Synthesis and properties of bis(heteroazulen-3-yl)methyl cations and bis(heteroazulen-3-yl)ketones

Article abstract of DOI:10.1016/S0040-4020(03)00546-5

The synthesis and properties of a novel type of bis(heteroazulen-3-yl)methyl cations, bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl cation salt and nitrogen analogues, (9a-c·PF₆^-) and (9a-c·BF₄^-), as well as bis(heteroazulen-3-yl)ketones (12a-d) are studied. The synthetic method was based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes (6a-d) with paraformaldehyde to afford the corresponding disubstituted methane derivatives 7a-d, followed by oxidative hydrogen abstraction with DDQ, and subsequent exchange of the counter-anion by using aq. HPF₆ or aq. HBF₄. In addition, the reaction of 7a-d with 2.2equiv. amounts of DDQ afforded carbonyl compounds 12a-d. The delocalization of the positive charge of 9a-c was evaluated by the ¹H and ¹³C NMR spectral data. The thermodynamic stability of cations 9a-c was evaluated to be in the order 9a<9b<9c on the basis of their reduction potentials measured by cyclic voltammetry (CV) and pK_R+ values (2.6-10.3) obtained spectrophotometrically. The reduction waves of cations 9a-c were irreversible, suggesting the dimerization of the radical species generated by one-electron reduction. This was demonstrated by the reduction of 9a·BF₄^- with Zn powder to give dimerized product 14a. In addition, the quenching of 9a·BF₄^- with MeOH/NaHCO₃ gives ether derivative 15a, which is proposed for the precursor for synthesizing tris(heteroazulene)-substituted methyl cations bearing two different heteroazulene-units.

Full text of DOI:10.1016/S0040-4020(03)00546-5

Tetrahedron 59 (2003) 4157–4165
Synthesis and properties of bis(heteroazulen-3-yl)methyl cations
and bis(heteroazulen-3-yl)ketones
*
Shin-ichi Naya and Makoto Nitta
Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan
Received 19 February 2003; revised 1 April 2003; accepted 3 April 2003
Abstract—The synthesis and properties of a novel type of bis(heteroazulen-3-yl)methyl cations, bis(2-oxo-2H-cyclohepta[b]furan-3-
yl)methyl cation salt and nitrogen analogues, (9a–c·PF26 ) and (9a–c·BF24 ), as well as bis(heteroazulen-3-yl)ketones (12a–d) are studied.
The synthetic method was based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes (6a–d) with
paraformaldehyde to afford the corresponding disubstituted methane derivatives 7a–d, followed by oxidative hydrogen abstraction with
DDQ, and subsequent exchange of the counter-anion by using aq. HPF6 or aq. HBF4. In addition, the reaction of 7a–d with 2.2 equiv.
amounts of DDQ afforded carbonyl compounds 12a–d. The delocalization of the positive charge of 9a–c was evaluated by the 1H and 13
C
NMR spectral data. The thermodynamic stability of cations 9a–c was evaluated to be in the order 9a,9b,9c on the basis of their reduction
potentials measured by cyclic voltammetry (CV) and pKRþ values (2.6–10.3) obtained spectrophotometrically. The reduction waves of
cations 9a–c were irreversible, suggesting the dimerization of the radical species generated by one-electron reduction. This was
demonstrated by the reduction of 9a·BF24 with Zn powder to give dimerized product 14a. In addition, the quenching of 9a·BF24 with
MeOH/NaHCO3 gives ether derivative 15a, which is proposed for the precursor for synthesizing tris(heteroazulene)-substituted methyl
cations bearing two different heteroazulene-units. q 2003 Published by Elsevier Science Ltd.
1. Introduction
3-yl)phenylmethyl cation and pyrrole analogues.12 Thus,
heteroazulenes 6a–c (Scheme 1) are demonstrated to
stabilize not only cations but also radical species and anions
on the basis of their pKRþ values and reduction potentials.12
The stabilizing effect can be ascribed to the electronic effect
expressed by p-electron donation and the steric effect of the
bulky heteroazulene-units. The reaction of 4a–c with the
hydroxide ion becomes unfavorable due to destabilization
of the corresponding alcohol owing to strain when the
central carbon is forced into sp3 hybridization. An
independent evaluation of these two stabilizing effects
seems to be difficult. In this relation, we have also reported
the synthesis and properties of heteroazulene-substituted
1,3-bismethyliumbenzene derivatives13 and 1,3,5-tris-
methyliumbenzene derivatives.14 In the studies, two or
three methylium units of dications or trications are twisted
against the central phenyl group, respectively, and no
conjugation among the methylium units is permitted. In
order to evaluate the electronic effect of heteroazulenes, we
have studied the synthesis and properties of (heteroazulen-
3-yl)tropylium ions (5a–d), which are expected to have
smaller steric effect.15 In these studies, we have established
that the electron-donating ability of heteroazulenes to the
tropylium ion is larger in the order 6d,6a,6b,6c
(Scheme 1).15 From this viewpoint, we investigated the
synthesis and properties of the bis(heteroazulen-3-yl)methyl
cations (9a–c), which are expected to have smaller steric
effect than the corresponding triheteroazulene-substituted
cations (4a–c), via bis(heteroazulene)-substituted methanes
Recently, Asao and co-workers have reported the synthesis
and properties of tris(azulene-3-yl)methyl cation and its
derivatives (1a–c).1 – 7 The pKRþ values of 1a–c
(pKRþ¼10.3–11.4) are remarkably higher than that of the
triphenylmethyl cation (pKRþ¼26.44).8 This feature shows
that azulenes have large stabilizing effect toward methyl
cations. In the studies, attempted hydride abstraction of
tri(azulen-3-yl)methanes with Ph3Cþ·PF62 experience
extrusion of an azulene moiety to give bis(azulen-3-
yl)methyl cations (2a–c) (Fig. 1). The pKRþ values of
2a–c (pKRþ¼2.1–8.7) are lower than those of 1a–c and
remarkably higher than that of the diphenylmethyl cation
(pKRþ¼213.3).9 Thus, methyl cations 3a–c have been
synthesized, and their crystal structures revealed that the
best plane of the isopropylbenzene ring twists by 40.18,
21.38, and 20.78, respectively, from the best plane of the
guaiazulenylmethylium substituent owing to the influence
of steric hindrance.10a,b On the other hand, we have studied
the synthesis and properties of heteroazulene analogues of
the triphenylmethyl cation, i.e. the tris(2-oxo-2H-cyclo-
hepta[b]furan-3-yl)methyl cation and pyrrole analogues
(4a–c),11 as well as the bis(2-oxo-2H-cyclohepta[b]furan-
Keywords: bis(heteroazulen-3-yl)methyl cation; bis(heteroazulen-3-
yl)ketone; pKRþ; redox potential.
