Synthesis and properties of 4,4'-bis[5-alkyl(aryl)benzoxazol-2-yl]-2- hydroxy(alkoxy)biphenyls

Article abstract of DOI:10.1134/S1070428006080100

Synthesis of 2-hydroxy(methoxy)-4,4'-biphenyldicarboxylic acids was developed. Based on their reactions with o-aminophenols we prepared for the first time 4,4'-bis[5-alkyl(aryl)benzoxazol-2-yl]-2-hydroxy-(alkoxy)biphenyls, which might be of interest as lu

Full text of DOI:10.1134/S1070428006080100

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2006, Vol. 42, No. 8, pp. 1164-1168. Ó Pleiades Publishing, Inc. 2006.
Original Russian Text Ó V.K. Ol’khovik, Yu.V. Matveenko, G.V. Kalechits, A.A. Pap, A.A. Zenyuk, 2006, published in Zhurnal Organicheskoi
Khimii, 2006, Vol. 42, No. 8, pp. 1185-1189.
Synthesis and Properties of 4,4'-Bis[5-alkyl(aryl)benzoxazol-2-yl]-
2-hydroxy(alkoxy)biphenyls
†
V.K. Ol’khovik, Yu.V. Matveenko, G.V. Kalechits,A.A. Pap, andA.A. Zenyuk
Institute of New Materials Chemistry, Belarussian Academy of Sciences, Minsk, 22014 Belarus’
e-mail: slavol@ichnm.basnet.by
Received June 8, 2005
Abstract—Synthesis of 2-hydroxy(methoxy)-4,4'-biphenyldicarboxylic acids was developed. Based on their
reactions with o-aminophenols we prepared for the first time 4,4'-bis[5-alkyl(aryl)benzoxazol-2-yl]-2-hydroxy-
(alkoxy)biphenyls, which might be of interest as luminescent dyes. Hydroxy compounds in a salt form possess
abnormally high Stokes shift.
DOI: 10.1134/S1070428006080100
Biphenyl derivatives are widely used as dyes, liquid-
crystalline materials, biologically active and medicinal
preparations [1–5]. Luminescent dyes with benzazole
moieties directly attached to a biphenyl fragment possess
a high practical potential. However the studies of their
synthesis are relatively few [6–9] and concern only
ubsubstituted biphenyls. The presence of electron-donor
or electron-acceptor substituents in the biphenyl fragment
of the molecule essentially affects the electron density
distribution in the molecule and consequently its spectral
and luminescent characteristics.
yield by alkylation of phenol III with methyl iodide in
alkaline medium.
&2 +
0H2 &
&2 +
&2 +
2+
2&+
62
We report here on the results of the synthesis of 2-
hydroxy(methoxy)-4,4'-biphenyldicarboxylic acids (III and
IV) and 4,4'-bis[5-alkyl(aryl)benzoxazol-2-yl]-2-
hydroxy(alkoxy)biphenyls prepared therefrom. Our
interest to this type compounds was attracted both by
the prospect to apply them as luminescent dyes and by
the accessibility of the initial raw material, namely,
dimethyl 4,4'-biphenyldicarboxylate.
&2 +
0H2 &
&2 +
&2 +
,,,
,
,9
,,
The synthesis of bisbenzoxazole derivatives VIa–VId,
VIIa, and VIIb was performed by condensation of 2-
hydroxy(methoxy)-4,4'-biphenyldicarboxylic acids (III and
IV) with alkyl(phenyl)aminophenols Va–Vd in a mixture
of diphenyl ether and pyridine, 4:1, at 250–260°C in the
presence of boric acid [9]. The yields of bisbenzoxazoles
VIa–VId and VIIa–VIIc reached 73–96%.
We developed a simple and efficient procedure for
introduction of a hydroxy group into the position 2 of
dimethyl 4,4'-biphenyldicarboxylate (I) providing the target
compound III in a 97% yield. The boiling of diester I
with excess chlorosulfonic acid led to the formation in a
quantitative yield of dibenzothiopnene-5,5'-dioxide-3,7-
dicarboxylic acid (II), which on treatment with sodium
hydroxide at 300°C transformed into 2-hydroxy-4,4'-
biphenyldicarboxylic acid (III) [10]. 2-Methoxy-4,4'-
biphenyldicarboxylic acid (IV) was obtained in a 94%
Alkoxy derivatives VIIa and VIIb were also prepared
by another procedure, a direct O-alkylation of 4,4'-
bis(benzoxazol-2-yl)-2-hydroxybiphenyls VIa and VIb
with alkyl halides or the corresponding dialkyl sulfates in
a solution of lower aliphatic alcohols.
The composition of compounds II–IV and also of
bisbenzoxazoles VIa–VId and VIIa–VIIc was confirmed
by elemental analyses; their structure was consistent with
†
Deceased.
1164

