Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via p

Article abstract of DOI:10.1021/ja010125j

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,π* triplet to the π,π* singlet which becomes lower in energy than the n,π* triplet by the complexation with metal ions

Full text of DOI:10.1021/ja010125j

7756
J. Am. Chem. Soc. 2001, 123, 7756-7766
Change in Spin State and Enhancement of Redox Reactivity of
Photoexcited States of Aromatic Carbonyl Compounds by
Complexation with Metal Ion Salts Acting as Lewis Acids. Lewis
Acid-Catalyzed Photoaddition of Benzyltrimethylsilane and
Tetramethyltin via Photoinduced Electron Transfer
,
†
†
†
†
Shunichi Fukuzumi,* Naoya Satoh, Toshihiko Okamoto, Kiyomi Yasui,
†
†
‡
‡
Tomoyoshi Suenobu, Yasuyo Seko, Mamoru Fujitsuka, and Osamu Ito
Contribution from the Department of Material and Life Science, Graduate School of Engineering, Osaka
UniVersity, CREST, Japan Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan, and
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, CREST, Japan Science
and Technology Corporation (JST), Sendai, Miyagi 980-8577, Japan
ReceiVed January 16, 2001. ReVised Manuscript ReceiVed April 24, 2001
Abstract: The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and
/
/
1
0-methylacridone) is changed from the n,π triplet to the π,π singlet which becomes lower in energy than
/
the n,π triplet by the complexation with metal ions such as Mg(ClO4)2 and Sc(OTf)3 (OTf ) triflate), which
act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited
states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)3 complex) as
compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant
increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the
photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes
leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds
via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence
quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments
indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from
benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.
Introduction
electrophiles has recently received considerable interest from
5-10
both synthetic and mechanistic viewpoints.
However, elec-
The addition of organosilane and organostannane reagents
with various carbonyl compounds has been one of the most
useful procedures for carbon-carbon bond formation. The
process requires the presence of strong Lewis acids such as TiCl4
which activate the electrophilic function of carbonyl compounds
trophiles employed for the photoaddition by organometallic
1
,2
(5) (a) Mariano, P. S. In Photoinduced Electron Transfer; Fox, M. A.,
Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 372. (b) Mariano,
P. S. Acc. Chem. Res. 1983, 16, 130. (c) Mattes, S. L.; Farid, S. Acc. Chem.
Res. 1982, 15, 80. (d) Mariano, P. S. Synthetic Organic Chemistry; Horspool,
W. M., Ed.; Plenum Press: London, 1984. (e) Fukuzumi, S.; Tanaka, T. In
Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier:
Amsterdam, The Netherlands, 1988; Part C, p 578.
toward nucleophilic attack by the organometallic reagent
controlling the acyclic stereoselection.1-4 On the other hand,
the photochemical carbon-carbon bond formation via photo-
induced electron transfer from organometallic reagents to various
(6) (a) Ohga, K.; Mariano, P. S. J. Am. Chem. Soc. 1982, 104, 617. (b)
Ohga, K.; Yoon, U. C.; Mariano, P. S. J. Org. Chem. 1984, 49, 213. (c)
Mizuno, K.; Ikeda, M.; Otsuji, Y. Tetrahedron Lett. 1985, 26, 461. (d)
Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Chem. Soc., Chem. Commun. 1986,
1553. (e) Kubo, Y.; Imaoka, T.; Shiragami, T.; Araki, T. Chem. Lett. 1986,
1749. (f) Cho, I.-S.; Tu, C.-L.; Mariano, P. S. J. Am. Chem. Soc. 1990,
112, 3594. (g) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.;
Gould, I. R. Tetrahedron Lett. 1993, 34, 2863. (h) Su, Z.; Mariano, P. S.;
Falvey, D. E.; Yoon, U. C.; Oh, S. W. J. Am. Chem. Soc. 1998, 120, 10676.
(i) Takahashi, Y.; Miyashi, T.; Yoon, U. C.; Oh, S. W.; Mancheno, M.;
Su, Z.; Falvey, D. F.; Mariano, P. S. J. Am. Chem. Soc. 1999, 121, 3926.
(7) (a) Takuwa, A.; Tagawa, H.; Iwamoto, H.; Soga, O.; Maruyama, K.
Chem. Lett. 1987, 1091. (b) Takuwa, A.; Nishigaichi, Y.; Iwamoto, H. Chem.
Lett. 1991, 1013.
*
To whom correspondence should be addressed: (e-mail) fukuzumi@
ap.chem.eng.osaka-u.ac.jp.
†
Osaka University.
Tohoku University.
‡
(
1) (a) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London,
1
981. (b) Mukaiyama, T. Angew. Chem., Int. Ed. Engl. 1977, 16, 817. (c)
Hosomi, A. Acc. Chem. Res. 1988, 21, 200. (d) Hoffmann, R. W. Angew.
Chem., Int. Ed. Engl. 1987, 26, 489.
(
2) (a) Pereyre, M.; Quitard, J.-P.; Rahm, A. Tin in Organic Synthesis;
Butterworth: London, 1987. (b) Yamamoto, Y. Acc. Chem. Res. 1987, 20,
43.
2
(
3) (a) Schinzer, D. SelectiVity in Lewis Acid Promoted Reactions;
(8) (a) Sakurai, H.; Sakamoto, K.; Kira, M. Chem. Lett. 1984, 1213. (b)
Nakadaira, Y.; Komatsu, N.; Sakurai, H. Chem. Lett. 1985, 1781. (c)
Nakadaira, Y.; Sekiguchi, A.; Funada, Y.; Sakurai, H. Chem. Lett. 1991,
327. (d) Mizuno, K.; Kobata, T.; Maeda, R.; Otsuji, Y. Chem. Lett. 1990,
1821. (e) Fukuzumi, S.; Kitano, T.; Mochida, K. Chem. Lett. 1989, 2177.
(f) Fukuzumi, S.; Kitano, T.; Mochida, K. Chem. Lett. 1990, 1774. (g)
Fukuzumi, S.; Kitano, T.; Mochida, K. J. Chem. Soc., Chem. Commun.
1990, 1236.
Kluwer: Dordrecht, The Netherlands, 1989. (b) Shambayati, S.; Crowe,
W. E.; Schreiber, S. L. Angew. Chem., Int. Ed. Engl. 1990, 29, 256.
(4) (a) Denmark, S. E.; Wilson, T. M.; Willson, T. M. J. Am. Chem.
Soc. 1988, 110, 984. (b) Denmark, S. E.; Weber, E. J.; Wilson, T. M.;
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1
0.1021/ja010125j CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/19/2001

Photoaddition of Benzyltrimethylsilane and Tetramethyltin
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7757
reagents have so far been limited to luminescent electron
acceptors such as iminium cations and cyanoaromatics, which
acetonaphthones are generally nonfluorescent but phosphores-
cent because of the fast intersystem crossing to generate the
0
/
26
have more positive one-electron reduction potentials (E red*) at
lowest n,π triplet excited state. However, it is found that the
0
/
the excited states than the one-electron oxidation potentials (E ox)
lowest excited state is completely changed from the n,π triplet
of organometallic reagents.5
-10
to the π,π singlet which becomes lower in energy than the
/
/
Since the lifetimes of excited states are usually very short
and accordingly any reaction of the excited state should be fast
enough to compete the decay of the excited state to the ground
state, there seems to be little chance for a catalyst to accelerate
further the reactions of excited states, which are already fast.
There are many cases, however, such that photochemical
reactions can be accelerated by some added substances which
n,π triplet due to the complexation with metal ions such as
Mg(ClO4)2 and Sc(OTf)3 (OTf ) triflate), which act as Lewis
27
acids. Efficient photoaddition of benzyltrimethylsilane (PhCH2-
SiMe3) with aromatic carbonyl compounds is made possible by
the complexation of the photoexcited states with metal ions.24
The catalytic mechanism for the photoaddition reaction is
revealed on the basis of the studies done on the complex
formation between carbonyl compounds and Lewis acids, the
quantum yield determinations, the fluorescence quenching by
electron donors, and direct detection of the reaction intermedi-
ates by means of laser flash photolysis experiments.
1
1-14
act as catalysts in the photochemical reactions.
The
photoexcitation induces significant enhancement in the reactivity
of electron transfer, and thereby photochemical reactions via
15-20
photoinduced electron transfer have been reported extensively.
There have been some examples for photoinduced electron-
2
1,22
Experimental Section
transfer reactions which are catalyzed significantly,
since
remarkable enhancement of the redox reactivity of photoexcited
Materials. 1-Naphthaldehyde (1-NA), 2-naphthaldehyde (2-NA),
1-acetonaphthone (1-AN), 2-acetonaphthone (2-AN), 10-methylacri-
done, and alkylbenzene derivatives were obtained commercially and
purified by the standard method.28 Benzyltrimethylsilane, trimethylsilyl
trifluoromethanesulfonate, and tetramethyltin were also obtained com-
mercially and used as received. A dimeric 1-benzyl-1,4-dihydronico-
states of flavin analogues due to the complex formation with
Mg(ClO4)2 was first reported.23 There remains a wealth of
important fundamental questions with regard to catalysis in
photoinduced electron transfer reactions, which has been only
partially explored in the past, and which certainly deserves much
more detailed attention.2
29
tinamide [(BNA)
triflate [Sc(OTf)
2
] was prepared according to the literature. Scandium
] was prepared by the following procedure according
1,22
3
30
We report herein the first systematic studies on the change
of the spin state as well as enhancement of redox reactivity of
photoexcited states of aromatic carbonyl compounds due to the
complexation with metal ions acting as Lewis acids.24 Enhance-
ment of fluorescence intensity has previously been reported for
to the literature. A deionized aqueous solution was mixed (1:1 v/v)
with trifluoromethanesulfonic acid (>99.5%, 10.6 mL) obtained from
the Central Glass, Co., Ltd., Japan. The trifluoromethanesulfonic acid
solution was slowly added to a flask which contained scandium oxide
2 3
(Sc O ) (>99.9%, 30 mmol) obtained from Shin Etsu Chemical, Co.,
2
5
Ltd., Japan. The mixture was refluxed at 100 °C for 3 days. After cen-
trifugation of the reaction mixture, the solution containing scandium
triflate was separated and water was removed by vacuum evaporation
for 40 h. Similarly, lutetium triflate and ytterbium triflate were prepared
by the reaction of lutetium oxide and ytterbium oxide with an aqueous
solution of trifluoromethanesulfonic acid. Lanthanum triflate was ob-
tained from Aldrich as the hexahydrate form and used after drying under
2
-quinolones complexed with BF3 acting as a Lewis acid.
