Structural effects on the OH--promoted fragmentation of methoxy-substituted 1-arylalkanol radical cations in aqueous solution: The role of oxygen acidity

Article abstract of DOI:10.1002/1521-3765(20010401)7:7<1408::AID-CHEM1408>3.0.CO;2-M

A kinetic and product study of the OH^--induced decay in H₂O of the radical cations generated from some di- and tri-methoxy-substituted 1-arylalkanols (ArCH(OH)R^·+) and 2- and 3-(3,4-dimethoxyphenyl) alkanols has been carried out by using pulse- and γ-radiolysis techniques. In the 1-arylalkanol system, the radical cation 3,4-(MeO)₂C₆H₃CH₂OH ^·+ decay at a rate more than two orders of magnitude higher than that of its methyl ether; this indicates the key role of the side-chain OH group in the decay process (oxygen acidity). However, quite a large deuterium kinetic isotope effect (3.7) is present for this radical cation compared with its α-dideuterated counterpart. A mechanism is suggested in which a fast OH deprotonation leads to a radical zwitterion which then undergoes a rate-determining 1,2-H shift, coupled to a side-chain-to-ring intramolecular electron transfer (ET) step. This concept also attributes an important role to the energy barrier for this ET, which should depend on the stability of the positive charge in the ring and, hence, on the number and position of methoxy groups. On a similar experimental basis, the same mechanism is suggested for 2,5-(MeO)₂C₆H₃CH₂OH ^·+ as for 3,4-(MeO)₂C₆H₃CH₂OH ^·+, in which some contribution from direct C-H deprotonation (carbon acidity) is possible. In fact, the latter process dominates the decay of the trimethoxylated system 2,4,5-(MeO)₃C₆H₂CH₂OH ^·+, which, accordingly, reacts with OH^- at the same rate as that of its methyl ether. Thus, a shift from oxygen to carbon acidity is observed as the positive charge is increasingly stabilized in the ring; this is attributed to a corresponding increase in the energy barrier for the intramolecular ET. When R = tBu, the OH^--promoted decay of the radical cation ArCH(OH)R^·+ leads to products of C-C bond cleavage. With both Ar = 3,4- and 2,5-dimethoxyphenyl the reactivity is three orders of magnitude higher than that of the corresponding cumyl alcohol radical cations; this suggests a mechanism in which a key role is played by the oxygen acidity as well as by the strength of the scissile C-C bond: a radical zwitterion is formed which undergoes a rate-determining C-C bond cleavage, coupled with the intramolecular ET. Finally, oxygen acidity also determines the reactivity of the radical cations of 2-(3,4-dimethoxyphenyl)ethanol and 3-(3,4-dimethoxyphenyl)propanol. In the former the decay involves C-C bond cleavage, in the latter it leads to 3-(3,4-dimethoxyphenyl) propanal. In both cases no products of C-H deprotonation were observed. Possible mechanisms, again involving the initial formation of a radical zwitterion, are discussed.

Full text of DOI:10.1002/1521-3765(20010401)7:7<1408::AID-CHEM1408>3.0.CO;2-M

FULL PAPER
Structural Effects on the OH -Promoted Fragmentation of
Methoxy-Substituted 1-Arylalkanol Radical Cations in Aqueous Solution:
The Role of Oxygen Acidity
Enrico Baciocchi,*[a] Massimo Bietti,*[b] Maria Francesca Gerini,[c] Laura Manduchi,[c]
Michela Salamone,[b] and Steen Steenken*[d]
Abstract: A kinetic and product study
of the OH -induced decay in H2O of the
radical cations generated from some di-
and tri-methoxy-substituted 1-arylalka-
for this ET, which should depend on the
stability of the positive charge in the ring
and, hence, on the number and position
of methoxy groups. On a similar exper-
imental basis, the same mechanism is
R ˆ tBu, the OH -promoted decay of
.
‡
the radical cation ArCH(OH)R leads
to products of C C bond cleavage. With
both Arˆ 3,4- and 2,5-dimethoxyphenyl
the reactivity is three orders of magni-
tude higher than that of the correspond-
ing cumyl alcohol radical cations; this
suggests a mechanism in which a key
role is played by the oxygen acidity as
well as by the strength of the scissile
.
‡
nols (ArCH(OH)R ) and 2- and 3-(3,4-
dimethoxyphenyl)alkanols has been car-
ried out by using pulse- and g-radiolysis
techniques. In the 1-arylalkanol system,
the radical cation 3,4-(MeO)2C6H3CH2-
.
‡
suggested for 2,5-(MeO)2C6H3CH2OH
.
‡
as for 3,4-(MeO)2C6H3CH2OH
, in
which some contribution from direct
C H deprotonation (carbon acidity) is
possible. In fact, the latter process
dominates the decay of the trimethoxyl-
ated system 2,4,5-(MeO)3C6H2CH2-
.
‡
OH decay at a rate more than two
orders of magnitude higher than that of
its methyl ether; this indicates the key
role of the side-chain OH group in the
decay process (oxygen acidity). How-
ever, quite a large deuterium kinetic
isotope effect (3.7) is present for this
radical cation compared with its a-
dideuterated counterpart. A mechanism
is suggested in which a fast OH depro-
tonation leads to a radical zwitterion
which then undergoes a rate-determin-
ing 1,2-H shift, coupled to a side-chain-
to-ring intramolecular electron transfer
(ET) step. This concept also attributes
an important role to the energy barrier
C C bond:
a radical zwitterion is
formed which undergoes a rate-deter-
mining C C bond cleavage, coupled
with the intramolecular ET. Finally,
oxygen acidity also determines the re-
activity of the radical cations of 2-(3,4-
dimethoxyphenyl)ethanol and 3-(3,4-di-
methoxyphenyl)propanol. In the former
the decay involves C C bond cleavage,
in the latter it leads to 3-(3,4-dimethoxy-
phenyl)propanal. In both cases no prod-
ucts of C H deprotonation were ob-
served. Possible mechanisms, again in-
volving the initial formation of a radical
zwitterion, are discussed.
.
‡
OH , which, accordingly, reacts with
OH at the same rate as that of its
methyl ether. Thus, a shift from oxygen
to carbon acidity is observed as the
positive charge is increasingly stabilized
in the ring; this is attributed to a
corresponding increase in the energy
barrier for the intramolecular ET. When
Keywords: acidity
electron transfer
radiolysis ´ radical ions
´
alcohols
´
´
pulse
[a] Prof. E. Baciocchi
Á
Dipartimento di Chimica, Universita ªLa Sapienzaº
Introduction
P. le A. Moro, 5, 00185 Rome (Italy)
Fax : (‡39)06-490421
Recently, we found convincing evidence that the OH -
promoted decay of the 4-methoxybenzyl alcohol radical
cation to the a-hydroxy-4-methoxybenzyl radical does not
involve the expected Ca H deprotonation (carbon acidity),
but is initiated by deprotonation at the alcoholic OH group
(oxygen acidity).[1, 2] The mechanism reported in Scheme 1
(R ˆ H) was suggested, in which the radical cation and the
OH first form an encounter complex, which is then O H
deprotonated to produce a benzyloxyl radical, presumably
through a radical zwitterion (paths b and c).
