Scandium ion-promoted reduction of heterocyclic N = N double bond. Hydride transfer vs electron transfer

Article abstract of DOI:10.1021/ja026592y

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH₂) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph₂Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)₃ (OTf = OSO₂-CF₃

Full text of DOI:10.1021/ja026592y

Published on Web 09/25/2002
Scandium Ion-Promoted Reduction of Heterocyclic NdN
Double Bond. Hydride Transfer vs Electron Transfer
Shunichi Fukuzumi,* Junpei Yuasa, and Tomoyoshi Suenobu
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
Received April 18, 2002
Abstract: Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH
Ph Tz), which contains a NdN double bond, occurs efficiently in the presence of Sc(OTf)
CF
2
) to 3,6-diphenyl-1,2,4,5-tetrazine
(
2
3
(OTf ) OSO
2
-
3+
3
) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc . The
observed second-order rate constant (kobs) increases with increasing Sc3 concentration to approach a
+
), the rate of Sc3 -promoted
+
limited value. When AcrH
hydride transfer exhibits the same primary kinetic isotope effect (k
concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP ) tetraphenylporphyrin
2
is replaced by the dideuterated compound (AcrD
2
3+
/k
H D
) 5.2(0.2), irrespective of Sc
2-
dianion) and 10,10′-dimethyl-9,9′-biacridine [(AcrH)
2
] to Ph
2
Tz, whereas no electron transfer from CoTPP
Tz occurs in the absence of Sc3 . In each case, the observed second-order rate constant
+
or (AcrH)
2
to Ph
2
of electron transfer (ket) shows a first-order dependence on [Sc3 ] at low concentrations and a second-
+
order dependence at higher concentrations. Such dependence of ket on [Sc3 ] is ascribed to formation of
+
•
-
3+
3+
1
:1 and 1:2 complexes between Ph
2
Tz and Sc at the low and high concentrations of Sc , respectively,
which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed
•
-
by the ESR spectrum in which the hyperfine structure is different from that of free Ph
2
Tz . The 1:2 complex
formation results in the saturated kinetic dependence of kobs on [Sc3 ] for the Sc3+-promoted hydride transfer,
+
3+
which proceeds via Sc -promoted electron transfer from AcrH
2
to Ph
2
Tz, followed by proton transfer from
•+
•-
3+
•
AcrH
2
to the 1:1 Ph
2
Tz -Sc complex and the subsequent facile electron transfer from AcrH to Ph
2
-
TzH . The effects of counteranions on the Sc3+-promoted electron transfer and hydride transfer reactions
•
are also reported.
trochemically.9,10 The effects of metal ions on hydride transfer
reactions from NADH analogues to substrates have particularly
Introduction
Nicotinamide adenine dinucleotide (NADH) is the most
important electron source in biological redox reactions, provid-
ing a hydride ion that is equivalent to two electrons and a proton.
Mechanisms of hydride transfer reactions of NADH analogues
with hydride acceptors such as carbonyl compounds and organic
cations have been extensively studied chemically1-8 or elec-
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*
To whom correspondence should be addressed. E-mail: fukuzumi@
ap.chem.eng.osaka-u.ac.jp.
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68, 1662. (b) Tanner, D. D.; Kharrat, A. J. Org. Chem. 1988, 53, 1646. (c)
Liu, Y.-C.; Li, B.; Guo, Q.-X. Tetrahedron 1995, 51, 9671. (d) Ohno, A.;
Ishikawa, Y.; Yamazaki, N.; Okamura, M.; Kawai, Y. J. Am. Chem. Soc.
1998, 120, 1186. (e) Kanomata, N.; Nakata, T. Angew. Chem., Int. Ed.
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1
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5
67.
(
3) (a) Fukuzumi, S. AdVances in Electron-Transfer Chemistry; Mariano, P.
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(
4) (a) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
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4
286.
1
2566 J. AM. CHEM. SOC. 2002, 124, 12566-12573
9
10.1021/ja026592y CCC: $22.00 © 2002 American Chemical Society

Scandium Ion-Promoted Reduction of Heterocyclic NdN
A R T I C L E S
attracted considerable interest in relation to the catalytic role
of metal ions in the redox reactions of nicotinamide coenzymes
in the native enzymatic system.1-4,11,12 Metal ions acting as
Lewis acids are known to promote electron-transfer reactions,
We report herein that hydride transfer from an NADH
analogue to 3,6-diphenyl-1,2,4,5-tetrazine (Ph2Tz), which would
otherwise be difficult to reduce, occurs efficiently in the
presence of Sc(OTf)3 (OTf ) OSO2CF3) in MeCN. 1,2,4,5-
Tetrazines containing a NdN double bond are valuable electron-
deficient 4π components that have been utilized in Diels-Alder
reactions with inverse electron demanding, making a large
21
where metal ions bind to the product radical anions produced
in the electron-transfer reactions.1
3-16
Both thermal and pho-
tochemical redox reactions that would otherwise be unlikely to
2
2-24
occur are made possible to proceed efficiently by the catalysis
variety of valuable compounds available.
