Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

Article abstract of DOI:10.1021/jo060678i

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH ₂)_nCO₂H, n = 2, 3) has been carried out at different pH values. Oxidation leads t

Full text of DOI:10.1021/jo060678i

Reactivity and Acid-Base Behavior of Ring-Methoxylated
Arylalkanoic Acid Radical Cations and Radical Zwitterions in
Aqueous Solution. Influence of Structural Effects and pH on the
Benzylic C-H Deprotonation Pathway†
Massimo Bietti* and Alberto Capone
Dipartimento di Scienze e Tecnologie Chimiche, UniVersita` “Tor Vergata”, Via della Ricerca Scientifica
1, I-00133 Rome, Italy
bietti@uniroma2.it
ReceiVed March 29, 2006
A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenyl-
propanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n ) 2, 3) has been carried out at different pH
values. Oxidation leads to the formation of aromatic radical cations (Ar•+(CH2)nCO2H) or radical zwitterions
-
(Ar•+(CH2)nCO2 ) depending on pH, and pKa values for the corresponding acid-base equilibria have
been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the
number of methoxy ring substituents and by increasing the distance between the carboxylic group and
the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H
deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis
results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated
arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for
the R-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic
solution, the second-order rate constants for reaction of the radical zwitterions with -OH (k-OH) have
been obtained. These values are similar to those obtained previously for the -OH-induced R-C-H
deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under
these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with
3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive
decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is
observed above pH 11.
Introduction
the decarboxylation reaction has attracted considerable in-
terest.1a,13-17 This reaction takes part in several chemical and
The fragmentation reactions of organic radical cations have
been intensively investigated in recent years.1 Among these
fragmentations, most attention has been devoted to the study
of the mechanistic1-10 and synthetic11,12 aspects of reactions
involving the cleavage of the C-H and C-C bonds. For what
concerns the C-C bond cleavages of organic radical cations
(1) See, for example: (a) Electron Transfer in Chemistry; Balzani, V.,
Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2 (Organic, Organometallic, and
Inorganic Molecules; Part 1: Organic Molecules). (b) Baciocchi, E.; Bietti,
M.; Lanzalunga, O. Acc. Chem. Res. 2000, 33, 243-251. (c) Mizuno, K.;
Tamai, T.; Sugimoto, A.; Maeda, H. AdV. Electron Transfer Chem. 1999,
6, 131-165. (d) Glass R. S. Top. Curr. Chem. 1999, 205, 1-87. (e)
Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. Engl. 1997, 36, 2550-
2589. Maslak, P. Top. Curr. Chem. 1993, 168, 1-46.
† Dedicated to Prof. Enrico Baciocchi on the occasion of his 75th birthday.
10.1021/jo060678i CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/14/2006
5260
J. Org. Chem. 2006, 71, 5260-5267

ReactiVity and Acid-Base BehaVior of Arylalkanoic Acid Radical Cations
biological processes18 and is involved moreover in important
processes such as the initiation of free-radical polymerization19
and the enhancement in the efficiency of silver halide photog-
raphy (two-electron sensitization).20 In particular, a large number
of studies on the generation and reactivity of arylalkanoic acid
radical cations has been carried out.21-31 In this context, we
SCHEME 1
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have recently shown that in aqueous solution the one-electron
oxidation of ring-dimethoxylated phenylethanoic acids leads to
the formation of aromatic radical cations or radical zwitterions
depending on pH, providing moreover pKa values for their acid-
base equilibria.32 The radical cations were observed to undergo
exclusive decarboxylation to give the corresponding benzyl
radicals (as shown in Scheme 1 for 3,4-dimethoxyphenyletha-
noic acid radical cation (1•+)) with rate constants that are
influenced by the substitution pattern of the aromatic ring, and
a significant increase in decarboxylation rate constant was
observed on going from the radical cations to the corresponding
radical zwitterions.
These results have been interpreted in terms of the reorga-
nization energy required for the side-chain to ring intramolecular
electron-transfer associated with decarboxylation, which is
influenced by the extent of stabilization of the positive charge
on the aromatic ring and hence by the relative position of
methoxy ring substituents. In this study, no evidence for the
formation of products deriving from benzylic C-H deprotona-
tion in the radical cations or in the radical zwitterions was
obtained.
Along this line, to obtain more information on the role of
structural effects on the acid-base behavior of arylalkanoic acid
radical cations and on the possible competition between
decarboxylation and benzylic C-H deprotonation, we have
investigated the effect of the distance between the carboxylic
group and the aromatic ring through a product and time-resolved
kinetic study at different pH values on the reactivity of the
radical cations generated after one-electron oxidation of 3-(4′-
methoxyphenyl)propanoic (2), 3-hydroxy-3-(4′-methoxyphenyl)-
propanoic (2a), 3-(3′,4′-dimethoxyphenyl)propanoic (3), 4-(4′-
methoxyphenyl)butanoic (4), and 4-(3′,4′-dimethoxyphenyl)-
butanoic acid (5) (Chart 1).
