Synthesis, molecular structure, and properties of 2-(2-hydroxyphenyl)-1- azaazulene

Article abstract of DOI:10.1002/ejoc.201101831

We herein report the synthesis of 2-(2-methoxyphenyl)-1-azaazulene (8) by a Suzuki-Miyaura cross-coupling reaction between 2-chloro-1-azaazulene and 2-methoxyphenylboronic acid and also by the condensation of tropone and the ylide derived from 1-(o-methox

Full text of DOI:10.1002/ejoc.201101831

FULL PAPER
DOI: 10.1002/ejoc.201101831
Synthesis, Molecular Structure, and Properties of 2-(2-Hydroxyphenyl)-1-
azaazulene
Mitsunori Oda,*[a] Ayumi Sugiyama,[a] Rie Takeuchi,[a] Yurie Fujiwara,[a] Ryuta Miyatake,[b]
Takako Abe,[b] and Shigeyasu Kuroda[b]
Keywords: Nitrogen heterocycles / Cross-coupling / Basicity / Acidity
We herein report the synthesis of 2-(2-methoxyphenyl)-1-
azaazulene (8) by a Suzuki–Miyaura cross-coupling reaction
between 2-chloro-1-azaazulene and 2-methoxyphenylbo-
tallographic analysis of 6 provided evidence for an intramo-
lecular hydrogen bond between the phenolic hydrogen atom
and the nitrogen atom of the azaazulenyl ring and also re-
ronic acid and also by the condensation of tropone and the vealed its coplanar ring system. We also report the acidity,
ylide derived from 1-(o-methoxyphenacyl)pyridinium iodide.
Demethylation of 8 afforded the title compound 6. X-ray crys-
basicity, and absorption and emission behavior of 6.
vestigate the efficiency of the combination of an imino ni-
trogen atom and a phenolic oxygen atom in a 1-azaazulenyl
ligand, we became interested in the title compound, 2-(2-
hydroxyphenyl)-1-azaazulene (6), the structure of which is
related to 8-hydroxyquinoline (7),[12] one of the most widely
used chelating reagents. In this paper we describe the syn-
thesis, molecular structure, and physical properties (for ex-
ample, absorption and emission behavior, acidity and basic-
ity) of 1-azaazulene 6 (Scheme 1).
Introduction
Azaazulenes, heteroatomic analogues of azulene, have
long been attractive to organic chemists for their basic phys-
ical and chemical properties and potential biological ac-
tivity.[1] Among the various azaazulenes, 1-azaazulenes have
been most studied because of their stability and facile ac-
cessibility. In addition to the classical synthetic methods in-
volving condensation reactions of troponoid substrates with
carbon fragments corresponding to the five-membered ring
of 1-azaazulene explored by Nozoe[2] and Nitta[3] and their
co-workers, Narasaka and co-workers have recently devel-
oped a novel synthesis of 1-azaazulenes by a palladium-
catalyzed amino-Heck reaction,[4] and Deprés and co-
workers have reported that cycloheptatriene/dichloroketene
adducts can be converted into 1-azaazulenes.[5] In addition
to the development of these synthetic methods, recent tran-
sition-metal-catalyzed cross-coupling and amination reac-
tions have facilitated the derivatization of halogen-substi-
tuted 1-azaazulenes to afford various functionalized 1-aza-
azulenes.[6,7] Meanwhile, interest in substituted 1-azaazul-
enes as ligands has recently increased. In 1998, Sugihara et
al. reported for the first time on the use of 8-(methylamino)-
3-phenyl-1-azaazulene (1)[8] as a bidentate ligand. Later, we
and Abe et al. reported the interactions of other 1-azaazul-
enes, 2,[9] 3,[10] 4,[11] and 5,[11] with metal ions. All these
ligands have only amino and imino nitrogen atoms. To in-
[a] Department of Chemistry, Faculty of Science, Shinshu
University,
Scheme 1. Title molecule 6 and structurally related compounds.
