Note: Descriptions are shown in the official language in which they were submitted.
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Bis-benzoxazolyl-stilbenes have gained technical
importance as optical brighteners, for example as spin
brighteners or as so-components in optical brighteners for
textile materials.
Up to now, they have been prepared, for example as
described in US-PS 3,586,673, by reacting 4-benzoxazolyl-
2,4'-stilbene-carboxylic acid in 1,2,4-trichlorobenzene
with phosphorus oxychloride to give the acid chloride, sub-
se~uently addin~ a solid aminophenol and heating the reac-
tion mixture. For purification the crude products obtained
in this manner must be recrystallized repeatedly. A similar
process is described in ~A-PS 23,028/69.
One-stage processes for the manufacture of the above
eompounds have been described in DE-OS 2,247,791, method
t5 A~ pages 16-17 and in DE-OS 2,315,955, Example 5. In both
eases the acid chloride is reacted with aminophenol in tri-
ehlorobenzene and the amine formed is subjected to cycli-
zation in the reaction mixture without previous lsolation.
In DE-OS 2,315~955 the aminophenol is dissolved in pyridine
and the acid chloxide is added in the orm of a solution
~n ~richlorobenzene. During the course o~ the reaction
the pyridine distils off. In the process of DE-OS
2,247,791 the aminophenol is added in the form of a hot
solution in trichlorobenzene. The processes yield products
that are highly contaminated and that must be purified by
recrystallization.
It has now been found, surprisingly, that bis-benzoxa-
zolyl-stilbenes of the formula defined below can be ob-
tained in a high purity in a one-stage process if a
solution of aminophenol in an aprotic, dipolar solvent is
added to the solution of a benzoxazolyl-stilbene-carboxylic
acid in an inert solvent. In this manner products are ob-
tained which can be directly used as optical brigtheners
without previous purification by recrystallization.
It is the objective of the present invention to
provide a simple and economical process for the manufacture
o bis-benzoxazolyl-stilbene compounds according to which
the desired products are obtained in a high purity and in a
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good yield. According to the invention this objective i5
obtained by adding the aminophenol not in solid form but
in the form of a solution in an aprotic,dipolar solvent.
It i5, therefore, the object of the present invention
to provide a process for the manufacture of bis-benzoxa-
zolyl-stilbene compounds of the formula I
R~
1 2 3
in which R , R and R , independent of one another, denot~
hydrogen, C1-Cg alkyl, carbo-C1-C4 alkoxy and tri3fluoro-
methyl, at least two of the radicals R , R and R
being hydrogen, by chlorination of an acid of the
formula II
1 ~ ~ _CEI=CH- ~ _COOH ~II)
in which R1 is as defined under formula I, in an inert
organic solvent ancl subsequent reaction with an amino-
phenol of the formula III
2 ~ R2 (III)
Ho~R3
in which R2 and R3 are as defined under formula I, which
comprises adding the aminophenol in the form of a solution
in an aprotic, dipolar solvent, preparing the compound of
the formula IV 2
R1 ~ ~ ~ ~ CH=~H- ~ C ~ (IV)
in which R1, R2 and R3 are as defined under formula I by
acylation and preparing therefrom the compound of the
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~ormula I ~y cyclization after addition of an acid
catalyst.
To carry out ~he process of the invention the compound
of the formula II is reacted in an inert or~anic solvent,
for example o-dichlorobenzene, trichlorobenzene, tetra-
; hydronaphthalene, optionally with the addition of acatalyst, for example a small amount of dimethyl formamide,
with a chlorination agent such as thionyl chloride to give
the acid chloride. The excess amount of chlorination agent
is subsequently distilled off up to an internal temperature
of 140 to 160C, a complete removal possibly being promoted
by passing an N2 current over the reaction mixture. To
this mixture, which is either a solution or a suspension,
a solution of a compound of the formula III in an aprotic,
dipolar solvent is then added dropwise at a temperature of
from 100 to 200C, preferably 150 to 160C and stirring
is continued at 100 to 200C, preferably 130 to 160C until
the conversion into the compound of the ~ormula IV is
complete, which can be percei~ed by the termination of the
hydrogen chloride generation.
5uitable aprotic solvents are ~,N-dialkyl-carboxylic
acid amides, for example dimethyl formamide, N-methyl-
pyrrolidone, or hexamethyl-phosphoric acid trisamide,
preferably, however, N-methylpyrrolidone. The aprotic,
dipolar solvent (solvent B) is used in an amount of from
3 to 30 and preferably 5 to 15-~ by volume, calculated on
the amount of inert organic solvent (solvent A) used for
the reaction of the compound of the formula II. As men-
tioned above, suitable inert organic solvents are, for
example, tetrahydronaphthalene, decahydronaphthalene or
methylnaphthalene, the boiling point of which should not
be below the reaction temperature.
The cyclization of compound IV to give compound I
is carried out without intermediate isolation by heating
the reaction mixture to a temperature of from 150 to
250C, preferably the boiling point of solvent A and op-
tionally in the presence of an acid catalyst. Suitable
catalysts are, for example, boric acid, zinc chloride,
. -
.
