Hydride transfer from 9-substituted 10-methyl-9,10-dihydroacridines to hydride acceptors via charge-transfer complexes and sequential electron- proton-electron transfer. A negative temperature dependence of the rates

Article abstract of DOI:10.1021/ja9941375

The reactivity of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) in the reactions with hydride acceptors (A) such as p-benzoquinone derivatives and tetracyanoethylene (TCNE) in acetonitrile varies significantly spanning a range of 10⁷ starting from R = H to Bu(t) and CMe₂COOMe. Comparison of the large variation in the reactivity of the hydride transfer reaction with that of the deprotonation of the radical cation (AcrHR·⁺) determined independently indicates that the large variation in the reactivity is attributed mainly to that of proton transfer from AcrHR·⁺ to A·^- following the initial electron transfer from AcrHR to A. The overall hydride transfer reaction from AcrHR to A therefore proceeds via sequential electron-proton-electron transfer in which the initial electron transfer to give the radical ion pair (AcrHR·⁺ A·^-) is in equilibrium and the proton transfer from AcrHR·⁺ to A·^- is the rate- determining step. Charge-transfer complexes are shown to be formed in the course of the hydride transfer reactions from AcrHR to p-benzoquinone derivatives. A negative temperature dependence was observed for the rates of hydride transfer reactions from AcrHR (R = H, Me, and CH₂Ph) to 2,3- dichloro-5,6-dicyano-p-benzoquinone (DDQ) in chloroform (the lower the temperature, the faster the rate) to afford the negative activation enthalpy (ΔH((+))(obs) = -32, -4, and -13 kJ mol^-1, respectively). Such a negative ΔH((+))(obs) value indicates clearly that the CT complex lies along the reaction pathway of the hydride transfer reaction via sequential electron- proton-electron transfer and does not enter merely through a side reaction that is indifferent to the hydride transfer reaction. The ΔH((+))(obs) value increases with increasing solvent polarity from a negative value (-13 kJ mol^-1) in chloroform to a positive value (13 kJ mol^-1) in benzonitrile as the proton-transfer rate from AcrHR·⁺ to DDQ·^- may be slower.

Full text of DOI:10.1021/ja9941375

4286
J. Am. Chem. Soc. 2000, 122, 4286-4294
Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines
to Hydride Acceptors via Charge-Transfer Complexes and Sequential
Electron-Proton-Electron Transfer. A Negative Temperature
Dependence of the Rates
Shunichi Fukuzumi,* Kei Ohkubo, Yoshihiro Tokuda, and Tomoyoshi Suenobu
Contribution from the Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan
ReceiVed NoVember 29, 1999
Abstract: The reactivity of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) in the reactions with hydride
acceptors (A) such as p-benzoquinone derivatives and tetracyanoethylene (TCNE) in acetonitrile varies
7
t
significantly spanning a range of 10 starting from R ) H to Bu and CMe2COOMe. Comparison of the large
variation in the reactivity of the hydride transfer reaction with that of the deprotonation of the radical cation
•
+
(
AcrHR ) determined independently indicates that the large variation in the reactivity is attributed mainly to
•
+
•-
that of proton transfer from AcrHR to A following the initial electron transfer from AcrHR to A. The
overall hydride transfer reaction from AcrHR to A therefore proceeds via sequential electron-proton-electron
•
+
•-
transfer in which the initial electron transfer to give the radical ion pair (AcrHR A ) is in equilibrium and
•
+
•-
the proton transfer from AcrHR to A is the rate-determining step. Charge-transfer complexes are shown
to be formed in the course of the hydride transfer reactions from AcrHR to p-benzoquinone derivatives. A
negative temperature dependence was observed for the rates of hydride transfer reactions from AcrHR (R )
H, Me, and CH2Ph) to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in chloroform (the lower the
q
temperature, the faster the rate) to afford the negative activation enthalpy (∆H obs ) -32, -4, and -13 kJ
-
1
q
mol , respectively). Such a negative ∆H obs value indicates clearly that the CT complex lies along the reaction
pathway of the hydride transfer reaction via sequential electron-proton-electron transfer and does not enter
q
merely through a side reaction that is indifferent to the hydride transfer reaction. The ∆H obs value increases
with increasing solvent polarity from a negative value (-13 kJ mol ) in chloroform to a positive value (13
kJ mol ) in benzonitrile as the proton-transfer rate from AcrHR to DDQ may be slower.
-
1
-
1
•+
•-
Introduction
discussed concerning two main possibilities, i.e., concerted
hydride transfer or sequential electron-proton-electron (equiva-
lent to a hydride ion) transfer. Since both processes involve the
formation of a formal positive charge in the transition state, it
has been difficult to differentiate between the mechanisms based
on the classical approach of electronic and substitution effects.1
We have previously reported that the distinction between the
Dihydronicotinamide adenine dinucleotide (NADH) and
analogues act as the source of two electrons and a proton, thus
formally transferring a hydride ion to a suitable substrate. The
1
mechanism of the hydride transfer has so far been extensively
studied by using NADH analogues in the reactions with various
3-15
2
-12
substrates.
In these investigations, the mechanism has been
(
7.
1) Stryer, L. Biochemistry, 3rd ed; Freeman: New York, 1988; Chapter
(9) Ohno, A.; Ishikawa, Y.; Yamazaki, N.; Okamura, M.; Kawai, Y. J.
Am. Chem. Soc. 1998, 120, 1186.
1
(
2) (a) Eisner, U.; Kuthan, J. Chem. ReV. 1972, 72, 1. (b) Stout, D. M.;
(10) (a) Pestovsky, O.; Bakac, A.; Espenson, J. H. J. Am. Chem. Soc.
1998, 120, 13422. (b) Pestovsky, O.; Bakac, A.; Espenson, J. H. Inorg.
Chem. 1998, 37, 1616.
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10224.
(12) (a) Kreevoy, M. M.; Ostovic, D.; Lee, I.-S. H.; Binder, D. A.; King,
G. W. J. Am. Chem. Soc. 1988, 110, 524. (b) Lee, I.-S. H.; Jeoung, E. H.;
Kreevoy, M. M. J. Am. Chem. Soc. 1997, 119, 2722.
(13) (a) Ohno, A.; Yamamoto, H.; Oka, S. J. Am. Chem. Soc. 1981, 103,
2041. (b) Ohno, A.; Shio, T.; Yamamoto, H.; Oka, S. J. Am. Chem. Soc.
1981, 103, 2045.
Meyer, A. I. Chem. ReV. 1982, 82, 223.
(
3) Fukuzumi, S.; Tanaka, T. Photoinduced Electron Transfer; Fox, M.
A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chapter 10.
4) (a) Sund, H. Pyridine-Nucleotide Dependent Dehydrogenase; Walter
de Gruyer: West Berlin, 1977. (b) Kellog, R. M. Top. Curr. Chem. 1982,
01, 111. Kellog, R. M. Angew. Chem. 1984, 96, 769. (c) Ohno, A.; Ushida,
(
1
S. Lecture Notes in Bioorganic Chemistry. Mechanistic Models of Asym-
metric Reductions; Springer-Verlag: Berlin, 1986; p 105. (d) Bunting, J.
W. Bioorg. Chem. 1991, 19, 456. (e) He, G.-X.; Blasko, A.; Bruice, T. C.
Bioorg. Chem. 1993, 21, 423. (f) Ohno, A. J. Phys. Org. Chem. 1995, 8,
5
67.
