In-cage formation of carbanions in photoinduced electron-transfer reaction of carboxylate ions

Article abstract of DOI:10.1021/ja981470k

To develop a novel method for the photochemical generation of carbanions, the photoinduced electron-transfer reaction of carboxylate salts was investigated. The photolysis of tetra-n-butylammonium or metal ion/crown ether salts of diphenylacetate (Ph₂CHCO₂^-) sensitized with 1- cyanonaphthalene or 1,4-dicyanonaphthalene in THF or benzene yielded Ph₂CH₂ as the protonated product of the corresponding carbanion (Ph₂CH^-). This reaction was not affected by the presence of oxygen, and nanosecond transient absorption spectroscopy allowed the observation of Ph₂CH^-, immediately after the laser pulse. The suggested reaction mechanism involves (1) photoexcitation of the sensitizer, (2) the one-electron oxidation of the carboxylate ion, (3) the decarboxylation of the resulting carboxyl radical generating Ph₂CH^. and (4) the back electron-transfer from the sensitizer radical anion to the radical to yield the carbanion, which occurs smoothly within cage of the geminate radical ion pair. When the sensitizer employed was 9, 10-dicyanoanthracene, which was a less negative reduction potential, Ph₂CH^. was observed by laser flash photolysis. The free energy change (- ΔG) on the electron transfer is the crucial factor determining the radical/anion selectivity.

Full text of DOI:10.1021/ja981470k

J. Am. Chem. Soc. 1998, 120, 12453-12458
12453
In-Cage Formation of Carbanions in Photoinduced Electron-Transfer
Reaction of Carboxylate Ions
Hajime Yokoi, Tomoharu Nakano, Wataru Fujita, Katsuya Ishiguro, and
Yasuhiko Sawaki*
Contribution from the Department of Applied Chemistry, Graduate School of Engineering,
Nagoya UniVersity, Chikusa-ku, Nagoya, 464-8603 Japan
ReceiVed April 29, 1998
Abstract: To develop a novel method for the photochemical generation of carbanions, the photoinduced electron-
transfer reaction of carboxylate salts was investigated. The photolysis of tetra-n-butylammonium or metal
-
ion/crown ether salts of diphenylacetate (Ph2CHCO2 ) sensitized with 1-cyanonaphthalene or 1,4-dicyano-
naphthalene in THF or benzene yielded Ph2CH2 as the protonated product of the corresponding carbanion
-
(
Ph2CH ). This reaction was not affected by the presence of oxygen, and nanosecond transient absorption
-
spectroscopy allowed the observation of Ph2CH immediately after the laser pulse. The suggested reaction
mechanism involves (1) photoexcitation of the sensitizer, (2) the one-electron oxidation of the carboxylate
ion, (3) the decarboxylation of the resulting carboxyl radical generating Ph2CH , and (4) the back electron-
•
transfer from the sensitizer radical anion to the radical to yield the carbanion, which occurs smoothly within
cage of the geminate radical ion pair. When the sensitizer employed was 9,10-dicyanoanthracene, which has
•
a less negative reduction potential, Ph2CH was observed by laser flash photolysis. The free energy change
(-∆G) on the electron transfer is the crucial factor determining the radical/anion selectivity.
•
Introduction
centered radicals (R ) are formed via decarboxylation of carboxyl
•
radicals (RCO2 ). These carbon radicals undergo dimerization
The photochemical generation of closed-shell ions is a topic
of current interest in organic photochemistry. For example,
1
photogenerated cations may act as an initiator of cation
2
polymerizations, and much attention has recently been focused
on “caged compounds” which lead to the photorelease of
biologically active ionic compounds.3 Another advantage of
photogeneration is that very fast reactions can be followed by
laser flash photolysis techniques. Carbanions are one of the
+
or further oxidation, yielding R-R or R , respectively. It is
4
most common reactive intermediates in organic syntheses, and
known that the photoinduced electron-transfer (PET) reaction
of carboxylic acids also gives carbon-centered radicals via
decarboxylation. In this reaction, carbon radicals are formed
hence the photogeneration of such ionic species may contribute
to progress in mechanistic organic chemistry. Only a few
examples are known for the photochemical formations of
7
-•
together with sensitizer radical anion (Sens ), and hence the
5
carbanions, most of which are based on the photodissociation
formation of carbanion might be expected if an effective electron
transfer between them takes place. However, the reported PET
reactions yielded a mixture of radical-sensitizer adducts and
of carboxylic acids.
A related reaction is the anodic oxidation of carboxylate ions
-
6
(
RCO2 ), i.e., the Kolbe reaction (eq 1), in which carbon-
8,9
other radical coupling products. While several synthetically
(
1) (a) Tolbert, L. M. In Handbook of Organic Photochemistry; Scaiano,
useful reactions have been explored by the photosensitized
J. C., Ed; CRC Press: Boca Raton, FL, 1989; Vol. 2, pp 23-34. (b) Das,
P. K. Chem. ReV. 1993, 93, 119-144. (c) McClelland, R. A. Tetrahedron
(7) Bowers, P. R.; McLauchlan, K. A.; Sealy, R. C. J. Chem. Soc., Perkin
Trans. 2 1976, 915-921.
1
996, 52, 6823-6858.
(2) Hofer, D. C.; Allen, R.; Wallraff, G.; Ito, H.; Breyfon, G.; Brock,
(8) (a) Noyori, R.; Kato, M.; Kawanisi, M.; Nozaki, H. Tetrahedron 1969,
25, 1125-1136. (b) Libman, J. J. Am. Chem. Soc. 1975, 99, 4139-4141.
