data displayed in Table 1 show that k2 increases with
decreasing the oxidation potential of the ferrocene donor
(0.29, - 0.02, and -0.10 V/SCE in MeCN for DMFc, OMFc,
and DcMFc, respectively),11c as expected for an ET process. For
Analogous behavior was observed in the reactions of the 4-X-
CumO• radicals with the ferrocene donors. In all cases the
1790
dx.doi.org/10.1021/jo102420p |J. Org. Chem. 2011, 76, 1789–1794

The Journal of Organic Chemistry
ARTICLE
Table 1. Second-Order Rate Constants (k2) for the Reactions
of 4-X-CumO• with Methylferrocenes (MenFc) Measured in
MeCN at T = 25 °Ca and Calculated Electron Affinities (EA)
of the Radicals (kcal/mol)
k2/M-1 s-1
radicalb
DMFc
OMFc
DcMFc
EA c
4-MeO-CumO•
d
d
d
d
1.3 ꢀ 109
1.0 ꢀ 109 e
1.6 ꢀ 109
1.6 ꢀ 109
2.0 ꢀ 109
3.3 ꢀ 109
4.2 ꢀ 109
7.5 ꢀ 109
2.4 ꢀ 109
2.4 ꢀ 109e
f
3.2 ꢀ 109
4.0 ꢀ 109
5.4 ꢀ 109
7.4 ꢀ 109
1.1 ꢀ 1010
104.6
4-t-Bu-CumO•
4-Me-CumO•
CumO• g
4-Cl-CumO•
4-CF3-CumO•
BnO• g
103.3
104.8
105.8
106.6
107.4
107.8
1.0 ꢀ 108
1.3 ꢀ 108
1.9 ꢀ 108
5.2 ꢀ 108
a Determined from the slope of the kobs vs [MenFc] plots. Average of at
least two values. Error <10%. kobs was measured following the buildup of
the pertinent ferrocenium ion (at 300 nm for DMFcþ, 330 nm for
OMFcþ, and 320 nm for DcMFcþ). b Generated by 266 nm LFP of the
parent tert-butyl 4-X-cumyl peroxides as described in eq 1. c Calculated
using (RO)B323LYP24/6-311þG(2d,2p)//B3LYP/6-31þG(d,p),25
with the PCM26 solvent model (MeCN), as implemented in Gaussi-
an-03.27 Zero-point energies (frequency scale factor = 0.9806) were
used in the calculation of EAs. d The high concentrations of DMFc
required for the kinetic study of the ET reactions prevented the
determination of the corresponding second-order rate constants. e kobs
was measured following the decay of 4-MeO-CumO• at 580 nm. f With
4-t-Bu-CumO• the kinetic study was limited to the reaction with OMFc.
g From ref 17.
Figure 2. Dependence of the second-order rate constant (k2) on the
Hammett σ substituent constant for the reactions of 4-X-CumO• with
MenFc. From the linear regression analysis DMFc (open circles) F =
0.52, r2 = 0.9996; OMFc (black circles) F = 0.64, r2 = 0.9734; DcMFc
(gray circles) F = 0.58, r2 = 0.9736.
CumO•, 4-Cl-CumO•, and 4-CF3-CumO•, k2 increases by a
factor of ca. 40 on going from DMFc to DcMFc. Relative to the
unsubstituted cumyloxyl radical CumO•, k2 values increase for
alkoxyl radicals with EW substituents (4-Cl and 4-CF3) and
decrease for those bearing ED substituents (4-Me, 4-t-Bu, and
4-MeO). Although the substituent effect is not very pronounced
(k2 values for OMFc and DcMFc are within a factor of 3 over the
range of substituents), this behavior is in line with the oxidizing
power of the ring-substituted cumyloxyl radicals that can be
derived from the calculated solvent-phase (MeCN) electron
affinity (EA) values displayed in Table 1.28 This trend in k2 is
well matched by the EA values, as evidenced by the reasonably
good correlations (r2 g 0.969) obtained between log k2 for the
reactions of 4-X-CumO• with OMFc and DcMFc and the EAs
(see the Supporting Information, Figure S16). The slightly lower
EA value obtained for 4-t-Bu-CumO• may reflect poor differ-
ential treatment of the bulky substituent group by the implicit
solvent model.
