Preparation, structures, and properties of new monocarbenium ion compounds stabilized by a 3-Guaiazulenyl group and an azobenzene unit: Comparative studies on three delocalized π-electron system

Article abstract of DOI:10.1246/bcsj.82.879

Reaction of guaiazulene (=7-isopropyl-1,4-dimethylazulene) with 4-hydroxyazobenzene-4-carbaldehyde in methanol in the presence of hexafluorophosphoric acid at 25°C for 2 h gives as high as 94% yield of a new monocarbenium ion compound (3-guaiazulenyl)[4-(

Full text of DOI:10.1246/bcsj.82.879

© 2009 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 82, No. 7, 879–890 (2009)
879
Preparation, Structures, and Properties of New Monocarbenium Ion
Compounds Stabilized by a 3-Guaiazulenyl Group and an Azobenzene
Unit: Comparative Studies on Three Delocalized ³-Electron Systems
Shin-ichi Takekuma,*1 Kenji Fukuda,1 Yasuko Kawase,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 February 18, 2009; E-mail: takekuma@apch.kindai.ac.jp
Reaction of guaiazulene (=7-isopropyl-1,4-dimethylazulene) with 4¤-hydroxyazobenzene-4-carbaldehyde in
methanol in the presence of hexafluorophosphoric acid at 25 °C for 2 h gives as high as 94% yield of a new mono-
carbenium ion compound (3-guaiazulenyl)[4-(4-hydroxyphenylazo)phenyl]methylium hexafluorophosphate. Similarly,
reactions of guaiazulene with 4¤-methoxyazobenzene-4-carbaldehyde and 4¤-(dimethylamino)azobenzene-4-carbaldehyde
under the same reaction conditions as the above afford the corresponding new (3-guaiazulenyl)[4-(4-methoxyphenylazo)-
phenyl]methylium and {4-[4-(dimethylamino)phenylazo]phenyl}(3-guaiazulenyl)methylium hexafluorophosphates in
97 and 95% yields. Along with an efficient preparation of the target monocarbenium ion compounds, comparative
studies on spectroscopic, chemical, and electrochemical properties of the unique products, indicating different delocalized
³-electron systems, are reported. Moreover, crystal structure of {4-[4-(dimethylamino)phenylazo]phenyl}(3-guai-
azulenyl)methylium tetrafluoroborate with an equiv of HBF4 molecule is documented.
As a series of basic studies on creation of novel functional
materials with a 3-guaiazulenyl (=5-isopropyl-3,8-dimethyl-
azulen-1-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 electrochemical properties of
delocalized mono- and dicarbenium ion compounds stabilized
by extended ³-electron systems possessing a 3-guaiazulenyl
[or an azulen-1-yl or a 3-(methoxycarbonyl)azulen-1-yl]
group.1-18 The products, with a 3-guaiazulenyl group, can be
readily (and quantitatively) obtained by the reactions of
naturally occurring guaiazulene19 (=7-isopropyl-1,4-dimethyl-
azulene) (1) with the corresponding aromatic (and ³-conju-
gated aliphatic) aldehydes in methanol in the presence of
hexafluorophosphoric acid (and tetrafluoroboric acid) in com-
parison with other azulenes.15,17 During the course of our basic
and systematic investigations, we found that the reaction of 1
with 4-aminobenzaldehyde in CH3OH in the presence of
hexafluorophosphoric acid at 25 °C for 2 h gave (4-amino-
phenyl)(3-guaiazulenyl)methylium hexafluorophosphate (16),
with an equiv of HPF6, in 56% yield. The elemental analysis
and the IR spectrum suggested the formation of 16, with an
equiv of molecular HPF6, forming a non-protonated H2N-4
group in the solid state, while the 1H, 19F, and 31P NMR spectra
indicated the formation of the protonated 17 in CH3CN
(Chart 1).20 Interestingly, a solution of the obtained mono-
carbenium ion compound 17, forming a protonated amino
¹
group (4-H3N+PF6 ), in CH3CN was allowed to stand at room
temperature for 48 h, gradually converting to 4-(3-guaiazul-
enyl)methylene-2,5-cyclohexadiene-1-iminium
hexafluoro-
phosphate (18), completely (Chart 1).20 From the structures
of the resulting products 16-18, it could be inferred that the
–
–
PF6
PF6
+
NH3
+
N
H
H
NH2
–
–
–HPF6
PF6
PF6
α
α
HPF6
+
+
H
H
H
in CH3CN
in CH3CN
16
17
18
Chart 1.
Published on the web July 10, 2009; doi:10.1246/bcsj.82.879

880
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
4
4"
R
−
1
PF6
1"
1
4 N
7
N
4'
R
2
H +
1
1
3'
α
N
4'
N
in CH3OH
with 60% HPF6
4
2, 5: R = OH
3, 6: R = OCH3
4, 7a: R = N(CH3)2
1'
OHC
7'
2–4
5–7a
Scheme 1. The reactions of 1 with 2-4 in methanol in the presence of hexafluorophosphoric acid at 25 °C for 2 h, yielding the
corresponding monocarbenium ion compounds 5-7a.
OH
OH
OH
−
PF6
+
N
N
N
−
+
N
N
−
N
PF6
+
H
H
H
PF6
5'
5
5''
OCH3
OCH3
OCH3
−
PF6
+
N
N
N
N
N
N
−
PF6
+
−
H
H
H
PF6
+
6'
6
6''
+
N
N
N
−
PF6
−
N
N
N
PF6
N
N
N
+
−
H
H
H
PF6
+
7a'
7a
Chart 2.
7a''
obtained 17, forming a 4-H3N+PF6 in CH3CN, owing to the
kinetic control was converted to the stabilized 18 owing to the
thermodynamic control (i.e., stability of conjugated ³-electron
system) in the solvent.20 In relation to the above investigations
on structures and properties, our interest has quite recently been
focused on the title chemistry: namely, on the reactions of 1
with azobenezene derivatives possessing an aldhyde group, i.e.,
2, 3, and 421-24 (Scheme 1), in methanol in the presence of
hexafluorophosphoric acid at 25 °C, expectantly producing the
corresponding new extended (and delocalized) monocarbenium
ion compounds 5, 6, and 7a with similar resonance structures
of the 3-guaiazulenylium ion and p-benzoquinoid structures to
those of 16, 17, and 18 (Charts 1 and 2). On the other hand,
azobenzenes in general are currently drawing an increasing
interest from the viewpoint of potential utilities as photo-
memories,25,26 optical switchings,27-29 and optoelectronics.30,31
We now wish to report a facile preparation as well as the
spectroscopic, chemical, and electrochemical properties of
the unique products 5-7a, indicating an apparently difference
between the delocalized ³-electron system of 7a and those of
5 and 6 (Chart 2), along with the crystal structure of the
monocarbenium tetrafluoroborate 7b compared with those of
structurally related known compounds 11-13 (Chart 3).
¹
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 JEOL GX-500 (500 MHz for 1H and 125 MHz for
1
13C) and JNM-ECA600 (600 MHz for H and 150 MHz for 13C)
cryospectrometer. 1H NMR spectra (¤ and J values) were assigned