The kinetic
of metal ions on the electron-transfer steps.1
3-16
Among metal
analysis of the Sc -promoted hydride transfer reactions involv-
ing kinetic deuterium isotope effects as compared with the
authentic electron-transfer reactions provides valuable informa-
3+
ions, rare-earth metal ions have particularly attracted consider-
able attention as much more effective Lewis acids than divalent
metal ions such as Mg2 and Zn in various carbon-carbon
+
2+
tion for the Sc -promoted electron-transfer step and the
3+
bond-forming reactions due to the strong affinity to carbonyl
subsequent proton-transfer step in the overall hydride transfer
reaction. The key intermediate for the Sc -promoted reduction
oxygen.1
7-19
In particular, scandium ion (Sc ) has recently been
3+
3+
reported to accelerate electron-transfer reduction of p-benzo-
quinone derivatives much more efficiently than any other metal
ion, including lanthanide ions.20 The reactions of NADH
analogues with p-benzoquinone derivatives, which are normally
good hydride acceptors, are also accelerated most remarkably
of Ph2Tz is found to be the complex formed between the radical
3+
anion of Ph2Tz and Sc , which is detected successfully by ESR
spectroscopy in this study. The effects of counterions have also
been studied to reveal the relation between the electron transfer
and the hydride transfer reactions.
3+ 20
by Sc . In this case, however, efficient [2 + 3] cycloaddition
reactions of NADH analogues with p-benzoquinone derivatives
rather than the hydride transfer reactions occur in the presence
of scandium triflate [Sc(OTf)3] in MeCN.20 Moreover, there has
so far been no report on the metal ion-promoted hydride transfer
from an NADH analogue to hydride acceptors other than
carbonyl compounds, which would be difficult to reduce without
the metal ion. The effects of counterions on the metal ion-
promoted electron transfer or hydride transfer have yet to be
examined.
Experimental Section
The standard procedures of experiments including product analysis
and kinetic measurements are given in the Supporting Information (S1).
2
Materials. The preparation of 10-methyl-9,10-dihydroacridine (AcrH )
4a
and the dideuterated compound (AcrD
The dimeric 1-benzyl-1,4-dihydronicotinamide dimer [(BNA)
prepared according to the literature procedure.25 Cobalt(II) tetraphen-
2
) was described previously.
2
] was
2
6
ylporphyrin (CoTPP) was prepared as given in the literature. 10,10′-
Dimethyl-9,9′-biacridine [(AcrH) ] was prepared by the one-electron
reduction of 10-methylacridinium perchlorate by hexamethylditin.
Scandium triflate [Sc(OTf) ] was prepared by the procedure reported
2
2
7
(
11) (a) Sigman, D. S.; Hajdu, J.; Creighton, D. J. In Bioorganic Chemistry;
3
van Tamelen, E. E., Ed.; Academic Press: New York, 1978; Vol. IV, p
20
13
2
elsewhere. Scandium triflate was characterized by C NMR in D O
3
85. (b) Gase, R. A.; Pandit, U. K. J. Am. Chem. Soc. 1979, 101, 7059. (c)
28
Ohno, A.; Yamamoto, H.; Oka, S.; J. Am. Chem. Soc. 1981, 103, 2041.
as compared with the data for Yb(OTf)
,1,1-Trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide, [Sc-
(NTf ], was obtained commercially from Aldrich. The scandium salt
of tetrakis(pentafluorophenyl)borate [Sc[B(C ] was prepared by
3
.
(
d) Ohno, A.; Shio, T.; Yamamoto, H.; Oka, S. J. Am. Chem. Soc. 1981,
1
1
03, 2045. (e) Powell, M. F.; Bruice, T. C. J. Am. Chem. Soc. 1983, 105,
014, 7139.
1
2 3
)
(
12) (a) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Chem. Soc., Perkin Trans.
6 5 4 3
F ) ]
2
1985, 371. (b) Ishikawa, M.; Fukuzumi, S. J. Chem. Soc., Faraday Trans.
990, 86, 3531.
the following procedure. A powder form of ion-exchange resin
(DOWEX 50WX8-200, 0.5 g) obtained from Ardrich was suspended
1
(
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(
(
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F.; Skorianetz, W. HelV. Chim. Acta 1969, 52, 169. (b) Skorianetz, W.;
Kov a¨ ts, E. sz. HelV. Chim. Acta 1972, 55, 1404. (c) Russell, G. A.; Konaka,
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5.
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(
4
c) Itoh, S.; Kawakami, H.; Fukuzumi, S. J. Am. Chem. Soc. 1997, 119,
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(
5
1
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J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002 12567

A R T I C L E S
Fukuzumi et al.
3
in an aqueous solution of Sc(OTf) (1.0 g, 200 mL) and stirred by the
3
+
magnetic stirrer for 1 day. The isolated Sc -exchanged resin was
washed with deionized water and dried under vacuum for one night.