Aromatic carbonyl compounds such as naphthaldehydes and
(
9) (a) Fukuzumi, S.; Fujita, M.; Otera, J.; Fujita, Y. J. Am. Chem. Soc.
1
1
992, 114, 10271. (b) Fukuzumi, S.; Fujita, M.; Otera, J. J. Org. Chem.
993, 58, 5405. (c) Mikami, K.; Matsumoto, S.; Ishida, A.; Takamuku, S.;
Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 1995, 117, 11134. (d) Mikami,
K.; Matsumoto, S.; Okubo, Y.; Fujitsuka, M.; Ito, O.; Suenobu, T.;
Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 2236.
vacuum evacuation for 40 h. Magnesium triflate [Mg(OTf)
tained from Aldrich and used as received. Anhydrous magnesium per-
chlorate [Mg(ClO ] was obtained from Nacalai Tesque. Potassium
2
] was ob-
(10) For thermal electron-transfer reactions involving main-group orga-
nometallics, see: (a) Kaim, W. Acc. Chem. Res. 1985, 18, 160. (b) Kochi,
J. K. Angew. Chem., Int. Ed. Engl. 1988, 27, 1227.
4 2
)
ferrioxalate used as an actinometer was prepared according to the lit-
erature,31 and purified by recrystallization from hot water. Tetrabutyl-
ammonium hexafluorophosphate used as a supporting electrolyte for
the electrochemical measurements was obtained commercially and pur-
(11) (a) Wubbels, G. G. Acc. Chem. Res. 1983, 16, 285. (b) Wubbels,
G. G.; Celander, D. W. J. Am. Chem. Soc. 1981, 103, 7669.
(
12) Albini, A. J. Chem. Educ. 1986, 63, 383.
(13) (a) Photocatalysis, Fundamentals and Applications; Serpone, N.,
Pelizzetti, E., Eds.; Wiley: New York, 1989. (b) Homogeneous Photoca-
talysis; Chanon, M., Ed.; Wiley: Chichester, UK, 1997.
28
ified by the standard method. Acetonitrile, propionitrile and dichlor-
omethane as solvents were purified and dried by the standard pro-
(
14) (a) Lewis, F. D.; Howard, D. K.; Oxman, J. D. J. Am. Chem. Soc.
cedure.28 Acetonitrile-d
was obtained from EURI SO-TOP, CEA,
1
983, 105, 3344. (b) Lewis, F. D.; Oxman, J. D. J. Am. Chem. Soc. 1984,
3
1
06, 466. (c) Lewis, F. D.; Oxman, J. D.; Gibson, L. L.; Hampsch, H. L.;
France.
Quillen, S. L. J. Am. Chem. Soc. 1986, 108, 3005. (d) Lewis, F. D.; Howard,
D. K.; Oxman, J. D.; Upthagrove, A. L.; Quillen, S. L. J. Am. Chem. Soc.
Reaction Procedures. Photoaddition of benzyltrimethylsilane with
aromatic carbonyl compounds was performed in the presence of Mg-
1
986, 108, 5964. (e) Lewis, F. D.; Quillen, S. L.; Hale, P. D.; Oxman, J.
D. J. Am. Chem. Soc. 1988, 110, 1261. (f) Lewis, F. D.; Barancyk, S. V.
J. Am. Chem. Soc. 1989, 111, 8653. (g) Lewis, F. D.; Elbert, J. E.;
Upthagrove, A. L.; Hale, P. D. J. Org. Chem. 1991, 56, 553. (h) Lewis, F.
D.; Barancyk, S. V.; Burch, E. L. J. Am. Chem. Soc. 1992, 114, 3866.
(25) Lewis, F. D.; Reddy, G. D.; Elbert, J. E.; Tillberg, B. E.; Meltzer,
J. A.; Kojima, M. J. Org. Chem. 1991, 56, 5311.
(26) Turro, N. J. Molecular Photochemistry; W. A. Benjamin, Inc.: New
York, 1967.
(
15) Fox, M. A.; Chanon, M., Eds. Photoinduced Electron Transfer;
(27) Heavy metal cations in zeolites have been reported to accelerate
spin-state interconversion (singlet-triplet intersystem crossing) of excited
guest molecules. See: (a) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.;
Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (b) Uppili,
S.; Marti, V.; Nikolaus, A.; Jockusch, S.; Adam, W.; Engel, P. S.; Turro,
N. J.; Ramamurthy, V. J. Am. Chem. Soc. 2000, 122, 11025.
(28) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Oxford, England, 1988.
(29) Wallenfels, K.; Gellerich, M. Chem. Ber. 1959, 92, 1406.
(30) (a) Forsberg, J. H.; Spaziano, V. T.; Balasubramanian, T. M.; Liu,
G. K.; Kinsley, S. A.; Duckworth, C. A.; Poteruca, J. J.; Brown, P. S.;
Miller, J. L. J. Org. Chem. 1987, 52, 1017. (b) Kobayashi, S.; Hachiya, I.
J. Org. Chem. 1994, 59, 3590. (c) Kobayashi, S.; Hachiya, I.; Ishitani, H.;
Araki, M. Synlett 1993, 472.
Elsevier: Amsterdam, The Netherlands, 1988; Parts A-D.
(
(
(
(
(
(
16) Julliard, M.; Chanon, M. Chem. ReV. 1983, 83, 425.
17) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
18) M u¨ ller, F.; Mattay, J. Chem. ReV. 1993, 93, 99.
19) Rathore, R.; Kochi, J. K. AdV. Phys. Org. Chem. 2000, 35, 193.
20) Ho, T.-I.; Ho, J.-H.; Wu, J.-Y. J. Am. Chem. Soc. 2000, 122, 8575.
21) Fukuzumi, S.; Itoh, S. In AdVances in Photochemistry; Neckers, D.
C., Volman, D. H., von B u¨ nau, G., Eds.; John Wiley & Sons: New York,
1
999; Vol. 25, p 107.
22) Fukuzumi, S. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, Germany, 2001; Vol. 4, pp 3-67.
23) (a) Fukuzumi. S.; Kuroda, S.; Tanaka, T. J. Am. Chem. Soc. 1985,
(
(
1
07, 3020. (b) Fukuzumi. S.; Kuroda, S.; Tanaka, T. Chem. Lett. 1984,
4
17.
(31) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A
1956, 235, 518. (b) Calvert, J. G.; Pitts, J. N. Photochemistry; Wiley: New
York, 1966; p 783.
(24) A preliminary report has appeared: Fukuzumi, S.; Okamoto, T.;
Otera, J. J. Am. Chem. Soc. 1994, 116, 5503.

7758 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Fukuzumi et al.
-
1
(
ClO
4
)
2
in MeCN. Typically, benzyltrimethylsilane (1.0 × 10 M) was
emission band at λmax ) 474 nm. The MeCN solutions were deaerated
by argon purging for 7 min prior to the measurements. Relative
fluorescence intensities were measured for MeCN solutions containing
added by means of a microsyringe to a CD
containing 2-NA (5.0 × 10 M) and Mg(ClO
3
CN solution (0.7 mL)
-
2
-1
4
)
2
(2.0 × 10 M) in a
-
4
square quartz cuvette (1 mm i.d.) and the solution was deaerated by
bubbling with argon gas for 5 min. The solution was irradiated with
monochromatized light from a xenon lamp of a Shimadzu RF-5000
spectrofluorophotometer for 6.5 h. The experimental details for other
photochemical reactions are given in the Supporting Information (S1).
an aromatic carbonyl compound (2.0 × 10 M) with a quencher at
various concentrations in the presence of 1.0 M Mg(ClO . Relative
fluorescence intensities were measured for MeCN solutions containing
4 2
)
-
4
-2
1-NA (3.0 × 10 M) with various alkylbenzenes (1.0 × 10 to 9.6
-
1
-2
× 10 M) in the presence of metal ions (Sc(OTf)
3
, 1.0 × 10 M;
1
-1
The products were identified by comparison of the H NMR spectra
Mg(ClO )
4 2
, 1.0 × 10 M). Relative fluorescence intensities were also
2
4,32
with those of the authentic samples.
