Spectral properties and reactivity of diarylmethanol radical cations in aqueous solution. Evidence for intramolecular charge resonance

Article abstract of DOI:10.1021/jo016287f

Spectral properties and reactivities of ring-methoxylated diarylmethane and diarylmethanol radical cations, generated in aqueous solution by pulse and γ-radiolysis and by the one-electron chemical oxidant potassium 12-tungstocobalt(III)ate, have been studied. The radical cations display three bands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to an intramolecular charge resonance interaction (CR) between the neutral donor and charged acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band compared to that of the UV and visible bands does not increase with increasing substrate concentration and that the position and intensity of this band is influenced by the ring-substitution pattern. In acidic solution (pH = 4), monomethoxylated diarylmethanol radical cations 1a^.+-1e^.+ decay by C_a-H deprotonation [k = (1.7-1.9) x 10⁴ S^-1] through the intermediacy of a ketyl radical, which is further oxidized in the reaction medium to give the corresponding benzophenones, as evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical cation 2^.+, C_a-H deprotonation is instead significantly slower (k = 6.7 × 10² s^-1). In basic solution, 1^.+-1e^.+ undergo ^-OH-induced deprotonation from the α-OH group with k_OH ≈ 1.4 × 10¹⁰ M^-1 s^-1, leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone.

Full text of DOI:10.1021/jo016287f

2632
J . Org. Chem. 2002, 67, 2632-2638
Sp ectr a l P r op er ties a n d Rea ctivity of Dia r ylm eth a n ol Ra d ica l
Ca tion s in Aqu eou s Solu tion . Evid en ce for In tr a m olecu la r Ch a r ge
Reson a n ce1
Massimo Bietti*,2a and Osvaldo Lanzalunga*,2b
Dipartimento di Scienze e Tecnologie Chimiche, Universita` Tor Vergata, Via della Ricerca Scientifica,
I-00133 Rome, Italy, and Dipartimento di Chimica, Universita` La Sapienza, P.le A. Moro, 5
I-00185 Rome, Italy
bietti@uniroma2.it
Received November 16, 2001
Spectral properties and reactivities of ring-methoxylated diarylmethane and diarylmethanol radical
cations, generated in aqueous solution by pulse and γ-radiolysis and by the one-electron chemical
oxidant potassium 12-tungstocobalt(III)ate, have been studied. The radical cations display three
bands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to
an intramolecular charge resonance interaction (CR) between the neutral donor and charged
acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band
compared to that of the UV and visible bands does not increase with increasing substrate
concentration and that the position and intensity of this band is influenced by the ring-substitution
pattern. In acidic solution (pH ) 4), monomethoxylated diarylmethanol radical cations 1a •+-1e•+
decay by CR-H deprotonation [k ) (1.7-1.9) × 104 s-1] through the intermediacy of a ketyl radical,
which is further oxidized in the reaction medium to give the corresponding benzophenones, as
evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical
cation 2•+, CR-H deprotonation is instead significantly slower (k ) 6.7 × 102 s-1). In basic solution,
1a •+-1e•+ undergo -OH-induced deprotonation from the R-OH group with kOH ≈ 1.4 × 1010 M-1
-
s-1, leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding
benzophenone.
Side-chain deprotonation of alkylaromatic radical cat-
2 in Chart 1), i.e., substrates containing an additional
aromatic ring on the benzylic carbon, both in acidic and
basic solution. For comparison, the radical cations of
diarylmethanes 3a and 3c have also been investigated.
The results of these studies are reported herein.
ions represents one of the main reaction pathways of
these important reactive intermediates, and the reactions
of this class of carbon acids attract continuous attention.3
In relation to our interest on this topic, we have recently
shown the existence of a pH-dependent mechanistic
change for the deprotonation reactions of 1-(4-methoxy-
phenyl)alkanol radical cations in aqueous solution. Ac-
cordingly, while in neutral and acidic solution 4-meth-
oxybenzyl alcohol radical cation undergoes CR-H depro-
tonation to give the corresponding benzylic radical, in
alkaline solution it reacts with -OH at a diffusion-
controlled rate undergoing O-H deprotonation.4,5 To
obtain a better understanding of this mechanistic picture,
we felt that useful information could be provided through
the study of the reactivity of mono- and dimethoxylated
diarylmethanol radical cations (compounds 1a -1e and
Resu lts
Gen er a tion of th e Ra d ica l Ca tion s. Radical cations
of substrates 1-3 were generated in aqueous solution by
means of radiation chemical techniques (pulse radiolysis
and steady-state 60Co γ-radiolysis) employing sulfate
radical anion (SO4•-) as the oxidant (method 1: eqs 1-4,
X ) H, OH):
hν, γ-ray
•
H2O
8 H+, OH, e-
(1)
aq
•OH + CH3C(CH3)2OH f H2O + •CH2C(CH3)2OH
(2)
(1) This paper is dedicated to Professor Enrico Baciocchi on the
occasion of his 70th birthday.