5) Fukuzumi, S. AdVances in Electron-Transfer Chemistry; Mariano,
P. S., Ed.; JAI Press; Greenwich, CT, 1992; pp 67-175.
6) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305 and references therein.
7) (a) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin
(14) (a) Powell, M. F.; Bruice, T. C. J. Am. Chem. Soc. 1983, 105, 1014,
7139. (b) Chipman, D. M.; Yaniv, R.; van Eikeren, P. J. Am. Chem. Soc.
1980, 102, 3244.
(15) (a) Carlson, B. W.; Miller, L. L. J. Am. Chem. Soc. 1985, 107, 479.
(b) Miller, L. L.; Valentine, J. R. J. Am. Chem. Soc. 1988, 110, 3982. (c)
Colter, A. K.; Plank, P.; Bergsma, J. P.; Lahti, R.; Quesnel, A. A.; Parsons,
A. G. Can. J. Chem. 1984, 62, 1780. (d) Verhoeven, J. W.; van Gerresheim,
W.; Martens, F. M.; van der Kerk, S. M. Tetrahedron 1986, 42, 975. (e)
Coleman, C. A.; Rose, J. G.; Murray, C. J. J. Am. Chem. Soc. 1992, 114,
9755.
(
(
(
Trans. 2 1989, 1811. (b) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J. Am.
Chem. Soc. 1989, 111, 1497. (c) Ishikawa, M.; Fukuzumi, S. J. Chem. Soc.,
Chem. Commun. 1990, 1353.
(8) Cheng, J.-P.; Lu, Y.; Zhu, X.; Mu, L. J. Org. Chem. 1998, 63, 6108.
1
0.1021/ja9941375 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/21/2000

ReactiVity of Substituted 10-Methyl-9,10-dihydroacridines
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4287
q
two mechanisms can be made by comparing the reactivities of
different types of NADH analogues which have different donor
abilities in the initial and second electron transfer in the
> 0) in eq 1, i.e., -∆HCT > ∆H1 . However, it is difficult to
examine the kinetics in such a system, since formation of strong
CT complexes, which is prerequisite to observe negative ∆H obs
values, is usually too fast to follow the reactions. Fine-tuning
of the strength of the CT complex and the reactivity seems
essential to observe the negative ∆H obs values.
We have previously shown that 9-substituted 10-methyl-9,10-
dihydroacridines (AcrHR) have similar one-electron donor
properties but quite different proton donor abilities in the
corresponding radical cations formed by the electron-transfer
q
6
electron-proton-electron sequence. Namely, the one-electron
donor ability between 1-benzyl-1,4-dihydronicotinamide (BNAH)
and 10-methyl-9,10-dihydroacridine (AcrH2) is rather similar,
as compared to the large difference in the one-electron donor
q
24,25
•
ability between the corresponding radicals, i.e., BNA and
•
6
AcrH . In such a case, the energetics of the initial electron
transfer is similar, while the energetics of overall hydride transfer
is quite different between the two NADH analogues. We have
shown clearly that the activation barrier is mainly determined
by the energetics of initial electron transfer rather than the
3
+
oxidation of AcrHR with Fe and that the deprotonation rate
varies significantly depending on the substituent R.2
6,27
In this study we have examined the change in the reactivities
of AcrHR having a variety of substituents R in the reactions
with hydride acceptors. The present study provides an excellent
opportunity to compare the reactivities of AcrHR in the hydride-
transfer reactions with those in the deprotonation of the
corresponding radical cations. By the proper choice of alkyl
(or phenyl) substituents in AcrHR the electron donor property
of AcrHR and the acid property of AcrHR can be systemati-
cally varied and finely tuned to cover a wide range of subtle
molecular effects. Such fine-tuning of the electron donor and
acid properties has enabled us to observe negative activation
enthalpies for the hydride-transfer reactions of AcrHR, which
indicates unequivocally that the CT complex is a true intermedi-
ate for the hydride-transfer reaction, lying on the reaction
6
energetics of overall hydride transfer.
The mechanistic discussion is further complicated by forma-
tion of charge-transfer (CT) complexes in the course of hydride-
transfer reactions from NADH analogues to p-benzoquinone
derivatives and tetracyanoethylene (TCNE).1
6,17
The CT com-
plexes have been implicated as intermediates in a variety of
reactions between electron donors (D) and acceptors (A), eq
•
+
18-20
1.
However, the mechanistic involvement of CT complexes
KCT
k1
D + A y z (D A) y
\
z products
(1)
has always been questioned by an alternative mechanism in
which the CT complex is merely an innocent bystander in an
21
28
otherwise dead-end equilibrium, eq 2. The two pathways in
pathway.
22
eqs 1 and 2 are kinetically indistinguishable. However, Kiselev
Experimental Section
KCT-1
k2
Materials. 9,10-Dihydro-10-methylacridine (AcrH
2
) was prepared
from 10-methylacridinium iodide (AcrH I ) by reduction with NaBH
in methanol and purified by recrystallization from ethanol.29 AcrH
(D A) y z D + A y\ z products
(2)
+
-
4
and Miller23 have shown that the two pathways in eqs 1 and 2
+ -
I
was prepared by the reaction of acridine with methyl iodide in acetone
and was converted to the perchlorate salt (AcrH ClO
of magnesium perchlorate to the iodide salt (AcrH I ) and purified by
recrystallization from methanol. 9-Alkyl (or phenyl)-9,10-dihydro-10-
methylacridine (AcrHR; R ) Me, Et, CH
by the reduction of AcrH I with the corresponding Grignard reagents
can be distinguishable by the temperature dependence of the
observed second-order rate constant (kobs) if one can observe a
negative temperature dependence. A negative activation enthalpy
could only arise when the CT complex lies along the reaction
+
-
4
) by the addition
+
-
6
2
Ph, and Ph) was prepared
q
pathway (eq 1), since for such a pathway, kobs ) k1KCT[∆H obs
+ -
q
)
2
∆H1 (>0) + ∆HCT(<0)], whereas for the other pathway (eq
27
i
t
(
RMgX). AcrHR (R ) Pr , Bu , CHPh2, and 1-CH
by the photoreduction of AcrH ClO with RCOOH in the presence
of NaOH in H
O-MeCN as described previously.30 AcrHR (R ) CH
COOEt, CMe(H)COOEt, and CMe COOMe) was prepared by the
with the corresponding ketene silyl acetals
, CMe(H)dC(OEt)OSiEt , and Me CdC(OMe)-
, respectively). 9-Substituted 10-methylacridinium perchlorate
2 10 7
C H ) was prepared
q
q
), kobs ) k[∆H obs ) ∆H2 (>0)]. Thus, the necessary condition
+
-
4
to observe a negative activation enthalpy for reactions involving
CT complexes is that the heat of formation of the CT complex
2
2
-
2
+
-
4
reduction of AcrH ClO
(CH dC(OEt)OSiEt
OSiMe
(
∆HCT < 0) is of greater magnitude than the activation enthalpy
q
2
3
3
2
for the passage of the CT complex to the transition state (∆H1
31
3
+
(
16) Fukuzumi, S.; Nishizawa, N.; Tanaka, T. J. Org. Chem. 1984, 49,
(
AcrR ClO
-
i
t
2
: R ) Me, Et, Pr , Bu , CHPh , and Ph) was prepared by
4
3
571.
the reaction of 10-methylacridone in dichloromethane with the corre-
sponding Grignard reagents (RMgX) and purified by recrystallization
from ethanol-diethyl ether.32 p-Benzoquinone derivatives (2,3-dichloro-
5,6-dicyano-p-benzoquinone (DDQ), p-chloranil, 2,6-dichloro-p-ben-
zoquinone, and chloro-p-benzoquinone) and tetracyanoethylene (TCNE)
(
17) Fukuzumi, S.; Kondo, Y.; Tanaka, T. J. Chem. Soc., Perkin Trans.