Corresponding author. Tel.: þ81-352-863236; fax: þ81-332-082735;
*
e-mail: nitta@waseda.jp
0040–4020/03/$ - see front matter q 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4020(03)00546-5

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S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
Figure 1.
(7a–c). The thermodynamic stability of cations 9a–c is
evaluated to be in the order 9a,9b,9c on the basis of their
reduction potentials and pKRþ values. To gain insight into
the reactivity of 9a, the one-electron reduction with Zn
powder and the quenching reaction with MeOH/NaHCO3
were carried out to result in the formation of radical-
coupling product 14a and ether 15a, respectively. On the
other hand, although bis(azulen-3-yl)ketone was syn-
thesized by the reaction of azulene with oxalyl chloride,16
bis(heteroazulen-3-yl)ketone has not been synthesized so
far. Since the alkyl group on the 1- or 3-position of the
azulene nucleus can be converted to a carbonyl function
upon treatment with 2 molar equiv. amounts of DDQ in the
presence of H2O,17 compounds 7a–d are allowed to react
with DDQ in the presence of H2O to give bis(heteroazulen-
3-yl)ketones (12a–d). In addition, we propose a possible
methodology for synthesizing heteroazulene-substituted
methyl cations bearing two different heteroazulene units.
We report herein the results in detail.
Scheme 1. Reagents and conditions: (i) (CH2)n, CH2Cl2–TFA (1:1), rt,
24 h; (ii) DDQ, CH2Cl2, rt, 1 h; (iii) 60% HPF6 or 42% HBF4, Ac2O, 08C,
1 h; (iv) DDQ (2.2 equiv.), H2O, CH2Cl2, CH3CN, rt, 24 h; (v) aq.
NaHCO3.
paraformaldehyde with 2 molar equiv. amounts of 2H-
cyclohepta[b]furan-2-one(6a),18 1,2-dihydro-N-phenylcyclo-
hepta[b]pyrrol-2-one (6b),19 1,2-dihydro-N-methylcyclo-
hepta[b]pyrrol-2-one (6c),20 and 2H-cyclohepta[b]thiophen-
2-one (6d)21 in CH2Cl2–TFA (5:1) at rt for 24 h afforded
bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methane (7a), bis(1,2-
dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)methane
(7b), bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]pyrrol-3-
yl)methane (7c), and bis(2-oxo-2H-cyclohepta[b]thiophen-3-
yl)methane (7d) in good yields, respectively (Scheme 1,
Table 1, runs 1, 4, 7, and 10). The compounds 7a–d are
powdery, orange or yellow crystals, the structures of which
1
were assigned on the basis of their IR, H and 13C NMR
2. Results and discussion
2.1. Synthesis
spectral data, as well as mass spectral data and elemental
analyses. The oxidative hydrogen abstraction of 7a–c with
1.2 molar equiv. amounts of DDQ in CH2Cl2 at rt to give
8a–c, followed by addition of aq. 60% HPF6 solution
afforded salts 9a–c·PF26 in the yields listed also in Table 1
(runs 1, 4, and 7). Although the yield of 9a·PF26 is poor, the
attempted reaction of 7a–c with DDQ and subsequent
Preparation of bis(heteroazulen-3-yl)methyl cations was
easily accomplished by the TFA-catalyzed electrophilic
substitution of heteroazulenes with paraformaldehyde and
subsequent oxidative hydrogen abstraction. The reactions of

S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
4159
Table 1. Results for the preparation of methane derivatives 7a–d and
methylium salts 9a–c·PF26 9a–c·BF42, and 12a–d
Run Compound 6
Substitution
Hydride abstraction
Product Yield Condition Product Yield
(%)
97
–
–
82
–
–
100
–
–
87
(%)
1
2
3
4
5
6
7
8
9
10
6a
–
–
6b
–
–
6c
–
–
6d
7a
–
–
7b
–
–
7c
–
–
7d
A
A
B
A
A
B
A
A
B
B
9a·PF26
9a·BF24
12a
15
87
71
90
99
76
78
100
72
71
9b·PF26
9b·BF24
12b
9c·PF26
9c·BF24
12c
12d
(A) 7a–c (0.25 mmol) and DDQ (1.2 equiv.) in CH2Cl2 (10 cm3). (B) 7a–d
(0.25 mmol), H2O (0.1 cm3), and DDQ (2.2 equiv.) in CH2Cl2 (10 cm3) and
CH3CN (10 cm3).
Figure 2. UV–vis spectra of 9a–c in CH3CN.
anion-exchange reaction with 42% aq. HBF4 in acetic
anhydride afforded salts 9a–c·BF24 in good yields, respect-
ively (Scheme 1, Table 1, runs 2, 5, and 8). Thus, the low
yield of 9a·PF26 would be attributable to its high solubility in
CH2Cl2. On the other hand, the reaction of 7a–d with
2.2 molar equiv. amounts of DDQ in moist CH3CN and
CH2Cl2 afforded bis(heteroazulen-3-yl)ketones (12a–d) in
good yields, respectively (Scheme 1, Table 1, runs 3, 6, 9,
and 10). These reactions are considered to proceed as
follows: the intermediates 8a–d, generated by the oxidative
hydrogen abstraction of 7a–d with DDQ, react with H2O to
give alcohols 10a–d. The subsequent oxidative hydrogen
abstraction of 10a–d with another DDQ gives 11a–d,
which is converted to give 12a–d by addition of aq.
