Tetracyanoanthraquinodimethanes with a chiral amide group: Preparation, properties, and charge-transfer photochirogenetic reaction with 1,2-dianisylacenaphthene-1,2-diol

Article abstract of DOI:10.1021/jo0505324

A series of butterfly-shaped tetracyanoanthraquinodimethanes (TCNAQs) with a chiral amide auxiliary 1a-f were prepared from the corresponding anthraquinones. They are stronger acceptors than the unsubstituted derivative and undergo one-wave two-electron r

Full text of DOI:10.1021/jo0505324

Tetracyanoanthraquinodimethanes with a Chiral Amide Group:
Preparation, Properties, and Charge-Transfer Photochirogenetic
Reaction with 1,2-Dianisylacenaphthene-1,2-diol†
Takanori Suzuki,*,‡ Koji Ichioka,‡ Hiroki Higuchi,‡ Hidetoshi Kawai,‡ Kenshu Fujiwara,‡
Masakazu Ohkita,§ Takashi Tsuji,‡ and Yasutake Takahashi|
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan,
Department of Applied Chemistry, Nagoya Institute of Technology, Nagoya 466-8555, Japan, and
Chemistry Department for Materials, Faculty of Engineering, Mie University, Tsu 514-8507, Japan
tak@sci.hokudai.ac.jp
Received March 16, 2005
A series of butterfly-shaped tetracyanoanthraquinodimethanes (TCNAQs) with a chiral amide
auxiliary 1a-f were prepared from the corresponding anthraquinones. They are stronger acceptors
than the unsubstituted derivative and undergo one-wave two-electron reduction. They form weak
electron-donor-acceptor (EDA) complexes with the title pinacol 2. Upon charge-transfer excitation
of these complexes, dihydro-TCNAQs 3 and 1,8-dianisoylnaphthalene 4 were efficiently formed,
the latter of which is the product of a retropinacol reaction via 2+•. Partial enantiodifferentiation
of rac-2 was realized during the photoreactions with 2-[(R)-1-phenylethylcarbamoyl]-TCNAQ 1a
in CD3CN. Thus, optically active (S,S)-(+)-pinacol 2 (12.3% ee at 54% conversion; 21.5% ee at 70%
conversion) was recovered from the photolyzates. This reaction represents a new and rare example
of the pseudokinetic resolution of tert-alcohol accompanied by C-C bond fission. Significant
differences in the association constants for the diastereomeric EDA complexes are responsible for
the observed enantiodifferentiation.
11,11,12,12-Tetracyano-9,10-anthraquinodimethane (TC-
NAQ) is the dibenzo analogue of 7,7,8,8-tetracyano-p-
quinodimethane (TCNQ), the latter of which is a well-
known electron acceptor that gives a plethora of electro-
conducting organic solids. A representative example is
the crystalline charge-transfer (CT) complex with tetra-
thiafulvalene (TTF), and research on organic conductors
was initiated by the discovery of this first molecular
metal.1 Although TTF2 and its selenium analogue3 have
often been used to provide strong donating properties for
more sophisticated molecules, easy deformation of the
TCNQ skeleton prevents the modified acceptors from
inheriting the intrinsic redox behavior.4 This is also the
case with TCNAQ, which is calculated to be a much
weaker acceptor as a result of the severely deformed
butterfly-shaped molecular geometry.5
The preparation of TCNAQ was first reported by
Misumi’s group.6 Soon thereafter, Hu¨nig and co-workers
succeeded in the direct conversion of anthraquinones to
TCNAQs7 by condensation reactions with malononitrile
(2) (a) Nielsen, M. B.; Lomholt, C.; Becher, J. Chem. Soc. Rev. 2000,
29, 153. (b) Segura, J. L.; Mart´ın, N. Angew. Chem., Int. Ed. 2001, 40,
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† Dedicated to Prof. Tsutomu Miyashi on the occasion of his 65th
birthday.
‡ Hokkaido University.
§ Nagoya Institute of Technology.
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6138. (b) Kenny, P. W. J. Chem. Soc., Perkin Trans. 2 1995, 907.
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1983, 1229.
| Mie University.
(1) Ferraris, J.; Cowan, D. O.; Walatka, V. V., Jr.; Perlstein, J. H.
