A facile preparation of (4-Aminophenyl)(3-guaiazulenyl)methylium hexafluorophosphate: Comparative studies on spectroscopic, chemical, and electrochemical properties of monocarbenium ion and p-benzoquinodimethane monoiminium ion compounds

Article abstract of DOI:10.1246/bcsj.82.585

Reaction of guaiazulene with 4-aminobenzaldehyde inCH₃OH in the presence of hexafluorophosphoricacidat 25 °C for 2 h gives the title compound, with an equivof HPF₆, in56% yield. Interestingly, a solution of the obtained new monocarbe

Full text of DOI:10.1246/bcsj.82.585

© 2009 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 82, No. 5, 585–593 (2009)
585
A Facile Preparation of (4-Aminophenyl)(3-guaiazulenyl)methylium
Hexafluorophosphate: Comparative Studies on Spectroscopic,
Chemical, and Electrochemical Properties of Monocarbenium Ion
and p-Benzoquinodimethane Monoiminium Ion Compounds
Shin-ichi Takekuma,*1 Naohiko Ijibata,1 Toshie Minematsu,2 and Hideko Takekuma1
1Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University,
3-4-1 Kowakae, Higashi-Osaka 577-8502
2School of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi-Osaka 577-8502
Received October 28, 2008; E-mail: takekuma@apch.kindai.ac.jp
Reaction of guaiazulene with 4-aminobenzaldehyde in CH3OH in the presence of hexafluorophosphoric acid at
25 °C for 2 h gives the title compound, with an equiv of HPF6, in 56% yield. Interestingly, a solution of the obtained
¹
new monocarbenium ion compound, forming a protonated amino group (4-H3N+PF6 ), in CH3CN is allowed to stand at
room temperature for 48 h, gradually converting to a new 4-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-1-iminium
hexafluorophosphate, completely. Furthermore, reductions of the above two products with NaBH4 in a mixed solvent of
EtOH and CH3CN afford 4-amino-1-(3-guaiazulenylmethyl)benzene, selectively. Comparative studies on spectroscopic,
chemical, and electrochemical properties of the monocarbenium ion compound with those of the 4-methylene-2,5-
cyclohexadiene-1-iminium ion compound, are reported.
Azulene and its derivatives have been studied to a consid-
erable extent over the past 50 y and an extremely large number
of the studies have been well documented and further, naturally
occurring guaiazulene (=7-isopropyl-1,4-dimethylazulene, 1)1
has been widely used clinically as an antiinflammatory and
antiulcer agent; however, nothing has really been used as other
industrial materials. As a series of our basic studies on creation
of novel functional materials with a 3-guaiazulenyl (=7-
isopropyl-1,4-dimethylazulen-3-yl) (or another azulen-1-yl)
group possessing a large dipole moment and on their potential
utility, we have been working on facile preparation and crystal
structures as well as spectroscopic, chemical, and electro-
chemical properties of delocalized mono- and dicarbenium ion
compounds stabilized by expanded ³-electron systems pos-
sessing a 3-guaiazulenyl [or an azulen-1-yl or a 3-(methoxy-
carbonyl)azulen-1-yl] group.2-18 During the course of our basic
and systematic investigations, we found the following interest-
ing results, namely, the reaction of 1 with 4-(dimethyl-
amino)benzaldehyde, (or 4-hydroxybenzaldehyde or 4-me-
thoxybenzaldehyde) in methanol in the presence of
tetrafluoroboric acid (or hexafluorophosphoric acid) at 25 °C
for 2 h gave the corresponding monocarbenium ion compound
A, quantitatively, possessing interesting resonance structures of
the 3-guaiazulenylium ion structure C and the p-benzoquinoid
structure B in acetonitrile (Figure 1)6,10,17 and further, the X-ray
crystallographic analyses of 10 and 12 could be achieved,
apparently indicating the crystal structures with the resonance
structures A-C in their single crystals.6,17 In relation to the
above results, we quite recently isolated the title compound (4-
aminophenyl)(3-guaiazulenyl)methylium hexafluorophosphate
(3a), with an equiv of HPF6, and 3b possessing a p-
benzoquinodimethane monoiminium ion (=4-methylene-2,5-
cyclohexadiene-1-iminium ion) structure, the products of
which enabled us to compare their properties as the first
example. On the other hand, in 2003 Cho et al. reported that the
1H and 13C NMR spectral data of 13C¡-labeled Malachite
Green indicated the existence of a resonance structure with a
cationic charge located in the central C¡ carbon atom, forming
a triphenylmethylium ion structure (Chart 1),19 and further, the
synthesis, chemical properties, material chemistry, and crystal
structure of Crystal Violet have been studied (Chart 1);20-24
however, none have really been documented for comparative
studies on the properties of the isolated triphenylmethylium ion
and p-benzoquinodimethane monoiminium ion compounds
(Chart 1). We now wish to report detailed studies on the title
chemistry, namely, a facile preparation as well as the spectro-
scopic, chemical, and electrochemical properties of the target
monocarbenium ion compound 3a, with the resonance structure
3c possessing a 3-guaiazulenylium ion, and the isolated 3b
possessing a p-benzoquinodimethane monoiminium ion struc-
ture with a view to comparative study.
Experimental
General. Thermal (TGA and DTA) and elemental analyses
were taken on a Shimadzu DTG-50H thermal analyzer and a
Yanaco MT-3 CHN corder. MS spectra were taken on a JEOL The
Tandem Mstation JMS-700 TKM data system. UV-vis and IR
spectra were taken on a Beckman DU640 spectrophotometer and a
Shimadzu FTIR-4200 Grating spectrometer. NMR spectra were
recorded with a JNM-ECA600 (600 MHz for 1H and 150 MHz for
Published on the web May 13, 2009; doi:10.1246/bcsj.82.585

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Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
3-Guaiazulenylmethylium Ion Compounds
R
R
4
R
X
X
1
α
3'
1'
H
H
H
4'
X
7'
C
A
B
10 : R = N(CH3)2 , X = BF4
11 : R = OH , X = PF6
12 : R = OCH3 , X = PF6
Figure 1. The monocarbenium ion structure A of 10-12, possessing the corresponding resonance structures of the 3-
guaiazulenylium ion structure C and the p-benzoquinoid structure B in CH3CN.
