Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D263/00—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings
  • C07D263/52—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings condensed with carbocyclic rings or ring systems
  • C07D263/62—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings condensed with carbocyclic rings or ring systems having two or more ring systems containing condensed 1,3-oxazole rings
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D263/00—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings
  • C07D263/52—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings condensed with carbocyclic rings or ring systems
  • C07D263/62—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings condensed with carbocyclic rings or ring systems having two or more ring systems containing condensed 1,3-oxazole rings
  • C07D263/64—Heterocyclic compounds containing 1,3-oxazole or hydrogenated 1,3-oxazole rings condensed with carbocyclic rings or ring systems having two or more ring systems containing condensed 1,3-oxazole rings linked in positions 2 and 2' by chains containing six-membered aromatic rings or ring systems containing such rings
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D413/00—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms
  • C07D413/02—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms containing two hetero rings
  • C07D413/06—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms containing two hetero rings linked by a carbon chain containing only aliphatic carbon atoms
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D413/00—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms
  • C07D413/02—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms containing two hetero rings
  • C07D413/10—Heterocyclic compounds containing two or more hetero rings, at least one ring having nitrogen and oxygen atoms as the only ring hetero atoms containing two hetero rings linked by a carbon chain containing aromatic rings

Definitions

• Bis-benzoxazol-2-yl-substituted compounds in which two benzoxazol-2-yl radicals are linked to one another via a system of conjugated double bonds have acquired industrial significance as dyes, UV absorbers and optical brighteners for natural, synthetic and semisynthetic fibers , They are used for example as a spin-on brightener, as a brightener for Polyolefinfasem or for textile applications.
• Benzoxazoles are generally prepared starting from 2-aminophenols by reaction with carboxylic acid derivatives, by Schiff 's base cyclization or 2-hydroxyanilides.
• the classical preparation processes require highly reactive carboxylic acid derivatives such as acid anhydrides, nitriles or acid halides such as acid chlorides or chlorinating reagents, very special starting materials and / or large amounts, i. at least stoichiometric amounts
• Excipients such as acidic catalysts or they can be performed only under very extreme reaction conditions such as long reaction times and high reaction temperatures using special catalysts and are therefore very expensive.
• large amounts of unwanted by-products such as acids and salts, which have to be separated from the product and disposed of, are formed in these production processes.
• the rising environmental awareness also requires the use of chlorinating reagents, hydrogen fluoride and metallic catalysts due to their corrosive Properties and the air and water pollution caused by them, or even to avoid them. But also the remaining in the products remainders of these by-products can cause in part very undesirable effects. For example, halide ions as well as acids lead to corrosion; Residues of metal salts are often toxicologically questionable.
• bisbenzoxazoles linked together by a conjugated double bond system are prepared by direct reaction of o-aminophenols with dicarboxylic acids whose carboxyl groups are linked by a conjugated double bond system, by microwave irradiation in the presence of dehydrating catalysts and low dielectric solvents Loss can be produced in high yields and with high purity.
• the invention relates to a process for preparing bisbenzoxazoles which are interconnected via a system of conjugated double bonds by reacting o-aminophenols with dicarboxylic acids, their carboxyl groups via a double bond or via a system of conjugated double bonds with one another are reacted to an ammonium salt and this ammonium salt is subsequently reacted in the presence of dehydrating catalysts and solvents with low dielectric loss under microwave irradiation to the benzoxazole.
• Bisbenzoxazoles which are linked together by a double bond or by a system of conjugated double bonds, are understood to mean compounds which have a double bond or a through-conjugated system of ⁇ -electrons between the nitrogen atoms of the terminal benzoxazole structures.
• This effetkonjugator system can be composed of olefinic and / or aromatic double bonds.
• Preferred bisbenzoxazoles correspond to the formula
• R 1 , R 2 , R 3 and R 4 independently of one another represent hydrogen, halogen, a hydroxyl, nitro, amino, sulfonic acid, carboxyl, or Carbon
• R 1 , R 2 , R 3 and R 4 are independently hydrogen, halogen, a hydroxyl, nitro, amino, sulfonic acid, carboxyl or
• Preferred halogen atoms are chlorine or bromine.
• Preferred amino groups are primary and secondary amino groups.
• R 1 , R 2 , R 3 and R 4 independently of one another are hydrogen or C 1 -C 6 -alkyl radicals, such as methyl or ethyl. One or two of these radicals are particularly preferably C 1 -C 6 -alkyl radicals, such as methyl or ethyl.