(2) (a) Universita´ Tor Vergata. (b) Universita´ La Sapienza. E-mail:
lanzalunga@uniroma1.it.
e- + S2O82- f SO42- + SO4
(3)
•-
(3) See, for example: (a) Electron Transfer in Chemistry (Organic,
Organometallic, and Inorganic Molecules; Part 1: Organic Molecules);
Balzani, V., Ed., Wiley-VCH: Weinheim, 2001; Vol. 2. (b) Baciocchi,
E.; Bietti, M.; Lanzalunga, O. Acc. Chem. Res. 2000, 33, 243-251. (c)
Parker, V. D.; Zhao, Y.; Lu, Y.; Zheng, G. J . Am. Chem. Soc. 1998,
120, 12720-12727. (d) Bockman, T. M.; Hubig, S. M.; Kochi, J . K. J .
Am. Chem. Soc. 1998, 120, 2826-2830. (e) Freccero, M.; Pratt, A.;
Albini, A.; Long, C. J . Am. Chem. Soc. 1998, 120, 284-297.
(4) Baciocchi, E.; Bietti, M.; Steenken, S. Chem. Eur. J . 1999, 5,
1785-1793.
aq
SO4•- + ArCH(X)Ar′ f SO42- + Ar•+CH(X)Ar′ (4)
The hydroxyl radical (•OH) is scavenged by 2-methyl-
2-propanol (eq 2) with k ) 6 × 108 M-1 s-1,6 while the
(5) Baciocchi, E.; Bietti, M.; Gerini, M. F.; Manduchi, L.; Salamone,
M.; Steenken, S. Chem. Eur. J . 2001, 7, 1408-1416
(6) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J .
Phys. Chem. Ref. Data 1988, 17, 513-886.
10.1021/jo016287f CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/16/2002

Diarylmethanol Radical Cations in Aqueous Solution
J . Org. Chem., Vol. 67, No. 8, 2002 2633
Ch a r t 1
hydrated electron (e-aq) reacts with the peroxydisulfate
anion, leading to the formation of SO4•- (eq 3), with k )
1.2 × 1010 M-1 s-1.6 SO4•- is known to react with anisole-
type derivatives by electron transfer to give the corre-
sponding radical cations (eq 4) with a rate constant k )
F igu r e 1. Time-resolved absorption spectra observed on
reaction of SO4•- with 1b (0.2 mM) at T ) 25 °C, recorded after
pulse radiolysis of an Ar-saturated aqueous solution (pH )
3.9), containing 0.1 M 2-methyl-2-propanol and 5 mM K2S2O8,
at 4 µs (filled circles), 32 µs (empty circles), 80 µs (filled
squares) and 280 µs (empty squares) after the 50 ns, 10 MeV
electron pulse. Inset: Time-resolved absorption spectra ob-
5 × 109 M-1 s-1 4,7,8
.
In the case of 2, Tl2+ was also used as the oxidant,
produced by irradiating N2O-saturated aqueous solutions
(method 2, eqs 1, 5-7):
e- + N2O + H2O f N2 + •OH + -OH
(5)
(6)
•-
served on reaction of SO4 with 1b (0.2 mM) at T ) 25 °C,
aq
recorded after pulse radiolysis of an O2-saturated aqueous
solution (pH ) 3.9), containing 0.1 M 2-methyl-2-propanol and
5 mM K2S2O8, at 4 µs (filled circles), 32 µs (empty circles), and
280 µs (empty squares) after the 50 ns, 10 MeV electron pulse.
•OH + Tl+ + H+ f Tl2+ + H2O
Tl2+ + ArCH(OH)Ar′ f Tl+ + Ar•+CH(OH)Ar′ (7)
Ta ble 1. Sp ectr a l Da ta for th e Ra d ica l Ca tion s of
Dia r ylm eth a n ols 1 a n d 2 a n d Dia r ylm eth a n es 3,
Gen er a ted by P u lse Ra d iolysis in Aqu eou s Solu tion
(p H ≈ 4.0)
The function of N2O is to scavenge e-aq, leading to the
formation of an additional hydroxyl radical (eq 5), with
k ) 9.1 × 109 M-1 s-1.9 Tl2+ is then produced by oxidation
λ
max/nm
•
of Tl+ by OH (eq 6) with k ) 1.2 × 1010 M-1 s-1 10
.
Also
radical cation
A
B
C
∆ODC/∆ODB
Tl2+ reacts with anisole-type derivatives by one-electron
transfer to give the corresponding radical cations (eq 7)
1a •+
1b•+
1c•+
1d •+
1e•+
2•+
290
290
290
290
290
300
300
300
440
450
450
450
450
420
440
440
980
800
680
690
560a
620
960
680
0.81
0.61
0.49
0.47
<0.42
0.45
with k ≈ 5 × 108 M-1 s-1 7
.
