Dication species stabilized by heteroazulenes: Synthesis and properties of 1,3- and 1,4-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyliumyl]-, bis[bis(1,2-dihydro-N-methyl-2-oxocyclohepta-[b]pyrrol-3-yl)methyliumyl]benzene, and their related dications

Article abstract of DOI:10.1039/b009373o

A general synthetic route to a novel type of heteroazulene analogue of exceptionally stable dimethyliumbenzenes (16a-c·2PF₆^- and 17a-c·2PF₆^-) bearing 1,3-di- and 1,4-dimethylium groups substituted with four 2H-cyclohepta-[b]furan-2-one 9a, four 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 9b, and two 9a and two 9b, is reported. The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 9a and 9b with isophthalaldehyde and terephthalaldehyde to afford the corresponding 1,3- and 1,4-dimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF₆ solution. In spite of the dicationic nature of 16a-c and 17a-c, they exhibited high stability with large pK_R+ values due to the stabilizing effect of the heteroazulene units; however, we could not determine pk_R+ and pK_R++ values separately. Thus, the pK_R+ values correspond to the average of the pH used to form neutralized dications and half-neutralized monocations. The electrochemical reduction of the cations exhibits irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the basis of the comparison with those of the related monocation species.

Full text of DOI:10.1039/b009373o

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Dication species stabilized by heteroazulenes: synthesis and
properties of 1,3- and 1,4-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-
yl)methyliumyl]-, bis[bis(1,2-dihydro-N-methyl-2-oxocyclohepta-
[b]pyrrol-3-yl)methyliumyl]benzene, and their related dications
2
Shin-ichi Naya and Makoto Nitta*
Department of Chemistry, School of Science and Engineering, and Materials Research
Laboratory for Bioscience and Photonics, Waseda University, Tokyo 169-8555, Japan
Received (in Cambridge, UK) 27th September 2000, Accepted 22nd December 2000
First published as an Advance Article on the web 26th January 2001
A general synthetic route to a novel type of heteroazulene analogue of exceptionally stable dimethyliumbenzenes
(16a–cؒ2PF6؊ and 17a–cؒ2PF6؊) bearing 1,3-di- and 1,4-dimethylium groups substituted with four 2H-cyclohepta-
[b]furan-2-one 9a, four 1,2-dihydro-N-methylcyclohepta[b]pyrrol-2-one 9b, and two 9a and two 9b, is reported.
The synthetic method is based on a single and stepwise TFA-catalyzed electrophilic aromatic substitution on the
heteroazulenes 9a and 9b with isophthalaldehyde and terephthalaldehyde to afford the corresponding 1,3- and
1,4-dimethylbenzene derivatives, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange
of the counter-anion by using aq. HPF6 solution. In spite of the dicationic nature of 16a–c and 17a–c, they exhibited
ϩ
high stability with large pKR values due to the stabilizing effect of the heteroazulene units; however, we could not
ϩ
ϩϩ
ϩ
determine pKR and pKR values separately. Thus, the pKR values correspond to the average of the pH used to
form neutralized dications and half-neutralized monocations. The electrochemical reduction of the cations exhibits
irreversible waves and low reduction peak potentials upon cyclic voltammetry (CV); the values are discussed on the
basis of the comparison with those of the related monocation species.
1,4-bis[bis(3-methylazulen-1-yl)methyliumyl]benzene 4, and
Introduction
their related derivatives. The dications are extraordinarily stable
The first aryl-stabilized carbodications, 1,3-bis(diphenylmethyl-
iumyl)benzene 1 and 1,4-bis(diphenylmethyliumyl)benzene 2 in
with high pKR values (11.5 for 3 and 11.2 for 4).4,5 The pKR
ϩ
ϩ
values seem to be reasonable because azulene derivatives stabil-
ize cations, i.e., triazulen-1-ylmethyl,6,12 diazulen-1-yl(phenyl)-
methyl,6,9,11–15 and azulen-1-yl(diphenyl)methyl cations6,9,11,12,14
and their derivatives, to a great extent as studied extensively by
Asao and his co-workers. Much of the motivation for studying
the properties of organic molecules stems from manipulation of
the primary chemical structure. Strategies for raising or lower-
ing the HOMO and LUMO levels include conjugation length
control, as well as the introduction of an electron-donating or
-withdrawing group to the parent molecular skeleton.