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
881
dissolved in dichloromethane (30 mL), which was washed with
distilled water, dried (MgSO4), and evaporated in vacuo. The
residue thus obtained was carefully separated by silica gel column
chromatography with hexane-ethyl acetate-benzene (80:15:5,
v/v/v) as an eluant. The crude product 3 was recrystallized from
hexane (several times) to provide pure 3 as stable crystals (59 mg,
245 ¯mol, 69% yield).
Compound 3:22 Orange needles, Rf = 0.28 on silica gel TLC
(hexane-AcOEt-benzene = 80:15:5, v/v/v); exact EI-MS (70 eV),
found: m/z 240.0901 (M+, 100%); calcd for C14H12O2N2: M+, m/z
240.0899.
−
3
6
3"
6"
BF4
4
N
2
4"
N
2"
1"
1
−
+
H
3
6
BF4
5
5"
4 N
2
1
α
N
3'
4'
2
H +
5
1
3'
α
1'
4'
7'
1'
7'
7b
11
Preparation of 4¤-(Dimethylamino)azobenzene-4-carbalde-
hyde (4). Compound 4 was prepared according to the procedures
reported in references No. 22 and 23: namely, to a solid of 4-
aminobenzaldehyde (50 mg, 412 ¯mol) was added an aqueous
solution (0.15 mL) of 12% HCl at ¹15 °C and further an aqueous
solution (2.0 mL) of NaNO2 (50 mg, 724 ¯mol) was added to the
above solution. The mixture was stirred for 15 min, yielding the
corresponding diazonium salt and then a solution of N,N-
dimethylaniline (60 ¯L, 495 ¯mol) was added to the aqueous
solution of the diazonium salt, turning the orange solution red, the
mixture was further stirred at room temperature for 30 min. After
the reaction, the solution was carefully neutralized with aq
NaHCO3 and then the mixture was extracted with dichloromethane
(10 mL © 3). The extract was washed with distilled water, dried
(MgSO4), and evaporated in vacuo. The residue thus obtained was
carefully separated by silica gel column chromatography with
hexane-ethyl acetate (80:20, v/v) as an eluant. The crude product
4 was recrystallized from hexane-dichloromethane (90:10, v/v)
(several times) to provide pure 4 as stable crystals (80 mg,
315 ¯mol, 76% yield).
−
PF6
3
6
1
+
3
6
2
1
N
4
2
1
4'
N
1
N
4
N
2
3'
5
7'
5
N
2
1'
12a)
13b)
Chart 3. a) For a comparative purpose, the numbering
scheme of the 1-azulenyl group of 12 was changed to that
of the 3-azulenyl group. b) For a comparative purpose, the
numbering scheme of the phenyl group of 13 was changed
to that of the 4-phenyl group.
using computer-assisted simulation (software: gNMR developed
by Adept Scientific plc) on a SONY VAIO PCV-HS80 personal
computer with a Pentium (R) 4 processor. Cyclic and differential
pulse voltammograms were measured with an ALS Model 600
electrochemical analyzer.
Preparation of 4¤-Hydroxyazobenzene-4-carbaldehyde (2).
Compound 2 was prepared according to procedures based on
reference No. 23: namely, to a solid of 4-aminobenzaldehyde
(50 mg, 412 ¯mol) was added an aqueous solution (0.15 mL) of
12% HCl at ¹15 °C and further an aqueous solution (2.0 mL)
of NaNO2 (50 mg, 724 ¯mol) was added to the above solution.
The mixture was stirred for 15 min, yielding the corresponding
diazonium salt. An aqueous solution (2.0 mL) of phenol (40 mg,
425 ¯mol) was then added to the aqueous solution of the
diazonium salt, turning the orange solution red, and the mixture
was further stirred at room temperature for 30 min. After the
reaction, the solution was carefully neutralized with aq NaHCO3
and then the mixture was extracted with dichloromethane
(10 mL © 3). The extract was washed with distilled water, dried
(MgSO4), and evaporated in vacuo. The residue thus obtained was
carefully separated by silica gel column chromatography with
hexane-ethyl acetate-benzene (60:30:10, v/v/v) as an eluant. The
crude product 2 was recrystallized from hexane (several times) to
provide pure 2 as stable crystals (77 mg, 340 ¯mol, 83% yield).
Compound 2:22 Yellow prisms, Rf = 0.22 on silica gel TLC
(hexane-EtOAc-benzene = 60:30:10, v/v/v); exact EI-MS
(70 eV), found: m/z 226.0737 (M+, 100%); calcd for C13H10-
O2N2: M+, m/z 226.0742.
Compound 4:23 Red prisms, Rf = 0.44 on silica gel TLC
(hexane-AcOEt = 80:20, v/v); exact EI-MS (70 eV), found: m/z
253.1236 (M+, 100%); calcd for C15H15ON3: M+, m/z 253.1215.
Preparation of (3-Guaiazulenyl)[4-(4-hydroxyphenylazo)-
phenyl]methylium Hexafluorophosphate (5). To a solution of
commercially available guaiazulene (1) (30 mg, 151 ¯mol) in
methanol (1.0 mL) was added a solution of 4¤-hydroxyazobenzene-
4-carbaldehyde (2) (30 mg, 132 ¯mol) in methanol (3.0 mL)
containing hexafluorophosphoric acid (60% aqueous solution,
0.15 mL). The mixture was stirred at 25 °C for 2 h, precipitating a
dark-green solid of 5 and then was centrifuged at 2.5 krpm for
1 min. The crude product 5 thus obtained was carefully washed
with diethyl ether, and was recrystallized from acetonitrile-diethyl
ether (10:50, v/v) (several times) to provide pure 5 as a red powder
(69 mg, 124 ¯mol, 94% yield).
Compound 5: Red powder, mp >180 °C [decomp., determined
by thermal analysis (TGA and DTA)]. Found: C, 61.01; H, 4.64;
N, 5.03%. Calcd for C28H27ON2F6P: C, 60.87; H, 4.93; N, 5.07%;
UV-vis -max (CH3CN) nm (log ¾), 232 (4.47), 343 (4.29), and 498
(4.56); IR ¯max (KBr) cm¹1, 3503 (O-H), 1601 (N=N), and 841,
¹
559 (PF6 ); exact FAB-MS (3-nitrobenzyl alcohol matrix), found:
m/z 407.2101; calcd for C28H27ON2: [M ¹ PF6]+, m/z 407.2124;
1
500 MHz H NMR (CD3CN): signals based on a 3-guaiazulenyl-
Preparation of 4¤-Methoxyazobenzene-4-carbaldehyde (3).
Compound 3 was prepared according to procedures described in
reference No. 23: namely, a solution of 2 (80 mg, 353 ¯mol) in
acetone (5.0 mL) was added to a suspension of K2CO3 (50 mg,
361 ¯mol) in acetone (5.0 mL) and further a solution of CH3I
(80 ¯L, 781 ¯mol) was added to the above solution. The mixture
was refluxed for 2 h under argon. After cooling to room temper-
ature, the solvent was evaporated in vacuo and then the residue was
methylium substituent: ¤ 1.45 (6H, d, J = 6.9 Hz, (CH3)2CH-7¤),
2.50 (3H, s, Me-1¤), 3.33 (3H, s, Me-4¤), 3.47 (1H, sept, J =
6.9 Hz, Me2CH-7¤), 7.95 (1H, s, H-2¤), 8.38 (1H, dd, J = 11.0,
2.3 Hz, H-6¤), 8.48 (1H, d, J = 11.0 Hz, H-5¤), 8.53 (1H, d,
J = 2.3 Hz, H-8¤), and 8.69 (1H, s, HC+-¡); signals based on a
4-(4-hydroxyphenylazo)benzene part: ¤ 6.90 (2H, ddd, J = 8.9,
3.1, 2.0 Hz, H-3¤¤,5¤¤), 7.69 (1H, s, HO-4¤¤), 7.78 (2H, ddd, J = 8.9,
3.1, 2.0 Hz, H-2¤¤,6¤¤), 7.90 (2H, ddd, J = 8.7, 2.3, 1.4 Hz, H-2,6),