(c) Epling, G. A.; Lopes, A. J. Am. Chem. Soc. 1977, 99, 2700-2704. (d)
Novak, M.; Miller, A.; Bruice, T. C. J. Am. Chem. Soc. 1980, 102, 1465-
1467. (e) Kurauchi, Y.; Ohga, K.; Nobuhara, H.; Morita, S. Bull. Chem.
Soc. Jpn. 1985, 58, 2711-2712. (f) Kurauchi, Y.; Nobuhara, H.; Ohga, K.
Bull. Chem. Soc. Jpn. 1986, 59, 897-905. (g) Maki, Y.; Sako, M.; Oyabu,
I.; Murase, T.; Kitade, Y.; Hirota, K. J. Chem. Soc., Chem. Commun. 1989,
1780-1782. (h) Tsujimoto, K.; Nakao, N.; Ohashi, M. J. Chem. Soc., Chem.
Commun. 1992, 336-337. (i) Okumura, Y.; Hata, N. Bull. Chem. Soc. Jpn.
1993, 66, 648-650. (j) Mella, M.; Freccero, M.; Soldi, T.; Fasani, E.; Albini,
A. J. Org. Chem. 1996, 61, 1413-1422.
P.; DiPietro, R. J. Photopolym. Sci. Technol. 1996, 9, 387-398 and
references therein.
(3) (a) Bioorganic Photochemistry; Morrison, H., Ed.; Wiley: New York,
1
5
993; Vol. 2. (b) Givens, R. S.; Kueper, L. W., III. Chem. ReV. 1993, 93,
5-66.
(
4) Buncel, E.; Durst, T. ComprehensiVe Carbanion Chemistry; Elsevi-
er: Amsterdam, 1980.
5) (a) Ireland, J. F.; Wyatt, P. A. H. AdV. Phys. Org. Chem. 1976, 12,
31-221. (b) Wan, P.; Shukla, D. Chem. ReV. 1993, 93, 571-584. (c)
Budac, D.; Wan, P. Can. J. Chem. 1996, 74, 1447-1464.
6) (a) Kolbe, H. Ann. Chem. 1849, 69, 257. (b) Compton, R. G.; Coles,
(
1
(
B. A.; Day, M. J. J. Electroanal. Chem. 1986, 200, 205-215. (c) Compton,
R. G.; Laing, M. E. J. Chem. Soc., Chem. Commun. 1988, 1320-1321. (d)
Ohno, T.; Fukumoto, T.; Hirashima, T.; Nishiguchi, I. Chem. Lett. 1991,
085-1088. (e) Torii, S. Electro-organic Syntheses; Kodansha: Tokyo,
985; Chapter 2, pp 51-74.
(9) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7729-
7731. (b) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 2239-
2240. (c) Kraeutler, B.; Jaeger, C. D.; Bard, A. J. J. Am. Chem. Soc. 1978,
100, 4903-4905. (d) Lai, C.; Kim, Y. I.; Wang, C. M.; Mallouk, T. E. J.
Org. Chem. 1993, 58, 1393-1399.
1
1
1
0.1021/ja981470k CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/18/1998

12454 J. Am. Chem. Soc., Vol. 120, No. 48, 1998
Yokoi et al.
Scheme 1
Table 1. Electrochemical and Photophysical Data of Sensitizers
and Radicals
red
E
1/2
E
s
-∆G
q
a
b
(
V vs SCE) (eV) (eV)
Sensitizers
,2,4,5-tetracyanobenzene (TCB)
,4-dicyanobenzene (DCB)
1
1
1
-0.65c
3.80c 1.7
d
d
-1.60
4.27
0.7
0.6
,4-dicyano-2,5-dimethylbenzene
-1.75e
-1.90e
3.89f
(
DMDCB)
,4-dicyano-2,3,5,6-tetramethylbenzene
TMDCB)
-cyanonaphthalene (CN)
,4-dicyanonaphthalene (DCN)
1
3.74f
0.3
(
decarboxylations10 in the presence of hydrogen donors, carban-
ion formation has never been intended in these studies.
-1.98dd
d
1
1
3.88
3.45
3.04
2.86
0.4
0.7
0.0
0.5
-1.28
8c,11
g
Direct photolysis of carboxylate anions produces carbanions,
9-cyanoanthracene (CA)
9,10-dicyanoanthracene (DCA)
-1.58
-0.89d
but their spectroscopic observation has been achieved only for
carbanions stabilized by electron-withdrawing substituents (e.g.,
p-nitrobenzyl anion)12 or by resonance with a carbonyl group.
Radicals
13
-1.40h
p-chlorobenzyl
p-methoxybenzyl
diphenylmethyl
R-naphthylmethyl
h
14
-1.82
During our study on the redox reactions of neutral radicals,
i
-1.14
-1.27
it has been elucidated that the reduction of benzylic radicals by
cyanoaromatic radical anions proceeds quite efficiently under
appropriate conditions. Thus, we tried to generate carbanions
via photosensitized decarboxylations. The strategy is to use
j
a
Redox potentials obtained at room temperature in CH
3
CN. b Est-
1
9
mated from Rehm-Weller relationship based on the assumption that
ox
-
18
c
d
e
E
1/2 (RCO
2
) is 1.5 V vs SCE.
From ref 17b. From ref 17c. From
i
-
+
carboxylate salts (RCO2 M ) in aprotic solvents in order to
ref 14. From ref 17d. From ref 17a. h From ref 23b. From ref 23a.
f
g
-
produce basic carbanions (R ). Fast and efficient carbanion
j
From ref 23c.
formation would be expected under appropriate conditions in
the photochemical redox reaction, as shown in Scheme 1. Four
elementary steps are involved here: (i) the excitation of an
electron-accepting sensitizer, (ii) the one-electron oxidation of
carboxylate ion, (iii) the decarboxylation of the carboxyl radical,
and (iv) the electron transfer from sensitizer radical anion to
the resulting carbon-centered radical.
relationship19 to be high for all sensitizers (i.e., -∆Gq g 0, as
listed in the last column in Table 1). Here, solvent effects were
neglected since the potential shifts in the sensitizers may be
-
mostly canceled by the ones in RCO2 .