E-mail: baciocchi@caspur.it
[b] Dr. M. Bietti, M. Salamone
Dipartimento di Scienze e Tecnologie Chimiche
Universita ªTor Vergataº, Via della Ricerca Scientifica
Á
00133 Rome (Italy)
Fax : (‡39)06-72594328
E-mail: bietti@uniroma2.it
[c] M. F. Gerini, L. Manduchi
Dipartimento di Chimica e Centro CNR per i Meccanismi di Reazione
Universita ªLa Sapienzaº, P. le A. Moro, 5, 00185 Rome (Italy)
Á
[d] Prof. S. Steenken
Max-Planck-Institut für Strahlenchemie
Stiftstrasse 34-36, 45413 Mülheim (Germany)
Fax : (‡49)208-306-3951
The benzyloxyl radical undergoes a formal 1,2-H atom shift
(from here on indicated as 1,2-H shift) to form the a-hydroxy
carbon radical (path d).[3, 4] The same mechanism holds for
E-mail: steenken@mpi-muelheim.mpg.de
1408
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Chem. Eur. J. 2001, 7, No. 7

1408±1416
mote position with respect to
the ring.[6] Thus, the 2-(4-me-
thoxyphenyl)ethanol radical
.
‡
cation (5 ) reacted with OH
at an almost diffusion-control-
led rate and formed products
of b-C C bond cleavage,
whereas 3-(4-methoxyphenyl)-
.
‡
propanol (6 ) was converted
into 3-(4-methoxyphenyl)pro-
panal. In the former case, in-
tramolecular electron transfer
(ET) coupled to C C bond
cleavage either in the encoun-
ter complex or in the radical
zwitterion was considered as a
possible mechanism. In the
second case, formation of an
alkoxyl radical,[6] which then
undergoes a 1,2-H atom shift,
was suggested.
Scheme 1. Proposed mechanism for the OH -promoted decay of 1-(4-methoxyphenyl)alkan-1-ol radical cations.
R ˆ Me, whereas when R ˆ Et, iPr or tBu, the benzyloxyl
radical can also undergo a b-fragmentation reaction produc-
ing 4-methoxybenzaldehyde and an alkyl radical (path e).
The evidence supporting the main features of this mecha-
nism, and particularly the role of the OH group, is as follows:
a) the reaction of both the 4-methoxybenzyl alcohol and the
1-(4-methoxyphenyl)-2,2-dimethylpropan-1-ol radical cations
In order to extend our investigation to systems other than
monomethoxylated ones, we also studied the OH -induced
decay of 3,4-dimethoxybenzyl alcohol (veratryl alcohol) and
1-(3,4-dimethoxyphenyl)-2,2-dimethylpropan-1-ol radical cat-
.
.
‡
‡
ions (7 and 8 , respectively, see below) in aqueous solution.
This study produced results quite similar to those obtained
with the monomethoxylated systems and led us to suggest that
the mechanism reported in Scheme 1 also holds true for the
more stable radical cations.[7] An intriguing result was,
.
.
‡
‡
(1 and 2 , respectively) with OH takes place at a diffusion-
.
‡
however, that the reactivity of 7 was significantly smaller
.
‡
than that of 8 and, hence, below the diffusion limit. This
contrasts with the situation observed in the monomethoxyl-
.
.
‡
‡
ated systems, where the reactions of both 1 and 2 were
diffusion controlled.[2]
Oxygen acidity represents a really novel aspect of the side-
chain reactivity of arylalkanol radical cations and the
definition of its scope and mechanism is certainly worth
further investigation. Thus, it seemed appropriate to get a
deeper mechanistic insight into the OH -promoted reactions
of ring-substituted di- and trimethoxybenzyl alcohol radical
cations with the aim of determining if and how oxygen acidity
is influenced by the number and position of ring methoxy
groups. An additional motive of interest is that dimethox-
ylated benzyl alcohol radical cations (particularly the 3,4-
dimethoxy systems) are structurally very closely related to the
radical cations supposed to be key intermediates in the
oxidative degradation of lignin,[7, 8] which is indeed triggered
1
controlled rate (k ˆ 1.2  1010 m 1 s ),[2] as expected for a
process that involves an oxygen acid; b) a significant lowering
of reactivity (k ˆ 2.5  108 m 1 s )[2] is observed on going from
1
.
.
‡
‡
1
to the corresponding methyl ether radical cation (3 )
which can only undergo direct Ca H bond cleavage and
c) formation of the 4-methoxycumyloxyl radical is observed
when the 4-methoxycumyl alcohol radical cation (4 ) is
.
‡
reacted with OH .[5] More im-
.
‡
portantly, the reaction of 4
with OH is diffusion control-
.
.
‡
‡
led as is that of 1 and 2 . The
cumyloxyl radical then under-
goes a b-fragmentation reaction
leading to 4-methoxyacetophe-
.
none by loss of Me (Scheme 2).
The oxygen acidity was even
found to play a role when the
OH group was in a more re-
Scheme 2. Formation of an intermediate cumyloxyl radical in the reaction of the 4-methoxycumyl alcohol radical
cation with OH leading to 4-methoxyacetophenone.
Chem. Eur. J. 2001, 7, No. 7
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1409

E. Baciocchi, M. Bietti, S. Steenken et al.
FULL PAPER
by the simple fragmentation reactions of these species. Thus,
this study could also have a bearing on lignin degradation, a
process of enormous practical importance.
In this paper we report on a pulse- and g-radiolysis study of
the reactions of a number of radical cations generated from
1-arylalkanols and some of the corresponding methyl ethers
(compounds 7 ± 18), 2-(3,4-dimethoxyphenyl)ethanol (19) and
2.0mm substrate, 0.5 ± 2.0mm K2S2O8 and 0.2m 2-methyl-2-
propanol at room temperature for the time necessary to
obtain a 40 ± 60% conversion with respect to peroxydisulfate.
In order to minimize overoxidation of the first-formed
products, most experiments were carried out with a sub-
strate/oxidant ratio ꢂ2:1.