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J. Org. Chem, Vol. 71, No. 14, 2006 5261

Bietti and Capone
In the LFP experiments, SO4•- was instead generated by 248
or 266 nm photolysis of argon saturated aqueous solutions
containing substrates 2a, 2-5 (0.1-1.0 mM), and K2S2O8 (0.1
M) as described in eq 8.24a,32
CHART 1
S2O82- 9hν8 2SO4
(8)
•-
The transients produced after PR or LFP of acidic aqueous
solutions (pH e 2) containing substrates 2-5 showed UV and
visible absorption bands centered around 290-310 and 420-
450 nm which are analogous to those observed for the cor-
responding mono- and dimethoxylated aromatic radical
cations2c,32,35,39 and are thus assigned to the arylalkanoic acid
radical cations 2•+-5•+, formed by SO4•- or Tl2+ induced one-
electron oxidation of the neutral substrates as described in eq
9.
Results and Discussion
Spectral Properties and Acid-Base Behavior. Radical
cations of substrates 2-5 have been generated in aqueous
solution by pulse radiolysis (PR) and laser flash photolysis (LFP)
+ SO4•-(Tl2+) f
Ar(CH ) CO H
2
n )22n-3
employing sulfate radical anion (SO4•-) as the oxidant. SO4
•-
•+Ar(CH2)nCO2H + SO42-(Tl+) (9)
is a strong oxidant that is known to react with ring-methoxylated
aromatic compounds via electron transfer to give the corre-
sponding radical cations with k g 5 × 109 M-1s-1 (eq 1).33-35
By increasing the pH of the solution to ∼7, the spectra
obtained after SO4•--induced one-electron oxidation of acids
2-5 were similar to those obtained in acidic solution.40 How-
ever, as described previously for ring-methoxylated phenyle-
thanoic acid radical cations,32 a broadening of the radical cation
visible absorption band was observed. This behavior is attributed
to the formation of the radical zwitterions -2•+--5•+.41 As an
example, Figure 1 shows the time-resolved absorption spectra
observed after 248 nm LFP of argon-saturated aqueous solutions
containing 2 (1 mM) and K2S2O8 (0.1 M) at pH ) 2.0 (filled
circles) and pH ) 7.0 (empty circles). A broadening of the
visible absorption band is observed on going from 2•+ to -2•+.
Identical spectra were obtained after PR of argon saturated
aqueous solutions (pH ) 2.0 and 7.3) containing K2S2O8 (5
mM), 2-methyl-2-propanol (0.1 M), and 2 (0.5 mM).
SO4•- + ArR f SO4
+
•+ArR
(1)
2-
In the PR experiments, SO4•- was generated by radiolysis of
argon-saturated aqueous solutions containing substrates 2-5
(0.1-0.5 mM), K2S2O8 (5-10 mM), and 2-methyl-2-propanol
(0.1 M), according to eqs 2-4.
By measuring the absorption at a suitable wavelength (where
the difference in absorption between radical cation and radical
zwitterion is sufficiently large) as a function of pH, the pKa
values for the acid-base equilibria between the radical cations
and the corresponding radical zwitterions (eq 10) were deter-
mined.
Radiolysis of water leads to the formation of the hydroxyl
radical (•OH) and the hydrated electron (eaq-) (eq 2). The former
is scavenged by 2-methyl-2-propanol (eq 3; k ) 6 × 108 M-1
s-1),36 while eaq- reacts with the peroxydisulfate anion leading
•-
to the formation of SO4 (eq 4; k ) 1.2 × 1010 M-1 s-1).36
In acidic solution, Tl2+ was also used as the oxidant, produced
by PR of N2O saturated aqueous solutions. The function of N2O
is to scavenge eaq-, leading to the formation of an additional
(36) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J.
Phys. Chem. Ref. Data 1988, 17, 513-886.
hydroxyl radical (eq 5), with k ) 9.1 × 109 M-1 s-1 37
.
Tl2+ is
(37) Janata, E.; Schuler, R. H. J. Phys. Chem. 1982, 86, 2078-2084.
(38) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1984, 88, 3643-
3647. Asmus, K.-D.; Bonifacic, M.; Toffel, P.; O’Neill, P.; Schulte-
Frohlinde, D.; Steenken, S. J. Chem. Soc., Faraday Trans. 1 1978, 74,
1820-1826.
then produced by oxidation of Tl+ by •OH (eq 6) with k ) 1.2
× 1010 M-1 s-1 38
.
Also Tl2+ reacts with ring methoxylated
aromatic substrates by electron transfer to give the corresponding
(39) 2a•+ was generated exclusively by 266 nm LFP of an argon saturated
aqueous solutions (pH ) 1.7 or 6.6) containing 2a (1.0 mM) and K2S2O8
(0.1 M), and showed in both cases UV and visible absorption bands centered
at 290 and 445 nm.
radical cations (eq 7) with k ≈ 5 × 108 M-1 s-1 35
.
e-aq + N2O + H2O f N2 + •OH + -OH
•OH + Tl+ + H+ f Tl2+ + H2O
Tl2+ + ArR f Tl+ + •+ArR
(5)
(6)
(7)
(40) It is important to point out that at pH ≈ 7 one-electron oxidation
occurs on the deprotonated acids and not on the neutral substrates 2-5.