SYNTHESIS AND PROPERTIES OF 4,4'-BIS[5-ALKYL(ARYL)BENZOXAZOL-2-YL]-...
1165
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1
(VIb, VIIb), of the tert-butyl group C–C(CH3)3 in
compound VId, at 1.18 ppm, multiplet of the phenyl in
compound VIc is observed at 6.95–7.35 ppm. The
methoxy group protons in the spectra of compounds VIIa
and VIIb give rise to a singlet at 3.95 ppm, and the ethoxy
the IR and H NMR spectra. The absorption bands in
the IR spectra are in agreement with the assumed
structures. Comparing the spectra of the initial diacids
III and IV with those of 4,4'-bis[5-alkyl(aryl)benzoxazol-
2-yl]-2-hydroxy(alkoxy)biphenyls VIa–VId and VIIa–
VIIc synthesized it should be noted that the absorption
bands belonging to the aromatic structures were retained,
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the strong band of the ester group at 1730 cm disap-
peared, and appeared a strong absorption band at 1565–
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tion band in the region 1470–1480 cm is apparently due
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group are observed in the region 3460–3520 cm .
In the 1H NMR spectra of compounds VIa–VId and
VIIa–VIIc the signals from the protons at C3' and C6',
C3' and C5' of the biphenyl moiety appear as doublets at
8.15–8.25 ppm (4H) with a coupling constant 8.3 Hz.
The proton signals from the benzoxazolyl and the other
biphenyl protons are observed as a rule as multiplets in
the region 7.15–7.60 ppm. The singlets of the methyl group
of the benzoxazole fragment appear at 2.25–2.44 ppm
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Fluorescesce spectra of compound VIb. 1, DMSO, 2, DMSO +
NaOH.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 8 2006

OL’KHOVIK et al.
1166
Absorption and fluorescence spectra of compounds VIà–VIa, VIIà, and VIIb in the salt and protonated forms
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spectrometers BrukerAvance-400 (400 MHz), and Tesla
BS-587A and BS-567A with operating frequencies 80
and 100 MHz respectively using TMS as internal refer-
ence. IR spectra were recorded on a spectrophotometer
group in compound VIIc forms a characteristic triplet at
1.36 ppm and a quartet at 4.23 ppm.
The electron absorption spectra of bisbenzoxazoles
VIa–VId, VIIa, and VIIb contain a longwave band at
315–350 nm. These compounds are characterized by
strong fluorescence with a maximum in a range 415–
425 nm and can be applied as optical bleaching agents.
Compounds VIa–VId containing a free hydroxy group
in the position 2 exhibit the properties of fluorescent pH-
indicators, and in the salt forms their fluorescence maxima
shift to the yellow-green region (l 570–580 nm) at
virtually the same absorption frequency; thus the
compounds demonstrate an abnormally large Stokes shift
(»240 nm). The spectral characteristics of compounds
VIa–VId, VIIa, and VIIb are given in the table.
–1
Specord M-80 in a range 400–4000 cm from samples
pelletized with KBr. The purification and drying of solvents
and reagents was performed by common procedures.
The reaction progress was monitored by TLC on plastic
plated with silica gel 60 F254 (Merck Art. 7734).
Dibenzothiophene-5,5'-dioxide-3,7-dicarboxylic
acid (II). To 81 g (0.3 mol) of dimethyl 4,4'-biphenyl-
dicarboxylate (I) was added 116.5 g (1 mol) of chloro-
sulfonic acid, and the mixture was heated for 3 h at 130–
155°C till the end of hydrogen chloride liberation. The
excess acid was evaporated at a reduced pressure, and
the residue was treated with ice water. The separated
precipitate was filtered off, washed with a large volume
of water, and dried in a drying cabinet at 50°C. Yield
90 g (99%), mp >300°C (decomp.). 1H NMR spectrum
(DMSO-d6), d, ppm: 8.4 s (6Harom). Found, %: C 55.11;
H 2.52. C14H8O6S. Calculated, %: C 55.26; H 2.64.
M 304.27.
This fact apparently originates from the large
contribution of the canonical form A and alterations in
the structure of the excited state of the molecule. In this
case probably plays the role the increased coplanarity in
the tautomer form B resulting in the elongation of the
conjugation chain and consequently in the change in the
electron density distribution in the molecule.
2-Hydroxy-4,4'-biphenyldicarboxylic acid (III). In
a steel cylinder 400 g of NaOH was melted and heated
to 300°C. Sulfone II (90 g, 0.29 mol) was added thereto
by small portions at stirring, the mixture was then stirred
for 15 min at 315–320°C. The reaction mixture was cooled
to room temperature and treated with 1 N hydrochloric
acid. The precipitate formed was filtered off, washed
with water, and dried in a drying cabinet at 50°C. Yield
77 g (97%), mp >300°C (decomp.). 1H NMR spectrum
(DMSO-d6), d, ppm: 7.45 m (2H, H5, H6), 7.61 s (1H,
H3), 7.71 d (2H, H2' and H6, J 8.3 Hz), 7.99 d (2H, H3'
and H5, J 8.3 Hz), 10.2 br.s (1H, OH). Found, %: C 65.14;
H 3.93. C14H10O5. Calculated, %: C 65.10; H 3.91.
M 258.22.
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EXPERIMENTAL
2-Methoxy-4,4'-biphenyldicarboxylic acid (IV).
Into a round-bottom flask was charged 100 ml of CH3OH,
Melting points were measured on a Koeffler heating
1
block. H and 13C NMR spectra were registered on
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 8 2006