Product distributions of the oxidation reactions of the
4-methoxyphenyl derivatives 1 ± 4 and the 3,4-dimethoxy-
phenyl derivatives 7 ± 9, under analogous experimental con-
ditions, have been described previously.[2, 5±7]
The reaction of 11 led to 3,4-dimethoxyacetophenone (yield
.
5%, with respect to SO4 ) as the exclusive product; whereas,
under the same reaction conditions, 16 was practically
unreactive.
Compounds 12, 13, and 14 gave 2,5-dimethoxybenzalde-
hyde as the exclusive product, with the yields (52%, 85%, and
4%, respectively) reflecting the different reactivities of the
substrates (see later).[2] Compounds 17 and 18 led to 2,4,5-
trimethoxybenzaldheyde.
The reaction of 19 led to 3,3',4,4'-tetramethoxybibenzyl and
2-methyl-4-(3,4-dimethoxyphenyl)-2-butanol as the main
products. Traces of 3,4-dimethoxybenzaldehyde were also
formed. These products clearly indicate that the radical cation
undergoes C C bond cleavage that leads to the 3,4-dimeth-
oxybenzyl radical. Compound 20 gave 3-(3,4-dimethoxyphe-
nyl)propanal as the exclusive product (yield 10%, with
.
respect to SO4 ).
3-(3,4-dimethoxyphenyl)propanol (20). For comparison pur-
Pulse-radiolysis studies: The radical cations were produced
according to Equations (1) ± (4). In all cases they exhibited the
characteristic UV and visible absorption bands of methoxy-
benzene-type radical cations,[11, 12] at around 290 ± 310 and
425 ± 455 nm depending on the substitution pattern.
poses, the reactivity of 3,4- and 2,5-dimethoxytoluene radical
.
.
‡
‡
cations (10 and 15 , respectively) under the same reaction
conditions was also investigated.
As an example, the time-resolved spectra obtained by
oxidation of 13 at pH 10.4 are given in Figure 1. The spectra
Results
Generation of the radical cations: Radical cations of sub-
strates 7 ± 20 were generated in aqueous solution either by
pulse radiolysis or steady state g-radiolysis, with a sulfate
.
radical anion (SO4 ) as the oxidant [Eqs. (1) ± (4)].
.
‡
H2O H , OH ‡ eꢀaq†
(1)
(2)
(3)
(4)
.
.
OH ‡ CH3C(CH3)2OH ! H2O ‡ CH2C(CH3)2OH
.
2
2
eꢀaq† ‡ S2O8 ! SO4 ‡ SO4
.
.
‡
2
SO4 ‡ ArCH(OH)R ! SO4
‡
ArCH(OH)R
Radiolysis of water leads to the formation of the hydroxyl
.
radical ( OH) and the hydrated electron (eꢀaq†† [Eq. (1)]. The
former is scavenged by 2-methylpropan-2-ol [Eq. (2); k ˆ 6 Â
1
108 m 1 s ][9] while eꢀaq† reacts with the peroxydisulfate anion;
.
this leads to the formation of SO4 [Eq. (3); k ˆ 1.2 Â
.
Figure 1. Time-resolved absorption spectra observed on reaction of SO4
.
1010 m 1 s ].[9] SO4 then reacts with aromatic compounds[10]
to form the corresponding radical cations [Eq. (4); k ˆ 5 Â
109 m 1 s 1 with anisole and derivatives[11, 12]].
1
with 13 (0.1mm) and recorded by pulse radiolysis of an argon-saturated
aqueous solution (pH 10.4) containing 2-methylpropan-2-ol (0.1m) and
K2S2O8 (10mm), at 4 ( ), 15 ( ), 30 ( ) and 140 ms ( ) after the 300 ns,
&
*
*
^
.
‡
3 MeV electron pulse. Insets: a) First-order decay of 13 monitored at
.
‡
455 nm. b) First-order decay of 13 monitored at 310 nm (the residual
absorption is due to the formation of 2,5-dimethoxybenzaldehyde).
c) Buildup of absorption at 360 nm due to formation of 2,5-dimethoxy-
benzaldehyde (Ar), which is unchanged in the presence of O2 (inset d).
Product analysis: Product analysis of the oxidation reactions
of 1-arylalkanols was carried out after steady-state g-radiol-
ysis of Ar-saturated aqueous solutions (pH ꢁ 10) of 1.0 ±
1410
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Chem. Eur. J. 2001, 7, No. 7

Fragmentation of Radical Cations
1408±1416
.
‡
Table 3. Second-order rate constants (kOH [m 1 s 1]) for the OH -promot-
ed decay of 2- and 3-arylalkanol radical cations generated by pulse
radiolysis of the parent substrate in aqueous solution, measured at Tˆ
(25 Æ 0.1)8C.
show the characteristic absorptions due to 13 : the bands
being centered at 310 and 455 nm. These absorptions reach a
maximum at 4 ms after the pulse due to completion of radical
cation formation [Eq. (4)] and then decrease (see Figure 1
insets a and b) as a consequence of the first-order decay of the
radical cation. This decay is accompanied (inset c) by a
corresponding build up of optical density centered at 360 nm;
this indicates that a product is formed that has a higher
absorption at this wavelength than the radical cation itself
(two isosbestic points are visible at ꢁ330 and 390 nm). Inset d
shows that the absorption build up at 360 nm is unaffected by
O2; this suggests that the species responsible is not a radical. It
was identified as 2,5-dimethoxybenzaldehyde, formed after
1
substrate
kOH [m 1 s
]
.
.
‡
‡
.
.
4-MeOC6H4CH2CH2OH[a]
(5
(6
)
)
8.3 Â 109
1.7 Â 109
3.4 Â 108
2.1 Â 108
4-MeOC6H4CH2CH2CH2OH[a]
3,4-(MeO)2C6H3CH2CH2OH
3,4-(MeO)2C6H3CH2CH2CH2OH
‡
‡
(19
(20
)
)
[a] From ref. [6].
of the C H bond, can be compared with those of their methyl
ethers as well as with those of the previously investigated
4-methoxy-substituted system.[2] Considering the results for
.
‡
C C bond cleavage in 13 (Scheme 1), in agreement with
product studies (see above).
.
‡
The rates of decay of the radical cations were measured
spectrophotometrically by monitoring the decrease in optical
density at the visible absorption maximum of the radical
cations. The rates significantly increased upon addition of
OH . Clean first-order decays were observed and linear
dependencies were obtained of the observed rates (kobs) on
the concentrations of added OH . From the slopes of these
plots, the second-order rate constants for reaction of OH
with the radical cations (kOH ) were obtained. The data are
listed in Tables 1 and 2 in which previously determined kinetic
constants for the monomethoxylated systems and for some
3,4-dimethoxy substituted compounds are also displayed.
the 3,4-dimethoxybenzyl alcohol radical cation (7 ), one very
striking observation is that a significant kinetic deuterium
.