However, on the basis of the spectroscopic evidences presented, which are
indicative of the formation of aromatic radical zwitterions under these
conditions, the direct oxidation of the carboxylate function to give an
acyloxyl radical can be excluded. In addition, as it is well-known that
acyloxyl radicals undergo very rapid decarboxylation (see, for example,
ref 23), also the lack of decarboxylation products observed after one-electron
oxidation of the acids at pH ≈ 7 (see the product studies described below)
points against direct one-electron oxidation of the carboxylate function. We
thank a referee for drawing our attention on this point.
(33) Baciocchi, E.; Bietti, M.; Putignani, L.; Steenken, S. J. Am. Chem.
Soc. 1996, 118, 5952-5960.
(34) Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W. J. Am. Chem.
Soc. 1977, 99, 163-164.
(41) This notation represents an oversimplification because, as compared
to the radical cations, the corresponding radical zwitterions lack the presence
of the carboxylic proton.
(35) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J. Phys. Chem. 1975,
79, 2773-2779.
5262 J. Org. Chem., Vol. 71, No. 14, 2006

ReactiVity and Acid-Base BehaVior of Arylalkanoic Acid Radical Cations
•+Ar(CH ) CO H
+ H2O h •+Ar(CH2)nCO2- + H3O+ (10)
2
n )22n-3
Figure 2 shows the plot of ∆A (monitored at 510 nm) vs pH
for the acid-base equilibrium between 2•+ and -2•+.
The pKa values thus obtained for 2•+-5•+ are collected in
Table 1 together with the radical cation visible absorption band
maxima and the pKa values of the neutral acids 2-5. As a matter
of comparison, the corresponding data for 3,4-dimethoxyphe-
nylethanoic acid (1) and for its radical cation (1•+) are also
included.32
With 3,4-dimethoxyphenylethanoic acid (1) the increase in
acidity of the carboxylic proton observed on going from the
neutral substrate (pKa ) 4.33) to the corresponding radical cation
1•+ (pKa ) 3.49) has been rationalized in terms of the increased
electron withdrawing effect determined by the presence of an
electron hole on the aromatic ring.32
FIGURE 1. Time-resolved absorption spectra observed after 248 nm
LFP of argon-saturated aqueous solutions containing 0.1 M K2S2O8
and 1 mM 3-(4′-methoxyphenyl)propanoic acid (2), recorded at pH )
2.0 (filled circles), and pH ) 7.0 (empty circles), 1 µs after the 20 ns,
10 mJ laser flash.
Along this line, the observation that in the ring dimethoxylated
arylalkanoic acid series the radical cation pKa value increases
on going from 1•+ to 5•+ can be explained analogously in terms
of a decrease in the electron-withdrawing effect as the distance
between the carboxylic group and the electron-deficient aromatic
ring increases. Accordingly, on the basis of the pKa values
available for the neutral substrates, 4.33 for 1, 4.69 for 3, and
4.76 for 5 (see Table 1), the difference in pKa between the
neutral substrates and the corresponding radical cations (∆pKa)
decreases progressively on going from 1•+ to 5•+: ∆pKa ) 0.84,
0.57, and 0.32 for 1•+, 3•+, and 5•+, respectively. Quite
interestingly, the ∆pKa value obtained for 5/5•+ shows that even
though small, an effect on acidity is still present when the
positively charged aromatic ring is in the γ position with respect
to the carboxylic function. As a matter of comparison, a similar
effect on acidity has been also observed when comparing
butanoic acid (pKa ) 4.82) with 4-chlorobutanoic acid (pKa )
4.52).43
FIGURE 2. Plot of ∆A monitored at 510 nm vs pH for 2•+, generated
after 248 nm LFP of an argon-saturated aqueous solution containing
0.1 M K2S2O8 and 1 mM 2. From the curve fit: pKa ) 3.84 ( 0.02 (r
) 0.9995).
A slight decrease in acidity is also observed on going from
the monomethoxylated radical cations to the dimethoxylated
ones (compare 2•+ with 3•+ and 4•+ with 5•+), a behavior that
reasonably reflects the fact that the presence of two methoxy
ring substituents as in 3•+ and 5•+ allows a greater extent of
positive charge stabilization as compared to 2•+ and 4•+,
resulting in a weaker electron-withdrawing effect.
At pH ) 1.7 the reaction of 2 led to a 60% substrate
conversion after 1.5 min of irradiation (mass balance >99%).
Formation of 3-hydroxy-3-(4′-methoxyphenyl)propanoic acid
(2a) was observed accompanied by a small amount (∼2% of
the reaction products) of 4-methoxyacetophenone (Scheme 2).45
An analogous product distribution was observed when the
irradiation was carried out at pH ) 6.7.