882
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
and 7.94 (2H, ddd, J = 8.7, 2.3, 1.4 Hz, H-3,5); 125 MHz
13C NMR (CD3CN): ¤ 172.1 (C-7¤), 161.9 (C-4¤¤), 161.6 (C-8a¤),
158.0 (C-4¤), 154.8 (C-4), 153.6 (C-3a¤), 150.9 (C-5¤), 148.9
(HC+-¡), 147.4 (C-1¤¤), 146.7 (C-1¤), 145.1 (C-6¤), 141.1 (C-2¤),
140.9 (C-3¤), 139.8 (C-8¤), 138.0 (C-1), 135.0 (C-2,6), 126.3
(C-2¤¤,6¤¤), 124.0 (C-3,5), 116.9 (C-3¤¤,5¤¤), 40.2 (Me2CH-7¤), 29.7
(Me-4¤), 23.7 ((CH3)2CH-7¤), and 13.9 (Me-1¤).
Preparation of (3-Guaiazulenyl)[4-(4-methoxyphenylazo)-
phenyl]methylium Hexafluorophosphate (6). To a solution of
guaiazulene (1) (30 mg, 151 ¯mol) in methanol (1.0 mL) was added
a solution of 4¤-methoxyazobenzene-4-carbaldehyde (3) (30 mg,
124 ¯mol) in methanol (3.0 mL) containing hexafluorophosphoric
acid (60% aqueous solution, 0.15 mL). The mixture was stirred at
25 °C for 2 h, precipitating a red solid of 6 and then was centrifuged
at 2.5 krpm for 1 min. The crude product 6 thus obtained was
carefully washed with diethyl ether, and was recrystallized from
acetonitrile-diethyl ether (10:50, v/v) (several times) to provide
pure 6 as stable crystals (68 mg, 120 ¯mol, 97% yield).
2.53 (3H, s, Me-1¤), 3.36 (3H, s, Me-4¤), 3.49 (1H, sept, J =
7.0 Hz, Me2CH-7¤), 8.04 (1H, s, H-2¤), 8.41 (1H, dd, J = 11.0,
2.3 Hz, H-6¤), 8.51 (1H, d, J = 11.0 Hz, H-5¤), 8.59 (1H, d, J =
2.0 Hz, H-8¤), and 8.73 (1H, s, HC+-¡); signals based on a 4-[4-
(dimethylamino)phenylazo]benzene part: ¤ 3.48 (6H, s, Me2N-4¤¤),
7.85 (2H, ddd, J = 9.0, 2.2, 1.7 Hz, H-3,5), and 7.98 (2H, ddd,
J = 9.0, 2.2, 1.7 Hz, H-2,6); 125 MHz 13C NMR (CD3CN): ¤ 172.5
(C-7¤), 162.0 (C-8a¤), 158.9 (C-4¤), 154.5 (C-3a¤), 151.5 (C-5¤),
149.9 (HC+-¡), 147.3 (C-1¤), 145.9 (C-6¤), 142.0 (C-2¤), 141.1 (C-
3¤), 140.8 (C-8¤), 136.9 (C-2,6), 120.1 (C-3,5), 44.4 (Me2N-4¤¤),
41.2 (Me2CH-7¤), 30.7 (Me-4¤), 24.7 ((CH3)2CH-7¤), and 14.8
(Me-1¤).
Reduction of (3-Guaiazulenyl)[4-(4-hydroxyphenylazo)-
phenyl]methylium Hexafluorophosphate (5) with NaBH4. To
a solution of NaBH4 (11 mg, 290 ¯mol) in ethanol (2.0 mL) was
added a solution of 5 (40 mg, 72 ¯mol) in acetonitrile (2.0 mL).
The mixture was stirred at 25 °C for 30 min and then was
evaporated in vacuo. The residue thus obtained was dissolved in
hexane (5.0 mL), and was filtered. The filtrate was evaporated in
vacuo, giving a green pasty residue, which was carefully separated
by silica gel column chromatography with hexane-ethyl acetate
(80:20, v/v) as an eluant. The separated product 8 was recrystal-
lized from hexane (several times) to provide pure 4-(3-guaiazul-
enylmethyl)-4¤-hydroxyazobenzene as a green powder (25 mg,
61 ¯mol, 85% yield).
Compound 6: Red blocks, mp >132 °C [decomp., determined
by thermal analysis (TGA and DTA)]. Found: C, 61.41; H, 4.94;
N, 5.02%. Calcd for C29H29ON2F6P: C, 61.48; H, 5.16; N, 4.94%;
UV-vis -max (CH3CN) nm (log ¾), 232 (4.50), 346 (4.33), and 496
¹
(4.58); IR ¯max (KBr) cm¹1, 1601 (N=N) and 837, 556 (PF6 );
exact FAB-MS (3-nitrobenzyl alcohol matrix), found: m/z
421.2305; calcd for C29H29ON2: [M ¹ PF6]+, m/z 421.2280; 600
MHz 1H NMR (CD3CN): signals based on a 3-guaiazulenyl-
methylium substituent: ¤ 1.45 (6H, d, J = 6.9 Hz, (CH3)2CH-7¤),
2.51 (3H, s, Me-1¤), 3.35 (3H, s, Me-4¤), 3.48 (1H, sept, J =
6.9 Hz, Me2CH-7¤), 7.98 (1H, s, H-2¤), 8.41 (1H, dd, J = 11.2,
2.5 Hz, H-6¤), 8.51 (1H, d, J = 11.2 Hz, H-5¤), 8.56 (1H, d, J =
2.5 Hz, H-8¤), and 8.74 (1H, s, HC+-¡); signals based on a 4-(4-
methoxyphenylazo)benzene part: ¤ 3.88 (3H, s, MeO-4¤¤), 7.06
(2H, ddd, J = 8.9, 3.0, 2.0 Hz, H-3¤¤,5¤¤), 7.91 (2H, ddd, J = 8.9,
3.0, 2.0 Hz, H-2¤¤,6¤¤), 7.94 (2H, ddd, J = 8.3, 2.2, 1.9 Hz, H-2,6),
and 7.99 (2H, ddd, J = 8.3, 2.2, 1.9 Hz, H-3,5); 150 MHz
13C NMR (CD3CN): ¤ 172.2 (C-7¤), 164.1 (C-4¤¤), 161.7 (C-8a¤),
158.2 (C-4¤), 154.9 (C-4), 153.7 (C-3a¤), 151.0 (C-5¤), 149.0 (HC+-
¡), 147.9 (C-1¤¤), 146.7 (C-1¤), 145.1 (C-6¤), 141.2 (C-2¤), 141.0
(C-3¤), 139.9 (C-8¤), 138.2 (C-1), 135.0 (C-2,6), 126.1 (C-2¤¤,6¤¤),
124.0 (C-3,5), 115.6 (C-3¤¤,5¤¤), 56.5 (MeO-4¤¤), 40.3 (Me2CH-7¤),
29.7 (Me-4¤), 23.7 ((CH3)2CH-7¤), and 13.9 (Me-1¤).
Compound 8: Green powder, mp 155 °C [determined by thermal
analysis (TGA and DTA)]. Rf = 0.22 on silica gel TLC (hexane-
AcOEt = 80:20, v/v); UV-vis -max (CH2Cl2) nm (log ¾), 246
¹1
(4.58), 293 (4.70), 352 (4.56), and 620 (2.76); IR ¯max (KBr) cm
,
3414 (O-H) and 1593 (N=N); exact FAB-MS (3-nitrobenzyl
alcohol matrix), found: m/z 408.2190; calcd for C28H28ON2: M+,
m/z 408.2201; 600 MHz 1H NMR (CD3CN): signals based on a 3-
guaiazulenylmethyl group: ¤ 1.31 (6H, d, J = 6.9 Hz, (CH3)2CH-
7¤), 2.58 (3H, s, Me-1¤), 2.80 (3H, s, Me-4¤), 3.03 (1H, sept, J =
6.9 Hz, Me2CH-7¤), 4.65 (2H, s, H2C-3¤), 6.82 (1H, d, J = 10.7 Hz,
H-5¤), 7.31 (1H, dd, J = 10.7, 2.3 Hz, H-6¤), 7.42 (1H, s, H-2¤), and
8.13 (1H, d, J = 2.3 Hz, H-8¤); signals based on a 4-hydroxyazo-
benzene part: ¤ 6.93 (2H, ddd, J = 8.9, 2.7, 2.0 Hz, H-3¤¤,5¤¤), 7.15
(2H, ddd, J = 8.2, 2.4, 1.7 Hz, H-2,6), 7.57 (1H, s, HO-4¤¤), 7.71
(2H, ddd, J = 8.2, 2.4, 1.7 Hz, H-3,5), and 7.76 (2H, ddd, J = 8.9,
2.7, 2.0 Hz, H-2¤¤,6¤¤); 150 MHz 13C NMR (CD3CN): ¤ 160.8
(C-4¤¤), 151.7 (C-4), 147.5 (C-1), 147.3 (C-1¤¤), 146.3 (C-4¤), 141.9
(C-2¤), 140.2 (C-7¤), 138.8 (C-8a¤), 135.9 (C-6¤), 134.4 (C-8¤),
133.8 (C-3a¤), 130.0 (C-2,6), 127.1 (C-5¤), 126.1 (C-3¤), 125.5
(C-2¤¤,6¤¤), 125.3 (C-1¤), 123.3 (C-3,5), 116.7 (C-3¤¤,5¤¤), 38.3
(Me2CH-7¤), 37.4 (H2C-3¤), 26.8 (Me-4¤), 24.7 ((CH3)2CH-7¤), and
12.9 (Me-1¤). For comparative purposes on 1H and 13C NMR
signals, the numbering scheme of compound 8 was changed as
shown in Scheme 2.
Reduction of (3-Guaiazulenyl)[4-(4-methoxyphenylazo)-
phenyl]methylium Hexafluorophosphate (6) with NaBH4. To
a solution of NaBH4 (11 mg, 290 ¯mol) in ethanol (2.0 mL) was
added a solution of 6 (40 mg, 70 ¯mol) in acetonitrile (2.0 mL).
The mixture was stirred at 25 °C for 30 min and then was
evaporated in vacuo. The residue thus obtained was dissolved in
hexane (5.0 mL), and was filtered. The filtrate was evaporated in
vacuo, giving a green pasty residue, which was carefully separated
by silica gel column chromatography with hexane-ethyl acetate
(90:10, v/v) as an eluant. The separated product 9 was recrystal-
lized from hexane (several times) to provide pure 4-(3-guai-
azulenylmethyl)-4¤-methoxyazobenzene as green blocks (22 mg,
52 ¯mol, 74% yield).
Preparation of {4-[4-(Dimethylamino)phenylazo]phenyl}(3-
guaiazulenyl)methylium Hexafluorophosphate (7a).
To a
solution of guaiazulene (1) (30 mg, 151 ¯mol) in methanol
(1.0 mL) was added a solution of 4¤-(dimethylamino)azobenzene-
4-carbaldehyde (4) (30 mg, 118 ¯mol) in methanol (3.0 mL)
containing hexafluorophosphoric acid (60% aqueous solution,
0.15 mL). The mixture was stirred at 25 °C for 2 h, precipitating a
dark-blue solid of 7a and then was centrifuged at 2.5 krpm for
1 min. The crude product 7a thus obtained was carefully washed
with diethyl ether, and was recrystallized from acetonitrile-diethyl
ether (10:50, v/v) (several times) to provide pure 7a as stable
crystals (65 mg, 112 ¯mol, 95% yield).
Compound 7a: Green plates, mp >140 °C [decomp., determined
by thermal analysis (TGA and DTA)]. Found: C, 62.03; H, 5.23;
N, 6.94%. Calcd for C30H32N3F6P: C, 62.17; H, 5.57; N, 7.25%;
UV-vis -max (CH3CN) nm (log ¾), 233 (4.42), 414 (4.22), and 600
¹
(4.74); IR ¯max (KBr) cm¹1, 1620 (N=N) and 841, 559 (PF6 );
exact FAB-MS (3-nitrobenzyl alcohol matrix), found: m/z
434.2570; calcd for C30H32N3: [M ¹ PF6]+, m/z 434.2596; 500
MHz 1H NMR (CD3CN): signals based on a 3-guaiazulenyl-
methylium substituent: ¤ 1.45 (6H, d, J = 7.0 Hz, (CH3)2CH-7¤),

