FRIEDEL-CRAFT PROCESS FOR THE PREPARATION OF THIOXANTHONES

Abstract: This invention is directed to a process of preparing thioxanthones, represented by Formula 1, by a Friedel-Craft reaction giving higher yields and lower amount of waste, comprated to known processes. This invention also involves purification of the starting material, a 2-chlorothiobenzodyl chloride, by distillation.

Description: Friedel-Craft process for the preparation of Thioxanthones Technical field of the invention The invention relates to a process for the preparation of thioxanthen-9-one derivatives, of the following thioxanthones, of the Formula 1. Formula 1 Background of the invention Thioxanthones are useful intermediates for the preparation of pharmaceuticals in the field of psychoterapeutics. Another important use is as activators or sensitizers in the photopolymerisation of ethylenically unsaturated monomers. Basically thioxanthones are formed by cyclisation of 2-phenylthiobenzoic acid derivatives. As described by Smiles , J. Chem. Soc. (1911), 99, 645, this reaction can be performed in one step, by reacting 2,2'-dithiodibenzoic acid with an aromatic compound in sulfuric acid. This invention has been the most utilised for producing thioxanthones in an industrial scale. The main drawbacks of the process is that yields are poor, 40 - 60%, a large excess of sulfuric acid must be used, after reaction a large amount of diluted sulfuric acid has to be regenerated or disposed. Sulfonated aromatics are major biproducts. For that reason 2-chlorothiobenzoyl chloride (CTBC) or derivatives hereof has been considered as an obvious choice for a Friedel-Craft type reaction to form thioxanthones. This reaction is first revealed in patent US4101558 where 2-chlorothiobenzoyl - and 5-chloro-2-chlorobenzoyl chloride is reacted with benzene and chlorobenzene to form thioxanthone, 7-chlorothioxanthone and 2,7-dichlorothioxanthone with aluminium chloride as catalyst. In the examples of preparation, CTBC and the aromatic substrate are mixed in a suitable solvent and aluminium chloride is added in small portions giving rise to product yields from 70 - 91%. The patent claims do not specify in which order the Friedel-Craft catalyst and the two reactants are brought together. Belleau et al., Synth. Commun., (1983), 13, 977 - 984, have investigated different types of Lewis acids as catalyst to form thioxanthones from CTBC and ortho-/para- xylene or 1,4-dimethoxy substituted aromatic substrates. They found that in this case of activated aromatic substrates tin(IN)chloride was the most effective catalyst. The experimental section describes that CTBC in a chlorinated solvent is mixed with tin(IV) chloride, where after the substrate is added in one portion. Disclosure of invention It is an object of this invention to provide a facile process for the preparation of thioxanthones, specially 2-isopropyl- and 2-chlorothioxanthone in commercial quantities, good yield and high purity. To avoid the problems connected to the method using sulfuric acid as medium for reaction of 2,2'-dithiodibenzoic acid with an aromatic substrate, the reaction between CTBC and an aromatic substrate under Friedel-Craft conditions were pursued according to Reaction Schemel Reaction Scheme 1 CTBC is prepared by known methods where 2,2'-dithiodibenzoic acid or substituted derivatives hereof is transformed to the acid chloride by reaction with thionyl chloride or a similar reagent. The reaction is facilitated by using thionyl chloride as solvent and addition of a catalyst of the amide type like dimethylformamide or N-methyl-2-pyrrolidinon. The acid chloride is chlorinated with chlorine or sulfuryl chloride revealing CTBC. As CTBC where R t = R 2 = H also is an important starting material for the industrial production of the biocide, l,2-benzisothiazolin-3-one, investigations were done for purification of CTBC. Even the literature does not disclose examples of distilling the product, it was shown that this could be done by a vacuum distillation at, 165 - 170°C and 10 mm Hg, giving a light yellow product with m.p. 67 -68°C, from the brown crude starting material. CTBC was found to be heat sensitive by prolonged heating to about 200°C. To avoid decomposition in industrial scale, distillation should preferably be performed under mild conditions like falling- or wiped film evaporations. Experiments with CTBC wherein Rj = R 2 = H and substrate isopropylbenzene (cumene) wherein R 3 = i-C 3 H 7 , R = R 5 = H in 1,2-dichloroethane (DCE) as solvent and aluminium chloride as catalyst, were performed according to the conditions in US4101558. The content of isomers 2- and 4-isopropylthioxanthone in the organic phase were found to be 70% of theory, by HPLC, and 50% as isolated yield. The reaction could not be performed as described by Belleau et al. as CTBC disproportionated with aluminium chloride in DCE. Using the substrate, cumene, as solvent with aluminium chloride and adding CTBC to the slurry gave rise to formation of an insoluble compound, which probably is a reaction product from 1 mole CTBC with 2 moles cumene. Surprisingly it turned out that the reaction could be performed if a mixture of CTBC and cumene, with a small molar excess of cumene, in DCE, were added to a slurry of aluminium chloride, in small molar excess, and DCE. After water quench the total yield of 2- and 4-thioxanthone isomers in the organic phase were 89% of theory, by calibrated HPLC. By performing the process in the described way handling aluminium chloride in industrial scale has been facilitated, as the catalyst has not to be added during the process. The process has been extended to thioxanthones, according to Reaction Scheme 1, where Ri is different from, or equals R 2 both are represented by hydrogen, chlorine, bromine, alkyl, aryl and alkoxy radicals having 1 to 8 carbon atoms. R 3 ,Rt and R 5 can equals one another by pair or all three, or be different and be represented by hydrogen, chlorine, bromine, hydroxyl, alkyl, cycloalkyl, aryl, alkoxy, alkylcarbonylalkoxy, carbonylalkyl, carbonylalkoxy, carbamido or R 3 and Ε can form a 5, 6 or seven member ring fused to the aromatic ring. and R 5 can be radicals having 1 to 10 carbon atoms. Specific types of substituted t ioxanthone are 2-chloro, 4-chloro, 2-bromo, 4-bromo, 2,7-dicbloro, 2- hydroxy, 2-methyl, 4-methyl, 2,4-diethyl, 2-isopropyl, 2-carboethoxymethyl. The aromatic compound may include polynuclear aromatic compounds as biphenyl as naphthalene. The process is carried out in an organic solvent which are particularly known to be useful for Friedel-Craft reaction such as methylene chloride, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, etc. To perform the reaction, known Friedel-Craft catalyst as aluminium chloride, aluminium bromide, iron (III) chloride, tin(LT) chloride, tin(IN) chloride, iron(III) chloride can be used. Specially aluminium chloride. The reaction may preferably be carried out using substitutet or unsubstitutet CTBC : aromatic substrate : Friedel-Craft catalyst in the molar ratio 1 : 1 : 1 — 1 : 20 : 5. The molar ratio of said components is particularly advantageously as 1 : 1,2 : 1,6. The reaction may be performed at a temperature between 0°C and 80°C. Preferably 25°C. The method of isolating the thioxanthones from the organic reaction media may vary depending on the physical and chemical nature of the product. If the complex between the product and aluminium chloride is sparingly soluble, it can be an advantage to isolate the complex before decomposition. Preferably decomposition of the complex can be accomplished by adding the reaction mixture to water, diluted mineral acid or aqueous solutions of alkaline hydroxides like sodium, potassium, barium or calcium. The choice of decomposition medium depends on the base or acid nature of the product and of insoluble by-products, keeping the product in the organic phase and by-products in the water phase. Crude thioxanthones are isolated by evaporation of the solvent, and if necessary purified by methods as