NaHCO3.17 In addition, the reaction of 7d with even
1.2 molar equiv. amounts of DDQ afforded only 12d in
lower yield. Since the cation 8d would be unstable because
of the low electron-donating property of 6d as compared
with 6a–c,15 H2O-addition of 8d in the presence of stray
H2O is considered to proceed rapidly to give 10d (vide
infra). Thus, we could not isolate cation salt 9d·BF24 .
spectra, proton signals on the seven-membered ring of 9a–c
·PF26 and 9a–c·BF24 appear as broad signals. However,
these signals become sharp at 50–708C. Thus, rapid
conformational change of the heteroazulene moieties in
these cations occurs at high temperature in the NMR time
scale. This feature is completely different from that of
heteroazulene-substituted bulky methyl cations 4a–c,
which exhibit broad signals even at high temperature,4,5,7,8
thus, suggesting that the heteroazulene moieties of cations
9a–c experience less steric hindrance. The chemical shifts
of methylene protons of 7a–c and methylium protons of
9a–c as well as the chemical shift-difference (DdH) between
them, respectively, are summarized in Table 2. The 13C
NMR spectra of 9a–c were recorded and assigned by using
the C–H Cosy spectra. The chemical shifts of methylene
carbons of 7a–c and methylium carbons of 9a–c as well
as the chemical shift-difference (DdC) between them,
respectively, are also summarized in Table 2. The signal
of methylium carbons of 9a–c appear remarkably higher-
field than that of the diphenylmethyl cation (dc 200.2 at
2608C in SO2–SbF5).22 In the 1H NMR as well as in the 13
C
NMR, the chemical shift-differences (DdH and DdC) are
smaller in the order 9a.9b.9c, suggesting that the positive
charge is delocalized to the heteroazulene moiety more
largely in the order 9a,9b,9c. These features are similar
to the heteroazulene-substituted tropylium ions 5a–d.15
Consequently, it was clarified that the electron-donating
ability of heteroazulenes to the methyl cation is larger in the
order 6a,6b,6c.
2.2. Properties
The structures of 9a–c·PF26 , 9a–c·BF42, and 12a–d are
assigned on the basis of their spectral data and elemental
analyses. Mass spectra of the salts 9a–c·PF26 and 9a–c·BF42
ionized by FAB exhibit the correct Mþ2PF6 or Mþ2BF4
ion peaks, which are indicative of the cationic structure of
these compounds. The characteristic bands for the counter
ion PF26 are observed at 839 cm21 in the IR spectra of 9a–c·
PF26 , and the characteristic bands for the counter ion BF24 of
9a–c·BF24 are observed at 1084–1058 cm21. These fea-
tures also support the cationic nature of 9a–c. The UV–vis
spectra of cations 9a–c in acetonitrile are shown in Figure
2, and the longest wavelength absorption maxima of 9a–c
are also summarized in Table 2. The spectra of 9a–c are
similar to each other, and the longest wavelength absorption
maximum of 9a shows a blue-shift by 21 and 22 nm as
compared with those of 9b and 9c, respectively. Moreover,
the longest wavelength absorption maxima of 9a–c exhibit
blue-shift by 23, 38, and 37 nm as compared with those of
tris(heteroazulene)-substituted cations 4a–c (4a, 626 nm;
4b, 664 nm; 4c, 661 nm), respectively. In the 1H NMR
The UV–vis spectra of ketones 12a–d in acetonitrile are
shown in Figure 3 and the longest wavelength absorption
maxima of 12a–d are summarized in Table 2. The values
show a remarkable blue-shift as compared with those of
9a–c, respectively, and they are in the order
12a,12d,12b¼12c. The feature is similar to those of
5a–d. Furthermore, the chemical shifts of carbonyl-carbons
of 12a–d are also summarized in Table 2. While these
chemical shifts are slightly higher than that of benzo-
phenone (dC 195.2), the values are much lower than those of
1
cations 9a–c. In the H NMR spectra at rt, all protons of
12a–d appear as sharp signals. These features show that the
heteroazulene moiety undergoes fast conformational change
in the NMR time scale.

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S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
Table 2. The longest wavelength absorption maxima of 9a–c and 12a–d and 1H NMR and 13C NMR spectral data of 7a–c, 9a–c, and 12a–d
Compound
l
max/nm
1H NMR/d
13C NMR/d
(log 1/dm3 mol21 cm21
)
a
b
9
12
7
9
DdH
7
9
DdC
12
a
b
c
603 (4.73)
626 (4.82)
624 (4.77)
–
453 (4.63)
473 (4.63)
473 (4.62)
459 (4.46)
3.66
4.07
3.97
–
8.83
8.92
8.73
–
5.17
4.85
4.76
–
16.3
16.9
16.7
–
143.5
140.4
138.9
–
127.2
123.5
122.2
–
182.2
186.0
185.7
187.3
d
9a–c·PF26 were used for the measurement.
a
DdH: chemical shifts difference between dH (methine of 7) and dH (methylium of 9).
DdC: chemical shifts difference between dC (methine of 7) and dC (methylium of 9).
b
The reduction potentials of cations 9a–c are determined by
cations 4a–c show two reversible waves (E1red and E2red),
respectively, cations 9a–c as well as cations 5a–d exhibit
only one irreversible wave (E1red). The feature is similar to
that of azulene analogues of trisubstituted cations and
disubstituted cations 1a–c and 2a–c.4 Moreover, the
cations 3a–c showed similar feature.10a,b The irreversible
nature is probably due to the formation of methyl radicals
13a–c and their dimerization reactions (Scheme 2). In order
to confirm this point, the chemical reduction of 9a was
carried out (Scheme 3). The reaction of 9a with NaBH4
afforded 7a in 92% yield, while one-electron reduction of 9a
by using Zn afforded 7a and dimerized product 14a in 10%
and 54% yields, respectively; the latter compound 14a
plausibly arises from the dimerization of 13a. One-electron
reduction of 3b is also known to give dimerized product,
cyclic voltammetry (CV) in CH3CN. The reduction waves
are irreversible under the conditions of the CV measure-
ments, respectively; and thus, the peak potentials (E1red) are
summarized in Table 3 along with those of the reference
compound 4a–c11 and 5a–d.4 The values (E1red) of cation
9a–c are more positive in the order 9a.9b.9c, and they
are more positive than the corresponding tris(hetero-
azulene)-substituted cations (4a–c) and heteroazulene-
substituted tropyrium cations (5a–c), respectively. While
Figure 3. UV–vis spectra of 12a–d in CH3CN.
Scheme 2.
Table 3. Reduction potentials and pKRþ values of 9a–c, 4a–c, and 5a–d
compound
Reduction potentialsa
pKRþ
Db
E1red
E2red
9a
9b
9c
(20.27)
(20.51)
(20.52)
20.31
20.58
20.62
(20.50)
(20.65)
(20.66)
(20.49)
–
–
2.6
9.9
10.3
9.7
12.2
13.1
3.8
5.3
5.7
7.1
2.3
2.8
–
–
–
–
–
–
–
–
20.95
21.27
21.33
–
–
–
–
4ad
4bc
4cc
5ad
5bd
5cd
5dd
3.2
9a–c·PF26 were used for the measurement.