Based on this concept, we have studied previously the
synthesis and properties of heteroazulene analogues of
the triphenylmethyl cation, i.e., tris(2-oxo-2H-cyclohepta-
[b]furan-3-yl)-, tris(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]-
pyrrol-3-yl)-, and tris(1,2-dihydro-2-oxo-N-methylcyclohepta-
[b]pyrrol-3-yl)methyl cations, 5a–c,16 as well as bis(2-oxo-2H-
cyclohepta[b]furan-3-yl)(phenyl)methyl and bis(1,2-dihydro-2-
oxo-N-phenylcyclohepta[b]pyrrol-3-yl)(phenyl)methyl cations
6a–c and 7a–c.17 The MO calculations (AM1: MOPAC97)18
predict the heteroazulenes 9a and 9b have lower HOMO and
LUMO energies as compared with those of azulene 8. The
ϩ
cations are very stable with pKR values of 9.7–13.1 for 5a–c,
12.4–7.9 for 6a–c, 13.5–11.1 for 7a–c, which are considerably
solution were independently reported by Hart et al.1,2 and Volz
and Volz de Lecea3 more than three decades ago. It is remark-
able that Asao and his co-workers have reported recently the
synthesis and properties of azulene analogues of 1 and 2,
i.e., 1,3-bis[bis(3-methylazulen-1-yl)methyliumyl]benzene 3,
DOI: 10.1039/b009373o
J. Chem. Soc., Perkin Trans. 2, 2001, 275–281
This journal is © The Royal Society of Chemistry 2001
275

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Ϫ
Table 1 Results for the preparation of dimethylbenzene derivatives 12a–c and 13a–c, and dimethylium salts 16a–cؒ2PF6Ϫ and 17a–cؒ2PF6
Condensation
Product
Hydride abstraction
Run
Heteroazulene
Aldehyde
Yield (%)
Product
Yield (%)
1
2
3
4
5
9aa
9ba
9aa
9ba
9ab
10
10
11
11
10
12a
12b
13a
13b
14
12a
12c
15
88
100
83
100
74
13
98
81
9
100
16aؒ2PF6؊
16bؒ2PF6؊
17aؒ2PF6؊
17bؒ2PF6؊
—
91
70
96
69
—
—
94
—
—
75
—
6
7
9b
14
11
16cؒ2PF6؊
—
9ab
13a
13c
—
8
9b
15
17cؒ2PF6؊
a Four equiv. heteroazulene. b Two equiv. heteroazulene.
19
ϩ
larger than that of the triphenylmethyl cation (pKR = Ϫ6.44)
ϩ
and similar to that of the triazulen-1-ylmethyl cation (pKR
=
11.3)6–12 Furthermore, the electrochemical reduction of 5a–c
exhibited reversible waves and low reduction potentials, E1red
and E2red, upon cyclic voltammetry (CV), respectively, and they
are more positive as compared with those of the triazulen-1-
ylmethyl cation.6 Thus, heteroazulenes, such as 9a and 9b,
are suggested to stabilize not only cations but also radical
species and anions. Thus, in connection with our previous
study of heteroazulene-substituted methylium ions,16,18 we
studied an efficient synthesis of heteroazulene analogues of
dications 1–4, 1,3-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)-
methyliumyl]- and 1,3-bis[bis(1,2-dihydro-2-oxo-N-methyl-
cyclohepta[b]pyrrol-3-yl)methyliumyl]benzenes 16a and 16b,
and their 1,4-isomers 17a and 17b, as well as 1-[bis(2-oxo-2H-
cyclohepta[b]furan-3-yl)methyliumyl]-3-[bis(1,2-dihydro-2-oxo-
N-methylcyclohepta[b]pyrrol-3-yl)methyliumyl]benzene
16c
and its 1,4-analogue 17c. The dications exhibited quite large
ϩ
pKR values and low reduction potentials. We report herein the
results in detail.
Results and discussion
Synthesis
The reactions of four molar equivalent amounts of 2H-cyclo-
hepta[b]furan-2-one 9a20 and 1,2-dihydro-N-methylcyclo-
hepta[b]pyrrol-2-one 9b21,22 with one equivalent amount of
isophthalaldehyde 10 in CH2Cl2–TFA (10:1) at rt for 48 h
afforded 1,3-bis[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl]-
benzene 12a and the corresponding pyrrole derivative 12b
in moderate to good yields (Scheme 1; Table 1, Runs 1 and
2). Similarly, the reactions of compounds 9a and 9b with
terephthalaldehyde 11 yielded 1,4-bis[bis(2-oxo-2H-cyclo-
hepta[b]furan-3-yl)methyl]benzene 13a and the corresponding
pyrrole derivative 13b in similar yields (Table 1, Runs 3 and 4).
On the other hand, controlled reactions of two molar equiv-
alent amounts of 9a with one molar equivalent amount of
isophthalaldehyde 10 and terephthalaldehyde 11 in CH2Cl2–
TFA (10:1) at rt afforded the desired 3-[bis(2-oxo-2H-
cyclohepta[b]furan-3-yl)methyl]benzaldehyde 14 and 4-[bis(2-
oxo-2H-cyclohepta[b]furan-3-yl)methyl]benzaldehyde 15, in
addition to small amounts of 12a and 13a, respectively (Table 1,
Runs 5 and 7). The aldehydes 14 and 15 reacted with hetero-
azulene 9b in a similar fashion to afford 1-[bis(2-oxo-2H-
cyclohepta[b]furan-3-yl)methyl]-3-[bis(1,2-dihydro-2-oxo-N-
methylcyclohepta[b]pyrrol-3-yl)methyl]benzene 12c and the
corresponding 1,4-disubstituted isomer 13c in excellent yields,
respectively (Table 1, Runs 6 and 8). The compounds 12a–c and
13a–c formed powdery, orange or yellow crystals, the structures
Scheme 1 Reagents and conditions: i CH2Cl2–TFA (10:1), rt; ii CH2Cl2–
TFA (10:1), 9b, rt.
13a–c with DDQ in CH2Cl2 at rt for 1 h, followed by addition
of aqueous 60% HPF6 solution afforded stable dicationic salts
16a–cؒ2PF6؊ and 17a–cؒ2PF6؊ in the yields listed also in Table 1
(Scheme 2).
