Preparative synthesis of 4,4′-bis[5-alkyl(aryl)benzoxazol-2-yl] biphenyls

Article abstract of DOI:10.1023/B:RJAC.0000018685.80580.c6

An attempt was made to prepare 4,4′-bis[5-alkyl(aryl)benzoxazol-2-yl] biphenyls by reaction of dimethyl 4,4′-biphenyldicarboxylate with substituted o-aminophenols in a high-boiling solvent in the presence of boric acid.

Full text of DOI:10.1023/B:RJAC.0000018685.80580.c6

Russian Journal of Applied Chemistry, Vol. 76, No. 11, 2003, pp. 1792 1794. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 11,
2003, pp. 1842 1844.
Original Russian Text Copyright
2003 by Ol’khovik, Matveenko, Kalechits, Pap, Zenyuk, Skakovskii.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Preparative Synthesis
of 4,4 -Bis[5-alkyl(aryl)benzoxazol-2-yl]biphenyls
V. K. Ol’khovik, Yu. V. Matveenko, G. V. Kalechits, A. A. Pap,
A. A. Zenyuk, and E. D. Skakovskii
Institute of Chemistry of New Materials, National Academy of Sciences of Belarus, Minsk, Belarus
Received April 4, 2003
Abstract An attempt was made to prepare 4,4 -bis[5-alkyl(aryl)benzoxazol-2-yl]biphenyls by reaction of
dimethyl 4,4 -biphenyldicarboxylate with substituted o-aminophenols in a high-boiling solvent in the presence
of boric acid.
Compounds containing an oxazole fragment exhibit
a set of valuable properties [1]. In particular, benzoxa-
zoles are used in production of heat-resistant polymers
[2], scintillators [3, 4], and fluorescent whiteners [5
7]. Of considerable interest as optical whiteners are
bis(benzoxazoles) in which benzoxazole cores are
linked via an ethylene, aromatic, or heterocyclic
bridge. These compounds exhibit high heat resistance
and lightfastness; they are used for whitening of virtu-
ally all polymeric materials. Synthesis of new com-
pounds of this series and their study in the context of
specific applications remain urgent problems.
The goal of this study was development of a pro-
cedure for preparing 4,4 -bis[5-alkyl(aryl)benzoxazol-
2-yl]biphenyls ensuring high yield and purity of the
target product, involving no aggressive cyclization
agents and sophisticated purification procedures, and
starting from readily avaliable ester I. We studied the
reactions of I with various o-aminophenols IIa IId
in high-boiling solvents in the presence of boric acid
[11] (Table 1).
4,4 -Bis(benzoxazol-2-yl)biphenyl derivatives were
prepared by the scheme
R2
OH
Studies on synthesis of substituted benzoxazol-2-yl
compounds in which benzoxazole cores are linked to
biphenyl are relatively few [8, 9], although introduc-
tion of an additional aromatic core into the conjuga-
tion chain should enhance the whitening effect. One
of promising routes to bis(benzoxazoles) containing a
biphenyl bridge is that starting from dimethyl 4,4 -bi-
phenyldicarboxylate I, which became a cheap and
readily available substance after development of an
efficient procedure for its recovery from dimethyl
terephthalate production waste [10]. The known pro-
cedures for preparing 4,4 -bis(benzoxazol-2-yl)biphen-
yl derivatives are based on condensation of 4,4 -bi-
phenyldicarboxylic acid or its chloride with substi-
tuted o-aminophenols. The reaction is performed in
two steps (formation of acid arylamides followed by
cyclodehydration) or in one step, with polyphosphoric
acid used as solvent and cyclizing agent. The yields
of the target products, depending on the solvent and
cyclizing agent used, did not exceed 68% [9]. Isolation
of the pure target product involves prolonged (many
hours) extraction, which is a significant drawback.
MeO2 C
CO2 Me
+
R1
NH2
I
IIa IId
R1
R2
NH
O
HO
OH
O
HN
R2
R1
IVa IVd
O
N
R2
R1
R1
R2
N
O
IIIa IIId
The characteristics of IIIa IIId are listed in
Table 2.
As solvents we used dimethylformamide, dimethyl-
acetamide (DMA), diheptyl ether, diphenyl ether, and
1070-4272/03/7611-1792$25.00 2003 MAIK Nauka/Interperiodica