For the mechanistic analysis of the present study, four criteria
•
should be satisfied: (i) the reduction potential of R should be
We report here that the predicted reaction sequence of Scheme
known and should not be too negative, (ii) the resulting
1
is operative and carbanions are observable spectroscopically.
-
carbanion R should be observable in the visible range, (iii)
-
•
•
It is shown that the electron transfer from Sens to R occurs
efficiently within the geminate radical ion pair (in-cage).
the absorption spectrum of the carboxylate anion itself should
not overlap with that of the sensitizer, and (iv) the decarboxy-
•
lation of RCO2 should take place “instantaneously”. While the
Results and Discussion
decarboxylation is not so fast for primary alkyl, alkenyl, alkynyl,
2
0,21
Conditions for the Sensitized Photooxidations of Car-
and aryl carboxyls,
those of benzylic carboxyls proceed very
-
12
-10
22
boxylate Ions. We examined the photosensitized one-electron
fast on the time scale of 10 -10
s.
The benzylic
-
oxidation of carboxylate ions (RCO2 ) in aprotic solvents in
carboxylates employed here are listed in Chart 1, for which the
reduction potentials of corresponding benzyl radicals are known
order to develop a novel method of carbanion formation.
Solvents used were tetrahydrofuran (THF) and benzene; aceto-
nitrile (MeCN) could not be employed since it forms complexes
2
3
•
(Table 1). The one-electron reductions of R by sensitizer
radical anions, the key step iv in Scheme 1, are expected to
depend on the free energy changes (-∆G) for the electron
transfer, which can be estimated from the difference in one-
with crown ethers1 and has acidic methyl protons. Sensitizers
5
16
employed are listed in Table 1, together with their singlet exci-
red
14,17
•
tation energies (Es) and reduction potentials (E1/2 ) in MeCN.
electron reductions between R and sensitizer (eq 2).
Efficiencies of electron-transfer quenching of excited sensitizers
ox
∆G ) -E1/2 (sens) + E1/2red(R•)
red
by carboxylate anions (E1/2 ) +1.7-1.8 V vs NHE in MeCN
-
(2)
1
8
for R ) alkyl) could be evaluated from the Rehm-Weller
(
10) (a) Tossaint, O.; Capdevielle, P.; Maumy, M. Tetrahedron 1984,
(18) Eberson, L. AdV. Phys. Org. Chem. 1982, 18, 79-185.
4
0, 3229-3233. (b) Saito, I.; Ikehara, H.; Kasatani, R.; Watanabe, M.;
(19) (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271. (b) Weller,
A. Z. Phys. Chem. (Wiesbaden) 1982, 133, 93-98.
Matsuura, T. J. Am. Chem. Soc. 1986, 108, 3115-3117. (c) Okada, K.;
Okamoto, K.; Oda, M. J. Am. Chem. Soc. 1988, 110, 8736-8738. (d)
Hasebe, M.; Tsuchiya, T. Tetrahedron Lett. 1988, 29, 6287-6290.
(20) (a) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc.
1988, 110, 2877-2885. (b) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J.
Am. Chem. Soc. 1988, 110, 2886-2893. (c) Yamauchi, S.; Hirota, N.;
Takahara, S.; Misawa, H.; Sawabe, K.; Sakuragi, H.; Tokumaru, K. J. Am.
Chem. Soc. 1989, 111, 4402-4407. (d) Korth, H. G.; Chateauneuf, J.;
Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1990, 110, 5929-5931. (e)
Korth, H. G.; Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Org. Chem.
1991, 56, 2405-2410.
(
11) (a) Coyle, J. D. Chem. ReV. 1978, 78, 97-123. (b) Meiggs, T. O.;
Miller, S. I. J. Am. Chem. Soc. 1972, 94, 1989-1996. (c) Meiggs, T. O.;
Grossweiner, L. I.; Miller, S. I. J. Am. Chem. Soc. 1972, 94, 7981-7986.
(
12) Margerum, J. D. J. Am. Chem. Soc. 1965, 87, 3772-3773.
(13) Mart ´ı nez, J. L.; Scaiano, J. C. J. Am. Chem. Soc. 1997, 119, 11066-
1
1070.
(
14) Ishiguro, K.; Nakano, T.; Shibata, H.; Sawaki, Y. J. Am. Chem.
Soc. 1996, 118, 7255-7264.
15) Gokel, G. W.; Cram, D. J.; Liotta, C. L.; Harris, H. P.; Cook, F. L.
(21) Hillborn, J. W.; Pincock, J. A. J. Am. Chem. Soc. 1991, 113, 2683-
2686.
(
(22) (a) Falvey, D. E.; Schuster, G. B. J. Am. Chem. Soc. 1986, 108,
7419-7421. (b) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Org. Chem.
1997, 62, 2210-2221.
(23) (a) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem.
Soc. 1988, 110, 132-137. (b) Sim, B. A.; Milne, P. H.; Griller, D.; Wayner,
D. D. M. J. Am. Chem. Soc. 1990, 112, 6635-6638. (c) Milne, P. H.;
Wayner, D. D. M.; DeCosta, D. P.; Pincock, J. A. Can. J. Chem. 1992, 70,
121-127.
J. Org. Chem. 1974, 39, 2445-2446.
(
16) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.
(
17) (a) Eriksen, J.; Foote, C. S. J. Phys. Chem. 1978, 82, 2659-2662.