‡
isotope effect (KDIE, kH/kD ˆ 3.7) is found when 7 is
compared with its a-dideuterated counterpart. This indicates
that substantial cleavage of the b-C H bond occurs in the
rate-determining step of the reaction. This conclusion is not
consistent with our previous suggestion that the mechanism
.
‡
proposed for 1 , formation of a base/radical-cation encounter
complex in the rate-determining step (Scheme 1), can be
.
.
‡
‡
extended to 7 . In the case of 1 , the reaction was diffusion
controlled and, evidently, no KDIE was observed.
.
‡
The most simple explanation would be that 7 , which is a
.
‡
The second order rate constants for reaction of 19 and
20 with OH were measured analogously and are listed in
more stable radical cation and therefore a somewhat weaker
.
.
‡
‡
acid than 1 , no longer exhibits a kinetic oxygen acidity. Thus,
the reaction with OH might occur by direct C H deproton-
ation. However, this conclusion contrasts sharply with the
observation that the corresponding methyl ether radical
Table 3.
Discussion
.
‡
cation (9 ) reacts with OH by C H deprotonation with a
Benzyl alcohol radical cations: In Table 1, the kinetic data for
the dimethoxy- and trimethoxy-substituted benzyl alcohol
radical cations, whose decay ultimately involves the cleavage
rate constant more than two orders of magnitude smaller than
.
‡
1
that measured for 7 (7.9 Â 106 versus 1 Â 109 m 1 s ); this
clearly indicates that the OH
group also plays a crucial role in
the dimethoxylated system. Ac-
cordingly, in acidic solution
(pH ꢃ 5), where C H deproto-
nation is taking place exclusive-
Table 1. Second-order rate constants (kOH [m 1 s 1]) for the OH -promoted decay of ring methoxylated radical
cations ArR , generated by pulse radiolysis of the parent substrate in aqueous solution, measured at Tˆ (25 Æ
0.1)8C.
.
‡
Ar/R
CH3
CH2OH
CD2OH
CH2OMe
CD2OMe
4-MeOC6H4
5.5 Â 107[a]
1.2 Â 1010[b]
1.1 Â 1010[b]
2.5 Â 108[b]
1.4 Â 108[b]
.
.
‡
‡
ly, 7 and 9 exhibit a very
similar reactivity.[7] A different
mechanism for the OH -pro-
kH/kD ˆ 1.1
kH/kD ˆ 1.8
3,4-(MeO)2C6H3
2,5-(MeO)2C6H3
2,4,5-(MeO)3C6H2
2.1 Â 106
1.2 Â 105
±
1.0 Â 109[c]
2.9 Â 106
7.3 Â 104
2.7 Â 108
7.9 Â 106[c]
4.5 Â 105
5.8 Â 106
kH/kD ˆ 3.7
kH/kD ˆ 1.4
9.3 Â 105
2.8 Â 105
.
.
‡
‡
moted decay of 7 and 9 is
also indicated by the substantial
difference in the KDIE values:
kH/kD ˆ 3.2
kH/kD ˆ 1.6
±
6.7 Â 104
±
[a] From ref. [6]. [b] From ref. [3]. [c] From ref. [7].
.
.
‡
‡
3.7 for 7 and 1.4 for 9 . Thus
it is possible to suggest that the
.
‡
OH -promoted decay of 7 is
Table 2. Second-order rate constants (kOH [m 1 s 1]) for the OH -promot-
ed decay of methoxy-substituted 1-aryl-2,2-dimethylpropan-1-ol and cumyl
alcohol radical cations, generated by pulse radiolysis of the parent substrate
in aqueous solution, measured at Tˆ (25 Æ 0.1)8C.
also initiated by OH deprotonation even though the observed
KDIE tells us that substantial C H bond cleavage has
occurred in the transition state of the rate-determining step
of the overall deprotonation process.
Ar/R
CH(OH)tBu
C(CH3)2OH
The simplest way to deal with this apparently contradictory
.
‡
.
.
.
.
.
‡
‡
.
‡
‡
‡
1.3 Â 1010[a]
(4
(11
(16
)
)
)
1.2 Â 1010[b]
5.6 Â 106
1.6 Â 105
situation is by saying that, with 7 , the benzyloxyl radical is
formed in a fast step followed by a rate-determining 1,2-H
shift (Scheme 1, path d), a completely reversed situation from
4-MeOC6H4
3,4-(MeO)2C6H3
2,5-(MeO)2C6H3
(2
(8
)
)
8.3 Â 109[c]
‡
(13
)
1.5 Â 108
.
‡
that observed for 1 . However, this possibility appears
[a] From ref. [2]. [b] From ref. [5]. [c] From ref. [7].
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E. Baciocchi, M. Bietti, S. Steenken et al.
FULL PAPER
unlikely as it would have allowed us to observe the buildup of
the intermediate benzyloxyl radical, which was not the case.
Thus, the most reasonable hypothesis that would accommodate
both the observed C H KDIE and a mechanism involving
oxygen acidity is that a radical zwitterion is first formed in a
fast step and is then converted to the carbon-centered radical
by a rate-determining process in which intramolecular ET is
coupled to a 1,2-H shift (Scheme 3).
convert the radical zwitterion into the carbon-centered radical
in only one step . Therefore, if the above hypothesis is correct,
the value (3.7) of the KDIE measured for 7 would pertain to
the 1,2-H shift in the zwitterion, a process which reasonably
should exhibit features different from those of a direct OH -
induced C H deprotonation.[19, 20]
.
‡
Passing on to examine the results obtained with 2,5-
dimethoxybenzyl alcohol (12), we first noted that the decay
.
‡
of 12 is much slower (about
.
‡
300 times) than that of 7 .
However, the reaction exhibits
a
KDIE value (kH/kD ˆ 3.2,
.
‡
from the comparison of 12
with its a-dideuterated counter-
part) which is very close to that
observed for 7 . In this case
Scheme 3. Proposed mechanism for the OH -promoted decay of the veratryl alcohol radical cation.
.
‡
too, the value is significantly
The difference in the mechanistic behavior of the two
different from that found for the corresponding methyl ether
14 (1.6), which can only undergo direct C H deprotonation.
As expected, the latter value is very similar to that observed
.
.
.
‡
‡
‡
radical cations 1 and 7 may be attributed to the greater
energy barrier for the intramolecular side-chain-to-ring
electron transfer, which has to accompany the OH deproton-
ation in order to produce the benzyloxyl radical, in the
dimethoxylated system than in the monomethoxylated oneÐ
the ring-localized positive charge being more efficiently
stabilized in the former system than in the latter.[13] Thus, in
.