Figure 3. Optimized structure of the DcMFc/4-Me-CumO• encounter
complex. The intracomplex π-π distance between the nearest aromatic
rings of the two moieties is 4.1 Å. Key: blue = carbon, orange = iron,
red = oxygen, white = hydrogen. The methyl hydrogens on DcMFc have
been omitted for clarity.
global minimum encounter complex structure. However, our
previous work showed that interactions between the aromatic
ring of the arylcarbinyloxyl radical and the side of DcMFc is
higher in energy than the π-stacked arrangement. Figure 3 shows
the structure of the encounter complex between DcMFc and
4-Me-CumO•, which is characteristic of all the DcMFc/4-X-
CumO• complexes. As an additional example, the structure of the
DcMFc/4-t-Bu-CumO• encounter complex is shown in the
Supporting Information (Figure S17).
Figure 2 shows the plots of log k2 against the Hammett σ
substituent constant29 for the reactions of 4-X-CumO• with the
three ferrocene donors. Good correlations (r2 g 0.973) are
observed in all cases, indicating that variations in rate constants
are essentially determined by the inductive effect of the ring-
substituent. The very similar slopes observed for the three plots
(F comprised between 0.52 and 0.64) indicate that the substit-
uents exert comparable effects on the ET reactions of the
different ferrocene donors.
Our calculations predict that the DcMFc/4-X-CumO• en-
counter complexes are all strongly bound. Binding energies (BE)
for X = CF3, Cl, H, Me, t-Bu, and OMe are 7.5, 6.2, 5.4, 6.2, 6.5,
and 6.8 kcal/mol, respectively. Note that all substituents, regard-
less of ED or EW character, increase the BE. A very similar trend
in BEs was predicted for π-stacked C6H5X/C6H6 complexes
using high-level wave function methods,32 and was explained by
an analysis of the balance between electrostatic and dispersion
Calculations30 using dispersion-correcting potentials31 were
performed to explore the π-stacked encounter complexes formed
between the 4-X-CumO• radicals and DcMFc. It is important to
note that we did not search conformational space to find the
1791
dx.doi.org/10.1021/jo102420p |J. Org. Chem. 2011, 76, 1789–1794

The Journal of Organic Chemistry
ARTICLE
forces. As was predicted for the ET reactions between alkylfer-
rocenes and BnO•, the reactions involving substituted CumO•
are facilitated by the strong π-stacking: overlap of the aromatic
rings provides a conduit for the electron being transferred
between the Fe and the formal oxygen atom radical centers.
However, despite the indication of very similar reduction poten-
tials for BnO• and 4-CF3-CumO• (see Table 1),28 an up to
3-fold decrease in rate constant has been observed on going from
the former radical to the latter one. In our previous modeling on
the DcMFc/BnO• π-stacked complex, the C-O bond was found
to be oriented perpendicular to the plane of the aromatic ring.17
This arrangement allows for optimal (but weak) conjugation of
the formal, singly occupied O p-orbital with the BnO• ring and
ensures that the electron transferred from the DcMFc Fe center
pairs up with the (formal) unpaired electron on the O atom. In
the most stable conformation of the 4-X-CumO•/DcMFc en-
counter complexes, the C-O bond is essentially parallel to the
plane of the aromatic ring. However, the facile rotation about the
Ar-C(CH3)2O• bond allows for optimal overlap to be achieved.
This likely contributes, even though to a small degree, to the
kinetic barrier of the ET reactions involving 4-X-CumO•/
DcMFc systems.