PREPARATIVE SYNTHESIS OF 4,4 -BIS[5-ALKYL(ARYL)BENZOXAZOL-2-YL]BIPHENYLS
1793
Table 1. Yield of bis(benzoxazoles) IIIa IIId in cyclization of ester I with aminophenols IIa IId as influenced by
reaction conditions
Bihenyl : diphenyl
ether : pyridine
Diphenyl
ether : pyridine
Boric acid,
mol %
Reaction
T,
C
, h
Yield, %
I + IIa = IIa
I + IIb = IIIb
I + IIc = IIIc
I + IIa = IIIa
I + IIa = IIIa
I + IId = IIId
I + IIa = IIIa
I + IIa = IIIa
I + IIa = IIa
I + IIb = IIIb
I + IIc = IIIc
I + IIa = IIIa
1 : 5 : 1
2.3 : 10 : 1 (Dinil)
1 : 20 : 2
100
80
100
100
20
100
50
50
100
75
100
100
250
12
12
12
10
20
12
17
15
10
24
12
17
80.7
89.7
88.8
92.4
63.0
78.3
73.0
87.5
91.0
86.4
79.1
54.0
260
250
250
260
250
250
250
250
250
250
250
10 : 1
10 : 1
1 : 100 : 10
1 : 4 : 1
10 : 1
30 : 1
2.3 : 10 : 1 (Dinil)
1 : 50 : 2
Diheptyl ether : pyridine = 8 : 1
Table 2. Yields, constants, and elemental analyses of IIIa IIId
Found, %/Calculated, %
Com-
pound
Yield,
%
,
Molecular
weight
max
nm
R
Tm,
C
Formula
C
H
N
IIIa
R1 = Me,
R2 = H
92.4
89.7
88.8
78.3
336 338
305 306
293 295
336 338
437
440
447
435
81.54/80.75 4.67/4.84 6.27/6.23 C28H20N2O2
83.97/84.43 4.21/4.47 4.87/5.18 C38H24N2O2
82.18/81.57 7.06/6.44 5.10/5.60 C34H32N2O2
80.23/80.75 4.16/4.84 6.67/6.73 C28H20N2O2
416.48
540.62
500.4
IIIb
IIIc
IIId
R1 = Ph,
R2 = H
R1 = t-Bu,
R2 = H
R1 = H,
416.48
R2 = Me
Dinil (a mixture of 26.5% biphenyl and 73.5 iphenyl
ether). In amide solvents and diheptyl ether, the reac-
tion is very slow, because their boiling point is too
low for the efficient reaction. Table 1 shows that the
ratio between ester I and boric acid strongly affects
the yield of bis(benzoxazoles) IIIa IIId. Complete
conversion is attained with an equimolar amount of
the catalyst. At lower concentrations of boric acid and
optimal reaction temperatures, the yield is as low as
63%. The reaction can be accelerated by adding an
equimolar amount of pyridine; in this case, the maxi-
mal yield at 250 260 C is attained in 10 12 h. The
highest yield of IIIa IIId is attained in diphenyl ether
(91%) or Dinil (92%) at the component ratio I : (IIa
IId) : pyridine : Dinil = 1 : 2 : 1 : 5 and 100 mol %
amount of boric acid.
to 150 180 C, diluted with DMA, refluxed for 2 h,
and then cooled to 80 90 C. Under these conditions,
the reaction product crystallizes. It is filtered off and
washed on the filter with hot DMA. Compounds IIIa
IIId are colorless or light yellowish powders insoluble
in water and aqueous acids and alkalis and sparingly
soluble in organic solvents.
The reaction mechanism involves initial formation
of amides IVa IVd followed by their cyclization with
formation of the benzoxazole fragment, as demon-
strated by the example of reaction of I with IIa. The
IR spectrum of the intermediate amide IVa contains
1
NH stretching bands at 3320 and 3270 cm , NH
1
bending band at 1560 cm , and amide
1680 cm .
band at
C=O
1
The composition of benzoxazoles IIIa IIId was
confirmed by elemental analysis, and their structure
was determined by IR and H NMR spectroscopy.
Compounds IIIa IIId can be readily isolated from
the reaction mixture as follows. The mixture is cooled
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 11 2003