J. Am. Chem. Soc. 1973, 95, 948.
10.1021/jo0505324 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/10/2005
5592
J. Org. Chem. 2005, 70, 5592-5598

CT Photochirogenetic Reaction with Chiral TCNAQ
in the presence of TiCl4.8 Their detailed studies on redox
behavior revealed that TCNAQs undergo successive two-
electron capture nearly at the same potential7,9 which can
be accounted for by a drastic structural change upon
reduction.10 After these pioneering works, several groups,
including us, succeeded in preparing TCNAQs by differ-
ent methods,11-13 yet the TiCl4-assisted preparation is the
most versatile for obtaining a wide variety of function-
alized materials based on the TCNAQ skeleton,14 includ-
ing the intramolecular donor-acceptor compounds,15
photoconducting polymers,16 EL devices,17 and LB films.18
In addition to these examples, we recently found that
TCNAQs with a chiral ester group can serve as an
unprecedented molecular response system, by which an
electrochemical input is transduced into three-way spec-
tral outputs (UV-vis, fluorescence, and circular dichro-
ism).19 Although only a limited number of chiral electron
acceptors have been reported to date,20 we believe that
they may constitute an important class of compounds for
the development of novel materials such as electrochi-
roptical,21 mesophase,22 and NLO devices.23 In our con-
tinuing studies on chiral redox systems, we have found
that the newly designed TCNAQs 1 with a chiral amide
auxiliary can act as stereodifferentiating oxidants under
CT excitation conditions. In the first portion of this paper,
we report the preparation, redox properties, and X-ray
structures of the title acceptors 1. In the second part, we
SCHEME 1. Photoinduced Electron-Transfer
Reactions of TCNAQs 1 and Pinacol 2
show the photochemical reactions of TCNAQs 1 with rac-
1,2-dianisylacenaphthene-1,2-diol 2 that give the dihydro-
TCNAQs 3 and achiral 1,8-dianisoylnaphthalene 4
(Scheme 1). Racemic pinacol 2 was partially deracemized
by 1 upon irradiation and thus provides a new entry into
the less well-developed subject of photochirogenesis24
under photoinduced electron-transfer (PET) conditions.25,26
(7) Aumu¨ller, A.; Hu¨nig, S. Liebigs Ann. Chem. 1984, 618.
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Miyashi, T. In Chemistry of Nanomolecular Systems; Nakamura, T.,
Matsumoto, T., Tada, H., Sugiura, K., Eds.; Springer-Verlag: Heidel-
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Results and Discussion
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1. TCNAQs 1a-f with a Chiral Amide Auxiliary.
Preparation and Redox Properties. R-Substituted
benzylamines are easily accessible asymmetric mol-
ecules,27 and the benzylcarbamoyl-TCNAQs 1a-f were
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J. Org. Chem, Vol. 70, No. 14, 2005 5593

Suzuki et al.
TABLE 1. Selected Geometrical Parameters of 1a and
1f Determined by X-ray Analyses
SCHEME 2. Preparation of Chiral TCNAQs 1
1a (mol-1) 1a (mol-2)
1f
dihedral angle (deg) between
planes 1 and 2
designed so as to incorporate these asymmetric centers
with an (R)-configuration. To avoid unnecessary steric
repulsion between the dicyanomethylene group and the
amide moiety, the chiral substituents are attached at the
2-position of TCNAQ (Scheme 2). The corresponding
anthraquinones 5a-f were selected as precursors and
prepared in yields of 62-98% by the reactions of an-
thraquinone-2-carbonyl chloride with chiral amines ex-
cept for tert-amide 5e, which was more conveniently
prepared by N-methylation of 5a in quantitative yield.
Although 1-phenylethyl anthraquinone-2-carboxylate
could not be converted into the corresponding TCNAQ28
under the standard conditions (TiCl4/pyridine), the reac-
tion of phenylethylamide 5a with malononitrile proceeded
smoothly to give 2-[(R)-1-phenylethylcarbamoyl]-TCNAQ
1a in 61% yield. Similarly, 1b-f were obtained from the
corresponding quinones 5b-f in respective yields of 41-
97%. The newly prepared TCNAQs are stable crystalline
materials and were purified by chromatography and/or
recrystallization.
143.4
31.0
30.0
138.0
29.9
29.7
135.9
34.5
31.8
planes 3 and 4
planes 3 and 5
torsion angle (deg) around
C1-C2-CCO-OCO
11.5
169.8
164.0
147.7
177.7
136.2
143.1
175.2
176.7
OCO-CCO-NNH-HNH
HNH-NNH-C*-H*
SCHEME 3. Planar Chirality of Interconvertible
(R)- and (S)-TCNAQ
for 1f are listed in Table 1. Although all molecules of 1f
in a crystal adopt a planar chirality of (S) (Figure S3),
this may not be due to direct control by the chiral center
of the amide side chain but rather could arise from crystal
packing effects. In fact, a crystal of 1a contains both
diastereomers [(R)- and (S)-configuration of planar chiral-
ity] in a 1:1 ratio (Figures S1 and S2). The NMR spectra
of 1a and 1f with sharp resonances suggest that the
butterfly-shaped geometry is freely inverted in solution
(Scheme 3) and fixed to the planar chirality of (R) or (S)
upon crystallization, and this preference is mainly de-
termined by the crystal packing force.
The torsion angles around the amide moieties of 1a
and 1f exhibit considerable variation (Table 1), which
reflects the rotational and conformational flexibility of
the chiral side chain. A common feature observed in these
solid-state structures is the trans-conformation of the
-CONH- group (torsion angle 169.8-177.7°), which is in
accord with the preference reported for a series of
aromatic secondary amides.32
In contrast to that for the anthraquinone precursor 5a
red
red
(E1 ) -0.85 V, E2 ) -1.31 V vs SCE), the cyclic
voltammogram of TCNAQ 1a in MeCN shows only one
pair of reversible redox waves (Ered ) -0.32 V). Similarly,
other derivatives 1b-f undergo one-wave two-electron
reduction at a similar potential (-0.32 to -0.34 V),
showing that all of the newly prepared TCNAQs inherit
the intrinsic redox behavior of the parent TCNAQ.7,11,12
The slightly positive values of Ered compared to that for
the unsubstituted TCNAQ (-0.37 V) can be explained
by the electron-withdrawing ability of the carbamoyl
group, which is hardly affected by the difference in the
steric requirements of the chiral centers.