1
of which were assigned on the basis of their IR, H and 13C
NMR spectral data, as well as elemental analyses and mass
spectral data. The oxidative hydrogen abstraction of 12a–c and
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Fig. 1 UV–vis spectra of cations 16a–c in CH3CN.
Fig. 2 UV–vis spectra of cations 17a–c in CH3CN.
the series of 17a (612 nm), 17b (639 nm), and 17c (620 nm)
resemble each other. The longest wavelength absorption
maxima of dications 16a,c and 17a,c are even shorter than
those of the related monocations 6b,c (621 nm), while those
of 16b and 17b are shorter than those of similar phenyl-
substituted monocations 7b–e (652 nm). Thus, the UV–vis
spectra of dications 16a–c and 17a–c do not suggest the pres-
ence of appreciable conjugation among the methylium units.
This feature seems to be reasonable based on our previous study
considering the longest wavelength absorption maxima of 6b–e
(621 nm) and 7b–e (652 nm) as well as on the calculations of
the stable conformations of 6a–c and 7a–c with reference to the
dihedral angles, θ1, θ2, and θ3, which express deviation of
the plane of the phenyl groups and heteroazulenes from the
reference plane (the plane which is defined by the three aromatic
ipso carbons, Fig. 3).17
Scheme 2 Reagents and conditions: i (a) DDQ in CH2Cl2 (b) 60% aq.
HPF6.
Spectroscopic properties
The signals of the methine protons of methane derivatives
12a–c and 13a–c disappeared in the 1H NMR spectra of
16a–cؒ2PF6؊ and 17a–cؒ2PF4؊. Thus, the 1H NMR spectra also
support the dicationic structures of these compounds. Proton
signals on the seven-membered rings of 16a–cؒ2PF6؊ and 17a–cؒ
2PF6؊ appear as broad signals. Attempted measurement of
Dications 16a–c and 17a–c were fully characterized by the
spectral data, as shown in the Experimental section. The salts
16a–cؒ2PF6؊ and 17a–cؒ2PF6 were easily crystallized to give
؊
complexes containing CH2Cl2 or HPF6 molecules, respectively,
in the crystal lattice. Thus, some of the salts did not give satis-
factory analytical data; however, the mass spectra of the salts
1
؊
the H NMR spectra of 16bؒ2PF6؊ and 17bؒ2PF6 at temper-
atures ranging from rt to 70 ЊC (in CD3CN) exhibited no appre-
ciable change in the broad signals. Thus, slow conformational
change of the heteroazulene moieties of these cations occurs in
the 1H NMR time scale at these temperatures.
16a–cؒ2PF6؊ and 17a–cؒ2PF6 ionized by FAB exhibited the
؊
correct ion peaks, Mϩ Ϫ 2PF6, which are indicative of the
dicationic structure of these compounds. The characteristic
bands for the counter anion PF6Ϫ are observed at 836–843 and
837–843 cmϪ1 in the IR spectra of 16a–cؒ2PF6؊ and 17a–cؒ
2PF6؊, respectively. These features also support the dicationic
nature of the compounds. The UV–vis spectra of cations 16a–c
and 17a–c in CH3CN are shown in Figs. 1 and 2, respectively.
The longest wavelength absorption maxima of the series of
dications 16a (610 nm), 16b (630 nm), and 16c (612 nm), and
؉
Stability of the dications: pKR values and reduction potentials
The affinity of the carbocation toward hydroxide ions,
ϩ
expressed by the pKR value, is the most common criterion of
carbocation stability.23 The pKR values of the dications 16a–c
ϩ
and 17a–c were determined spectrophotometrically in buffer
J. Chem. Soc., Perkin Trans. 2, 2001, 275–281
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Table 2 pKR values and reduction potentialsa of dications 16a–c,b
ϩ
expected to give a non-Kekulé-type electronic structure upon
reduction. Actually, dication 16c, which has two different
methylium units, exhibited four reduction potentials, E1red
and 17a–cb
–
ϩ
Compd.
pKR
E1red
E2red
E3red
E4red
E4red (Table 2 and Scheme 3). The values of E1red and E2red are
16a
16b
16c
17a
17b
17c
6c
9.0
12.1
12.7
9.3
11.5
12.0
9.3
12.0
Ϫ0.33
Ϫ0.60
Ϫ1.05
Ϫ1.37
Ϫ0.34
Ϫ0.04
Ϫ0.30
Ϫ0.19
Ϫ0.62
Ϫ0.34
Ϫ0.50
Ϫ0.96
Ϫ1.06
Ϫ1.10
Ϫ0.95
Ϫ1.37
Ϫ1.34
Ϫ1.38
Ϫ1.32
Ϫ0.57
(Ϫ1.03)
(Ϫ1.29)
(Ϫ0.31)
(Ϫ0.53)
7c
3c
11.5 0.2
11.2 0.1
4c
a Peak potentials V vs. Ag/Agϩ. Reversible processes are shown in
parentheses. b 16a–cؒ2PF6؊ and 17a–cؒ2PF6؊ were used for the meas-
urement. c Ref. 4 and 5.