Asahi 3-1-1, Matsumoto, Nagano 390-8621, Japan
Fax: +81-263-37-3343
E-mail: mituoda@shinshu-u.ac.jp
[b] Department of Applied Chemistry, Graduate School of Science
and Engineering, University of Toyama,
3190 Gofuku, Toyama 930-8555, Japan
Results and Discussion
Compound 6 was synthesized by two methods via its
methoxy derivative 8, as shown in Scheme 2. Compound 8
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101831.
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M. Oda et al.
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was synthesized either by condensation[13] of tropone (9)
Furthermore, the detailed structure of 6 was determined
with pyridinium salt 10 in the presence of ammonium acet- by X-ray crystallographic analysis. Figure 2 shows ORTEP
ate or by Suzuki–Miyaura cross-coupling[14] between 2- drawings of 6, revealing its coplanar structure. Bond lengths
chloro-1-azaazulene (11)[2] and (2-methoxyphenyl)boronic show the convergence of C–C bonds in the seven-membered
acid (12). Note that [(A-taPhos)2PdCl2][15] (Scheme 3) and phenolic rings, which is consistent with the structure
served as an efficient palladium catalyst in the latter cou- determined on the basis of the NMR coupling constants.
pling reaction with 11, because it is known that 11 is notori- The distance between the oxygen atom and the nitrogen
ously less active in the Suzuki–Miyaura coupling reactions atom is 2.583 Å, fingerprinting the intramolecular hydrogen
than 2-bromo- and 2-iodo-1-azaazulenes.[10] The title com- bond.
pound 6 was obtained in 78% yield by demethylation of 8
in hydrobromic acid at reflux. In the 1H NMR spectrum of
6 in CDCl3, the hydrogen atom of the hydroxy group ap-
pears as a broad signal at δ = 14.22 ppm, which suggests
that it forms a hydrogen bond with the nitrogen atom of the
azaazulene ring. Other resonances appear in the aromatic
region, and their vicinal coupling constants suggest that
there is no C–C bond alternation in either the phenol ring
or the seven-membered ring of the azaazulenyl moiety. The
1
signals in the H and 13C NMR spectra were assigned on
the basis of 2D NMR spectra and NOE experiments, as
shown in Figure 1.
Figure 2. X-ray structure of 6. Bond lengths [Å]: N1–C2 1.365(3),
C2–C3 1.401(3), C3–C3a 1.390(3), C3a–C4 1.388(3), C4–C5
1.378(3), C5–C6 1.394(4), C6–C7 1.382(4), C7–C8 1.386(3), C8–
C8a 1.388(4), C3a–C8a 1.470(3), C8a–N1 1.353(3), C2–C1Ј
1.456(3), C1Ј–C2Ј 1.403(3), C2Ј–C3Ј 1.376(3), C3Ј–C4Ј 1.385(4),
C4Ј–C5Ј 1.367(4), C5Ј–C6Ј 1.392(3), C6Ј–C1Ј 1.408(3), C2Ј–O7Ј
1.353(3).
The UV/Vis spectra of 3, 6, and 2-phenyl-1-azaazulene
(13; Scheme 4)[13] in acetonitrile are shown in Figure 3.
Four absorption bands were observed for these 1-azaazul-
enes, and the long-wavelength absorption of 6 displays a
slight blueshift with a substantial hyperchromic effect com-
pared with those of 3 and 13. This blueshift and hyperchro-
mic effect can be attributed to the intramolecular proton-
ation resulting from the presence of the hydrogen bond. The
long-wavelength absorption maximum of 6 appears to de-
pend on the polarity of the solvent. The maximum shows a
Scheme 2. Synthesis of 6.
redshift in more polar solvents (Figure 4; left). The differ-
ence between the dependence on protic and aprotic solvents
is not clearly observed for 6. This is in contrast to the case
of 3. Although a similar redshift is observed for 3, the shift
is different in the two series of protic and aprotic solvents
(Figure 4; right). The phenomenon seen for 6 must be due
to the restricted conformational change around the C–C
Scheme 3. Structure of [(A-taPhos)2PdCl2].
single bond between the 1-azaazulenyl and phenol rings,
1
Figure 1. H and 13C NMR signal assignment (left) and NOE experiment (right) for 6.