.
.
‡
‡
‡
for 9 , the methyl ether of 7 . It is also found that 12 is
.
‡
more reactive than 14 , however the difference is much
.
.
‡
‡
smaller than that observed between 7 and 9
Thus, we also suggest that the OH -promoted decay of 12
involves initial OH deprotonation, the mechanism being the
.
.
‡
.
.
‡
‡
the radical zwitterion formed from 7 the energetic cost for
this intramolecular ET is likely to be so high as to require
assistance from the 1,2-H shift. With the coupled intra-
molecular ET-1,2-H shift the (stabilized) benzyl radical is
formed directly, bypassing the less-stable benzyloxyl radical.
same as that proposed for 7 . The lower reactivity with
.
‡
respect to 7 is probably due to the larger stabilization of the
positive charge in the 2,5-dimethoxy system as compared with
the 3,4-dimethoxy one.[22, 23] In other words, the intramolec-
ular ET coupled to the 1,2-H shift should require more energy
in the radical zwitterion derived from 12 than in that derived
from 7 . Even though OH deprotonation seems to remain the
main reaction channel for the decay of 12 , the possibility
that direct C H deprotonation begins to compete with this
system cannot be excluded. As already noted, this possibility
is suggested by the relatively small difference in reactivity
between 12 and its methyl ether 14 . If this is actually the
case, it would indicate that the stabilization of the positive
charge in the ring depresses the oxygen acidity more than the
C H acidity. Also in line with this conclusion is the
.
.
‡
‡
In other words, while 1 undergoes stepwise intramolecular
.
.
‡
‡
ETand 1,2-H shift, in 7 the two processes are coupled, due to
.
the higher barrier for the intramolecular ET.[15]
‡
In this respect, it is interesting to note that the role of the
energy associated with the intramolecular ET in determining
the reaction paths of radical zwitterions has already been
recognized in a study concerning the conversion of aromatic
carboxylate radical zwitterions into the corresponding acyl-
oxyl radicals.[16] It was observed that this process is much more
efficient with the benzoate radical zwitterion than with the
4-methoxybenzoate one. In that case too, the difference was
attributed to an energetically more costly intramolecular ET
in the methoxylated system.
.
.
‡
‡
.
‡
observation that 12 is about 300 times less reactive than
.
‡
7 , whereas when direct C H deprotonation occurs (methyl
ethers and toluenes), the difference in reactivity between the
3,4- and the 2,5-dimethoxy system is less than 20. (Compare
with 14 and 10 with 15 .)
The results obtained with the 2,4,5-trimethoxybenzyl alco-
hol radical cation (17 ) are also consistent. With this
substrate, no difference in reactivity between 17 and its
methyl ether (18 ) is observed (Table 1); this indicates that
The 1,2-H shift suggested to occur in the radical zwitterion
.
.
.
.
.
‡
‡
‡
‡
‡
formed from 7 is unprecedented. However, it has features
similar to those of the well-established 1,2-H shift in alkoxyl
radicals[3, 4, 17] being favored by the solvent water as illustrated
in Scheme 4.
9
.
‡
.
‡
.
‡
As a matter of fact, the proposed mechanism (Scheme 3)
differs from the mechanism in Scheme 1 in that it incorporates
the electronic and structural reorganization necessary to
both species react by direct C H deprotonation, with no role
.
‡
played by the OH group. Practically, in 17 the positive
charge is so efficiently stabilized by the three methoxy
groups[24] that intramolecular ET in the radical zwitterion is
very unfavorable. It is possible that the OH group is still
involved in a side process, but the progress of the reaction is
predominantly determined by C H deprotonation.
The observation that the stabilization of the positive charge
in the ring affects oxygen more than carbon acidity can
reasonably be explained by the fact that, in the latter case,
Scheme 4. Water-assisted 1,2-H shift in the radical zwitterion of veratryl
alcohol.
1412
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Chem. Eur. J. 2001, 7, No. 7

Fragmentation of Radical Cations
1408±1416
there is a direct overlap between the C H bond and the
aromatic p-system. This might significantly lower the ener-
getic barrier for the intramolecular ET and make it fairly
insensitive to the effect of electron-donating substituents in
the ring. In the radical zwitterion, on the other hand, the
negatively charged oxygen and the positively charged aro-
matic ring are separated by a methylene group and a higher
energy barrier for the intramolecular ET is expected.
well as by a decrease in the OH-deprotonation rate of the
radical cations themselves. However, both hypotheses are
unlikely. An increase in the rate of b-cleavage of the
.
.
‡
‡
intermediate benzyloxyl radical as we move from 4 to 11
.
‡
and 16 can be excluded since the rates of b-cleavage of some
ring-substituted cumyloxyl radicals have been found to be
practically unaffected by the nature and number of substitu-
ents.[30] Likewise, a significantly lower rate of OH deprotona-
.
.
.
‡
‡
‡
We have already discussed the competition between oxygen
and carbon acidity in 1-(4-methoxyphenyl)alkanol radical
cations.[2] Our conclusion was that with relatively weak bases,
the largely thermodynamically favored C H deprotonation
prevails; with strong bases, however, the driving force is no
longer a problem and the route involving OH deprotonation
takes over, due to the lower intrinsic barrier of this process
with respect to that of C H deprotonation.[25±29] In the light of
the present results this view has to be somewhat modified, the
main effect of the base being that of promoting the formation
of the radical zwitterion in a process with a low intrinsic
barrier. Hence, the follow-up of the reaction seems to depend
upon the energetics of the intramolecular side-chain-to-ring
electron transfer in the radical zwitterion. When the energy
barrier for the intramolecular ET is relatively low, the
reaction proceeds via the radical zwitterion, as suggested for
mono- and dimethoxy-substituted benzyl alcohol radical
cations, and the reaction is controlled by oxygen acidity.
However, when this barrier becomes high (2,4,5-trimethoxy
substituted system), the radical zwitterion is still formed but it
is no longer a productive reaction intermediate and direct
C H deprotonation in the radical cation takes over.
tion with 11 and 16 than with 4 is made unlikely by the
.
.
‡
‡
results obtained with the a-tert-butyl derivatives 8 and 13
which were found to react with OH at rates three orders of
magnitude higher than those measured for 11 and 16
.
.
‡
‡
,
respectively (Table 2). This large difference in rate is incom-
patible with a slow OH-deprotonation step as very similar
.
.
‡
‡
oxygen acidities can be assumed for 8 and 11 as well as for
.
.
‡
‡
13 and 16
.
Thus, it seems reasonable to suggest a mechanism resem-
.
.