P r od u ct An a lysis. Product analysis was carried out
under acidic conditions (pH ) 4.0) for substrates 1a -e
and 2 using potassium 12-tungstocobalt(III)ate, K5[Co-
(III)W12O40] (from now on indicated as Co(III)W), a
genuine one-electron chemical oxidant,11,12 to generate
the radical cations. 4-Methoxy-4′-X-benzophenones (5a -
e) and 3,4-dimethoxy-4′-methylbenzophenone were the
exclusive products observed.
3a •+
3c•+
0.39
0.29
a
Shoulder.
absorption spectra observed on reaction of SO4•- with 1b
in an argon-saturated aqueous solution (pH ) 3.9),
recorded after 4, 32, 80, and 280 µs.
Product analysis of 1a was also studied under both
acidic (pH ) 4.0) and basic (pH ) 10.0) conditions,
generating the oxidant by means of steady-state 60Co
γ-radiolysis. Argon-saturated aqueous solutions contain-
ing 1a (1 mM), K2S2O8 (0.5 mM), and 2-methyl-2-propanol
(0.2 M) were irradiated at room temperature with a 60Co
γ-source at dose rates of 0.5 Gy s-1 for the time necessary
to obtain a 40% conversion with respect to peroxydisul-
fate. 4,4′-Dimethoxybenzophenone (5a ) was the exclusive
product observed under both conditions.
Sp ectr a l P r op er ties. Spectral information about
radical cations 1•+-3•+ was obtained using the pulse
radiolysis technique, generating the radical cations by
reaction of 1-3 with SO4•-, as described in eqs 1-4. As
an example, in Figure 1 are displayed the time-resolved
The spectrum after 4 µs (filled circles), showing three
absorption bands centered around 290, 450, and 800 nm,
can be reasonably assigned to 1b•+. Interestingly, as
compared to 1-(4-methoxyphenyl)alkanol radical cations,
which display only two bands around 290 and 450 nm,4
an additional band is now present. Decay of 1b•+ is
accompanied by the formation of an intermediate char-
acterized by a sharp absorption band at 340 nm and a
weak absorption around 550 nm (32 µs, empty circles)
which is quenched by oxygen (inset), followed by the
formation of a stable product which shows a strong
absorption at 295 nm (280 µs, full squares) and is
unaffected by oxygen.
An analogous behavior was observed also with 1a •+
,
(7) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J . Phys. Chem.
1975, 79, 2773-2777.
1c•+, 1d •+, and 1e•+. However, the position of the long
wavelength band of the radical cations was found to be
influenced by the nature of the neutral ring substituent
(Table 1).
(8) Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W. J . Am.
Chem. Soc. 1977, 99, 163-164.
(9) J anata, E.; Schuler, R. H. J . Phys. Chem. 1982, 86, 2078-2084.
(10) Schwarz, H. A.; Dodson, R. W. J . Phys. Chem. 1984, 88, 3643-
3647.
(11) Weinstock, I. A. Chem. Rev. 1998, 98, 113-170.
(12) Eberson, L. J . Am. Chem. Soc. 1983, 105, 3192-3199.
The absorption spectrum recorded after reaction of
•-
SO4 with 2 displays the characteristic UV and visible

2634 J . Org. Chem., Vol. 67, No. 8, 2002
Bietti and Lanzalunga
Ta ble 2. Ra te Con sta n ts for th e Un ca ta lyzed Deca y of
Ra d ica l Ca tion s 1•+ a n d 2•+ Gen er a ted by P u lse
Ra d iolysis in Aqu eou s Solu tion (p H E 4), Mea su r ed a t
T ) 25 °C
radical cationa
k
dec/s-1 b
radical cationa
k
dec/s-1 b
1a•+
1b•+
1c•+
1.8 × 104
1.9 × 104
1.9 × 104
1d•+
1e•+
2•+
1.7 × 104
1.8 × 104
6.7 × 102 c
a
Radical cations generated by method 1 from argon-saturated
b
aqueous solutions at pH ) 4.0. Dose ≈ 1 Gy/pulse. Error e 5%.
c Generated by method 2 from N2O-saturated aqueous solutions
at pH ) 3.5. Dose ≈ 1 Gy/pulse.
absorption bands of 3,4-dimethoxybenzyl alcohol radical
cation centered around 300 and 420 nm13 and a broad
absorption band centered around 620 nm and can be
reasonably assigned to 2•+
.
F igu r e 2. Time-resolved absorption spectra observed on
reaction of SO4•- with 1a (0.2 mM) at T ) 25 °C, recorded after
pulse radiolysis of an Ar-saturated aqueous solution (pH )
9.5), containing 0.1 M 2-methyl-2-propanol, 1 mM Na2B4O7,
and 5 mM K2S2O8, at 1.5 µs (filled circles), 26 µs (empty circles),
and 332 µs (empty squares) after the 50 ns, 10 MeV electron
pulse.