1794
OL’KHOVIK et al.
The IR spectra are consistent with the suggested
ture for 3 h until the volatiles (methanol, water, pyri-
dine, etc.) were completely distilled off. Then the
temperature was raised to 250 260 C and kept at this
level for 10 12 h. After the reaction completion,
the mixture was cooled to 150 180 C, diluted with
100 ml of DMA, and refluxed for 2 h. After cooling
to 80 90 C, the precipitate was filtered off, washed
on the filter with hot DMA, and dried in a vacuum.
structures. Transformation of I into IIIa IIId results
1
in disappearance of a strong band at 1730 cm and
1
appearance of a strong band at 1565 1580 cm , with
the bands of aromatic structures being preserved. The
1
band at 1565 1580 cm belongs to vibrations of the
N=C O fragment in the ring [12]. A strong band at
1
1470 1480 cm is apparently due to conjugation of
the biphenyl fragment with the benzoxazole core of
the molecule. The stretching vibration bands of the
C H bonds of the aromatic rings are observed at
CONCLUSIONS
1
3060 3020 cm .
(1) 4,4 -Bis[5-alkyl(aryl)benzoxazol-2-yl]biphenyls
are formed in quantitative yield by the reactions of
dimethyl 4,4 -biphenyldicarboxylate with substituted
o-aminophenols in a high-boiling solvent (diphenyl
ether or Dinil) in the presence of boric acid and pyri-
dine at 250 260 C for 10 12 h.
1
In the H NMR spectra of IIIa IIId, the protons
of the biphenyl fragment give doublets at 7.73 7.78
(4H) and 8.15 8.21 ppm (4H), with the coupling con-
stant of 8.0 Hz. The benzoxazolyl protons give multi-
5
6
plets at 7.15 7.60 ppm (4,6,7-H, 6H). The C CH
3
3
protons in IVa give a singlet at 2.24 ppm; C CH
(2) High-purity substituted bis(benzoxazoles) can
be prepared by this procedure in one step in high
yield.
5
protons in IVd, a singlet at 2.26 ppm; C C(CH )
protons in IIIc, a singlet at 1.08 ppm (18H); and C
3 3
5
Ph protons in IIIb, a multiplet at 6.92 7.38 ppm
(10H). Bis(benzoxazoles) IIIa IIId are luminophores
emitting in the blue range, with the fluorescence max-
imum at 435 440 nm (Table 1).
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The H NMR spectra were recorded on a Tesla
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were taken on a Specord M-80 spectrophotometer in
the range 400 4000 cm using KBr pellets. The
fluorescence spectra were measured on a Fluorat 02-
Panorama spectrometer,
points were measured with a Boetius stage.
3
1
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exc
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4,4 -Bis[5-alkyl(aryl)benzoxazol-2-yl]biphenyls
IIIa IIId. The reaction vessel was charged with
30.0 g (0.11 mol) of ester I, 0.22 mol of substituted
aminophenol IIa IId, 100 ml of Dinil, 10 ml of pyri-
dine, and 6.2 g (0.1 mol) of boric acid; a nitrogen
flow was passed. The mixture was heated with contin-
uous stirring to 160 170 C and kept at this tempera-
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