V vs Ag/AgNO3; a mean value of the cathodic and anodic peaks.
a
Irreversible processes were shown in parentheses.
pKRþ difference between 9 and 4.
Ref. 11.
Ref. 15.
b
c
d
Scheme 3. Reagents and conditions: (i) NaBH4, CH3CN, rt, 1 h; (ii) Zn,
CH3CN, rt, 0.5 h.

S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
4161
1,2-di(3-guaiazulenyl)-1,2-diphenylehtane.10b Thus, the
E1red of 9a–c would become to be irreversible. This fact
suggests that the steric hindrance for dimerization would be
small for 9a–c. The structure of 14a is assigned on the basis
of the IR, 1H and 13C NMR spectral data as well as
elemental analysis and mass spectral data.
The affinity of the carbocation towards the hydroxide ion,
expressed by the pKRþ value, is the most common criterion
of carbocation stability.23 The pKRþ values of the cations
9a–c are obtained spectrophotometrically and are sum-
marized also in Table 3, together with those of the reference
compounds 4a–c11 and 5a–d.13 The neutralization of the
cations 9a–c is not completely reversible. This feature is
ascribed to the instability of the neutralized products under
the conditions of the pKRþ measurement. Rapid (after 10 s)
acidification of an alkaline solution (ca. pH 14) of 9a–c
with TFA regenerated the absorption maxima of the cations
in the visible regions in ca. 80% yield. Although the pKRþ
values of 9a–c are smaller than those of 4a–c, respectively,
the values become larger in the order 9ap9b,9c. Thus, the
stabilizing ability of the heteroazulenes 6a–c to the methyl
cations is considered to be larger in the order 6ap6b,6c.
This feature has also been observed in the pKRþ values of
5a–c (Table 3).13 The pKRþ value of 5d is smaller than that
of 5a. Thus, the salt 8d would be unstable and collapse to
10d in the presence of stray water (vide supra, Scheme 1).
The pKRþ difference (D) between 9a–c and 4a–c are also
summarized in Table 3. The D values for cations 9a and 4a
are large (7.1), while those of cations 9b,c and 4b,c are
small D (2.3 and 2.8, respectively). This feature suggests
that heteroazulenes 6b,c have large stabilizing ability
arising from the electronic effect. While heteroazulene 6a
has small stabilizing ability arising from the electronic
effect as compared with 6b,c, a large stabilizing effect by
steric effect in trisubstituted cation 4a–c is clearly
suggested.
Scheme 4. Reagents and conditions: (i) MeOH, CH2Cl2, NaHCO3, rt, 0.5 h;
(ii) 42% HBF4, Ac2O, 08C, 1 h; (iii) 6a, CH2Cl2–TFA (1:1), rt, 5 min; (iv)
6a, NaHCO3, CH3CN, rt, 1 h.
of the methyl cation derivatives 9a–c is clarified to be in the
order 9a,9b,9c based on the reduction potentials and the
pKRþ values. Furthermore, electronic effect and steric effect
in the cation-stabilizing heteroazulene-units are discussed.
The irreversibility of reduction waves of 9a–c was
suggested by dimerization of the postulated radical species
generated by Zn-reduction. In addition, quenching of cation
9a with MeOH/NaHCO3 giving 16a is proposed to provide
a new preparative method of trisubstituted methylcations
bearing two different heteroazulene units. Further studies
concerned with this project will be continued.
4. Experimental
4.1. General
The reaction of 3a with MeONa/MeOH gives the 1-iso-
propyl-4-(3-guaiazulenylmethoxymethyl)benzene.10a Thus,
the neutralized product 15a (90% yield) is isolated by the
reaction of 9a with MeOH/NaHCO3 (Scheme 4). The
structure was clearly identified on the basis of the spectro-
scopic data as well as elemental analysis. Compound 15a
regenerated 9a in good yield upon treatment with aq. HBF4
in Ac2O. On the other hand, the reaction of 15a with 6a in
the presence of TFA afforded 16a, which was a precursor of
cation 4a4 in 97% yield. Since the reaction of 6a with 9a in
the presence of NaHCO3 gives 16a along with 7a and 12a,
both of which plausibly arise from a disproportionation
reaction of 10a generated from 9a and stray water, the
present method is applicable to the preparation of
trisubstitutedmethyl cations bearing mixed heteroazulene-
units.
IR spectra were recorded on a HORIBA FT-710 spec-
trometer. Mass spectra and high-resolution mass spectra
were run on JMS-AUTOMASS 150 and JMS-SX102A
spectrometers. Unless otherwise specified, 1H NMR spectra
and 13C NMR spectra were recorded on JNM-lambda 500
spectrometers using CDCl3 as the solvent, and the
chemical shifts are given relative to internal SiMe4 standard:
J-values are given in Hz. Mps were recorded on a Yamato
MP-21 apparatus and are uncorrected. The heteroazulenes,
2H-cyclohepta[b]furan-2-one (6a),18 1,2-dihydro-N-
phenylcyclohepta[b]pyrrol-2-one (6b),19 1,2-dihydro-N-
methylcyclohepta[b]pyrrol-2-one (6c),20 and 2H-cyclo-
hepta[b]thiophen-2-one (6d),21 were prepared as described
previously.
3. Conclusion
4.2. General synthetic procedure for heteroazulene-
substituted methane derivatives (7a–d)
Convenient preparations of fairly stable bis(heteroazulen-3-
yl)methyl salts (9a–c·PF26 ) and (9a–c·BF42), and
bis(heteroazulen-3-yl)ketones (12a–d) were accomplished.
The delocalization of the positive charge of cations 9a–c
was suggested by the 1H and 13C NMR spectra. The stability
A solution of heteroazulene 6a–d (2 mmol) and para-
formaldehyde (30 mg, 1 mmol) in a mixture of CH2Cl2
(10 mL) and trifluoroacetic acid (2 mL) was stirred at rt for
24 h. After the reaction was completed, the mixture was

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S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
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/AcOEt (1:1) as the eluent to give the products
7a–d (Table 1, run 1, 4, 7, and 10).