The absorption spectra of 3a •+ and 3c•+, observed after
reaction of SO4•- with 3a and 3c, show three absorption
bands corresponding to those observed for 1a •+ and 1c•+
Decay of 3a •+ and 3c•+ is accompanied in both cases by
the formation of a species characterized by a sharp
absorption band at 340 nm and a weak absorption around
550 nm, which is quenched by oxygen. No stable product
absorbing around 295 nm was observed in this case. The
spectral data for radical cations 1•+-3•+ are collected in
Table 1.
.
Kin etic Stu d ies. In acidic solution, the rate of decay
of the radical cations 1•+ and 2•+ was measured spectro-
photometrically by following the change in optical density
at the wavelengths corresponding to the UV and visible
absorption maxima: 290-300, 420-450, and 600-1000
nm (see Table 1). Under these conditions the radical
cations were found to decay by a first-order reaction and
the same values of the rates were determined for the
three absorption bands, fully supporting their assignment
to the same transient species, i.e., the radical cation. The
rate constants for the uncatalyzed decay of radical cations
1•+ and 2•+ are reported in Table 2.
F igu r e 3. Effect of [-OH] on the rate of decay of 1a •+. Slope
) 1.4 × 107 mM-1 s-1, intercept ) 5.1 × 104 s-1, r2 ) 0.9987.
The reactivity of radical cations 1a •+-1e•+ was studied
also in alkaline solution. Under these conditions, the
radical cations decay at a significantly faster rate as
compared to the acidic solution, as indicated in Figure
2, displaying the time-resolved absorption spectra ob-
Discu ssion
Sp ectr a l P r op er ties. The UV-vis absorption spectra
of diarylmethanol radical cations 1a •+-1e•+ (Table 1)
closely resemble the spectrum of 4-methoxybenzyl alcohol
radical cation, displaying two absorption bands centered
at 290 and 450 nm.4 However, the broad band in the long
wavelength region is only present in the spectra of
diarylmethanol radical cations. NIR absorption bands are
characteristic of dimeric aromatic radical cations and
have been ascribed to a charge-resonance transition.15,16
In our case, the origin of this band can be reasonably
attributed to the interaction between the neutral donor
ring and the oxidized acceptor arene moieties due to
strong (π,π)-orbital overlap, an example of intramolecular
charge resonance (CR) absorption.17 The intramolecular
nature of this interaction is supported by the observation
that the relative intensity of the CR band compared to
served on reaction of SO4 with 1a at pH ) 9.5.14
•-
Decay of 1a •+ is accompanied by the rapid formation
of an intermediate characterized by a sharp absorption
band at 340 nm and a broad absorption around 570 nm
(26 µs, empty circles), which is quenched by oxygen,
followed by the formation of a stable product that shows
a strong absorption at 295 nm (332 µs, filled squares)
and is unaffected by oxygen.
The decay rates of radical cations 1a •+-1e•+ were
measured spectrophotometrically following the decrease
in optical density at 450 nm, as a function of the
concentration of -OH. Clean first-order decays and linear
dependencies of the observed rates (kobs) on the concen-
trations of added -OH were obtained. In Figure 3, the
effect of [-OH] on the rate of decay of 1a •+ is displayed.
From the slope of these plots, the second-order rate
constants for reaction of -OH with the radical cations
(15) Le Mague`res, P.; Lindeman, S. V.; Kochi, J . K. J . Chem. Soc.,
Perkin Trans. 2 2001, 1180-1185. Kochi, J . K.; Rathore, R.; Le
Mague`res, P. J . Org. Chem. 2000, 65, 6826-6836. Inokuchi, Y.; Naitoh,
Y.; Ohashi, K.; Saitow, K.; Yoshihara, K.; Nishi, N. Chem. Phys. Lett.
1997, 269, 298-304. Delcourt, M. O.; Rossi, M. J . J . Phys. Chem. 1982,
86, 3233-3239.
(kOH ) were determined as kOH ) 1.4 × 1010 M-1 s-1
.
-
-
(13) Bietti, M.; Baciocchi, E.; Steenken, S. J . Phys. Chem. A 1998,
102, 7337-7342.
(14) With time scales <2 µs, the detection system did not allow the
analysis of the region below 330 nm.
(16) Rathore, R.; Lindeman, S. V.; Kochi, J . K. J . Am. Chem. Soc.
1997, 119, 9393-9404.

Diarylmethanol Radical Cations in Aqueous Solution
J . Org. Chem., Vol. 67, No. 8, 2002 2635
F igu r e 4. Time-resolved vis-NIR absorption spectra of 1a •+
-
F igu r e 5. Plot of the CR band energy (ECR) vs the difference
in ionization potentials (IPD - IPA) between 4-XC6H4CH3 (IPD)
(taken from refs 22 and 23). and 4-CH3OC6H4CH3 (IPA) (8.18
eV, taken from ref 22). r2 ) 0.992.