An aqueous solution of lithium tetrakis(pentafluorophenyl)borate diethyl
-
2
ether complex (2.9 × 10 M, 50 mL) obtained from Tokyo Chemical
Industry Co., Ltd. was added dropwise to the glass short column packed
3
+
with the Sc -exchanged resin. The aqueous solution of the scandium
borate salt (Sc[B(C ) trickled down the column and was concen-
trated by the vacuum evaporation followed by drying under vacuum
for one night. The white powder of Sc[B(C with water of
6 5 4 3
F ) ]
6 5 4 3
F ) ]
crystallization was identified by Shimadzu TOF mass spectrometer,
AXIMA-CFR (see Supporting Information S2). Acetonitrile used as a
29
solvent was purified and dried by the standard procedure. Acetonitrile-
and dimethyl sulfoxide-d were obtained from EURI SO-TOP, CEA,
France.
d
3
6
-3
Figure 1. Spectral change in the reaction of AcrH2 (1.7 × 10 M) with
Ph Tz (1.7 × 10-3 M) in the presence of Sc3+ (1.4 × 10-2 M) in deaerated
2
Electrochemical Measurements. Electrochemical measurements
MeCN at 298 K (time interval, 400 s).
were performed on a BAS 100B electrochemical analyzer in deaerated
-
2
(
1.4 × 10 M), hydride transfer from AcrH2 to Ph2Tz occurs
+
-
MeCN containing 0.10 M n-Bu
4
N ClO
4
(TBAP) as a supporting
+
efficiently to yield 10-methylacridinium ion (AcrH ) and
dihydrotetrazine (Ph2TzH2) in MeCN at 298 K (eq 1, for the
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
were recorded with respect to an Ag/AgNO
3
(0.01 M) reference
electrode. The second-harmonic alternating current voltammetry
3
0
(
2 3
SHACV) measurements of Ph Tz in the presence of Sc(OTf) were
carried out with a BAS 100B electrochemical analyzer in deaerated
MeCN containing 0.10 M TBAP as a supporting electrolyte at 298 K.
0
The E red values (vs Ag/AgNO
3
) are converted into those vs SCE by
3
1
addition of 0.29 V.
ESR Measurements. Ph
M in 1.0 mL) and purged with argon for 10 min. Sc(OTf)
M in 1.0 mL) and Sc[B(C
dissolved in deaerated MeCN. The Ph
Tz solution (200 µL) and Sc3
-
3
2
2
Tz was dissolved in MeCN (1.0 × 10
-
3
(8.8 × 10
-
1
6
F
5
)
4
]
3
(2.0 × 10 M in 1.0 mL) were
+
2
solution (200 µL) were introduced into the ESR cell (1.8 mm i.d.)
-
2
containing (BNA)
2
(2.0 × 10 M) and mixed by bubbling with Ar
2
gas through a syringe with a long needle. The ESR spectra of the Ph
Tz -2Sc complex were recorded on a JEOL JES-RE1XE spec-
-
product analysis, see Experimental Section).3
4-37
As the reaction
•
-
3+
proceeds, the absorption band due to Ph2Tz (λmax ) 540 nm)
trometer under irradiation of a high-pressure mercury lamp (USH-
decreases and this is accompanied by the appearance of a new
1
005D) focusing at the sample cell in the ESR cavity at 298 K. The
+
•
-
absorption band (λmax ) 358 nm) due to the formation of AcrH
2
ESR spectrum of Ph Tz was also measured in the absence of Sc-
3
+
(
OTf)
3
under otherwise the same experimental conditions. The mag-
(Figure 1). In the absence of Sc , no reaction has occurred
nitude of modulation was chosen to optimize the resolution and signal-
to-noise (S/N) ratio of the observed spectra under nonsaturating
microwave power conditions. The g values were calibrated using an
between AcrH and Ph Tz.
The spectral titration in which the absorbance due to Ph Tz
at λ ) 540 nm is plotted against [AcrH2]/[Ph2Tz]0 (see
2
2
2
2
+
Mn marker.
Theoretical Calculations. Density functional calculations were
performed on a COMPAQ DS20E computer using the spin-restricted
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
32
B3LYP functional for the open shell molecule. The B3LYP geometry
•
-
+
of Ph Tz -2H was determined using the 6-31G(3d,3p) basis set and
2
3
3
2
the Gaussian 98 program. The 〈S 〉 value was determined as 0.761,
indicating a good representation of the doublet state.
Results and Discussion
3+
Sc -Promoted Hydride Transfer from AcrH2 to Ph2Tz.
98 (ReVision A.7) Gaussian, Inc.; Pittsburgh, PA, 1998.
-
3
1
Upon addition of AcrH2 (1.7 × 10 M) to a deaerated MeCN
solution of Ph2Tz (1.7 × 10 M) in the presence of Sc(OTf)3
2 2
(34) Formation of Ph TzH was confirmed by comparing the H NMR data (see
-
3
Experimental Section) with the reported values. See: Nagy, J.; Nyitrai, J.;
Kolonits, P.; Lempert, K.; Gergely, A.; P a´ rk a´ nyi, L.; K a´ lm a´ n, A. J. Chem.
Soc., Perkin Trans. 1 1988, 3267. Three positional isomers (1,4-, 1,2-, and
3,6-isomers) are expected to be formed as dihydro-3,6-diphenyl-1,2,4,5-
(
29) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals;
Butterworth-Heinemann: Oxford, 1988.
1
tetrazine on the basis of the H NMR data (see Experimental Section);
(
30) The SHACV method provides a superior approach to directly 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,
however, the 1,4-isomer (eqs 1 and 7) is the most likely candidate, since
it was calculated to be thermodynamically more stable than the 1,2- and
3,6-isomer by 6.7 and 28.0 kcal mol- , respectively at the B3LYP/6-31G*
level.