Fig. 3
solutions prepared in 20% aqueous CH3CN and summarized in
Table 2 along with those of reference compounds 3, 4, 6c, and
7c.4,5,17 We could not determine pKR and pKR values separ-
ately. The two methylium units of the dications 16a–c and 17a–c
were neutralized simultaneously by using buffer solutions of
ϩ
ϩϩ
ϩ
pH ranging from 8.3 to 14.0. Thus, the pKR values correspond
to the average of the pH used to form neutralized dications
and half-neutralized monocations. The neutralization of the
dications 16a–c and 17a–c is not completely reversible. This
feature may be ascribed to the instability of the neutralized
ϩ
products under the conditions of the pKR measurement.
Immediate acidification of an alkaline solution of 16a–c and
17a–c with TFA regenerated the absorption maxima of the
cations in the visible regions to 85–90%. As expected, the
ϩ
heteroazulenes effectively stabilize the dications, and the pKR
values of 16a–c and 17a–c are extremely high as compared with
ϩϩ
ϩ
those of dications 1 and 2 (1: pKR Ϫ9.9 and pKR Ϫ7.9; 2:
2,3
ϩϩ
ϩ
ϩ
pKR Ϫ10.5 and pKR Ϫ8.1). Although the pKR values of
16a and 17a are similar to that of monocation 6c, the values of
16b,c and 17b,c are similar to that of 7c. The pyrrole substitu-
ent 9b stabilizes dications 16b,c and 17b,c slightly more as com-
pared with azulene analogues 3 and 4 (Table 2). The relatively
high stability of the dications 16b,c and 17b,c as compared with
those of 16a and 17a is attributable to the low electronegativity
of the nitrogen atom as compared with the oxygen atom in the
five-membered ring of the heteroazulene unit.
The reduction potentials of dications 16a–c and 17a–c were
determined by cyclic voltammetry (CV) in CH3CN. The reduc-
tion waves of both series of 16a–c and 17a–c were irreversible
under the conditions of CV measurements, and thus, the peak
potentials are summarized in Table 2 together with those of the
reference monocations 6c and 7c.17 The reduction behavior of
dications is affected by the heteroazulene units and their substi-
tution patterns. The reduction of a series of dications 16a–c is
Scheme 3
similar to those of E1red of related monocations 6c and 7c,
respectively, and the values of E3red and E4red are close to the
values of E2red of monocations 6c and 7c, respectively. Thus,
the values of E1red–E4red for 16c could correspond to those
obtained in the process giving 18c, 19c, 20c, and 21c, respect-
ively. On the other hand, dications 16a and 16b, both of which
have the same two methylium units, exhibited two-step reduc-
tion peaks at Ϫ0.33 and Ϫ1.05, and Ϫ0.60 and Ϫ1.37; the
former values are very similar to those of E1red and E2red for
monocation 6c, as well as E1red and E3red for 16c, respectively.
These last two values for 16b, are similar to those of E1red
and E2red for monocation 7c, as well as E2red and E4red for
dication 16c, respectively. Thus, the reduction process of 16a,b
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J. Chem. Soc., Perkin Trans. 2, 2001, 275–281

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is rationalized to proceed via two steps, two-electron reductions
affording diradical species 19a,b and dianions 21a,b, respect-
ively (Scheme 3). Furthermore, this feature indicates that 16a–c
give non-Kekulé-type electronic structures upon reduction and
reflects the absence of conjugation between the two methylium
units. On the other hand, the CV of the series 17a–c exhibited
four reduction potentials, and they are suggestive of four steps,
four-electron reduction of 17a–c giving 22a–c, 23a–c, 24a–c,
and 25a–c, respectively (Scheme 3), as in the case of dication
16c. The less negative reduction potentials, E1red and E2red, for
17a–c, as compared with those of E1red and E2red for 16a–c, as
well as E1red for 6c and 7c, respectively, are attributable to the
destabilization arising from the through-bond electronic repul-
sion of the two methylium units in 17a–c. The reduction poten-
tials, E3red and E4red, for 17a–c are similar, and also similar to
those of dication 16c. This feature may be ascribed to the
contribution of a common closed-shell structure for 23a–c.
In summary, efficient synthesis of two series of fairly stable
heteroazulene-substituted dications 16a–c and 17a–c has been
(Found: C, 59.9; H, 2.7. C44H26O8ؒ2CH2Cl2 requires C, 59.96;
H, 3.06%).
Preparation of 1,3- and 1,4-bis[bis(1,2-dihydro-2-oxo-N-
methylcyclohepta[b]pyrrol-3-yl)methyl]benzenes 12b and 13b
A solution of 9b (4 mmol) and 10 or 11 (1 mmol) in a mixture
of CH2Cl2 (10 cm3) and TFA (2 cm3) was stirred at rt for 48 h.
After the reaction was complete, the mixture was poured into
aqueous NaHCO3 solution. The mixture was extracted with
CH2Cl2, and the extract was dried over Na2SO4 and concen-
trated in vacuo. The resulting residue was purified through
column chromatography on Al2O3 by using hexane–ethyl
acetate (1:1) as the eluent to give the products 12b or 13b. The
results are summarized in Table 1.