The similar BEs and intracomplex π-π distances obtained for
the encounter complexes are in full agreement with the kinetic
data displayed in Table 1, that show identical values of k2 for the
reactions of 4-Me-CumO• and 4-t-Bu-CumO• with OMFc. This
indicates that the effect of the t-Bu substituent is essentially
inductive in nature, as these two substituents are characterized by
very close σ values (- 0.170 and -0.197, respectively)29 and that
steric effects due to the presence of the bulky t-Bu substituent do
not play any significant role on the ET process.
separation, results in adiabatic ET with unit probability and
therefore very high rate constants for (endergonic) inner-sphere
ET.33,35
Along this line, the computed π-stacked encounter complexes
between the 4-X-CumO• radicals and DcMFc are characterized
by separations around 4 Å and interaction energies on the order
of 6 kcal/mol. Moreover, the measured second-order rate con-
stants for the (endergonic) ET reactions of the 4-X-CumO•/
MenFc couples approach in all cases the diffusion limit (k2 g
1.0 ꢀ 109 M-1 s-1). These observations strongly support the
hypothesis that the reactions between 4-X-CumO• radicals and
MenFc can also be described according to eq 3 in terms of a
(nonbonded) inner-sphere ET mechanism that proceeds
through the reversible formation of a complex followed by ET
within the complex (eq 4).36,37
KEC
•
•
kET
4-X-CumO þ Me Fc a ½4-X-Cumo , Me Fcꢂ B
n
n
4-X-Cumo- þ MenFcþ
ð4Þ
This hypothesis is in line with our previous observation that
the ET reactions of BnO• and CumO• with ferrocene donors do
not follow a Marcus-type behavior.17 Additional experimental
and computational studies with highly hindered ferrocene
donors may provide useful mechanistic information on the role
of structural effects on these ET reactions.
’ EXPERIMENTAL SECTION
The formation of strongly bound π-stacked encounter com-
plexes has been previously observed by Kochi and co-workers for
the ET reactions between alkylaromatic donors (ArH) and
photoactivated quinone acceptors (Q*).33,34 The authors ob-
served that increases in the donor-acceptor distance caused by
steric hindrance in the donor induce a changeover in the ET
mechanism owing to the substantial decrease in the extent of
donor/acceptor orbital overlap. Accordingly, with unhindered
donors significant electronic coupling between donor and ac-
ceptor can be achieved, leading to tight complexes characterized
by interaction energies significantly greater than 1 kcal/mol and
donor/acceptor separations e4 Å. Under these conditions,
deviations from the Marcus equation were observed and the
reactions were described in terms of a (nonbonded) inner-sphere
ET process characterized by the formation of a discrete encoun-
ter complex preceding the ET transition state (eq 3).
Materials. Spectroscopic-grade acetonitrile was used in the kinetic
experiments. Commercial samples of 1,10-dimethylferrocene (DMFc),
1,10,2,20,3,30,4,40-octamethylferrocene (OMFc), and decamethylferro-
cene (DcMFc) were of the highest commercial quality available (g99%)
and were further purified by sublimation prior to use. tert-Butyl 4-X-
cumyl peroxides (X = Me, OMe, Cl, CF3) were available from a previous
work.20 The synthesis of tert-butyl 4-tert-butylcumyl peroxide is de-
scribed in the Supporting Information.
Laser Flash Photolysis Studies. LFP experiments were carried
out with a laser kinetic spectrometer using the fourth harmonic
(266 nm) of a Q-switched Nd:YAG laser, delivering 8 ns pulses. The laser
energy was adjusted to e10 mJ/pulse by the use of the appropriate filter.
A 3 mL Suprasil quartz cell (10 mm ꢀ 10 mm) was used in all
experiments. Nitrogen-saturated acetonitrile solutions of tert-butyl
4-X-cumyl peroxides (between 2 and 10 mM) were employed. These
peroxide concentrations were chosen in order to ensure prevalent
absorption of the 266 nm laser light by the precursor peroxides in the
presence of the ferrocene donors. All the experiments were carried out at
T = 25.0 ( 0.5 °C under magnetic stirring. First-order or pseudo-first-
order rate constants were obtained by averaging four to eight individual
values and were reproducible to within 5%.