1
1
423. (b) Bond, A. M.; Smith, D. E. Anal. Chem. 1974, 46, 1946. (d)
(35) The proton required for formation of Ph
in MeCN. The acidity of water in the presence of Sc(OTf)
increases as compared with that in the absence of Sc(OTf)
3
2
TzH
2
comes from water contained
in acetonitrile-
, as indicated
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
d
3
1
by the low-field shift of H NMR signal due to water.
(36) Since AcrH is known to be oxidized by oxygen in the presence of acids
2
3
7
(
31) Mann, C. K.; Barnes, K. K. In Electrochemical Reaction in Nonaqueous
Systems; Marcel Dekker: New York, 1970.
in MeCN, oxygen was excluded in the reaction procedure.
(37) Fukuzumi, S.; Chiba, M.; Ishikawa, M.; Ishikawa, K.; Tanaka, T. J. Chem.
Soc., Perkin Trans 2 1989, 1417.
(
32) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
12568 J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002

Scandium Ion-Promoted Reduction of Heterocyclic NdN
A R T I C L E S
Figure 2. Plots of kobs vs [Sc(OTf)3] for reduction of Ph2Tz by (a) AcrH2
-
4
-4
(
O, 2.5 × 10 M) and (b) AcrD2 (0, 2.0 × 10 M) in the presence of
Sc(OTf)3 in deaerated MeCN at 298 K.
Figure 3. Plots of ket vs [Sc(OTf)3] for electron transfer from (a) CoTPP-
-
6
-5
(
O, 2.1 × 10 M) and (b) (AcrH)2 (0, 8.4 × 10 M) to Ph2Tz in the
Supporting Information S3) confirms the stoichiometry in eq
presence of Sc(OTf) , and (c) for electron transfer from CoTPP (4, 2.1 ×
3
-
6
1
0
M) to Ph2Tz in the presence of Sc[B(C6F5)4]3 in MeCN at 298 K.
1
1
, where 1 equiv of AcrH2 reacts with 1 equiv of Ph2Tz to yield
+
equiv of AcrH and Ph2TzH2.
dependence of the observed electron-transfer rate constant (ket)
3
+
The rates of Sc -promoted hydride transfer reaction were
determined by monitoring the appearance of absorbance due to
AcrH (λmax ) 358 nm, ꢀmax ) 1.9 × 10 M cm ). The
rates obeyed pseudo-first-order kinetics in the presence of large
excess Ph2Tz and Sc3 relative to the concentration of AcrH2
3
+
on [Sc ] was also examined for electron transfer from CoTPP
3+
to Ph2Tz at various concentrations of Sc . The results are quite
different from those of the hydride transfer reaction, as shown
+
4
-1
-1
3+
in Figure 3a, where the ket value increases with increasing [Sc ]
+
to exhibit a mixture of a first-order and second-order dependence
on [Sc ] (vide infra).
(
see Supporting Information S5). The pseudo-first-order rate
constant (k1) increases proportionally with Ph2Tz concentration
see Supporting Information S5). Thus, the rate exhibits a
3
+
It was confirmed that there was no interaction between Ph2-
(
3+
Tz and Sc , as indicated by the lack of spectral change of Ph2-
second-order kinetics showing a first-order dependence on each
reactant concentration.
3
+ 38
Tz in the presence of Sc . In such a case, the acceleration of
electron transfer from CoTPP to Ph2Tz is ascribed to the
The dependence of the observed second-order rate constant
3
+
•-
complexation of Sc with Ph2Tz . The contribution of the
3
+
3+
(
kobs) on [Sc ] was examined for the Sc -promoted hydride
3
+
second-order dependence of ket on [Sc ] in Figure 2a suggests
that Ph2Tz may form not only a 1:1 complex (n ) 1 in eq 2)
3+
transfer reaction at various concentrations of Sc . The kobs value
•
-
3+
increases with an increase in [Sc ] to approach a limited value,
as shown in Figure 2a. When AcrH2 is replaced by the
dideuterated compound (AcrD2), a large kinetic primary isotope
effect is observed as shown in Figure 2b.
Sc3+-Promoted Electron-Transfer Reduction of Ph2Tz.
When AcrH2 is replaced by a one-electron reductant such as
3
+
but also a 1:2 complex (n ) 2 in eq 2) with Sc . The complex
•
-
3+
formation of Ph2Tz and Sc should result in the positive shift
of the one-electron reduction potential of Ph2Tz (Ered), and the
Nernst equation may be given by eq 3,
0
3+
3+
Ered ) E + (2.3RT/F) log K [Sc ](1 + K [Sc ]) (3)
red
1
2
2
-
CoTPP (TPP
) tetraphenylporphyrin dianion), electron
transfer from CoTPP to Ph2Tz also occurs efficiently in the
presence of Sc(OTf)3 in MeCN (eq 2), whereas no electron
0
where E red is the one-electron reduction potential of Ph2Tz in
3+
the absence of Sc , and K1 and K2 are the formation constants
•
-
3+
for the 1:1 and 1:2 complexes between Ph2Tz and Sc ,
3+
respectively. Since Sc has no effect on the oxidation potential
of CoTPP, the free energy change of electron transfer from
CoTPP to Ph2Tz in the presence of Sc (∆Get) can be expressed
3+
by eq 4,
0
3+
3+ 2
∆
G ) ∆G - (2.3RT) log(K [Sc ] + K K [Sc ] ) (4)
et et 1 1 2
0
3+
where ∆G et is the free energy change in the absence of Sc .