Na2SO4 and concentrated. The resulting residue was
dissolved in CH2Cl2 and ether was added to the solution.
The precipitates were collected by filtration, washed with
ether to give the salts 9a–c·PF62 (Table 1, run 1, 4, and 7).
4.3.1. Bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl
hexafluorophosphate (9a·PF26 ). Reddish-brown powder;
1
4.2.1. Bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methane
(7a). Yellow powder; mp 244–2458C (from EtOH); 1H
NMR (500 MHz) d 3.66 (2H, s, CH2), 6.78 (1H, dd, J¼9.5,
8.5 Hz, H-6), 6.89 (1H, d, J¼8.9 Hz, H-8), 6.92 (1H, dd,
J¼9.5, 8.5 Hz, H-7), 7.07 (1H, dd, J¼11.4, 8.5 Hz, H-5),
7.76 (1H, d, J¼11.4 Hz, H-4); 13C NMR (125.7 MHz) d
16.3, 107.9, 113.6, 127.6, 130.7, 132.0, 134.9, 148.5, 157.5,
170.2; IR (KBr) n 1731, 1257 cm21; MS (rel. int.) m/z 304
(Mþ, 94.2), 220 (100%). Anal. calcd for C19H12O4: C,
74.99; H, 3.97. Found: C, 74.7; H, 3.5.
mp 221–222 (from CH3CN/Et2O); H NMR (500 MHz,
CD3CN, 508C) d 8.31 (2H, dd, J¼10.1, 9.4 Hz, H-6), 8.39
(4H, d, J¼10.2 Hz, H-4, 8), 8.49 (2H, dd, J¼10.2, 10.1 Hz,
H-7), 8.51 (2H, dd, J¼10.2, 9.4 Hz, H-5), 8.83 (1H, s, CH);
13C NMR (125.7 MHz, CD3CN, 508C) d 108.6, 129.2,
132.6, 139.0, 142.7, 143.5, 145.3, 146.4, 151.1, 165.5; IR
(KBr) n 1734, 1261, 839 cm21; MS (FAB) m/z 303
(Mþ2PF6). HRMS calcd for C19H11O4PF6: 303.0657
(M2PF6). Found: 303.0682 (Mþ2PF6).
4.3.2. Bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]-
(9b·PF26 ).
4.2.2. Bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]-
pyrrol-3-yl)methane (7b). Reddish-orange powder; mp
261–2628C (from AcOEt); 1H NMR (500 MHz) d 4.07 (2H,
s, CH2), 6.68 (2H, d, J¼8.8 Hz, H-8), 6.78 (2H, dd, J¼10.8,
8.2 Hz, H-6), 6.83 (2H, dd, J¼10.8, 8.8 Hz, H-7), 7.06 (2H,
dd, J¼11.3, 8.2 Hz, H-5), 7.33 (4H, d, J¼8.3 Hz, Ph-2, 6),
7.46 (2H, t, J¼7.3 Hz, Ph-4), 7.54 (4H, dd, J¼8.3, 7.3 Hz,
Ph-3, 5), 8.28 (2H, d, J¼11.3 Hz, H-4); 13C NMR
(125.7 MHz) d 16.9, 112.5, 112.9, 128.5, 128.6, 128.8,
129.2, 129.5, 130.3, 131.1, 134.6, 141.5, 145.2, 169.1; IR
(KBr) n 1711 cm21; MS (FAB) m/z 455 (MþþH). Anal.
calcd for C31H22N2O2: C, 81.92; H, 4.88; N, 6.16. Found: C,
81.5; H, 4.7; N, 6.0.
pyrrol-3-yl)methyl
hexafluorophosphate
Reddish-brown powder; mp 187–1888C (from CH2Cl2/
Et2O, decomp.); 1H NMR (500 MHz, CD3CN, 508C) d 7.49
(4H, d, J¼8.2 Hz, Ph-2, 6), 7.64 (2H, t, J¼7.2 Hz, Ph-4),
7.68 (4H, dd, J¼8.2, 7.2 Hz, Ph-3, 5), 7.80 (2H, d,
J¼10.3 Hz, H-8), 7.99 (2H, dd, J¼10.1, 9.6 Hz, H-6),
8.13 (2H, dd, J¼10.1, 10.0 Hz, H-5), 8.18 (2H, dd, J¼10.3,
9.6 Hz, H-7), 8.26 (2H, d, J¼10.0 Hz, H-4), 8.92 (1H, s,
CH); 13C NMR (125.7 MHz, CD3CN, 508C) d 113.4, 125.8,
129.2, 129.6, 131.2, 131.3, 134.2, 138.4, 139.7, 140.4,
142.1, 144.5, 146.7, 155.8; IR (KBr) n 1696, 839 cm21; MS
(FAB) m/z 453 (Mþ2PF6). HRMS calcd for
C31H21N2O2PF6: 453.1604 (M2PF6). Found: 453.1588
(Mþ2PF6). Anal. calcd for C31H21N2O2PF6: C, 62.21; H,
3.54; N, 4.68. Found: C, 62.8; H, 3.2; N, 4.8.
4.2.3. Bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]-
pyrrol-3-yl)methane (7c). Yellow powder; mp 256–
1
2578C (from EtOH); H NMR (500 MHz) d 3.53 (6H, s,
4.3.3. Bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]-
(9c·PF26 ).
CH3), 3.97 (2H, s, CH2), 6.79 (2H, d, J¼9.0 Hz, H-8), 6.80
(2H, dd, J¼10.8, 8.6 Hz, H-6), 6.96 (2H, dd, J¼10.8,
9.0 Hz, H-7), 7.06 (2H, dd, J¼11.2, 8.6 Hz, H-5), 8.13 (2H,
d, J¼11.3 Hz, H-4); 13C NMR (125.7 MHz) d 16.7, 26.5,
110.8, 113.6, 128.1, 128.6, 129.8, 130.5, 140.9, 144.6,
169.2; IR (KBr) n 1663 cm21; MS (FAB) m/z 331 (MþþH).
Anal. calcd for C21H18N2O2: C, 76.34; H, 5.49; N, 8.48.