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2-(2-Hydroxyphenyl)-1-azaazulene
that is, 6 exists mainly as the conformer with the intramo-
lecular hydrogen bond. Therefore, the conformation of 6 is
limited and less affected by solvent polarity. On the other
hand, 3 exists as conformer A in aprotic solvents, which
is the most stable structure based on DFT calculations,[16]
whereas 3 exists in protic solvents as conformer B forming
two intermolecular hydrogen bonds with the solvent
(Scheme 5). This conformational change around the C–C
single bond between the 1-azaazulenyl and pyridyl rings in
3 leads to the difference in the two series of protic and apro-
tic solvents.
Figure 4. Dependence of the long-wavelength absorption maxima
of 3 (right) and 6 (left) on the dielectric constants of solvents. Sym-
bols for solvents are as follows: Ethylene glycol (᭹), methanol (᭺),
ethanol (᭡), 1-butanol (Δ), acetonitrile (᭿), acetone (ᮀ), ethyl acet-
ate (♦), and dioxane (᭛).
Scheme 4. Structures of 13 and 14.
Scheme 5. Two major conformers, A and B, for 3 and the doubly
hydrogen-bonded structure of conformer B.
of 6 were determined by the titration method.[17] Because
the solubility of 6 in water is very poor, measurements were
carried out in 50% aqueous EtOH. The values, including
those of structurally related compounds such as phenol,[18]
pyridine,[18,19] 3, 7,[20,21] 13, and 2-(2-hydroxyphenyl)pyr-
Figure 3. UV/Vis spectra of 3, 6, and 13 in acetonitrile in the region
of 200–600 nm (top) and the expanded region of 400–600 nm (bot-
tom).
The UV/Vis spectrum of 6 varies with pH, as shown in
Figure 5. The long-wavelength maximum shows a clear
blueshift in acidic solution and a redshift in basic solution.
Based on these changes, the acidity (pKa) and basicity (pKb) Figure 5. UV/Vis spectra of 6 in acidic, neutral, and basic solutions.
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M. Oda et al.
FULL PAPER
idine (14),[22] are shown in Table 1. Although different sol- enes 3 and 13. It is clear that the 2-hydroxyphenyl group in
vent systems were used to measure the acidity and basicity 6 and 14 masks the basicity of imines rather than increasing
of these compounds, the following conclusions were drawn it by the electron-donating resonance effect.
from these data. Compound 6 is a weaker acid than phenol,
Spectral changes were also observed for 6 in the presence
7, and 14. Because the nitrogen atom at the 1-position of of 100 equiv. of metal ions (Figure 6). Among the metal
1-azaazulene is more electronegative than the nitrogen atom ions examined, Al3+, Sc3+, Pb2+, and Hg2+ led to a pro-
of pyridine, the phenolate anion of 6 must be more destabi- nounced blueshift and hyperchromic effect. Compound 6
lized. Compound 14 is a weaker base than pyridine, and it exhibits emissions at 444 and 561 nm upon excitation at 390
can be seen that 6 shows a similar tendency to 1-azaazul- and 495 nm, respectively (Figure 7). The emission quantum
yield of 6 was determined to be Φ = 1.4ϫ10–5 by compari-
son with that of anthracene.[23] However, its emission quan-
Table 1. Acidity and basicity of 6 and related compounds.
tum yield was found to be markedly higher in the presence
Compound
Acidity
pKa
Basicity
of metal ions (Table 2). The quantum yields of 6 in the pres-
ence of Pb2+, Al3+, and Sc3+ ions are 103 times higher than
the quantum yield determined in the absence of ions, which
suggests the potential for sensing these metal ions by 6.