guaiazulenylmethylium substituent: ¤ 1.46 (6H, d, J = 6.8 Hz,
H3C
CH3
Cl
H3C
CH3
Cl
N
N
(CH3)2CH-7¤), 2.53 (3H, br s, Me-1¤), 3.37 (3H, s, Me-4¤), 3.51
(1H, sept, J = 6.8 Hz, Me2CH-7¤), 7.98 (1H, br s, H-2¤), 8.45 (1H,
dd, J = 11.5, 2.3 Hz, H-6¤), 8.55 (1H, d, J = 11.5 Hz, H-5¤), 8.60
(1H, d, J = 2.3 Hz, H-8¤), and 8.73 (1H, br s, HC-¡); signals based
on a protonated 4-aminophenyl group: ¤ 6.90 (2H, br s, 4-
α
α
H3C
H3C
¹
H3N+PF6 ), 7.76 (2H, ddd, J = 8.8, 2.5, 1.0 Hz, H-3,5), and 7.98
N
N
R
R
(2H, ddd, J = 8.8, 2.5, 1.0 Hz, H-2,6). A signal based on three
protons of the 4-H3N+ group was not observed; 1H NMR (600
MHz, CD2Cl2), signals based on a 3-guaiazulenylmethylium sub-
stituent: ¤ 1.47 (6H, d, J = 6.9 Hz, (CH3)2CH-7¤), 2.59 (3H, br d,
J = 1.0 Hz, Me-1¤), 3.33 (3H, s, Me-4¤), 3.37 (1H, sept, J = 6.9 Hz,
Me2CH-7¤), 8.11 (1H, br s, H-2¤), 8.11 (1H, dd, J = 11.2, 2.0 Hz,
H-6¤), 8.16 (1H, d, J = 11.2 Hz, H-5¤), 8.44 (1H, d, J = 2.0 Hz, H-
8¤), and 8.65 (1H, br s, HC-¡); signals based on a 4-aminophenyl
group: ¤ 6.89 (2H, ddd, J = 8.8, 2.5, 1.0 Hz, H-3,5) and 7.80 (2H,
ddd, J = 8.8, 2.5, 1.0 Hz, H-2,6). The H2N-4 signal was not
observed; 1H NMR (600 MHz, CF3COOD), signals based on a
3-guaiazulenylmethylium substituent: ¤ 1.52 (6H, d, J = 6.9 Hz,
(CH3)2CH-7¤), 2.51 (3H, br s, Me-1¤), 3.40 (3H, s, Me-4¤), 3.48
(1H, sept, J = 6.9 Hz, Me2CH-7¤), 7.81 (1H, br s, H-2¤), 8.43 (1H,
dd, J = 11.2, 2.0 Hz, H-6¤), 8.57 (1H, d, J = 11.2 Hz, H-5¤), 8.65
(1H, d, J = 2.0 Hz, H-8¤), and 8.74 (1H, br s, HC-¡); signals based
on a deuterated 4-aminophenyl group: ¤ 7.74 (2H, br d, J =
8.0 Hz, H-3,5) and 7.84 (2H, br d, J = 8.0 Hz, H-2,6). The signal
of the deuterated amino group was not observed; 13C NMR
(150 MHz, CD3CN): ¤ 172.5 (C-7¤), 161.7 (C-8a¤), 161.1 (C-4),
158.3 (C-4¤), 153.7 (C-3a¤), 151.2 (C-5¤), 148.1 (HC-¡), 146.9 (C-
1¤), 145.3 (C-6¤), 140.8 (C-2¤), 140.1 (C-8¤), 136.1 (C-1), 135.5 (C-
2,6), 121.0 (C-3,5), 40.4 (Me2CH-7¤), 29.8 (Me-4¤), 23.8 ((CH3)2-
CH-7¤), and 13.9 (Me-1¤). The C-3¤ signal was included in the
other signals; 13C NMR (150 MHz, CF3COOD): ¤ 176.3 (C-7¤),
159.5 (C-4¤), 155.2 (C-3a¤), 152.6 (C-5¤), 148.2 (HC-¡), 146.5 (C-
6¤), 143.8 (C-1¤), 141.9 (C-2¤), 140.3 (C-8¤), 137.4 (C-1), 135.4 (C-
2,6), 133.0 (C-3¤), 126.3 (C-3,5), 42.3 (Me2CH-7¤), 30.0 (Me-4¤),
24.4 ((CH3)2CH-7¤), and 14.0 (Me-1¤). The C-4 and C-8a¤ signals
were included in the solvent signals; 19F NMR (470 MHz, CD3CN),
a signal based on a hexafluorophosphate: ¤ ¹72.7 (d, JF,P = 707.0
CH3
CH3
Malachite Green : R = H
Crystal Violet : R = N(CH3)2
Chart 1.
13C) cryospectrometer and a JNM-ECA500 (470 MHz for 19F and
202 MHz for 31P) cryospectrometer at 25 °C. 1H NMR spectra were
assigned using computer-assisted simulation (software: gNMR
developed by Adept Scientific plc) on a DELL Dimension XPS
T500 personal computer with a Pentium III processor. Cyclic and
differential pulse voltammograms were measured with an ALS
Model 600 electrochemical analyzer.
Preparation of (4-Aminophenyl)(3-guaiazulenyl)methylium
Hexafluorophosphate (3a).
To a solution of commercially
available guaiazulene (1) (158 mg, 0.80 mmol) in methanol
(2.0 mL) was added a solution of commercially available 4-
aminobenzaldehyde (2) (91 mg, 0.75 mmol) in methanol (2.0 mL)
containing hexafluorophosphoric acid (60% aqueous solution,
70 µL). The mixture was stirred at 25 °C for 2 h, precipitating a
reddish-brown powder and then was centrifuged at 2.6 krpm for
2 min. The crude product thus obtained was carefully washed with
diethyl ether, giving pure 3a (252 mg, 0.42 mmol, 56% yield) with
an equiv of HPF6.
Compound 3a: Reddish-brown powder, mp >200 °C (decomp.)
[determined by thermal analysis (TGA and DTA)]. Found: C,
44.62; H, 4.35; N, 2.48%. Calcd for C22H24NF6P + HPF6: C,
44.53; H, 4.25; N, 2.36%; UV-vis -max (CH3CN)/nm (log ¾): 234
(4.46), 293 (4.18), 342 (4.16), 424 (4.28), and 522 (4.67); UV-vis
-
(CF3COOH)/nm (log ¾): 276 (4.28), 325 (4.14), 364 (4.35),
max
¹
443 (4.31), 525sh (3.74), and 650 (2.54); FT-IR (KBr) ¯max (cm¹1):
see Figure 6a; exact FAB-MS (matrix: 3-nitrobenzyl alcohol),
found: m/z 302.1879; calcd for C22H24N: [M ¹ PF6]+, m/z
302.1909; 1H NMR (600 MHz, CD3CN), signals based on a 3-
Hz, PF6 ); 31P NMR (202 MHz, CD3CN), a signal based on a
¹
hexafluorophosphate: ¤ ¹143.4 (sept, JP,F = 707.0 Hz, PF6 ).