‡
‡
bling that proposed for 7 and 12 . As illustrated in
Scheme 5 for the 3,4-dimethoxy derivatives, OH deprotona-
tion leads to a radical zwitterion that undergoes intramolec-
ular ET coupled with C C bond cleavage in the slow step and
forms the carbonyl product directly. Through this concerted
process the energy of the transition state can be lowered by
ˆ
partial formation of a ring-conjugated C O double bond. As
.
‡
observed for 4 , no benzyloxyl radical is formed, probably
due to the higher energy barrier for intramolecular ET in
dimethoxylated radical zwitterions already discussed. Since
C C bond cleavage has progressed to some extent in the
transition state of the rate-determining step, the observation
.
‡
that the radical cations undergoing C tBu cleavage (8 and
13 ) are much more reactive than those undergoing C Me
cleavage (11 and 16 ) is easily accounted for on the basis of
the much higher stability of tBu as compared with Me .
The higher reactivity of 11 relative to 16 is also
consistent with the above proposal since, as already discussed,
the intramolecular ET should be energetically more costly in
the radical zwitterion from the latter than in that from the
former. In this respect, it is noteworthy that practically the
.
‡
.
.
‡
‡
Oxygen acidity and C C bond cleavage: The effects discussed
above also have a significant bearing on the OH -promoted
C C bond cleavage reaction of 1-arylalkanol radical cations
illustrated in Schemes 1 (paths e) and 2. Accordingly, for this
process as well, the introduction of a second methoxy group
induces a change in mechanism. This can be clearly seen by
.
.
.
.
‡
‡
.
.
‡
‡
the fact that in the cumyl alcohol series, 11 and 16 are
converted into the corresponding acetophenones at a much
lower rate (more than three orders of magnitude) than the
.
.
‡
‡
same difference in reactivity is also observed for 8 /13
.
.
‡
monomethoxylated 4 (Table 2). The latter reacts with OH
Reactivity of 2- and 3-(3,4-dimethoxyphenyl)alkanol radical
cations: The data displayed in Table 3 clearly show that the
OH group continues to play a key role in the OH -induced
decay of 3,4-dimethoxy-substituted phenylalkanol radical
cations even when two or three methylene groups are
interposed between the OH group and the ring. Accordingly,
at a diffusion-controlled rate,[5] whereas the rate constants for
.
.
‡
‡
1
11 and 16 are as low as 5.6 Â 106 and 1.6 Â 105 m 1 s ,
respectively. Moreover, while formation of the intermediate
benzyloxyl radical was clearly observed in the reaction of 4
no such evidence was obtained for either 11 or 16 . This
indicates that with the last two radical cations an intermediate
benzyloxyl radical is either not formed at all or it decomposes
at a rate much faster than that of its formation. If the latter
possibility is true, the much smaller reactivity of 11 and 16
compared with 4 can be explained by an increase in the rate
of b-fragmentation of the intermediate benzyloxyl radical as
.
‡
,
.
.
‡
‡
.
.
‡
‡
both 19 and 20 are about 100 times more reactive than the
.
‡
3,4-dimethoxytoluene radical cation (10 ) which decays by
C H deprotonation. Further strong support for the hypoth-
esis of OH deprotonation comes from the nature of the
.
.
‡
‡
.
.
‡
‡
reaction productsÐwith 19 products derived from C C
bond cleavage in the radical cation are observed, whereas the
Scheme 5. Proposed mechanism for the OH -promoted decay of the 3,4-dimethoxycumyl alcohol radical cation.
Chem. Eur. J. 2001, 7, No. 7
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1413

E. Baciocchi, M. Bietti, S. Steenken et al.
FULL PAPER
.
‡
decay of 20 leads to 3-(3,4-dimethoxyphenyl)propanal. No
product coming from a-C H deprotonation was observed
with either radical cation.
A very similar outcome has been observed for the reactions
.
.
‡
‡
of the corresponding 4-methoxy derivatives 5 and 6 ; this
suggests that the main features of the mechanisms proposed
for these compounds should also hold true when a second
Scheme 8. Proposed ring-closing/ring-opening mechanism for the conver-
sion of the 3-(4-dimethoxyphenyl)propan-1-ol radical zwitterion into the
alkoxyl radical.
.
‡
methoxy group is added to the aromatic ring.[6] For 19
,
formation of a radical zwitterion, which then undergoes a
rate-determining C C bond cleavage coupled with intra-
molecular ET, can be suggested (Scheme 6).
appears to be a more likely possibility for the conversion of
the zwitterion from 20 (and perhaps also that from 6 ) into
the alkoxyl radical.[6, 31, 32]
.
.
‡
‡
Conclusion
From the results reported in this paper it is evident that
oxygen acidity plays an important role in the OH -promoted
decay of dimethoxy-substituted 1-arylalkanol radical cations
[ArCH(OH)R ], a reaction which is also likely to play an
important role in the oxidative degradation of lignin in
alkaline aqueous solution.
More importantly, these results have provided a more
complete mechanistic picture for the fragmentation of
1-arylalkanol radical cations in alkaline aqueous solution in
which, as illustrated in Scheme 9, a key role is played by the
Scheme 6. Proposed mechanism for the OH -promoted decay of the
2-(3,4-dimethoxyphenyl)ethanol radical cation.
.
‡
.
‡
With 5 , the rate of reaction with OH is diffusion
controlled, probably owing to the lower energetic cost of
the intramolecular ET in the radical zwitterion formed from
the monomethoxylated system. The possible mechanism for
.
‡
20 is illustrated in Scheme 7.
Scheme 9. General mechanistic Scheme for the fragmentation of 1-aryl-
alkanol radical cations in alkaline aqueous solution.
radical zwitterion intermediate I, as well as by the energy
barrier for the side-chain-to-ring intramolecular ET in the
zwitterion itself. Such a barrier, the height of which should
depend upon the degree of stabilization of the positive charge
on the aromatic ring, appears to determine the concerted or
stepwise (via the oxyl radical II) nature of the conversion of I
into a carbon radical (III) or a carbonyl compound (IV). On
this basis, when the energy barrier for the intramolecular ET is
relatively small (Arˆ 4-MeOC6H4), conversion of I into III or
IV occurs via II (path a), which then undergoes a 1,2-H shift
and/or b-fragmentation (paths b or c, respectively).
When Arˆ 3,4- or 2,5-(MeO)2C6H3, the energy barrier for
the conversion of I into II is higher, and the reaction proceeds
through paths e and d, in which the intramolecular ET is
concerted with bond breaking. Finally, when Arˆ 2,4,5-
(MeO)3C6H2, the energy barrier for the intramolecular ET
becomes so high that carbon acidity (direct C H deproton-
ation) takes over, the radical zwitterion being formed only in
an unproductive side-equilibrium, if at all.