2
1984, 673.
(18) (a) Mulliken, R. S.; Person, W. B. Molecular Complexes; A Lecture
and Reprint Volume; Wiley-Interscience: New York, 1969. (b) Foster, R.
Organic Charge-Transfer Complexes; Academic Press: New York, 1969.
(
19) (a) Kochi, J. K. AdV. Phys. Org. Chem. 1994, 29, 185. (b) Kochi,
J. K. Chimica 1991, 45, 277. (b) Kochi, J. K. Angew. Chem., Int. Ed. Engl.
988, 27, 1227. (c) Kochi, J. K. Acc. Chem. Res. 1992, 25, 39. (d) Kochi,
J. K. Acta Chem. Scand. 1990, 44, 409.
20) (a) Chanon, M.; Tobe, M. Angew. Chem., Int. Ed. Engl. 1982, 21,
. (b) Jones, G., II In Photoinduced Electron Transfer; Fox, M. A., Chanon,
(25) Brominations of some alkenes in nonpolar media were reported to
have negative activation energies: Sergeev, G. B.; Serguchev, Yu. A.;
Smirnov, V. V. Russ. Chem. ReV. 1973, 42, 697.
(26) (a) Fukuzumi, S.; Kitano, T. Chem. Lett. 1990, 1275. (b) Fukuzumi,
S.; Mochizuki, S.; Tanaka, T. J. Chem. Soc., Dalton Trans. 1990, 695.
(27) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J.
Am. Chem. Soc. 1993, 115, 8960.
(28) A preliminary report on the observation of a negative activation
enthalpy for a hydride transfer reaction of a hydride donor other than NADH
analogues has appeared: (a) Zaman, K. M.; Yamamoto, S.; Nishimura, N.;
Maruta, J.; Fukuzumi, S. J. Am. Chem. Soc. 1994, 116, 12099. (b)
Yamamoto, S.; Sakurai, T.; Liu, Y.; Sueishi, Y. Phys. Chem. Chem. Phys.
1999, 1, 833.
1
(
1
M., Eds.; Elsevier: Amsterdam, 1988; Part A, p 245. (c) Chanon, M.;
Rajzmann, M.; Chanon, F. Tetrahedron 1990, 46, 6193. (d) Julliard, M.;
Chanon, M. Chem. ReV. 1983, 83, 425.
(21) Sustmann, R.; Korth, H.-G.; N u¨ chter, U.; Siangouri-Feulner, I.;
Sicking, W. Chem. Ber. 1991, 124, 2811.
22) (a) Fukuzumi, S.; Mochida, K.; Kochi, J. K. J. Am. Chem. Soc.
(
1
979, 101, 5961. (b) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1980,
02, 2141. (c) Fukuzumi, S.; Kochi, J. K. Tetrahedron 1982, 38, 1035.
1
(
23) Kiselev, V. D.; Miller, J. G. J. Am. Chem. Soc. 1975, 97, 4036.
(29) Roberts, R. M. G.; Ostovic, D.; Kreevoy, M. M. Faraday Discuss.
Chem. Soc. 1982, 74, 257.
(24) A negative temperature dependence of the rates of photoinduced
electron-transfer reactions of ruthenium(II) complexes has been reported:
a) Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S. J. Am. Chem.
Soc. 1987, 109, 2506. (b) Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke,
S. J. Phys. Chem. 1989, 93, 5757.
(30) Fukuzumi, S.; Kitano, T.; Tanaka, T. Chem. Lett. 1989, 1231.
(31) Otera, J.; Wakahara, Y.; Kamei, H.; Sato, T.; Nozaki, H.; Fukuzumi,
S. Tetrahedron Lett. 1991, 32, 2405.
(
(32) Bernthsen, A. Ann. 1884, 224, 1.

4288 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Fukuzumi et al.
were obtained commercially and purified by the standard methods.33
Acetonitrile and benzonitrile used as a solvent were purified and dried
by the standard procedure.33 Chloroform and 1,2-dichloroethane
Ag ) were converted to values vs SCE by adding 0.29 V. All
electrochemical measurements were carried out under an atmospheric
pressure of argon.
+
36
(
spectral grade) were obtained commercially from Wako Pure Chemi-
Theoretical Calculations. Theoretical calculations were performed
using the MOPAC program (Ver. 6) which is incorporated in the
MOLMOLIS program (Ver. 2.8) by Daikin Industries, Co. Ltd. The
PM3 Hamiltonian was used for the semiempirical MO calculations.
Final geometries and energetics were obtained by optimizing the total
molecular energy with respect to all structural variables. The heats of
cals and used without further purification.
-
2
Reaction Procedure. Typically, AcrHR (4.0 × 10 M) and DDQ
-
2
37
(
6.0 × 10 M) were added to an NMR tube that contained deaerated
3
CD
3
CN solution (0.60 cm ) under an atmospheric pressure of argon.
1
The oxidized products of AcrHR were identified by the H NMR spectra
by comparing with those of authentic samples. The H NMR measure-
ments were performed using a JNM-GSX-400 (400 MHz) NMR
spectrometer. H NMR (CD
1
formation (∆H ) were calculated with the restricted Hartree-Fock
f
(RHF) formalism using a key word “PRECISE”.
1
+
-
4
3
CN): AcrMe ClO
, δ 3.48 (s, 3H), 4.74
+
-
(
s, 3H), 7.9-8.9 (m, 8H); AcrEt ClO
.95 (q, 2H), 4.71 (s, 3H), 7.9-8.9 (m, 8H); AcrPh ClO
H), 7.5-8.6 (m, 13H); AcrCH
H), 7.5-8.9 (m, 13H); AcrCHPh
H), 7.5-8.9 (m, 18H); Acr(1-CH
s, 2H), 7.5-8.9 (m, 15H); AcrCH COOEt ClO , δ 1.09 (t, 3H, J )
.0 Hz), 2.41 (q, 2H, J ) 8.0 Hz), 4.76 (s, 3H), 5.02 (s, 2H), 7.7-8.7
4
, δ 1.52 (t, 3H, J ) 7.5 Hz),
Results and Discussion
+
-
3
3
2
1
(
8
4
, δ 4.83 (s,
Reactions of AcrHR with Hydride Acceptors. It has
previously been reported that hydride transfer reactions from
10-methyl-9,10-dihydroacridine (AcrH2) as well as 1-benzyl-
1,4-dihydronicotinamide (BNAH) to hydride acceptors (A) such
+
-
-
2
Ph ClO
2
4
, δ 4.79 (s, 3H), 5.35 (s,
+
ClO
4
, δ 4.70 (s, 3H), 5.96 (s,
+
-
2
C
10
H
7
) ClO
4
, δ 4.80 (s, 3H), 5.72
+
-
2 4
6
,38
as p-benzoquinone derivatives
(TCNE)
and tetracyanoethylene
1
7,38
(
m, 8H).
occur efficiently (eq 1) followed by a subsequent
fast electron transfer from the reduced product (AH ) to A (eq
-
Spectral and Kinetic Measurements. The reactions of AcrHR with
DDQ and TCNE in deaerated MeCN were monitored with a Shimadzu
UV-2200, 160A spectrophotometer or a Hewlett-Packard 8453 diode
array spectrophotometer when the rates were slow enough to be
determined accurately. The rates were determined from appearance of
2
) and the disproportionation of the resulting radical (eq 3).