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
883
4"
R
1
N
1"
N
4
α 1
2
H2C
NaBH4
3'
4'
5–7a
8: R = OH
9: R = OCH3
10: R = N(CH3)2
in a mixed solvent
of EtOH and CH3CN
1'
7'
8–10a)
Scheme 2. The reductions of 5-7a with NaBH4 in a mixed solvent of EtOH and CH3CN at 25 °C for 30 min, yielding the
corresponding hydride-reduction products 8-10. a) For a comparative purposes on 1H and 13C NMR signals, the numbering
schemes of compounds 8-10 were changed as shown in Scheme 2.
Compound 9: Green blocks, mp 122 °C [determined by thermal
analysis (TGA and DTA)]. Rf = 0.30 on silica gel TLC (hexane-
Me2CH-7¤), 4.67 (2H, s, H2C-3¤), 6.82 (1H, d, J = 11.0 Hz, H-5¤),
7.28 (1H, dd, J = 11.0, 2.0 Hz, H-6¤), 7.43 (1H, s, H-2¤), and 8.11
(1H, d, J = 2.0 Hz, H-8¤); signals based on a 4-(dimethylamino)-
azobenzene part: ¤ 3.07 (6H, s, Me2N-4¤¤), 6.77 (2H, ddd, J = 9.0,
3.0, 2.0 Hz, H-3¤¤,5¤¤), 7.14 (2H, ddd, J = 8.6, 2.0, 2.0 Hz, H-2,6),
7.71 (2H, ddd, J = 8.6, 2.0, 2.0 Hz, H-3,5), and 7.82 (2H, ddd,
J = 9.0, 3.0, 2.0 Hz, H-2¤¤,6¤¤); 150 MHz 13C NMR (CD2Cl2): ¤
152.1 (C-4¤¤), 150.8 (C-4), 145.0 (C-4¤), 144.8 (C-1), 143.1 (C-1¤¤),
140.7 (C-2¤), 138.7 (C-7¤), 137.5 (C-8a¤), 134.4 (C-6¤), 133.0
(C-8¤), 132.7 (C-3a¤), 128.6 (C-2,6), 125.8 (C-5¤), 124.8 (C-3¤),
124.3 (C-2¤¤,6¤¤), 123.9 (C-1¤), 121.7 (C-3,5), 111.1 (C-3¤¤,5¤¤), 39.8
(Me2N-4¤¤), 37.3 (Me2CH-7¤), 36.6 (H2C-3¤), 26.1 (Me-4¤), 23.9
((CH3)2CH-7¤), and 12.2 (Me-1¤). For comparative purposes on 1H
and 13C NMR signals, the numbering scheme of compound 10 was
changed as shown in Scheme 2.
Preparation of {4-[4-(Dimethylamino)phenylazo]phenyl}(3-
guaiazulenyl)methylium Tetrafluoroborate (7b). To a solution
of guaiazulene (1) (30 mg, 151 ¯mol) in methanol (1.0 mL) was
added a solution of 4¤-(dimethylamino)azobenzene-4-carbaldehyde
(4) (30 mg, 118 ¯mol) in methanol (3.0 mL) containing tetrafluo-
roboric acid (42% aqueous solution, 0.15 mL). The mixture was
stirred at 25 °C for 2 h, precipitating a dark-blue solid of 7b and
then was centrifuged at 2.5 krpm for 1 min. The crude product 7b
thus obtained was carefully washed with diethyl ether, and was
recrystallized from acetonitrile-diethyl ether (10:50, v/v) (several
times) to provide pure 7b as stable single crystals (58 mg, 95 ¯mol,
81% yield) with an equiv of HBF4 molecule.
Compound 7b: Green plates. Found: C, 59.43; H, 5.69; N,
7.14%. Calcd for C30H33N3B2F8: C, 59.15; H, 5.46; N, 6.90%.
X-ray Crystal Structure of {4-[4-(Dimethylamino)phenyl-
azo]phenyl}(3-guaiazulenyl)methylium Tetrafluoroborate (7b).
A total 7096 reflections with 2ªmax = 55.0° were collected on a
Rigaku AFC-5R automated four-circle diffractometer with graphite
monochromated Mo K¡ radiation (- = 0.71069 ¡, rotating anode:
50 kV, 180 mA) at ¹75 °C. The structure was solved by direct
methods (SIR92)32 and expanded using Fourier techniques
(DIRDIF94).33 Non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included but not refined. The final cycle of
full-matrix least-squares refinement was based on F2. All calcu-
lations were performed using the teXsan crystallographic software
package.34 Crystallographic data have been deposited with Cam-
bridge Crystallographic Data Center: Deposition number CCDC-
669773 for compound No. 7b. Copies of the data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge, CB2 1EZ, U.K.; Fax: +44 1223 336033;
e-mail: deposit@ccdc.cam.ac.uk).
EtOAc = 90:10, v/v); UV-vis -max (CH2Cl2) nm (log ¾), 246
¹1
(4.57), 293 (4.69), 353 (4.55), and 618 (2.74); IR ¯max (KBr) cm
,
1601 (N=N); exact FAB-MS (3-nitrobenzyl alcohol matrix),
found: m/z 422.2388; calcd for C29H30ON2: M+, m/z 422.2358;
1
600 MHz H NMR (CD3CN): signals based on a 3-guaiazulenyl-
methyl group: ¤ 1.31 (6H, d, J = 6.8 Hz, (CH3)2CH-7¤), 2.58
(3H, s, Me-1¤), 2.81 (3H, s, Me-4¤), 3.03 (1H, sept, J = 6.8 Hz,
Me2CH-7¤), 4.66 (2H, s, H2C-3¤), 6.83 (1H, d, J = 10.8 Hz, H-5¤),
7.32 (1H, dd, J = 10.8, 2.0 Hz, H-6¤), 7.42 (1H, s, H-2¤), and 8.13
(1H, d, J = 2.0 Hz, H-8¤); signals based on 4-methoxyazobenzene:
¤ 3.85 (3H, s, MeO-4¤¤), 7.05 (2H, ddd, J = 9.1, 3.1, 2.2 Hz,
H-3¤¤,5¤¤), 7.16 (2H, ddd, J = 8.4, 2.3, 1.9 Hz, H-2,6), 7.73 (2H,
ddd, J = 8.4, 2.3, 1.9 Hz, H-3,5), and 7.84 (2H, ddd, J = 9.1, 3.1,
2.2 Hz, H-2¤¤,6¤¤); 150 MHz 13C NMR (CD3CN): ¤ 163.1 (C-4¤¤),
151.7 (C-4), 147.8 (C-1), 147.6 (C-1¤¤), 146.3 (C-4¤), 141.9 (C-2¤),
140.2 (C-7¤), 138.8 (C-8a¤), 135.9 (C-6¤), 134.4 (C-8¤), 133.8
(C-3a¤), 130.0 (C-2,6), 127.2 (C-5¤), 126.0 (C-3¤), 125.3 (C-2¤¤,6¤¤),
125.3 (C-1¤), 123.3 (C-3,5), 115.3 (C-3¤¤,5¤¤), 56.3 (MeO-4¤¤), 38.3
(Me2CH-7¤), 37.4 (H2C-3¤), 26.8 (Me-4¤), 24.7 ((CH3)2CH-7¤), and
12.9 (Me-1¤). For comparative purposes on 1H and 13C NMR
signals, the numbering scheme of compound 9 was changed as
shown in Scheme 2.
Reduction of {4-[4-(Dimethylamino)phenylazo]phenyl}(3-
guaiazulenyl)methylium Hexafluorophosphate (7a) with
NaBH4. To a solution of NaBH4 (10 mg, 264 ¯mol) in ethanol
(2.0 mL) was added a solution of 7a (40 mg, 69 ¯mol) in
acetonitrile (2.0 mL). The mixture was stirred at 25 °C for 30 min
and then was evaporated in vacuo. The residue thus obtained was
dissolved in dichloromethane (5.0 mL), and was filtered. The
filtrate was evaporated in vacuo, giving a green pasty residue,
which was carefully separated by silica gel column chromatog-
raphy with hexane-ethyl acetate (90:10, v/v) as an eluant. The
separated product 10 was recrystallized from dichloromethane-
acetonitrile (10:50, v/v) (several times) to provide pure 4¤-
dimethylamino-4-(3-guaiazulenylmethyl)azobenzene as green
blocks (22 mg, 50 ¯mol, 72% yield).
Compound 10: Green blocks, mp 223 °C [determined by
thermal analysis (TGA and DTA)]. Rf = 0.28 on silica gel TLC
(hexane-EtOAc = 90:10, v/v); UV-vis -max (CH2Cl2) nm (log ¾),
248 (4.54), 293 (4.71), 413 (4.55), and 620 (2.74); IR ¯max (KBr)
cm¹1, 1597 (N=N); exact FAB-MS (3-nitrobenzyl alcohol matrix),
found: m/z 435.2665; calcd for C30H33N3: M+, m/z 435.2675;
1
600 MHz H NMR (CD2Cl2): signals based on a 3-guaiazulenyl-
methyl group: ¤ 1.35 (6H, d, J = 7.0 Hz, (CH3)2CH-7¤), 2.62
(3H, s, Me-1¤), 2.84 (3H, s, Me-4¤), 3.03 (1H, sept, J = 7.0 Hz,