(
b) Mattes, S. L.; Farid, S. Org. Photochem. 1983, 6, 233-236. (c)
Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449. (d) Suzuki,
M.; Ikeno, T.; Osoda, K.; Narasaka, K.; Suenobu, T.; Fukuzumi, S.; Ishida,
A. Bull. Chem. Soc. Jpn. 1997, 70, 2267-2277.

Cage ET Reaction of Carboxylate Ions
J. Am. Chem. Soc., Vol. 120, No. 48, 1998 12455
Chart 1
benzene as well (run 4). When the photoreaction was carried
out under an oxygen atmosphere, Ph2CdO was obtained in 39%
yield, but Ph2CH2 was still formed as the major product in 57%
yield (run 6). Thus, the predominant pathway is not affected
25
by molecular oxygen, a well-known radical scavenger, indicat-
•
ing that most of Ph2CH is reduced instantaneously within the
geminate radical ion pair.
We were interested in the sensitization with 9,10-dicyanoan-
•
thracene (DCA) because the one-electron reduction of R by
-
•
DCA is estimated to be endothermic by 0.25 eV. Contrary
to the prediction, the DCA-sensitized photooxidations of 1a
proceeded similarly, affording Ph2CH2 as the major product
in addition to a small amount of 1,1,2,2-tetraphenylethane
(
(Ph2CH)2). An interesting point about this observation will
be discussed later.
b) Substituted Phenylacetates (2b and 3b). The photo-
(
oxidations were also investigated for substituted phenylacetates.
The 1,2,4,5-tetracyanobenzene (TCB)- or DCA-sensitized pho-
tooxidations of K/18-crown-6 salts of p-chloro- (2b) and
p-methoxyphenylacetates (3b) in benzene also gave p-chloro-
and p-methoxytoluene, respectively, but in much lower yields
-
+
Several carboxylate salts (RCO2 M ) with various R’s and
M’s were prepared from the corresponding carboxylic acids by
adding equimolar bases or by cation exchange. The tetra-n-
butylammonium salt (1a) was soluble in THF and benzene, and
metal carboxylates (M ) Li, Na, and K) could also be dissolved
by stirring with an equimolar amount of 12-crown-4 (Li), 15-
crown-5 (Na), or 18-crown-6 (K). Several carboxylate salts such
as potassium 9-methyl-9-fluorenecarboxylate could not be
employed since their spontaneous thermal decarboxylation
proceeded rapidly at room temperature.24
(
Table 2, runs 10, 11, 14, and 15). A substantial amount of
bibenzyl, (p-MeOC6H4CH2)2, was detected for the reaction of
b. The low mass balance in these cases might be due to
3
coupling reactions of benzyl radicals with the cyanoaromatic
26
radical anion; in fact, a coupling product was actually identi-
fied in the products from 2b (see run 11 in Table 2). These
Products of the Photooxidation of Carboxylates. (a)
Diphenylacetates (1). The fluorescence of 1-cyanonaphthalene
CN) was found to be quenched efficiently by tetra-n-butylam-
coupling reactions may be avoided by use of sterically hin-
17d,27
dered sensitizers,
such as 1,4-dicyano-2,5-dimethylbenzene
(
(DMDCB) or 1,4-dicyano-2,3,5,6-tetramethylbenzene (TMDCB).
monium diphenylacetate (1a) in THF, with a bimolecular rate
It was found that the selectivities for formation of the toluenes
were significantly improved (runs 12, 13, 16, and 17) with these
hindered sensitizers. The remarkable effect of TMDCB as the
sensitizer might be, in addition to the steric hindrance, due to
its more negative reduction potential facilitating electron transfer
9
-1 -1
constant of 9.0 × 10 M s . When an argon-saturated THF
solution of 10 mM 1a and 5 mM CN was irradiated (>300
nm) for 2 h, 58% of 1a was converted to diphenylmethane (5)
in 100% selectivity (run 1 in Table 2).
-
•
from TMDCB to benzyl radicals.
(c) 1-Naphthylacetates (4). Similar sensitized photolyses
of the K/18-crown-6 salt of 1-naphthalenecarboxylate
-
(
NpCH2CO2 , 4b), which has an absorption band at ∼340 nm,
were carried out with both CA and DCA as sensitizer using a
filter solution (i.e., >350 nm). The photooxidations of 4b were
effective, affording 1-methylnaphthalene (NpCH3 (10)) as the
major product (runs 18 and 19), and the dimeric product,
Similar photoreactions of 1 were carried out with various
sensitizers, solvents, and counterions, the results being sum-
marized in runs 1-9 of Table 2. The use of naphthalene, instead
of cyanoaromatic sensitizers, where the one-electron oxidation
of carboxylate 1a is slightly endothermic (-∆Gq < 0), resulted
in practically no reaction. Thus, it is apparent that the
decarboxylation is initiated by photoinduced one-electron oxida-
tion of 1, not by energy transfer. The CN-sensitized photooxi-
dation of K/18-crown-6 (1b) and Na/15-crown-5 salts (1c) in
THF proceeded similarly (runs 7 and 9), but the reaction was
significantly decelerated by the addition of 10% H2O. In
addition, the Li salt of 1 in THF or the Na salt in MeOH did
not quench the fluorescence of CN at all. Thus, the present
photodecarboxylation is effective only for naked carboxylate
ions, that is, in the absence of ion-pairing with a countercation
or hydrogen-bonding with protic solvents.
(
NpCH2)2, was not detected. When anthracene was employed
as the sensitizer, no reaction took place, 4b being recovered
quantitatively.
The predominant formation of NpCH2-H in the DCA
sensitization seems to be anomalous because the one-electron
(25) Maillard, B.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc. 1983,
1
05, 5095-5099.