+
4
-1
the absorbance due to AcrR (λmax ) 358 nm, ꢀmax ) 1.80 × 10 M
-
1
•-
3
cm ) or the radical anion (DDQ : λmax ) 585 nm, ꢀmax ) 5.6 × 10
cm ; TCNE : λmax ) 457 nm, ꢀmax ) 5.67 × 10 M cm ).
The kinetic measurements for faster reactions such as the reaction of
AcrH or AcrHCH Ph with DDQ were carried out with a Union RA-
03 stopped-flow spectrophotometer which was thermostated at 298
-1
-1
•-
3
-1
-1 34,35
M
2
2
1
K under deaerated conditions. The concentration of AcrHR or a hydride
acceptor was maintained at more than 15-fold excess of the other
reactant to attain pseudo-first-order conditions. Pseudo-first-order rate
constants were determined by a least-squares curve fit using an NEC
microcomputer. The first-order plots of ln(A
∞
∞
- A) vs time (A and A
are the final absorbance and the absorbance at the reaction time,
respectively) were linear for three or more half-lives with the correlation
coefficient F > 0.999. In each case, it was confirmed that the rate
constants derived from at least five independent measurements agreed
within an experimental error of (5%.
The transient CT spectra of complexes formed between AcrHR and
p-benzoquinone derivatives with half-lives <10 s were obtained by
plotting the initial rise of the absorbance against the wavelength with
a stopped flow spectrophotometer. The CT spectra of stable complexes
When AcrH is replaced by 9-substituted analogues (AcrHR),
essentially the same reactions (eqs 1-3) occur to give the overall
stoichiometry as given by eq 4. A typical example of the UV-
2
such as the AcrHCH
sured with a Hewlett-Packard 8452 or a Hewlett-Packard 8453 diode
array spectrophotometer. The formation constant (KCT) of the AcrHCH
2
Ph-chloro-p-benzoquinone complex were mea-
2
-
Ph-chloro-p-benzoquinone complex was determined from the depen-
dence of the initial rise of the absorbance at λmax ) 530 nm due to the
CT complex on the concentration of chloro-p-benzoquinone in MeCN
at various temperatures.
The ESR spectra of DDQ and TCNE•- formed as final products
in the reactions of AcrHR with DDQ and TCNE, respectively, were
measured with a JEOL X-band spectrometer (JES-RE1XE). The g
values and the hyperfine coupling constants were calibrated with a Mn2
marker.
•-
+
Cyclic Voltammetry. Cyclic voltammetry measurements were
performed at 298 K on a BAS 100 W electrochemical analyzer in
deaerated MeCN containing 0.1 M Bu NClO (TBAP) as supporting
4 4
vis spectral change in the reaction of AcrHCH2Ph with DDQ
is shown in Figure 1. The spectral titration shown in Figure 2
•
-
electrolyte. A conventional three-electrode cell was used with a platinum
where [DDQ ]/[DDQ]0 is plotted against [AcrHR]/[DDQ]0
confirms the stoichiometry in eq 4 where 2 equiv of AcrHR
2
working electrode (surface area of 0.3 mm ) and a platinum wire as
•-
the counter electrode. The Pt working electrode (BAS) was routinely
polished with a BAS polishing alumina suspension and rinsed with
acetone before use. The measured potentials were recorded with respect
reacts with 3 equiv of DDQ to yield 2 equiv of DDQ (67%
yield). The formation of DDQ•- was also confirmed by the ESR
spectrum (g ) 2.0054), which showed the hyperfine structure
3
to the Ag/AgNO (0.01 M) reference electrode. All potentials (vs Ag/
(36) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non-
(
33) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
aqueous Systems; Marcel Dekker: New York, 1990.
(37) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221.
(38) (a) Colter, A. K.; Saito, G.; Sharom, F. J.; Hong, A. P. J. Am. Chem.
Soc. 1976, 98, 7833. (b) Colter, A. K.; Saito, G.; Sharom, F. J. Can. J.
Chem. 1977, 55, 2741.
Chemicals; Butterworth-Heinemann: Oxford, 1988.
(
34) Iida, Y. Bull. Chem. Soc. Jpn. 1971, 44, 1777.
(35) Webster, O. W.; Mahler, W.; Benson, R. E. J. Am. Chem. Soc. 1962,
8
4, 3678.

ReactiVity of Substituted 10-Methyl-9,10-dihydroacridines
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4289
Figure 1. Electronic absorption spectra observed in the reaction of
-
6
-5
-5
-5
AcrHCH
×
2
Ph (0, 9.3 × 10 , 2.8 × 10 , 3.7 × 10 , 4.7 × 10 , 5.6
10 , 6.5 × 10 , and 7.5 × 10 M) with DDQ (8.3 × 10 M) in
deaerated MeCN at 298 K.
(1)
-
5
-5
-5
-5
Figure 3. Plots of the pseudo-first-order rate constants (k ) vs
•
-
[
AcrHMe] (O) or [DDQ] (b) for formation of DDQ in the reaction
of AcrHMe with DDQ in MeCN at 298 K. Either AcrHMe or DDQ is
used in a large excess.
•+
d
Table 1. Rate Constants (k ) for Deprotonation of AcrHR ,
0
One-Electron Oxidation Potentials (E ox) of AcrHR, and Rate
Constants (kobs) of Hydride Transfer Reactions from AcrHR to DDQ
and TCNE in MeCN at 298 K
1
E0ox(vs SCE),a
k
obs, M- s-1
AcrHR,
R )
d
k ,
1
-
s
V
DDQ
TCNE
6
5
4
4
4
3
3
2
2
H
Me
Ph
6.4
1.1
4.1
0.81
0.84
0.88
0.84
0.84
0.85
0.89
0.84
0.84
0.92
0.86
0.92
1.5 × 10
1.1 × 10
6.5 × 10
4.0 × 10
1.1 × 10
6.9 × 10
6.9 × 10
5.3 × 10
4.5 × 10
2.0 × 10
1.3 × 10
1.3 × 10
1.0 × 10
7.0
-1
7.9 × 10
3.0
Et
0.49
CH
CH
CH
CHPh
Pr
CMe(H)COOEt
Bu
2
CMe COOMe
0.17b
5.4 × 10
3.3 × 10
2.3 × 10
1.5 × 10
7.0 × 10
3.4 × 10
<5 × 10
<1 × 10
-1
-1
-1
-2
-4
2
Ph
2
2
C
10
H
7
c
c
c
c
c
c
c
COOEt
2
i
-
3
t
-1
-5
-4
-
1
•
-
Figure 2. Plot of the ratio of the DDQ concentration to the initial
a
Taken from ref 27. b The deprotonation rate constant separated from
-
4
•-
•+
concentration of DDQ (1.0 × 10 M), [DDQ ]/[DDQ]
of the initial concentration of AcrHR to DDQ, and [AcrHR]/[DDQ]
for the reaction of AcrHR (R ) Me (O), CH COOEt (b), 1-CH
4), and CMe(H)COOEt (9)) with DDQ.
