A R T I C L E S
Fukuzumi et al.
prepared as given in the literature.21 The dimeric 1-benzyl-1,4-
dihydronicotinamide [(BNA)2] was prepared according to the litera-
ture.22 9,10-Dihydro-10-methylacridine (AcrH2) was prepared from 10-
methylacridinium iodide (AcrH+I-) by reduction with NaBH4 in
methanol and purified by recrystallization from ethanol.23 Scandium
triflate [Sc(OTf)3] was purchased from Pacific Metals Co., Ltd.
(Taiheiyo Kinzoku). Lanthanum triflate [La(OTf)3] was obtained from
Aldrich in hexahydrate form. Yttrium triflate [Y(OTf)3], europium
triflate [Eu(OTf)3], ytterbium triflate [Yb(OTf)3], and lutetium triflate
[Lu(OTf)3] were prepared as follows.24 A deionized aqueous solution
was mixed (1:1 v/v) with trifluoromethanesulfonic acid (>99.5%, 10.6
mL) obtained from Central Glass, Co., Ltd., Japan. The trifluo-
romethanesulfonic acid solution was slowly added to a flask which
contained the corresponding metal oxide (>99.9%, 30 mmol). The
mixture was refluxed at 100 °C for 3 days. After centrifugation of the
reaction mixture, the solution containing metal triflate was separated
and water was removed by vacuum evaporation. Yttrium oxide,
europium oxide, and ytterbium triflate were purchased from Shin Etsu
Chemical, Co., Ltd., Japan. Lutetium oxide was obtained from Nichia
Corporation, Japan. Metal triflates were dried under vacuum evacuation
at 403 K for 40 h prior to use. Magnesium perchlorate [Mg(ClO4)2]
was obtained from Wako Pure Chemical Ind. Ltd., Japan. Calcium
perchlorate [Ca(ClO4)2] was obtained from Nacalai Tesque, Japan.
Acetonitrile (MeCN) used as a solvent was purified and dried by the
standard procedure.20
as Lewis acids are also known to accelerate electron-transfer
reactions of carbonyl compounds significantly.13-15 Metal ions
can also accelerate hydride-transfer reactions from NADH
(dihydronicotinamide adenine dinucleotide) model compounds
to carbonyl compounds via metal ion-catalyzed electron transfer
as the rate-determining step.16,17 However, it has yet to be
clarified how the Lewis acid catalysis in electron-transfer
reactions of carbonyl compounds such as p-benzoquinones is
correlated with the catalysis in Diels-Alder reactions and
hydride-transfer reactions of the same p-benzoquinones acting
as dienophiles and hydride acceptors, respectively.
We report, herein, that a variety of metal ions act as effective
catalysts to accelerate Diels-Alder reactions of anthracenes with
p-benzoquinone derivatives, which have been regarded as inert
or weak dienophiles.18 Scandium triflate (Sc(OTf)3) is shown
to be by far the most reactive as compared to other metal ions.19
An extensive comparison of the catalysis of a series of metal
ions has been performed for the first time among the electron-
transfer reactions, the Diels-Alder reactions, and the hydride-
transfer reactions of p-benzoquinones acting as electron accep-
tors, dienophiles, and hydride acceptors, respectively. The direct
spectroscopic detection of complexes formed between the
corresponding semiquinone radical anions and metal ions,
combined with the detailed kinetic analysis of the catalytic
effects of metal ions, provides a confirmative basis to delineate
the common catalytic mechanism of metal ions in each reaction.
Reaction Procedure and Analysis. Typically, an [2H3]acetonitrile
(CD3CN) solution (0.7 cm3) containing an anthracene derivative (1.0
× 10-2 M) and p-benzoquinone derivatives (2.0 × 10-2 M) in the
presence (1.0 M) and absence of Mg(ClO4)2 in an NMR tube sealed
with a rubber septum was deaerated by bubbling with argon gas through
a stainless steel needle for 5 min and was mixed. Several hours later,
Experimental Section
Materials. Anthracene and its derivatives (9,10-dimethylanthracene,
9-methylanthracene, 9-ethylanthracene, 9-benzylanthracene, and 9-bro-
moanthracene) were obtained commercially. p-Benzoquinone and its
derivatives (2,5-dimethyl-p-benzoquinone, 2,5-dichloro-p-benzoquinone,
p-fluoranil, and p-chloranil) were also obtained commercially and
purified by the standard methods.20 Decamethylferrocene (Sigma) and
1,1′-dimethylferrocene (Tokyo Kasei Organic Chemicals) were com-
mercially available. Cobalt(II) tetraphenylporphyrin (CoTPP) was
1
1
the reaction solution was analyzed by H NMR spectroscopy. The H
NMR measurements were performed using a Japan Electron Optics
JNM-GSX-400 (400 MHz) NMR spectrometer at 300 K. 1H NMR
(CD3CN in the presence of 1.0 M Mg(ClO4)2). 1a: δ 1.98 (s, 6H),
2.85 (s, 2H), 6.12 (s, 2H), 7.22-7.31 (m, 6H), 7.45-7.49 (m, 2H).
1b: δ 1.91 (s, 3H), 2.01 (s, 3H), 3.29 (s, 1H), 6.50 (s, 1H), 7.24-7.39
(m, 6H), 7.50-7.57 (m, 2H). 1c: δ 0.88 (s, 3H), 1.59 (s, 3H), 1.85 (s,
3H), 1.90 (s, 3H), 2.39 (s, 1H), 5.87 (s, 1H), 7.15-7.32 (m, 6H), 7.42-
7.50 (m, 2H). 1d: δ 1.98 (s, 3H), 2.77 (d, 1H, J ) 8.8 Hz), 3.24 (dd,
1H, J ) 2.9, 8.8 Hz), 4.73 (d, 1H, J ) 2.9 Hz), 6.17, 6.26 (ABq, 2H,
J ) 10.26 Hz), 7.17-7.27 (m, 6H), 7.43-7.50 (m, 2H). 1e: δ 3.21 (s,
2H), 4.86 (s, 2H), 6.40 (s, 2H), 7.04-7.24 (m, 6H), 7.43-7.50 (m,
2H).
(11) (a) Doyle, M. P.; Phillips, I. M.; Hu, W. J. Am. Chem. Soc. 2001, 123,
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Volman, D. H., von Bu¨nau, G., Eds.; Wiley: New York, 1998; Vol. 25,
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Spectral Measurements. Transient absorption spectra of 1:1 and
1:2 complexes of semiquinone radical anions (X-Q•-) with Mg2+ were
measured by using a Union RA-103 stopped-flow spectrophotometer.
The transient absorption spectra of the Mg2+ complexes were obtained
by plotting the initial absorbances in the kinetic curves against the
wavelengths in the electron-transfer reduction of p-benzoquinone
derivatives (2.4 × 10-3 M) by [Fe(Me5C5)2] or [Fe(MeC5H4)2] (2.4 ×
10-4 M) in the presence of various concentrations of Mg2+ in deaerated
MeCN at 298 K.
(17) Fukuzumi, S.; Fujii, Y.; Suenobu, T. J. Am. Chem. Soc. 2001, 123, 10191.
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Kinetic Measurements. Kinetic measurements were performed on
a Hewlett-Packard 8453 photodiode array spectrophotometer. Rates of
Diels-Alder reactions of anthracene derivatives (6.2 × 10-4-1.3 ×
10-3 M) with p-benzoquinone derivatives (1.8 × 10-2-3.8 × 10-1 M)
in the presence and absence of metal ion were monitored by measuring
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