SYNTHESIS AND PROPERTIES OF 4,4'-BIS[5-ALKYL(ARYL)BENZOXAZOL-2-YL]-...
1167
1
10 ml of H2O, 4.0 g of KOH, 2.58 g of phenol III, and
5 ml of methyl iodide. The reaction mixture was boiled
for 14 h. On completion of the reaction the mixture was
diluted with 100 ml of 20% HCl solution. The separated
precipitate was filtered off and recrystallized from a mix-
ture CH3OH–H2O, 10:1. Yield 2.6 g (94.5%), mp 330–
332°C (subl.). 1H NMR spectrum (DMSO-d6), d, ppm:
3.83 s (3H, OMe), 7.44–7.75 m (5Harom), 7.99 d (2H, H3'
and H5, J 8.3 Hz). Found, %: C 65.03; H 3.87. C15H13O5.
Calculated, %: C 65.12; H 3.90. M 273.25.
(decomp.). H NMR spectrum (CF3CO2D), d, ppm:
1.18 s [18H, C–(CH3)3], 7.2–8.3 m (13Harom). Found, %:
C 79.01; H 6.25; N 5.43. C34H32N2O3. Calculated, %:
C 79.04; H 6.24; N 5.42. M 516.64.
4,4'-Bis(benzoxazol-2-yl)-2-methoxybiphenyl
(VIIa). a. Analogously to the synthesis of compound VIa
0.67 g (0.0025 mol) of acid IV and 0.55 g (0.005 mol) of
o-aminophenol Va in a mixture of 20 ml of diphenyl ether,
3 ml of pyridine, and 0.15 g (0.025 mol) of boric acid was
heated at 160–170°C for 1 h and at 250–260°C for 3 h.
After an appropriate workup we obtained compound VIIa
in an yield 0.87 g (84%).
4,4'-Bis(benzoxazol-2-yl)-2-hydroxybiphenyl
(VIa). General procedure. In a flask equipped with
a mechanical agitator, reflux condenser, and a thermom-
eter was heated at 160–170°C while constant stirring for
2 h a mixture of 2.72 g (0.01 mol) of compound III,
2.18 g (0.02 mol) of o-aminophenol, 40 ml of diphenyl
ether, 10 ml of pyridine, and 0.15 g (0.025 mol) of boric
acid. After 2 h the temperature was raised to 250–260°C,
and heating at this temperature continued for 6 h. Then
the reaction mixture was cooled, diluted with 50 ml of
acetic acid, and boiled for 30 min. The precipitated
crystals were filtered off and dried. Yield 3.9 g (95%),
mp >300°C (decomp.). 1H NMR spectrum (DMSO-d6),
d, ppm: 7.25–8.00 m (13Harom), 8.25 d (2H, H3' and H5'
of biphenyl, J 8.3 Hz ). Found, %: C 77.20; H 3.98;
N 6.90. C26H16N2O3. Calculated, %: C 77.22; H 3.99;
N 6.93. M 404.42.
b. To a solution of 0.4 g (0.01 mol) of compound VIa
in 20 ml of ethanol was added 0.48 g (0.12 mol) of NaOH,
and 0.1 ml (0.01 mol) of dimethyl sulfate. The reaction
mixture was boiled at reflux for 1 h, then it was cooled
and diluted with 100 ml of water. The precipitate was
filtered off and recrystallized from acetic acid. Yield
370 mg (88.7%).
c. To a solution of 0.2 g (0.005 mol) of compound VIa
in 100 ml of ethanol containing 0.24 g (0.05 mol) of NaOH
was added 0.4 ml of methyl iodide, and the mixture was
boiled for 30 h. Then the reaction mixture was poured
into 100 ml of 5% hydrochloric acid, the precipitate was
filtered off and recrystallized from acetic acid. Yield
1
175 mg (84%), mp 193–195°C. H NMR spectrum