0
vs the ratio
the rate constant for the C-C bond cleavage of AcrHCH Ph ; see ref
2
43. c Too small to be separated from the rate constant for the C-C
0
•
+
bond cleavage of AcrHR accurately.
2
2
C
10
H
7
(
plot of k(1) vs [A] gives the rate constant (kobs) of the hydride
(
1)
due to two equivalent nitrogens (aN ) 0.058 mT) in agreement
transfer from AcrHR to A (eq 5), and the slope of k vs
[AcrHR] gives (3/2)kobs (eq 6).
with the literature value.39 The same 2:3 stoichiometry was
•-
obtained for the reaction of AcrHR with TCNE to yield TCNE ,
the formation of which was confirmed by the absorption
spectrum as well as the ESR spectrum.1
(
1)
k
) kobs[A]
(5)
(6)
7,35,40
•
-
(1)
The rates of formation of the radical anion (A ) in the
presence of a large excess of AcrHR or the hydride acceptor
k
) (3/2)kobs[AcrHR]
4
1
(
A) obeyed the pseudo-first-order kinetics. The value of the
The kobs values for the reactions of a series of AcrHR with
hydride acceptors (DDQ and TCNE) are listed in Table 1. The
kobs values for the reactions of AcrHR with DDQ vary
significantly depending on the substituent R in AcrHR. The
magnitude spans a range of 10 starting from R ) H to Bu and
CMe2COOMe. Similar change in the reactivity with R is
observed for the reactions of AcrHR with TCNE (Table 1). Such
a significant decrease in the reactivity by the introduction of a
substituent at the C-9 position can hardly be reconciled by a
concerted hydride transfer mechanism. The alkyl or phenyl
group at the C-9 position is known to be in a boat axial
conformation, and thereby the hydrogen at the C-9 position is
located at the equatorial position, where steric hindrance due
(1)
pseudo-first-order rate constant (k ) in an excess of AcrHR
(
eq 5) is 1.5-fold larger than the k(1) value in an excess of A at
the same concentration (eq 6). A typical example is shown in
Figure 3, where the k( values are plotted against [AcrHMe] or
1)
7
t
[DDQ]. This agrees with the stoichiometry in eq 4 used in excess
for the reaction of AcrHMe with DDQ. Thus, the slope of the
(39) (a) Gordon, D.; Hove, M. J. J. Chem. Phys. 1973, 59, 3419. (b)
Corvaja, C.; Pasimeni, L.; Brustalon, M. Chem. Phys. 1976, 14, 177. (c)
Grampp, G.; Landgraf, S.; Rasmussen, K. J. Chem. Soc., Perkin Trans. 2
1
999, 1897.
40) Phillips, W. D.; Powell, J. C.; Weissman, S. I. J. Chem. Phys. 1960,
3, 626.
41) It was confirmed that the rates were not affected by the room light.
(
3
(

4290 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Fukuzumi et al.
Scheme 1
to the axial substituent is minimized in the hydride transfer
reactions. Moreover, the introduction of an electron-donating
t
substituent such as R ) Bu would activate the release of a
negatively charged hydride ion if the concerted hydride transfer
should take place. The remarkable decrease in the reactivity
with the increasing electron-donor ability of R (Table 1) rather
indicates that the reactivity is determined by the process in which
a positive charge is released.
Comparison of the Reactivities in Hydride Transfer
•
+
Reactions of AcrHR and Deprotonation of AcrHR . We
have previously succeeded in detecting transient absorption and
•
+
ESR spectra of AcrHR produced by the electron-transfer
3+
oxidation of AcrHR with [Fe(phen)3] (phen ) 1,10-phenan-
27
throline). It has been found based on the product analysis that
•
+
there are two pathways for the decay of AcrHR : one is the
•
+
C(9)-H bond cleavage (deprotonation) to give AcrR and H ,
+
and the other is the C(9)-C bond cleavage to give AcrH and
•
27,42
R as shown in Scheme 1.
In the case of R ) H, Me, Et,
Ph, and CH2COOEt, the C(9)-H bond is cleaved exclusively
Figure 4. (a) Plots of kobs for the reaction of AcrHR with DDQ (O)
•
+
+
27
and TCNE (b) vs k for deprotonation of AcrHR in MeCN at 298 K.
to yield only AcrR . In such a case the decay rate constant of
d
•
+
(b) Plots of kobs for the reaction of AcrHR with DDQ (O) and TCNE
AcrHR corresponds to the deprotonation rate constant (kd).
The kd values thus determined from the decay of AcrHR are
0
•
+
(b) vs k
d
K
et
.
•+
also listed in Table 1. In contrast, the C(9)-C bond of AcrHR
Scheme 2
t
is cleaved selectively in the case of R ) Bu and CMe2COOMe
when the deprotonation rates were too slow to be deter-
mined.2
7,42
In the case of R ) CH2Ph, 1-CH2C10H7, CMe(H)-
COOEt, Pr , and CHPh2, both the C-H and C-C bonds of
i
•
+
AcrHR are cleaved to yield two types of products shown in
Scheme 1.27 In this case the decay rate constant of AcrHR•+
includes the rate constants of two pathways. In the case of R )
CH2Ph, the deprotonation rate constant (kd) could be separated
from the rate constant of the C-C bond cleavage based on the
electron transfer from AcrHR to A, followed by proton transfer
•+
•-
from AcrHR to A in the radical ion pair and the subsequent
•
+
43
reaction of AcrHCH2Ph with a base. The kd value of
•
•
electron transfer from AcrR to AH , and that the proton-transfer
step may be involved as a rate-determining step. Such a
•
+
AcrHCH2Ph is also listed in Table 1.
44
The log kobs values of the hydride transfer from AcrHR to a
hydride acceptor (A: DDQ or TCNE in eq 4) are plotted against
sequential electron-proton-electron transfer leads to the overall
+
-
hydride transfer to yield AcrR and AH (Scheme 2).
•
+
Since the one-electron reduction potential of DDQ (E0red(vs
the log kd values of the deprotonation of AcrHR in Figure
a, where a reasonably good linear correlation between them
6
0
45
4
SCE) ) 0.51 V) or TCNE (E red(vs SCE) ) 0.22 V) is less
is observed except for R ) Ph and CH2COOEt. Such a linear
correlation indicates that the hydride transfer proceeds via
positive than the one-electron oxidation potential of examined
0
6,46
AcrHR (E (vs SCE) ) 0.81-0.92 V in Table 1), the back
ox
•-
•+
electron transfer from A to AcrHR may be much faster than
t •+
(
42) The C(9)-C bond cleavage of AcrHBu
generated by the
the proton transfer from AcrHR to A•- (kb . kp in Scheme
1). In such a case, the observed rate constant (kobs) of the overall
hydride transfer is given by eq 9, where Ket ) ket/kb, provided
•+
t
electrochemical oxidation of AcrHBu has also been reported: Anne, A.;
Fraoua, S.; Moiroux, J.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1996, 118, 3938.
•
+
(
43) The kd value of AcrHCH2Ph was determined from the kd value of
•
+
AcrHMe and the ratio of the observed second-order rate constant for the
•
•
•
+
that an electron transfer from AcrR to AH in the final step in
Scheme 1 is much faster than the proton transfer from AcrHR
proton transfer from AcrHCH2Ph to 3,5-dichloropyridine to that from
AcrHMe ; see ref 27. The kd values of AcrHR•+ in which the C(9)-C
•
+
•+
bond is cleaved exclusively have not been determined accurately.