Product Studies. The oxidation reactions of substrates 2a
and 2-5 were carried out in argon-saturated aqueous solution
•-
(pH ) 1.7 or 6.7) at room temperature, employing SO4 as
the oxidant, generated by steady-state 254 nm photolysis as
described in eq 8. Irradiation times were chosen in such a way
as to avoid complete substrate consumption. In some experi-
ments, potassium 12-tungstocobalt(III)ate (Co(III)W) was used
as the oxidant. Co(III)W is a well-known one-electron oxidant
able to convert ring-methoxylated aromatic substrates into the
corresponding radical cations via outer-sphere electron transfer.44
The formation of 2a can be explained in terms of benzylic
C-H deprotonation of the radical cation 2•+ (generated by SO4
•-
induced oxidation of 2 as described in eq 9) to give a substituted
benzyl radical. Oxidation of this radical in the reaction medium
followed by reaction with the solvent water leads to 2a, as
shown in Scheme 3, in line with the deprotonation reactions of
a variety of alkylaromatic radical cations described pre-
viously.2b,d,5,6,7c,9,10
1
After workup, the reaction mixtures were analyzed by H
NMR using diphenylmethanol as internal standard and the
reaction products were generally identified by comparison with
authentic samples.
(45) It is important to point out that when the reaction mixture of 2 was
analyzed by GC and GC-MS, the formation of 4-methoxystyrene as major
product, accompanied by smaller amounts of 4-methoxybenzaldehyde,
4-methoxyacetophenone and 4-methoxyphenylacetaldehyde was observed.
No evidence for the formation of hydroxy acid 2a was instead obtained. A
similar outcome was also observed when an authentic sample of 2a was
analyzed by GC and GC-MS. Accordingly, the formation of the products
described above from the reaction mixture of 2 under these analytical
conditions can be reasonably explained in terms of the thermal decomposi-
tion of the first formed 2a.
(42) Kortu¨m, G.; Vogel, W.; Andrussow, K. Dissociation Constants of
Organic Acids in Aqueous Solution; Butterworth: London, 1961.
(43) Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to
Organic Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, 1998.
(44) (a) Weinstock, I. A. Chem. ReV. 1998, 98, 113-170. (b) Baciocchi,
E.; Bietti, M.; Mattioli, M. J. Org. Chem. 1993, 58, 7106-7110. (c) Eberson,
L. J. Am. Chem. Soc. 1983, 105, 3192-3199.
J. Org. Chem, Vol. 71, No. 14, 2006 5263

Bietti and Capone
TABLE 1. Visible Absorption Band Maxima for Radical Cations 1•+-5•+ and Corresponding pKa Values for Their Acid-Base Equilibria,
Measured at Room Temperature
f
radical cation a
λ
max (vis)b (nm)
λmonitc (nm)
pKad (radical cation)
pKae (neutral acid)
∆pKa
1•+
2•+
3•+
4•+
5•+
420 (450)g
445
420
450
420
460
510
450
480
450
3.49 ( 0.05
3.87 ( 0.03
4.12 ( 0.04
4.28 ( 0.03
4.44 ( 0.03
4.33
4.69
4.69h
4.76i
4.76i
0.84
0.82
0.57
0.48
0.32
a Generated by 248 nm LFP as described above (eqs 8 and 9). b Radical cation visible absorption band maxima. c Monitoring wavelength for pKa
determination. d Based on the average of at least two independent measurements. e Taken from ref 42. f ∆pKa ) pKa (neutral acid) - pKa (radical cation).
g Visible absorption band maximum for the corresponding radical zwitterion -1•+ h This value refers to 3-(4′-methoxyphenyl)propanoic acid (2). However,
.
on the basis of the very similar pKa values measured for 2 and for 3-phenylpropanoic acid (pKa ) 4.69 and 4.66, respectively) and for 4-methoxyphenylethanoic
acid and 3,4-dimethoxyphenylethanoic acid (1) (pKa ) 4.36 and 4.33, respectively), the same pKa value (pKa ) 4.69) can be reasonably assumed for 2 and
3. i This value refers to 4-phenylbutanoic acid. However, on the basis of the considerations outlined in footnote h above, the same pKa value (pKa ) 4.76)
can be reasonably assumed also for 4 and 5.
SCHEME 2
SCHEME 4
SCHEME 3
deprotonation (Scheme 4, path a) or intramolecular electron
transfer from the carboxyl group to the ring (either direct (path
b) or via formation of a discrete σ-bonded cyclic intermediate
(paths c and d)) to give an acyloxyl radical, precursor of the
•
The formation of 4-methoxyacetophenone in small amounts
can be instead explained in terms of the further oxidation of
the first formed â-hydroxy acid 2a (see below).
decarboxylated radical PhCH2CH2 (path e).
According to this scheme, the observation that 2•+ undergoes
instead exclusive benzylic C-H deprotonation at both pH 1.7
and 6.7 can be rationalized in terms of the increased stability
of 2•+ as compared to 3-phenylpropanoic acid radical cation
determined by the presence of a methoxy ring substituent. This
increased stabilization of the positive charge on the aromatic
ring results in an increase of the energy barrier for the
intramolecular electron-transfer step (Scheme 4, path b or c-d),
thus disfavoring the formation of the intermediate acyloxyl
radical and hence of the products deriving from the decarboxy-
lation pathway.