(26) (a) Albini, A.; Fasani, E.; Sulpizio, A. J. Am. Chem. Soc. 1984,
06, 3562-3566. (b) Albini, A.; Fasani, E.; Mella, M. J. Am. Chem. Soc.
986, 108, 4119-4125. (c) Albini, A.; Spreti, S. J. Chem. Soc., Perkin
1
1
Trans. 2 1987, 1175-1179. (d) Albini, A.; Sulpizio, A. J. Org. Chem. 1989,
5
Pieta, S. J. Am. Chem. Soc. 1989, 111, 5773-5777. (f) d’Alessandro, N.;
Albini, A.; Mariano, P. S. J. Org. Chem. 1993, 58, 937-942. (g) Mella,
M.; d’Alessandro, N.; Feccero, M.; Albini, A. J. Chem. Soc., Perkin Trans.
4, 2147-2152. (e) Sulpizio, A.; Albini, A.; d’Alessandro, N.; Fasani, E.;
2
1993, 515-519. (h) Mella, M.; Freccero, M.; Albini, A. J. Org. Chem.
Sensitizations with 1,4-dicyanonaphthalene (DCN) and 9-cy-
anoanthracene (CA) resulted in less selectivity (runs 2 and 3),
where substantial amounts of sensitizers were consumed during
the photooxidation but the corresponding products could not
be identified. The CN-sensitized photooxidation took place in
1994, 59, 1047-1052. (i) Albini, A.; Mella, M.; Freccero, M. Tetrahadron
1
994, 50, 575-607. (j) Fagnoni, M.; Mella, M.; Albini, A. J. Am. Chem.
Soc. 1995, 117, 7877-7881. (k) Fasani, E.; Mella, M.; Albini, A. J. Chem.
Soc., Perkin Trans. 2 1995, 449-452. (l) Mella, M.; Freccero, M.; Albini,
A. Tetrahedron 1996, 52, 5533-5548.
(27) (a) Yamada, S.; Nakagawa, Y.; Watabiki, O.; Suzuki, M.; Ohashi,
M. Chem. Lett. 1986, 361-364. (b) Gassman, P. G.; De Silva, S. A. J. Am.
Chem. Soc. 1991, 113, 9870-9872. (c) Osoda, K.; Pannecoucke, X.;
Narasaka, K. Chem. Lett. 1995, 1119-1122.
(
24) Hunter, D. H.; Hamity, M.; Patel, V.; Perry, R. A. Can. J. Chem.
1
978, 56, 104-113.

12456 J. Am. Chem. Soc., Vol. 120, No. 48, 1998
Yokoi et al.
Table 2. Products on the Continuous Irradiation of Carboxylate and Sensitizer Systemsa
selectivity (%)
others
ndc
nd
nd
-
-∆G (eV)b
run no.
RCO
2
R-
CH-
solvent
THF
sens
CN
CA
DCN
CN
DCA
CN
CN
CA
CN
DCA
TCB
TMDCB
DMDCB
DCA
TCB
TMDCB
DMDCB
CA
DCA
CA
DCA
CA
R-H
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
1a
Ph
2
100
21
28
73
55
57
74
34
100
26
12
78
81
12
6.1
52
21
64
52
+0.84
+0.44
+0.14
+0.84
-0.25
+0.84
+0.84
+0.44
+0.84
-0.51
-0.75
+0.50
+0.35
-0.93
-1.17
+0.08
-0.07
+0.31
-0.38
+0.31
-0.38
+0.31
-0.38
benzene
nd
R-R, 13
benzened
THF
Ph
nd
2
CdO, 39
1b
Ph
2
CH-
CH-
R-R, 1.4
1c
2b
Ph
2
THF
benzene
nd
nd
0
1
2
3
4
5
6
7
8
9
0
1
2
3
p-ClC
6
H
4
CH
2
-
e
6, 19
nd
nd
R-R, 23
3b
4b
p-MeOC
6
H
4
CH
2
-
benzene
R-R, 5.2
e
R-R, 1.2; 7, 8.9
e
e
R-R, 3.2; 8, 22; 9, 15
nd
R-NpCH
2
-
benzene
benzenef
benzened
nd
nd
nd
g
23
g
44
45
6.7
NpCHO, 12
NpCHO, 44
DCA
a
Argon-saturated solutions of carboxylate salt (0.3-5.0 mM) and sensitizer (0.2-5.0 mM) were irradiated with a 300-W medium-pressure
mercury lamp through an appropriate filter (>350 nm for 1-3; >300 nm for 4 and 5). Conversions of carboxylates were mostly over 60% yield.
b
Estimated from eq 1. c nd, not detected (<0.5%). Under oxygen atmosphere. e See Chart 2. 1% D
d
f
O was added. g 1-Methylnaphthalene-R-d
2
(
NpCH D), over 80%.
2
reduction of NpCH2 by DCA-• is endothermic by 0.38 eV (see
•
Table 2). Since 10 may be formed either by the protonation of
-
•
NpCH2 or via hydrogen abstraction of NpCH2 , the origin of
hydrogen in 10 was examined by deuterium labeling. When
1% D2O was added to the solvent benzene, 1-methylnaphtha-
lene-R-d (NpCH2D) was obtained in over 80% D isotropic
•
purity, indicating the predominance of the reduction of NpCH2
-
•
-
by DCA followed by protonation of NpCH2 with water.
It is interesting to note that a large difference in the effects
of O2 on CA- and DCA-sensitized photooxidations was ob-
served. CA-sensitized reaction of 4b was not significantly
affected by the presence of molecular oxygen, affording 10
(45%) as the major product. However, in the case of DCA
sensitization, the formation of 10 was suppressed to only one-
eighth, and the major product was NpCHO (44%).
