4078
J. Am. Chem. Soc. 1997, 119, 4078-4079
age of the C-C bond according to eq 1. In contrast, the decay
of 2•+ leads to the R-hydroxybenzyl type radical 3• (λmax at 300
nm), Via cleavage of the C-H bond (eq 2). In agreement with
the pulse radiolysis results, steady state γ-irradiation of 1 and
2 gave 4-methoxybenzaldehyde and 4-methoxyacetophenone,
Base-Catalyzed C-H Deprotonation of
4-Methoxybenzyl Alcohol Radical Cations in
Water: Evidence for a Carbon-to-Oxygen
1,2-H-Shift Mechanism
respectively, the latter deriving from oxidation of 3•, in the
Enrico Baciocchi,*,1a Massimo Bietti,1b and
Steen Steenken*,1b
2- 5
presence of S2O8
.
The rate constants of reactions 1 and 2 in water, measured
by the decrease of the 450 nm absorption of the radical cation,6
turned out to be 1.8 × 105 and 3.7 × 104 s-1, respectively; i.e.,
the C-C bond cleavage (of 1•+) is ca. 5 times faster than the
deprotonation reaction (of 2•+). With both 1•+ and 2•+, the
decay rates were found to increase in the presence of OH-
(Figure 1), whereby the rate constants for the OH--catalyzed
process are almost identical (k ≈ 5 × 109 M-1 s-1) for the two
radical cations. At first sight, this is a very surprising result in
view of the fact that with 1•+ the base catalysis can only involve
the O-H bond whereas with 2•+ the C-H bond is expected to
be involved.
Dipartimento di Chimica, UniVersita´ La Sapienza
Piazzale A. Moro 5, I-00185 Roma, Italy
Max-Planck-Institut fu¨r Strahlenchemie
D-45413 Mu¨lheim, Germany
ReceiVed January 27, 1997
Fragmentation reactions of radical cations, and particularly
those involving C-H and C-C bond cleavage, are attracting
continuous interest due to their many practical and theoretical
implications.2 In this research area, we recently reported that,
in aqueous solution, the cleavage of a C-C bond in alkyl-
aromatic radical cations is assisted by the presence of an R-OH
group.3 Base catalysis was observed, and a hydrogen-bonded
transition state was suggested. In an effort to throw more light
on this phenomenon, we have now discovered that an R-OH
group can assist as well the cleavage of a C-H bond (C-H
deprotonation). The results of our study, which permit more
general conclusions to be drawn on the nature of the R-OH
group assistance in fragmentation reactions of radical cations,
are presented herewith.
To gain further information on this phenomenon, we also
studied the radical cations 4•+ and 5•+, where the only possible
fragmentation reaction is C-H deprotonation, as described in
eqs 3 and 4 and as experimentally observed.7
The starting point was the similar effect of OH- on the
fragmentation reactions of the radical cations 1•+ and 2•+, the
former of which involves the cleavage of the C-C bond (eq 1,
B symbolizes a base) and the latter one, however, that of the
C-H bond (eq 2).
The kinetic study of these reactions showed that in the
absence of NaOH the rate of deprotonation of 4•+ (7.5 × 104
s-1) is similar to that of 5•+ (1.0 × 105 s-1), as expected since
the OH and OMe R-substituents should exhibit very similar
electronic effects. However, the OH--catalyzed reaction is
drastically different for the two compounds as clearly visible
from the slopes of the corresponding plots in Figure 1. The
OH--catalyzed deprotonation of 4•+ (squares) is 27 times faster
than that of 5•+ (triangles) (5.4 × 109 Vs 2.0 × 108 M-1 s-1),
although with 4•+ and 5•+ the same bond is (finally) broken.
Even more interestingly, the second-order rate constant for the
OH--catalyzed decay of 4•+ is almost identical with those for
the case of 1•+ and 2•+, as indicated above. These values (5 ×
109 M-1 s-1) are characteristic for reaction of OH- with protons
bonded to a heteroatom such as oxygen.8 The kinetic isotope
effect was also studied, using 4-MeOC6H4-CD2OH and
-CD2OMe instead of 4 and 5. With the radical cation from
the alcohol (-CD2OH), the rate constant for reaction with OH-
was 5.3 × 109 M-1 s-1, i.e., the same as that from the
The two radical cations were generated in H2O from 1 or 2
using SO4•-, and their reactions were studied by the pulse
radiolysis technique, as previously described.3 Both radical
cations showed the characteristic absorption bands centered
around 290 and 450 nm.4 It was observed that 1•+ decayed to
form 4-methoxybenzaldehyde (absorption at 285 nm) by cleav-
(1) (a) Universita´ La Sapienza. (b) Max-Planck-Institut.
(2) For some of the most recent papers on the subject see: Anne, A.;
Fraoua, S.; Moiroux, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1996, 118, 3938-
3945. Gaillard, E. R.; Whitten, D. G. Acc. Chem. Res. 1996, 29, 292-297.
Burton, R. D.; Bartberger, M. D.; Zhang, Y.; Eyler, J. R.; Schanze, K. S.
J. Am. Chem. Soc. 1996, 118, 5655-5664. Maslak, P.; Chapman, W. H.
Jr. J. Org. Chem. 1996, 61, 2647-2656. de Lijser, H. J. P.; Arnold, D. R.
J. Phys. Chem. 1996, 100, 3996-4010. Anne, A.; Fraoua, S.; Hapiot, P.;
Moiroux, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1995, 117, 7412-7421.
Maslak, P.; Chapman, W. H. Jr.; Vallombroso, T. M. Jr.; Watson, B. A. J.
Am. Chem. Soc. 1995, 117, 12380-12389. Baciocchi, E.; Del Giacco, T.;
Elisei, F. J. Am. Chem. Soc. 1993, 115, 12290-12295. Anne, A.; Hapiot,
P.; Moiroux, J.; Neta, P.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114,
4694-4700. Steenken, S.; McClelland, R. A. J. Am. Chem. Soc. 1989, 111,
4967-4973.
(5) Irradiations were carried out on argon-saturated aqueous solutions
containing the substrate, K2S2O8 (substrate/oxidant ratio g2), and 0.2 M
2-methyl-2-propanol at room temperature, using a 60Co γ-source at dose
rates of 0.3-0.9 Gy s-1, for the time necessary to obtain a 40% conversion
with respect to peroxydisulfate. Products were identified and quantitatively
determined by HPLC (comparison with authentic samples).
(6) Experiments were carried out at 25 °C by pulse-irradiating argon-
saturated aqueous solutions containing the substrate, peroxydisulfate, and
2-methyl-2-propanol. For the kinetic study of the radical cation decay, the
ionic strength of the solution was buffered with 0.5 M Na2SO4, and 1 mM
Na2HPO4 was added to avoid undesired pH changes upon irradiation. The
decay followed first-order kinetics. Second-order rate constants for the base-
induced process were obtained from the slope of the plots of kobs Vs [NaOH].
(7) The two radical cations were generated from 4 and 5, respectively,
and their decay kinetically studied as described in ref 6. In both cases, the
decay of the radical cation, monitored at 450 nm, produced the correspond-
ing carbon-centered radical (absorption at about 300 nm), as described in
eqs 3 and 4.
(3) Baciocchi, E.; Bietti, M.; Putignani, L.; Steenken, S. J. Am. Chem.
Soc. 1996, 118, 5952-5960.
(4) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J. Phys. Chem. 1975,
79, 2773-2779.
(8) Eigen, M. Angew. Chem. 1963, 75, 489-508.
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