No homo-coupling products
measured for an MeCN solution containing AcrCO-Sc(OTf)
3
-1
(5.0 ×
1
-5
-3
such as dibenzyl have been detected. The isolated yield was 70%. H
NMR measurements were performed with a Japan Electron Optics
10 M) with various electron donors (2.0 × 10 to 7.0 × 10 M) in
-3 -2
3 2 2
the presence of Sc(OTf)
8.0 × 10 to 8.0 × 10 M) or a CH
Cl
1
-4
JNM-GSX-400 (400 MHz) NMR spectrometer at 298 K. H NMR
solution containing AcrCO-Me
3
SiOTf (1.0 × 10 M) with various
-1
3
-
3
(
CD
3
CN, 298 K) δ (Me
.14 (s, 9H), 3.13 (dd, 1H, J ) 13.9, 6.1 Hz), 3.15 (dd, 1H, J ) 13.9,
.6 Hz) 5.05-5.11 (m, 1H), 7.14-7.22 (m, 5H), 7.46-7.58 (m, 3H),
.79-7.92 (m, 4H); (C10 [1-CH(CH Ph)(OSiMe )], 65%) 0.14 (s, 9H),
.11 (dd, 1H, J ) 13.9, 8.4 Hz), 3.24 (dd, 1H, J ) 13.9, 4.6 Hz),
.67-5.72 (m, 1H), 7.26-7.33 (m, 5H), 7.50-8.26 (m, 7H); (C10 [2-
Ph)(OSiMe )], 72%) 0.14 (s, 9H), 1.62 (s, 3H), 3.15, 3.20
ABq, 2H, J ) 13.2 Hz), 7.14-7.19 (m, 5H), 7.46-7.52 (m, 3H), 7.82-
4
Si, ppm): (C10
7
H [2-CH(CH
2
Ph)(OSiMe
3
)])
electron donors (1.9 × 10 to 7.7 × 10 M) in the presence of Me
-
-
3
0
7
7
3
5
SiOTf (2.0 × 10 M). There was no change in the shape but there
was a change in the intensity of the fluorescence spectrum by the
addition of a quencher. The Stern-Volmer relationship (eq 1) was
H
7
2
3
obtained for the ratio of the fluorescence intensities (I
0
/I) in the absence
H
7
CMe(CH
(
7
1
(
4
(
2
3
I /I ) 1 + K [D]
(1)
0
q
.90 (m, 4H); (C10
.76 (s, 3H), 3.46 (s, 2H), 7.34-7.44 (m, 5H), 7.54-8.12 (m, 7H);
[1-CH(Me)(OSnMe )], 100%) 0.21 (s, 9H), 1.09 (d, 3H, J )
7 2 3
H [1-CMe(CH Ph)(OSiMe )], 32%) 0.17 (s, 9H),
and presence of electron donors and the concentrations of donors used
as quenchers [D]. The fluorescence lifetimes (τ) of the metal ion
complexes of aromatic carbonyl compounds were determined in
10
C H
7
3
.6 Hz), 4.97 (m, 1H), 7.16-7.27 (m, 4H), 7.56-7.77 (m, 3H); (Acr-
2 2
deaerated MeCN and CH Cl at 298 K by single photon counting using
+
CH
2
Ph) , 100%) 4.79 (s, 3H), 5.35 (s, 2H), 8.9-7.5 (m, 13H).
a Horiba NAES-1100 time-resolved spectrofluorophotometer. The
-1
Spectral Measurements. The formation of metal ion complexes with
naphthaldehydes and acetonaphthones was examined from the change
in the UV-vis spectra in the presence of various concentrations of
metal ions (M ) by using a Hewlett-Packard 8452A diode array
spectrophotometer. The formation constants were determined from
are the absorbance
at λmax in the presence of the metal ion and the absorbance at the same
observed quenching rate constants k ()K τ ) were obtained from the
q
q
q
K and τ values.
Electrochemical Measurements. Electrochemical measurements
n+
were performed on a BAS 100B electrochemical analyzer in deaerated
+
-
4 6 6
MeCN containing 0.10 M n-Bu N PF (TBAPF ) as a supporting
-1
n+ -1
linear plots of (A - A
0
)
vs [M
]
, where A and A
0
electrolyte at 298 K. The platinum working electrode was polished with
BAS polishing alumina suspension and rinsed with acetone before use.
The counter electrode was a platinum wire. The measured potentials
wavelength in the absence of the metal ion, respectively.
Quantum Yield Determinations. A standard actinometer (potassium
3
were recorded with respect to an Ag/AgNO (0.01 M) reference
3
1
ferrioxalate) was used for the quantum yield determinations. In the
case of photoaddition of PhCH SiMe with aromatic carbonyl com-
electrode. The second-harmonic alternating current voltammetry
(SHACV)33 measurements of alkylbenzenes, 2-NA, 1-NA, AcrCO, and
2
3
pounds, an MeCN solution (3.0 mL) containing an aromatic carbonyl
the AcrCO-Sc(OTf)
3
complex were carried out with a BAS 100B
electrochemical analyzer in deaerated MeCN containing 0.10 M
-
4
-3
-3
compound (5.1 × 10 - 1.0 × 10 M), PhCH
2
SiMe
3
(7.1 × 10 to
-
1
as a supporting electrolyte at 298 K. The E0red values (vs Ag/
) are converted into those vs SCE by addition of 0.29 V.
1
.7 × 10 M), and Mg(ClO
4
)
2
(1.0 M) in a square quartz cuvette (10
TBAPF
AgNO
6
3
4
mm i.d.) was deaerated thoroughly with a stream of argon for 7 min,
and it was irradiated with monochromatized light of λ ) 340 nm from
a Shimadzu RF-5000 fluorescence spectrophotometer with the slit width
of 20 nm. Under the conditions of actinometry experiments, both the
actinometer and aromatic carbonyl compounds absorbed essentially all
the incident light of λ ) 340 nm. The light intensity of monochroma-
3
Laser Flash Photolysis. Triplet-triplet transient absorption spectra
of 1-NA and 1-NA-Mg(ClO complexes were measured by laser
flash photolysis of the MeCN solution containing 1-NA (1.0 × 10
M) in the absence and presence of Mg(ClO (1.0 M), respectively.
To observe transient absorption spectra in the photochemical reaction
4 2
)
-
3
4 2
)
-
6
tized light of λ ) 340 nm was determined as 7.98 × 10 einstein
of the 1-NA-Mg(ClO
4
)
2
complex with PhCH
2
SiMe
3
, a deaerated
SiMe
(1.7 ×
-
3
-1
-4
dm with the slit width of 20 nm. The photochemical reaction
s
MeCN solution containing 1-NA (2.8 × 10 M), PhCH
2
3
-
1
was monitored using a Hewlett-Packard 8452A diode-array spectro-
photometer. The quantum yields for the photoaddition of benzyltrim-
ethylsilane to 2-NA, 1-NA, 2-AN, and 1-AN in the presence of
4 2
10 M), and Mg(ClO ) (1.0 M) was excited by a Nd:YAG laser
(Continuum, Surelite II-10) at 355 nm with the power of 15 mJ at 298
K. For the detection of transient absorption spectra in the photochemical
Mg(ClO
4
)
2
(1.0 M) in deaerated MeCN were determined from the
reaction of the AcrCO-Sc(OTf)
3
2 3
complex with PhCH SiMe , a
3
-1
-1
-4
decrease in absorbance at λ ) 346 nm (ꢀ ) 2.3 × 10 M cm ), λ
deaerated MeCN solution containing AcrCO (1.0 × 10 M), PhCH
2
-
3
-1
-1
3
-1
-2
)
M
350 nm (ꢀ ) 3.7 × 10 M cm ), λ ) 342 nm (ꢀ ) 1.9 × 10
SiMe
3
(2.5 × 10 M), and Sc(OTf) (8.0 × 10 M) was excited by
3
-
1
-1
3
-1
-1
cm ), and λ ) 350 nm (ꢀ ) 1.1 × 10 M cm ), respectively.
an optical parametric oscillation (Continuum Surelite OPO, fwhm 4
ns, 440 nm) pumped by a Nd:YAG laser (Continuum, Surelite II-10)
with the power of 10 mJ. For the photoinduced electron transfer from
The detailed procedures for other photochemical reactions are given
in the Supporting Information (S1-S2).
(
BNA)
complex, (BNA)
SiMe
spectra were recorded using fresh solutions in each laser excitation.
2
to the 1-NA-Mg(ClO
4
)
2
complex and the AcrCO-Sc(OTf)
3
Fluorescence Quenching. Quenching experiments of the fluores-
cence of metal ion complexes of aromatic carbonyl compounds were
carried out by using a Shimadzu spectrofluorophotometer (RF-5000).
The excitation wavelengths for 2-NA, 1-NA, 2-AN, and 1-AN were
-
4
2
(2.0 × 10 M) was employed in place of PhCH
2
-
3
under otherwise the same experimental conditions. The transient
3
75, 375, 370, and 350 nm in the presence of Mg(ClO
4 2
) (1.0 M) in
(
33) The SHACV method provides a superior approach to directly
deaerated MeCN, respectively. The excitation wavelength for 1-NA in
the presence of Sc(OTf)
MeCN. The monitoring wavelengths were those corresponding to the
maxima of the emission bands. The excitation wavelength of AcrCO-
Sc(OTf)
The monitoring wavelength was corresponding to the maximum of the
evaluating the one-electron redox potentials in the presence of a follow-up
chemical reaction, relative to the better-known dc and fundamental harmonic
ac methods. See: (a) McCord, T. G.; Smith, D. E. Anal. Chem. 1969, 41,
-
2
3
(1.0 × 10 M) was 380 nm in deaerated
1
423. (b) Bond, A. M.; Smith, D. E. Anal. Chem. 1974, 46, 1946. (c)
Wasielewski, M. R.; Breslow, R. J. Am. Chem. Soc. 1976, 98, 4222. (d)
Arnett, E. M.; Amarnath, K.; Harvey, N. G.; Cheng, J.-P. J. Am. Chem.