Transient Absorption Spectra of Intermediates Generated
by Laser Flash. Nanosecond laser flash photolysis (LFP) was
carried out in order to clarify the primary photoproduct on the
sensitized photooxidation of diphenylacetate salts (1). LFP
(XeCl, 308 nm, 10 ns) of 1b (20 mM) and CN (0.2 mM) in
THF showed an absorption band of an intermediate with its
maximum at 450 nm immediately after the laser pulse, as shown
-
•
28
in Figure 1a. The absorption bands of CN (λmax ) 370 nm)
•
29
and of Ph2CH (λmax ) 325 nm) were practically absent in
-
+
this spectrum. The spectrum quite resembles that of Ph2CH K /
1
8-crown-6 in THF (λmax ) 448 nm),30 but the assignment was
not conclusive because the triplet excited state of CN has a
similar T-T absorption band around 450 nm.31 Next, the LFP
experiment was done with TMDCB (0.2 mM), which led to
the production of an almost identical transient absorption
spectrum (Figure 1b). Thus, in both cases, the diphenylmethyl
Figure 1. Absorption spectra of transients obtained from laser flash
of sensitizers and diphenylacetate salts in solution saturated with argon.
(a) 0.2 mM CN and 20 mM potassium diphenylacetate/18-crown-6
(1b) in THF at 1 µs after excitation (308 nm, XeCl). (b) 0.2 mM
TeMDCB and 20 mM 1b in THF at 1 µs after excitation (308 nm,
XeCl). (c) 0.1 mM DCA and 10 mM tetra-n-butylammonium di-
phenylacetate (1a) in benzene at 300 ns after excitation (355 nm, Nd:
YAG THG).
(28) Shida, T. Electronic Absorption Spectra of Radical Ions; Physical
Sciences Data 34; Elsevier: Amsterdam, 1988.
(
(
29) Faria, J. L.; Steenken, S. J. Phys. Chem. 1993, 97, 1924-1930.
30) Buncel, E.; Menon, B. C.; Colpa, J. P. Can. J. Chem. 1979, 57,
-
carbanion (Ph2CH ) was actually generated within the pulse
9
1
99-1005.
duration of excimer laser, i.e., <10 ns. This observation
confirms that the reduction of the carbon radical by electron
(
31) (a) Mac, M.; Wirz, J.; Najbar, J. HelV. Chim. Acta 1993, 76, 1319-
331. (b) Nouchi, G. J. Chim. Phys. Phys.-Chim. Biol. 1969, 66, 554-565.

Cage ET Reaction of Carboxylate Ions
J. Am. Chem. Soc., Vol. 120, No. 48, 1998 12457
Chart 2
Scheme 2
lecular rate constant will reach the diffusion limit when -∆G
•
g +0.5 eV. This case corresponds to the ET between Ph2CH
-
•
-•
and CA (-∆G ) +0.84 eV) or TMDCB (-∆G ) +0.80
eV). On the other hand, the predicted rate constant for the
reduction of Ph2CH by DCA (-∆G ) -0.25 eV) falls to
10 M s . Such a slow electron transfer would not compete
with the dissociation of radical ion pairs into, as observed, free
radical and ions as depicted in Scheme 2.
•
-•
7
-1 -1
∼
transfer proceeds smoothly in-cage, i.e., within the geminate
radical ion pair.
In contrast, LFP (third harmonic of Nd:YAG laser, 355 nm,
Electron-transfer processes within geminate radical ion pairs
32,33
have been well studied both by the free ion yields
high-speed time-resolved spectroscopy, demonstrating that the
and by
∼
6 ns) of 10 mM 1a and 0.1 mM DCA in benzene resulted in
34
a quite different spectrum, as shown in Figure 1c, which
35
first-order ET rates show the Marcus inverted region and
•
-•
indicated the generation of Ph2CH (λmax ) 325 nm) and DCA
36
distance dependence. Since the ET in the present case is a
28,32
(
λmax ) 335, 500, 580, 640, and 700 nm).
The absorption
charge shift in less polar solvents, some difference may be
expected in the behavior of intermediates and in ET parameters
from a charge recombination between radical cations and anions.
-
of Ph2CH was not observed at all, which might be understand-
•
-•
able because the reduction of Ph2CH by DCA is endothermic,
-
∆G < 0. The kinetic trace of diphenylmethyl radical followed
37
38
Related cases have been studied by Pincock, Gould et al.,
-•
a rapid second-order decay due to its dimerization, while DCA
39
and Schuster el al. More detailed examination of the electron
did not decay within the time window of the experiment, i.e.,
•
-•
transfer within the R /Sens pair will be possible if the first-
order rate constants are determined, e.g., by picosecond transient
absorption spectroscopy.
100 µs.
Free Energy Change (-∆G) Dependence of Anion/Radical
Selectivities. It was shown that irradiation of argon-saturated
solutions of sensitizers in the presence of carboxylate salts (R-
Finally, some mention should be made here of the out-of-
•
cage ET process. As stated above, the geminate pair of (R /
-
+
CO2 M ) led to the predominant formation of R-H, which was
-•
Sens ) dissociates into free radical and radical ions when the
-
formed by protonation of carbanions (R ) as indicated by the
in-cage ET is endothermic (-∆G > 0). In the case of DCA,
incorporation of deuterium from D2O. The products of irradia-
tion under oxygen and the LFP study showed that the formation
•
-•
the predicted rate for the reduction of Ph2CH by DCA is only
7
-1 -1
∼
10 M
s
according to the Rehm-Weller equation. A
question here is why the slow ET reaction becomes the major
one despite quite fast coupling of R . This could well be
-
•
of either carbanion (R ) or radical (R ) depends on the energy
•
gap of the electron transfer from sensitizer anion radical to R .