Found: C, 76.0; H, 5.0; N, 8.3.
pyrrol-3-yl)methyl
hexafluorophosphate
Reddish-brown powder; mp 206–2078C (from CH2Cl2/
Et2O, decomp.); 1H NMR (600 MHz, CD3CN, 508C) d 3.59
(6H, s, Me), 7.97 (2H, dd, J¼10.1, 9.6 Hz, H-6), 8.06 (2H, d,
J¼9.8 Hz, H-8), 8.07–8.12 (4H, m, H-4, 5), 8.27 (2H, dd,
J¼10.1, 9.8 Hz, H-7), 8.73 (1H, s, CH); 13C NMR
(150.9 MHz, CD3CN, 508C) d 27.5, 112.5, 124.2, 136.8,
138.1, 138.9, 141.0, 143.4, 145.5, 154.7, 165.8; IR (KBr) n
1701, 839 cm21; MS (FAB) m/z 329 (Mþ2PF6). HRMS
calcd for C21H17N2O2PF6: 329.1290 (M2PF6). Found:
329.1287 (Mþ2PF6). Anal. calcd for C21H17N2O2PF6: C,
53.17; H, 3.02; N, 5.96. Found: C, 53.2; H, 3.6; N, 5.9.
4.2.4. Bis(2-oxo-2H-cyclohepta[b]thiophen-3-yl)methane
(7d). Dark orange needles; mp 230–2318C (from CH2Cl2/
EtOH); 1H NMR (500 MHz) d 3.90 (2H, s, CH2), 6.83–6.89
(4H, m, H-6, 8), 7.02–7.06 (2H, m, H-5), 7.26–7.28 (2H, m,
H-7), 7.90 (2H, d, J¼11.6 Hz, H-4); 13C NMR (150.9 MHz)
d 20.3, 125.4, 130.3, 131.0, 131.4, 133.2, 133.4, 150.2,
153.1, 190.2; IR (KBr) n 1628 cm21; MS (FAB) m/z 337
(MþþH). Anal. calcd for C19H12O2S2þ1/2H2O: C, 66.06;
H, 3.79. Found: C, 65.7; H, 3.3.
4.4. Preparation of methylium tetrafluoroborates (9a–c·
BF24 )
To a stirred solution of 7a–c (0.25 mmol) in CH2Cl2
(10 mL) was added DDQ (70 mg, 0.30 mmol), and the
mixture was stirred at rt for 1 h. After evaporation of the
CH2Cl2, the residue was dissolved in Ac2O (5 mL) and 42%
HBF4 (1 mL) at 08C and the mixture was stirred for 1 h. To
the mixture was added Et2O (100 mL) and the precipitates
were collected by filtration to give 9a–c·BF24 (Table 1, run
2, 5, and 8).
4.3. General synthetic procedure for methylium
hexafluorophosphates (9a–c·PF26 )
To a stirred solution of 7a–c (0.25 mol) in CH2Cl2 (10 mL)
was added DDQ (70 mg, 0.3 mmol) and the mixture was
stirred at rt for 1 h until the reaction completed. To the
reaction mixture was added 60% aqueous HPF6 (1 mL)
solution and the resulting mixture was filtered. The filtrate
was extracted with CH2Cl2 and the extract was dried over
4.4.1. Bis(2-oxo-2H-cyclohepta[b]furan-3-yl)methyl
tetrafluoroborate (9a·BF42). Greenish yellow powder; mp
224–2258C (from CH2Cl2/Et2O, decomp.); 1H NMR

S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
4163
(600 MHz, CD3CN, 708C) d 8.31 (2H, t, J¼9.7 Hz, H-6),
8.38 (2H, d, J¼10.2 Hz, H-4), 8.39 (2H, d, J¼10.0 Hz,
H-8), 8.48 (2H, dd, J¼10.0, 9.7 Hz, H-7), 8.50 (2H, dd,
J¼10.2, 9.7 Hz, H-5), 8.82 (1H, s, CH); 13C NMR
(150.9 MHz, CD3CN, 708C) d 111.9, 130.1, 137.5, 137.6,
139.8, 143.6, 144.3, 146.2, 147.2, 168.3; IR (KBr) n 1806,
1792, 1769, 1736, 1261, 1060 cm21; MS (FAB) m/z 303
(Mþ2BF4). HRMS calcd for C19H11O4BF4: 303.0657
(M2BF4). Found: 303.0645 (Mþ2BF4). Anal. calcd for
C19H11O4BF4: C, 58.50; H, 2.84. Found: C, 58.6; H, 2.5.
d, J¼9.5 Hz, H-8), 7.81 (2H, dd, J¼11.2, 9.3 Hz, H-5), 8.51
(2H, d, J¼11.2 Hz, H-4); 13C NMR (150.9 MHz, DMSO-
d6) d 105.7, 119.9, 129.3, 135.1, 137.0, 140.7, 152.1, 158.0,
165.1, 182.2; IR (CHCl3)/nmax 1776, 1744, 1469,
1268 cm21; MS (rel. int.) m/z 318 (Mþ, 72), 173 (100%).
Anal. calcd for C19H10O5þ1/5H2O: C, 68.78; H, 3.13.
Found: C, 68.9; H, 2.9.
4.5.2. Bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]-
pyrrol-3-yl)ketone (12b). Orange prisms; mp 273–2758C
1
(from CH2Cl2/EtOH); H NMR (500 MHz) d 7.09 (2H, d,
4.4.2. Bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]-
pyrrol-3-yl)methyl tetrafluoroborate (9b·BF24 ). Reddish-
brown powder; mp 232–2348C (from CH3CN/Et2O,
decomp.); 1H NMR (600 MHz, CD3CN, 708C) d 7.52
(4H, d, J¼8.3 Hz, Ph-2, 6), 7.66 (2H, t, J¼7.4 Hz, Ph-4),
7.70 (4H, dd, J¼8.3, 7.4 Hz, Ph-3, 5), 7.82 (2H, d,
J¼10.5 Hz, H-8), 8.01 (2H, t, J¼9.7 Hz, H-6), 8.15 (2H,
dd, J¼10.5, 9.7 Hz, H-7), 8.20 (2H, dd, J¼10.5, 9.7 Hz,
H-5), 8.29 (2H, d, J¼10.5 Hz, H-4), 8.95 (1H, s, CH); 13C
NMR (150.9 MHz, CD3CN, 708C) dC 113.2, 121.0, 125.7,
129.1, 129.4, 131.0, 131.2, 134.0, 139.6, 140.2, 142.0,
144.3, 155.5, 166.4; IR (KBr) n 1701, 1685, 1084 cm21; MS
(FAB) m/z 453 (Mþ2BF4). HRMS calcd for
C31H21N2O2BF4: 453.1604 (M2BF4). Found: 453.1628
(Mþ2BF4). Anal. calcd for C31H21N2O2BF4þH2O: C,
66.69; H, 4.18; N, 5.02. Found: C, 66.4; H, 3.9; N, 4.8.