Preparation of 4-(3-Guaiazulenyl)methylene-2,5-cyclohexa-
diene-1-iminium Hexafluorophosphate (3b). A solution of 3a

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
587
(25 mg, 42 µmol), with an equiv of HPF6, in CH3CN (2.0 mL) was
allowed to stand at 25 °C for 48 h, turning the red solution into a
blue solution, gradually. The blue solution thus obtained was
evaporated in vacuo, giving pure 3b (23 mg, 42 µmol, 100% yield)
with 1/2 equiv of HPF6 and 1/2 equiv of CH3CN.
on a 4-aminophenyl group: ¤ 3.94 (2H, br s, H2N-4), 6.49 (2H, ddd,
J = 8.6, 2.5, 1.0 Hz, H-3,5), and 6.71 (2H, ddd, J = 8.6, 2.5, 1.0 Hz,
H-2,6); 13C NMR (150 MHz, CD3CN): ¤ 148.4 (C-4), 147.0 (C-4¤),
142.7 (C-2¤), 140.4 (C-7¤), 139.4 (C-8a¤), 136.2 (C-6¤), 134.8 (C-
8¤), 134.2 (C-3a¤), 133.1 (C-1), 130.4 (C-2,6), 128.6 (C-3¤), 127.4
(C-5¤), 125.7 (C-1¤), 114.4 (C-3,5), 38.9 (Me2CH-7¤), 37.2 (1-CH2-
3¤), 27.5 (Me-4¤), 25.4 ((CH3)2CH-7¤), and 13.5 (Me-1¤).
Compound 3b: Dark-green solid, mp >130 °C (decomp.)
[determined by thermal analysis (TGA and DTA)]. Found: C,
51.21; H, 4.57; N, 3.67%. Calcd for C22H24NF6P + 1/2HPF6 +
1/2CH3CN: C, 51.07; H, 4.85; N, 3.88%; UV-vis -max (CH3CN)/
nm (log ¾): 235 (4.53), 293 (4.19), 352 (4.17), 423 (4.17), and 593
(4.71); UV-vis -max (CF3COOH)/nm: the spectrum coincided
with that of 3a in CF3COOH; FT-IR (KBr) ¯max (cm¹1): see
Figure 6b; exact FAB-MS (matrix: 3-nitrobenzyl alcohol),
found: m/z 303.1970; calcd for C22H25N: [M ¹ PF6 + H]+, m/z
303.1987; 1H NMR (600 MHz, CD3CN): signals based on a 3-
guaiazulenyl group: ¤ 1.42 (6H, d, J = 7.0 Hz, (CH3)2CH-7¤), 2.56
(3H, d, J = 1.0 Hz, Me-1¤), 3.28 (3H, s, Me-4¤), 3.38 (1H, sept,
J = 7.0 Hz, Me2CH-7¤), 8.17 (3H, br s, H-2¤,5¤,6¤), and 8.51 (1H,
br s, H-8¤); signals based on a p-benzoquinodimethane mono-
iminium ion part: ¤ 6.55 (2H, br s, H2N+-1), 6.85 (2H, ddd,
J = 8.8, 2.5, 1.0 Hz, H-2,6), 7.85 (2H, ddd, J = 8.8, 2.5, 1.0 Hz,
Results and Discussion
Preparation and Spectroscopic Properties of 3a. The
target monocarbenium ion compound 3a was prepared in
methanol according to the procedures shown in Figure 2 and
Experimental section. The structure of the product 3a was
established on the basis of elemental analysis and spectroscopic
1
data [UV-vis, IR, exact FAB-MS, H and 13C NMR including
2D NMR (i.e., H-H COSY, HMQC, and HMBC), and 19F and
31P NMR].
Compound 3a¢HPF6 (56% yield) was obtained as a reddish-
brown powder (decomp. >200 °C), while a solution of 3a in
acetonitrile was red. The UV-vis (CH3CN) spectrum showed
that the spectral pattern of 3a was close to those of structurally
related compounds 1110 and 1217 (Figure 3), while the longest
absorption wavelength of 3a (-max 522 nm, log ¾ = 4.67)
revealed a bathochromic shift in comparison with those of
11 (-max 510 nm, log ¾ = 4.67) and 12 (-max 507 nm, log ¾ =
1
H-3,5), and 8.70 (1H, s, HC-4); H NMR (600 MHz, CD2Cl2 or
CF3COOD): the signals coincided with those of 3a in CD2Cl2 or
CF3COOD; 13C NMR (150 MHz, CD3CN): ¤ 163.5 (C-7¤), 157.2
(C-1), 155.4 (C-8a¤), 155.1 (C-4¤), 153.6 (HC-4), 151.0 (C-3a¤),
145.1 (C-5¤), 142.7 (C-6¤), 142.1 (C-2¤), 140.8 (C-1¤), 140.1 (C-
3,5), 138.8 (C-8¤), 132.8 (C-3¤), 125.6 (C-4), 116.5 (C-2,6), 39.6
(Me2CH-7¤), 30.1 (Me-4¤), 24.0 ((CH3)2CH-7¤), and 13.5 (Me-1¤);
13C NMR (150 MHz, CF3COOD): the signals coincided with those
of 3a in CF3COOD.
NH2
HPF6
PF6
NH2
Reduction of 3a (or 3b) with NaBH4. To a solution of NaBH4
(54 mg, 1.42 mmol) in ethanol (5.0 mL) was added a solution of 3a
(150 mg, 0.25 mmol), with an equiv of HPF6, in acetonitrile
(4.0 mL). The mixture was stirred at 25 °C for 1 h and then
evaporated in vacuo. The residue thus obtained was dissolved in
dichloromethane and filtered. The reaction solution was evaporated
in vacuo, giving a greenish-blue pasty residue, which was carefully
separated by silica gel column chromatography with hexane-ethyl
acetate (4:1, vol/vol) as an eluant, giving pure 4-amino-1-(3-
guaiazulenylmethyl)benzene (4) (76 mg, 0.25 mmol, 100% yield).