Quite interestingly, previous studies carried out on 3-phe-
nylpropanoic acid radical cation showed the formation of
products deriving from both the decarboxylation and benzylic
C-H deprotonation pathways.24b,c,26a In particular, by means
of product studies Walling observed the formation of styrene,
ethylbenzene, â-phenylethyl acetate, phenylacetaldehyde, and
2-chromanone as major reaction products after one-electron
oxidation of 3-phenylpropanoic acid in acetic acid and
acetonitrile.26a It was proposed that styrene, ethylbenzene,
â-phenylethyl acetate, and phenylacetaldehyde derive from the
decarboxylation of the first formed radical cation,46 whereas
2-chromanone derives from the C-H deprotonation pathway.47
Gilbert carried out instead an EPR study at different pH values
on the one-electron oxidation of 3-phenylpropanoic acid pro-
At pH ) 1.7, the reaction of 2a led to a 35% substrate
conversion after 30 s of irradiation (mass balance >99%).
Formation of 3-oxo-3-(4′-methoxyphenyl)propanoic acid (2k)
as the major reaction product, accompanied by 4-methoxyac-
etophenone was observed. The 1H NMR analysis of the reaction
mixture showed a decrease in the amount of 2k on standing at
room temperature, accompanied by a corresponding increase
in the amount of 4-methoxyacetophenone.
moted by •OH or SO4•- 24b,c The signals deriving from the C-H
.
deprotonated benzylic radical (PhCH•CH2CO2H) were observed
•
at pH < 1.8 with OH and < 1.5 with SO4•-. Above these pH
values, these signals were replaced in both cases by those
1
In particular, the H NMR spectrum recorded immediately
•
assigned to the decarboxylated radical PhCH2CH2 . This be-
after workup of the reaction mixture (Figure 3, spectrum a,
showing the 2.5-5.2 ppm range) shows the presence of the
substrate 2a (65%: characterized by the double doublet at 5.09
ppm, the singlet at 3.79 ppm and the multiplet at 2.77 ppm)
accompanied by 2k (33%: characterized by the singlets at 4.00
and 3.89 ppm) and 4-methoxyacetophenone (2%: characterized
havior was explained in terms of the formation of an aromatic
radical cation which, depending on pH, undergoes C-H
(46) In this work, the reaction products have been determined by GC.
However, as described above for 2, it cannot be excluded that a different
product distribution would have been observed by 1H NMR analysis of the
reaction mixture.
(47) However, 2-chromanone could also be formed following different
reactive pathways in the radical cation, not involving direct benzylic C-H
deprotonation (see, for example, ref 28).
1
by the singlets at 3.87 and 2.55 ppm). The H NMR spectrum
recorded after heating the same sample for 1 h at T ) 50 °C
(Figure 3, spectrum b) shows in addition to the signals due to
5264 J. Org. Chem., Vol. 71, No. 14, 2006

ReactiVity and Acid-Base BehaVior of Arylalkanoic Acid Radical Cations
SCHEME 6
SCHEME 7
SCHEME 8
The exclusive formation of 5-(4′-methoxyphenyl)oxa-2-cyclo-
pentanone (4c) was observed. Formation of the same product
was also observed when the oxidation of 4 was carried out at
pH ) 1.7 or 6.7 employing Co(III)W (Scheme 7: Ar )
4-MeOC6H4, ox ) hν/K2S2O8 or Co(III)W).
The formation of 4c following one-electron oxidation of 4
can be explained in terms of C-H deprotonation of the radical
cation 4•+ to give a substituted benzyl radical. Oxidation of this
radical in the reaction medium followed by intramolecular
nucleophilic attack leads to 4c (Scheme 8, Ar ) 4-MeOC6H4),
as described previously for the one-electron oxidation of
4-phenylbutanoic acid.26b,29,30
FIGURE 3. 1H NMR spectra recorded in CDCl3 after 254 nm steady-
state photolysis of an aqueous solution (pH ) 1.7) containing 2 mM
3-hydroxy-3-(4′-methoxyphenyl)propanoic acid (2a) and 0.1 M K2S2O8.
(a) Spectrum recorded immediately after workup of the reaction mixture.
(b) Spectrum recorded on the same solution as in a after heating for 1
h at T ) 50 °C.
SCHEME 5
At pH ) 1.7, 1.5 min irradiation of the ring-dimethoxylated
acids 3 and 5 in the presence of K2S2O8 resulted in both cases
in an almost quantitative (g95%) recovery of the parent
compound. No oxidation products were detected under these
conditions. The reactions of 3 and 5 were thus carried out
employing Co(III)W as the oxidant.
2a the almost complete disappearance of the signals due to 2k
(2%), accompanied by an increase in intensity of the signals
due to 4-metoxyacetophenone (33%).
The reaction of 3 with Co(III)W was carried out only at pH
) 1.7, and the exclusive formation of 3-hydroxy-3-(3′,4′-
dimethoxyphenyl)propanoic acid (3a) (see for analogy Scheme
2 describing the reaction of 2) was observed. Formation of 3a
can be explained in terms of C-H deprotonation of the radical
cation 3•+ as described in Scheme 3 for the oxidation of 2.