(CF3CO2D), d, ppm: 3.95 s (3H, OMe), 7.30–7.95 m
(13Harom), 8.25 d (2H, H3' and H5' of biphenyl, J 8.2 Hz).
Found, %: C 77.52; H 4.34; N 6.70. C27H18N2O3.
Calculated, %: C 77.49; H 4.34; N 6.69. M 418.45.
Likewise were prepared compounds VIb–VId.
4,4'-Bis(5-methylbenzoxazol-2-yl)-2-hydroxybi-
phenyl(VIb). The reaction mixture was heated for 4 h
at 160–170°C and 5 h at 250–260°C. Yield 96%, mp 345–
4,4'-Bis(5-methylbenzoxazol-2-yl)-2-methoxy-
biphenyl (VIIb) was prepared as described in method
a. Yield 0.8 g (73%), mp 310–312°C. 1H NMR spectrum
(CF3CO2D), d, ppm: 2,44 s (6H, 2CH3), 3.95 s (3H,
OCH3), 7.23 d (2H, H2' and H6' of biphenyl, J 8.12 Hz),
7.5–7.95 m (9Harom), 8.22 d (2H, H3' and H5' of biphenyl,
J 8.12 Hz). Found, %: C 78.00; H 4.93; N 6.28.
C29H22N2O3. Calculated, %: C 77.76; H 4.97; N 6.27.
M 446.50.
1
346°C. H NMR spectrum (CF3CO2D), d, ppm: 2.25 s
(6H, 2CH3), 7.2–8.3 m (13Harom). Found, %: C 77.74;
H 4.68; N 6.47. C28H20N2O3. Calculated, %: C 77.76;
H 4.66; N 6.48. M 432.48.
4,4'-Bis(5-phenylbenzoxazol-2-yl)-2-hydroxy-
biphenyl (VIc) was obtained from 0.64 g (0.0025 mol)
of compound III and 0.93 g (0.005 mol) of 2-amino-4-
phenylphenol (Vc). Yield 1.2 g (87%), mp >365°C
(decomp.). 1H NMR spectrum (CF3CO2D), d, ppm: 6.95–
7.35 m (10H, 5-Ph-H in benzoxazol-2-yl), 7.45 d (2H,
H2' and H6', J 8.3 Hz), 7.65–7.85 m (9Harom), 8.15 d (2H,
H3' and H5' of biphenyl, J 8.3 Hz). Found, %: C 82.04;
H 4.39; N 5.11. C38H24N2O3. Calculated, %: C 82.00;
H 4.35; N 5.03. M 556.62.
4,4'-Bis(benzoxazol-2-yl)-2-ethoxybiphenyl
(VIIc). To a solution of 0.4 g (0.01 mol) of compound
VIa in 100 ml of ethanol containing 0.48 g (0.12 mol) of
NaOH was added 1 ml of ethyl bromide, and the mixture
was boiled for 30 min. Then the reaction mixture was
poured into 100 ml of 5% hydrochloric acid, the precipitate
was filtered off and recrystallized from acetic acid. Yield
0.32 g (73.5%), mp 154°C. 1H NMR spectrum (CF3CO2D),
d, ppm: 1.36 t (3H, OCH2CH3, J 6 Hz), 4.23 q (2H,
OCH2CH3, J 6 Hz), 7.30–7.95 m (13Harom), 8.25 d (2H,
4,4'-Bis(5-tert-butylbenzoxazol-2-yl)-2-hydroxy-
biphenyl (VId) was obtained from 0.516 g (0.002 mol)
of acid III and 0.67 g (0.004 mol) of 2-amino-4-tert-
butylphenola (Vd). Yield 0.82 g (78%), mp >300°C
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 8 2006

OL’KHOVIK et al.
1168
H3' and H5' of biphenyl, J 8.2 Hz). Found, %: C 77.72;
H 4.65; N 6.45. C28H20N2O3. Calculated, %: C 77.76;
H 4.66; N 6.48. M 432.48.
5. Gelisser, C., Bormans, G. and de Groot, T. J. Labele, Com-
pounds and Radiopharm., 1999, vol. 42, p. 5185.
6.Clemens, A.H. , WIPO Patent 9411485, 1995; Chem. Abstr.,
1995, vol. 122, 242797v.
7. Dell’Armo, L.A. and Lala, R.J., EPV Patent 368383, 1990;
Chem. Abstr., 1990, vol. 113, 174513m.
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