•
When the electron transfer is exothermic (-∆G > 0), it takes
explained by the persistent radical ion effect of the relatively
place smoothly within the geminate pair of radical/sensitizer
-•
stable radical ion DCA . In fact, the spectra of long-living
•
-•
radical anion (R /Sens ) and is not affected by the presence of
-•
DCA was observed by laser flash spectroscopy (see Figure
molecular oxygen. In contrast, when -∆G < 0, the geminate
1
c) and also after 5 min irradiation of DCA and 1a, as shown
•
-•
•
-•
pair of R /Sens dissociates into free R and Sens , which are
trapped effectively by oxygen to afford mainly oxygenated
products. Thus, it is apparent that the reduction potentials of
sensitizers have a crucial role in determining the in-cage
production of carbanions. In other words, the product selectivity
depends on the free energy change (-∆G) for the one-electron
in the Supporting Information. Although the ET reduction of
•
-•
•
R by DCA is not as fast as the coupling of R , the steady-
-•
state concentration of DCA is high enough to suppress the
(33) (a) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S. J. Am. Chem. Soc.
1
987, 109, 3794-3796. (b) Gould, I. R.; Moser, J. E.; Ege, D.; Farid, S. J.
Am. Chem. Soc. 1988, 110, 1991-1993. (c) Vauthey, E.; Suppan, P.;
Haselbach, E. HelV. Chim. Acta 1988, 71, 93-99. (d) Kikuchi, K.;
Takahashi, Y.; Koike, K.; Wakamatsu, K.; Ikeda, H.; Miyashi, T. Z. Phys.
Chem. (Munich) 1990, 167, 27-39. (e) Grampp, G.; Hetz, G. Ber. Busenges.
Phys. Chem. 1992, 96, 198-200.
•
reduction of R by sensitizer radical anions.
The rate constant of bimolecular electron transfer (ket) can
be estimated from the Rehm-Weller relationship19 according
to
(34) (a) Asahi, T.; Mataga, N. J. Phys. Chem. 1989, 93, 6575-6578.
b) Asahi, T.; Mataga, N. J. Phys. Chem. 1991, 95, 1956-1963.
(
(
35) (a) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978. (b) Marcus,
kdiff
R. A. Annu. ReV. Phys. Chem. 1964, 15, 155-196. (c) Eberson, L. In
AdVances in Physical Organic Chemistry; Gold, V., Bethell, D., Eds;
Academic Press: London, 1982; Vol. 18, pp 79-185.
ket )
(4)
q
1
+ 0.25[exp(∆G /RT) + exp(∆G/RT)]
(
36) (a) Marcus, R. A.; Siders, P. J. Phys. Chem. 1982, 86, 622-630.
q
2
2
∆
G )
x
(∆G/2) + (∆G(0)) + ∆G/2
(5)
(b) Kakitani, T.; Yoshimori, A.; Mataga, N. J. Phys. Chem. 1992, 96, 5385-
5
392. (c) Tachiya, M.; Murata, S. J. Phys. Chem. 1992, 96, 8441-8444.
q
(37) Pincock, J. A. Acc. Chem. Res., 1997, 30, 43-49.
Here, kdiff is the diffusion-controlled rate constant, and ∆G
and ∆G (0) are the activation free energy and that at ∆G ) 0,
respectively. If the electron transfer between R and aromatic
(38) (a) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S.; Goodman, J.
q
L.; Herman, M. S. J. Am. Chem. Soc. 1989, 111, 1917-1919. (b) Todd,
W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R. J. Am.
Chem. Soc. 1991, 113, 3601-3602.
•
radical ions follows the Rehm-Weller relationship, the bimo-
(39) (a) Murphy, S.; Zou, J. B.; Miers, J. B.; Ballew, R. M.; Dlott, D.
(
32) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc.
D.; Schuster, G. B. J. Phys. Chem. 1993, 97, 13152-13157. (b) Murphy,
S.; Schuster, G. B. J. Phys. Chem. 1995, 99, 511-515.
1
990, 112, 4290-4301.

12458 J. Am. Chem. Soc., Vol. 120, No. 48, 1998
Yokoi et al.
dimerization. This is an interesting example of Ingold-Fischer
50 mmol, excess) were dissolved in water and stirred with a magnetic
stirrer. The solution was extracted three times with CH Cl (200 mL).
2 2
After being dried over anhydrous sodium sulfate, the solvent was
removed by rotary evaporation to yield a viscous oil that solidified
upon standing, yielding white needle crystals. 1H NMR (200 MHz) in
persistent radical effect.40 The DCA has been detected
-•
spectroscopically in some PET reactions.4
1,42
It should be
stressed here that the persistent radical ion effect may play an
important role in controlling reaction selectivities.
CDCl
m, 8 H), 5.06 (s, 1 H), 7.28 (m, 10 H).
Continuous and Pulse Irradiations. Since all sensitizers should
3
: δ 0.92 (t, J ) 7 Hz, 12 H), 1.31 (m, 8 H), 1.52 (m, 8 H), 3.16
(
Conclusion
We have shown that the generation of carbanions from
carboxylate ions is achieved by photoinduced electron transfer.
This reaction is potentially useful not only in the conversion of
arylacetic acids (RCO2H) to RH but as a method to generate
carbanions photochemically. The key process in the formation
of either carbanion is the electron transfer from sensitizer anion
radical to radical intermediates proceeding in-cage and depend-
ing on the free energy changes. When the electron transfer is
exothermic (-∆G > 0), it takes place smoothly within the
be excited at longer wavelength than carboxylates, the excitation of
carboxylate salts or photoproducts was avoided in sensitized photolyses
by the use of appropriate filters. Continuous irradiations were done
with a 300-W medium-pressure mercury lamp through a Pyrex filter
(
i.e., >300 nm) or 10% KNO filter solution (i.e., >350 nm). Typically,
3
a 10-mL solution of carboxylate (∼5 mM) including sensitizer (∼3
mM) in a test tube with a septum rubber cap was purged with argon
for 10 min and was irradiated at ambient temperature.