The reaction of 5 with Co(III)W was carried out at pH ) 1.7
and 6.7. In both cases, the exclusive formation of 5-(3′,4′-
dimethoxyphenyl)oxa-2-cyclopentanone (5c) was observed
(Scheme 7: Ar ) 3,4-(MeO)2C6H3, ox ) Co(III)W). Formation
of 5c can be explained in terms of C-H deprotonation of 5•+
(Scheme 8: Ar ) 3,4-(MeO)2C6H3, ox ) Co(III)W), as
discussed above for the corresponding reaction of 4.
This behavior strongly suggests that 4-methoxyacetophenone
derives from the spontaneous decarboxylation of the first formed
â-keto acid 2k, as described in Scheme 5, in full agreement
with the reported decarboxylation kinetics of 2k (in aqueous
solution at T ) 35.6 °C, τ ) 1930 s).48
An analogous behavior was observed when 2a was irradiated
at pH ) 6.7 even though, as compared to the reaction carried
out at pH ) 1.7, a significantly higher substrate conversion was
observed (85% after 30 s irradiation).
The formation of 2k after one-electron oxidation of 2a can
be explained in terms of C-H deprotonation of the radical cation
2a•+ (generated by SO4•--induced oxidation of 2a as described
in eq 9) to give a substituted R-hydroxy benzyl radical.
Oxidation of this radical in the reaction medium followed by
deprotonation leads to 2k, as shown in Scheme 6, in line with
the deprotonation reactions of a variety of 1-arylalkanol radical
cations described previously.1b,2b,c,49
Time-Resolved Kinetic Studies. Acidic Solution (pH ≈
1.7). The decay rates of the radical cations 2a•+ and 2•+-5•+
were measured spectrophotometrically following the decrease
in optical density at the visible absorption band maxima
(between 420 and 450 nm). With 2•+, 2a•+, and 4•+, the decay
was observed to follow first-order kinetics in a reaction that,
on the basis of the product analysis results described above, is
assigned to benzylic C-H deprotonation.
At pH ) 1.7, the reaction of 4 led to a 60% substrate
conversion after 1.5 min of irradiation (mass balance >99%).
(48) Hay, R. W.; Tate, K. R. Aust. J. Chem. 1970, 23, 1407-1413.
(49) Baciocchi, E.; Bietti, M.; Steenken, S. Chem. Eur. J. 1999, 5, 1785-
1793.
The decay of radical cations 3•+ and 5•+ was instead observed
to be influenced by the radiation chemical dose or laser intensity
J. Org. Chem, Vol. 71, No. 14, 2006 5265

Bietti and Capone
TABLE 2. First-Order Rate Constants (k) Measured Following the
Decay of Radical Cations 2•+, 2a•+, and 3•+-5•+ Generated by PR
or LFP of the Parent Substrates in Aqueous Solution (pH ≈ 1.7),
Measured at T ) 25 °C
radical cation
oxidant
Tl2+ b
ka/s-1
2•+
1.5 × 103
1.8 × 103
6.5 × 103
<5 × 102
<50
•- c
SO4•- c
2a•+
3•+
SO4
Tl2+ b
•- c
SO4
4•+
5•+
Tl2+ b
1.7 × 103
2.1 × 103
<5 × 102
<50
•- c
SO4
Tl2+ b
•- c
SO4
a Monitored following the decay of absorption at the radical cation visible
absorption band maxima (between 420 and 450 nm). Error e 10%.
b Generated by PR (dose e 1 Gy/pulse) of N2O-saturated aqueous solutions
containing the substrate (0.5 mM) and Tl2SO4 (2.0 mM) as described in
eqs 2 and 5-7. c Generated by LFP of argon saturated aqueous solutions
containing the substrate (0.1-1.0 mM) and K2S2O8 (0.1 M) as described
in eqs 1 and 8.
FIGURE 4. Plot of kobs against concentration of NaOH for the reaction
of radical zwitterion -1•+. From the linear regression analysis: intercept
) 7.2 × 104 s-1, slope ) 2.9 × 106 M-1 s-1, r2 ) 0.9991.
TABLE 3. Second-Order Rate Constants for the -OH-Catalyzed
(k-OH) Decay of Radical Zwitterions -1•+--5•+ Generated by Pulse
Radiolysis of the Parent Substrates in Aqueous Solution, Measured
at T ) 25 °C
and accordingly, only an upper limit for their decay rate constant
could be determined. Also the decay of 3•+ and 5•+ is assigned
to benzylic C-H deprotonation on the basis of the product
analysis results described above. The rate constants thus obtained
are collected in Table 2.