Soc. 1990, 112, 344. (e) Patz, M.; Mayr, H.; Maruta, J.; Fukuzumi, S. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1225.
3
3 2 2
and AcrCO-Me SiOTf was 413 nm in MeCN and CH Cl .
(
32) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J.
(34) Mann, C. K.; Barnes, K. K. In Electrochemical Reaction in
Nonaqueous Systems; Marcel Dekker: New York, 1970.
Am. Chem. Soc. 1993, 115, 8960.

Photoaddition of Benzyltrimethylsilane and Tetramethyltin
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7759
Figure 1. Spectral change observed upon addition of Mg(ClO
4
)
2
to
an MeCN solution of 2-naphthaldehyde (6.5 × 10 M) in a 1 mm
quartz cell ([Mg(ClO ] ) 0, 0.20, 0.40, 0.60, 0.80, and 1.0 M) and
the fluorescence spectrum of the Mg(ClO complex (broken line) in
the presence of Mg(ClO (1.0 M) in MeCN at 298 K.
-
3
Figure 2. T-T absorption spectra by laser-flash photolysis of
4 2
)
-
3
1
-naphthaldehyde (1.0 × 10 M) in the absence (O) and presence (b)
-7
4 2
4 2
)
of Mg(ClO
)
(1.0 M) in deaerated MeCN at 298 K at 2.0 × 10
s
4 2
)
after laser irradiation at λ ) 355 nm.
Table 1. Formation Constants (K), Fluorescence Maxima (λmax),
Fluorescence Lifetimes (τ), the One-Electron Reduction Potentials
0
/
(E
red ) of the Singlet Excited States of Mg(ClO
)
4 2
and Sc(OTf)
3
Complexes of Aromatic Carbonyl Compounds, and Intrinsic
q
0
Barriers of Electron Transfer (∆G )
a
E0redb,cvs
*
q
,b
aromatic carbonyl-
K,
M
λ
max
,
τ,
∆G
0
-
1
-1
metal ion salt complex
nm
0.17 437
2.8 487 10.0 2.11 (0.83)
0.27 440 10.3 1.87 (0.90)
432 3.3 1.90 (0.60)
0.51 430 11.8 1.77 (0.65)
ns
SCE,
V
kcal mol
1
1
2
1
2
-NA-Mg(ClO
-NA-Sc(OTf)
4 2
-NA-Mg(ClO )
4
)
2
6.7 1.97 (0.83)
2.3
2.3
2.3
2.7
2.3
3
-AN-Mg(ClO
-AN-Mg(ClO
4
)2
)
4
2
a
Determined from the spectral change of aromatic carbonyl com-
b
4 2 3
pounds in the presence of Mg(ClO ) and Sc(OTf) . Determined by
adaptation of the free energy relationship for photoinduced electron-
c
transfer reactions (see text). Values in parentheses are those for the
triplet excited states of uncomplexed compounds.
Figure 3. Plots of the intensity ratio of fluorescence (O) at λmax
)
3
All experiments were performed at 298 K. The detailed procedures for
the measurements are available in the Supporting Information (S2-
S3).
487 nm at 298 K: phosphorescence (b) at λmax ) 548 nm vs [Sc(OTf) ]
-
3
7f o7 r K1 .- naphthaldehyde (1.5 × 10 M) in 2-methyltetrahydrofuran at
Phosphorescence Experiments. The phosphorescence spectra were
measured on a Hitachi 850 fluorescence phosphorescence spectropho-
tometer. Typically, a 2-methyltetrahydrofuran solution (1 mL) contain-
(Figure 1). When Mg(ClO ) is replaced by Sc(OTf) , the
4
2
3
fluorescence maximum (λmax) is red-shifted to 510 nm. Fluo-
rescence is generally observed for other Mg(ClO4)2-carbonyl
complexes. The florescence lifetimes (τ) of the Mg(ClO4)2-
carbonyl complexes in MeCN at 298 K were determined by
single-photon counting (see Experimental Section). The λmax
and τ values are also listed in Table 1.
-
3
ing 1-NA (1.5 × 10 M) in the presence of various concentrations of
-
3
-1
Sc(OTf)
3
(1.0 × 10 to 2.9 × 10 M) in the capillary cell was
degassed by bubbling with argon gas for 15 min. The solution was
irradiated with monochromatized light (λ ) 360 nm) from a xenon
lamp and phosphorescence spectra were measured at 77 K. The
phosphorescence spectra were measured from 400 to 600 nm.
The absence of the triplet excited state for the 1-NA-Mg-
ClO4)2 complex is confirmed as the disappearance of the
Results and Discussion
(
triplet-triplet absorption of 1-NA at 500 nm in the presence of
.0 M Mg(ClO4)2 in MeCN (Figure 2). The change in spin state
Change in Spin State of Photoexcited States by Complex-
ation with Metal Ions. Addition of Mg(ClO4)2 to an MeCN
solution of 2-naphthaldehyde (2-NA) results in a red shift of
ca. 20 nm in the absorption band as shown in Figure 1. The
appearance of a new absorption band at 360 nm is ascribed to
the 1:1 complex formation between Mg(ClO4)2 and the carbonyl
compound. Mg(ClO4)2 also forms complexes with other aro-
matic carbonyl compounds, 2-naphthaldehyde (2-NA), 1-acet-
onaphthone (1-AN), and 2-acetonaphthone (2-AN). The for-
mation constants (K) at 298 K are determined from the spectral
changes in the presence of various concentrations of Mg(ClO4)2
1
of the lowest excited state from the triplet to the singlet due to
the complexation with Sc(OTf)3 has also been demonstrated
clearly in Figure 3, where the phosphorescence intensity at 548
nm measured at 77 K decreases accompanied by the increased
fluorescence intensity at 487 nm measured at 298 K with
increasing Sc(OTf)3 concentration in 2-methyltetrahydrofuran.
The change in the lowest excited state may be caused by the
complexation of aromatic carbonyl compounds with Mg(ClO4)2.
The nonbonding orbitals are more stabilized by the complex
formation with Mg(ClO4)2 than π-orbitals due to the stronger
interaction between nonbonding electrons and Mg(ClO4)2. Thus,
(see Experimental Section), and the K values are listed in Table
1
.
/
Although aromatic carbonyl compounds are nonfluorescent,
irradiation of the new absorption band of 2-NA in the presence
of Mg(ClO4)2 in MeCN causes strong fluorescence at 440 nm
the π,π excited state becomes the lowest excited state in the
/
Mg(ClO4)2 complex as compared with the lowest n,π triplet
excited state in the uncomplexed carbonyl compound. The

7760 J. Am. Chem. Soc., Vol. 123, No. 32, 2001 Fukuzumi et al.
a
Table 2. Fluorescence Quenching Rate Constants (k ) of the Mg(ClO ) and Sc(OTf) Complexes of Aromatic Carbonyl Compounds by
q
4 2 3
0
Electron Donors in Deaerated MeCN at 298 K and One-Electron Oxidation Potentials (E ox) of the Donors
b
(M-1 s-1) of the Mg(ClO
k
q
4
)
2
complex of
E0ox vs SCE, V
c
1-NA
2-NA
1-AN
2-AN
alkylbenzene
toluene
2.20
d
(
d
(
d
d
d
d
d
d
d
d
d
d
8
2.0 × 10 )
ethylbenzene
2.14
2.02
9
1.2 × 10 )
8
m-xylene
6.1 × 10
9
(
3.2 × 10 )
9
o-xylene
p-cymene
p-xylene
1.98
1.96
1.93
(4.2 × 10 )
d
d
d
9
8
8
8
8
2.6 × 10
2.7 × 10
3.5 × 10
9
7
3.2 × 10
4.3 × 10
3.5 × 10
5.1 × 10
9
(
5.6 × 10 )
9
7
8
1
1
,2,3-trimethylbenzene
,2,4-trimethylbenzene
1.88
1.79
4.0 × 10
e
e
8.0 × 10
9.9 × 10
9
9
9
9
5.5 × 10
3.9 × 10
3.6 × 10
9
(
7.9 × 10 )
9
9
9
9
9
1
1
,2,3,4-tetramethylbenzene
,2,3,5-tetramethylbenzene
1.71
1.71
6.3 × 10
5.3 × 10
5.2 × 10
e
e3 .8 × 10
e
(
e
4.6 × 10
9
8.2 × 10 )
9
pentamethylbenzene
1.58
6.3 × 10
e
5.5 × 10
a
] ) 1.0 × 10 M. b The experimental errors are within (10%; [Mg(ClO
-2
Values in parentheses are those for the Sc(OTf)
3
complex; [Sc(OTf)
3
4 2
) ]
c
)
3
1.0 M. Measured by the second harmonic ac voltammetry using an Ag/AgNO (0.01 M) reference electrode and converted to the value vs SCE.
All values obtained in deaerated MeCN containing 0.10 M tetrabutylammonium perchlorate at 298 K using a Pt working electrode and a Pt wire
auxiliary electrode at a scan rate of 4 mV s , ∆E(ac amplitude) ) 25 mV, f(ac frequency) ) 25 Hz. d Too slow to determine accurately. Not
-
1
e
determined.
picosecond studies by Boldridge et al.35 clearly show that the
/
triplet π,π state of naphthaldehydes is populated through the
/
/
triplet n,π state formed from the first excited singlet n,π state
by the fast intersystem crossing. Since the singlet-triplet energy
/
/
gap is substantially larger in the π,π state than the n,π state,
the singlet π,π state being the lowest excited state in the Mg-
/
(ClO4)2 complex becomes strongly fluorescent.