KEC
Q-• þ ArHþ•
ð3Þ
kET
ꢁ
ꢁ
þ ArH a ½Q , ArHꢂ B
Q
With hindered donors, weak coupling between donor and
acceptor was observed leading to interaction energies e1 kcal/
mol and donor/acceptor separations g4.5 Å. Under these condi-
tions, the predicted Marcus outer-sphere ET behavior was observed.
Significantly higher ET rate constants were observed for the
unhindered donor/acceptor couples as compared to the hin-
dered ones, indicating that tight encounter complexes must
experience a significant predisposition toward ET, which allows
even endergonic ET reactions to occur at rate constants that
approach the diffusion limit. The strong electronic coupling
between the donor and the acceptor, as is indicated by the small
The photochemical stability of the ferrocene donors at the laser
excitation wavelength (266 nm) was checked by LFP of acetonitrile
solutions containing donor concentrations comparable to the highest
concentrations employed in the kinetic experiments. No evidence for
the photolytic decomposition of the ferrocene donors or for the
formation of the pertinent ferrocenium ions was obtained in these
experiments.
Second-order rate constant for the reactions of the cumyloxyl radicals
with the ferrocene donors (MenFc) were obtained from the slopes of the
kobs (measured following the buildup of the ferrocenium ion band
between 300 and 330 nm, and with the 4-methoxycumyloxyl radical also
1792
dx.doi.org/10.1021/jo102420p |J. Org. Chem. 2011, 76, 1789–1794

The Journal of Organic Chemistry
ARTICLE
by following the decay of the visible absorption band at 580 nm) vs
[MenFc] plots.
Fresh solutions were used for every MenFc concentration. Correla-
tion coefficients were in all cases >0.992. The given rate constants are the
average of at least two independent experiments, typical errors being
<10%.
(14) Shao, J.; Geacintov, N. E.; Shafirovich, V. J. Phys. Chem. B 2010,
114, 6685–6692.
(15) (a) Magri, D. C.; Workentin, M. S. Chem.—Eur. J. 2008,
14, 1698–1709. (b) Donkers, R. L.; Workentin, M. S. J. Am. Chem.
Soc. 2004, 126, 1688–1698. (c) Donkers, R. L.; Tse, J.; Workentin, M. S.
Chem. Commun. 1999, 135–136.
(16) (a) Donkers, R. L.; Maran, F.; Wayner, D. D. M.; Workentin,
M. S. J. Am. Chem. Soc. 1999, 121, 7239–7248. (b) Antonello, S.;
Musumeci, M.; Wayner, D. D. M.; Maran, F. J. Am. Chem. Soc. 1997,
119, 9541–9549. (c) Workentin, M. S.; Maran, F.; Wayner, D. D. M.
J. Am. Chem. Soc. 1995, 117, 2120–2121.
’ ASSOCIATED CONTENT
S
Supporting Information. Details of the synthesis of tert-
b
butyl 4-tert-butylcumyl peroxide. NMR spectra. Time-resolved
absorption spectra. Plots of kobs vs [MenFc] for the reactions of
the 4-X-CumO• with the MenFc. Plots of log k2 vs EA (4-X-
CumO•). Computational details and data for the 4-X-CumO•-
DcMFc complexes and the electron affinities of 4-X-CumO•.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(17) Bietti, M.; DiLabio, G. A.; Lanzalunga, O.; Salamone, M. J. Org.
Chem. 2010, 75, 5875–5881.
(18) LFP of tert-butyl 4-X-cumyl peroxides also produces the tert-
butoxyl radical (t-BuO•). This radical, however, does not interfere with
our measurements because, as mentioned above, the ET rate constants
for its reaction with the ferrocene donors are at least 2 orders of
magnitude lower than those measured for the corresponding reactions
of CumO•.17
(19) (a) Avila, D. V.; Ingold, K. U.; Di Nardo, A. A.; Zerbetto, F.;
Zgierski, M. Z.; Lusztyk, J. J. Am. Chem. Soc. 1995, 117, 2711–2718.