Scheme 7. Proposed mechanism for the OH -promoted decay of the
3-(3,4-dimethoxyphenyl)propan-1-ol radical cation.
A radical zwitterion is formed, from which an alkoxyl
radical is produced; this then undergoes a 1,2-H shift leading
to an a-hydroxy carbon radical, the precursor of the final
3-arylpropanal.[6] However, in view of the previous discussion,
it is difficult to imagine that the radical zwitterion is converted
into an alkoxyl radical by a side-chain-to-ring intramolecular
ET. In line with this, such a possibility was excluded for the
zwitterions formed from 1-(3,4-dimethoxyphenyl)alkanol
radical cations and a fortiori it should be excluded when the
negatively charged oxygen is further removed from the
aromatic ring. Thus, as already suggested, the sequence of
ring-closing/ring-opening equilibria illustrated in Scheme 8
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Chem. Eur. J. 2001, 7, No. 7

Fragmentation of Radical Cations
1408±1416
Oxygen acidity appears to also play a major role in the
reactivity of 2- and 3-arylalkanol radical cations, in which the
OH group is separated from the ring by two or three
methylene groups, respectively. With these substrates, the
same reaction mechanism is suggested for both 4-methoxy-
and 3,4-dimethoxy-substituted systems.
Acknowledgement
Part of this work was carried out with the financial support of the European
Union (Contract No. QLK5-CT-1999 ± 01277). E.B., M.B., M.F.G., L.M.,
and M.S. also thank the Ministero dellꢁUniversita
Á
e della Ricerca
Scientifica e Tecnologica (MURST) for financial support.
[1] E. Baciocchi, M. Bietti, S. Steenken, J. Am. Chem. Soc. 1997, 119,
4078 ± 4079.
[2] E. Baciocchi, M. Bietti, S. Steenken, Chem. Eur. J. 1999, 5, 1785 ± 1793.
[3] B. C. Gilbert, R. G. G. Holmes, H. A. H. Laue, R. O. C. Norman, J.
Chem. Soc. Perkin Trans. 2 1976, 1047 ± 1052.
[4] P. E. Elford, B. P. Roberts, J. Chem. Soc. Perkin Trans. 2 1996, 2247 ±
2256.
[5] E. Baciocchi, M. Bietti, O. Lanzalunga, S. Steenken, J. Am. Chem. Soc.
1998, 120, 11516 ± 11517.
[6] E. Baciocchi, M. Bietti, L. Manduchi, S. Steenken, J. Am. Chem. Soc.
1999, 121, 6624 ± 6629.
[7] M. Bietti, E. Baciocchi, S. Steenken, J. Phys. Chem. A 1998, 102,
7337 ± 7342.
[8] See for example: a) H. E. Schoemaker, K. Piontek, Pure Appl. Chem.
1996, 68, 2089 ± 2096; b) V. L. Pardini, C. Z. Smith, J. H. P. Utley, R. R.
Vargas, H. Viertler, J. Org. Chem. 1991, 56, 7305 ± 7313; c) R.
DiCosimo, H.-C. Szabo, J. Org. Chem. 1988, 53, 1673 ± 1679.
[9] G. V. Buxton, C. L. Greenstock, W. P. Helman, A. B. Ross, J. Phys.
Chem. Ref. Data 1988, 17, 513 ± 886.
Experimental Section
Reagents: Potassium peroxydisulfate, sodium hydroxide, disodium tetra-
borate decahydrate, potassium thiocyanate, and 2-methyl-2-propanol were
of the highest commercial quality available. Milli-Q-filtered (Millipore)
water was used for all solutions.
3,4-Dimethoxytoluene (10), 2,5-dimethoxybenzyl alcohol (12), 2,5-di-
methoxytoluene (15), 2,4,5-trimethoxybenzyl alcohol (17), 2-(3,4-dimethoxy-
phenyl)ethanol (19) and 3-(3,4-dimethoxyphenyl)propanol (20) (Aldrich)
were used as received. 3,4-Dimethoxybenzyl alcohol (7) was purified by
distillation. 1-(3,4-Dimethoxyphenyl)-2,2-dimethylpropan-1-ol (8) was
available from previous work.[7]
3,4-Dimethoxycumyl alcohol (11) and 2,5-dimethoxycumyl alcohol (16)
were prepared by reaction of the pertinent arylmagnesium bromide with
acetone in anhydrous tetrahydrofuran, purified by column chromatography
(silica gel, eluent hexane/ethyl acetate 3:1) and identified by GC-MS and
1H NMR.
1-(2,5-Dimethoxyphenyl)-2,2-dimethylpropan-1-ol (13) was prepared by
reduction of 2,5-dimethoxyphenyl tert-butyl ketone with NaBH4[33], puri-
fied by column chromatography (silica gel, eluent hexane/ethyl acetate in
gradient), and identified by GC-MS. 1H NMR (CDCl3): d ˆ 0.93 (s, 9H;
3CH3), 3.77 (s, 6H; 2OCH3), 6.77 ± 6.88 (m, 3H; 2,5-MeOC6H3). 2,5-
[10] P. Neta, V. Madhavan, H. Zemel, R. W. Fessenden, J. Am. Chem. Soc.
1977, 99, 163 ± 164.
[11] E. Baciocchi, M. Bietti, L. Putignani, S. Steenken, J. Am. Chem. Soc.
1996, 118, 5952 ± 5960.
[12] P. OꢁNeill, S. Steenken, D. Schulte-Frohlinde, J. Phys. Chem. 1975, 79,
2773 ± 2777.
Dimethoxyphenyl tert-butyl ketone was synthesized by
a literature
procedure.[34]
[13] From the comparison between the reduction potentials of the anisole
radical cation (1.62 V/NHE) and the 1,2-dimethoxybenzene radical
cation (1.44 V/NHE), measured by pulse radiolysis in aqueous
solution,[14] a similar difference can be expected between the reduction
a,a-[D2]-3,4-Dimethoxybenzyl alcohol and a,a-[D2]-2,5-dimethoxybenzyl
alcohol were synthesized by reduction of the pertinent benzoic acid with
LiAlD4.[33] The corresponding methyl ethers were prepared by reaction of
these alcohols with methyl iodide and sodium hydride in anhydrous
tetrahydrofuran.
.
.
‡
‡
potentials of 7 and 1
.
Â
[14] M. Jonsson, J. Lind, T. Reitberger, T. E. Eriksen, G. Merenyi, J. Phys.
Chem. 1993, 97, 11278 ± 11282.