884
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
Crystallographic data for 7b: C30H33N3B2F8 (FW 609.22), green
plate (the crystal size, 0.30 © 0.30 © 0.50 mm ), triclinic, P1 (#2),
Compound 5 was obtained as a red powder. The UV-vis
spectrum is shown in Figure 1a. A comparative study of the
UV-vis spectrum of 5 with that of 1 (Figure 1b) showed that the
characteristic UV-vis absorption bands based on 1 were not
observed, indicating the formation of the structure 5 with an
extended ³-electron system formed by a 3-guaiazulenylmeth-
ylium ion and a 4-hydroxyazobenzene. The longest absorption
wavelength appeared at 498 nm (log ¾ = 4.56). The IR spectrum
showed four specific bands from O-H at ¯max 3503 cm¹1, N=N
3
ꢀ
a = 10.202(2) ¡, b = 20.324(3) ¡, c = 7.249(1) ¡, ¡ = 99.56(1)°,
¢ = 94.74(2)°, £ = 97.42(1)°, V = 1461.5(4) ¡3, Z = 2, Dcalcd
=
¹3
¹1
1.382 g cm
,
®(Mo K¡) = 1.16 cm
,
Scan width = (1.21 +
0.30 tan ª)°, Scan mode = ½¹2ª, Scan rate = 8.0° min¹1, mea-
sured reflections = 7096, observed reflections = 6717, No. of
parameters = 388, R1 = 0.081, wR2 = 0.240, and Goodness of Fit
Indicator = 1.56.
¹
at ¯max 1601 cm¹1, and PF6 at ¯max 841 and 559 cm¹1. The
Results and Discussion
formula C28H27ON2 for [M ¹ PF6]+ ion was determined by
exact FAB-MS spectrum. An elemental analysis confirmed the
formula C28H27ON2F6P. The 1H NMR spectrum showed signals
originating from a 3-guaiazulenylmethylium ion structure with
a resonance structure of a 3-guaiazulenylium ion, and showed
Preparation and Spectroscopic Properties of 5, 6, and 7a.
The target monocarbeniun ion compounds 5, 6, and 7a were
prepared in methanol according to the procedures shown in
Scheme 1, Table 1, and Experimental Section. The structures
of the products 5-7a were established on the basis of elemental
analysis and spectroscopic data [UV-vis, IR, exact FAB-MS,
signals originating from
a 4-(4-hydroxyphenylazo)phenyl
group, the signals of which were carefully assigned using
computer-assisted simulation based on first-order analysis along
with H-H COSY (Tables 2 and 3). The 13C NMR spectrum
exhibited 23 carbon signals assigned by HMQC and HMBC
(Tables 4 and 5). Thus, the elemental analysis and the spec-
troscopic data for 5 led to the structure (3-guaiazulenyl)[4-(4-
hydroxyphenylazo)phenyl]methylium hexafluorophosphate.
Compound 6 was obtained as red blocks. The character-
istic UV-vis absorption bands coincided with those of 5
(Figure 1a). The IR spectrum showed three specific bands
1
and H and 13C NMR including 2D NMR (i.e., H-H COSY,
HMQC, and HMBC)].
Table 1. The Yield (%) of the Products 5-7a Obtained from
the Reactions of 1 with 2-4 in CH3OH in the Presence of
Hexafluorophosphoric Acid
Substituent
Entry
Temp/°C Time/h Product Yield/%a)
R
¹1
¹
1
2
3
OH
OCH3
N(CH3)2
25
25
25
2
2
2
5
6
7a
94
97
95
resulting from N=N at ¯max 1601 cm and PF6 at ¯max 837
and 556 cm¹1, the wavenumbers of which coincided with
those of 5. The formula C29H29ON2 for [M ¹ PF6]+ ion was
determined by exact FAB-MS spectrum. Elemental analysis
a) Isolated yield.
¹1
¹1
Figure 1. (a) The UV-vis spectra of 5, 6, and 7a in CH3CN. Concentrations, 5: 0.10 g L (181 ¯mol L¹1), 6: 0.10 g L
(177 ¯mol L¹1), and 7a: 0.10 g L (173 ¯mol L¹1). Length of the cell, 0.1 cm each. log ¾ values are given in parenthesis. (b) The
UV-vis spectrum of 1 in CH3CN. Concentration, 1: 0.075 g L¹1 (379 ¯mol L¹1). Length of the cell, 0.1 cm. 1: -max/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).
¹1
Table 2. The 1H NMR Chemical Shifts (¤) for the 3-Guaiazulenylmethylium Ions of 5-7a and the 3-Guaiazulenylmethyl
Groups of 8-10 in CD3CN at 25 °C
Compound
HC-¡
Me-1¤
H-2¤
Me-4¤
H-5¤
H-6¤
Me2CH-7¤ (CH3)2CH-7¤
H-8¤
5
6
7a
8
9
10b)
8.69
8.74
8.73
4.65a)
4.66a)
4.67a)
2.50
2.51
2.53
2.58
2.58
2.62
7.95
7.98
8.04
7.42
7.42
7.43
3.33
3.35
3.36
2.80
2.81
2.84
8.48
8.51
8.51
6.82
6.83
6.82
8.38
8.41
8.41
7.31
7.32
7.28
3.47
3.48
3.49
3.03
3.03
3.03
1.45
1.45
1.45
1.31
1.31
1.35
8.53
8.56
8.59
8.13
8.13
8.11
a) H2C-3¤. b) Measurement solvent: CD2Cl2.

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
885
1
confirmed the formula C29H29ON2F6P. The H NMR spectrum
showed signals from a 3-guaiazulenylmethylium ion with a
resonance structure of a 3-guaiazulenylium ion, and displayed
signals from a 4-(4-methoxyphenylazo)phenyl group, the
signals of which were carefully assigned using similar
techniques to those of 5 (Tables 2 and 3). The 13C NMR
spectrum exhibited 24 carbon signals assigned by HMQC and
HMBC (Tables 4 and 5). Thus, the elemental analysis and the
spectroscopic data for 6 led to the structure (3-guaiazulenyl)[4-
(4-methoxyphenylazo)phenyl]methylium hexafluorophosphate.
Compound 7a was obtained as green plates. The UV-vis
spectrum showed that the spectral pattern of 7a resembled those
of 5 and 6; however, the longest absorption wavelength of 7a
showed a large bathochromic shift (¦102 and 104 nm) and a
large hyperchromic effect (¦ log ¾ = 0.18 and 0.16) in com-
structure of a 3-guaiazulenylium ion, and revealed three signals
orginating from the H-2,6, H-3,5, and Me2N-4¤¤ protons of a
4-[4-(dimethylamino)phenylazo]phenyl group, the signals of
which were carefully assigned using similar techniques to those
of 5 (Tables 2 and 3). Furthermore, the broadened signals
resulting from the H-2¤¤,6¤¤ and H-3¤¤,5¤¤ protons were observed
at 25 °C (Figure 2), and could not be unambiguously assigned.
The 13C NMR spectrum exhibited 18 carbon signals assigned
by HMQC and HMBC (Tables 4 and 5); however, the six
signals from the C-1, C-4, C-1¤¤, C-2¤¤,6¤¤, C-3¤¤,5¤¤, and C-4¤¤
carbons were not observed. The 1H and 13C NMR spectral data
suggested the existence of rotational stereoisomers for 7a¤¤ at
25 °C (Chart 2). Then, along with the above analyses, the
1
variable-temperature H NMR spectra of 7a in CD3CN at 70,
40, 25, 0, and ¹40 °C were measured (Figure 2). As a result, it
was found that the H-2¤¤,6¤¤ and H-3¤¤,5¤¤ proton signals were
not observed completely at 0 °C and corresponding broadened
signals were observed at 25 and 40 °C. However, two signals
(¤ 7.92 and 7.28 for the H-2¤¤,6¤¤ and H-3¤¤,5¤¤ protons) were
observed at 70 °C and four signals (¤ 7.96, 7.69, 7.27, and 7.08
for the H-2¤¤, 6¤¤, 3¤¤, and 5¤¤ protons) were observed at ¹40 °C.
These results support the formation of rotational stereoisomers
of 7a¤¤ under the measurement conditions. All the signals
observed at ¹40 °C showed up-field shifts in comparison with
those observed at other temperatures, resulting from the
difference between the ring-current effect of the yielded
rotational stereoisomers. Thus, the elemental analysis and the
spectroscopic data for 7a led to the structure {4-[4-(dimethyl-
parison with those of 5 and 6 (Figure 1a). The IR spectrum
showed three specific bands from N=N at ¯max 1620 cm
which was higher than those of 5 and 6, and PF6 at ¯max 841
and 559 cm¹1, which coincided with those of 5 and 6. The
formula C30H32N3 for [M ¹ PF6]+ ion was determined by exact
FAB-MS spectrum. Elemental analysis confirmed the formula
¹1
,
¹
1
C30H32N3F6P. The H NMR spectrum showed signals from a
3-guaiazulenylmethylium ion structure with a resonance
1
Table 3. The H NMR Chemical Shifts (¤) for the Azoben-
zenes of 5-10 in CD3CN at 25 °C
Compound
H-2,6
H-3,5
H-2¤¤,6¤¤
H-3¤¤,5¤¤
5
6
7a
8
9
10a)
7.90
7.94
7.98
7.15
7.16
7.14
7.94
7.99
7.85
7.71
7.73
7.71
7.78
7.91
®
6.90
7.06
®
amino)phenylazo]phenyl}(3-guaiazulenyl)methylium
fluorophosphate.
hexa-
b)
b)
Reductions of 5-7a with NaBH4 and Comparative
1
Studies of H and 13C NMR Spectral Parameters of 5-10.
7.76
7.84
7.82
6.93
7.05
6.77
The reduction of 5 with NaBH4 in a mixed solvent of ethanol
and acetonitrile at 25 °C for 30 min gave as high as 85% yield
of 8 (Scheme 2 and Table 6). Compound 8 was obtained as a
green powder. A comparative study of the UV-vis spectrum of
a) Measurement solvent: CD2Cl2. b) The broadened signal,
which could not be assigned, was observed (see Figure 2).
Table 4. Selected 13C NMR Chemical Shifts (¤) for the 3-Guaiazulenylmethylium Ions of 5-7a and the
3-Guaiazulenylmethyl Groups of 8-10 in CD3CN at 25 °C
Compound HC-¡
C-1¤
C-2¤
C-3¤
C-3a¤
C-4¤
C-5¤
C-6¤
C-7¤
C-8¤
C-8a¤
5
6
7a
8
9
10b)
148.9
149.0
149.9
146.7
146.7
147.3
141.1
141.2
142.0
141.9
141.9
140.7
140.9
141.0
141.1
126.1
126.0
124.8
153.6
153.7
154.5
133.8
133.8
132.7
158.0
158.2
158.9
146.3
146.3
145.0
150.9
151.0
151.5
127.1
127.2
125.8
145.1
145.1
145.9
135.9
135.9
134.4
172.1
172.2
172.5
140.2
140.2
138.7
139.8
139.9
140.8
134.4
134.4
133.0
161.6
161.7
162.0
138.8
138.8
137.5
37.4a) 125.3
37.4a) 125.3
36.6a) 123.9
a) H2C-3¤. b) Measurement solvent: CD2Cl2.
Table 5. The 13C NMR Chemical Shifts (¤) for the Azobenzenes of 5-10 in CD3CN at 25 °C
Compound
C-1
C-2,6
C-3,5
C-4
C-1¤¤
C-2¤¤,6¤¤
C-3¤¤,5¤¤
C-4¤¤
5
6
7a
8
9
10a)
138.0
138.2
®
135.0
135.0
136.9
130.0
130.0
128.6
124.0
124.0
120.1
123.3
123.3
121.7
154.8
154.9
®
147.4
147.9
®
126.3
126.1
®
116.9
115.6
®
161.9
164.1
®
b)
b)
b)
b)
b)
b)
147.5
147.8
144.8
151.7
151.7
150.8
147.3
147.6
143.1
125.5
125.3
124.3
116.7
115.3
111.1
160.8
163.1
152.1
a) Measurement solvent: CD2Cl2. b) The signal was not observed.