For 12b. Yellow powder; mp 302–304 ЊC (from CHCl3);
δH (500 MHz) 3.43 (12H, s, Me), 6.03 (2H, s, CH), 6.75 (4H, d,
J 8.6, H-8), 6.76 (4H, dd, J 10.2, 8.5, H-6), 6.83 (4H, dd, J 11.0,
8.5, H-5), 6.96 (4H, dd, J 10.2, 8.6, H-7), 7.04 (1H, s, Ph-2), 7.08
(2H, d, J 7.6, Ph-4, 6), 7.18 (1H, t, J 7.6, Ph-5), 7.71 (4H, d,
J 11.0, H-4); δC (125.7 MHz) 26.4, 35.9, 110.9, 114.3, 125.9,
128.1, 128.4, 128.7, 128.8, 129.7, 130.2, 140.0, 141.0, 144.8,
168.7; νmax (CHCl3)/cmϪ1 1663; m/z (FAB) 735 (Mϩ ϩ 1)
(Found: C, 77.7 H, 4.7; N, 7.5. C48H38N4O4ؒ¹H2O requires C,
ϩ
accomplished. Their stabilities were determined by their pKR
values and the reduction potentials measured by CV. The pKR
ϩ
values of dications 16a and 17a were shown to be smaller than
those of azulene analogues 3 and 4, while the values of 16b,c
and 17b,c are larger than those of 3 and 4. Further studies
concerning the synthesis and properties of stable hetero-
azulene-substituted polycations are underway.
¯
²
77.50; H, 5.28; N, 7.53%).
For 13b. Yellow powder; mp >330 ЊC (from TFA–EtOH);
δH (500 MHz) 3.52 (12H, s, Me), 6.13 (2H, s, CH), 6.81 (4H, d,
J 8.7, H-8), 6.82 (4H, dd, J 8.7, 8.2, H-6), 6.94–7.01 (8H, m,
H-5, 7), 7.10 (4H, s, Ph-2, 3, 5, 6), 7.80 (4H, d, J 11.2, H-4);
δC (125.7 MHz) 26.5, 35.4, 111.2, 114.4, 128.0, 128.8, 128.8,
129.9, 130.4, 137.6, 141.0, 144.8, 168.8; νmax (CHCl3)/cmϪ1
1663; m/z (FAB) 735 (Mϩ ϩ 1) (Found: C, 72.0; H, 5.1; N, 6.5.
C48H38N4O4ؒCH2Cl2 requires C, 71.79; H, 4.92; N, 6.83%).
Experimental
IR spectra were recorded on a Horiba FT-710 spectrometer.
Mass spectra and high-resolution mass spectra were run on
JMS-AUTOMASS 150 and JMS-SX102A spectrometers.
1
Unless otherwise specified, H NMR spectra and 13C NMR
spectra were recorded on a JNM-lambda 500 spectrometer
using CDCl3 as the solvent, and the chemical shifts are given
relative to internal SiMe4 standard: J-values are given in Hz.
The abbreviations, Fr and Py, in the 1NMR data denote 2-oxo-
2H-cyclohepta[b]furan-3-yl and 1,2-dihydro-N-methyl-2-oxo-
cyclohepta[b]pyrrol-3-yl moieties, respectively. Mps were
recorded on a Yamato MP-21 apparatus and are uncorrected.
The heteroazulenes, 2H-cyclohepta[b]furan-2-one 9a20 and
1,2-dihydro-N-phenylcyclohepta[b]pyrrol-2-one 9b21,22 were
prepared as described previously.
Preparation of 3- and 4-[bis(2-oxo-2H-cyclohepta[b]furan-3-yl)-
methyl]benzaldehydes 14 and 15
A solution of 9 (2 mmol) and 10 or 11 (1 mmol) in a mixture of
CH2Cl2 (20 cm3) and TFA (2 cm3) was stirred at rt for 1 h. The
reaction mixture was poured into aqueous NaHCO3 solution.
The mixture was extracted with CH2Cl2, and the extract was
dried over Na2SO4 and concentrated in vacuo. The resulting
residue was purified through column chromatography on Al2O3
by using hexane–ethyl acetate (1:1) as the eluent to give the
products 14 and 12a, or 15 and 13a. The results are summarized
in Table 1.
Preparation of 1,3- and 1,4-bis[bis(2-oxo-2H-cyclohepta[b]-
furan-3-yl)methyl]benzenes 12a and 13a
For 14. Yellow powder; mp 127–128 ЊC (from CH2Cl2–
EtOH); δH (500 MHz) 5.76 (1H, s, CH), 6.83–6.88 (2H, m, H-6),
6.99–7.04 (6H, m, H-5, 7, 8), 7.51 (2H, d, J 11.3, H-4), 7.51
(2H, d, J 4.8, Ph-4,6), 7.72 (1H, s, Ph-2), 7.81 (1H, t, J 4.8,
Ph-5), 9.97 (1H, s, CHO); δC (125.7 MHz) 34.7, 108.1, 114.6,
128.0, 128.5, 128.7, 129.5, 131.3, 132.6, 133.7, 135.2, 136.9,
138.3, 148.9, 157.7, 169.4, 192.2; νmax (CHCl3)/cmϪ1 1742, 1707;
m/z (FAB) 408 (Mϩ) (Found: C, 74.6; H, 3.8. C26H16O5ؒ¹H2O
A solution of 9a (4 mmol) and 10 or 11 (1 mmol) in a mixture
of CH2Cl2 (10 cm3) and TFA (2 cm3) was stirred at rt for 48 h.