•-
to A .
(44) The proton transfer cannot precede the initial electron transfer from
AcrHR to DDQ, since no deprotonation of AcrHR occurs in the presence
of pyridine which is a much stronger base than DDQ.
kobs ) k K
(9)
p
et
(
45) Fukuzumi, S.; Tokuda, Y. J. Phys. Chem. 1992, 96, 8409.
•
•
The fast electron transfer from AcrH to AH is well supported
by the highly negative one-electron oxidation potential of AcrH•
(46) Hapiot, P.; Moiroux, J.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1990,
1
12, 1337.

ReactiVity of Substituted 10-Methyl-9,10-dihydroacridines
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4291
0
0
obtained from the E ox value of AcrHR and the E red value of
0
A by eq 10. When the difference in the K et values for the
0
0
0
K
) exp[-F(E - E red)/RT]
(10)
et
ox
AcrHR-DDQ and AcrHR-TCNE systems is included in the
plots between log kobs and log kd, the two separate linear
correlations and deviation from the linear lines in Figure 4a
are remarkably merged into a single line with a slope of unity
as shown in Figure 4b where the log kobs values are plotted
0
49
against the log kdK et values. Thus, it can be concluded that
the overall hydride transfer from AcrHR to A proceeds via
sequential electron-proton-electron transfer in which the initial
electron transfer to produce the radical ion pair is in equilibrium
•
+
•-
and the proton transfer from AcrHR to A is the rate-
determining step (Scheme 2). The observed primary kinetic
isotope effects (kH/kD) of the overall hydride transfer from AcrH2
and the 9,9′-dideuterated compound (AcrD2) to p-benzoquinone
Figure 5. Cyclic voltammograms of (a) AcrPri +ClO
-
(1.0 × 10
-3
4
+
-
-3
M) and (b) AcrCH
2
Ph ClO
4
(1.0 × 10 M) in deaerated MeCN
50
derivatives (Q) can be attributed to those of the proton-transfer
containing TBAP (0.10 M) with a Pt electrode at 298 K; sweep rate
•
+
•+
•-
-
1
step from AcrH2 and AcrD2 to Q , since the variation of
5
0 mV s
.
kH/kD with p-benzoquinone derivatives has been well correlated
Table 2. One-Electron Reduction Potentials (E0red) of AcrR ClO
+
-
4
with the difference in the pKa values between AcrH2•+ and QH•
Determined by the Cyclic Voltammograms in MeCN at 298 K and
the Sweep Rates
(
∆
∆pKa) and the maximum value (kH/kD ) 10.4) is obtained at
5
0
pKa ) 0.
AcrR+,
R )
E0red(vs SCE),
sweep rate,
1
It should be noted that A•- is formed as a final product by
-
V
V s
15000a
0.05
-
the subsequent fast reaction of AH with A (eq 2) and the
disproportionation reaction of AH (eq 3) after the overall
H
PhCH
Ph
Et
Bu
-0.46
-0.50
-0.55
-0.57
-0.59
-0.63
•
2
hydride transfer reaction (eq 1). Such fast reactions following
0.05
1
•-
the hydride transfer to produce A have precluded the detection
t
•-
1
of A in the course of the hydride-transfer reaction. Conversely
i
Pr
0.05
the detection of radical ions in hydride-transfer reactions does
not necessarily mean the involvement of an electron-transfer
step in the hydride-transfer reactions.
a
Taken from ref 46.
E0ox ) -0.46 V),46 which is much more negative than the one-
CT Complex Formed between AcrHR and p-Benzo-
quinone Derivatives. Although there is an excellent single linear
(
0
electron reduction potential of A (E red(vs SCE) ) 0.51 and
0
0
correlation between log kobs and log kdK et, the kobs values, which
0
.22 V for DDQ and TCNE, respectively), and these E red values
9
correspond to the kpKet values in eq 9, are 10 times larger than
are even less positive than the reduction potential of the
0
•
47
0
the corresponding kdK et values (Figure 4). The reason for such
protonated form of the radical anion (AH ). The E ox values
•
a huge difference in the absolute values may be 2-fold. First,
of 9-substituted 10-methylacridinyl radicals (AcrR : R ) Ph,
Et, CH2Ph, Pr , and Bu ) are readily determined from the cyclic
voltammograms of AcrR , since AcrR is much more stable
than AcrH (R ) H). The typical cyclic voltammograms are
shown in Figure 5 and the E ox values of AcrR are listed in
Table 2. The E ox value of AcrR is more negative than the
value of AcrH . Thus, electron transfer from AcrR to AH is
•
+
•-
i
t
the rate constant of proton transfer from AcrHR to A (kp)
+
•
may be much larger than the spontaneous deprotonation rate
•
+
•-
•
constant of AcrHR (kd), since A acts as a base. This may
t •+
•-
0
•
be the reason why the proton transfer from AcrHBu to A
preceded the cleavage of the C(9)-C bond of AcrHBu
leading to the overall hydride transfer reaction (eq 4). Second,
t •+
0
•
•
•
•
0
the Ket value for the radical ion pair formation in Scheme 2
highly exergonic irrespective of the type of R (e.g., ∆G et <
0
-1
•
•
may also be larger than the K et value for formation of free
-
97 kJ mol for the AcrCH2Ph -DDQH system). On the other
•
+
•-
•
+
•-
radical ions (AcrHR and A ), since there may be a significant
hand, the proton transfer from AcrH2 to DDQ is known to
•+
•-
-
1
6
t •+
Coulombic interaction between AcrHR and A in the radical
be endergonic (27 kJ mol ). The pKa value of AcrHBu
6
may be slightly larger than the pKa value of AcrH2•+, since the
ion pair.
t •
Formation of a radical ion pair is usually preceded by
formation of a charge-transfer (CT) complex between an electron
difference in the calculated heat of formation between AcrBu
t •+
-1
and AcrHBu
is slightly (2.3 kJ mol ) larger than that
1
8-22,51
•
•+ 48
donor and acceptor.
The observation of CT complexes
between AcrH and AcrH2
.
Thus, the proton transfer from
AcrHR to A should be rate determining as compared to fast
•+
•-
is difficult in fast reactions such as hydride transfer from AcrH2
•
•
to DDQ because of the instability of the CT complex. When
electron transfer from AcrR to AH as shown in Scheme 2.
According to eq 9, deviation from linear correlations between
log kobs and log kd may be ascribed to the difference in the Ket
value, since the proton-transfer rate constant (kp) in Scheme 2
may be in parallel with the deprotonation rate constant (kd) in
Scheme 1. The equilibrium constant for electron transfer from
49) The kobs values are about 1010 times larger than the corresponding
(
0
kdK et values (Figure 4b). Such a large difference may originate from the
•
+
much larger rate constant of proton transfer from AcrHR to a strong base
•
-
(
A ) than the spontaneous deprotonation rate constant kd, combined with
the larger Ket value for the radical ion pair formation, in which the large
work term is included,6 than the K0 value for the free radical ion formation.
et
0
•+
•-
AcrHR to A (K et) to produce free AcrHR and A can be
(50) Ishikawa, M.; Fukuzumi, S. J. Chem. Soc., Faraday Trans. 1990,
6, 3531.
8
(
47) Rich, P. R.; Bendall, D. S. Biochim. Biophys. Acta 1980, 592, 506.