J¼9.4 Hz, H-8), 7.16 (2H, dd, J¼9.8, 9.2 Hz, H-6), 7.24
(2H, dd, J¼9.8, 9.4 Hz, H-7), 7.37 (4H, d, J¼8.0 Hz, o-Ph),
7.46 (2H, t, J¼7.4 Hz, p-Ph), 7.46 (2H, dd, J¼11.1, 9.2 Hz,
H-5), 7.52 (4H, dd, J¼8.0, 7.4 Hz, m-Ph), 8.86 (2H, d,
J¼11.1 Hz, H-4); 13C NMR (125.7 MHz) d 112.7, 116.3,
128.7, 128.8, 129.6, 130.1, 131.7, 133.8, 134.2, 135.9,
146.4, 147.3, 166.2, 186.0; IR (CHCl3) n 1684, 1458 cm21
;
MS (rel. int.) m/z 468 (Mþ, 100%). Anal. calcd for
C31H20N2O3þ1/3H2O: C, 78.47; H, 4.39; N, 5.90. Found:
C, 78.6; H, 4.2; N, 5.8.
4.5.3. Bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]-
pyrrol-3-yl)ketone (12c). Orange powder; mp.3008C
1
(from CH2Cl2/EtOH); H NMR (500 MHz) d 3.58 (6H, s,
Me), 7.20 (2H, dd, J¼10.0, 9.7 Hz, H-6), 7.24 (2H, d,
J¼9.5 Hz, H-8), 7.42 (2H, dd, J¼10.0, 9.5 Hz, H-7), 7.44
(2H, dd, J¼11.0, 9.7 Hz, H-5), 8.80 (2H, d, J¼11.0 Hz,
H-4); 13C NMR (150.9 MHz) d 26.6, 113.1, 115.0, 129.8,
131.2, 133.5, 135.7, 146.4, 146.7, 166.4, 185.7; IR (CHCl3)
n 1670, 1593, 1468 cm21; MS (FAB) m/z 345 (MþþH).
HRMS calcd for C21H16N2O3: 345.1239 (MþH). Found:
345.1246 (MþþH). Anal. calcd for C21H16N2O3þ1/2H2O:
C, 71.38; H, 4.85; N, 7.93. Found: C, 71.3; H, 4.3; N, 7.9.
4.4.3. Bis(1,2-dihydro-N-methyl-2-oxocyclohepta[b]-
pyrrol-3-yl)methyl tetrafluoroborate (9c·BF24 ). Reddish-
brown powder; mp 241–2448C (from CH3CN/Et2O,
decomp.); 1H NMR (600 MHz, CD3CN, 708C) d 3.64
(6H, s, Me), 8.06 (2H, t, J¼9.7 Hz, H-6), 8.15 (2H, d,
J¼10.0 Hz, H-8), 8.20 (2H, dd, J¼10.0, 9.7 Hz, H-7), 8.28–
8.35 (4H, m, H-4, 5), 8.85 (1H, s, CH); 13C NMR
(150.9 MHz, CD3CN, 708C) d 28.6, 113.5, 125.6, 137.8,
139.4, 140.2, 142.2, 144.5, 147.0, 155.8, 167.2; IR (KBr) n
1719, 1694, 1084 cm21; MS (FAB) m/z 329 (Mþ2BF4).
HRMS calcd for C21H17N2O2BF4: 329.1290 (M2BF4).
Found: 329.1291 (Mþ2BF4). Anal. calcd for C21H17N2O2-
BF4þHBF4: C, 50.05; H, 3.60; N, 5.56. Found: C, 50.1; H,
3.9; N, 5.5.
4.5.4. Bis(2-oxo-2H-cyclohepta[b]thiophen-3-yl)ketone
(12d). Orange powder; mp 274–2778C (from CH2Cl2/
EtOH, decomp); 1H NMR (500 MHz) d 7.19–7.23 (4H, m,
H-6, 7), 7.33–7.37 (2H, m, H-5), 7.69 (2H, d, J¼10.1 Hz,
H-8), 8.57 (2H, d, J¼11.6 Hz, H-4); 13C NMR (150.9 MHz)
d 125.0, 132.4, 133.3, 133.6, 136.0, 137.0, 152.9, 156.8,
187.0, 187.3; IR (CHCl3) n 1672, 1643, 1449 cm21; MS
(FAB) m/z 351 (MþþH). HRMS calcd for C19H10O3S2:
351.0150 (MþH). Found: 351.0126 (MþþH). Anal. calcd
for C19H10O3S2þH2O: C, 61.94; H, 3.28. Found: C, 61.6; H,
2.8.
4.5. Preparation of bis(heteroazulen-3-yl)ketones
(12a–d)
To a stirred solution of 7a–d (0.5 mmol) in H2O (0.1 mL),
CH2Cl2 (10 mL), and CH3CN (10 mL) was added DDQ
(257 mg, 1.1 mmol) and the mixture was stirred at rt for
24 h. After evaporation of the solvent, the residue was
dissolved in Ac2O (5 mL) and 42% HBF4 (1 mL) at 08C
and the mixture was stirred for 1 h. To the mixture
was added Et2O (100 mL) and the precipitates were
collected by filtration. Then the precipitates were dissolved
in aqueous NaHCO3 solution, extracted with CH2Cl2, and
the extract was dried over Na2SO4 and concentrated in
vacuo to give the products 12a–d (Table 1, run 3, 6, 9, and
10).
4.6. Reduction of 9a·BF24 with NaBH4
A solution of 9a·BF24 (19.5 mg, 0.05 mmol) and NaBH4
(1.9 mg, 0.05 mmol) in CH3CN (1 mL) was stirred at rt for
1 h. To the mixture was added saturated aqueous NH4Cl
solution, and the mixture was extracted with CH2Cl2. The
extract was dried over Na2SO4 and concentrated in vacuo to
give 7a (14.0 mg, 92%).