886
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
1
Figure 2. The partial variable-temperature H NMR spectra of 7a in CD3CN at 70, 40, 25, 0, and ¹40 °C.
Table 6. The Yield (%) of the Products 8-10 Obtained from
the Reductions of 5-7a with NaBH4 in a Mixed Solvent of
EtOH and CH3CN at 25 °C for 30 min
Table 7. The Selected UV-Vis and IR Spectral Data of 1
and 5-10
¹1
¯
max/cm
(N=N)
Compound
-max/nm (log ¾)
Substituent
Entry
Temp/°C Time/h Product Yield/%a)
R
1
244 (4.39), 284 (4.61), 348 (3.65),
600 (2.68)
1
2
3
OH
OCH3
N(CH3)2
25
25
25
30
30
30
8
9
10
85
74
72
5
6
7a
8
232 (4.47), 343 (4.29), 498 (4.56)
232 (4.50), 346 (4.33), 496 (4.58)
233 (4.42), 414 (4.22), 600 (4.74)
246 (4.58), 293 (4.70), 352 (4.56),
620 (2.76)
1601
1601
1620
1593
a) Isolated yield.
8 with that of 1 (Figure 1b) showed that the characteristic UV-
vis absorption bands from 1 were observed (Table 7). The IR
spectrum showed specific bands originating from O-H at ¯max
3414 cm¹1, which was much lower than that of 5 (¦¯max
89 cm¹1), and N=N at ¯max 1593 cm¹1, which was slightly
lower than that of 5 (¦¯max 8 cm¹1) (Table 7). The molecular
formula C28H28ON2 was determined by exact FAB-MS
9
246 (4.57), 293 (4.69), 353 (4.55),
618 (2.74)
248 (4.54), 293 (4.71), 413 (4.55),
620 (2.74)
1601
1597
10
methylium ion of 5 displayed slight up-field shifts in
comparison with those of 8, the other carbon signals for that
of 5 showed down-field shifts in comparison with those of 8.
The order of larger down-field shift was HC-¡ (¦¤ 111.5) >
C-7¤ (31.9) > C-5¤ (23.8) > C-8a¤ (22.8) > C-1¤ (21.4) > C-3a¤
(19.8) > C-3¤ (14.8) > C-4¤ (11.7) > C-6¤ (9.2) > C-8¤ (5.4) >
Me-4¤ (2.9) > Me2CH-7¤ (1.9) > Me-1¤ (1.0). The H-2¤¤,6¤¤ and
H-3¤¤,5¤¤ proton signals for the 4-hydroxyazobenzene of 5
coincided with those of 8; however, the other proton signals for
5 were shifted down-field in comparison with those of 8. The
order of larger down-field shift was H-2,6 (¦¤ 0.75) > H-3,5
(0.23) > HO-4¤¤ (0.12). Although the C-1, C-2,6, C-3,5, and
C-4 carbon signals for the 4-hydroxyazobenzene of 5 revealed
up- and down-field shifts in comparison with those of 8, the
C-1¤¤ and C-3¤¤,5¤¤ carbon signals for that of 5 coincided with
those of 8 and the C-2¤¤,6¤¤ and C-4¤¤ carbon signals for that of 5
showed slight down field shifts in comparison with those of 8.
1
spectrum. The H NMR spectrum showed signals from a 3-
guaiazulenylmethyl-substituted 4-hydroxyazobenzene at the
C-4¤ position, the signals of which were carefully assigned
using H-H COSY and computer-assisted simulation based on
first-order analysis (Tables 2 and 3). The 13C NMR spectrum
exhibited 23 carbon signals assigned by HMQC and HMBC
(Tables 4 and 5). Thus, the spectroscopic data for 8 led to the
molecular structure 4-(3-guaiazulenylmethyl)-4¤-hydroxyazo-
benzene (Scheme 2), in which a hydride-ion attached to the
HC+-¡ carbon atom of 5, selectively. Comparative studies of
1
the chemical shifts for the H and 13C NMR signals of 5 with
those of 8 showed that the Me-1¤ proton signal for the 3-
guaiazulenylmethylium ion of 5 displayed a slight up-field shift
in comparison with that of 8; however, the other proton signals
of 5 showed down-field shifts in comparison with those of 8.
The order of larger down-field shift was HC-¡ (¦¤ 4.04) > H-
5¤ (1.66) > H-6¤ (1.07) > Me-4¤, H-2¤ (0.53, each) > Me2CH-
7¤ (0.44) > H-8¤ (0.40) > (CH3)2CH-7¤ (0.14). Although the
C-2¤ and (CH3)2CH-7¤ carbon signals for the 3-guaiazulenyl-
Comparing the H and 13C NMR chemical shifts of 5 to those
of 8, the resonance structures of 5 can be inferred as illustrated
in Chart 2.
1