1e•+ observed on reaction of SO4•- with 1a -e (0.2 mM) at T )
25 °C, recorded after pulse radiolysis of an Ar-saturated
aqueous solution (pH ) 3.9-4.1), containing 0.1 M 2-methyl-
2-propanol and 5 mM K2S2O8, at ca. 4 µs after the 50 ns, 10
MeV electron pulse.
In agreement with the theory, an analogous effect is
observed with 2•+, which contains two methoxy groups
in the charged acceptor ring, i.e., by lowering IPA.
Accordingly, the CR absorption band is now centered at
620 nm, a value which is significantly blue-shifted with
respect to that observed for the corresponding mono-
methoxylated diarylcarbinol 1b•+ (800 nm).
A CR interaction is also observed with diarylmethane
radical cations 3a •+ and 3c•+, the CR absorption band
corresponding to that observed for 1a •+ and 1c•+ (Table
1), indicating that the position of this band is not affected
by the presence of an R-OH group.
that of the UV and visible bands does not increase with
increasing substrate concentration in the range 0.1-0.4
mM.
The observation of a CR band in diarylmethanol radical
cations indicates that there must be sufficient overlap
between the orbitals involved in the interaction of the
two aromatic rings, even though the geometry of the
system does not fulfill the requirement of a cofacial
arrangement between donor and acceptor with an inter-
planar separation equal to or shorter than the van der
Waals contacts necessary for efficient charge transfer and
optimum electronic coupling.16,18 The attribution of the
broad long wavelength band to an intramolecular CR
interaction is further supported by the effect of the
neutral ring substituent on the position and intensity of
this band (Table 1). In Figure 4 are displayed the vis-
Rea ctivity. In acidic solution, decay of radical cations
1a •+-1e•+ is accompanied by the formation of an inter-
mediate characterized by a sharp absorption band at 340
nm and a weak absorption around 550 nm, which is
quenched by oxygen followed by the formation of a stable
product that shows a strong absorption at 295 nm and
is unaffected by oxygen. A similar situation was also
observed in basic solution, where, however, the visible
absorption band of the intermediate formed after decay
of the radical cations is now centered at 570 nm and is
significantly more intense than that observed in acidic
solution. On the basis of these observations, the results
of product analysis, indicating the formation of ben-
zophenones as exclusive reaction products, and literature
NIR spectra of 1a •+-1e•+
.
The position of the band centered around 450 nm is
almost unaffected by varying the neutral ring substitu-
ent; however, an increase in the electron-donor properties
of these substituents results in a significant red shift of
the CR band and in an increase of the relative intensity
of the CR band with respect to the 450 nm band.
Charge resonance between an aromatic donor and a
radical cation acceptor may be interpreted in terms of
the classical Mulliken charge transfer theory.15,20 Along
this line, a good linear correlation is obtained when the
energy of the CR band (ECR) for 1a •+-1e•+ is plotted
against the difference in ionization potentials (IPD - IPA)
between 4-XC6H4CH3 and 4-CH3OC6H4CH3,21 taken as
models of the two aromatic moieties of 1a -1e. ECR
increases linearly by increasing the electron-withdrawing
properties of the donor ring substituent (IPD) (Figure 5).
data,24,25 the intermediate formed after decay of 1a •+
-
1e•+ can be reasonably assigned to the ketyl radical
4-MeOC6H4C(•)OHC6H4X (4•) in acidic solution and to the
corresponding ketyl radical anion 4-MeOC6H4C(•)O(-)-
C6H4X (4•-) in basic solution, while the stable product is
assigned in both cases to the benzophenone 4-MeOC6H4-
COC6H4X (5). The attribution of the intermediates to the
ketyl radical 4• and to the ketyl radical anion 4•- is
confirmed by comparison with the spectra of 4a • and 4a •-
(Figure 6), generated independently after pulse radiolysis
of an argon-saturated aqueous solution containing
(4-MeOC6H4)2CO (5a ) and 2-methyl-2-propanol (to scav-
(17) A vis-NIR CR band has been observed for the radical cations
of diphenylmethane, generated by γ-radiolysis of a glassy isopentane-
butyl chloride mixture at 77 K18 and of 1,3-bis(aryl)propanes generated
by pulse radiolysis in 1,2-dichloroethane.19
(18) Badger, B.; Brocklehurst, B. Trans. Faraday Soc. 1969, 65,
2583-2587.
(19) Takamuku, S.; Komitsu, S.; Toki, S. Radiat. Phys. Chem. 1989,
34, 553-559.
(20) Mulliken, R. S. J . Am. Chem. Soc. 1952, 74, 811-824.
(21) The ionization potentials of4-CH3OC6H4CH3 (8.18 eV),4-CH3C6H4-
CH3 (8.44 eV), C6H5CH3 (8.82 eV), and 4-ClC6H4CH3 (8.69 eV) are
taken from ref 22. The ionization potential of 4-CF3C6H4Me (9.13 eV)
has been estimated by the correlation of the ionization potential of
ring-substituted toluenes vs Hammett σ values.23
•
enge OH, as described in eq 2).