Molecular and Crystal Structures. X-ray structural
analyses of (R)-phenylethylamide 1a and (R)-cyclohexyl-
ethylamide 1f have shown that the TCNAQ skeleton in
them adopts a severely deformed butterfly-shaped ge-
ometry, as observed in the unsubstituted TCNAQ.31
Selected dihedral angles for the two crystallographically
independent molecules of 1a (mol-1 and mol-2) and those
In a crystal of cyclohexylethylamide 1f, molecules are
connected by intermolecular CO‚‚‚HN hydrogen bonds
(O‚‚‚H, 1.99 Å; O‚‚‚N, 2.92 Å; O‚‚‚H-N, 165.9°) to form
an infinite network along the a axis (Figure 1). In this
one-dimensional array, the cyclohexyl moiety of a certain
(27) (a) Pallavicini, M.; Valoti, E.; Villa, L.; Piccolo, O. Tetrahe-
dron: Asymmetry 1997, 8, 1069. (b) Pohland, A.; Sullivan, H. R. J.
Am. Chem. Soc. 1953, 75, 5898.
(28) Consistent with the facile nucleophilic transformation of the
dicyanomethylene group (ref 29), TCNAQs are prone to be hydrolyzed
under alkaline conditions to reproduce anthraquinones (ref 12a). This
is the major reason their condensations with malononitrile do not
proceed in the absence of TiCl4. When this Lewis acid induces the
degradation of substrates as in the case of this phenylethyl ester, other
conditions such as dry pyridine/MS4A (refs 12a, 30) seem promising.
With this method, we could prepare 2-[(R)-1-phenylethylcarbonyloxy]-
TCNAQ (1i), which cannot be obtained by the protocol using TiCl4.
(29) Falk, H.; Vaisburg, A. F. Monatsh. Chem. 1994, 125, 549.
(30) Yamaguchi, S.; Hanafusa, T.; Tanaka, T.; Sawada, M.; Kondo,
K.; Irie, M.; Tatemitsu, H.; Sakata, Y.; Misumi, S. Tetrahedron Lett.
1986, 21, 2411.
(31) (a) Schubert, U.; Hu¨nig, S.; Aumu¨ller, A. Liebigs Ann. Chem.
1985, 1216. (b) Heimer, N. E.; Mattern, D. L. J. Am. Chem. Soc. 1993,
115, 2217.
(32) (a) Azumaya, I.; Kagechika, H.; Fujiwara, Y.; Itoh, M.; Yamagu-
chi, K.; Shudo, K. J. Am. Chem. Soc. 1991, 113, 2833. (b) Bragg, R. A.;
Clayden, J.; Morris, G. A.; Pink, J. H. Chem. Eur. J. 2002, 8, 1279.
5594 J. Org. Chem., Vol. 70, No. 14, 2005

CT Photochirogenetic Reaction with Chiral TCNAQ
butterfly-shaped cavity of the neighboring TCNAQ skel-
eton is another common feature. Such inclusion behavior
was previously observed when we carried out X-ray
analyses34 on crystalline CT complexes of TCNAQs.35
These results prompted us to explore CT complexation
of chiral TCNAQs 1 with rac-1,2-dianisylacenaphthene-
1,2-diol 2. We anticipated that the electron-donating
naphthalene nucleus or one of the anisyl groups of 2
would be incorporated in the electron-deficient cavity of
1, and hydrogen bonds between the CO of 1 and HO of 2
may enhance the association.36 It would be of special
interest to clarify if 1 with a chiral amide group could
differentiate enantiomers of pinacol 2 to result in the
diastereoselective formation37a of electron-donor-acceptor
(EDA) complexes in solution.38 We envisaged that the
diastereoselectivity, if any, could be evaluated through
the photoreaction of the EDA complexes, where 1 could
serve as a chiral oxidant toward 2. The results will be
shown in the next section.
2. CT Excitation Reaction of TCNAQs 1a-f and
Pinacol 2.
FIGURE 1. Molecular arrangement in the crystal of 1f
determined by the X-ray analysis. Short CO‚‚‚NH contacts are
shown by broken lines.
Photoreaction of TCNAQ and Pinacol. Excitation
of the CT absorption band (CT excitation) is one of the
two common protocols in photoinduced electron transfer
(PET) reactions. Although the photosensitized reaction
is generally a more effective alternative in terms of
quantum yield,39 the close proximity of the radical ions
that result from CT excitation is preferable from the
viewpoint of stereodifferentiation by the chiral auxiliary
of 1. The TCNAQ skeleton is of special interest in PET
reactions40 since the change in geometry upon electron
capture retards back electron-transfer (BET), which
suppresses the main cause of the ineffectiveness of the
reaction in the CT excitation protocol. In fact, we previ-
ously found41 that 2,6-dibromo-substituted TCNAQ 1h
acts as a strong oxidant under photoirradiation condi-
tions to induce the retropinacol reaction39,42 of 1,2-
diarylacenaphthene-1,2-diols such as 2. In these reac-
tions, the acceptor was converted to a 2,6-dibromo
(34) Kabuto, C.; Fukazawa, Y.; Suzuki, T.; Yamashita, Y.; Miyashi,
T.; Mukai, T. Tetrahedron Lett. 1986, 27, 925.