radical zwitteriona
pH rangeb
k-OHc (M-1 s-1
)
Quite interestingly, the rate constants measured for 2•+ and
4•+ and for 2a•+ are very similar to those measured previously
in acidic aqueous solution for benzylic C-H deprotonation of
the structurally related radical cations 4-MeOC6H4(CH2)3OH•+
and 4-MeOC6H4(CH2)4OH•+ (k ) 1.8 × 103 and 1.4 × 103
-1•+
-2•+
-3•+
-4•+
-5•+
11.3-12.6
10.3-11.4
11.2-12.5
10.3-11.4
11.2-12.5
2.9 × 106
2.8 × 107
9.0 × 105
4.4 × 107
8.5 × 105
•+
s-1, respectively)50 and for 4-MeOC6H4CH(OH)CH2CH3 (k
a The radical zwitterions were generated by PR (dose e 5 Gy/pulse) of
an argon-saturated aqueous solution containing the substrate (0.5-1.0 mM),
K2S2O8 (10 mM), 2-methyl-2-propanol (0.1 M), and Na2B4O7 (1 mM) as
described in eqs 2-4 and 9. b pH range employed for the determination of
) 5.4 × 103 s-1).49 The similarity of these values provides
additional support to the assignment of the rate constants
displayed in Table 2 to the benzylic C-H deprotonation
reaction. For what concerns instead the dimethoxylated radical
cations 3•+ and 5•+, even though due to instrumental limits only
an upper limit for their deprotonation rate constants could be
determined, the limits reported in Table 2 (k < 50 s-1) can be
reasonably compared with the rate constant obtained for R-C-H
deprotonation of 3,4-dimethoxybenzyl alcohol radical cation (k
) 17 s-1). As discussed previously for alkylaromatic radical
cations,1b,2c,d,5 stabilization of the radical cation (that is on going
from the monomethoxylated radical cations 2•+ and 4•+ to the
corresponding dimethoxylated ones 3•+ and 5•+) leads to a
significant decrease in the deprotonation rate constant.
Basic Solution (pH > 10). By monitoring the decay of radical
zwitterions -1•+--5•+ at the visible absorption band maxima
(between 420 and 450 nm) a significant increase in rate was
observed when -OH was added to the solution, and by plotting
the observed rates (kobs) vs concentration of added base, a linear
dependence was observed for all radical zwitterions (Figure 4,
showing the plot for the reaction between -1•+ and -OH).
From the slopes of these plots, the second-order rate constants
for reaction of -OH with the radical zwitterions (k-OH) were
determined. The rate constants thus obtained are collected in
Table 3.
k-OH.
c Obtained from the slopes of the kobs vs [NaOH] plots, where kobs
has been obtained from the decay of absorption at the radical zwitterion
visible absorption band maxima (between 420 and 450 nm). Average of at
least two independent determinations. Error e10%.
to those determined previously for the -OH-induced R-C-H
deprotonation of 4-methoxytoluene and 3,4-dimethoxytoluene
radical cations (k-OH ) 5.5 × 107 and 2.1 × 106 M-1 s-1
,
respectively)2c and can be thus assigned to the same process,
i.e., -OH-induced benzylic C-H deprotonation in -2•+--5•+.
With -1•+, the second-order rate constant for reaction with
-OH has been determined as k-OH ) 2.9 × 106 M-1 s-1, a
value that is very similar to the one determined previously for
the -OH-induced R-C-H deprotonation of 3,4-dimethoxytolu-
ene radical cation discussed above.2c Quite importantly, the
intercept of the kobs against [NaOH] plot (7.2 × 104 s-1, see
Figure 4), which represents the rate constant for the uncatalyzed
reaction of -1•+, is very close to the decarboxylation rate
constant measured previously for this radical zwitterion between
pH 6 and 10 (k ) 6.5 × 104 s-1).32 On the basis of these
observations, it is reasonable to propose that above pH 11 -1•+
undergoes competition between benzylic C-H deprotonation
and decarboxylation, with the former process that becomes the
major fragmentation pathway around pH 12.5.
The data collected in Table 3 show that very similar second-
order rate constants k-OH have been obtained for the mono-
methoxylated radical zwitterions -2•+ and -4•+, as well as for
the dimethoxylated ones -3•+ and -5•+. These values are similar
In conclusion, by means of product and time-resolved studies
additional information on the acid-base behavior and side-chain
fragmentation reactivity of ring-methoxylated arylpropanoic and
arylbutanoic acid radical cations and radical zwitterions in
aqueous solution have been obtained. The radical cation pKa
values increase by increasing the number of methoxy ring
(50) Baciocchi, E.; Bietti, M.; Manduchi, L.; Salamone, M.; Steenken,
S. J. Am. Chem. Soc. 1999, 121, 6624-6629.
5266 J. Org. Chem., Vol. 71, No. 14, 2006

ReactiVity and Acid-Base BehaVior of Arylalkanoic Acid Radical Cations
ments, the solution containing 0.5-1.0 mM substrate, 10 mM
potassium peroxydisulfate and 0.1 M 2-methyl-2-propanol was
saturated with argon and 1 mM sodium tetraborate was added to
avoid undesired pH variations upon irradiation.
substituents and by increasing the distance between the car-
boxylic group and the aromatic ring. The radical cations or
radical zwitterions undergo benzylic C-H deprotonation as the
exclusive side-chain fragmentation pathway over the pH range
1.7-12.5. Very interestingly, with 3,4-dimethoxyphenylethanoic
acid radical zwitterion, that was previously observed to undergo
decarboxylation as the exclusive fragmentation pathway up to
pH 10, competition between decarboxylation and benzylic C-H
deprotonation is observed above pH 11.