Thus, electron transfer from CoTPP to Ph2Tz becomes more
3+
favorable energetically with an increase in concentration of Sc .
If such a change in the energetics is directly reflected in the
3
+
(
38) No complex formation between Ph Tz and Sc in acetonitrile was
2
1 13 1
confirmed by the H and C NMR titration. H NMR (CD
absence of Sc(OTf) : δ 8.60 (dd, J ) 8.0, 1.5 Hz, 4H), 7.68 (m, 6H). H
NMR (CD CN) in the presence of 0.38 M Sc(OTf) : δ 8.60 (dd, J ) 8.0,
1.5 Hz, 4H), 7.68 (m, 6H). C NMR (CD CN) in the absence of
Sc(OTf) : δ 128.56 (s), 128.75 (s), 130.11 (s), 130.61 (s), 164.97 (s).
NMR (CD CN) in the presence of 0.07 M Sc(OTf) : δ 128.56 (s), 128.75
(s), 130.11 (s), 130.61 (s), 164.97 (s).
3
CN) in the
1
3
3
3
1
3
transfer from CoTPP to Ph2Tz occurred without Sc(OTf)3. The
electron-transfer rates obeyed a second-order kinetics, showing
a first-order dependence on each reactant concentration. The
3
1
3
3
C
3
3
J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002 12569

A R T I C L E S
Fukuzumi et al.
Table 1. Rate Constants (k0K1) of Sc3+-Promoted Electron
Scheme 1
Transfer from CoTPP and (AcrH)2 to Ph2Tz and the Formation
•
-
3+
Constants (K2) of the Ph2Tz -2Sc Complexes in Deaerated
MeCN at 298 K
electron donor
k K
0 1
,a M-2 s-1
K
2
,a M-1
5
3
CoTPP
9.4 × 10
1.8 × 10
8.2 × 10
8.5 × 10
(AcrH)2
a
Determined from the dependence of kobs on [Sc3+] based on eq 6. The
experimental error is 10%.
-
3
as -0.91 V (vs SCE). The addition of Sc(OTf)3 (4.1 × 10
M) to an MeCN solution of Ph2Tz results in a largely positive
shift of the one-electron reduction potential (+1.04 V), as shown
in Figure 4b, where the second harmonic ac voltammogram
39,40
affords the one-electron reduction potential (+0.13 V).
•-
3+
Detection of the Ph2Tz -2Sc Complex. The 1:2 complex
•
-
3+
formation between Ph2Tz and Sc has been confirmed by
•
-
3+
the observation of the ESR spectrum of the Ph2Tz -2Sc
•
-
complex, which is compared with that of Ph2Tz (vide infra).
The Ph2Tz was produced by photoinduced electron transfer
from dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2] to
Ph2Tz in MeCN at 233 K (Scheme 1). The (BNA)2 is known
to act as a unique electron donor to produce the radical anions
•
-
41
Figure 4. (a) Cyclic voltammogram of Ph2Tz (1.0 × 10-3 M) in deaerated
MeCN containing 0.10 M TBAP with Pt electrode at 298 K and (b) SHACV
4
2
•-
of electron acceptors. The ESR spectrum of Ph2Tz thus
obtained under photoirradiation exhibits nine hyperfine lines
as shown in Figure 5a, indicating that four nitrogens are
equivalent [a(4N) ) 5.1 G]. The addition of Sc(OTf)3 (4.4 ×
-
3
-3
of Ph2Tz (1.0 × 10 M) in the presence of Sc(OTf)3 (4.1 × 10 M) in
deaerated MeCN containing 0.10 M TBAP with Pt electrode at 298 K.
transition state of electron transfer, the dependence of the
-2
3
+
10 M) to the (BNA)2-Ph2Tz system results in a drastic change
observed rate constant of electron transfer (ket) on [Sc ] is
•
-
in the hyperfine pattern of Ph2Tz due to the complexation
derived from eq 4, as given by eq 5,
3
+
with Sc (Scheme 1), as shown in Figure 5b. The ESR
spectrum is well-reproduced by the computer simulation spec-
trum with the hfc values of a(2N) ) 6.6, 5.0 G and a(6H) )
5.0 G (Figure 5c). The four equivalent nitrogens of Ph2Tz•
are split into two sets of two equivalent nitrogens due to the
3+
3+ 2
k ) k K ([Sc ] + K [Sc ] )
(5)
et
0
1
2
-
3+
where k0 is the rate constant in the absence of Sc . From eq 5
is derived eq 6,
3+
complexation with two Sc ions. The absence of superhyperfine
lines due to the scandium nucleus indicates that the interaction
between Ph2Tz and two Sc ions is largely electrostatic rather
3+
3+
k /[Sc ] ) k K (1 + K [Sc ])
(6)
et
0
1
2
•-
3+
•-
3+
3+
than covalent. The complexation of Ph2Tz with two Sc ions
the validity of which is confirmed by the linear plot of ket/[Sc ]
3
+
3+
vs [Sc ] for the Sc -promoted electron transfer from CoTPP
to Ph2Tz (see Supporting Information S6). From the slope and
intercept is obtained the K2 value which is listed in Table 1
together with the k0K1 value. The solid lines in Figure 3 are
drawn on the basis of eq 5 with these parameters.