pKb1
pKb2
3
6
7.98[a]
11.58[a]
12.7[b]
11.3[b]
9.99[c]
8.7[b]
Phenol
7
11.30[d]
11.54[e]
9.67[d]
10.03[e]
7.18[a]
9.9[f]
13
14
Pyridine
10.2[f]
8.99[c]
8.9[g]
8.7[h]
[a] Measured in 50% aqueous CH3CN; taken from ref.[10] [b] Mea-
sured in 50% aqueous EtOH; this work. [c] Measured in water;
taken from ref.[17] [d] Measured in 75% aqueous EtOH; taken from
ref.[20] [e] Measured in 50% aqueous dioxane; taken from ref.[21] [f]
Measured in water; taken from ref.[22] [g] Measured in water; taken
from ref.[18] [h] Measured in 80% aqueous EtOH; taken from ref.[19]
Figure 7. UV/Vis and emission spectra of 6.
Table 2. Photophysical data of 6 in the presence of metal ions in
acetonitrile.
abs
em
Metal cation[a]
λmax
[nm]
εmax
λmax
Φ
[m–1 cm–1]
[nm]
Na+
Ag+
390
390
462
477
459
461
456
456
468
457
456
11501
11339
9034
5291
13132
7995
14853
14881
7406
14512
14945
445
444
526
525
527
526
526
526
528
528
526
8.0ϫ10–4
4.4ϫ10–4
5.0ϫ10–3
5.7ϫ10–3
9.7ϫ10–3
1.4ϫ10–3
1.2ϫ10–3
1.2ϫ10–2
1.3ϫ10–3
1.3ϫ10–2
1.1ϫ10–2
Mg2+
Ca2+
Zn2+
Ni2+
Hg2+
Pb2+
Cd2+
Sc3+
Al3+
[a] Perchlorate salts were used except for the scandium ion for
which the triflate salt was used.
Conclusions
It has been demonstrated that the novel 1-azaazulene de-
rivative 6 with a phenol functional group at the 2-position
can be synthesized by two simple methods via its methoxy
derivative 8. The structure of 6 was confirmed by spectro-
scopic and X-ray crystallographic analyses, the presence of
its aromatic and coplanar nature as well as an intramolecu-
lar hydrogen bond being supported by a relatively intense
Figure 6. UV/Vis spectra of 6 in the presence of metal ions in aceto-
nitrile (top) and the expanded spectra of the area in the rectangle
(bottom).
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2-(2-Hydroxyphenyl)-1-azaazulene
1452 (s), 1433 (s), 1404 (s), 1282 (m), 1254 (s), 1240 (w), 1159 (w),
1065 (m), 1020 (s), 820 (m), 760 (s), 752 (s), 742 (s) cm–1. UV
(acetonitrile): λmax [logε] = 211 [4.35], 234 [3.95] 283 [4.43], 369
[3.96], 383 [3.99], 491 [3.27], 507 [3.27], 550 (sh) [2.92 m–1 cm–1] nm.
MS (70 eV): m/z (%) = 235 (62) [M]+, 234 (100), 206 (54), 205 (58),
204 (53), 190 (15), 178 (18), 130 (22), 102 (26), 89 (15), 63 (17).
C16H13NO·0.2H2O (238.9): calcd. C 80.45, H 5.65, N 5.86; found
C 80.37, H 5.52, N 5.97.
absorption in the visible light range and by the X-ray dif-
fraction data. The long-wavelength absorption band of 6
shifts in the presence of several metal ions and, its emission
upon excitation was clearly enhanced.