(22) Fukuzumi, S.; Kochi, J . K. J . Am. Chem. Soc. 1981, 103, 7240-
7252.
(23) Lias, S. G.; J ackson, J .-A. A.; Argentar, H.; Liebman, J . F. J .
Org. Chem. 1985, 50, 333-338.
(24) Baciocchi, E.; Del Giacco, T.; Elisei, F. J . Am. Chem. Soc. 1993,
115, 12290-12295.
(25) Bartl, J .; Steenken, S.; Mayr, H.; McClelland, R. A. J . Am.
Chem. Soc. 1990, 112, 6918-6928.

2636 J . Org. Chem., Vol. 67, No. 8, 2002
Bietti and Lanzalunga
Sch em e 1
) 7.5 × 103 s-1, statistically corrected for the number of
R-hydrogen atoms),4 indicating that the effect of the
additional aromatic ring on the deprotonation rate con-
stant is small and consequently that of the ring substi-
tuents are negligible. This observation is somewhat
surprising, as one would have expected a larger increase
in deprotonation rate due to the higher stability of the
diarylcarbinyl radical as compared to the arylcarbinyl
one.30 However, deprotonation reactions of alkylaromatic
radical cations have been described to proceed through
an early transition state.4,24,32,33
Decay of 2•+, assigned on the basis of product studies
to CR-H deprotonation, is much slower than that of 1b•+
,
a result that can be explained with the increased stability
of the radical cation and in line with the relative
deprotonation rate of 4-methoxy and 3,4-dimethoxybenzyl
alcohol radical cations.4,13
Decay of diarylmethane radical cations 3a •+ and 3c•+
is assigned to C-H deprotonation, as also in this case
the formation of the diarylmethyl radical characterized
by a sharp absorption band at 340 nm and a weak
absorption around 550 nm was observed.24,25,34
F igu r e 6. Time-resolved absorption spectra recorded after
pulse radiolysis of an Ar-saturated aqueous solution containing
0.05 mM (4-MeOC6H4)2CO (5a ), 1 M 2-methyl-2-propanol, 1
mM KH2PO4 (pH ) 5.0), or 1 mM Na2B4O7 (pH ) 11.0) and
20% MeCN (to solubilize 5a ) at 4 µs after the 50 ns, 10 MeV
electron pulse. pH ) 5.0 (filled circles), pH ) 11.0 (empty
circles).
Sch em e 2
•+
-
The values of kOH measured for the reactions of 1a
-
)
1e•+ with -OH, are all very similar (≈1.4 × 1010 M-1 s-1
and in agreement with values determined previously for
1-(4-methoxyphenyl)alkanol radical cations,4,35 indicating
interaction of -OH with the R-OH function of the radical
cation, as described in Scheme 3.
The radical cation undergoes diffusion-controlled O-H
deprotonation to give a radical zwitterion which, directly
or through the intermediacy of a benzyloxyl radical,5
undergoes a 1,2-H atom shift to give the ketyl radical 4•.
This radical is then deprotonated to give the correspond-
ing radical anion (4•-), which is finally oxidized to
benzophenone 5. It might also be suggested that the
radical zwitterion undergoes -OH-induced CR-H depro-
tonation, directly leading to the ketyl radical anion, which
accordingly was observed spectroscopically; however,
such an hypothesis seems unlikely, since it should lead
to an overall reaction that is second-order in base, which
has not been observed experimentally. Moreover, in the
4a •- is formed after reaction of 5a with e- (Scheme
aq
1, path a) followed, in acidic solution, by rapid protona-
tion of the ketyl radical anion to give 4a • (path b).13,26,27
In acidic solution, decay of 1a •+-1e•+, which is also
unaffected by oxygen, can thus be attributed to depro-
tonation from the diaryl substituted carbon, leading to
the corresponding carbon-centered radical 4•, which is
then oxidized in the reaction medium, finally leading to
5 (Scheme 2), a typical reaction of arylalkanol radical
cations.4,29
The deprotonation rate constants for 1a •+-1e•+ are
almost identical (k ≈ 1.8 × 104 s-1) and close to the value
measured for 4-methoxybenzyl alcohol radical cation (k
•
(30) The difference in stability between C6H5CH2 and (C6H5)2CH•
is 4 kcal mol-1. However, this difference is likely to decrease in the
presence of an OH group at the radical center.31
(31) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982,
33, 493-532.
(26) Konya, K. G.; Paul, T.; Lin, S.; Lusztyk, J .; Ingold, K. U. J .
Am. Chem. Soc. 2000, 122, 7518-7527.
(32) Amatore, C.; Kochi, J . K. Adv. Electron-Transfer Chem. 1991,
1, 55-148.