(
39) The cyclic voltammogram of Ph
irreversible cathodic wave due to the instability of the Ph
complex. Although the one-electron reduction potential determined by
2
Tz in the presence of Sc(OTf)
3
shows an
•-
3+
2
Tz -Sc
3
0
•-
SHACV may involve uncertainty due to the instability of the Ph
2
Tz
-
3+
Sc complex, the observed large positive shift as compared to that in the
3
+
absence of Sc is unmistakable.
(
40) An even larger positive shift (+2.02 V) of the reversible one-electron
reduction potential of a naphthoquinone derivative, determined by CV, has
been reported to be caused by the complexation of the radical anion with
The positive shift of Ered caused by the complexation of
Ph2Tz• with Sc was verified by the electrochemical measure-
-
3+
3
+
Sc (7.0 mM) in MeCN. See: Fukuzumi, S.; Okamoto, K.; Imahori, H.
Angew. Chem., Int. Ed. 2002, 41, 620. For other examples, see ref 14.
41) Patz, M.; Kuwahara, Y.; Suenobu, T.; Fukuzumi, S. Chem. Lett. 1997, 567.
42) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.; Fujitsuka, M.;
Ito, O. J. Am. Chem. Soc. 1998, 120, 8060.
ments. The cyclic voltammogram of Ph2Tz exhibits a reversible
redox wave, as shown in Figure 4a. The one-electron reduction
potential of Ph2Tz is determined from the half-wave potential
(
(
12570 J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002

Scandium Ion-Promoted Reduction of Heterocyclic NdN
A R T I C L E S
Scheme 2
titration (see Supporting Information S8) confirms the stoichi-
ometry in eq 7, where 1 equiv of (AcrH)2 reacts with 1 equiv
+
of Ph2Tz to yield 2 equiv of AcrH and Ph2TzH2. In this case,
3+
the initial Sc -promoted electron transfer from (AcrH)2 to Ph2-
+
Tz is followed by the facile C-C bond cleavage to give AcrH
•
•
0
and AcrH (Scheme 2). Since AcrH (E ox vs SCE ) -0.46
V) is much stronger reductant than (AcrH)2 (E ox vs SCE )
4
e
0
4
3
•
Figure 5. ESR spectra of an MeCN solution containing (BNA)2 (2.0 ×
0.62 V), the rapid electron transfer from AcrH to the Ph2-
-
2
-4
•-
3+
+
1
0
M) and Ph2Tz (5.0 × 10 M) in the absence (a) and presence (b) of
Tz -2Sc complex occurs to yield the final products, AcrH
-
2
Sc(OTf)3 (4.4 × 10 M) under irradiation with a high-pressure mercury
3+
+
35
and Ph2TzH2 after replacement of Sc by H (Scheme 2).
lamp at 233 K. (c) Computer-simulated spectrum for condition b with g )
1
2
3+
2
0
.0041, a(2N ) ) 6.6 G, a(2N ) ) 5.0 G, a(6H) ) 5.0 G, and ∆Hmsl )
The rates of Sc -promoted electron transfer from (AcrH)2
to Ph2Tz were determined by monitoring the appearance of
.40 G.
+
absorbance due to AcrH as in the case of the hydride transfer
also results in the structural change such that the phenyl group
becomes coplanar with the heterocycle ring when the hyperfine
lines due to six equivalent protons appear due to the spin
from AcrH2 to Ph2Tz. However, the dependence of the observed
second-order rate constant (ket) on [Sc ] is different from that
for the Sc -promoted hydride transfer reaction in Figure 2a,
but it is virtually the same as that for the Sc -promoted electron
transfer from CoTPP to Ph Tz (Figure 3a): the k value
increases linearly with [Sc ] to show a first-order dependence
3
+
3
+
3+
delocalization on the phenyl group in the Sc complex. Such
3+
coplanarity in the Sc3 complex is supported by the B3LYP
+
•
-
+
theoretical calculation of the protonated species Ph2Tz -2H
2
et
3
+
(see Supporting Information S7).
3
+
on [Sc ] at low concentrations, changing to a second-order
dependence at high concentrations (Figure 3b). The same linear
Electron Transfer vs Hydride Transfer. The acridine dimer,
0,10′-dimethyl-9,9′-biacridine [(AcrH)2], is known to act as
1
3
+
3+
3+
an electron donor, in contrast with the case of the monomer,
plot of ket/[Sc ] vs [Sc ] is applied for the Sc -promoted
electron transfer from (AcrH)2 to Ph2Tz to afford the K2 value
AcrH2, which is a hydride donor.4
3,44
Electron transfer from
-
1
(
AcrH)2 to Ph2Tz occurs efficiently in the presence of Sc(OTf)3
as 85 M (Figure 4b). This value agrees with the K2 value (82
+
-1
3+
in MeCN at 298 K to yield AcrH and Ph2TzH2 (eq 7), which
are the same as the products of the Sc -promoted hydride
transfer reaction from AcrH2 to Ph2Tz (eq 1).35 The spectral
M ) derived from the Sc -promoted electron transfer from
CoTPP to Ph2Tz (Figure 4a). Such an agreement indicates that
the rate-determining step for the two-electron reduction of Ph2-
3+
3
+
Tz by (AcrH)2 is indeed the Sc -promoted electron transfer
from (AcrH)2 to Ph2Tz (Scheme 2).