(35) Mukai, T.; Suzuki, T.; Yamashita, Y. Bull. Chem. Soc. Jpn.
1985, 58, 2433.
FIGURE 2. Molecular arrangement in the crystal of 1a
determined by the X-ray analysis. Short CO‚‚‚NH and C‚‚‚C
(π-π) contacts are shown by broken lines.
(36) When rac-pinacol 2 was recrystallized from EtOH, the EtOH-
solvated crystal was obtained. X-ray analysis showed that the hydroxy
groups of 2 are involved in hydrogen bonds with the solvent. Similarly,
hydrogen bonds between 2 and solvent are present in the EtOH solvate
of optically pure (S,S)-2.
(37) (a) Both CT interaction and hydrogen bonds are considered to
be important factors for obtaining high diastereoselectivity upon the
separation of enantiomers by chiral HPLC: Perkle, W. H.; Pochapsky,
T. C. J. Am. Chem. Soc. 1987, 109, 5975. Perkle, W. H.; Pochapsky, T.
C.; Mahler, G. S.; Corey, D. E.; Reno, D. S. J. Org. Chem. 1986, 51,
4991. Perkle, W. H.; Welch, C. J. Tetrahedron: Asymmetry 1994, 5,
777. (b) In the present case, diastereoselectivities caused by CT
interaction and hydrogen bonds would be mismatched to induce lower
selectivity in less polar CDCl3.
(38) Many attempts to isolate crystalline complexes were unsuc-
cessful.
(39) Perrier, S.; Sankararaman, S.; Kochi, J. K. J. Chem. Soc., Perkin
Trans. 2 1993, 825.
(40) (a) Janssen, R. A. J.; Christiaans, M. P. T.; Hare, C.; Mart´ın,
N.; Sariciftci, N. S.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1995, 103,
8840. (b) Imahori, H.; Hasegawa, M.; Taniguchi, S.; Aoki, M.; Okada,
T.; Sakata, Y. Chem. Lett. 1998, 721. (c) Zerza, G.; Scharber, M. C.;
Brabec, C. J. Sariciftci, N. S.; Gomez, R.; Segura, J. L.; Mart´ın, N.;
Srdanov, V. I. J. Phys. Chem. A 2000, 104, 8315. (d) Zerza, G.; Cravino,
A.; Neugebauer, H.; Sariciftci, N. S.; Go´mez, R.; Segura, J. L.; Mart´ın,
N.; Svensson, M.; Andersson, M. R. J. Phys. Chem. A 2001, 105, 4172.
(e) Donkers, R L.; Workentin, M. S. J. Am. Chem. Soc. 2004, 126, 1688.
(41) Miyashi, T.; Suzuki, T. Kokagaku 1990, 14, 42.
molecule is accommodated in the butterfly-shaped cavity
made by the neighboring TCNAQ. In the case of 1a, the
crystal structure composed of two crystallographically
independent molecules is more complicated but shares
some common features with 1f. There is a hydrogen bond
between CO in mol-1 and HN in mol-2 (1.91 Å, 2.89 Å,
168.2°) to form a dyad (Figure 2), where the phenyl group
of mol-1 is deeply included in the cavity of mol-2 with a
face-to-face π overlap33 and van der Waals contacts of
C‚‚‚C (3.29 and 3.41 Å). The dyad of 1a is further
connected along the b axis by another hydrogen bond
(2.02 Å, 2.96 Å, 162.4°) to form an infinite lattice.
The hydrogen-bonded network is a common structural
motif observed in crystals of 1a and 1f, which may be
related to the geometrical variations about the amide
auxiliary in the solid-state molecular structures. The
accommodation of a phenyl or cyclohexyl group in the
(33) Crystalline 1a exhibits a yellow color, whereas 1f is nearly
colorless. Weak CT interaction through a face-to-face π overlap in the
former can explain this difference.
J. Org. Chem, Vol. 70, No. 14, 2005 5595

Suzuki et al.
SCHEME 4. EDA Complexation between 1 and
Enantiomers of 2
FIGURE 3. UV-vis absorption of 1a (2 × 10-2 mol dm-3), rac-2
(2 × 10-2 mol dm-3), and their mixture (2 × 10-2 mol dm-3
each) in CH3CN.
SCHEME 5. Thermal Isomerization of
Dihydro-TCNAQ 3
unprecedented technique for the pseudo-kinetic resolu-
tion of pinacols under photochemical conditions.