The same procedure was used for the synthesis of methyl ethers 9, 14, and
18 from the corresponding alcohols (7, 12, and 17, respectively). All methyl
[15] OH deprotonation in the radical cation, with a concerted 1,2-H shift
might explain the data as well. However, we prefer to suggest the
intervention of a radical zwitterion since, in basic media, a rapid
equilibrium of this species with the radical cation should immediately
be established. Accordingly, the pKa value for OH dissociation in the
radical cation should be similar to or lower than that of water.
[16] B. Ashworth, B. C. Gilbert, R. G. G. Holmes, R. O. C. Norman, J.
Chem. Soc. Perkin Trans. 2 1978, 951 ± 956.
[17] Recently, in a very detailed study, a two step mechanism for the 1,2-H
atom shift has been proposed. See ref. [18].
[18] K. G. Konya, T. Paul, S. Lin, J. Lusztyk, K. U. Ingold, J. Am. Chem.
Soc. 2000, 122, 7518 ± 7527.
1
ethers showed the expected GC-MS and H NMR spectra.
Product analysis: g-Irradiations were carried out with a panorama 60Co g-
source (Nuclear Engineering) at dose rates of ꢄ0.5 Gys 1. In a typical
experiment, 5 mL of an argon-saturated aqueous solution (pH ꢁ 10)
containing the substrate (1.0 ± 2.0mm), potassium peroxydisulfate (0.5 ±
2.0mm), disodium tetraborate decahydrate (1.0mm) and 2-methylpropan-
2-ol (0.2m) was irradiated at room temperature for the time necessary to
obtain a 40 ± 60% conversion of peroxydisulfate. Reaction products were
identified and quantitatively determined by GC-MS and HPLC (compar-
ison with authentic samples). Blank experiments were performed under
every condition and showed the presence of only negligible amounts of
products.
[19] It might also be suggested that the radical zwitterion undergoes direct
Ca H deprotonation; however, such a deprotonation should be
promoted by OH ; this would lead to a reaction that was second
order with respect to base; this has not been observed.
Pulse Radiolysis: The pulse-radiolysis experiments were performed by
using a 3-M eV van de Graaff accelerator which supplied 300 ns pulses with
doses such that 1 ± 3mm radicals were produced.
A thermostatable
continuous-flow cell was employed in all experiments. The pulse-radiolysis
setup and the methods of data handling have been described elsewhere.[35]
Dosimetry was performed with N2O-saturated KSCN aqueous solutions
[20] We have also measured the solvent deuterium isotope effect (kH O/
2
.
‡
kD O) for the decay of 7 and found a (preliminary) value of about 0.7,
2
which might be consistent with a fast pre-equilibrium involving the
formation of the radical zwitterion (specific base catalysis).[21] Instead
a kH O/kD O value of 1.5 has been observed for the 1,2-H shift in the
.
.
1
(10mm) by taking G( OH) ˆ 6.0  10 7 molJ
and
e
((SCN)2 ) ˆ
7600m 1 cm at 480 nm.[36] Experiments were performed by using argon-
saturated aqueous solutions containing the substrate (0.2 ± 1.0mm), peroxy-
disulfate (10mm), and 2-methylpropan-2-ol (0.1m). The pH of the solutions
was adjusted with NaOH or HClO4 and sodium tetraborate (1mm) was
added to avoid undesired pH variations upon irradiation. The temperature
1
2
2
benzyloxyl radical. See ref. [18].
[21] P. M. Laughton, R. E. Robertson in Solute ± Solvent Interactions (Eds:
J. F. Coetzee, C. D. Ritchie), Dekker, New York, 1969, pp. 473 ± 478.
[22] The reduction potential of the 1,4-dimethoxybenzene radical cation in
aqueous solution (1.30 V/NHE) is 0.14 V lower than that of 1,2-
dimethoxybenzene radical cation. See ref. [14].
[23] The possibility that the 2-MeO group exerts some stereoelectronic
effect by interfering with the required colinearity of the various
orbitals involved in the intramolecular electron transfer should also be
considered. We thank one of the referees for this suggestion.
of the solutions was kept constant at 25 Æ 0.18C. The observed rates (kobs
)
were obtained by averaging eight to 14 values, each consisting of an average
of ten to 30 shots, and were reproducible to within 3%.
The second order rate constants for reaction of the radical cations with
OH (kOH ) were obtained from the slopes of the plots of kobs versus the
concentration of NaOH.
Chem. Eur. J. 2001, 7, No. 7
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1415

E. Baciocchi, M. Bietti, S. Steenken et al.
FULL PAPER
[24] This is based on the reduction potential of the 1,2,4-trimethoxyben-
zene radical cation, measured by cyclic voltammetry in aqueous
solution (1.14 V/NHE). See ref. [7].
[25] M. Eigen, Angew. Chem. 1963, 75, 489 ± 508; Angew. Chem. Int. Ed.
Engl. 1964, 3, 1 ± 19.
[32] In favor of this mechanism is the observation that 2-[1,1-D2]-(4-
methoxyphenoxy)ethanol isomerizes to 2-[2,2-D2]-(4-methoxyphen-
oxy)ethanol when reacted with K2S2O8 at 808C both at pH 4.0 and at
pH 10.0. Such an isomerization certainly involves a sequence of ring-
closing/ring-opening reactions. E. Baciocchi, M. F. Gerini, unpublish-
ed results.
[33] A. D. N. Vaz, M. J. Coon, Biochemistry 1994, 33, 6442 ± 6449.
[34] R. Martin, Bull. Soc. Chim. Fr. 1979, 8, 373 ± 380.
[35] V. Jagannadham, S. Steenken, J. Am. Chem. Soc. 1984, 106, 6542 ±
6551.
[26] a) R. A. Marcus, J. Phys. Chem. 1968, 72, 891 ± 899; b) A. O. Cohen,
R. A. Marcus, J. Phys. Chem. 1968, 72, 4249 ± 4256.
Â
[27] A. Anne, S. Fraoua, V. Grass, J. Moiroux, J.-M. Saveant, J. Am. Chem.
Soc. 1998, 120, 2951 ± 2958.
[28] E. Baciocchi, T. Del Giacco, F. Elisei, J. Am. Chem. Soc. 1993, 115,
12290 ± 12295.
[29] C. J. Schlesener, C. Amatore, J. K. Kochi, J. Am. Chem. Soc. 1984, 106,
7472 ± 7482.
[36] R. H. Schuler, A. L. Hartzell, B. Behar, J. Phys. Chem. 1981, 85, 192 ±
199.
[30] M. Bietti, M. Salamone, S. Steenken, unpublished results.
[31] M. J. Davies, B. C. Gilbert, Advances in Detailed Reaction Mechanism,
Vol. 1, JAI, 1991, pp. 35 ± 81, and references therein.
Received: August 16, 2000 [F2679]
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Chem. Eur. J. 2001, 7, No. 7