3+
The saturated dependence of kobs with respect to [Sc ] for
3+
the Sc -promoted hydride transfer from AcrH2 to Ph2Tz (Figure
a) normally results from the complex formation between Ph2-
2
3
+
Tz and Sc , which enhances the electrophilicity of Ph2Tz to
accelerate the hydride transfer reaction. However, as noted
3+
earlier, there was no interaction between Ph2Tz and Sc . Only
•
-
the one-electron reduced species, i.e., Ph2Tz , can form the
3+
complex with Sc (vide supra). In such a case, the dependence
(
43) Fukuzumi, S.; Tokuda, Y. J. Phys. Chem. 1992, 96, 8409.
44) The acridine dimer [(AcrH) ] is employed instead of (BNA)
(
2
2
, since (AcrH)
2
]
3
+
has no interaction with Sc , whereas the carbonyl oxygen of [(BNA)
2
3
+ 20
forms a complex with Sc
of (AcrH) is similar to that of (BNA)
of (AcrH)
.
The electron-transfer oxidation mechanism
2
2
, although the oxidation potentials
•
• 42,43
2
and AcrH are much higher than those of (BNA)
2
and BNA .
J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002 12571

A R T I C L E S
Fukuzumi et al.
Scheme 3
3+
of kobs on [Sc ] and the observed primary kinetic isotope effects
in Figure 2 can be best explained by Scheme 3 (vide infra).
Electron transfer from AcrH2 to Ph2Tz is highly endergonic,
0
4e
0
judging from the E ox value of AcrH2 (0.81 V) and the E red
value of Ph2Tz (-0.91 V), and no reaction occurs between
Figure 6. Plots of kobs vs (a) [Sc(NTf2)3] and (b) [Sc[B(C6F5)4]3] for the
reaction of Ph Tz with AcrH (a: 2.5 × 10-4 M, b: 4.9 × 10-6 M) in the
-
3
AcrH2 and Ph2Tz. In the presence of Sc(OTf)3 (4.1 × 10 M),
however, the reduction potential of Ph2Tz is shifted to +0.13
V when electron transfer from AcrH2 to Ph2Tz becomes much
less endergonic as compared with electron transfer without
Sc(OTf)3. Although the free energy change of electron transfer
is still positive, electron transfer is followed by proton transfer
2
2
presence of Sc(NTf2)3 and Sc[B(C6F5)4]3 in deaerated MeCN at 298 K.
for AcrH2 and k-1/k1kD and k2/k1kD values for AcrD2. The solid
lines in Figure 2 are drawn on the basis of eq 10 with these
parameters. The kH/kD value obtained from the ratio of the slopes
(
5.0) agrees with that from the intercepts (5.4).
Effects of Counteranions. The Lewis acidity of Sc is
•
+
•-
3+
from AcrH2 to the Ph2Tz -Sc complex and the subsequent
3+
•
•
electron transfer from AcrH to Ph2TzH may be highly
exergonic to yield the final products, AcrH and Ph2TzH2
expected to be regulated by counterions. When Sc(OTf)3 is
replaced by the weaker Lewis acid, Sc(NTf2)3, the k2 value is
expected to be smaller, but the k-1 value may be larger than
+
(Scheme 3). The overall reaction is exergonic and the hydride
transfer reaction occurs irreversibly when the observed second-
order rate constant (kobs) may be given by eq 8,
3+
the value of Sc(OTf)3. In such a case, k-1 . k2[Sc ] when eq
1
0 is reduced to eq 8. This expectation is confirmed as shown
in Figure 6a, where the kobs value of the Sc(NTf2)3-promoted
hydride transfer from AcrH2 to Ph2Tz increases linearly with
3+
kobs ) k (k /k )[Sc ]
(8)
H
1
-1
[
Sc(NTf2)3] up to large concentrations, where the kobs value of
3
+
which predicts that the kobs value increases linearly with [Sc ].
the Sc(OTf)3-promoted reaction exhibits the saturated depen-
dence (Figure 2). Conversely, when Sc(OTf)3 is replaced by a
stronger Lewis acid, Sc[B(C6F5)4]3, the kobs value reaches a
constant value at the much smaller concentration, as shown in
Figure 6b, as compared to the kobs value of the Sc(OTf)3-
promoted reaction. The use of such a bulky counteranion
B(C6F5)4 results in the weaker interaction between Sc and
the counteranion, thereby increasing the Lewis acidity of Sc .
The weak nucleophilicity of B(C6F5)4 has been well-
documented.