After the reaction was complete, the mixture was poured into
aqueous NaHCO3 solution. The mixture was extracted with
CH2Cl2, and the extract was dried over Na2SO4 and concen-
trated in vacuo. The resulting residue was purified through
column chromatography on Al2O3 by using hexane–ethyl
acetate (1:1) as the eluent to give the products 12a or 13a. The
results are summarized in Table 1.
¯
²
requires C, 74.81; H, 4.10%).
For 15. Yellow powder; mp 191–192 ЊC (from AcOEt);
δH (500 MHz) 5.75 (1H, s, CH), 6.83–6.89 (2H, m, H-6), 6.99–
7.05 (6H, m, H-5, 7, 8), 7.40 (2H, d, J 8.2, Ph-2, 6), 7.50 (2H,
d, J 11.4, H-4), 7.95 (2H, d, J 8.2, Ph-3, 5); δC (125.7 MHz)
35.2, 108.1, 114.6, 128.1, 128.3, 130.2, 131.3, 132.6, 135.2,
144.2, 148.8, 157.7, 169.4, 191.7; νmax (CHCl3)/cmϪ1 1746, 1705;
m/z (FAB) 408 (Mϩ) (Found: C, 76.1; H, 3.6. C26H16O5 requires
C, 76.46; H, 3.95%).
For 12a. Orange powder; mp 280–282 ЊC (from CH2Cl2–
EtOH); δH (500 MHz) 5.61 (2H, s, CH), 6.76–6.80 (4H, m, H-6),
6.90–6.96 (12H, m, H-5, 7, 8), 7.03 (1H, s, Ph-2), 7.15 (2H, d,
J 7.8, Ph-4, 6), 7.30 (1H, t, J 7.8, Ph-5), 7.40 (4H, d, J 11.7, H-4);
δC (125.7 MHz) 35.2, 108.8, 114.0, 126.4, 126.8, 128.1, 129.3,
130.9, 132.2, 134.8, 137.7, 148.6, 157.5, 169.2; νmax (CHCl3)/
cmϪ1 1735, 1268; m/z (rel. int.) 682 (Mϩ, 100%) (Found: C, 75.8;
2
–
H, 3.8. C44H26O8ؒ3H2O requires C, 76.07; H, 3.97%).
Preparation of 1-[bis(1,2-dihydro-2-oxo-N-methylcyclohepta-
[b]pyrrol-3-yl)methyl]-3-[bis(2-oxo-2H-cyclohepta[b]furan-3-
yl)methyl]benzene 12c and its 1,4-analogue 13c
For 13a. Yellow powder; mp 304–305 ЊC (from CH2Cl2–
EtOH); δH (500 MHz) 5.69 (2H, s, CH), 6.79–6.84 (4H, m, H-6),
6.93–7.01 (12H, m, H-5, 7, 8), 7.18 (4H, s, Ph), 7.47 (4H, d,
J 11.3, H-4); δC (125.7 MHz; DMSO-d6) 34.7, 108.0, 114.1,
127.0, 128.0, 131.1, 132.9, 135.1, 136.1, 147.8, 156.6, 168.0; νmax
(CHCl3)/cmϪ1 1745, 1271; m/z (rel. int.) 682 (Mϩ, 100%)
A solution of 14 or 15 (1 mmol) and 9b (2 mmol) in a mixture
of CH2Cl2 (10 cm3) and TFA (2 cm3) was stirred at rt for 6 h.
After the reaction was complete, the mixture was poured into
J. Chem. Soc., Perkin Trans. 2, 2001, 275–281
279

View Article Online
aqueous NaHCO3 solution. The mixture was extracted with
CH2Cl2, and the extract was dried over Na2SO4 and concen-
trated in vacuo. The resulting residue was purified through
column chromatography on Al2O3 by using hexane–ethyl
acetate (1:1) as the eluent to give the product 12c or 13c. The
results are summarized in Table 1.
For 17aؒ2PF6Ϫ. Dark-brown powder; mp >330 ЊC (from
CH2Cl2–Et2O); δH (500 MHz; CD3CN) 7.88–7.90 (4H, br m),
7.91 (4H, s, Ph), 8.18–8.22 (4H, br m), 8.36–8.39 (4H, br m),
8.43–8.49 (8H, br m); νmax (KBr)/cmϪ1 1763, 1262, 839;
m/z (FAB) 681 (Mϩ ϩ 1 Ϫ 2PF6) (Found: Mϩ ϩ 1 Ϫ 2PF6,
681.1605. C44H24O8P2F12 requires M ϩ 1 Ϫ 2PF6 681.1530)
(Found: C, 47.2; H, 2.0. C46H32N2O6P2F12ؒHPF6 requires C,
47.33; H, 2.26%).