(33) A role may be also played by stereoelectronic effects, since in
the presence of a CR interaction between the two aromatic rings (see
text) it is more difficult to reach the conformation required for
deprotonation, where the scissile C-H bond is collinear with the
aromatic π system.3b
(34) No kinetic studies were carried out for these radical cations.
(35) Baciocchi, E.; Bietti, M.; Lanzalunga, O.; Steenken, S. J . Am.
Chem. Soc. 1998, 120, 11516-11517.
(27) In both ethanol and 2-propanol,
a
pKa ) 9.25 has been
determined for (C6H5)2CH(•)OH.28
(28) Autrey, T.; Kandanarachchi, P.; Franz, J . A. J . Phys. Chem. A
2001, 105, 5948-5953.
(29) Baciocchi, E.; Belvedere, S.; Bietti, M.; Lanzalunga, O. Eur. J .
Org. Chem. 1998, 1, 299-302. Walling, C.; El-Taliawi, G. M.; Zhao, C.
J . Org. Chem. 1983, 48, 4914-4917. Trahanovsky, W. S.; Cramer, J .
J . Org. Chem. 1971, 36, 1890-1893.

Diarylmethanol Radical Cations in Aqueous Solution
J . Org. Chem., Vol. 67, No. 8, 2002 2637
Sch em e 3
drol (1e)39 were prepared by reaction of 4-methoxyphenylmag-
nesium bromide with 4-chlorobenzaldehyde, 4-methylbenzal-
dehyde, and 4-trifluoromethylbenzaldehyde, respectively.
3,4-Dimethoxy-4′-methylbenzhydrol (2) was synthesized in
a similar way by reaction of 3,4-dimethoxyphenylmagnesium
bromide with 4-methylbenzaldehyde. 1H NMR: δ 7.27-7.12
(m, 4H, ArH), 6.93-6.78 (m, 3H, ArH), 5.76 (s, 1H, CH), 3.85
(s, 3H, OCH3), 3.84 (s, 3H, OCH3), 2.33 (s, 3H, CH3). MS: m/z
(rel intensity) M+ 258, 185, 139 (100), 119, 91, 77.
presence of -OH, deprotonation of 4• to give the ketyl
radical anion is expected to be a relatively fast process.36
Con clu sion s
The absorption spectra of ring-methoxylated diaryl-
methane and diarylmethanol radical cations display, in
addition to the characteristic UV and visible bands of
methoxybenzene radical cations, a broad band in the vis-
NIR region of the spectrum. This band has been assigned
to an intramolecular charge resonance (CR) interaction
between the donor and acceptor rings on the basis of the
observation that the relative intensity of the CR band to
that of the UV and visible bands does not increase with
increasing substrate concentration and by the effect of
the ring substituents on the position and intensity of this
band. In acidic solution the radical cations undergo C-H
deprotonation, leading to the corresponding diarylmethyl
radicals. In the case of diarylmethanol radical cations,
the intermediate ketyl radical is further oxidized to give
the corresponding benzophenones, as evidenced by both
spectroscopic and product studies. In basic solution,
diarylmethanol radical cations undergo -OH-induced
deprotonation from the R-OH group, a typical reaction
of 1-(4-methoxyphenyl)alkanol radical cations.
Co(III)W was prepared using the literature procedure,12 with
some modifications.40
Rea ction P r od u cts. All products were identified by com-
parison with authentic specimens. 4,4′-dimethoxybenzophen-
one (5a ) and 4-methoxybenzophenone (5c) were commercially
available. 4-Methoxy-4′-methylbenzophenone (5b),41 4-chloro-
4′-methoxybenzophenone (5d ),38 4-methoxy-4′-trifluorometh-
ylbenzophenone (5e),42 and 3,4-dimethoxy-4′methylbenzophen-
one [1H NMR: δ 7.67-7.35 (m, 7H, ArH), 3.96 (s, 3H, OCH3),
3.94 (s, 3H, OCH3), 2.44 (s, 3H, CH3)] were prepared by
oxidation of the corresponding alcohols with chromic acid or
J ones reagent.
P r od u ct An a lysis. γ-Ra d iolysis. Argon-saturated aqueous
solutions containing 1a (1 mM), K2S2O8 (0.5 mM), and 2-meth-
yl-2-propanol (0.2 M) both in acidic (pH ) 4.0) and basic (pH
) 10.0) solution were irradiated at room temperature with a
60Co γ-source at dose rates of 0.5 Gy s-1 for the time necessary
to obtain a 40% conversion with respect to peroxydisulfate.
4,4′-Dimethoxybenzophenone was the exclusive product ob-
served under both acidic and basic conditions. Reaction
1
Exp er im en ta l Section
products were identified by GC-MS and H NMR by compari-
1
son with authentic specimens. Yields were determined by H
Ma ter ia ls. Potassium peroxydisulfate, sodium hydroxide,
disodium tetraborate decahydrate, and 2-methyl-2-propanol
were of the highest commercial quality available. Milli-Q-
filtered water was used for all solutions. 4,4′-Dimethoxyben-
zhydrol (1a ) was used as received. 4,4′-Dimethoxydiphenyl-
methane (3a ) and 4-methoxydiphenylmethane (3c) were
available from a previous work.24
NMR (using bibenzil as the internal standard) and referred
to the starting material. A good material balance (>95%) was
observed in all the experiments.