Enantiodifferentiating Photoreaction of TCNAQ
1a and rac-2. Upon the admixture of 2-[(R)-1-phenyl-
ethylcarbamoyl]-TCNAQ 1a (2 × 10-2 mol dm-3) and
rac-2 (2 × 10-2 mol dm-3) in CH3CN, a new absorption
band appeared in the visible region, which was assigned
to the CT band from the pinacol donor 2 (Eox ) +1.24 V)
to 1a (Figure 3). The association constant (KCT(rac)) was
determined to be 2.9 mol-1 dm3 by using the Benesi-
Hildebrand equation46 at 23 °C. Significantly different
values were obtained upon complexation of 1a with
enantiomers of 2: KCT ) 4.5 mol-1 dm3 for (R,R)-(-)-2
and 1.6 mol-1 dm3 for (S,S)-(+)-2. Therefore, the CT
excitation reaction between 1a and rac-2 may consume
(R,R)-(-)-2 faster than (S,S)-(+)-2 in CH3CN. To excite
the EDA complex selectively, the irradiation wavelength
is adjusted to λ > 435 nm by using a glass cutoff filter in
the subsequent photoreactions.
derivative of 9,10-bis(dicyanomethyl)anthracene (dihy-
dro-TCNAQ, 3),43 whereas the pinacols were transformed
into the corresponding 1,8-diaroylnaphthalenes via a
facile mesolytic bond fragment44 of pinacol cation radi-
cals. With these results in mind, we designed here a novel
asymmetric photochemistry based on the preferential
formation of diastereomeric EDA complexes between
chiral TCNAQ 1 and (R,R)- or (S,S)-2 in solution (Scheme
4).45 Although both enantiomers of 2 are transformed into
achiral diketone 4, the recovered pinacol 2 would become
optically active when photoirradiation is stopped before
the reactions are complete. This study should provide an
Upon irradiation of a 0.6 mL CD3CN solution of
TCNAQ 1a and rac-pinacol 2 (2 × 10-2 mol dm-3 each)
by a 500-W Xe lamp at 0 °C, 1a and 2 were consumed
gradually with the concomitant formation of dihydro-
TCNAQ 3a43b and diketone 4. No other species were
observed in the 1H NMR spectra of the reaction mixtures,
which shows that the retropinacol reaction under PET
conditions proceeded smoothly as in the case of dibromo-
TCNAQ 1h.41 Nearly half of the pinacol 2 was consumed
(42) (a) Reichel, L. W.; Griffin, G. W.; Muller, A. J.; Das, P. K.; Ege,
S. N. Can. J. Chem. 1984, 62, 424. (b) Sankararaman, S,; Kochi, J. K.
J. Chem. Soc., Chem. Commun. 1989, 1800. (c) Zhang, W.; Yang, L.;
Wu, L.-M.; Liu, Y.-C.; Liu, Z.-L. J. Chem. Soc., Perkin Trans. 2 1998,
1189. (d) Gan, H.; Leinhos, U.; Gould, I. R.; Whitten, D. G. J. Phys.
Chem. 1995, 99, 3566.
(43) (a) Very low solubility and strong C-H acidity hampered full
characterization of the dibromo derivative of dihydro-TCNAQ 3h.
Another obstacle is its easy isomerization to 2,6-dibromo-9,10-dihydro-
9-dicyanomethyl-10-dicyanomethyleneanthracene by a thermal 1,5-H
shift (Scheme 5), which is facilitated by polar solvents in which 3h
can be dissolved. (b) Because of difficulties similar to those with
dibromide 3h, dihydro-TCNAQs 3a-f with an amide side chain could
not be isolated or fully characterized. We confirmed their formation
and quantity based on the 1H NMR analyses of the reaction mixtures.
Although 3a-f are more soluble than 3h, the pale yellow color of 3a-f
turned deep violet upon contact with SiO2, which is characteristic of
the dianionic species of TCNAQs 12- (ref 19). The anionic dyes were
strongly adsorbed on SiO2 and were difficult to elute before air
oxidation of 12- to 1.
1
within 1 h on the basis of H NMR analyses, and the
reaction mixture was separated using SiO2 TLC followed
by HPLC. Only negligible amounts of 1a, 2, and 4 were
lost during the separation.43b To confirm that the pho-
toreaction of chiral TCNAQ 1a and the enantiomers of 2
proceeds with a difference in efficiency, the enantiomeric
excess (ee) of the recovered 2 was determined by chiral
HPLC, which gave a value of 12.3% ee [rich in (S,S)-(+)-
2]47 at 54% conversion. Nearly identical values were
obtained in two additional runs of similar experiments.
A higher value of 21.5% ee was obtained at 70% conver-
sion upon prolonged irradiation (3 h) in CD3CN. Although
the observed enantiodifferentiation is far from perfect,
(44) Maslak, P.; Chapman, W. H., Jr.; Narvaez, J. N.; Vallombroso,
T. M., Jr.; Watson, B. A. J. Am. Chem. Soc. 1995, 117, 12380.
(45) Besides the difference in the association constants (KCT(R,R) vs
KCT(S,S)), two other factors may contribute to the deracemization of 2.
One is the difference in the reaction rates (k1) from the radical ion
pairs (1-•/2+•) to the products (3 and 4), and the other is the difference
in the rates for BET (k2). The former reaction consists of very facile
intramolecular processes such as conformational change of 1-• (ref
7-12) or C-C bond cleavage of 2+• (refs 39, 42), so that the relative
(46) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1960, 82,
2134.
orientation of radical ions would not greatly affect the rate (k1(R,R)
)
(47) The optical resolution of sterically hindered tert-alcohols is a
difficult task. The first successful nonenzymatic kinetic resolution was
reported only very recently (ref 48). This is also the case for rac-pinacol
2, which had not been resolved before. There was no information about
the chiroptical properties or absolute configurations of the enantiomers.