.:
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p-toluenesulfonic acid and polyphosphoric acids, which
are preferably used in an amount of from 0.5 to 5 %,
calculated on the amount of compound II used.
When the cyclization is terminated, which can be
perceived by the termination of the water separation, the
reaction product is isolated in usual manner, for example
by suction filtration after cooling to room temperature~
Prior to filtration a low molecular weight alcohol, for
example methanol or ethanol, may be added to the reaction
mixture.
~ he compounds of the formula IV used as starting
components are known from literature and can be produced
by processes described in 1iterature (cf. JA-PS 40,581/65,
JA-PS 44-6979, JA-PS 7045/68, DE-OS 2,306,050, DE-OS
1,594,829 and DE-OS 2,129,818).
Preferred compounds to be produced by the process
described above are compounds of the formula I a
~3 ~ ~ 0~3 '
in which R1 , R2 , R3 and R4 , independent of one another,
denote hydrogen or C1-C4 alkyl and at least one of the
radicals R to R is hydrogen.
In the process according to the invention the compounds
of the formula I are obtained in an excellent purity and
in an almost quantitative yield. Normally, no further
purification by recrystallization is necessary. This is
an especially important feature as bis-benzoxazolyl-
stilbenes of the formula I generally have a poor solubility
and, therefore, large amounts of solvent would be required
for recrystallization. As compared therewith, all known
processes, in which the aminophenol is not added in the
form of a solution in an aprotic, dipolar solvent, yield
dark yellow or brown products which possess a distinctly
.~ lesser brightening effect and, therefore, have to be
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redissolved.
The following examples illustrate the invention,
the parts are by weight unless otherwise stated and the
melting points are not corrected.
E X A M P L E 1:
. _ __ _ _
177 Parts of thionyl chloride are added over a period
of approximately 1 hour to a suspension of 170 parts of
4-(2-benzoxazolyl-2-4'-stilbe,ne~carboxylic acid in 1150
parts of tetrahydronaphthalene, which suspension contains
~ parts of dimethyl formamide. The reaction mixture is
heated to 110C within a period of about 2 hours and
stirring at that temperature is continued for 1 hour. Next,
the thionyl chloride in excess is distilled off in a
nitrogen current down to an internal temperature of
150C. At said temperature a solution of 65 parts of
2-amino-4-methylphenol iIl 125 parts of N-methylpyrrolidone
is added dropwise and the reaction mixture is slowly
heated to 170C within 3 hours. After addition of 5 parts
of p-toluene-sulfonic acid, the mixture is heated to
~0 reflux temperature and the reaction water is separated
over a water separator. When the water separation is
terminated, the reaction mixture is cooled to 100C,
320 parts of methanol are added dropwise and the mixture
is refluxed for another hour. After cooling to room tempe-
rature, the mixture is filtered with suction and theresidue is washed with methanol. After drying, 206 g
(96 ~ of theory) of the compound
,
~ > ~ CII=CH- ~ _,,N ~ ~ C~I3
are obtained in the form of a light yellow powder having
the following melting properties:
the product sinters at 266 to 270C, it has a liquid-
crystalline transition at 316 to 321C and it melts above
370C. The tinctorial properties can be improved slightly
only by repeated recrystallization from N-methylpyrrolidone
with clarification with active carbon.
,
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E X A M P L E 2:
The reaction is carried out as described in Example 1
with the exception that 5 parts of phosphorus pentoxide
are used instead of p-toluenesulonic acid. The same
compound is obtained in the same quality but in a slightly
smaller yield.
E X A M P L E 3:
The reaction is carried out as described in Example 1
with the exception that 1,2,4~trichlorobenzene is used
as solvent instead of tetrahydronaphthalene. The same
compound is obtained in an equally good quality in a
yield of 92 % of the theory.
E X A M P L E 4:
In the manner described in Examples 1 to 3 further
optical brighteners o the formula Ia as specified in
the following table can be prepared.
,
a~ ;:; ~ ~ u~ o ~ n
~ --1 C ~ r~ ~ G~ co
up ~ o o ll o o o o
~ ~ ~ ~ ~ ~ ~ ~
~--~ - ~
O _ ~ ~ ~ ~ ~ ~r
-1~ lo ~ W C~ O CO ~
QI ~ ~ CO Ct) 0;~ G~ CO G~
~ _ _______.
>~ ~ 0 ~ ~ ~ O ~r
~ ~; ~ t~ ~ ~ r
k ~ ~'1 ~ r~l 1`'1 ~) t`')
. .~ ~
\~ ~? O '
~, ~3 ~ r~ ~ ~ r~ ~ ~7
~r~ ~1 N ~ ~ I ~) ~ (~
~11 ''1._
~;~ h
, R; ~ ~ ~ ~ ~ ~ r
U ; ~; r-l h ~ t~ ~ O
~: . . ~ _
~ ~ ~ ~ r~ - rJo ~D O
l~ ~ ~ rl ~ 1
~ . ~ .
~ ~ dP ~
. .. _ .
~: ~ . ~ ~
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~1
_ ~ ~ 5 X 3~
~ _ _ ,~ _
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