• Preferred fused aliphatic rings are 5- or 6-membered.
• Preferred fused mononuclear or polynuclear aromatic hydrocarbon rings are mono-, di-, tri- or polycyclic, such as, for example, benzene or naphthalene systems.
• R 1 , R 2 , R 3 , R 4 and Z have the abovementioned meaning.
• R 1 , R 2 , R 3 and R 4 are independently hydrogen, halogen, a hydroxyl, nitro, amino, sulfonic acid, carboxyl or
• Carboxamide Preferred halogen atoms are chlorine or bromine.
• Preferred amino groups are primary and secondary amino groups.
• R 1 , R 2 , R 3 and R 4 are independently hydrogen or Ci-C ⁇ -alkyl radicals such as methyl or ethyl. Particularly preferably, one or two of these radicals are alkyl radicals such as methyl or ethyl.
• 2-aminophenols in which two adjacent radicals R 1 and R 2 , R 2 and R 3 or R 3 and R 4 is an optionally substituted, fused cycloaliphatic, especially 5- to 6-membered hydrocarbon ring or form a mononuclear or polynuclear aromatic hydrocarbon ring such as benzene or naphthalene.
• Suitable aminophenols are, for example, 1-amino-2-naphthol, 2-aminophenol and 2-amino-4-methylphenol. Particularly preferred is 2-aminophenol.
• the double bonds linking the carboxyl groups are transubstituted.
• Z is an aromatic system having one or more, such as, for example, two, three, four or more fused aromatic rings.
• the aromatic systems may contain heteroatoms such as N, S and / or O.
• the carboxyl groups are preferably attached to the same aromatic ring but not ortho to each other.
• the carboxyl groups are preferably bonded in the meta and especially in the para position of an aromatic ring, for example in 1,4-naphthalene.
• the carboxyl groups may also be attached to various rings, such as in the 1, 5-position of the naphthalene.
• the double bond or the double bonds are preferably transesterically substituted.
• the carboxyl groups are here preferably in para-position to the attachment sites of the aromatic nuclei such as in 4,4'-bipyridine.
• Z is a hydrocarbon radical containing a system of at least one aromatic ring and at least one olefinic double bond conjugated thereto.
• the double bond or the double bonds are preferably transesterically substituted.
• Suitable hydrocarbon radicals Z are the ethylene radical, the butadiene radical, the benzene radical, the naphthalene radical, the anthracene radical, the phenanthrene radical, the pyridine radical, the furan radical, the thiophene radical, the
• Biphenylrest the styrene residue, the bisstyrene residue and the stilbene residue.
• Particularly preferred are the ethylene radical, the thiophene radical, the furan radical, the naphthalene radical, the stilbene radical, the biphenyl radical and the bisstyrene radical.
• Z corresponds to the following structural units:
• Y and Y ' may be H or Ci-Ci 2 alkyl groups and X may be O, S or NR 5 , wherein R 5 is hydrogen, Ci-C 30 alkyl, C 6 -C 3 O-AIyI, hydroxyl or C 1 -C 2 O-hydroxyalkyl.
• radicals may have one or more substituents such as, for example, halogen atoms, hydroxyl, nitro, amino, sulfonic acid, sulfonic acid ester, carboxamide or acylamino groups, and / or C 1 -C 2 -alkyl-, C 2 -C 2 - Alkenyl, Ci-Ci 2 alkoxy, phenoxy, C 7 -C 12 -Alkylaryl-, Ci-Ci 2 -alkylsulfonyl, arylsulfonyl, Ci-Ci 2 -Carboxyalkyl- and Ci-Ci 2 -Carbonklamidalkylreste carry.
• substituents such as, for example, halogen atoms, hydroxyl, nitro, amino, sulfonic acid, sulfonic acid ester, carboxamide or acylamino groups, and / or C 1 -C 2 -alkyl-, C 2 -C 2
• the dicarboxylic acids used in the process according to the invention comprise a hydrocarbon radical Z containing a fully conjugated system of ⁇ -electrons between two carboxylic acid functions.
• Z has the meanings outlined above.