Experimental Section
General: Melting points were measured with a Yanaco M-3 instru-
ment. IR spectra were recorded with a JEOL Diamond-20 spec-
trometer. UV/Vis spectra were recorded with a Shimadzu UV-2550
spectrometer. Emission spectra were recorded with a Shimadzu RT-
5300PC spectrometer. 1H and 13C NMR spectra were recorded
with a JEOL λ400 spectrometer by using tetramethylsilane as in-
ternal standard. Mass spectra were recorded with a JMS-700 mass
spectrometer. Column chromatography was performed with Merck
neutral alumina-90. 2-Methoxyacetophenone was purchased from
Tokyo Chemical Industry, Inc. (2-Methoxyphenyl)boronic acid and
[(A-taPhos)2PdCl2] were purchased from Aldrich Chem. Co. Tro-
pone was prepared by selenium oxide oxidation of cycloheptatri-
ene.[24] Compound 3 was prepared from tropone according to our
previously reported method.[10] Compound 11 was prepared from
2-aminotropone according to the method of Nozoe and co-
workers.[2] Compound 13 was prepared from tropone according to
the method of Sugimura et al.[13]
2-(2-Methoxyphenyl)-1-azaazulene (8) from 2-Chloro-1-azaazulene:
A reaction vessel with a mixture of 11 (100 mg, 0.61 mmol), 12
(111 mg, 0.73 mmol), [(A-taPhos)2PdCl2] (13 mg, 18.4 μmol), and
Cs2CO3 (397 mg, 1.22 mmol) in DMF/H2O (10:1; 3.5 mL) was
evacuated and flushed with argon (5ϫ). This mixture was heated
in an oil bath at 140 °C under argon by using a balloon for 12 h.
The resulting reaction mixture was poured into a 10% EDTA/
NaHCO3 solution (30 mL) and extracted three times with toluene
(20 mL). The combined organic layers were washed with a 1 n
NaOH aqueous solution and brine, and dried with MgSO4. The
solvent was removed, and the residue was purified by chromatog-
raphy to give 97 mg of 8 (67% yield).
2-(2-Hydroxyphenyl)-1-azaazulene (6): A solution of 8 (470 mg,
2.00 mmol) in 48% HBr (40 mL) was heated at reflux for 12 h. The
resulting reaction mixture was carefully poured into an aqueous
solution of NaHCO3 and extracted with dichloromethane (5ϫ
20 mL). The combined organic layers were washed with brine. Af-
ter drying with Na2SO4, the solvent was removed under reduced
pressure, and the residue was purified by chromatography (Al2O3;
20% ethyl acetate/hexane) to give 344 mg (78% yield) of 6 as ver-
1-(2-Methoxyphenacyl)pyridinium Iodide (10): A solution of 2-
methoxyacetophenone (2.01 g, 13.4 mmol) and iodine (3.39 g,
13.3 mmol) in pyridine (50 mL) was heated at reflux under nitrogen
for 22 h. Solids formed were removed by filtration, and the filtrate
was concentrated. The residue was crystallized from dichlorometh-
ane to give 3.71 g (80% yield) of 10 as buff microcrystals. M.p.