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
887
The NaBH4-reduction of 6 under the same reaction
conditions as for 5 afforded as high as 74% yield of 9
(Scheme 2 and Table 6). Compound 9 was obtained as green
blocks. A comparative study on the UV-vis spectrum of 9
with that of 1 (Figure 1b) showed the characteristic UV-vis
absorption bands from 1 (Table 7). The IR spectrum showed a
specific band originating from N=N at ¯max 1601 cm¹1, which
coincided with that of 6 (Table 7). The molecular formula
C29H30ON2 was determined by exact FAB-MS spectrum. The
1H NMR spectrum showed signals from a 3-guaiazulenylmeth-
yl-substituted 4-methoxyazobenzene at the C-4¤ position, the
signals of which were carefully assigned using similar
techniques to those of 8 (Tables 2 and 3). The 13C NMR
spectrum exhibited 24 carbon signals assigned by HMQC
and HMBC (Tables 4 and 5). Thus, the spectroscopic data for
9 led to the molecular structure 4-(3-guaiazulenylmethyl)-4¤-
methoxyazobenzene (Scheme 2), in which a hydride-ion
also attached to the HC+-¡ carbon atom of 6, selectively.
Comparative studies of the chemical shifts of the 1H and
13C NMR signals of 6 with those of 9 showed that the Me-1¤
proton signal for the 3-guaiazulenylmethylium ion of 6 shifted
slightly up-field in comparison with that of 9; however, the
other proton signals for 6 showed down-field shifts in
comparison with those of 9. The order of larger down-field
shift was HC-¡ (¦¤ 4.08) > H-5¤ (1.68) > H-6¤ (1.09) > H-2¤
(0.56) > Me-4¤ (0.54) > Me2CH-7¤ (0.45) > H-8¤ (0.43) >
(CH3)2CH-7¤ (0.14). Although the C-2¤ and (CH3)2CH-7¤
carbon signals for the 3-guaiazulenylmethylium ion of 6
displayed slight up-field shifts in comparison with 9, the other
carbon signals for 6 showed down-field shifts in comparison
with those of 9. The order of larger down-field shift was HC-¡
(¦¤ 111.6) > C-7¤ (32.0) > C-5¤ (23.8) > C-8a¤ (22.9) > C-1¤
(21.4) > C-3a¤ (19.9) > C-3¤ (15.0) > C-4¤ (11.9) > C-6¤
(9.2) > C-8¤ (5.5) > Me-4¤ (2.9) > Me2CH-7¤ (2.0) > Me-1¤
(1.0). The H-2¤¤,6¤¤, H-3¤¤,5¤¤, and MeO-4¤¤ proton signals for
the 4-methoxyazobenzene of 6 coincided with those of 9, while
the other proton signals for 6 shifted down-field in comparison
with those of 9. The order of larger down-field shift was H-2,6
(¦¤ 0.78) > H-3,5 (0.26). The C-1¤¤, C-3¤¤,5¤¤, and MeO-4¤¤
carbon signals for the 4-methoxyazobenzene of 6 coincided
with those of 9; however, the C-1 carbon signal for 6 displayed
an up-field shift in comparison with that of 9 and the other
carbon signals of 6 showed down-field shifts in comparison
with those of 9. The order of larger down-field shift was C-2,6
(¦¤ 5.0) > C-4 (3.2) > C-4¤¤ (1.0) > C-2¤¤,6¤¤ (0.8) > C-3,5
(0.7). Similar to the case of 5, comparing the 1H and 13C NMR
chemical shifts of 6 to those of 9, the resonance structures of 6
can be inferred as illustrated in Chart 2.
amino)azobenzene at the C-4¤ position, the signals of which
were carefully assigned using similar techniques to those of 8
(Tables 2 and 3). The 13C NMR spectrum exhibited 24 carbon
signals assigned by HMQC and HMBC techniques (Tables 4
and 5). Thus, the spectroscopic data for 10 led to the molecular
structure 4¤-(dimethylamino)-4-(3-guaiazulenylmethyl)azoben-
zene, in which a hydride-ion also attached to the HC+-¡ carbon
atom of 7a, selectively. Comparative studies of the chemical
1
shifts for the H and 13C NMR signals of 7a with those of 10
showed broadening H-2¤¤,6¤¤ and H-3¤¤,5¤¤ proton signals of 7a
which could not be unambiguously assigned, and the C-1, C-4,
C-1¤¤, C-2¤¤,6¤¤, C-3¤¤,5¤¤, and C-4¤¤ carbon signals of 7a were
not observed, suggesting the existence of rotational stereo-
isomers for 7a¤¤ (Chart 2); however, the corresponding signals
of 10 were observed. The Me-1¤ proton signal for the 3-
guaiazulenylmethylium ion of 7a revealed a slight up-field shift
in comparison with that of 10; however, the other proton
signals for 7a showed down-field shifts in comparison with
those of 10. The order of larger down-field shift was HC-¡ (¦¤
4.06) > H-5¤ (1.69) > H-6¤ (1.13) > H-2¤ (0.61) > Me-4¤
(0.52) > H-8¤
(0.48) > Me2CH-7¤
(0.46) > (CH3)2CH-7¤
(0.10). All the carbon signals for the 3-guaiazulenylmethylium
ion of 7a were shifted down-field in comparison with those
of 10. The order of larger down-field shift was HC-¡
(¦¤ 113.3) > C-7¤ (33.8) > C-5¤ (25.7) > C-8a¤ (24.5) > C-1¤
(23.4) > C-3a¤ (21.8) > C-3¤ (16.3) > C-4¤ (13.9) > C-6¤
(11.5) > C-8¤ (7.8) > Me-4¤ (4.6) > Me2CH-7¤ (3.9) > Me-1¤
(2.6) > C-2¤ (1.3) > (CH3)2CH-7¤ (0.8). The observed proton
signals for the 4-(dimethylamino)azobenzene of 7a showed
down-field shifts relative to those of 10. The order of larger
down-field shift was H-2,6 (¦¤ 0.84) > Me2N-4¤¤ (0.41) > H-
3,5 (0.14). Although the C-3,5 carbon signal for the 4-
(dimethylamino)azobenzene of 7a was up-field compared to
that of 10, the other observed carbon signals for 7a showed
down-field shifts in comparison with those of 10. The order of
larger down-field shift was C-2,6 (¦¤ 8.3) > Me2N-4¤¤ (4.6).
From the variable-temperature 1H NMR spectral data of 7a and
1
comparative studies of the H and 13C NMR chemical shifts of
7a with those of 5, 6, and 10 (Tables 2-5), the resonance
structures of 7a can be inferred as illustrated in Chart 2.
X-ray Crystal Structure of 7b. Although it was very
difficult to obtain a single crystal of 5-7a suitable for X-ray
crystallographic analysis, recrystallization of the monocarbe-
nium tetrafluoroborate 7b from a mixed solvent of acetonitrile
and diethyl ether provided a single crystal suitable for that
purpose. Therefore, the crystal structure of 7b could be
determined by means of X-ray diffraction at ¹75 °C. The
ORTEP drawing 7b possessing an equiv of HBF4 is shown in
Figure 3. Comparing the selected bond lengths of 7b to those
of structurally related known compounds [4-(dimethylamino)-
phenyl](3-guaiazulenyl)methylium tetrafluoroborate5 (11), 4-
(1-azulenylazo)-1-methylpyridium hexafluorophosphate35 (12),
and azobenzene36-38 (13) (Chart 3) are shown in Tables 8 and
9. From dihedral angles between least-squares planes, the plane
of the 4-[4-(dimethylamino)phenylazo]phenyl group twisted by
11.6° from that of the 3-guaiazulenylmethylium ion structure,
because of the influence of steric hindrance and repulsion
between the hydrogen atoms of the C-6 and C-2¤ positions, the
angle of which was smaller than that of 11 (20.7°). Similarly
The NaBH4-reduction of 7a under the same reaction
conditions as for 5 gave as high as 72% yield of 10 (Scheme 2
and Table 6). Compound 10 was obtained as green blocks. A
comparative study of the UV-vis spectrum of 10 with that of 1
(Figure 1b) showed characteristic UV-vis absorption bands of
1 (Table 7). The IR spectrum showed a specific band from
N=N at ¯max 1597 cm¹1, which was lower than that of 7a
(¦¯max 23 cm¹1), however coincided with those of 8 and 9
(Table 7). The molecular formula C30H33N3 was determined by
exact FAB-MS spectrum. The 1H NMR spectrum showed
signals from a 3-guaiazulenylmethyl-substituted 4-(dimethyl-

