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.;
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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-
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(
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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.
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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
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1
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(
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