Change in the Redox Potentials of Photoexcited States by
Complexation with Metal Ions. The free energy change of
photoinduced electron transfer from electron donors to the
0
singlet excited states (∆G et in eV) is given by eq 2, where e is
0
0
/
elementary charge, E ox and E red are the one-electron oxidation
potential of electron donors and the one-electron reduction
0
et
) e(E - E /)
0
ox
0
red
∆G
(2)
Figure 4. Plots of E0ox - (∆G et/e) vs e/∆G et for electron transfer
q
q
potentials of the excited states of electron acceptors, respectively.
The dependence of the activation free energy change of
from benzene derivatives to the singlet excited states of Mg(ClO )
4 2
complexes of 1-naphthaldehyde (O), 2-naphthaldehyde (b), 1-acet-
onaphthone (∆), and 2-acetonaphthone (2), and the singlet excited states
q
0
photoinduced electron transfer (∆G et) on ∆G et has well been
36
q
established as given by eq 3, where ∆G 0 is the intrinsic barrier
of Sc(OTf)
3
complexes of 1-naphthaldehyde (0); see eq 5 in text.
that represents the activation free energy change when the
2
(see Experimental Section). The kq value increases with a
G* ) (∆G /2) + [(∆G /2) + (∆G ) ]
0
et
0
et
2
* 2 1/2
(3)
∆
0
et
0
decrease in the E ox values of alkyl benzene donors to reach a
diffusion-limited value as expected from eqs 2-4 (Table 2).
The E red values (vs SCE) of the singlet excited states of the
q
q
driving force of electron transfer is zero, i.e., ∆G et ) ∆G 0 at
0
/
0
q
∆
G et ) 0. The ∆G et values are obtained from the fluorescence
2+
Mg -carbonyl complexes can be determined from the depen-
quenching rate constant (kq) by eq 4, where Z is the collision
0
dence of kq on E ox as follows. From eqs 2-4 is derived a linear
11
-1 -1
frequency that is taken as 1 × 10
M
s , kB is the Boltzmann
0
q
q
-1
relation between E ox - (∆G et/e) and (∆G et/e) as given by
3
6
constant, and kdiff is the diffusion rate constant in MeCN.
0
/
q
eq 5. Thus, the unknown values of E red and ∆G 0 can be
determined from the intercept and slope of the linear plots of
G* ) (2.3k T/e)log[Z(kq - kdiff-1)]
-1
(4)
∆
et
B
0
- (∆G* /e) ) E
0
*
red
- (∆G /e) /(∆G*et/e) (5)
*
2
E
ox
et
0
The quenching rate constants (kq) of the photoinduced electron
transfer from a series of alkylbenzene electron donors to the
Mg(ClO4)2 complexes of naphthaldehydes and acetonaphthones
were determined from the fluorescence quenching of the Mg-
E ox - (∆G et/e) vs (∆G et/e) as shown in Figure 4.37 The
0
q
q
-1
0
/
q
E red and ∆G 0 values of the Mg(ClO4)2-carbonyl complexes
0
thus obtained are listed in Table 1. The E red values of ground
(
ClO4)2-carbonyl complexes by alkylbenzenes as listed in Table
states of the carbonyl compounds were determined by the cyclic
voltammograms and the second-harmonic alternating current
(
35) Boldridge, D. W.; Justus, B. L.; Scott, G. W. J. Chem. Phys. 1984,
8
0, 3179.
(
9
-1
36) (a) Rehm, A.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73,
34. (b) Rehm, A.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(37) The kdiff value is taken as 8.2 × 10 M s-1, which is the maximum
8
value in Table 2.

Photoaddition of Benzyltrimethylsilane and Tetramethyltin
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7761
voltammograms (see Experimental Section).33 The E red values
0
/
Table 3. Fluorescence Lifetime (τ) and Emission Maxima (λmax) of
AcrCO, AcrCO-M(OTf) (M ) Mg, La, Lu, and Sc) in MeCN,
3
of the triplet excited states are obtained by adding the triplet
excitation energies as also listed in Table 1. The comparison
and AcrCO-Me
3 2 2
SiOTf in CH Cl and Lewis Acidity of Metal Ion
3
8
Salts (∆E)a
0
/
of the E red values between the triplet excited states of
uncomplexed carbonyl compounds and the singlet excited states
of the Mg(ClO4)2 complexes in Table 1 reveals the remarkable
positive shifts (ca. 1.2 V) of the E red values of the singlet
excited states of the Mg(ClO4)2-carbonyl complexes as com-
pared to those of the triplet excited states of uncomplexed
Lewis acid
Mg(OTf)2
∆E, eV
τ, ns
λ
max, nm
0
6.1
12.8
14.4
15.2
16.9
20.3
413
430
446
461
474
474
0
/
0.33
0.50
0.51
0.68
La(OTf)
Lu(OTf)
Sc(OTf)
Me SiOTf
3
3
3
0
/
carbonyl compounds. Such large positive shifts of the E red
3
values result in a significant increase in the reactivity of the
Mg(ClO4)2 complexes vs uncomplexed carbonyl compounds in
the photoinduced electron-transfer reactions as shown in Table
a
Derived from the gzz-values of ESR spectra of superoxide-metal
39
ion salt complexes as the quantitative measure of the Lewis acidity.
2
.
complex with AcrCO in competition with the coordination of
MeCN which is an abundant weak base. When AcrCO forms
the complex with Sc(OTf)3, the fluorescence maximum is also
red-shifted from 413 to 474 nm and the fluorescence lifetime
becomes longer in the AcrCO-Sc(OTf)3 complex (16.9 ns for
The promoting effect of Mg(ClO4)2 on the photoinduced
electron-transfer reactions of aromatic carbonyl compounds is
certainly related to the Lewis acidity of the metal ion salt. We
have recently reported that the gzz-values of ESR spectra of
superoxide-metal ion complexes are highly sensitive to the
Lewis acidity of a variety of metal ions and that the binding
energies (∆E) readily derived from the gzz-values provide the
quantitative experimental measure of Lewis acidity of a wide
variety of metal ions.39 The ∆E values have been shown to be
directly correlated with the promoting effects of metal ion salts
in electron-transfer reactions.39 It has been found that Sc(OTf)3
is the strongest Lewis acid among monovalent, divalent, and
trivalent metal ion salts.39 Thus, the rate constants of the
photoinduced electron transfer from a series of alkylbenzene
electron donors to the Sc(OTf)3 complex of 1-NA were also
determined from the fluorescence quenching of the Sc(OTf)3
complex of 1-NA by alkylbenzenes as listed in Table 2.
Although no quenching of the fluorescence of an Mg(ClO4)2
complex of 1-NA occurs with toluene, which is the weakest
electron donor among alkylbenzene donors employed in this
study, the fluorescence of the Sc(OTf)3 complex of 1-NA is
efficiently quenched by toluene (Table 2). The E red and ∆G 0
values of the singlet excited state of the 1-NA-Sc(OTf)3
complex are also determined from the intercept and slope of
the linear plots of E ox - (∆G et/e) vs (∆G et/e) (Figure 4).
The comparison of the E red values in Table 1 reveals the further
positive shift (0.14 V) of the E red value of the singlet excited
states of the 1-NA-Sc(OTf)3 complex as compared with the
value of the singlet excited state to the 1-NA-Mg(ClO4)2
complex. The overall positive shift from the value of the triplet
excited state of 1-NA is as large as 1.3 V, which corresponds
to 10 times acceleration in terms of the rate constant of
photoinduced electron transfer in the endergonic region (∆G et
1
/
1
/
the AcrCO -Sc(OTf)3 complex and 6.1 ns for AcrCO , see
Supporting Information, S5). Similarly the fluorescence maxima
(λmax) and the lifetimes (τ) are changed when AcrCO forms
the complexes with various Lewis acids (Mg(ClO4)2, La(OTf)3,
Lu(OTf)3, and Me3SiOTf). The results are summarized in Table
3
, where the binding energies (∆E) derived from the gzz-values
of ESR spectra of superoxide-metal ion complexes are given
39
as the quantitative measure of Lewis acidity of the metal ion.
The stronger the Lewis acidity of metal ions, the more red-
shifted is the λmax value, and the longer is the excited-state
lifetime (τ). The AcrCO-Sc(OTf)3 complex in MeCN and the
AcrCO-Me3SiOTf complex in CH2Cl2 have the longest λmax
value. Thus, we have examined the change in the redox
potentials of the singlet excited state of AcrCO by complexation
with Sc(OTf)3 in MeCN and Me3SiOTf in CH2Cl2 (vide infra).
The fluorescence quenching rate constants (kq) of the pho-
toinduced electron transfer from a series of electron donors to
the AcrCO-Sc(OTf)3 complex in MeCN and the AcrCO-Me3-
SiOTf complex in CH2Cl2 were determined from the fluores-
cence quenching by the donors (see Experimental Section). The
kq values are constant independent of [Sc(OTf)3] or [Me3SiOTf]
in the concentration region where all AcrCO molecules form
the complex. The kq values are summarized in Table 4, together
0
/
q
0
q
q
-1
0
/
0
/
0
with the E ox values of donors in MeCN and CH2Cl2.
0
Plots of log kq vs E ox for the AcrCO-Sc(OTf)3 complex in
MeCN and the AcrCO-Me3SiOTf complex in CH2Cl2 are
shown in Figure 5. In each case, the kq value increases with
2
2
0
0
decreasing the E ox value to reach a diffusion-limited value. The
4
0
plots of Figure 5 are fitted using eqs 2-4 as shown by the solid
>
0).