The optical resolution procedures for 2 are given in Supporting
Information.
k1(S,S)). Since electrons can transfer from one to another over a
considerable distance, there is only a slight chance that the rates of
BET differ in the diastereomeric contact ion pairs (k2(R,R) ) k2(S,S)). From
the above points of view, the enantiodifferentiation observed in the
present work can be ascribed mainly to the preferential formation of
diastereomeric EDA complexes.
5596 J. Org. Chem., Vol. 70, No. 14, 2005

CT Photochirogenetic Reaction with Chiral TCNAQ
TABLE 2. Conversion Ratio of Pinacol 2 and
photoreactions were carried out in CD3CN. The optical
purity decreased drastically when the photoreactions of
1b and 1c were conducted in CDCl3, which is consistent
with the solvent effects observed for 1a. On the other
hand, neither bicyclic amide 1d nor tert-amide 1e induced
more than 1% ee for the recovered pinacol 2. Since
aromatic tertiary amides prefer a geometry different from
that of the secondary ones,32 the loss of stereoselectivity
may be related to such a drastic geometrical difference
around the chiral amide group. This may also be the case
for 1d, which has a stereogenic center confined in a ring
structure. Notably, the aromatic ring is not essential at
Enantiomeric Excessa of Recovered 2 after Irradiationb
of EDA Complexes Consisting of TCNAQ 1a-f and rac-2
(R)-R1R2C*H-NR3-CO-
group of 1
a
b
c
d
e
f
R1
R2
R3
Ph
Me
H
Ph
Et
H
Ph
c
c
H
Ph
Me
Me
cHex
Me
H
iPr
H
in CD3CN
conv (%)
ee (%)
54
48
48
55
51
d
+12.3 +12.2 +11.5 -0.6 +0.8
in CDCl3
conv (%)
ee (%)
56
50
50
52
50
49
the asymmetric center since alicyclic amide 1f (R1
)
+2.7 +1.4 +1.6 +0.8 <0.5 +2.3
cHex, R2 ) Me) induced a similar optical purity for 2 as
1a-c in CDCl3. This suggests that the π-type substituent
at the chiral center plays only a slight role in the
diastereoselective complexation of 1 with (R,R)-(-)-2. A
secondary amide group with a large difference in steric
demand between R1 and R2 should be a promising chiral
auxiliary for attaining a higher enantiodifferentiation of
pinacol 2.50
a Values with a plus sign indicate that the recovered 2 is rich
in (S,S)-(+)-2. b Irradiated by a 500-W Xe lamp through a glass
cutoff filter (λ > 435 nm) at 0 °C. [1] ) [rac-2] ) 2 × 10-2 mol
dm-3. Irradiation time is 1 h (a-g in CD3CN), 2 h (a, d, e in
CDCl3), 2.5 h (c in CDCl3), or 2.7 h (b in CDCl3). c (1R)-Indanyl.
d Because of the low solubility, the reaction could not be conducted
under comparable conditions.
this reaction provides a new and rare example of asym-
metric photoreactions under CT excitation conditions.25,49
When the photoreactions of 1a and rac-2 were con-
ducted in CDCl3, 2 was converted to 4 more slowly than
in CD3CN. After irradiation for 2 h, 56% of pinacol 2 was
consumed, and the enriched (+)-2 that was recovered
exhibited only marginal optical purity (2.7% ee deter-
mined by both chiral HPLC and CD spectrum). The much
lower enantiodifferentiation attained in CDCl3 is consis-
tent with the fact that the KCT values of 1a with
enantiomers of 2 are nearly the same in CHCl3: KCT(R,R)
) 0.9 mol-1 dm3; KCT(S,S) ) 1.3 mol-1 dm3. Comparisons
of KCT values in CHCl3 and CH3CN suggest that the
hydrogen bonds between 1a and 2 in the former solvent,
if any, do not enhance the association or increase the
enantiodifferentiation.37b To further investigate the ob-
served solvent effects as well as the stereogenic effects
of chiral centers, CT excitation reactions of EDA com-
plexes were carried out for other combinations of TCNAQ
1b-f with 2.
Conclusion
In this work, we prepared a series of TCNAQs 1 with
a chiral benzylcarbamoyl auxiliary, which act as strong
oxidants to induce the retropinacol reaction of 2 upon the
photoirradiation of EDA complexes. Although the asym-
metric centers of 1 do not strongly affect the planar
chirality of the TCNAQ skeleton or the conformations of
chiral side chains in the solid state, some of them play
an important role in realizing the partial deracemization
of 2 through the pseudo-kinetic resolution of sterically
hindered tert-alcohol47 in solution. This reaction can serve
as a novel prototype of photochirogenesis using the CT
excitation protocol.51
Experimental Section
Preparation of TCNAQs 1a-f. To a solution of 2-[(R)-1-
phenylethylcarbamoyl]anthraquinone 5a (1.42 g, 4.00 mmol)
and malononitrile (1.32 g, 20.0 mmol) in dry CHCl3 was added
TiCl4 (8.00 mL, 72.0 mmol) followed by dry pyridine (16 mL)
dropwise over 10 min at 0 °C. After the mixture was refluxed
for 8 min, the solvent was removed under reduced pressure.