• Suitable dicarboxylic acids for the process according to the invention are, for example, fumaric acid, maleic acid, hexadiene-1,6-dicarboxylic acid, benzene-1,4-dicarboxylic acid, naphthalene-1,4-dicarboxylic acid, naphthaene-1,5-dicarboxylic acid, anthracene-1, 4-dicarboxylic acid. dicarboxylic acid, thiophene-2,5-dicarboxylic acid, furan-2,5-dicarboxylic acid, stilbene-4,4'-dicarboxylic acid and
• Biphenyl-4,4'-dicarboxylic acid Particularly preferred are fumaric acid, benzene-1, 4-dicarboxylic acid, py ⁇ idine-2,5-dicarboxylic acid, naphthalene-1, 4-dicarboxylic acid, stilbene-4,4'-dicarboxylic acid and biphenyl-4,4'-dicarboxylic acid.
• the dehydrating catalysts required for the successful implementation of the process according to the invention are generally acidic, inorganic, organometallic or organic catalysts or mixtures of several of these catalysts.
• acidic inorganic catalysts for the purposes of the present invention include boric acid, sulfuric acid, phosphoric acid, polyphosphoric acid, phosphonic acid, hypophosphorous acid, aluminum sulfate hydrate, alum, acidic silica gel, acidic aluminum hydroxide and zinc chloride.
• boric acid sulfuric acid
• phosphoric acid polyphosphoric acid
• phosphonic acid hypophosphorous acid
• aluminum sulfate hydrate alum
• acidic silica gel acidic aluminum hydroxide
• zinc chloride has proven particularly useful the use of boric acid, phosphoric acid, polyposphoric acid or zinc chloride.
• aluminum compounds of the general formula AI (OR 5 ) 3 and in particular titanates of the general formula Ti (OR 5 ) 4 are used as acidic inorganic catalysts.
• the radicals R 5 may each be identical or different and are independently selected from Ci-Cio-alkyl radicals, for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert.
• n-pentyl iso-pentyl, sec-pentyl, neo-pentyl, 1, 2-dimethylpropyl, iso-amyl, n-hexyl, sec-hexyl, n-heptyl, n-octyl, 2-ethylhexyl , n-nonyl or n-decyl, C3-Ci2 cycloalkyl, for example cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cycloundecyl and cyclododecyl; preferred are cyclopentyl, cyclohexyl and cycloheptyl.
• the radicals R 5 in Al (OR 5 ) 3 or Ti (OR 5 ) 4 are preferably identical and selected from isopropyl, butyl and 2-ethylhexyl.
• Preferred acidic organometallic catalysts are, for example, selected from dialkyltin oxides (R 5 ⁇ SnO wherein R 5 is as defined above.
• R 5 ⁇ SnO dialkyltin oxides
• a particularly preferred representatives of acidic organometallic catalysts di-n-butyltin oxide, which as so called oxo-tin or as Fascat ® Brands is commercially available.
• Preferred acidic organic catalysts are acidic organic compounds with, for example, phosphate groups, sulfonic acid groups, sulfate groups or phosphonic acid groups.
• Particularly preferred sulfonic acids contain at least one sulfonic acid group and at least one saturated or unsaturated, linear, branched and / or cyclic hydrocarbon radical having 1 to 40 carbon atoms and preferably having 3 to 24 carbon atoms.
• aromatic sulfonic acids especially alkylaromatic monosulfonic acids having one or more C 1 -C 8 -alkyl radicals and, in particular, those having C 3 -C 22 -alkyl radicals.
• Suitable examples are methanesulfonic acid, butanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, xylenesulfonic acid, 2-mesitylenesulfonic acid, 4-ethylbenzenesulfonic acid, isopropylbenzenesulfonic acid, 4-butylbenzenesulfonic acid, 4-octylbenzenesulfonic acid; Dodecylbenzenesulfonic acid, didodecylbenzenesulfonic acid, naphthalenesulfonic acid.
• Acid ion exchangers can also be used as acidic organic catalysts, for example poly (styrene) -sulfonic acid-containing polyols which are crosslinked with about 2 mol% of divinylbenzene.
• boric acid particularly preferred for carrying out the process according to the invention are boric acid, phosphoric acid, polyphosphoric acid and zinc chloride.
• boric acid and titanates of the general formula Ti (OR 5 ) 4 such as titanium tetrabutylate and T ⁇ tantetraisopropylat.
• acidic inorganic, organometallic or organic catalysts 0.01 to 10.0% by weight, preferably 0.05 to 5.0% by weight, for example 0.1 to 2.0% by weight, are used. % Catalyst based on the reaction mixture.
• the microwave irradiation is carried out in the presence of acidic solid catalysts.