Similarly, as in the case of 3a, to a solution of NaBH4 (9 mg,
0.24 mmol) in ethanol (5.0 mL) was added a solution of 3b (25 mg,
46 µmol), with 1/2 equiv of HPF6 molecule and 1/2 equiv of
CH3CN molecule, dissolved in acetonitrile (4.0 mL). The mixture
was stirred at 25 °C for 1 h and then evaporated in vacuo. The
residue thus obtained was dissolved in dichloromethane and
filtered. The reaction solution was evaporated in vacuo, giving a
greenish-blue pasty residue, which was carefully separated by
silica gel column chromatography with hexane-ethyl acetate (4:1,
vol/vol) as an eluant, giving pure 4 (11 mg, 36 µmol, 78% yield)
and several chromatographically inseparable products.
α
1
H
in CH3OH
with 60% HPF6
at r.t. for 2 h
CHO
2
3a
Figure 2. The reaction of 1 with 2 in methanol in the
presence of hexafluorophosphoric acid (60% aqueous
solution) at 25 °C for 2 h, giving 3a, with an equiv of
HPF6, as a reddish-brown powder.
Compound 4: Greenish-blue paste [Rf = 0.06 on silica gel
TLC (hexane/EtOAc = 4:1, vol/vol)]; UV-vis -max (CH3CN)/nm
(log ¾): 202 (4.61), 249 (4.42), 290 (4.46), 354 (3.65), 370
(3.56), 392 (2.79), and 626 (2.41); FT-IR (KBr) ¯max (cm¹1): see
Figure 9; exact EI-MS (70 eV), found: m/z 303.2013; calcd for
1
C22H25N: M+, m/z 303.1987; H NMR (600 MHz, CD3CN), sig-
nals based on a 3-guaiazulenylmethyl group: ¤ 1.31 (6H, d, J =
6.8 Hz, (CH3)2CH-7¤), 2.56 (3H, s, Me-1¤), 2.80 (3H, s, Me-4¤), 3.01
(1H, sept, J = 6.8 Hz, Me2CH-7¤), 4.40 (2H, s, 1-CH2-3¤), 6.78 (1H,
d, J = 10.7 Hz, H-5¤), 7.28 (1H, dd, J = 10.7, 2.0 Hz, H-6¤), 7.35
(1H, br s, H-2¤), and 8.08 (1H, d, J = 2.0 Hz, H-8¤); signals based
Figure 3. The UV-vis spectra of 3a and 3b in CH3CN.
¹1
¹1
Concentrations, 3a: 169 µmol L and 3b: 185 µmol L
.
Length of the cell, 0.1 cm each. 3a: -max 522 nm (log ¾ =
4.67). 3b: -max 593 nm (log ¾ = 4.71).

588
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
3-Guaiazulenylmethylium Ion Compounds
Figure 4. The UV-vis spectrum of 3a in CF3COOH.
Concentration, 3a: 169 µmol L¹1. Length of the cell,
0.1 cm. 3a: -max 443 nm (log ¾ = 4.31).
CF3COO
NH3
PF6
α
H
in CF3COOH
at r.t.
3d
NH2
3a (or 3b)
Figure 6. The IR spectra of 3a (a) and 3b (b). 3a: ¯max
(KBr) (cm¹1): a, 3476; b, 1632; c, 1609; d, 1528; e, 1443;
f, 1400; g, 1111; h, 1049. 3b: ¯max (KBr) (cm¹1): i, 3476; j,
1666; k, 1593; l, 1528; m, 1443; n, 1404; o, 1115; p, 1049;
q, 667.
NaBH4
H2C
in a mixed solvent of
EtOH and CH3CN
at r.t. for 1 h
4
signal appeared at ¤ 6.90 (br s). The 19F and 31P NMR
(CD3CN) spectra showed a signal based on a hexafluorophos-
phate, indicating the existence of two equivalent PF6 . There-
Figure 5. The protonation of 3a (or 3b) in CF3COOH at
25 °C and the reduction of 3a (or 3b) with NaBH4 in a
mixed solvent of EtOH and CH3CN at 25 °C for 1 h.
¹
fore, the elemental analysis and the IR spectrum suggested the
formation of 3a, with an equiv of HPF6, forming a non-
1
4.58). Although a solution of 3a in acetonitrile was red, a
solution of 3a in trifluoroacetic acid was reddish-orange. The
UV-vis (CF3COOH) spectral pattern of 3a (Figure 4), forming
a protonated amino group (see 3d in Figure 5), changed in
comparison with that of 3a in acetonitrile. The IR (KBr)
spectrum showed specific bands based on the N-H bond (¯max
3476 and 3382 cm¹1) and the C-N bond (¯max 1338 cm¹1) of a
4-aminophenylmethylium ion structure. The specific bands
protonated H2N-4 group at the solid state,25 while the H, 19F,
and 31P NMR spectra indicated the formation of the protonated
3a (i.e., 4-H3N+PF6 ) in the solvent. Although the H-2¤ and
¹
H-5¤,6¤ proton signals of the 3-guaiazulenylmethylium ion
structure showed slight up- and down-field shifts in comparison
with those of 11 and 12, the other signals [i.e., HC-¡, Me-1¤,4¤,
(CH3)2CH-7¤, and H-8¤] coincided with those of 11 and 12
(Table 1). The 13C NMR (CD3CN) spectrum exhibited 18
carbon signals assigned by HMQC and HMBC techniques
(Table 2). The carbon signals (C-3a¤,8¤) of the 3-guaiazulenyl-
methylium ion structure coincided with those of 11 and 12;
however, the signals (C-1¤,4¤,5¤,6¤,7¤,8a¤) showed down-field
shifts and the signals (HC-¡ and C-2¤) revealed up-field shifts
in comparison with those of 11 and 12 (Table 2). Furthermore,
¹
¹1
based on the counter anion (PF6 : ¯max 844 and 559 cm
)
coincided with those of 1110 and 1217 (Figure 6a). The formula
C22H24N for the monocarbenium ion structure [M ¹ PF6]+ was
determined by exact FAB-MS spectrum. An elemental analysis
confirmed the formula C22H25NF12P2 (i.e., C22H24NF6P +
HPF6). The 1H NMR (CD3CN) spectrum showed signals based
on a 3-guaiazulenylmethylium ion structure with a resonance
structure of a 3-guaiazulenylium ion, and revealed signals
based on a protonated 4-aminophenylmethylium ion structure,
the signals (¤ and J values) of which were carefully assigned