4.7. Reduction of 9a·BF24 with Zn
To a solution of 9a·BF24 (195 mg, 0.5 mmol) in CH3CN
(20 mL) was added powdery Zn (325 mg, 5.0 mmol) and the
mixture was stirred at rt for 24 h. After filtration, the filtrate
was concentrated in vacuo. The resulting residue was
separated by column chromatography on SiO2 (hexane/
4.5.1. Bis(2-oxo-2H-cyclohepta[b]furan-3-yl)ketone
(12a). Orange powder; mp.3008C (from CH2Cl2/EtOH);
1H NMR (500 MHz, DMSO-d6) d 7.54 (2H, dd, J¼10.3,
9.3 Hz, H-6), 7.71 (2H, dd, J¼10.3, 9.5 Hz, H-7), 7.78 (2H,

4164
S. Naya, M. Nitta / Tetrahedron 59 (2003) 4157–4165
AcOEt¼1:2) to give 7a (15 mg, 10%) and 14a (83 mg,
was washed by H2O and the CH2Cl2 extract was dried over
Na2SO4, concentrated in vacuo to give a mixture of
6a,7a,12a, and 16a (47 mg) in a molar ratio in 29:10:6:84
54%).
1
4.7.1. 1,1,2,2-Tetrakis(2-oxo-2H-cyclohepta[b]furan-3-
yl)ethane (14a). Orange powder; mp 282–2838C (from
CH2Cl2/EtOH, decomp); 1H NMR (500 MHz) d 6.25 (2H, s,
CH), 6.81 (4H, dd, J¼10.2, 8.5 Hz, H-6), 6.82 (4H, d,
J¼9.4 Hz, H-8) 6.89 (4H, dd, J¼10.2, 9.4 Hz, H-7), 7.16
(4H, dd, J¼11.6, 8.5 Hz, H-5), 8.18 (4H, br s, H-4); 13C
NMR (125.7 MHz) d 29.3, 106.6, 114.1, 128.8, 131.4,
132.1, 135.3, 143.6, 157.4, 169.6; IR (KBr) n 1742,
1273 cm21; MS (FAB) m/z 607 (MþþH). HRMS calcd
for C38H22O8: 607.1393 (MþH). Found: 607.1388
(MþþH). Anal. calcd for C38H22O8þ3/2H2O: C, 72.03;
H, 3.98. Found: C, 71.8; H, 3.7.
as determined by H NMR spectrum.
4.12. Determination of pKR1 value of methyl cations
9a–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 mL portions of the stock
solution, prepared by dissolving 3–5 mg of cation 9a–c·
PF26 in CH3CN (20 mL), were diluted to 10 mL with the
buffer solution (8 mL) and CH3CN (1 mL). The UV–vis
spectrum was recorded for each cation 9a–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 (597 nm for 9a;
609 nm for 9b; 605 nm for 9c) of each cation 9a–c was
plotted against pH to give a classical titration curve, whose
midpoint was taken as the pKRþ value.
4.8. Reaction of 9a·BF24 with MeOH
To a suspension of 9a·BF24 (39 mg, 0.1 mmol) and NaHCO3
(24 mg, 0.3 mmol) in CH3CN (1 mL) was added MeOH
(3 mL) and the mixture was stirred at rt for 0.5 h. After
evaporation of the solvent, the resulting residue was purified
through column chromatography on SiO2 using hexane/
AcOEt (1:1) as the eluent to give 15a (30 mg, 90%).
4.8.1. Bis(2-oxo-2H-cyclohepta[b]furan-3-yl)-methoxy-
methane (15a). Orange needles; mp 174–1758C (from
EtOH); 1H NMR (500 MHz) d 3.43 (3H, s, Me), 5.62 (1H, s,
CH), 6.85–6.91(1H, m, H-6), 6.98–7.03(2H, m, H-7, 8), 7.16
(1H, dd, J¼11.3, 8.5 Hz, H-5), 8.04 (1H, d, J¼11.3 Hz, H-4);
13C NMR (125.7 MHz) d 57.1, 70.2, 107.0, 114.7, 128.4,
131.3, 132.4, 135.4, 149.0, 157.4, 168.1; IR (KBr) n 1749,
1271 cm21;MS(rel. int.)m/z334(Mþ, 20),303(100%). Anal.
calcd for C20H14O5: C, 71.85; H, 4.22. Found: C, 71.4; H, 4.2.
4.13. Cyclic voltammetry of methyl cations 9a–c
The reduction potentials of 9a–c were determined by means
of 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.
Nitrogen was bubbled through an acetonitrile solution
(4 mL) of each compound (0.5 mmol dm23) and Bu4NClO4
(0.1 mol dm23) to deaerate it. The measurements were
made at a scan rate of 0.1 V s21 and the voltammograms
were recorded on a WX-1000-UM-019 (Graphtec Co) X-Y
recorder. Immediately after the measurements, ferrocene
(0.1 mmol) (E1/2¼þ0.083) 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 scan, and they are
summarized in Table 3.
4.9. Reaction of compound 15a with HBF4
The compound 15a (84 mg, 0.25 mmol) was dissolved in a
mixture of Ac2O (5 mL) and aq. 42% HBF4 (1 mL), and the
mixture was stirred at 08C for 1 h. To the mixture was added
Et2O (100 mL) and the precipitates were collected by
filtration to give 9a·BF24 (98 mg, 100%).
4.10. Reaction of 15a with 6a
To a solution of 15a (10 mg, 0.03 mmol) and 6a (4 mg,
0.03 mmol) in CH2Cl2 (1 mL) was added TFA (0.2 mL) and
the mixture was stirred at rt for 5 min. 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, concentrated in vacuo to
give the product 16a (13 mg, 97%), which was identical
with the authentic sample.11
Acknowledgements
Financial support from a Waseda University Grant for
Special Research Project and 21COE ‘Practical Nano-
chemistry’ from MEXT, Japan is gratefully acknowledged.
We thank the Materials Characterization Central
Laboratory, Waseda University, for technical assistance
with the spectral data and elemental analyses.
4.11. Reaction of 9a with 6a
To a solution of 9a (39 mg, 0.1 mmol) and 6a (15 mg,
0.1 mmol) in CH3CN (2 mL) was added NaHCO3 (83 mg,
1.0 mmol), and the mixture was stirred at rt for 1 h. To the
mixture was added MeOH (1 mL) and the mixture was
further stirred at rt for 0.5 h. After evaporation of the
solvent, the resulting residue was dissolved in CH2Cl2 and
filtered to remove excess NaHCO3 and NaBF4. The filtrate
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