• the solid catalyst is suspended in the ammonium salt optionally mixed with solvent or, in particular in the case of continuous processes, the optionally solvent-displaced ammonium salt is passed over a fixed bed catalyst and exposed to the microwave radiation.
• Suitable solid catalysts are zeolites, silica gel and montmorillonite and (partially) crosslinked polystyrenesulphonic acids, which may optionally be impregnated with catalytically active metal salts.
• Suitable acidic ion exchanger based on polystyrene sulfonic acids that can be used as solid phase catalysts are for example available from the company Rohm & Haas under the trademark Amberlyst ®.
• the presence of solvents is necessary.
• the starting materials are suspended and at least partially dissolved, which favors their implementation. Furthermore, this improves the removal of excess heat, for example by means of evaporative cooling.
• all solvents can be used which are inert under the reaction conditions used and do not react with the starting materials or the products formed.
• An important factor in the selection of suitable solvents is their polarity, which on the one hand determines the dissolving properties and on the other hand determines the extent of the interaction with microwave radiation.
• a particularly important factor in the selection of suitable solvents is their dielectric loss ⁇ . " The dielectric loss ⁇ " describes the proportion of microwave radiation which is converted into heat during the interaction of a substance with microwave radiation.
• solvents having ⁇ " values below 1 are aromatic and / or aliphatic hydrocarbons, such as, for example, toluene, xylene, ethylbenzene, tetralin, naphthalene, ethylnaphthalene, biphenyl, diphenyl ether, hexane, cyclohexane, decane, pentadecane, decalin and mixtures thereof, and commercial hydrocarbon mixtures such as petroleum fractions, kerosene, solvent naphtha, ® Shellsol AB, ® Solvesso 150 ® Solvesso 200 ® Exxsol, ® Isopar ® and Shellsol- types.
• aromatic and / or aliphatic hydrocarbons such as, for example, toluene, xylene, ethylbenzene, tetralin, naphthalene, ethylnaphthalene, biphenyl, diphen
• Solvent mixtures which have ⁇ " values preferably below 10 and especially below 1 are likewise preferred for carrying out the process according to the invention
• the process according to the invention is also possible in solvents having ⁇ " values of 10 and higher, but this requires special measures to maintain the maximum temperature and often leads to reduced yields.
• their proportion of the reaction mixture is preferably between 2 and 95 wt .-%, especially between 10 and 90 wt .-% and in particular between 20 and 80 wt .-%, such as between 30 and 70 wt .-%.
• the process is particularly suitable for the preparation of 1,4-bis (benzoxazol-2'-yl) benzene, 1,4-bis (benzoxazol-2'-yl) naphthalene, 4,4'-bis ( benzoxazol-2'-yl) stilbene, 4,4'-bis (5-methylbenzoxazol-2 1 -yl) stilbene, 1, 2-bis (5-methylbenzoxazol-2'-yl) -ethylene, and 2 , 5-bis- (benzoxazol-2'-yl) thiophene.
• dicarboxylic acid and o-aminophenol can be reacted with one another in any ratio.
• molar ratios between dicarboxylic acid and o-aminophenol 10: 1 to 1: 20, preferably from 2: 1 to 1: 5, especially from 1, 0: 2.2 to 1, 2: 2, 0 and in particular 1, 0: 2.0.
• o-aminophenol molar ratios of o-aminophenol to dicarboxylic acid of at least 2.01: 1, 00, such as between 2.1: 1, 0 and 10: 1 to work.
• the dicarboxylic acid is converted virtually quantitatively to Bisbenzoxazol. This process is particularly advantageous when the o-aminophenol used is volatile. Volatile here means that the amine, optionally under reduced pressure, can be separated by distillation from Bisbenzoxazol.
• Bisbenzoxazoles are prepared by reacting dicarboxylic acid and o-aminophenol to form the ammonium salt and subsequently irradiating the ammonium salt with microwaves.
• the ammonium salt is usually formed as an intermediate after the mixing of the starting materials, which may have been mixed with solvents and / or heated, partly also only during the heating of the suspension of the educts under microwave irradiation. It is preferably not isolated but used directly for further reaction.
• the temperature rise caused by the microwave irradiation is limited to a maximum of 320 ° C. by regulation of the microwave intensity and / or cooling of the reaction vessel.