The residue was treated with aqueous HCl (6 mol dm-3, 200
mL) and extracted with CHCl3. The combined organic layers
were dried over anhydrous Na2SO4. After filtration and
removal of solvent, the residue was chromatographed on SiO2
and further purified by recrystallization from CH3CN to give
11,11,12,12-tetracyano-2-[(R)-1-phenylethylcarbamoyl]-9,10-
anthraquinodimethane 1a (1.09 g) as yellow needles in 61%
yield. Other TCNAQs 1b-f were similarly prepared by start-
Enantiodifferentiating Photoreactions of TCNAQ
1b-f and rac-2. When EDA complexes composed of
1b-f and rac-2 were irradiated with a 500-W Xe lamp
through a glass cutoff filter (λ > 435 nm) at 0 °C, nearly
half of the pinacol 2 was consumed within 1 h in CD3CN
and within 2-2.7 h in CDCl3. The recovered pinacol 2
was enriched with the (S,S)-(+)-enantiomer in all cases
except one (1d in CD3CN), which shows that the stereo-
genic effects of chiral benzylcarbamoyl groups with (R)-
configuration endow 1 with a diastereomeric preference
for (R,R)-(-)-2 upon EDA complexation. On the other
hand, the degree of this preference differs considerably
depending on the steric requirements of chiral centers,
as shown in Table 2.
(50) (a) According to this idea, 2-[(R)-1-(2-naphthyl)ethyl]-TCNAQ
(R1 ) 1-naphthyl, R2 ) Me) (1j) was prepared, yet very low solubility
in common solvents hampered its use in photoreactions. (b)
A
preliminary study showed that a C2-symmetric TCNAQ derivative with
two (R)-phenylethylamide groups at 2,6-positions (1k) could also be
used for an enantiodifferentiating photoreaction, yet the recovered 2
exhibited a similar degree (10.7% ee at 52% conversion by 0.5 h of
irradiation in CD3CN at 0 °C) of optical activity as with 1a, which has
only one chiral amide group.
(R)-Phenylpropylamide 1b (R1 ) Ph, R2 ) Et) and (R)-
phenylisobutylamide 1c (R1 ) Ph, R2 ) iPr) exhibit a
similar degree of enantiodifferentiation as 1a (R1 ) Ph,
R2 ) Me), and pinacol 2 was recovered with an optical
purity of more than 10% ee at 48% conversion when the
(51) Treatment of dihydro-TCNAQs with Pd/C in CH2Cl2 caused its
dehydrogenation to the corresponding TCNAQs. Therefore, the cata-
lytic oxidation of 2 to 4 by dibromo-TCNAQ 1h under CT excitation
conditions could be realized (turnover > 10) when the photoreaction
was conducted in the presence of Pd/C (ref 41). A similar protocol could
make it possible in the future to conduct the kinetic resolution of rac-2
using a catalytic amount of chiral TCNAQs.
(48) Nishimura, T.; Matsumura, S.; Maeda, Y.; Uemura, S. Tetra-
hedron Lett. 2002, 43, 3037.
(49) Suzuki, T.; Fukushima, T.; Yamashita, Y.; Miyashi, T. J. Am.
Chem. Soc. 1994, 116, 2793.
J. Org. Chem, Vol. 70, No. 14, 2005 5597

Suzuki et al.
ing from the corresponding anthraquinones 5b-f. Analytical
and spectral data are given in Supporting Information.
Preparation of Racemic Diol rac-2. This material was
obtained in 62% yield from acenaphthenequinone and 4-meth-
oxyphenyllithium by modifying the reported procedure.52
Preparation of Anthraquinones 5a-d and 5f. To a
solution of (R)-(+)-1-phenylethylamine (2.62 g, 21.7 mmol) in
50 mL of dry CHCl3 was added anthraquinone-2-carbonyl
chloride (6.45 g, 23.8 mmol). After the mixture was refluxed
for 3 h, the solvent was removed under reduced pressure. The
residue was recrystallized from CH3CN to give 2-[(R)-1-
phenylethylcarbamoyl]-9,10-anthraquinone 5a (6.69 g) as yel-
low needles in 87% yield. Other quinones, 5b-d and 5f, were
similarly prepared by starting from the corresponding chiral
amines with an (R)-configuration. Most of the amines are
commercially available, and (R)-1-phenylisobutylamine was
prepared as described in the literature.27 Analytical and
spectral data are given in Supporting Information.
Preparation of Anthraquinone 5e. To a solution of
secondary amide 5a (500 mg, 1.41 mmol) in dry THF (15 mL)
was added NaH (60% in oil, 113 mg, 2.83 mmol) at 50 °C. After
gas evolution ceased, CH3I (2.7 mL, 43.4 mmol) was added to
the red solution at 40 °C, and the mixture was stirred for 15
h at this temperature. The mixture was diluted with water
and extracted with CHCl3. The combined organic layers were
dried over anhydrous Na2SO4. After filtration and removal of
the solvent, the residue was chromatographed on SiO2 to give
2-[methyl-(R)-1-phenylethylcarbamoyl]-9,10-anthraquinone 5e
(516 mg) as a yellow solid in 99% yield. Analytical and spectral
data are given in Supporting Information.