using H-H COSY technique and computer-assisted simulation
based on first-order analysis (Table 1). The protonated H2N-4
1
the H and 13C NMR signals of 3a in trifluoroacetic acid-d1,
forming a deuterated H2N-4 group, are shown in Tables 1
1
1
and 2 (see 3ac) for H and 3ab) for 13C). The H and 13C NMR
(CF3COOD) chemical shifts for the HC+-¡ carbenium ion
center of 3a coincided with those signals in acetonitrile-d3
and further, those of the other proton and carbon signals in

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
589
1
Table 1. Selected H NMR Chemical Shifts (¤) for 3a, 3b,a) 4, and 10-12 in CD3CN (or CD2Cl2 or CF3COOD)
Compound
HC-¡
Me-1¤
H-2¤
Me-4¤
H-5¤
H-6¤
(CH3)2CH-7¤ Me2CH-7¤
H-8¤
H-2,6
H-3,5
3a
3b
8.73
8.70
2.53
2.56
2.59
2.51
2.56
2.59
2.53
2.53
2.53
7.98
8.17
8.11
7.81
7.35
8.18
7.81
8.08
8.09
3.37
3.28
3.33
3.40
2.80
3.27
3.42
3.33
3.34
8.55
8.17
8.16
8.57
6.78
8.07
8.60
8.43
8.45
8.45
8.17
8.11
8.43
7.28
8.11
8.45
8.37
8.36
1.46
1.42
1.47
1.52
1.31
1.43
1.53
1.44
1.45
3.51
3.38
3.37
3.48
3.01
3.37
3.47
3.46
3.47
8.60
8.51
8.44
8.65
8.08
8.49
8.66
8.56
8.57
7.98
7.85
7.80
7.84
6.71
7.94
7.94
7.83
7.90
7.76
6.85
6.89
7.74
6.49
6.97
7.84
7.04
7.16
3ab) (or 3bb)) 8.65
3ac) (or 3bc))
8.74
4.40d)
8.70
8.74
8.72
8.74
4
10
10c)
11
12
a) For comparative purposes, the numbering scheme of the 4-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-1-iminium ion structure
3b was changed to that of the 1-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-4-iminium ion structure as shown in Table 1. b) In
CD2Cl2. c) In CF3COOD. d) 1-CH2-3¤.
Table 2. Selected 13C NMR Chemical Shifts (¤) for 3a, 3b,a) 4, and 10-12 in CD3CN (or CF3COOD)
Compound HC-¡ C-1¤
C-2¤
C-3¤ C-3a¤ C-4¤
C-5¤
C-6¤
C-7¤
C-8¤ C-8a¤ C-1 C-2,6 C-3,5 C-4
d)
3a
148.1 146.9 140.8
®
153.7 158.3 151.2 145.3 172.5 140.1 161.7 136.1 135.5 121.0 161.1
3b
153.6 140.8 142.1 132.8 151.0 155.1 145.1 142.7 163.5 138.8 155.4 125.6 140.1 116.5 157.2
d)
d)
3ab) (or 3bb)) 148.2 143.8 141.9 133.0 155.2 159.5 152.6 146.5 176.3 140.3
®
137.4 135.4 126.3
®
4
37.2c) 125.7 142.7 128.6 134.2 147.0 127.4 136.2 140.4 134.8 139.4 133.1 130.4 114.4 148.4
10
11
12
153.0 139.5 142.0 131.8 149.8 154.3 143.6 142.1 161.4 138.5 153.8 124.9 140.0 115.0 156.8
151.7 144.6 141.8 137.3 153.3 157.1 149.1 144.3 169.3 139.6 159.6 128.6 137.6 118.3 163.3
151.3 144.9 141.7 137.8 153.4 157.3 149.4 144.5 169.7 139.7 159.9 129.2 137.1 116.4 165.1
a) For comparative purposes, the numbering scheme of the 4-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-1-iminium ion structure 3b
was changed to that of the 1-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-4-iminium ion structure as shown in Table 2. b) In
CF3COOD. c) 1-CH2-3¤. d) The signal was included in the other (or solvent) signals.
trifluoroacetic acid-d1 more resembled those of 3a in acetoni-
trile-d3 in comparison with those of 3b in acetonitrile-d3.
Moreover, the 1H NMR (CF3COOD) chemical shifts of 3a
coincided with those of 10, forming a deuterated (CH3)2N-4
group, in trifluoroacetic acid-d1 (see 10c) in Table 1). Along
with the 19F and 31P NMR spectral data, comparative studies on
the above 1H and 13C NMR spectral data led to the structure (4-
aminophenyl)(3-guaiazulenyl)methylium hexafluorophosphate
for 3a, with the resonance structure 3c possessing a 3-
guaiazulenylium ion, forming a protonated amino group (4-
¹
H3N+PF6 ). A similar reaction pathway to that of the previous
monocarbenium ion compound16 can be inferred for the
formation of 3a.
Preparation and Spectroscopic Properties of 3b.
A
Figure 7. The time-dependent UV-vis spectra; reaction
conditions: a solution of 3a (1.0 mg, 1.69 µmol), with an
equiv of HPF6, in CH3CN (10 mL) was allowed to stand at
room temperature for 22 h, gradually converting to 3b;
length of the cell: 0.1 cm; interval time: 2 h.
solution of 3a, with an equiv of HPF6, in CH3CN was allowed
to stand at 25 °C for 48 h, gradually turning the red solution
into a blue solution. The blue solution thus obtained was
evaporated in vacuo, giving pure 3b (100% yield), with 1/2
equiv of HPF6 and 1/2 equiv of CH3CN, as a solid. The struc-
ture of the product 3b was established on the basis of elemental
analysis and similar spectroscopic analyses to those of 3a.
Along with the above results, the time-dependent UV-visible
(CH3CN) spectra of 3a, forming a protonated amino group (4-
(CH3)2CO, or C5H5N (pyridine)] was converted to a solution of
3b in each solvent, rapidly, as a result of dehydrogen hexafluo-
rophosphate. The difference between the two is as follows: the
permittivities (¾) of CH2Cl2, (CH3)2CO, and C5H5N are smaller
than that of CH3CN, and are 7.77 at 10 °C, 20.7 at 25 °C, and
12.3 at 25 °C. The 1H NMR (CD2Cl2) signals of 3a, converting
to 3b in the solvent, are shown in Table 1 (see 3ab)).