(b) Avila, D. V.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1992,
114, 6576–6577.
(20) Baciocchi, E.; Bietti, M.; Salamone, M.; Steenken, S. J. Org.
Chem. 2002, 67, 2266–2270.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: bietti@uniroma2.it; gino.dilabio@nrc.ca; osvaldo.
lanzalunga@uniroma1.it.
(21) Baciocchi, E.; Bietti, M.; Lanzalunga, O.; Steenken, S. J. Am.
Chem. Soc. 1998, 120, 11516–11517.
(22) Frey, J. E.; Du Pont, L. E.; Puckett, J. J. J. Org. Chem. 1994,
59, 5386–5392.
’ ACKNOWLEDGMENT
Financial support from the Ministero dell’Istruzione
dell’Universitꢀa e della Ricerca (MIUR) is gratefully acknowl-
edged. We thank Lorenzo Stella for the use of LFP equipment
and Alessandra Gentili for performing an Enhanced Resolution
ESI(þ) experiment.
(23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(24) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(25) This model was described in: DiLabio, G. A.; Pratt, D. A.;
LoFaro, A. D.; Wright, J. S. J. Phys. Chem. A 1999, 103, 1653–1661.
(26) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669–681 and references cited therein.
’ REFERENCES
(27) Gaussian 03, Revision D.01: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam,
J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian,
H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann,
R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz,
J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.;
Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian, Inc., Pittsburgh PA, 2004.
(1) Jomovꢁa, K.; Kysel, O.; Madden, J. C.; Morris, H.; Enoch, S. J.;
Budzak, S.; Young, A. J.; Cronin, M. T. D.; Mazur, M.; Valko, M. Chem.
Phys. Lett. 2009, 478, 266–270.
(2) Goldstein, S.; Samuni, A. J. Phys. Chem. A 2007, 111, 1066–1072.
(3) Fukuzumi, S.; Shimoosako, K.; Suenobu, T.; Watanabe, Y. J. Am.
Chem. Soc. 2003, 125, 9074–9082.
(4) (a) Brinck, T.; Lee, H.-N.; Jonsson, M. J. Phys. Chem. A 1999,
103, 7094–7104. (b) Jonsson, M. J. Phys. Chem. 1996, 100, 6814–6818.
(5) (a) Das, T. N.; Dhanasekaran, T.; Alfassi, Z. B.; Neta, P. J. Phys.
Chem. A 1998, 102, 280–284. (b) Alfassi, Z. B.; Khaikin, G. I.; Neta, P.
J. Phys. Chem. 1995, 99, 265–268.
(6) Jovanovic, S. V.; Jankovic, I.; Josimovic, L. J. Am. Chem. Soc.
1992, 114, 9018–9021.
(7) Litwinienko, G.; Ingold, K. U. Acc. Chem. Res. 2007, 40, 222–230.
(8) Steenken, S.; Neta, P. In The Chemistry of Phenols; Rappoport, Z.,
Ed.; John Wiley & Sons: New York, 2003; Chapter 16, pp 1107-1152.
(9) Itoh, S.; Kumei, H.; Nagatomo, S.; Kitagawa, T.; Fukuzumi, S.
J. Am. Chem. Soc. 2001, 123, 2165–2175.
(28) We have recently shown that the difference in the calculated
solvent-phase EAs between CumO• and t-BuO• (ΔEA(CumO•
-
t-BuO•)) parallels the analogous ΔE° values.17 Accordingly, by taking
into account the reduction potential of CumO• (E°CumO•/CumO- = -
0.19 V/SCE in MeCN),16a the ΔEA values obtained from Table 1 can
provide reasonable estimates of the E° values of the 4-X-CumO• radicals.
(29) Swain, C. J.; Lupton, E. C., Jr. J. Am. Chem. Soc. 1968, 90,
4328–4337.
(10) Armstrong, D. A.; Sun, Q.; Schuler, R. H. J. Phys. Chem. 1996,
100, 9892–9899.