Oxid a tion s w ith Co(III)W. The oxidant Co(III)W (2 mM)
and the substrate (2 mM) were magnetically stirred in 10 mL
of an argon-degassed 50 mM sodium tartrate buffer solution
(pH ) 4.0), at 50 °C. After 48 h (6 h in the oxidation of 2), the
products of the reaction were extracted with ethyl ether and
dried over anhydrous Na2SO4. 4-Methoxy-4′-X-benzophenones
(5a -5e) and 3,4-dimethoxy-4′-methylbenzophenone were the
exclusive products observed.43 Yields are as follows: 5a , 27%;
5b, 24%; 5c, 22%; 5d, 19%; 3,4-dimethoxy-4′-methylbenzophen-
one, 38%.
4-Methoxybenzhydrol (1c)37 was prepared by reduction of
4-methoxybenzophenone with NaBH4, purified by column
chromatography (silica gel, eluent hexane/ethyl acetate 5:1).
4-Chloro-4′-methoxybenzhydrol (1d ),38 4-methoxy-4′-meth-
ylbenzhydrol (1b),39 and 4-methoxy-4′-trifluoromethylbenzhy-
(36) The equilibrium between the ketyl radical -O2CC6H4CH(•)OH
and the corresponding radical dianion -O2CC6H4CH(•)O- was found
26
to be catalyzed by secondary phosphate with k ≈ 108 M-1 s-1
.
Thus,
(40) Baciocchi, E., Crescenzi, M., Fasella, E.; Mattioli, M. J . Org.
Chem. 1992, 57, 4684-4689.
the spectra displayed in Figure 2 show only the conversion of 1a •+ into
the corresponding ketyl radical anion, since this process is significantly
slower than the equilibrium described in Scheme 1.
(37) Guijarro, D.; Yus, M. Tetrahedron 2000, 56, 1135-1138.
(38) Mindl, J .; Vecera, M. Collect. Czech. Chem. Commun. 1972, 37,
1143-1149.
(41) Atkinson, G. E.; Fischer, P. M.; Chan, W. C. J . Org. Chem. 2000,
65, 5048-5056.
(42) Uehara, F.; Sato, M.; Kaneko, C.; Kurihara, H. J . Org. Chem.
1999, 64, 1436-1441.
(43) Due to the lower solubility of 1e as compared to 1a -d , the
exclusive reaction product 5e was not quantitatively determined.
(39) Kelly, D. P.; J enkins, M. J . J . Org. Chem. 1984, 49, 409-413.

2638 J . Org. Chem., Vol. 67, No. 8, 2002
Bietti and Lanzalunga
P u lse Ra d iolysis. The pulse radiolysis experiments were
performed using a 10 MeV electron linear accelerator that
supplied 50 ns pulses with doses such that 1-3 µM radicals
were produced. Experiments were performed at room temper-
ature using argon- or oxygen-saturated aqueous solutions
containing the substrate (0.05-1.0 mM), peroxydisulfate (5-
10 mM), and 2-methyl-2-propanol (0.1-1.0 M). The pH of the
solutions was adjusted with NaOH or HClO4, and for the
experiments carried out at pH > 9, 1 mM sodium tetraborate
was added to avoid undesired pH variations upon irradiation.
A flow system was employed in all the experiments. The
observed rates (kobs) were obtained by averaging 6-12 values,
each consisting of an average of 5-15 shots and were repro-
ducible to within 5%.
The ketyl radical and radical anion, (4-MeOC6H4)2C(•)OH
(4a •) and (4-MeOC6H4)2C(•)O- (4a •-), were generated after
pulse radiolysis of an argon-saturated aqueous solution (pH
)
5.0 and 11.0, respectively) containing 0.05 mM
(4-MeOC6H4)2CO (5a), 1 M 2-methyl-2-propanol, 1 mM Na2B4O7,
and 20% MeCN (to solubilize 5a ).
Ack n ow led gm en t. Thanks are due to the Ministero
dell’Istruzione, dell’Universita` e della Ricerca (MIUR)
and the Consiglio Nazionale delle Ricerche (CNR) for
financial support. Pulse and γ-radiolysis experiments
were performed at the Paterson Institute for Cancer
Research Free Radical Research Facility, Manchester,
UK, with the support of the European Commission
through the Access to Large-Scale Facilities activity of
the TMR Program.
The second-order rate constants for reaction of the radical
cations with -OH (kOH ) were obtained from the slopes of the
-
plots of kobs vs the concentration of NaOH.
J O016287F