888
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
Table 9. The Selected Bond Lengths (¡) for the
4-(Dimethylamino)phenyl Groups of 7b and 11 and
the Azobenzenes of 7b and 13
Atom
7b
11
13
C¡-C1
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
C4-N1
N1-N2
N2-C1¤¤
C1¤¤-C2¤¤
C2¤¤-C3¤¤
C3¤¤-C4¤¤
C4¤¤-C5¤¤
C5¤¤-C6¤¤
C6¤¤-C1¤¤
C4¤¤-N3
1.439(6)
1.405(6)
1.377(6)
1.393(6)
1.403(6)
1.375(6)
1.405(6)
1.400(5)
1.299(5)
1.324(6)
1.423(6)
1.335(6)
1.446(6)
1.431(7)
1.349(7)
1.438(6)
1.341(6)
1.414(7)
®
®
®
1.382(3)
1.384(3)
1.387(2)
1.389(2)
1.384(3)
1.391(2)
1.428(2)
1.247(2)
®
®
®
®
®
®
®
®
1.405(7)a)
1.372(8)b)
1.390(8)c)
1.403(8)d)
1.360(7)e)
1.404(8)f)
1.359(7)g)
®
®
Figure 3. The ORTEP drawing of 7b with an equiv of
HBF4 (see BF4 drawn right) (30% probability thermal
ellipsoids). Measurement temperature: ¹75 °C.
®
®
®
®
Table 8. The Selected C-C Bond Lengths (¡) for the
3-Guaiazulenylmethylium Ions of 7b and 11 and the
3-Azulenyl Group of 12
®
a) C1-C2. b) C2-C3. c) C3-C4. d) C4-C5. e) C5-C6. f) C6-
C1. g) C4-N.
Atom
7b
11
12a)
C1¤-C2¤
C2¤-C3¤
C3¤-C3a¤
C3a¤-C4¤
C4¤-C5¤
C5¤-C6¤
C6¤-C7¤
C7¤-C8¤
C8¤-C8a¤
C8a¤-C1¤
C3a¤-C8a¤
C3¤-C¡
1.348(6)
1.442(6)
1.503(6)
1.390(6)
1.407(6)
1.379(7)
1.412(6)
1.382(6)
1.399(6)
1.446(6)
1.445(6)
1.360(6)
1.351(7)
1.448(7)
1.457(7)
1.385(7)
1.406(7)
1.373(8)
1.386(8)
1.379(8)
1.392(7)
1.416(7)
1.465(7)
1.396(7)
1.363(11)
1.424(11)
1.425(11)
1.384(12)
1.392(12)
1.384(12)
1.382(13)
1.391(13)
1.392(11)
1.423(12)
1.455(12)
®
however, the bond length was longer than that of 13 (1.247 ¡).
The C4¤¤-N3 bond length (1.341 ¡) was shorter than the C4-N
bond length of 11 (1.359 ¡). The N2-C1¤¤ (1.324 ¡), C2¤¤-C3¤¤
(1.335 ¡), C5¤¤-C6¤¤ (1.349 ¡), and C4¤¤-N3 (1.341 ¡) bond
lengths were shorter than the C¡-C1 (1.439 ¡), C2-C3
(1.377 ¡), C5-C6 (1.375 ¡), and C4-N1 (1.400 ¡) bond
lengths. The C4-N1 (1.400 ¡) bond length was shorter than
that of 13 (1.428 ¡). In conclusion, it can be inferred that the
C-C and C-N bond lengths based on the X-ray crystallographic
analysis of 7b, compared with those of 11-13, lead to a crystal
structure with similar resonance structures to those of 7a
illustrated in Chart 2. From the viewpoint of structural organic
chemistry, it is noteworthy that the crystal structure determi-
nation of 16, with an equiv of HPF6, has not yet been achieved;
however, the crystal structure of 7b, with an equiv of HBF4, has
been determined.
a) For a comparative purpose, the numbering scheme of the
1-azulenyl group of 12 was changed to that of the 3-azulenyl
group (see Chart 3).
to the case of 11, the 3-guaiazulenylmethylium ion clearly
underwent bond alternation between single and double bonds,
suggesting a similar resonance structure to 7a¤ (Chart 2). The
4-[4-(dimethylamino)phenylazo]phenyl group also clearly un-
derwent bond alternation between the single and double bonds,
suggesting a similar resonance structure to 7a¤¤ (Chart 2). The
average C-C bond length of the seven-membered ring of the
3-guaiazulenyl group (1.402 ¡) coincided with those of 11
(1.399 ¡) and 12 (1.398 ¡), the bond length of which was
shorter than that of the parent azulene (1.412 ¡),39 and was
longer than that of the azulenium ion structure (1.38 ¡).40,41
The C-C bond length of the five-membered ring of the 3-
guaiazulenyl group appreciably varied between 1.348 and
1.503 ¡; in particular, the C1¤-C2¤ bond length (1.348 ¡) was
characteristically shorter than the average C-C bond length for
the five-membered ring (1.436 ¡), which coincided with the C-
C bond lengths observed for the five-membered rings of 11 and
12. The C3¤-C¡ bond length (1.360 ¡) was also characteristi-
cally shorter than the C1-C¡ bond length (1.439 ¡). The N=N
bond length (1.299 ¡) coincided with that of 12 (1.295 ¡);
Electrochemical Behavior of 5-7a.
We have been
interested further in comparison of the electrochemical proper-
ties of the monocarbenium ion compounds 5-7a. The electro-
chemical behavior of 5-7a was, therefore, measured by means
of CV and 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 5 and 6 underwent one-electron
reduction at a potential of ¹0.15 V (Ep) by DPV [¹0.21 V (Epc,
irreversible) by CV] for 5 and that of ¹0.06 V (Ep) by DPV
[¹0.11 V (Epc, irreversible) by CV] for 6, presumably generat-
ing the corresponding electrochemically unstable radical
species (HC●-1), i.e., (3-guaiazulenyl)[4-(4-hydroxyphenyl-
azo)phenyl]methyl and (3-guaiazulenyl)[4-(4-methoxyphenyl-
azo)phenyl]methyl radical species. Therefore, 6 is slightly
more susceptible to reduction than 5. Interestingly, three
reduction potentials observed by DPV were positioned at the Ep
values of 0.02, ¹0.13, and ¹0.40 V for 7a and the correspond-
ing three irreversible reduction potentials determined by CV
were located at the values of ¹0.04, ¹0.18, and ¹0.45 V (Epc

S. Takekuma et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
889
each) as shown in Figure 4. Thus, an apparent difference
between the electrochemical behavior of 7a with rotational
stereoisomers of 7a¤¤ and that of 5 and 6 without rotational
stereoisomers of 5¤¤ and 6¤¤ was observed (Chart 2). The
reduction potentials of 7a coincided with those of (2E)-1-
azulenyl-3-phenyl-2-propenylium hexafluorophosphate (14)
[¹0.01 V (Ep) by DPV and ¹0.08 V (Epc) by CV]15 (Chart 4),
5, (2E)-1-(3-guaiazulenyl)-3-phenyl-2-propenylium hexafluo-
rophosphate (15) [¹0.16 V (Ep) by DPV and ¹0.23 V (Epc) by
CV]15 (Chart 4), 17 [¹0.19 V (Ep) by DPV and ¹0.25 V (Epc)
by CV],20 11 [¹0.39 V (Ep) by DPV and ¹0.47 V (Epc) by
CV],5 and 18 [¹0.43 V (Ep) by DPV and ¹0.49 V (Epc) by
for 14, the HC-¡ and HC-1 carbon atoms for 5, 15, and 17,
and the nitrogen atoms of the dimethylamino and amino groups
for 11 and 18, forming a 1-azulenylium ion structure, a 3-
guaiazulenylmethylium ion structure, and a p-benzoquinodi-
methane monoiminium ion structure, respectively, and further
that the three reduction potentials of 7a are observed owing to
the formation of the resonance structures 7a¤ and 7a¤¤ with
rotational stereoisomers (Chart 2). A plausible electron-transfer
mechanism of 7a derived from the CV and DPV data are
noteworthy, and is currently under intensive investigation.
Conclusion
1
CV]20 (Table 10). From the H and 13C NMR spectral param-
We have reported the following five interesting points in this
paper: namely, (i) the reaction of guaiazulene (1) with 4¤-
hydroxyazobenzene-4-carbaldehyde (2) in methanol in the
presence of hexafluorophosphoric acid at 25 °C for 2 h gave
as high as 94% yield of the target monocarbenium ion
compound (3-guaiazulenyl)[4-(4-hydroxyphenylazo)phenyl]-
methylium hexafluorophosphate (5). Similarly, the reactions
of 1 with 4¤-methoxyazobenzene-4-carbaldehyde (3) and
4¤-(dimethylamino)azobenzene-4-carbaldehyde (4) under the
same reaction conditions as for 2 afforded the correspond-
ing monocarbenium ion compounds (3-guaiazulenyl)[4-
(4-methoxyphenylazo)phenyl]methylium hexafluorophosphate
(6) and {4-[4-(dimethylamino)phenylazo]phenyl}(3-guaiazul-
enyl)methylium hexafluorophosphate (7a) in 97 and 95%
eters, it can be inferred that the positive charges of 14, 5, 15,
17, 11, and 18 are mainly localized at the seven-membered ring
R3
H
R2
PF6
R1
R3
8'
H
3
6"
3"
1
1'
5'
5"
4"
R2
1"
2"
2
3'
PF6
R1
R3
H
Figure 4. The cyclic (a) and differential pulse (b) voltam-
mograms of 7a (3.0 mg, 5.2 ¯mol) 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
R2
R1
PF6
¹1
electrodes; scan rates 100 mV s at 25 °C under argon.
14: R1 = R2 = R3 = H
15: R1 = R3 = CH3, R2 = CH(CH3)2
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 electro-
chemical measurement conditions as for 7a.
Chart 4.
Table 10. The Reduction Potential(s) (Epc/V) of 5-7a, 11, 14, 15, 17, and 18 by Means of CV under the Same
Electrochemical Measurement Conditions as for 7a
Compound
5
6
7a
11
14
15
17
18
E1red/V
E2red/V
E3red/V
¹0.21
¹0.11
¹0.04
¹0.18
¹0.45
¹0.47
¹0.08
¹0.23
¹0.25
¹0.49

890
Bull. Chem. Soc. Jpn. Vol. 82, No. 7 (2009)
New Delocalized ³-Electron Systems
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yields; (ii) the reduction of 5 with NaBH4 in a mixed solvent of
ethanol and acetonitrile at 25 °C for 30 min gave as high as
85% yield of 4-(3-guaiazulenylmethyl)-4¤-hydroxyazobenzene
(8), in which a hydride-ion attached to the HC+-¡ carbon atom
of 5, selectively. Similarly, the NaBH4-reductions of 6 and 7a
under the same reaction conditions as for 5 afforded the
corresponding hydride-reduction products 4-(3-guaiazulenyl-
methyl)-4¤-methoxyazobenzene (9) and 4¤-dimethylamino-4-(3-
guaiazulenylmethyl)azobenzene (10) in 74 and 72% yields;
(iii) along with comparative studies of the H and 13C NMR
1
chemical shifts of 5-7a with those of 8-10, apparently
indicating the difference between the delocalized ³-electron
system of 7a and those of 5 and 6 owing to the influence of
the substituted HO-, CH3O-, or (CH3)2N- group at the C4¤¤
position, the variable-temperature 1H NMR studies of 7a in
acetonitrile-d3 at 70, 40, 25, 0, and ¹40 °C, supporting the
formation of rotational stereoisomers for 7a¤¤, were reported;
(iv) although X-ray crystallographic analysis of 5-7a has not
yet been achieved because of difficulty in obtaining a single
crystal suitable for that purpose, the crystal structure of {4-[4-
(dimethylamino)phenylazo]phenyl}(3-guaiazulenyl)methylium
tetrafluoroborate (7b) with an equiv of HBF4 was determined
by X-ray diffraction at ¹75 °C, supporting the formation of 7b,
with similar resonance structures to those of 7a, in the single
crystal; and (v) an apparent difference between the electro-
chemical behavior of 7a and that of 5 and 6 was observed.
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
28 O. Srinivas, N. Mitra, A. Surolia, N. Jayaraman, J. Am.
Chem. Soc. 2002, 124, 2124.
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