Electron-Transfer Reaction of the Phthalimide-N-oxyl Radical
ensure adiabatic behaviors.36 Certainly, this a very important
problem that should deserve both specific experimental and
theoretical investigations; nevertheless a possible degree of
nonadiabaticity of ferrocene self-exchange reactions should not
significantly influence our results (particularly the self-exchange
electron-transfer rate obtained for the PINO/PINO- couple),39
as Nelsen and Pladziewicz have recently reported a very large
number of data showing that the Marcus cross relation remains
valid also when applied to couples with Hab as small as 0.01
a model of peroxyl radicals also in electron-transfer reactions.
Unfortunately, examination of the literature provided us with
very discordant data for the self-exchange electron-transfer
•
-
reorganization energies of peroxyl radicals (λROO /ROO ). Thus,
Fukuzumi et al.37 have recently reported a very high λROO /ROO
•
-
value (185-189 kcal mol-1) for cumylperoxyl radical in EtCN,
a value that is extremely different than that measured by us for
PINO in CH3CN (49.1 kcal mol-1). However, much lower
-
•
λROO /ROO values, not much different from that of PINO, are
reported for other alkylperoxy radicals, albeit under different
conditions. Accordingly, a value of 72 kcal mol-1 has been
kcal mol-1 41
.
The λPINO/PINO value of 49.1 kcal mol-1, calculated above,
is significantly lower than that (>60 kcal mol-1) estimated in
our study of the PINO-promoted N-demethylation of N,N-
dimethylanilines.7 We have no doubt that the value reported
herewith is the most reliable one. As already mentioned, the
-
reported by Jovanovic at al.38 for λROO /ROO of methylperoxyl,
•
-
cyclohexylperoxyl, and (dimethyl)hydroxyethylperoxyl radicals.
Moreover, λROO /ROO values in the 60-70 kcal mol-1 range can
be calculated from an estimate of 40 ( 4 kcal mol-1 for the
reorganization energy for ET reactions between closed-shell
organics and several alkyl peroxyl radicals.45,46 Another estimate
•
-
+•
self-exchange reorganization energy for the ArNMe2/ArNMe2
couple is not known with certainty, and moreover the PINO-
promoted N-demethylation reaction is a two-step reaction: A
reversible electron transfer from the N,N-dimethylaniline to
PINO followed by deprotonation of the anilinium radical cation.
Thus, it cannot be excluded that with the less electron-rich
substrates rates might be somewhat influenced by the second
step involving the deprotonation of the N,N-dimethylaniline
radical cation.
for λROO /ROO (g45 kcal mol-1) is possible from the rate constant
(2.1 × 106 M-1 s-1) for the reaction of N,N-dimethylaniline
with t-BuOO• reported by Das et al.47,48 Although the above
values are obtained in water, still lower values are probably to
be expected in CH3CN and EtCN since the solvent reorganiza-
tion energy accompanying the electron transfer should be larger
in water than in a less polar solvent.29 Furthermore, it is likely
that in aqueous solvents electron transfer is concerted with
proton transfer,49 and this should lead to a greater reorganization
energy.50
•
-
-
Interestingly, our λPINO/PINO value is very close to the value
determined by Espenson and his associates for the self-exchange
hydrogen atom transfer reaction of the same species (λPINO/NHPI
) 49 kcal/mol-1).42 Similar intrinsic barriers for self-exchange
ET and HAT reactions have been observed for other systems43
and have been explained by suggesting that in HAT reactions
the larger inner-sphere reorganization energy associated with
O-H bond cleavage may be partially offset by the absence of
solvent reorganization energy. The latter is instead present in
ET processes. Anyway, whatever the explanation, an important
conclusion is that for PINO the choice between exhibiting ET
or HAT reactivity should be principally dictated by thermody-
namics.
In conclusion, given the uncertainty in the self-exchange ET
reorganization energy value for the peroxyl radicals,51
a
meaningful comparison of the intrinsic reactivity in ET processes
of these radicals with that of PINO is not possible at present.
Further studies, aimed at obtaining additional and, hopefully,
definitive information on the electon-transfer reactivity of
peroxyl radicals, are in progress in our laboratories.