¹
H3N+PF6 ), apparently indicated that 3a (the longest specific
band: -max 523 nm) was gradually converted to 3b (the longest
specific band: -max 596 nm) (Figure 7) as a result of deproto-
nation (Figure 8). Therefore, the solvent served as a base, the
permittivity (¾) of which is 38.8 at 20 °C. Moreover, a solution
of 3a¢HPF6 dissolved in an organic solvent [i.e., CH2Cl2,
Compound 3b¢1/2HPF6¢1/2CH3CN (100% yield) was
obtained as a dark-green solid (decomp. >130 °C), while a

590
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
3-Guaiazulenylmethylium Ion Compounds
NH3
PF6
NH3
NH2
PF6
PF6
4
PF6
1
α
H
3'
1'
H
H
HPF6
in CH3CN
4'
PF6
7'
3c
3a
3b
¹
Figure 8. The formation of 3b via 3a, with the resonance structure 3c, forming a 4-H3N+PF6 in CH3CN.
solution of 3b in CH3CN was blue. The UV-vis (CH3CN)
spectrum showed that the longest absorption wavelength of 3b
(-max 593 nm, log ¾ = 4.71) revealed a bathochromic shift (¦
71 nm) and a slight hyperchromic effect (¦ log ¾ = 0.04) in
comparison with that of 3a (Figure 3), and showed that the
spectral pattern of 3b was close to that of 10,6 while the longest
absorption wavelength of 3b revealed a hypsochromic shift (¦
45 nm) and a hypochromic effect (¦ log ¾ = 0.17) in compar-
ison with that of 10 (-max 638 nm, log ¾ = 4.88). The UV-vis
(CF3COOH) spectrum coincided with that of 3a, forming a
protonated amino group, in the solvent (Figure 4). The IR
(KBr) spectrum showed specific bands based on the N-H
bond (¯max 3476 and 3379 cm¹1) and the C-N bond (¯max
1346 cm¹1) of a p-benzoquinodimethane monoiminium ion
¹
¹1
structure and the counter anion (PF6 : ¯max 848 and 559 cm
)
and further, the IR spectral pattern was almost the same as that
of 3a (Figure 6b). The formula C22H25N for the protonated
monocation part [M ¹ PF6 + H]+ was determined by exact
FAB-MS spectrum. An elemental analysis confirmed the
formula C22H24NF6P with 1/2 equiv of HPF6 and 1/2 equiv
Figure 9. The IR spectrum of 4. 4: ¯max (KBr) (cm¹1): a,
3221; b, 3020; c, 2924; d, 2866; e, 1616; f, 1466; g, 1408;
h, 1362; i, 1315.
1
of CH3CN. The H NMR (CD3CN) spectrum showed signals
Thus, the elemental analysis and the spectroscopic data for 3b
led to the structure 4-(3-guaiazulenyl)methylene-2,5-cyclohexa-
diene-1-iminium hexafluorophosphate illustrated in Figure 8.
From the structures of the resulting products 3a and 3b, it can
based on a 3-guaiazulenyl group and a p-benzoquinodimethane
monoiminium ion framework, the signals of which were
carefully assigned using similar techniques to those of 3a
(Table 1). The iminium ion (H2N+=) signal appeared at ¤ 6.55
(br s). Therefore, the elemental analysis and the IR and
1H NMR spectra suggested the formation of a p-benzoquinodi-
methane monoiminium ion structure. Although the proton
signals (Me-4¤, H-5¤,6¤,8¤, Me2CH-7¤, and H-2,3,5,6) and the H-
2¤ signal of 3b showed up- and down-field shifts in comparison
with those of 3a, the other signals [i.e., HC-¡, Me-1¤, and
(CH3)2CH-7¤] of 3b coincided with those of 3a (Table 1).
Furthermore, the 1H NMR (CD3CN) chemical shifts of 3b were
almost the same as those of 10 (Table 1), suggesting the forma-
tion of a p-benzoquinodimethane monoiminium ion structure.
The 13C NMR (CD3CN) spectrum exhibited 19 carbon signals
assigned by HMQC and HMBC techniques (Table 2). The
signals (C-1¤,3a¤,4¤,5¤,6¤,7¤,8¤,8a¤ and C-1,3,4,5) of 3b showed
up-field shifts; however, the signals (HC-¡, C-2¤, and C-2,6)
revealed down-field shifts in comparison with those of 3a
(Table 2). Furthermore, the 1H NMR (CD2Cl2) and 1H and
13C NMR (CF3COOD) spectra of 3b coincided with those of 3a
in CD2Cl2 and CF3COOD (see 3bb) and 3bc) in Table 1 and
3bb) in Table 2). Moreover, the 13C NMR (CD3CN) chemical
shifts of 3b were almost the same as those of 10 (Table 2).
be inferred that the obtained 3a, forming a 4-H3N+PF6 in
¹
CH3CN, owing to the kinetic control is converted to the
stabilized 3b owing to the thermodynamic control (i.e., stability
of conjugated ³-electron system) in the solvent (Figure 8).
Reductions of 3a and 3b with NaBH4. The reduction of
3a with NaBH4 in a mixed solvent of ethanol and acetonitrile at
25 °C for 1 h gave 4 in 100% yield (Figure 5). Similarly, as in
the case of 3a, the NaBH4-reduction of 3b under the same
reaction conditions as for 3a gave 4 in 78% yield (Figure 5),
along with several chromatographically inseparable products,
the molecular structure of which was established on the basis of
exact EI-MS spectrum and similar spectroscopic analyses
to those of 3a. The UV-vis spectrum showed specific bands
based on a 3-guaiazulenyl group.26 The IR (KBr) spectrum
showed specific bands based on the N-H bond (¯max 3413
and 3375 cm¹1), the aromatic C-C bond (¯max 1616, 1515,
and 1466 cm¹1), and the C-N bond (¯max 1269 cm¹1) of a 4-
aminobenzene part (Figure 9). The molecular formula C22H25N
was determined by exact EI-MS spectrum. The 1H NMR
spectrum revealed signals based on a 3-guaiazulenylmethyl-
substituted 4-aminobenzene at the C-1 position, the signals of

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
591
Figure 10. Cyclic and differential pulse voltammograms of 3b (3.0 mg, 5.55 µmol), with 1/2 equiv of HPF6 and 1/2 equiv of
CH3CN, [see (a), (b)] and 3a (3.0 mg, 5.06 µmol), with an equiv of HPF6, [see (c), (d)] in 0.1 M [n-Bu4N]PF6, CH3CN (10 mL) at a
glassy carbon (ID: 3 mm) and a platinum wire served as the working and auxiliary electrodes; scan rates 100 mV s¹1 at 25 °C under
argon. For comparative purposes, the oxidation potential using ferrocene as a standard material showed +0.42 V (Ep) by DPV and
+0.40 V (E1/2) by CV under the same electrochemical measurement conditions as for 3a and 3b.
which were carefully assigned using similar techniques to those
of 3a (Table 1). The H2N-4 signal appeared at ¤ 3.94 (br s).