• Proven particularly suitable to carry out the reaction at temperatures between 100 and 300 0 C and in particular 150-245 0 C, for example at temperatures between 170 and 23O 0 C. has
• the duration of the microwave irradiation depends on various factors such as the reaction volume, the geometry of the reaction space and the desired degree of conversion. To achieve a conversion of more than 70% and sometimes more than 80% such as more than 90%, the microwave irradiation is usually over a period of less than 200 minutes, preferably between 0.1 minutes and 180 minutes and especially between 1 and 90 minutes such as between 5 and 30 minutes.
• the intensity (power) of the microwave radiation is adjusted so that the reaction mixture reaches the desired reaction temperature in the shortest possible time. For the subsequent maintenance of the temperature, the reaction mixture with reduced and / or pulsed power be further irradiated.
• the reaction mixture In order to maintain the maximum temperature with maximum possible microwave irradiation, it has proven useful to cool the reaction mixture by means of cooling jacket, cooling tubes located in the reaction chamber by intermittent cooling between different irradiation zones and / or by boiling cooling via external heat exchangers.
• the reaction mixture is cooled as soon as possible after completion of the microwave irradiation to temperatures below 12O 0 C, preferably below 100 0 C and especially below 50 0 C.
• the reaction is carried out at pressures between 0.1 and 200 bar and especially between 1 bar (atmospheric pressure) and 100 bar.
• Pressures between 0.1 and 200 bar and especially between 1 bar (atmospheric pressure) and 100 bar.
• Working in closed vessels in which above the boiling point of the starting materials or products, of the optionally present solvent and / or above the reaction water formed during the reaction has worked is particularly useful.
• the pressure which builds up due to the heating of the reaction batch is sufficient to successfully carry out the process according to the invention.
• the method according to the invention is under atmospheric pressure, as it sets, for example, in an open vessel worked.
• an inert protective gas such as, for example, nitrogen, argon or helium.
• the microwave irradiation is usually carried out in devices which have a reaction space made of a material that is as far as possible transparent to microwaves, into which microwave radiation generated in a microwave generator is coupled by suitable antenna systems.
• Microwave generators such as the magnetron and the klystron are known in the art.
• Microwaves are electromagnetic waves having a wavelength between about 1 cm and 1 m and frequencies between about 300 MHz and 30 GHz. This frequency range is suitable in principle for the method according to the invention.
• microwave radiation with the frequencies of 915 MHz, 2.45 GHz, 5.8 GHz or 27.12 GHz released for industrial, scientific and medical applications is preferably used. It can be used both in mono or quasi monomode as well as in multimode.
• the reaction vessel to be irradiated microwave power is particularly dependent on the geometry of the reaction space and thus the reaction volume and the duration of the required irradiation. It is usually between 100 W and several 100 kW and in particular between 200 W and 100 kW such as between 500 W and 70 kW. It can be applied at one or more points of the reactor. It can be generated by one or more microwave generators.
• the reaction can be carried out batchwise or, preferably, continuously, for example in a flow tube. It can also be carried out in semi-batch processes such as continuously operated stirred reactors or cascade reactors.
• the reaction is carried out in a closed vessel, wherein the forming condensate and optionally starting materials and, if present, solvents lead to a pressure build-up. After completion of the reaction, the pressure can be reduced by relaxation to volatilize and separate water of reaction and optionally solvent and excess reactants and / or cooling of the Reaction product can be used.
• the process according to the invention can likewise be carried out successfully in an open vessel with boiling-cooling and / or removal of the water of reaction.
• the process according to the invention is carried out in a discontinuous microwave reactor.
• the microwave irradiation is carried out in a stirred vessel.
• cooling elements such as cold fingers or cooling coils or flanged to the reaction vessel reflux condenser for Siedekühlung the reaction medium.
• the microwave is here preferably operated in multimode.
• the discontinuous embodiment of the method according to the invention allows rapid as well as slow heating rates by varying the microwave power and in particular maintaining the temperature for longer periods of time such as several hours.
• the reactants and, if appropriate, solvents and further auxiliaries can be initially introduced into the reaction vessel before the beginning of the microwave irradiation.
• the dicarboxylic acid is suspended by stirring before the addition of o-aminophenol in the solvent, preferably at temperatures above 50 0 C, for example, between 100 0 C and 150 0 C by stirring or brought into solution.
• the reactants and solvents or parts thereof are supplied to the reaction vessel only during the irradiation with microwaves.
• the discontinuous microwave reactor is operated with continuous feed of educts optionally suspended or dissolved in solvent and simultaneous discharge of reaction material in the form of a semi-batch or cascade reactor.