Experimental Section
Starting Materials. CH3CN (spectrophotometric grade) was
distilled over CaH2. Commercial samples of dicumyl peroxide and
N-hydroxyphthalimide were used as received. Ferrocene FcH (1),
ferroceneacetonitrile FcCH2CN (4), ferrocenemethanol FcCH2OH
(5), ethylferrocene FcEt (6), 1,1′-dimethylferrocene FcMe2 (7),
octamethylferrocene (8), and decamethylferrocene (9) are com-
mercially available and were further purified by sublimation
(compounds 1, 4, 5, 7, 8, and 9) or by column chromatography on
silica gel using toluene as eluent (compound 6). Ethyl ferro-
cenecarboxylate, FcCO2Et (2), was prepared by esterification of
Since PINO and peroxyl radicals have similar reduction
potentials,44 it would be also of great interest to compare the
self-exchange electron-transfer reorganization energy of these
species in order to establish if and to what extent PINO can be
(36) It should also be noted that in ref 34 the value measured for the
self-exchange rate of the ferrocene/ferrocenium couple (9 × 106 M-1 s-1
)
is twice larger than that reported in ref 12. However, also using this value,
λPINO/PINO is 50.2 kcal mol-1, which is a value not significantly different
-
from that in the text. For our calculations, we have preferred the value
reported in ref 12, since this is the value used in previous studies to
determine the reorganization energy for peroxyl radicals (vide infra).37,38
(37) Fukuzumi, S.; Shimoosako, K.; Suenobu, T.; Watanabe, Y. J. Am.
Chem. Soc. 2003, 125, 9074-9082.
(45) Jonsson, M. J. Phys. Chem. A 1996, 100, 6814-6818.
(46) Self-exchange ET energies for most closed-shell organics lie between
10 and 20 kcal mol-1; see: Eberson, L. Electron Transfer Reactions in
Organic Chemistry; Springer-Verlag: Berlin, 1986; Chapter 4, Table 5.
(47) Das, T. D. A.; Dhanasekaran, T.; Alfassi, Z. B.; Neta, P. J. Phys.
Chem. A 1998, 102, 280-284.
(38) Jovanovic, S. V.; Jankovic, I.; Josimovic, L. J. Am. Chem. Soc.
1992, 114, 9018-9021.
(39) It should also be mentioned that very recently Mayer and his
associates40 have reported that there is not much difference between adiabatic
and nonadiabatic reorganization energies for Hab values around 0.028 kcal
(48) Assuming that the self-exchange ET energy for N,N-dimethylaniline
is e15 kcal mol-1 9
.
mol-1
.
(49) Neta, P.; Huie, R. E.; Maruthamuthu, P.; Steenken, S. J. Phys. Chem.
1989, 93, 7654-7659.
(40) Rhile, I. J.; Markle, T. F.; Nagao, H.; DiPasquale, A. G.; Lam, O.
P.; Lockwood, M. A.; Rotter, K.; Mayer, J. M. J. Am. Chem. Soc. 2006,
128, 6075-6088.
(50) Sjo¨din, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarstro¨m, L.
J. Am. Chem. Soc. 2000, 122, 3932-3936. Rhile, I. J.; Mayer, J. M. J. Am.
Chem. Soc. 2004, 126, 12718-12719. Costentin, C.; Evans, D. H.; Robert,
M.; Save`ant, J.-M.; Singh, P. S. J. Am. Chem. Soc. 2005, 127, 12490-
12491. Sjo¨din, M.; Styring, S.; Wolpher, H.; Xu, Y.; Sun, L.; Hammarstro¨m,
L. J. Am. Chem. Soc. 2005, 127, 3855-3863. Rhile, I. J.; Markle, T. F.;
Nagao, H.; DiPasquale, A. G.; Lam, O. P.; Lockwood, M. A.; Rotter, K.;
Mayer, J. M. J. Am. Chem. Soc. 2006, 128, 6075-6088.
(41) Nelsen, S. F.; Pladziewicz, J. R. Acc. Chem. Res. 2002, 35, 247-
254.
(42) Cai, Y.; Koshino, N.; Saha, B.; Espenson, J. H. J. Org. Chem. 2005,
70, 238-243.
(43) Roth, J. P.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122,
5486-5498.
•
-
(44) For example, the reduction potential of cumylperoxyl radical (0.65
V vs SCE in CH3CN)37 is almost identical to that of PINO.
(51) For the problems involved in the precise evaluation of λROO /ROO
values see ref 49.
J. Org. Chem, Vol. 72, No. 23, 2007 8753