(51) (a) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 7290.
(b) Fukuzumi, S.; Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1980, 102,
2928. (c) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1982, 104, 7599.
•
•+
t
(48) The heats of formation of AcrR and AcrHR (R ) H and Bu )
were calculated by the PM3 method.

4292 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Fukuzumi et al.
Figure 6. Electronic absorption spectra of CT complexes of (a) AcrH
2
Figure 7. Plot of CT transition energies (hνCT) of AcrHR-Q
-
2
-2
(
5.9 × 10 M) with chloro-p-benzoquinone (6.0 × 10 M), (b)
0
complexes (b) and other known CT complexes of Q (O) vs E ox
-
-
2
-2
AcrHCH
M), (c) AcrHCH
(
1
2
Ph (1.2 × 10 M) with chloro-p-benzoquinone (6.0 × 10
0
red. The hνCT and E0ox - E0red values are given in the Supporting
E
-
2
2
Ph (1.2 × 10 M) with 2,6-dichloro-p-benzoquinone
Information.
-
2
-2
6.0 × 10 M), (d) AcrH
2
(5.9 × 10 M) with p-chloranil (1.0 ×
-
2
t
-3
-2
0
M), (e) AcrHBu (6.0 × 10 M) with p-chloranil (1.0 × 10
0
0
the free energy change of electron transfer ∆G et/F ) E ox -
t
-3
-2
M), and (f) AcrHBu (6.0 × 10 M) with DDQ (1.0 × 10 M) in
0
E red. The hνmax values of the examined AcrHR-p-benzo-
quinone derivative complexes are consistent with those of other
known CT complexes in the correlation with ∆G et/F values
MeCN at 298 K.
0
DDQ is replaced by a weaker electron acceptor such as chloro-
p-benzoquinone, a new broad absorption band, which is
characteristic of an intermolecular CT transition, is readily
observed upon mixing an MeCN solution of AcrH2 with that
of chloro-p-benzoquinone as shown in Figure 6a. When AcrH2
is replaced by AcrHCH2Ph, a broad absorption band with the
same absorption maximum (λmax ) 540 nm) is observed (Figure
(Figure 7). Thus, the observed CT complexes in the course of
hydride-transfer reactions from AcrHR to p-benzoquinone
derivatives (Q) are classified as donor-acceptor complexes of
a quite general kind as shown in eq 11.
KCT
hνmax
•
+
•-
AcrHR + Q { } (AcrHR Q)
8 (AcrHR Q ) (11)
6
b). The λmax values are shifted to longer wavelengths when
The formation constant KCT of the CT complex formed
between AcrHCH2Ph and chloro-p-benzoquinone (ClQ) in
MeCN was determined from an increase in the CT absorbance
chloro-p-benzoquinone is replaced by 2,6-dichloro-p-benzo-
quinone and p-chloranil which are stronger electron acceptors
than chloro-p-benzoquinone as shown in Figure 6, lines c and
d, respectively. A stopped-flow technique was used for the
detection of an unstable CT complex formed between AcrHCH2-
Ph and p-chloranil (see the Experimental Section). The CT
(
A) at λmax with an increase in the quinone concentration [Q]
56
according to the Benesi-Hildebrand equation (eq 12), where
is the extinction coefficient. The KCT values were determined
ꢀ
-
1
t
at various temperatures. From the plot of ln KCT vs T shown
A ) ꢀK [AcrHR][ClQ]/(1 + K [ClQ]) (12)
in Figure 8 is determined the heat of formation of the CT
complex is significantly stabilized when AcrHBu is employed
as an electron donor which has the least reactivity toward
t
CT
CT
hydride acceptors (Table 1). Thus, the CT spectra of AcrHBu -
t
p-chloranil and AcrHBu -DDQ complexes are readily observed
as shown in Figure 6, lines e and f, respectively.52
-
1
complex (∆HCT ) -29 kJ mol ).
The CT transition energies (hνmax) observed in Figure 6 are
compared with those of other known CT complexes formed
between a variety of electron donors and p-benzoquinone
Negative Temperature Dependence of the Rates of Hy-
dride Transfer. The decay of the transient CT band observed
in the course of the hydride transfer from AcrH2 to p-chloranil
in Figure 6d coincides completely with the rise of the absorption
band due to the product (see the Supporting Information).
However, such a coincidence does not necessarily mean that
the CT complex is an intermediate for the hydride transfer
reaction as discussed in the Introduction. Whether the observed
CT complex is a real intermediate for the hydride transfer
reaction or a merely innocent bystander in an otherwise dead-
end equilibrium could only be distinguishable by the temperature
dependence of the rate if one can observe the negative
temperature dependence.
16,18,53
derivatives
in Figure 7 where the hνmax values are plotted
against the difference between the one-electron oxidation
potentials of electron donors1
potential of p-benzoquinone derivatives, which is related to
6,54,55
and the one-electron reduction
6
t
-3
(
52) The examined concentration of AcrHBu (6.0 × 10 M) was smaller
-2
than the AcrH2 concentration (5.9 × 10 M) because of the lower solubility
t
t
of AcrHBu , when the CT absorbances of the AcrHBu -p-chloranil and
t
AcrHBu -DDQ complexes in Figure 6, lines e and f, respectively, are smaller
than that of the AcrH2-p-chloranil complex (Figure 6d).
(
53) (a) Foster, R.; Thomson, T. J. Trans. Faraday Soc. 1962, 58, 860.
(
b) Zweig, A.; Lancaster, J. E.; Neglia, M. T.; Jura, W. H. J. Am. Chem.
Soc. 1964, 86, 4130.
54) Fukuzumi, S.; Hironaka, K.; Nishizawa, N.; Tanaka, T. Bull. Chem.
Soc. Jpn. 1983, 56, 2220.
55) (a) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in
(
The kobs values for the hydride transfer reaction from
AcrHCH2Ph to DDQ in different solvents were determined at
various temperatures and they are listed in Table 3. From the
Arrhenius plots shown in Figure 9 are obtained the activation
(
Nonaqueous Systems; Marcel Dekker: New York, 1970. (b) Seo, E. T.;
Nelsonm, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.; Adams, R.
N. J. Am. Chem. Soc. 1966, 84, 3498. (c) Bock, C. R.; Connor, J. A.;
Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K.
J. Am. Chem. Soc. 1979, 101, 4815.
q
q
enthalpies (∆H obs), and the activation entropies (∆S obs) as also
(56) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.

ReactiVity of Substituted 10-Methyl-9,10-dihydroacridines
J. Am. Chem. Soc., Vol. 122, No. 18, 2000 4293
Table 3. Rate Constants of Hydride Transfer Reaction from AcrHCH
2
Ph to DDQ in Various Solvents at Different Temperatures, Dielectric
Constants, and Activation Parameters
obs, M- s-1
1
k
q
q
∆
H
obs
,
∆S obs
,
-
1
-1
-1
solvent
ꢀ
298 K
308 K
318 K
328 K
kJ mol
J K mol
5
4
4
4
5
4
4
4
4
4
4
4
4
4
4
4
CHCl
3
4.8
10
25
38
1.3 × 10
5.5 × 10
1.0 × 10
1.1 × 10
1.1 × 10
5.8 × 10
1.2 × 10
1.2 × 10
9.8 × 10
6.1 × 10
1.4 × 10
1.4 × 10
8.0 × 10
6.4 × 10
1.7 × 10
1.4 × 10
-13
4
13
7
-160
-110
-89
CH
2
ClCH
2
Cl
PhCN
MeCN
-110
a
The experimental errors are within (5%.