Pulse irradiation was done with a SL248G Nd:YAG laser and carried
3
out by flowing the solution through a 3 × 1 × 0.1 cm quartz cell in
•
-•
geminate radical ion pair (R /Sens ) and is not affected by
order to ensure that each laser pulse irradiated a fresh volume of
solution.
external radical scavengers such as oxygen. In contrast, when
•
-
∆G < 0, the ET proceeds in bulk between free radical (R )
After irradiation, an internal standard was added to the solutions.
Then it was washed with 1 N HCl and treated with diazomethane for
the methylation of carboxylates. The products were identified by GC/
MS, and the conversion of the carboxylates (methyl esters) and the
yield of products were determined at appropriate intervals of time by
GLC.
-
•
and sensitizer radical anion (Sens ). The former in-cage
formation of carbanions is clean and may provide a useful
method to prepare “naked” benzyl-type carbanions.
Experimental Section
Time-Resolved Absorption Spectroscopy. Laser kinetic spectros-
copy experiments were carried out using a nanosecond laser system
similar to that previously described.46 The system consisted of a 150-W
Xe flash lamp (XF-80, Tokyo Instruments), a SPEX 270M monochro-
mator, and a Hamamatsu R-1221HA photomultiplier tube. The CCD
detector (ICCD-1024-ML∆G-E, Princeton Instruments) was controlled
by a detector controller (ST-135, Princeton Instruments) and a pulse
generator (PG-200, Princeton Instruments). The system was controlled
by a PC-9801 computer which was interfaced (GPIB) to the detector
controller. The delay time of this system was controlled by two digital
delay/pulse generators (DG-535, Stanford Research system). The
excitation source was a Physik MINex XeCl excimer laser (308 nm,
1H NMR spectra were recorded with a Varian Gemini-200 (200
MHz) NMR spectrometer. GC/MS analyses were carried out with a
Shimadzu QP-5000 mass spectrometer using a 0.2-mm × 25-m capillary
column of CBP1-M50-025 (Shimadzu). GLC analyses were performed
with a Shimadzu GC-14A gas chromatograph, using a 2.5-mm × 1-m
column of Carbowax 300M, 2% on Chromosorb WAW (GL Sciences).
Materials. Benzene and THF (Tokyo Kasei) were distilled over
sodium. Crown ethers were received from Tokyo Kasei and used
without further purification. 1,2,4,5-Tetracyanobenzene (TCB), 1,4-
dicyanobenzene (DCB), naphthalene, anthracene, and 9-cyanoan-
thracene (CA) were received from Tokyo Kasei and recrystallized from
ethanol. 1-Cyanonaphthalene (Tokyo Kasei) was distilled under
∼
10 ns, ∼15 mJ/pulse) or the third harmonic (355 nm, ∼6 ns, ∼25
43
reduced pressure. 1,4-Dicyano-2,5-dimethylbenzene (DMDCB), 1,4-
dicyano-2,3,5,6-tetramethylbenzene (TMDCB), 1,4-dicyanonaphthalene
mJ/pulse) from a Spectron SL248G Nd:YAG laser. The argon-saturated
2
sample solutions were irradiated in a 4 × 1 cm cell made of quartz.
4
4
45
(
DCN), and 9,10-dicyanoanthracene (DCA) were prepared by the
reported procedures. Metal salts of carboxylates were prepared from
the corresponding carboxylic acid by adding equimolar base such as
LiOH, NaOH, or KOH in MeOH and precipitated by the addition of
Acknowledgment. We thank Mrs. T. Watanabe and T.
Imura for technical assistance (glass blowing). This work was
supported in part by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports and Culture of Japan.
1
ether. The precipitate was filtered and washed with ether. H NMR
analysis revealed that the purity of salts of carboxylates was ∼100%.
Tetra-n-butylammonium Diphenylacetate (1a). Tetra-n-butylam-
monium bromide (13 g, 40 mmol) and sodium diphenylacetate (12 g,
Supporting Information Available: Absorption spectrum
of benzene solution of 0.15 mM DCA and 3 mM 1a under argon
(
40) Fischer, H. J. Am. Chem. Soc. 1986, 108, 3925-3927.
-
•
-3
(41) (a) Schaap, A. P.; Zaklika, K. A.; Kaskar, B.; Fung, L. W.-M. J.
(Figure 2); DCA of 2 × 10 mM was detected by the
characteristic absorption at 450-750 nm (1 page, print/PDF).
See any current masthead page for ordering information and
Web access instructions.
Am. Chem. Soc. 1980, 102, 389-391. (b) Spada, L. T.; Foote, C. S. J. Am.
Chem. Soc. 1980, 102, 391-393.
(42) (a) Kellett, M. A.; Whitten, D. G.; Gould, I. R.; Bergmark, W. R.
J. Am. Chem. Soc. 1991, 113, 358-359. (b) Freccero, M.; Mella, M. Albini,
A. Tetrahadron 1994, 50, 2115-2130.
JA981470K
(43) Suzuki, H.; Hanafusa, T. Syntheses 1974, 6, 432.
(
44) Heiss, L.; Paulus, E. F.; Rehling, H. Liebigs Ann. Chem. 1980,
1
583-1596.
(
(46) Nojima, T.; Ishiguro, K.; Sawaki, Y. J. Org. Chem. 1997, 62, 6911-
6917.
45) Dufraisse, C. Bull. Soc. Chim. Fr. 1947, 701-705.