159–160 °C. 1H NMR (CDCl3): δ = 4.07 (s, 3 H), 6.57 (s, 2 H),
7.02 (t, J = 7.9 Hz, 1 H), 7.05 (d, J = 7.9 Hz, 1 H), 7.60 (t, J =
7.9 Hz, 1 H), 7.88 (d, J = 7.9 Hz, 1 H), 8.10 (m, 2 H), 8.61 (t, J =
7.8 Hz, 1 H), 9.15 (d, J = 5.8 Hz, 2 H) ppm. 13C NMR (CDCl3):
δ = 56.9, 70.8, 112.4, 121.0, 122.6, 127.7, 131.1, 136.7, 146.0, 146.2,
1
milion prisms. M.p. 160–162 °C. H NMR (CDCl3): δ = 6.96 (td,
J = 7.8, 1.2 Hz, 1 H), 7.10 (dd, J = 8.4, 1.2 Hz, 1 H), 7.35 (ddd, J
= 8.4, 7.8, 1.2 Hz, 1 H), 7.63 (t, J = 9.4 Hz, 1 H), 7.70 (s, 1 H),
7.73 (t, J = 9.4 Hz, 1 H), 7.78 (t, J = 9.4 Hz, 1 H), 7.96 (dd, J =
7.8, 1.2 Hz, 1 H), 8.49 (d, J = 9.4 Hz, 1 H), 8.55 (d, J = 9.4 Hz, 1
H), 14.22 (br. s, 1 H) ppm. IR (KBr): ν
= 3446 (w), 1616 (m),
˜
max
1589 (m), 1571 (w), 1541 (w), 1508 (m), 1473 (s), 1441 (m), 1412
(s), 1304 (m), 1290 (w), 1275 (w), 1261 (s), 1216 (m), 1117 (w), 1041
(w), 1034 (w), 939 (w), 885 (w), 874 (w), 858 (w), 831 (s), 800 (s),
754 (s), 748 (s), 739 (s), 725 (m) cm–1. UV (acetonitrile): λmax [logε]
= 211 [4.42], 237 [4.10], 281 [4.55], 311 [4.23], 375 [3.90], 390 [3.97],
479 [3.60], 495 [3.61], 532 (sh) [3.29 m–1 cm–1] nm. MS (70 eV): m/z
(%) = 222 (17) [M + 1]+, 221 (100) [M]+, 193 (41), 192 (19), 191
(14), 167 (11), 97 (11), 84 (13). C15H11NO·0.2H2O (224.9): calcd.
C 80.12, H 5.11, N 6.23; found C 80.09, H 4.96, N 6.42. The 6·HBr
salt was obtained in quantitative yield as orange microcrystals by
160.5, 189.0 ppm. IR (KBr): ν
= 3465 (m), 3388 (m), 3093 (w),
˜
max
3049 (w), 3030 (w), 1672 (s), 1635 (m), 1599 (s), 1489 (s), 1483 (s),
1468 (s), 1454 (w), 1439 (m), 1344 (m), 1286 (s), 1242 (s), 1209 (s),
1194 (m), 1178 (m), 1157 (s), 1117 (w), 1020 (s), 991 (m), 783 (w),
775 (s), 752 (m), 678 (m) cm–1. MS (70 eV): m/z (%) = 228 (1)
[C14H14NO2]+, 227 (2), 135 (100), 127 (10), 105 (10), 91 (30), 77
(21), 71 (38), 63 (15), 51 (26). C14H14INO2 (355.2): calcd. C 47.34,
H 3.97, N 3.94; found C 47.29, H 4.21, N 3.95.
2-(2-Methoxyphenyl)-1-azaazulene (8) from Tropone: A solution of
10 (3.58 g, 10.1 mmol), tropone (530 mg, 5.00 mmol), and ammo-
nium acetate (5.40 g, 70.0 mmol) in acetic acid (20 mL) was heated
at reflux under nitrogen for 12 h. The solvent was evaporated, and
the residue was poured into an aqueous solution of NaHCO3 and
extracted with ethyl acetate (5ϫ 50 mL). The combined organic
layers were washed with a thiosulfate solution and brine. After dry-
ing with Na2SO4, the solvent was removed under reduced pressure,
and the residue was purified by chromatography (Al2O3; 20% ethyl
acetate/hexane) to give 681 mg (58% yield) of 8 as red prisms. M.p.
135–136 °C. 1H NMR (CDCl3): δ = 4.01 (s, 3 H), 7.05 (dd, J = 8.4,
1.0 Hz, 1 H), 7.15 (t, J = 7.4, 1.0 Hz, 1 H), 7.42 (ddd, J = 8.4, 7.4,
1.6 Hz, 1 H), 7.56 (t, J = 10.1 Hz, 1 H), 7.70 (t, J = 10.1 Hz, 1 H),
7.74 (t, J = 10.1 Hz, 1 H), 8.07 (s, 1 H), 8.50 (d, J = 10.1 Hz, 1 H),
8.63 (dd, J = 7.4, 1.6 Hz, 1 H), 8.65 (d, J = 10.1 Hz, 1 H) ppm.