In contrast to the case of naphthaldehydes and acetonaph-
thones, irradiation of the absorption band of 10-methylacridone
AcrCO) results in fluorescence at 413 nm in MeCN. Thus,
lines, which agree with the experimental results. The best fit
0
/
lines shown in Figure 5 give the E red values (1.64 and 1.89 V
q
-1
vs SCE) and ∆G 0 value (2.6 and 2.6 kcal mol ) for the Sc-
OTf)3-AcrCO complex in MeCN and the AcrCO-Me3SiOTf
(
(
AcrCO is chosen as an aromatic carbonyl compound to examine
the change in the redox potentials of the singlet excited state
by complexation with Lewis acids. When Sc(OTf)3 is added to
an MeCN solution of AcrCO, the absorption bands of 10-
methylacridone are red-shifted due to the 1:1 complex formation
between AcrCO and Sc(OTf)3 (see Supporting Information, S4).
The formation constant (K) is determined from the spectral
0
/
complex in CH2Cl2, respectively. As compared to the E red
1
*
0
/
1
*
value of AcrCO (1.13 V), the E red value of the AcrCO -
Sc(OTf)3 complex is significantly shifted to the positive direction
0
/
(
0.51 V). The E red value of the Me3SiOTf-AcrCO complex
in CH2Cl2 is further shifted (0.76 V). This indicates that Me3-
SiOTf acts as a stronger Lewis acid than Sc(OTf)3.
5
-1
The large positive shift in the one-electron reduction potential
of AcrCO-Sc(OTf)3 in MeCN is also observed in the ground
state as indicated by the electrochemical study. Figure 6 shows
change as 1.2 × 10 M (see Experimental Section). Thus,
the Lewis acidity of Sc(OTf)3 is strong enough to form the
33
(38) Herkstroeter, W. G.; Lamola, A. A.; Hammond, G. S. J. Am. Chem.
the second harmonic ac voltammograms (SHACV) of AcrCO
Soc. 1964, 86, 4537.
0
in the absence and presence of Sc(OTf)3. The E red value of
(
39) Fukuzumi, S.; Ohkubo, K. Chem. Eur. J. 2000, 6, 4532.
AcrCO (-1.92 V) is shifted to -1.21 V when AcrCO forms
the complex with Sc(OTf)3. The zero-zero excitation energy
(40) The difference in the log ket value is given by the following
0
equation: ∆log ket ) -∆∆G et/(2.3kBT) ) 22 (at 298 K).

7762 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Fukuzumi et al.
0
Table 4. Oxidation Potentials (E ox) of Electron Donors and Fluorescence Quenching Rate Constants (k
q
) of the AcrCO-Sc(OTf)
3
Complex
-
5
(
5.0 × 10 M) and the AcrCO-Me
3
SiOTf Complex by Electron Donors in MeCN and in CH Cl at 298 K, Respectively
2 2
a
k
q
,b M-1 s-1
no.
donor
E0ox vs SCE, V
1
0
10
1
2
3
4
5
6
7
8
9
1
10-methyl-9,10-dihydroacridine
9,10-dimethylanthracene
anthracene
hexamethylbenzene
pentamethylbenzene
1,2,4,5-tetramethylbenzene
1,2,3,5-tetramethylbenzene
1,2,3,4-tetramethylbenzene
1,2,4-trimethylbenzene
1,2,3-trimethylbenzene
0.80 (0.90)
1.05
1.19
1.49 (1.60)
1.58 (1.68)
1.63 (1.75)
1.71 (1.77)
(1.81)
1.79 (1.89)
(1.98)
1.6 × 10 (1.5 × 10 )
1
1
9
0
0
1.4 × 10
1.3 × 10
9
5.8 × 10 (8.2 × 10 )
9
9
4.2 × 10 (7.6 × 10 )
9 9
1.4 × 10 (6.6 × 10 )
8 9
3.8 × 10 (4.7 × 10 )
9
(4.4 × 10 )
7
8
2.8 × 10 (8.2 × 10 )
7
0
(9.1 × 10 )
a
. b Values in parentheses are those for the AcrCO-Me
SiOTf complex in CH Cl .
3 2 2
Values in parentheses are determined in CH
2
Cl
2
agrees with the value (1.88 V) evaluated from the fluorescence
quenching in Figure 5.
Lewis Acid-Catalyzed Photoaddition of Benzyltrimethyl-
0
/
silane via Photoinduced Electron Transfer. Since the E red
values of the singlet excited states of naphthaldehyde- and
acetonaphthone-Mg(ClO4)2 complexes (Table 1) become higher
0
9a
than the E ox values of PhCH2SiMe3 (1.38 V), the photoin-
duced electron transfer from PhCH2SiMe3 to the singlet excited
states of Mg(ClO4)2-carbonyl complexes would occur ef-
ficiently. In fact, the fluorescence of Mg(ClO4)2-carbonyl
complexes is quenched efficiently by PhCH2SiMe3 in MeCN
at 298 K. The quenching rate constants (kq) were determined
from the fluorescence quenching as listed in Table 5 (see
Supporting Information, S6).
q
The ∆G et values of photoinduced electron transfer from
PhCH2SiMe3 to the singlet excited states can be evaluated by
vs E0ox for the fluorescence quenching of
AcrCO (5.0 × 10 M) by various electron donors in the presence of
Sc(OTf)
(4.0 × 10 M) in deaerated MeCN (b) and in the presence
of Me Cl (2) at 298 K.
q
Figure 5. Plots of log k
q
using eqs 2-4, since the ∆G 0 value for the photoinduced
-
5
electron transfer of organosilanes such as PhCH2SiMe3 has
-
2
3
-1 9a
previously been determined as 4.6 kcal mol . The ket values
-
3
3
SiOTf (2.0 × 10 M) in deaerated dry CH
2
2
which correspond to the kq values in eq 4 are calculated as listed
in Table 5, where the ket values indeed agree with the kq values.
Such an agreement indicates that the fluorescence quenching
of the Mg(ClO4)2-carbonyl complexes by PhCH2SiMe3 occurs
via electron transfer from PhCH2SiMe3 to the singlet excited
states of the Mg(ClO4)2-carbonyl complexes.
The numbers refer to compounds in Table 4. The solid lines are drawn
based on eqs 2-4.
No photochemical reaction of naphthaldehydes or acetonaph-
thones with PhCH2SiMe3 has occurred, as expected from the
0
9a
0
higher E ox value of PhCH2SiMe3 than the E red values of the
triplet excited states in Table 1. In the presence of Mg(ClO4)2
(
0.94 M), however, irradiation of a deaerated MeCN solution
-1
-3
containing PhCH2SiMe3 (1.7 × 10 M) and 1-NA (1.4 × 10
M) with monochromatized light of λ ) 350 nm results in the
decrease in the absorbance due to 1-NA, accompanied by the
increase in the absorbance due to the photoproduct with a clean
isosbestic point at λ ) 282 nm (see Supporting Information,
S7). The photoproduct is identified as the benzyl adduct as
2
4,41,42
shown in eq 6 (see Experimental Section).
The benzyl
adducts are also obtained in the photochemical reactions of other
carbonyl compounds (2-NA, 1-AN, 2-AN) with PhCH2SiMe3
in the presence of Mg(ClO4)2 in MeCN (see Experimental
Section).
-3
Figure 6. SHACVs of (a) AcrCO (5.0 × 10 M) in deaerated MeCN
and (b) AcrCO in the presence of Sc(OTf)
(5.0 × 10 M) in deaerated
EtCN, 0.10 M TBAPF at 298 K. Scan rate ) 4 mV s
-2
3
-
1
6
.
1
/
of AcrCO -Sc(OTf)3 (∆E0,0) can be obtained as 2.81 V from
the absorption maximum (413 nm, 3.00 eV) and the fluorescence
0
/
maximum (474 nm, 2.62 eV). Then, the E red value of
1
*
AcrCO -Sc(OTf)3 is determined as 1.60 eV by subtracting
0
the ∆E0,0 value from the E red value of the ground-state complex.
This value agrees with the value (1.64 V) evaluated from the
The quantum yields (Φ) of the photoaddition reactions in
0
/
fluorescence quenching in Figure 5. Similarly, the E red value
the presence of Mg(ClO ) (1.0 M) increase with an increase
4
2
1
/
of AcrCO -Me3SiOTf is determined as 1.91 eV, which also
in [PhCH2SiMe3] to reach a constant value (Φ∞) (see Supporting

Photoaddition of Benzyltrimethylsilane and Tetramethyltin
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7763
SiMe and Me Sn
Table 5. Fluorescence Quenching Rate Constants (k
to the Singlet Excited States of Mg(ClO
q
), Rate Constants (ket) of Photoinduced Electron Transfer from PhCH
2
3
4
4
)
2
- and Sc(OTf)
3
-Carbonyl Complexes, and Observed Rate Constants (kobs) and Limiting Quantum
Yields (Φ
∞
) for Photoaddition of PhCH
2
SiMe and Me Sn with Mg(ClO
3
4
4
)
2
- and Sc(OTf)
3
-Carbonyl Complexes in Deaerated MeCN at 298 K
,a M-1 s-1
k
et,b M-1 s-1
k
obs,c M-1 s-1
Φ
c
reactant pair compd
k
q
∞
9
9
9
-2
-2
-1
-1
-1
PhCH
PhCH
PhCH
PhCH
2
2
2
2
SiMe
SiMe
SiMe
SiMe
3
3
3
3
/1-NA-Mg(ClO
/2-NA-Mg(ClO
/1-AN-Mg(ClO
/2-AN-Mg(ClO
4
4
)
)
2
2
4.9 × 10
4.3 × 10
3.7 × 10
3.0 × 10
1.8 × 10
4.3 × 10
3.6 × 10
3.8 × 10
2.6 × 10
1.6 × 10
5.4 × 10
4.6 × 10
3.9 × 10
3.5 × 10
1.5 × 10
9.6 × 10
7.1 × 10
1.0 × 10
1.1 × 10
3.3 × 10
9
9
9
8
9
9
9
7
9
9
9
8
4
)
)
2
2
4
Me
4
Sn/1-NA-Sc(OTf)
3
8
(
1.0 × 10 )
8
8
8
-2
PhCH
2
SiMe
3
/AcrCO-Sc(OTf)
3
7.9 × 10
9.6 × 10
8.9 × 10
3.9 × 10
a
b
Determined from Stern-Volmer plots for the fluorescence quenching (eq 1, see Supporting Information, S6). Calculated based on the free
energy relationship for the photoinduced electron transfer using eqs 2-4. The value in parentheses is that calculated based on the Marcus equation
c
(
eq 10, see text). Determined from the dependence of Φ on the substrate concentration.