Laser Flash Photolysis. The radical cations of interest were
generated at room temperature by direct laser flash photolysis (LFP)
of argon-saturated aqueous solutions, containing the substrate (0.2-
1.0 mM) and K2S2O8 (0.1 M), using a 248 nm excimer laser (KrF*)
providing 20 ns pulses, or the fourth harmonic (266 nm) of a
Q-switched Nd:YAG laser providing 8 ns pulses. The laser energy
was adjusted in both cases to e10 mJ/pulse (output power of the
laser) by the use of the appropriate filter. The concentration of
K2S2O8 was such that this species absorbed most of the 248 or 266
nm radiation. The 248 nm LFP experiments were carried out at
room temperature and the solutions were flowed through a 2 mm
(in the direction of the laser beam) by 4 mm (in the direction of
the analyzing light, 90° geometry) Suprasil quartz cell. The 266
nm LFP experiments were carried out at T ) 25 ( 0.5 °C under
magnetic stirring employing a 3 mL Suprasil quartz cell (10 mm
× 10 mm). The stability of the solutions to the experimental
conditions was checked spectrophotometrically by comparing the
spectrum of the solution before irradiation with that obtained after
irradiation.
Experimental Section
Product Studies. Oxidations with SO4•-. Irradiations were
performed employing a photochemical reactor equipped with 8 ×
15 W lamps with emission at 254 nm. The reactor was a cylindrical
flask equipped with a water cooling jacket thermostated at T ) 25
°C. Irradiation times were chosen in such a way as to avoid
complete substrate consumption. In a typical experiment 20 mL of
an argon saturated aqueous solution (pH ) 1.7 or 6.7) containing
the substrate (2 mM) and K2S2O8 (0.1 M) were irradiated for times
varying between 30 and 90 s. Blank experiments performed in the
absence of irradiation showed the formation of negligible amounts
of reaction products.
Rate constants, determined employing 266 nm LFP, were
obtained by averaging at least 8 values and were reproducible to
within 10%. pKa values for the acid-base equilibria between the
radical cations and the corresponding radical zwitterions were
determined employing 248 nm LFP by plotting ∆A as a function
of pH at a fixed wavelength (where the difference in absorption
between radical cation and radical zwitterion is sufficiently large)
in the pH range 1-8. The ∆A vs pH curve was fitted to the
Oxidations with Co(III)W. Five milliliters of an argon-saturated
aqueous solution (pH ) 1.7 or 6.7) containing the substrate (5 mM)
and Co(III)W (substrate/Co(III)W ratio ) 0.5-2) were stirred at T
) 25 °C until complete conversion of the oxidant.
With both oxidizing systems, the reaction mixture was acidified
with 2 N HCl (only for the experiments at pH ) 6.7) and extracted
with diethyl ether (3 × 10 mL), and the combined organic extracts
were dried over anhydrous sodium sulfate. The reaction mixtures
(pH-pKa
equation: ∆A ) [∆Ao + ∆A110(pH-pK )]/1 + 10
). Two or
three independent pKa determinations were carried out for every
radical cation.
a
1
were analyzed by H NMR using diphenylmethanol as internal
standard, and the reaction products were generally identified by
comparison with authentic samples. Good to excellent mass
balances (g95%) were obtained in all experiments.
Acknowledgment. This paper is dedicated to Professor
Enrico Baciocchi on the occasion of his 75th birthday. Financial
support from the Ministero dell’Istruzione dell’Universita` e della
Ricerca (MIUR) is gratefully acknowledged. Pulse radiolysis
experiments were performed at the Free Radical Research
Facility, Daresbury Laboratory, Warrington, UK, under the
support of the European Commission’s Transnational Access
to Major Research Infrastructures. We thank Prof. Basilio
Pispisa and Prof. Lorenzo Stella for the use of and assistance
in the use of LFP equipment.
Time-Resolved Studies. Pulse Radiolysis. The pulse radiolysis
experiments were performed using a 10 MeV electron linear
accelerator which supplied 300 ns pulses with doses such that 1-3
µM radicals were produced. Experiments were performed at room
temperature using argon-saturated aqueous solutions containing the
substrate (0.1-1.0 mM), peroxydisulfate (5-10 mM), and 2-meth-
yl-2-propanol (0.1 M). Alternatively, N2O-saturated aqueous solu-
tions (pH e 2) containing the substrate (0.5 mM) and thallium(I)
sulfate (2.0 mM) were employed. The pH of the solutions was
adjusted with NaOH or HClO4. For the experiments at pH ≈ 7, 2
mM Na2HPO4 was added. A flow system was employed in all the
experiments. The first-order rate constants (k) were obtained by
averaging 4-8 values, each consisting of the average of 3-10 shots
and were reproducible to within 10%.
Supporting Information Available: Details on the synthesis
and characterization of arylalkanoic acid 2a and of the reaction
products 3a, 4c, and 5c. This material is available free of charge
via the Internet at http://pubs.acs.org.
The second-order rate constants for reaction of the radical cations
with -OH (k-OH) were obtained from the slopes of the plots of the
observed rates (kobs) vs concentration of NaOH. For these experi-
JO060678I
J. Org. Chem, Vol. 71, No. 14, 2006 5267