13C NMR (CDCl3): δ = 55.5, 111.4, 115.6, 121.1, 124.1, 128.4,
1
reaction of 6 with 48% HBr in ethanol. M.p. Ͼ230 °C (dec.). H
NMR (CDCl3): δ = 7.12 (t, J = 7.7 Hz, 1 H), 7.22 (d, J = 7.7 Hz,
1 H), 7.51 (t, J = 7.7 Hz, 1 H), 8.15 (d, J = 7.7 Hz, 1 H), 8.28 (s,
1 H), 8.39 (t, J = 9.8 Hz, 1 H), 8.47 (t, J = 9.8 Hz, 1 H), 8.56 (t, J
= 9.8 Hz, 1 H), 9.21 (d, J = 9.8 Hz, 1 H), 9.23 (d, J = 9.8 Hz, 1
H), 11.35 (s, 1 H), 14.43 (s, 1 H) ppm. 13C NMR (CDCl3): δ =
112.6, 114.4, 117.2, 120.2, 130.0, 133.3, 134.0, 136.6, 136.8, 141.7,
143.9, 144.8, 147.3, 157.4, 166.0 ppm. IR (KBr): ν
= 3437 (s),
˜
max
3078 (s), 1608 (s), 1589 (w), 1566 (m), 1537 (m), 1491 (m), 1460 (s),
1452 (s), 1439 (s), 1398 (w), 1379 (w), 1333 (m), 1279 (w), 1252 (w),
1238 (m), 1159 (m), 829 (m), 825 (m), 760 (s), 741 (m), 694 (w)
cm–1. UV (acetonitrile): λmax [logε] = 211 [4.49], 238 [4.18], 282
[3.10], 375 (sh) [4.00], 390 [4.06], 473 [3.66], 496 [3.68], 532 (sh)
[3.36 m–1 cm–1] nm. C15H12BrNO·1/3H2O (308.2): calcd. C 58.47, H
4.41, N 4.55; found C 58.42, H 4.22, N 4.45.
129.3, 130.9, 131.3, 134.6, 134.9, 136.2, 147.8, 157.4, 158.6, Determination of the Acidity and Basicity of 6: The acidity and
164.2 ppm. IR (KBr): ν = 3513 (w), 3477 (w), 3456 (w), 3440 basicity of 6 were determined from the titration curves based on
˜
max
(w), 3348 (w), 1599 (m), 1579 (w), 1498 (m), 1487 (m), 1464 (s), pH-dependent absorption spectra in 50% aqueous ethanol solu-
Eur. J. Org. Chem. 2012, 2231–2236
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tions by a curve-fitting method using the KaleidaGraph pro-
gram.[25] For acidity, the absorption peaks at 389, 445, and 495 nm
were used, and for basicity, the absorption peak at 456 nm. Boronic
acid buffer solutions were used in the range of pH = 2.09–8.53 and
NaOH solutions in the range of pH = 11.1–13.9.
X-ray Crystallographic Analysis of 6: Diffraction measurements
were carried out with a Rigaku AFC7R diffractometer. The crystal
data are as follows: monoclinic, space group, P21/c (#14), a =
11.168(3), b = 6.039(2), c = 16.659(2) Å, β = 103.43(2)°, V =
1092.8(5) Å3, Z = 4, R = 0.057, wR = 0.114, and R1 = 0.046
[IϾ2.0σ(I)]. CCDC-814552 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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1
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Financial support from the Faculty of Science in Shinshu Univer-
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Received: December 21, 2011
Published Online: February 23, 2012
2236
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Eur. J. Org. Chem. 2012, 2231–2236