Scheme 1
SiMe3] can be derived as given by eq 8, which agrees with the
observed dependence of Φ on [PhCH2SiMe3] (eq 7). The
observed rate constant (kobs) and the limiting quantum yield (Φ∞)
correspond to the rate constant (ket) of photoinduced electron
transfer from PhCH2SiMe3 to the Mg(ClO4)2-carbonyl com-
plexes and kp/(kp + kb), respectively.
Φ ) [k /(k + k )]k τ[PhCH SiMe ]/
p
p
b
et
2
3
(
1 + k τ[PhCH SiMe ]) (8)
et
2
3
When PhCH2SiMe3 is replaced by a much weaker electron
donor such as tetramethyltin (Me4Sn), no photoaddition of Me4-
Sn with the 1-NA-Mg(ClO4)2 complex has occurred. In the
case of the 1-NA-Sc(OTf)3 complex, however, the photoad-
dition of Me4Sn occurs efficiently to yield the methyl adduct
(see Experimental Section and eq 9).
Information, S8) in accordance with eq 7. From the linear plots
-
1
-1
of Φ vs [PhCH2SiMe3] are obtained the values of Φ∞ and
Φ ) Φ∞kobsτ[PhCH SiMe ]/(1 + k τ[PhCH SiMe ]) (7)
2
3
obs
2
3
the rate constants (kobs), which are also listed in Table 5. The
kobs values agree well with both the kq and ket values. Such an
agreement strongly indicates that the photoaddition reactions
proceed via photoinduced electron transfer from PhCH2SiMe3
to the singlet excited states of the Mg(ClO4)2-carbonyl
complexes (ket), followed by the cleavage of the Si-C bond in
the radical cation4 and the radical coupling with the carbonyl
radical anion (kp) to yield the adduct in competition with the
back electron transfer to the reactant pair (kb) as shown in
Scheme 1 for the case of the PhCH2SiMe3/2-NA-Mg(ClO4)2
It has been reported that no photoinduced electron transfer
from Me4Sn to the singlet excited state of a flavin analogue
0
/
48
0
/
(
E red ) 1.96 V) occurs. Since this E red value is nearly the
same as the value of the singlet excited state of the 1-NA-
0
/
Mg(ClO4)2 complex (E red ) 1.98 V), no photoinduced electron
transfer from Me4Sn to the singlet excited state of the 1-NA-
Mg(ClO4)2 complex would occur. In fact, no fluorescence
quenching of the 1-NA-Mg(ClO4)2 complex has occurred by
Me4Sn. In contrast to the Mg(ClO4)2 complex, the fluorescence
of the 1-NA-Sc(OTf)3 complex is quenched efficiently by Me4-
Sn. The quenching rate constant kq was determined from the
fluorescence quenching as listed in Table 5.
3,44
45,46
system.
The rapid cleavage of the Si-C bond in the radical
is consistent with relatively large quantum yields
cation4
3,44
(Table 5) which are higher than one would normally expect for
singlet radical ion chemistry for which energy wasting back
electron transfer usually dominates.47
(
45) A referee pointed out the possibility of a photoinduced electron
By application of the steady-state approximation to the
reactive species in Scheme 1, the dependence of Φ on [PhCH2-
transfer from the carbonyl compound to the Lewis acid, since some Lewis
9
a,46
acids are known to undergo electron-transfer reactions with substrates.
The radical cation thus formed may be responsible for the subsequent
electron transfer from PhCH2SiMe3 in Scheme 1. In the present case,
however, there is no indication of the occurrence of photoinduced electron
transfer between the carbonyl compound and the Lewis acid in the
fluorescence spectrum (Figure 1) and in the laser flash photolysis experi-
ments (Figure 2).
(46) (a) Barton, D. H. R.; Dalko, P. I.; G e´ ro, S. D. Tetrahedron Lett.
1992, 33, 1883. (b) Barton, D. H. R.; Haynes, R. K.; Leclerc, G.; Magnus,
P. D.; Menzies, I. D. J. Chem. Soc., Perkin Trans. 1 1975, 2055. (c) Barton,
D. H. R.; Haynes, R. K.; Magnus, P. D.; Menzies, I. D. J. Chem. Soc.,
Chem. Commun. 1974, 511.
(41) In this case, no homo-coupling products such as dibenzyl have been
detected (see Experimental Section) in contrast with the case of the
photoalkylation of 10-methylacridinium ion by a bulky electron donor such
as diphenylmethane via photoinduced electron transfer in which the hetero-
coupling between diphenylmethyl radical and acridinyl radical is retarded
due to the steric effects to yield the homo-coupling product (13% yield.).42
(
42) Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. J. Am. Chem.
Soc. 1996, 118, 8566.
43) (a) Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould, I. R.; Todd,
(
W. P.; Mattes, S. L. J. Am. Chem. Soc. 1989, 111, 8973. (b) Cermenati, L.;
Freccero, M.; Venturello, P.; Albini, A. J. Am. Chem. Soc. 1995, 117, 7869.
(47) A referee suggested to add this discussion.
(
44) Dockery, K. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould,
(48) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Chem. Soc., Perkin Trans.
2 1986, 25.
I. R.; Todd, W. P. J. Am. Chem. Soc. 1997, 119, 1876.

7764 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Fukuzumi et al.
The kobs and Φ∞ values were also determined from the
Scheme 2
dependence of Φ on [Me4Sn] (see Supporting Information, S9)
as listed in Table 5, where the kobs value agrees with the kq
value. The rate constant (ket) of photoinduced electron transfer
from Me4Sn to the singlet excited state of the 1-NA-Sc(OTf)3
q
complex can be evaluated by using eqs 2-4, since the ∆G 0
value for the electron transfer of Me4Sn has previously been
-
1 48-50
determined as 10.2 kcal mol .
The ket value thus evaluated
7
-1
is also listed in Table 5, where the ket value (1.6 × 10 M
-1
q
s ) is smaller than the kq and kobs values. When the ∆G 0 value
is large, the Marcus equation (eq 10)51 is known to fit better
with the experimental results than the Rehm-Weller equation
4
8-52
(eq 3).
q
G
q
0
0
et
q
0
2
∆
) ∆G [1 + (∆G /4∆G )]
(10)
et
The calculated ket value (1.0 × 10 M s-1) using eq 10
8
-1
instead of eq 3 is given in parenthesis in Table 5. This value
agrees with the experimental values (kq and kobs).53 Such an
agreement indicates that the photoaddition reaction proceeds
via photoinduced electron transfer from Me4Sn to the singlet
excited state of the 1-NA-Sc(OTf)3 complex, followed by the
cleavage of the Sn-C bond in the radical cation and the radical
coupling with the carbonyl radical anion as is the case for the
photoaddition of PhCH2SiMe3 in Scheme 1.
Photoaddition of PhCH2SiMe3 with AcrCO is also made
possible when AcrCO forms the complex with Sc(OTf)3 and
Me3SiOTf (vide infra). The photoirradiation of the absorption
band of the AcrCO-Sc(OTf)3 complex in the presence of
PhCH2SiMe3 with monochromatized visible light (λ ) 413 nm)
results in an efficient photochemical reaction of the AcrCO-
Sc(OTf)3 complex with PhCH2SiMe3 to yield 9-benzyl-10-
methylacridinium ion as shown in eq 10 (see Experimental
-1 9a,54
mol ).
(
The ket value agrees with both the kq and kobs values
Table 5). Such agreements strongly indicate that the photoad-
dition reaction proceeds via photoinduced electron transfer from
1
/
PhCH2SiMe3 to AcrCO -Sc(OTf)3 as shown in Scheme 2. The
1
/
drastically enhanced electron acceptor ability of the AcrCO -
1
/
Sc(OTf)3 complex as compared to AcrCO (vide supra) makes
1
/
it possible for electron transfer from PhCH2SiMe3 to AcrCO -
Sc(OTf)3 to occur efficiently to produce the radical ion pair
•+
•-
(
PhCH2SiMe3 AcrCO -Sc(OTf)3). The Si-C bond is readily
•+
•-
cleaved by the reaction of PhCH2SiMe3 with AcrCO -Sc-
(
OTf)3 in the radical ion pair to yield the siloxy adduct (kp) in
•-
competition with the back electron transfer from AcrCO -
•
+
Sc(OTf)3 to PhCH2SiMe3 . The carbon-oxygen bond of the
siloxy adduct is readily cleaved by an acid to yield the 9-benzyl-
1
0-methylacridinium ion as the final product.