The stronger Lewis acidity of Sc[B(C6F5)4]3 than Sc(OTf)3
is confirmed by the much higher reactivity in the Sc[B(C F ) ] -
promoted electron transfer from CoTPP to Ph2Tz, as shown in
Figure 3c. The ESR spectrum of the Ph Tz -2Sc[B(C F ) ]
complex is observed under photoirradiation of the (BNA)2-
Ph Tz system in the presence of Sc[B(C F ) ] (0.1 M) in MeCN
at 233 K, as shown in Figure 7a, which is different from the
3+
•-
3+
At high concentrations of Sc , however, the Ph2Tz -Sc
complex is converted to the 1:2 complex Ph2Tz -2Sc , the
basicity of which is reduced significantly as compared to the
:1 complex, as shown in Scheme 3. In such a case, proton
transfer from AcrH2 to the Ph2Tz -2Sc complex is retarded
when the overall rate would reach a limited value. Under such
experimental conditions, kobs is given by eq 9,
•
-
3+
1
•+
•-
3+
-
3+
3+
-
3+
3+
kobs ) k k [Sc ]/(k + k + k [Sc ])
(9)
1
H
-1
H
2
4
5
which predicts that the kobs value exhibits a saturated dependence
with respect to [Sc ]. According to eq 9, there would be no
primary kinetic isotope effect under the conditions such that kH
3+
6 5 4 3
•
-
3+
3+
.
k-1 + k2[Sc ] when kobs ) k1[Sc ]. On the other hand, the
2
6 5 4 3
primary kinetic isotope effect would be the same independent
3+
3+
of [Sc ] under the conditions such that kH , k-1 + k2[Sc ].
The latter case is consistent with the experimental observation
in Figure 2. Under such conditions, eq 9 is reduced to eq 10,
2
6 5 4 3
•-
spectrum of the Ph2Tz -2Sc(OTf)3 complex in Figure 5b. The
ESR spectrum is well-reproduced by the computer simulation
spectrum with the hfc values of a(2N) ) 6.1, 4.7 G and a(6H)
3+
3+
kobs ) k k [Sc ]/(k + k [Sc ])
(10)
1
H
-1
2
)
4.7 G. These hfc values are smaller than those of the Ph2-
3
+
•-
which agrees with the saturated dependence of kobs on [Sc ]
Tz -2Sc(OTf) complex, since the stronger interaction between
3
3
+
•-
in Figure 2. Such a saturated dependence of kobs on [Sc ] can
Ph Tz and Sc[B(C F ) ] results in more delocalization of spin
to the carbon atoms.
In conclusion, Sc can promote both electron transfer from
CoTPP and (AcrH)2 to Ph2Tz and hydride transfer from AcrH2
2
6 5 4 3
-
1
3+ -1
be expressed by a double-reciprocal plot of kobs on [Sc ]
3
+
according to eq 11.
kobs- ) (k /k k )[Sc ] + (k /k k )
1
3+ -1
(11)
-1
1
H
2
1 H
(
45) (a) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.;
Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223. (b) Jia, L.; Yang, X.-
M.; Marks, T. J. Organometallics 1997, 16, 842. (c) Ewart, S. W.; Sarsfield,
M. J.; Williams, E. F.; Baird, M. C. J. Organomet. Chem. 1999, 579, 106.
From the slope and intercept of the linear plots (See Supporting
Information S9) are obtained the k-1/k1kH and k2/k1kH values
12572 J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002

Scandium Ion-Promoted Reduction of Heterocyclic NdN
A R T I C L E S
proton transfer and electron transfer. The overall reactivity is
regulated by the initial electron transfer and the subsequent
proton-transfer processes, in which the formation of a 1:2
•
-
3+
complex between Ph2Tz and Sc accelerates the electron
transfer but decelerates the proton-transfer process. Such ac-
3+
celerating and decelerating effects of Sc are controlled by the
3
+
counterions, which alter the Lewis acidity of Sc . Thus, the
3+
balance between the accelerating effects of Sc on the electron-
transfer step and the decelerating effects on the subsequent
proton-transfer step is of importance to control the overall
reactivity of the hydride transfer reaction of NADH analogues.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (Nos. 11228205
and 13031059) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting Information Available: Supporting Experimental
Section (S1), TOF/MS spectrum for Sc[B(C6F5)4]3‚12H2O (S2),
UV-vis spectral change in the reaction between Ph2Tz and
AcrH2 (S3), time course of the absorption change and the first-
order plot (S4), plots of pseudo-first-order rate constant vs [Ph2-
Tz] (S5), plots of ket/[Sc(OTf)3] vs [Sc(OTf)3] (S6), optimized
Figure 7. (a) ESR spectrum of an MeCN solution containing (BNA)2 (2.0
•-
+
structure of Ph2Tz -2H (S7), UV-vis spectral change in the
-
2
-4
×
10 M) and Ph2Tz (5.0 × 10 M) in the presence of Sc[B(C6F5)4]3
-
1
-1
reaction between Ph2Tz and (AcrH)2 (S8), and plots of kobs
(
1.0 × 10 M) under irradiation with a high-pressure mercury lamp at
1
-1
2
33 K. (b) Computer-simulated spectrum with g ) 2.0039, a(2N ) ) 6.1
vs [Ph2Tz] (S9). This material is available free of charge via
the Internet at http://pubs.acs.org.
2
G, a(2N ) ) 4.7 G, a(6H) ) 4.7 G, and ∆Hmsl ) 0.40 G.
3
+
to Ph2Tz. The hydride transfer reaction proceeds via the Sc -
promoted electron transfer from AcrH2 to Ph2Tz, followed by
JA026592Y
J. AM. CHEM. SOC.
9
VOL. 124, NO. 42, 2002 12573