For 12c. Yellow powder; mp 234–235 ЊC (from CHCl3–
AcOEt); δH (500 MHz) 3.46 (6H, s, NMe), 5.57 (1H, s, FrCH),
6.08 (1H, s, PyCH), 6.72–6.76 (2H, m, Fr-6), 6.77 (2H, d, J 9.6,
Fr-8), 6.79 (2H, d, J 9.5, Py-8), 6.84–6.92 (8H, m, Fr-5, 7, Py-5,
6), 6.98 (2H, dd, J 10.0, 9.8, Py-7), 7.03 (1H, s, Ph-2), 7.09 (1H,
d, J 7.6, Ph-4), 7.14 (1H, d, J 7.8, Ph-6), 7.24 (1H, dd, J 7.8, 7.6,
Ph-5), 7.34 (2H, d, J 11.6, Fr-4), 7.76 (2H, d, J 11.4, Py-4);
δC (125.7 MHz) 26.4, 35.2, 35.7, 109.0, 111.2, 113.7, 114.1,
125.5, 126.8, 127.5, 128.3, 128.5, 128.8, 128.9, 130.0, 130.5,
130.8, 131.9, 134.5, 137.2, 140.5, 141.0, 144.7, 148.5, 157.6,
168.6, 169.3; νmax (CHCl3)/cmϪ1 1744, 1670, 1268; m/z (FAB)
709 (Mϩ ϩ 1) (Found: C, 77.2; H, 4.1; N, 3.9. C46H32N2O6ؒ
¹H2O requires C, 76.97; H, 4.63; N, 3.9%).
For 17bؒ2PF6Ϫ. Dark-brown powder; mp >330 ЊC (from
CH2Cl2–Et2O); δH (500 MHz; CD3CN) 3.59 (12H, s, Me), 7.67
(4H, br s), 7.82–7.96 (12H, br m), 7.96–8.05 (4H, br m), 8.15–
8.22 (4H, br m); νmax (KBr)/cmϪ1 1685, 837; m/z (FAB) 732
(Mϩ Ϫ 2PF6) (Found: Mϩ Ϫ 2PF6, 732.2650. C48H36N4O4P2F12
requires M Ϫ 2PF6, 732.2760) (Found: C, 47.0; H, 3.0; N, 4.5.
C48H36N4O4P2F12ؒ³HPF6 requires C, 46.43; H, 3.04; N, 4.51%).
¯
²
For 17cؒ2PF6Ϫ. Dark-brown powder; mp 196–198 ЊC (from
CH3CN–Et2O); δH (500 MHz; CD3CN) 3.55–3.62 (6H, br m),
7.65–8.45 (24H, br m); νmax (KBr)/cmϪ1 1735, 1685, 1262, 843;
m/z (FAB) 706 (Mϩ Ϫ 2PF6) (Found: Mϩ Ϫ 2PF6, 706.2128.
C46H30N2O6P2F12 requires M Ϫ 2PF6 706.2094) (Found: C,
54.5; H, 2.7; N, 3.0. C46H30N2O6P2F12 requires C, 55.43; H, 3.03;
N, 2.81%).
¯
²
For 13c. Yellow powder; mp 291–292 ЊC (from CHCl3–
AcOEt); δH (500 MHz) 3.46 (6H, s, NMe), 5.57 (1H, s, FrCH),
6.08 (1H, s, PyCH), 6.72–6.76 (2H, m, Fr-6), 6.77 (2H, d, J 9.6,
Fr-8), 6.79 (2H, d, J 9.5, Py-8), 6.84–6.92 (8H, m, Fr-5, 7, Py-5,
6), 6.98 (2H, dd, J 10.0, 9.8, Py-7), 7.03 (1H, s, Ph-2), 7.09 (1H,
d, J 7.6, Ph-4), 7.14 (1H, d, J 7.8, Ph-6), 7.24 (1H, dd, J 7.8, 7.6,
Ph-5), 7.34 (2H, d, J 11.6, Fr-4), 7.76 (2H, d, J 11.4, Py-4);
δC (125.7 MHz) 26.4, 35.2, 35.7, 109.0, 111.2, 113.7, 114.1,
125.5, 126.8, 127.5, 128.3, 128.5, 128.8, 128.9, 130.0, 130.5,
130.8, 131.9, 134.5, 137.2, 140.5, 141.0, 144.7, 148.5, 157.6,
168.6, 169.3; νmax (CHCl3)/cmϪ1 1745, 1675, 1268; m/z (FAB)
ϩ
Determination of pKR values of dications 16a–c and 17a–c
Buffer solutions of slightly different acidities were prepared
by mixing aqueous solutions of KH2PO4 (0.1 M) and NaOH
(0.1 M) (for pH 6.0–8.0), Na2B4O7 (0.025 M) and HCl (0.1 M)
(for pH 8.2–9.0), Na2B4O7 (0.025 M) and NaOH (0.1 M) (for
9.2–10.8), Na2HPO4 (0.05 M) and NaOH (0.1 M) (for pH
11.0–12.0), and KCl (0.2 M) and NaOH (0.1 M) (for pH
12.0–14.0) in various portions. For the preparation of sample
solutions, 1 cm3 portions of the stock solution, prepared by
709 (Mϩ ϩ 1) (Found: C, 70.0; H, 3.8; N, 3.4. C46H32N2O6ؒ
2
–
3CHCl3 requires C, 70.19; H, 4.31; N, 3.67%).
Ϫ
dissolving 3–5 mg of cation 16a–cؒPF6 in MeCN (20 cm3),
were diluted to 10 cm3 with the buffer solution (8 cm3) and
MeCN (1 cm3). The UV–vis spectrum was recorded for each
cation 16a–c and 17a–c in 10 different buffer solutions.