• the process according to the invention is carried out in a continuous microwave reactor.
• reaction mixture is passed through a pressure-resistant reaction tube which is inert to the reactants and largely transparent to microwaves and which is installed in a microwave oven.
• This reaction tube preferably has a diameter of one millimeter to about 50 cm, preferably between 2 mm and 35 cm and especially between 5 mm and 15 cm, for example between 10 mm and 5 cm.
• Reaction tubes are understood here to be vessels whose ratio of length to diameter is greater than 5, preferably between 10 and 100,000, particularly preferably between 20 and 10,000, for example between 30 and 1,000.
• the reaction tube is configured in the form of a double-walled tube, through the interior and exterior space of which the reaction mixture can be passed successively in countercurrent, for example to increase the temperature control and energy efficiency of the process.
• the length of the reaction tube is to be understood as the total flow through the reaction mixture route.
• the reaction tube is surrounded over its length by at least one, but preferably several such as, for example, two, three, four, five, six, seven, eight or more microwave radiators.
• the microwave radiation preferably takes place via the tube jacket.
• the microwave irradiation takes place by means of at least one antenna via the tube ends.
• the reaction tube is usually provided at the inlet with a metering pump and a pressure gauge and at the outlet with a pressure-holding valve and a heat exchanger.
• the reaction tube mixing or conveying elements such as augers, feed screws or static mixers may contain.
• the educts o-aminophenol and dicarboxylic acid, the latter preferably diluted with solvent, are preferably mixed only shortly before they enter the reaction tube. With further preference the educts are fed to the process according to the invention in liquid form.
• the reaction conditions are adjusted so that the maximum reaction temperature is reached as quickly as possible and the residence time at maximum temperature so short remains that as few incidental or subsequent reactions occur as possible.
• the continuous microwave reactor is preferably operated in monomode or quasi-monomode.
• the residence time in the reaction tube is preferably between 0.1 second and 90 minutes, more preferably between one second and 60 minutes and in particular between 10 seconds and 30 minutes, such as between 20 seconds and 10 minutes.
• the reaction mixture can be run through the reactor several times to complete the reaction, optionally after intermediate cooling. It has proven particularly useful if the reaction product immediately after leaving the reaction tube z. B. is cooled by jacket cooling or relaxation.
• bisbenzoxazoles prepared by the process according to the invention are obtained as crystal suspension and can be separated by filtration and if appropriate, separate washing with solvent in a purity which is sufficient for further use.
• An optional multiple recrystallization or reprecipitation is usually not required ..
• they can be further purified by conventional purification methods such as recrystallization, optionally in the presence of bleaching agents such as bleaching earth or activated carbon, falling over or by chromatographic methods.
• the process according to the invention makes it possible to produce the bisbenzoxazoles interconnected in a high yield and with high purity by means of a conjugated double bond system in a very rapid and cost-effective manner.
• the bisbenzoxazoles prepared according to the invention are particularly suitable as
• Dyes, UV absorbers and optical brighteners for natural, synthetic and semi-synthetic fibers, plastics and paper Dyes, UV absorbers and optical brighteners for natural, synthetic and semi-synthetic fibers, plastics and paper.
• the reactions under microwave irradiation were carried out in a single mode microwave reactor of the "Discover" type from CEM at a frequency of 2.45 GHz Cooling of the reaction vessels was carried out by means of compressed air Temperature measurement had to take place due to the pressure conditions in the reaction vessels via an IR sensor take place at the bottom of the cell. Comparative tests with a dipping into the reaction mixture of glass fiber optics, it was found that the temperature in the reaction medium in the temperature range relevant here about 50 to 8O 0 C above the temperature measured by the IR sensor at the base of the cuvette.
• Ammonium salt was pumped through the inlet tube into the cuvette and the residence time in the irradiation zone was adjusted by modifying the pumping capacity.
• the determination of the reaction conversion was carried out by means of HPLC by determining the content of target product against a calibration curve recorded with reference substance.
• the HPLC separation was carried out on a RP-column (Nucleodur 100-5 C18 ®) with a solvent system consisting of acetonitrile, isopropanol and water in the ratio 45:45:10.
• the detection was carried out via a UV detector at 254 nm. Water determinations were carried out by Karl Fischer titration.
• reaction mixture was cooled over a short Liebig condenser at 100 0 C, followed -naphthalene crystallized out on further slow cooling 1, 4-bis (benzoxazol-2'-yl) in the form of yellow needles.

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