Direct Detection of Radical Ion Intermediates. Formation
of the radical ion pair in photoinduced electron transfer from
1
/
PhCH2SiMe3 to the (1-NA) -Mg(ClO4)2 complex (Scheme 1)
1
/
and the AcrCO -Sc(OTf)3 complex (Scheme 2) was confirmed
by the laser flash experiments. The transient absorption spectra
obtained after the laser pulse excitation of the PhCH2SiMe3/1-
NA-Mg(ClO4)2 system are shown in Figure 7a. The transient
absorption band at 520 nm in Figure 7a may correspond to the
Section). Essentially the same results were obtained when the
photochemical reaction was performed in the presence of Me3-
SiOTf in dry CH2Cl2 instead of Sc(OTf)3 in MeCN.
As is the case of Mg(ClO4)2- and Sc(OTf)3-catalyzed pho-
toaddition reactions of PhCH2SiMe3 and Me4Sn described
above, the kq, kobs, and Φ∞ values are determined as listed in
Table 5 (see Supporting Information, S10). The rate constants
•
+
reported spectrum of PhCH2SiMe3 , since no other possible
•
intermediate such as PhCH2 has the absorption band in this
5
5,56
wavelength region.
be attributed to the transient spectrum of the 1-NA -Mg-
ClO4)2 complex. To confirm this assignment, a photoinduced
electron transfer from a dimeric 1-benzyl-1,4-dihydronicotina-
The additional band around 600 nm may
•-
(
57
mide [(BNA)2] to the 1-NA-Mg(ClO4)2 complex was exam-
ined by means of the laser flash photolysis. The (BNA)2 is
(
ket) of photoinduced electron transfer from PhCH2SiMe3 to the
1
/
singlet excited state complex [ AcrCO -Sc(OTf)3] can be
q
evaluated by using eqs 2-4 and the ∆G 0 value for the
0
/
(
54) The E red value (1.60 V) evaluated from the electrochemical
measurements was used for the calculation.
55) Fukuzumi, S.; Fujita, M.; Noura, S.; Ohkubo, K.; Suenobu, T.; Araki,
Y.; Ito, O. J. Phys. Chem. A 2001, 105, 1857.
photoinduced electron transfer of PhCH2SiMe3 (4.6 kcal
(
(
49) Fukuzumi, S.; Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1980,
•+
1
02, 2928.
(56) The lifetime of PhCH2SiMe3 in Figure 7 is apparently inconsistent
(
(
50) Eberson, L. AdV. Phys. Org. Chem. 1982, 18, 79.
51) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
with the large second-order rate constant of the Si-C bond cleavage of
•+
9
-1 -1
PhCH2SiMe3 with MeCN (3.2 × 10 M
s ) in dichloromethane, which
•+
R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
leads to a short lifetime of the free radical cation (PhCH2SiMe3 ) in neat
q
0
q
-9
44
(
52) When the ∆G 0 value is small such that ∆G et < -4∆G 0, however,
MeCN (<10 s). However, the Si-C bond cleavage rate of PhCH2-
3
6
•+
eq 3 fits better with experimental results than eq 10.
SiMe3 in the cage (Scheme 2) may be much slower than the rate for the
(
53) The ket values for the PhCH2SiMe3/Mg(ClO4)2-carbonyl systems
free radical cation as observed in photoinduced electron transfer from
in Table 5, calculated based on eq 10, are somewhat larger than those
PhCH2SiMe3 to 10-methylacridinium ion.55
9
-1 -1
calculated based on eq 3 (e.g., 7.2 × 10 M
s
for the PhCH2SiMe3/1-
(57) Patz, M.; Kuwahara, Y.; Suenobu, T.; Fukuzumi, S. Chem. Lett.
1997, 567.
NA-Mg(ClO4)2 system).

Photoaddition of Benzyltrimethylsilane and Tetramethyltin
J. Am. Chem. Soc., Vol. 123, No. 32, 2001 7765
Figure 7. (a) Transient absorption spectra observed in the photoreaction
Figure 8. (a) Transient absorption spectra observed in the photore-
of 1-naphthaldehyde-Mg(ClO
4
)
2
complex formed between 1-naph-
thaldehyde (2.8 × 10 M) and Mg(ClO (1.0 M) with PhCH SiMe
1.7 × 10 M) at 1.2 (b) and 4.0 µs (2) after laser excitation in
deaerated MeCN at 298 K. (b) Transient absorption spectra observed
in the photoreduction of 1-naphthaldehyde-Mg(ClO complex formed
between 1-naphthaldehyde (2.8 × 10 M) and Mg(ClO (1.0 M) by
BNA)
-4
duction of AcrCO-Sc(OTf) complex formed between AcrCO (5.0 x
4
)
2
2
3
3
-
5
-2
-1
-
1
10 M) and Sc(OTf) (8.0 × 10 M) by PhCH SiMe (2.5 × 10
(
3
2
3
M) at 50 µs after laser excitation in deaerated MeCN at 298 K. (b)
Transient absorption spectra observed in the photoreduction of AcrCO-
4 2
)
-
4
-
4
Sc(OTf) complex formed between AcrCO (1.0 × 10 M) and Sc-
)
2
3
4
-
2
-2
-
4
(OTf) (4.0 × 10 M) by (BNA) (1.0 × 10 M) at 50 µs after laser
(
2
(2.0 × 10 M) at 2.0 (b) and 12 µs (2) after laser excitation
3
2
excitation in deaerated MeCN at 298 K.
in deaerated MeCN at 298 K.
•
-
known to act as a unique two-electron donor to produce the
radical anions of electron acceptors.57,58 The transient absorption
spectra after the laser pulse excitation of the (BNA)2/1-NA-
Mg(ClO4)2 system are shown in Figure 7b, where the absorption
absorption bands at 520 and 600 nm due to the 1-NA -Mg-
(ClO4)2 complex are overlapped with the absorption band at
•
+
ca. 500 nm due to PhCH2SiMe3 in Figure 7a.
The transient absorption spectrum is also obtained after the
laser pulse excitation of the PhCH2SiMe3/AcrCO-Sc(OTf)3
system as shown in Figure 8a. The transient absorption spectrum
after the laser pulse excitation of the (BNA)2/AcrCO-Sc(OTf)3
system is also shown in Figure 8b, where the absorption band
•
-
bands at 520 and 600 nm are assigned to those of the 1-NA -
Mg(ClO4)2 complex. It has been reported that the radical cation
of (BNA)2 cannot be detected at the microsecond time scale
•+
due to the facile oxidation of (BNA)2 , which readily dissoci-
•
+
+ 57,58
ates to BNA and BNA , then to 2 equiv of BNA .
Thus,
•-
at 600 nm is assigned to that of the AcrCO -Sc(OTf)3
the stoichiometry of the photoinduced electron transfer from
complex. Thus, the transient absorption spectrum in Figure 8a
(BNA)2 to the 1-NA-Mg(ClO4)2 complex is given by eq 11.
•+
consists of the absorption band due to PhCH2SiMe3 at 500
nm, which is overlapped with the absorption band due to the
•
-
AcrCO -Sc(OTf)3 complex at 600 nm.
Summary and Conclusions.
As demonstrated above, photoinduced electron transfer reac-
tions of aromatic carbonyl compounds are remarkably acceler-
ated by the complexation with metal ions acting as Lewis acids.
A change in spin state of the lowest excited states from the
triplet to the singlet is generally observed for aromatic carbonyl
compounds. Photochemical redox reactions which would oth-
erwise be unlikely to occur are allowed to proceed efficiently
via Lewis acid-catalyzed photoinduced electron transfer by
complexation of the photoexcited states with Lewis acids. The
one-electron reduction potentials of both the ground state and
excited state of aromatic carbonyl compounds are shown to be
remarkably shifted to the positive direction by complexation
with Lewis acids.
+
Since BNA has no absorption band in the visible region, only
the radical anion part is detected in Figure 7b. Thus, the
(
58) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.;
Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 1998, 120, 8060.

7766 J. Am. Chem. Soc., Vol. 123, No. 32, 2001
Fukuzumi et al.
Acknowledgment. This work was partially supported by a
ing of the Mg(ClO4)2-carbonyl complexes (S6); UV-vis
Grant-in-Aid for Scientific Research Priority Area (Nos. 11228205
and 13031059) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. Our special thanks are due to
Dr. A. Ishida of Institute of Industrial Science, Osaka University
for his help at the early stage of this study in the laser flash
photolysis experiments and phosphorescence measurements.
spectroscopy for the photoaddition of benzyltrimethylsilane with
1-NA (S7); dependence of the quantum yield (Φ) on [PhCH -
2
-
1
-1
SiMe ] and plot of Φ vs [PhCH SiMe ] for the photoad-
3
2
3
dition of PhCH SiMe with Mg(ClO ) -carbonyl complexes
2
3
4 2
(
S8); dependence of the quantum yield (Φ) on [Me4Sn] and
-1
-1
plot of Φ vs [Me4Sn] for the photoaddition of Me4Sn with
the 1-NA-Sc(OTf)3 complex (S9); dependence of the quantum
Supporting Information Available: Experimental proce-
dures for photochemical reactions; quantum yield determinations
and laser flash photolysis (S1-S3); UV-visible spectral
changes for the complex formation between AcrCO and Sc-
-1
-1
yield (Φ) on [PhCH2SiMe3] and plot of Φ vs [PhCH2SiMe3]
for the photoaddition of PhCH2SiMe3 with the AcrCO-Sc-
OTf)3 complex (S10) (PDF). This material is available free of
(
charge via the Internet at http://pubs.acs.org.
(OTf)3 (S4); fluorescence decay curve of the AcrCO-Sc(OTf)3
complex (S5); Stern-Volmer plots for the fluorescence quench-
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