Immediately after recording the spectrum, the pH of each
solution was determined on a pH meter calibrated with
standard buffers. The observed absorbance at the specific
absorption wavelengths (609 nm for 16a; 622 nm for 16b; 622
nm for 16c, 609 nm for 17a, 624 nm for 17b, 620 nm for 17c)
of each cation was plotted against pH to give a classical
General synthetic procedure for the 1,3- and 1,4-bis[bis(hetero-
azulene-substituted)methyliumyl]benzene bis(hexafluoro-
phosphates) 16a–cؒ2PF؊ and 17a–cؒ2PF6؊
To a stirred solution of bis[bis(heteroazulene-substituted)-
methylbenzenes 12a–c or 13a–c (0.2 mol) in CH2Cl2 (10 cm3)
was added DDQ (140 mg, 0.6 mmol) and the mixture was
stirred at rt for 1 h until the reaction was complete. To
the reaction mixture was added 60% aqueous HPF6 solution
(2 cm3) and the resulting mixture was filtered. The filtrate was
extracted with CH2Cl2 and the extract was dried over Na2SO4
and concentrated. The resulting residue was dissolved in
CH2Cl2 and ether was added to the solution. The precipitated
crystals were collected by filtration, washed with ether to give
ϩ
titration curve, whose midpoint was taken as the pKR value.
The results are summarized in Table 2.
Cyclic voltammetry of dications 16a–c and 17a–c
Ϫ
Ϫ
the salts 16aؒ2PF6Ϫ, 16bؒ2PF6Ϫ, and 16cؒ2PF6 or 17aؒ2PF6
,
The reduction potentials of 16a–c and 17a–c were determined
by means of a CV-27 voltammetry controller (BAS Co). A
three-electrode cell was used consisting of Pt working and
counter electrodes and a reference Ag/AgNO3 electrode. Nitro-
gen was bubbled through an acetonitrile solution (4 cm3) of
each compound (0.5 mmol dmϪ3) and Bu4NClO4 (0.1 mol
dmϪ3) to deaerate it. The measurements were made at a scan
rate of 0.1 V sϪ1 and the voltammograms were recorded on a
WX-1000-UM-019 (Graphtec Co) X-Y recorder. Immediately
after the measurements, ferrocene (0.1 mmol, E1/2 = ϩ0.083)
was added as the internal standard, and the observed peak
potentials were corrected with reference to this standard. The
compounds exhibited no reversible reduction wave: each of the
reduction potentials was measured through independent scans,
and they are summarized in Table 2.
Ϫ
17bؒ2PF6Ϫ, and 17cؒ2PF6
.
For 16aؒ2PF6Ϫ. Dark-brown powder; mp 209–210 ЊC (from
CH2Cl2–Et2O); δH (500 MHz; CD3CN) 7.53–8.43 (24H, br m);
νmax (KBr)/cmϪ1 1749, 1261, 839; m/z (FAB) 681 (Mϩ ϩ
1 Ϫ 2PF6) (Found: M ϩ 1 Ϫ 2PF6, 681.1589. C44H24O8P2F12
requires M ϩ 1 Ϫ 2PF6 681.1530) (Found: C, 49.7; H, 2.1.
C44H24O8P2F12ؒ³CH2Cl2 requires C, 49.77; H, 2.48%).
¯
²
For 16bؒ2PF6Ϫ. Dark-brown powder; mp 235–236 ЊC (from
CH2Cl2–Et2O); δH (500 MHz; CD3CN) 3.53 (12H, br m, Me),
7.58–7.71 (2H, m), 7.79–7.98 (18H, br m), 8.15 (4H, t, J 10.3);
νmax (KBr)/cmϪ1 1685, 839; m/z (FAB) 732 (Mϩ Ϫ 2PF6)
(Found: Mϩ Ϫ 2PF6, 732.2672. C48H36N4O4P2F12 requires
M Ϫ 2PF6 732.2760) (Found: C, 51.6; H, 3.0; N, 5.0. C48H36-
N4O4P2F12ؒ³CH2Cl2 requires C, 51.70; H, 3.42; N, 4.87%).
¯
²
For 16cؒ2PF6Ϫ. Dark-brown powder; mp 204–205 ЊC (from
acetone–Et2O); δH (500 MHz; CD3CN) 3.39, 3.48, 3.51, 3.57
(6H, br s, Me), 7.71 (2H, t, J 7.9), 7.78–8.07 (12H, br m), 8.12–
8.33 (8H, br m), 8.38 (2H, t, J 10.1); νmax (KBr)/cmϪ1 1752,
1685, 1262, 843; m/z (FAB) 706 (Mϩ Ϫ 2PF6) (Found:
Mϩ Ϫ 2PF6, 706.2126. C46H30N2O6P2F12 requires M Ϫ 2PF6,
706.2094) (Found: C, 53.5; H, 2.6; N, 3.2. C46H30N2O6-
P2F12ؒ¹CH2Cl2 requires C, 53.75; H, 3.01; N, 2.70%).
Acknowledgements
Financial support from Waseda University Grant for Special
Research Project is gratefully acknowledged. We thank the
Materials Characterization Central Laboratory, Waseda
University, for technical assistance with the spectral data and
elemental analyses.
¯
²
280
J. Chem. Soc., Perkin Trans. 2, 2001, 275–281

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12 S. Ito, S. Kikuchi, N. Morita and T. Asao, J. Org. Chem., 1999, 64,
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281