The 13C NMR spectrum exhibited 19 carbon signals assigned
using similar techniques to those of 3a (Table 2). Thus, the
spectroscopic data for 4 led to the molecular structure 4-amino-
1-(3-guaiazulenylmethyl)benzene, in which a hydride ion
attached to each HC-¡ carbon atom of 3a and 3b, respectively.
¹0.19 and ¹0.45 V for 3a, whose currents showed 3.1 and
1.5 µA, and further, the corresponding two irreversible reduc-
tion potentials determined by CV were located at the values of
¹0.25 and ¹0.51 V (Epc each) as shown in Figures 10c and
10d. From the result of the DPV datum [i.e., the ratio of each
current (in µA) of the two potentials] for the sample 3a
(Figure 10d), it can be inferred that a solution of ca. 1/3 equiv
of 3a¢HPF6 in CH3CN in the presence of 0.1 M [n-Bu4N]PF6
was converted to a solution of 3b in the solvent as a result of
dehydrogen hexafluorophosphate. An electron-transfer mecha-
nism of 3a with 3b based on CV datum can be inferred as
illustrated in Figure 11: namely, 3a undergoes one-electron
reduction at a potential of ¹0.25 V, generating the correspond-
ing electrochemically unstable radical-species 3a● and further,
the existent 3b in the solution of 3a undergoes one-electron
reduction at a potential of ¹0.51 V, whose reduction potential
coincided with that of the isolated 3b, also generating 3a●.
Furthermore, the reduction potential of the monocarbenium ion
compound 3a coincided with that of (3-guaiazulenyl)phenyl-
methylium hexafluorophosphate [¹0.29 V by CV (¹0.20 V by
DPV)];10 however, 3a was susceptible to reduction as com-
pared with 3b and 10-12,6,10,17 owing to a difference in
electron affinity based on each delocalized ³-electron system.
Moreover, comparing the 1H and 13C NMR signals and the
reduction potential of 10 to those of 3a and 3b, it can be
inferred that the positive charge of 10 in CH3CN is mainly
localized at the nitrogen atom of the 4-(dimethylamino)benzene
part, forming the p-benzoquinodimethane monoiminium ion
1
The chemical shifts for the H and 13C NMR signals of 3a and
3b compared with those of 4 and 10-12 are shown in Tables 1
and 2, leading to the formation of 3a, possessing a 3-
guaiazulenylmethylium ion structure with the resonance struc-
ture 3c of a 3-guaiazulenylium ion, and the formation of 3b,
possessing a p-benzoquinodimethane monoiminium ion struc-
ture, illustrated in Figure 8.
Electrochemical Behavior of 3a and 3b. We have been
interested further in the electrochemical properties of 3a and 3b
with a view to comparative study. The electrochemical behav-
ior of 3a, with an equiv of HPF6, and 3b, with 1/2 equiv of
HPF6 and 1/2 equiv of CH3CN, was therefore measured by
means of cyclic voltammetry (CV) and differential pulse
valtammetry (DPV) [Potential (in volt) vs. SCE] in CH3CN
containing 0.1 M [n-Bu4N]PF6 as a supporting electrolyte. As a
result, it was found that 3b underwent one-electron reduction at
a potential of ¹0.49 V (Epc, irreversible) by CV [¹0.43 V (Ep)
by DPV] (Figures 10a and 10b), whose reduction potential
coincided with that of 10,6 generating an electrochemically
unstable radical-species 3a● (Figure 11), while two reduction
potentials observed by DPV were positioned at the Ep values of

592
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
3-Guaiazulenylmethylium Ion Compounds
NH3
NH2
PF6
NH3
PF6
PF6
−
+e
PF6
PF6
−
−
HPF6
−e
Epc = −0.25 V
H
H
H
+e
−
−e
Epc = −0.51 V
3a
3b
3a
Figure 11. A plausible electron-transfer mechanism based on the CV datum of 3a, in the presence of 3b, possessing an equiv of
HPF6.
structure B, while the positive charge apparently is transferred
to the HC-¡ carbon atom or the seven-membered ring, forming
the 3-guaiazulenylmethylium ion structure A or the 3-guaia-
zulenylium ion structure C (Figure 1).
Table 3. Selected Bond Lengths (¡) of the X-ray Crystal
and Optimized Structures for 10 and of the Optimized (4-
Aminophenyl)(3-guaiazulenyl)methylium Ion Structure 3a)
Atom
10 (X-ray) 10 (PM3) 10 (AM1) 3 (PM3) 3 (AM1)
The Optimized (4-Aminophenyl)(3-guaiazulenyl)methyl-
ium Ion Structure 3. In a previous paper,6 we reported the
crystal structure of 10, leading to the formation of 4-(3-
guaiazulenyl)methylene-2,5-cyclohexadiene-1-dimethyliminium
tetrafluoroborate (B) with the two resonance structures of A and
C (Figure 1). From a comparative study on the C-C and C-N
bond lengths of the crystal structure of 10 with those of the
optimized 4-(3-guaiazulenyl)methylene-2,5-cyclohexadiene-1-
dimethyliminium ion structure calculated by a WinMOPAC
(Ver. 3.0) program using PM3 (or AM1) as a semiempirical
Hamiltonian, those bond lengths calculated using PM3 more
resembled those of the crystal structure of 10 in comparison
with those calculated using AM1 (Table 3). Referring to the
above results, the optimized (4-aminophenyl)(3-guaiazulenyl)-
methylium ion structure 3 has been calculated using PM3 (or
AM1) (Table 3), because the X-ray crystal structures of 3a and
3b27 have not yet been achieved. From the bond lengths of 3
along with the spectroscopic and CV/DPV data and the
NaBH4-reductions of 3a and 3b, it can be inferred that
similarly as in the case of 10, the positive charge of 3 is mainly
localized at the nitrogen atom of the 4-aminobenzene part,
forming a p-benzoquinodimethane monoiminium ion structure
owing to the thermodynamic control (i.e., stability of con-
jugated ³-electron system), while the positive charge appa-
rently is transferred to the HC-¡ carbon atom or the seven-
membered ring, forming a 3-guaiazulenylmethylium ion or
3-guaiazulenylium ion structure.