(11) (a) Baciocchi, E.; Bietti, M.; Lanzalunga, O.; Lapi, A.; Raponi,
D. J. Org. Chem. 2010, 75, 1378–1385. (b) Baciocchi, E.; Bietti, M.;
Di Fusco, M.; Lanzalunga, O.; Raponi, D. J. Org. Chem. 2009, 74,
5576–5583. (c) Baciocchi, E.; Bietti, M.; Di Fusco, M.; Lanzalunga, O.
J. Org. Chem. 2007, 72, 8748–8754.
(30) Using the B971 functional: Hamprecht, F. A.; Cohen, A. J.;
Tozer, D. J.; Handy, N. C. J. Chem. Phys. 1998, 109, 6264–6271 as
implemented in ref 27, along with 6-31þG(d,p) basis sets.
(31) (a) DiLabio, G. A. Chem. Phys. Lett. 2008, 455, 348–353. (b)
Mackie, I. D.; DiLabio, G. A. J. Phys. Chem. A 2008, 112, 10968–
10976.
(12) Galli, C.; Gentili, P.; Lanzalunga, O. Angew. Chem., Int. Ed.
2008, 47, 4790–4796.
(13) (a) Blinco, J. P.; Hodgson, J. L.; Morrow, B. J.; Walker, J. R.;
Will, G. D.; Coote, M. L.; Bottle, S. E. J. Org. Chem. 2008, 73, 6763–6771.
(b) Manda, S.; Nakanishi, I.; Ohkubo, K.; Yakumaru, H.; Matsumoto,
K.; Ozawa, T.; Ikota, N.; Fukuzumi, S.; Anzai, K. Org. Biomol. Chem.
2007, 5, 3951–3955.
(32) Sinnokrot, M. O.; Sherrill, C. D. J. Am. Chem. Soc. 2004,
126, 7690–7697.
1793
dx.doi.org/10.1021/jo102420p |J. Org. Chem. 2011, 76, 1789–1794

The Journal of Organic Chemistry
ARTICLE
(33) Hubig, S. M.; Rathore, R.; Kochi, J. K. J. Am. Chem. Soc. 1999,
121, 617–626.
(34) For a critical discussion of the role of structural effects on
the ET reactions of π-stacked systems, see: Rosokha, S. V.; Kochi, J. K.
J. Am. Chem. Soc. 2007, 129, 3683–3697.
(35) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985,
811, 265–322.
(36) In ref 33, the kinetic plots of kobs vs [ArH] for unhindered
donors showed an asymptotic behavior diagnostic of a preequilibrium
step that precedes ET. From these plots the preequilibrium constant
(KEC) and the intrinsic ET rate constant (kET) could be obtained. Linear
plots of kobs vs [MenFc] have been instead obtained for the reactions of
the 4-X-CumO• radicals, where the relatively strong absorption of the
ferrocene donors at the laser excitation wavelength limited the study to
[MenFc] e 5 mM (see the Supporting Information). Accordingly, at
these low concentrations the measured second order rate constant can
be expressed as k2 = KECkET. On the basis of this kinetic scheme, and on
the data collected in Table 1, spectral evidence for the formation of an
encounter complex would require significantly higher concentrations of
the ferrocene donors. See, for example: Rathore, R.; Hubig, S. M.; Kochi,
J. K. J. Am. Chem. Soc. 1997, 119, 11468–11480.
(37) A similar inner-sphere mechanism has been proposed for the
ET reactions of the carbonate radical anion (CO3•-) with oligonucleotides
•
and of the nitrate radical (NO3 ) with alkylaromatic compounds. See: Lee,
Y. A.; Yun, B. H.; Kim, S. K.; Mergolin, Y.; Dedon, P. C.; Geacintov, N. E.;
Shafirovich, V. Chem.—Eur. J. 2007, 13, 4571–4581. Del Giacco, T.;
Baciocchi, E.; Steenken, S. J. Phys. Chem. 1993, 97, 5451–5456.
1794
dx.doi.org/10.1021/jo102420p |J. Org. Chem. 2011, 76, 1789–1794