Determination of Association Constants (KCT). In the
presence of a large excess of pinacol 2, KCT values of the EDA
complexes of 1a were determined in CH3CN and CHCl3 by a
spectrophotometric procedure based on the Benesi-Hilde-
brand relationship46 at 23 °C. In all cases, linear correlations
were observed, indicating a 1:1 molar ratio for the EDA
complex. In these measurements, the concentration of 1a is 2
× 10-3 mol dm-3 and those of 2 are 0.10-0.15 mol dm-3. Errors
are 5% (rac-2-1a), 15% [(R,R)-2-1a] and 20% [(S,S)-2-1a],
respectively, for the values obtained in MeCN. Those for the
values measured in CHCl3 are 10%. Molar extinction coef-
ficients for [(R,R)-2-1a] and [(S,S)-2-1a] are 200 and 400 at
440 nm in MeCN, respectively. The CT absorption bands for
the diastereomeric EDA complexes are given in the Supporting
Information.
× 10-2 mol dm-3) in 0.6 mL of CD3CN was immersed in a cold
bath (0 °C) and irradiated by a 500-W Xe lamp through a
Corning 3-72 glass cutoff filter (λ > 435 nm). The progress of
the reaction was monitored by 1H NMR spectroscopy. After 1
h, the mixture was separated by SiO2 preparative TLC (MeOH:
CHCl3 ) 1:30) followed by HPLC (SiO2; AcOEt/CHCl3 ) 1:150)
to isolate 1a, 2, and 4.53 The enantiomeric excess of recovered
2 was determined by CHIRALCEL-OD-H (0.46 cm φ × 25 cm;
EtOH/n-hexane ) 1:1; flow rate of 0.5 mL min-1). The
retention time of (R,R)-(-)-2 is 9.7 min and that of (S,S)-(+)-2
is 13.4 min (separability factor R ) 1.38; resolution Rs ) 1.64;
see Figure S7). Other EDA complexes were similarly irradi-
ated, and the photolyzates were analyzed as described above.
The CD spectrum of recovered 2 was also measured when
necessary to determine the small ee values precisely. The
temperature-dependence of the photoreaction could not be
investigated because of the limited solubility of 1 at low
temperature. At higher temperatures, subsequent transforma-
tion of 3a into four isomers by thermal 1,5-H shift43 made the
reaction too complicated to analyze.
Redox Potential Measurements. Redox potentials were
measured by cyclic voltammetry in dry MeCN containing 0.1
mol dm-3 Et4NClO4 as a supporting electrolyte. Ferrocene
undergoes 1e-oxidation at +0.38 V under the same conditions.
All of the values shown in the text are in E/V vs SCE.
Acknowledgment. This work was supported by
grants from the Ministry of Education, Science and
Culture, Japan (nos. 14654139, 15350019). We thank
Prof. Tamotsu Inabe (Hokkaido University) for use of
the X-ray structure analysis system. MS spectra were
measured by Dr. Eri Fukushi and Mr. Kenji Watanabe
at the GC-MS & NMR Laboratory (Faculty of Agricul-
ture, Hokkaido University). Elemental analyses were
conducted by Ms. Akiko Maeda at the Center for
Instrumental Analysis (Hokkaido University).
Supporting Information Available: Physical data of
TCNAQs 1 and AQs 5 and details of X-ray analyses. ORTEP
drawings (Figures S1-S6) and structural data for the X-ray
analyses (positional and thermal parameters, bond distances
and angles in CIF format) of 1a, 1f, rac-2‚EtOH, (S,S)-(+)-2‚
EtOH, and (S,S,S,S)-7; and the HPLC chart of (+)-2 (Figure
S7). CT absorption bands of [(S,S)-(+)-2-1a] and [(R,R)-(-)-
2-1a] (Figure S8). Procedure of optical resolution of pinacol
2. This material is available free of charge via the Internet at
http://pubs.acs.org.
CT Excitation Reactions of EDA Complexes. The typi-
cal procedures are as follows. An NMR tube containing a
degassed solution of 1a (2.0 × 10-2 mol dm-3) and rac-2 (2.0
JO0505324
(52) (a) Bachmann, W. E.; Chu, E. J. J. Am. Chem. Soc. 1936, 58,
1118. (b) 1H NMR (300 MHz, CDCl3) δ 7.89 (2H, d, J ) 8.3 Hz), 7.64
(2H, dd, J ) 6.9, 8.3 Hz), 7.37 (2H, d, J ) 6.9 Hz), 7.17 (4H, AA′XX′),
6.88 (4H, AA′XX′), 3.82 (6H, s), 2.15 (2H, s).
(53) (a) Letsinger, R. L.; Lansbury, P. T. J. Am. Chem. Soc. 1956,
78, 2648. (b) 1H NMR (300 MHz, CDCl3) δ 8.04 (2H, dd, J ) 7.2, 2.1
Hz), 7.80 (4H, AA′XX′), 7.58-7.49 (4H, m), 6.85 (4H, AA′XX′), 3.84 (6H,
s).
5598 J. Org. Chem., Vol. 70, No. 14, 2005