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
C1-N
C4-C¡
C¡-C3¤
C1¤-C2¤
C2¤-C3¤
C3¤-C3a¤
C3a¤-C4¤
C4¤-C5¤
C5¤-C6¤
C6¤-C7¤
C7¤-C8¤
C8¤-C8a¤
C8a¤-C1¤
C8a¤-C3a¤
1.390
1.372
1.405
1.404
1.360
1.403
1.359
1.414
1.396
1.351
1.448
1.457
1.385
1.406
1.373
1.386
1.379
1.392
1.416
1.465
1.415
1.376
1.415
1.408
1.378
1.414
1.409
1.429
1.368
1.367
1.453
1.468
1.392
1.404
1.376
1.401
1.388
1.388
1.455
1.444
1.432
1.375
1.417
1.413
1.377
1.430
1.372
1.422
1.371
1.376
1.458
1.467
1.392
1.403
1.380
1.398
1.391
1.386
1.461
1.452
1.412
1.378
1.414
1.407
1.380
1.411
1.400
1.432
1.366
1.366
1.454
1.470
1.392
1.404
1.375
1.401
1.388
1.388
1.456
1.443
1.429
1.376
1.417
1.412
1.378
1.428
1.360
1.422
1.369
1.375
1.459
1.468
1.392
1.404
1.380
1.399
1.391
1.386
1.462
1.451
a) The following MO calculation program and calculation
conditions were used (i.e., the software: WinMOPAC Ver. 3.0
developed by Fujitsu Ltd., Japan; semiempirical Hamiltonian:
PM3 or AM1; and keywords: CHARGE = 1, PRECISE,
VECTORS, ALLVEC, BONDS, GEO-OK, EF, PL, LET, T =
10D, GNORM = 10¹4, and SCFCRT = 10¹10). The final value
of the Gradient Norm of the optimized structure; PM3: 0.011
and AM1: 0.009 for 10; PM3: 0.010 and AM1: 0.011 for 3.
Conclusion
We have reported the following four interesting points in this
paper: namely, (i) the reaction of guaiazulene (1) with 4-
aminobenzaldehyde (2) in CH3OH in the presence of hexa-
fluorophosphoric acid at 25 °C for 2 h gave (4-aminophenyl)(3-
guaiazulenyl)methylium hexafluorophosphate (3a) possessing
the resonance structure 3c of a 3-guaiazulenylium ion, with an
equiv of HPF6, in 56% yield; (ii) a solution of the obtained new
monocarbenium ion compound 3a, forming a protonated amino
fluorophosphate (3b), completely, as a result of deprotonation
and further, a solution of 3a¢HPF6 dissolved in an organic
solvent [i.e., CH2Cl2, (CH3)2CO, or C5H5N (pyridine)] was
converted to a solution of 3b in each solvent as a result of
dehydrogen hexafluorophosphate, rapidly; (iii) the reductions
of 3a (and 3b) with NaBH4 in a mixed solvent of EtOH and
CH3CN afforded 4-amino-1-(3-guaiazulenylmethyl)benzene
(4) (100 and 78% yields from 3a and 3b, respectively), in
which a hydride ion attached to each HC-¡ carbon atom of 3a
and 3b, selectively; and (iv) comparative studies on the
spectroscopic, chemical, and electrochemical properties of the
¹
group (4-H3N+PF6 ), in CH3CN was allowed to stand at room
temperature for 48 h, gradually converting to a new 4-(3-
guaiazulenyl)methylene-2,5-cyclohexadiene-1-iminium hexa-

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 5 (2009)
593
Minematsu, H. Takekuma, Tetrahedron 2006, 62, 3732.
14 S. Takekuma, K. Sonoda, C. Fukuhara, T. Minematsu,
Tetrahedron 2007, 63, 2472.
15 S. Takekuma, K. Tone, M. Sasaki, T. Minematsu, H.
Takekuma, Tetrahedron 2007, 63, 2490.
isolated 3a and 3b with those of structurally related compounds
10-12 were reported as the first example, whose studies
apparently indicated the difference between the formation and
properties of 3a and those of 3b.
16 S. Takekuma, K. Mizutani, K. Inoue, M. Nakamura, M.
Sasaki, T. Minematsu, K. Sugimoto, H. Takekuma, Tetrahedron
2007, 63, 3882.
17 S. Takekuma, M. Tamura, T. Minematsu, H. Takekuma,
Tetrahedron 2007, 63, 12058.
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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25 In relation to the formation of 3a, with an equiv of HPF6,
forming a non-protonated H2N-4 group at the solid state, the
crystal structure of {4-[4-(dimethylamino)phenylazo]phenyl}(3-
guaiazulenyl)methylium tetrafluoroborate, with an equiv of HBF4,
forming a non-protonated (CH3)2N-4 group could be determined
by means of X-ray diffraction, recently; details will be reported
elsewhere.
7
M. Nakamura, M. Sasaki, H. Takekuma, T. Minematsu, S.
Takekuma, Bull. Chem. Soc. Jpn. 2003, 76, 2051.
S. Takekuma, K. Sasaki, M. Nakatsuji, M. Sasaki, T.
Minematsu, H. Takekuma, Bull. Chem. Soc. Jpn. 2004, 77, 379.
M. Nakatsuji, Y. Hata, T. Fujihara, K. Yamamoto, M.
8
9
Sasaki, H. Takekuma, M. Yoshihara, T. Minematsu, S. Takekuma,
Tetrahedron 2004, 60, 5983.
26 Guaiazulene (1): UV-vis -max (CH3CN)/nm (log ¾): 213
(4.10), 244 (4.39), 284 (4.61), 301sh (4.03), 348 (3.65), 365 (3.46),
600 (2.68), 648sh (2.61), and 721sh (2.20).
27 The recrystallization of 3a, providing a stable single crystal
suitable for X-ray crystallographic analysis, is very difficult;
however, the recrystallization of 3b, providing a stable single
crystal suitable for that purpose, is possible, and is currently under
intensive investigation.
10 S. Takekuma, Y. Hata, T. Nishimoto, E. Nomura, M.
Sasaki, T. Minematsu, H. Takekuma, Tetrahedron 2005, 61, 6892.
11 S. Takekuma, K. Takahashi, A. Sakaguchi, Y. Shibata, M.
Sasaki, T. Minematsu, H. Takekuma, Tetrahedron 2005, 61, 10349.
12 S. Takekuma, K. Takahashi, A. Sakaguchi, M. Sasaki, T.
Minematsu, H. Takekuma, Tetrahedron 2006, 62, 1520.
13 S. Takekuma, M. Hirosawa, S. Morishita, M. Sasaki, T.