Figure 8. Plot of ln KCT vs T- for the temperature dependence of
1
KCT of the CT complex formed between AcrHCH
2
Ph and chloro-p-
benzoquinone in MeCN.
Figure 10. Arrhenius plots of kobs for the reaction of AcrHR [R ) (a)
-
5
H, (b) Me, (c) Et, and (d) CH
2
Ph; 1.1 × 10 M] with DDQ (2.0 ×
-
4
1
0
3
M) in CHCl .
Table 4. Rate Constants of Hydride Transfer Reaction from
AcrHR to DDQ in CHCl
Activation Parameters
3
at Different Temperatures and the
kobs, M s-1
a
-1
q
q
AcrHR,
R )
∆H obs,
∆S obs,
-1
-
1
298 K
308 K
318 K
328 K kJ mol J K mol
7
7
7
H
b
2.3 × 10 1.6 × 10 1.1 × 10
-32
-4
1
-170
-100
-95
6
6
6
6
5
Me 2.0 × 10 1.8 × 10 1.8 × 10 1.7 × 10
5
5
5
Et
7.5 × 10 7.6 × 10 7.7 × 10 7.8 × 10
a
The experimental errors of kobs are within (5%. b Too fast to be
determined accurately.
q
The ∆H obs value of AcrHR increases in the following order:
-
1
-1
R ) H (-32 kJ mol ) < R ) Me (-4 kJ mol ) < R ) Et (1
-
1
kJ mol ), as listed in Table 4.
The observed negative ∆H obs values, which should be equal
to ∆HCT + ∆H 1 (kobs ) k1KCT), could only arise when the CT
complex lies along the reaction pathway (vide supra, eq 1). The
q
q
∆
HCT values for the AcrHR-DDQ complexes may be more
-
1
negative than the observed ∆HCT value (-29 kJ mol ) for the
AcrH2 complex with chloro-p-benzoquinone, which is a weaker
electron acceptor than DDQ. Thus, the heat of formation of the
CT complex (∆HCT < 0) may be of greater magnitude than the
activation enthalpy for the passage of the CT complex to the
Figure 9. Arrhenius plots of kobs for the reaction of AcrHCH
2
Ph (1.1
-
5
-4
×
10 M) with DDQ (2.0 × 10 M) in (a) CHCl
3
, (b) CH
2
ClCH Cl,
2
(
c) PhCN, and (d) MeCN.
listed in Table 3. The kobs value in CHCl3, which is the least
polar solvent among the examined solvents, is the largest. The
q
q
transition state (∆H1 > 0) in eq 1, i.e., -∆HCT > ∆H1 when
q q q
q
-1
negative ∆H obs value (-13 kJ mol ) is obtained in CHCl3,
and this means the lower the temperature, the faster the rate of
the ∆H obs values (∆H obs ) ∆HCT + ∆H 1) become negative.
As demonstrated by a single correlation between log kobs and
q
q
0
q
hydride transfer. The ∆H obs and ∆S obs values of other AcrHR
derivatives (R ) H, Me, and Et) were also determined from
the temperature dependence of kobs. The Arrhenius plots are
shown in Figure 10, where a negative temperature dependence
is clearly observed for the hydride transfer reaction of AcrH2.
log(kdK et) in Figure 4b, the ∆H 1 value for the hydride transfer
reaction consists of the sum of the activation enthalpies for
electron transfer from AcrHR to DDQ in the CT complex and
proton transfer from AcrHR to DDQ in the radical ion pair
in Scheme 2. Thus, the largest negative ∆H obs value (-32 kJ
•
+
•-
q

4294 J. Am. Chem. Soc., Vol. 122, No. 18, 2000
Fukuzumi et al.
-
1
mol ) is obtained for the reaction of AcrH2 with DDQ when
both electron transfer and proton transfer are fastest among
correlated with the large variation in the deprotonation rate
•
+
constant of AcrHR combined with the small variation in the
electron-transfer reactivity of AcrHR. On the basis of these
results it is concluded that the overall hydride transfer proceeds
via a CT complex formed between AcrHR and the hydride
acceptor (A), electron transfer from AcrHR to A in the CT
q
examined AcrHR and p-benzoquinone derivatives and the ∆H 1
value is therefore minimized. An increase in the ∆H obs value
q
-
1
of AcrHR in the order R ) H (-32 kJ mol ) < R ) Me (-4
-
1
-1
kJ mol ) < R ) Et (1 kJ mol ) in Table 4 can be well
•
+
•-
accounted for by an increase in the activation enthalpy for the
complex, proton transfer from AcrHR to A , and electron
•+
•-
•
•
+
-
proton-transfer step from AcrHR to DDQ , since the depro-
tonation rate constant of AcrHR decreased in the same order
Table 1). The solvent effects on ∆H obs are more complicated
than the substituent effects. The more polar the solvent, the more
favorable the charge transfer and electron-transfer steps, but the
less favorable the proton-transfer step following the electron-
transfer step. These two opposite effects may be optimized in
CHCl3 so as to achieve the smallest ∆H 1 value resulting in
transfer from AcrR to AH to yield AcrR and AH . The
overall reactivity is determined by the three consecutive steps,
i.e., the CT complex formation, the electron-transfer, and the
proton-transfer steps, since the electron transfer in the final step
is much faster than the previous proton-transfer step. The initial
electron-transfer step in the CT complex may be facilitated by
the charge-transfer interaction in the CT complex, since such
charge-transfer interaction should result in a decrease in the
difference of nuclear configurations before and after the electron-
transfer step. Thus, this study has provided the first compre-
hensive and confirmative understanding of the mechanism of
sequential electron-proton-electron transfer via CT com-
•
+
q
(
5
7
q
q
the successful observation of the negative ∆H obs value in this
solvent (Table 2).
Summary and Conclusions
5
8
plexes.
The observed negative temperature dependence of the rate
of hydride transfer from AcrH2 to DDQ gave unequivocal
evidence for the role of the observed CT complex as an actual
intermediate in the hydride-transfer reaction. The magnitude of
the observed rate constant for the reactions of AcrHR with a
hydride acceptor (DDQ or TCNE) varies significantly depending
on the type of substituent R in AcrHR at the C-9 position and
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (Nos.
1
1228205, 11166241, 11136229, and 11555230) from the
Ministry of Education, Science, Culture and Sports, Japan.
7
t
Supporting Information Available: Decay and the rise of
absorbances at 710 and 550 nm due to the AcrH2-p-chloranil
complex and p-chloranil radical anion, respectively, in the
reaction of AcrH2 with p-chloranil (Figure S1) and a table of
spans a range of 10 starting from R ) H to Bu and CMe2-
COOMe. Such large variation in the rate constant is well
(
(
57) Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1985, 107, 6452.
58) Theoretical confirmation of the participation of a CT complex as a
real intermediate has recently been reported for the Diels-Alder reaction
of anthracene with TCNE in which an electron-transfer process following
the CT complex formation plays an important role in determining the overall
reactivity; Wise, K. E.; Wheeler, R. A. J. Phys. Chem. A 1999, 103,
279.
0
0
hνCT and E ox - E red values plotted in Figure 7 (Table S1)
PDF). This material is available free of charge via the Internet
(
at http://pubs.acs.org.
2
0c
8
JA9941375