crystallisation, chromatographi etc. Examples Reaction mixtures and products were analysed by HPLC chromatographi. Specific compounds were characterised by comparison with authentic samples. HPLC conditions: Example 1 2-Chlorothiobenzoyl chloride (CTBC) 2,2'-dthiodibenzoic acid, assay 95%, 100 g, 0.3105 moles, and l-methyl-2- pyrrolidinone, 1.0 g, 0.010 moles, was added to thionyl chloride 260 g, 2.185 moles. The stirred slurry was heated slowly to reflux, 80°C, until it has become a clear solution and HCl evolution had ceased. The mixture was cooled to 50°C and sulfuryl chloride, 48.6 g, 0.3600 moles, was slowly introduced. Stirring was continued for 30 min, where after volatiles were evaporated off on a rotary evaporator leaving crude, CTBC, 140.0 g, as a light brown solid. The product was vacuum distilled through a short Nigreux column at 165 -170°C, 10 mmHg, affording CTBC, 119.5g, 0.5773 moles, as a light yellow crystalline product in 93% yield of theory. Purity was estimated to 98.5 % by derivatisation with ammonia to l,2-benzisothiazolin-3-one and running HPLC of the solution. Example 2 2-/4-isopropylthioxanthone (ITX) Crude CTBC, 95% assay, 7.6 g, 0.0349 moles, and cumene, 4.6 g, 0.0383 moles, were dissolved in 1,2-dichloroethane (DCE), 70 g. This solution was added over 45 min to a stirred slurry of aluminium chloride, 7.3 g, 0.0548, in DCE, 50 g covered with nitrogen, with temperature kept at 20 - 25°C. Temperature was raised to 30°C for 30 min, where after the reaction mixture was poured into 5% hydrochloric acid, 100 mL, with vigorous stirring. The organic phase was separated and washed once with water. HPLC on the organic phase revealed a content of 7.5 g, 0.0295 moles ITX. Corresponding to 85% yield of theory. Nolatiles were evaporated off in vacuo leaving crude ITX as an orange oil, 9.44 g, which was crystallised from methanol, 28 g, leaving ITX, 6.11 g, 0.0240 moles, 69% yield of theory, as light yellow crystals, with purity, by HPLC, 97.3% and m.p. 73 - 74°C. Example 3 ITX from distilled CTBC The experiment in Example 2 was repeated using distilled CTBC. Results were nearly similar, except for purity, which was estimated, by HPLC, to 99.2%. Example 4 2-/4-chlorothioxanthone (CTX Crude CTBC, 95 % assay, 0.0481 moles, and chlorobenzene, 6.7 g, 0.0588 moles, dissolved in DCE, 16 g, was added over 30 min to a stirred slurry of aluminium chloride in DCE, 150 g, covered with nitrogen, keeping temperature at 40 -^Φ5°C. The reaction mixture was stirred for a further 30 min at 40°C and poured into stirred water keeping pH at 12 with 50 w/w% sodium hydroxide. The DCE phase was separated and washed once with water. HPLC run revealed a content of 11.1 g, 0.0450 moles of CTX corresponding to 94% yield of theory. The solvent was evaporated on rotary evaporator to a weight of 20.1 g. Separated crystals was isolated, washed with a little ethanol and dried affording CTX, 8.6 g, 0.0349 moles, with purity, by HPLC, 98.8% and m.p. 143 - 145°C. Yield of theory was 73%.

Claims: Claims 1. Process for the preparation of thioxanthones represented by Formula 1. Formula 1 2. Process according to claim 1 which comprises adding a mixture of nearly equal molar amounts of 2-chlorothiobenzoyl chlorides represented, by Formula 2, Formula 2 and an aromatic compound represented by Formula 3. Formula 3 to a slurry of an effective amounts of a Friedel-Craft catalyst. 3. Process according to claim 2 which comprises use of a Friedel-Craft catalyst presented in the amount of 1.0 to 2.5 moles for each moles of the 2-chlorothiobenzoyl chloride represented by Formula 2. 4. Process according to claim 2 which comprises use of a Friedel-Craft catalyst presented by aluminium chloride, aluminium bromide, iron (IH) chloride, tin(TI) chloride, tin(IN) chloride or iron(H[) chloride. Preferably aluminium chloride. 5. Process according to claim 2which comprises use of a Friedel-Craft catalyst specially presented in the amount of 1.1 to 1.6 moles for each moles of the 2-chlorothiobenzoyl chloride represented by Formula 2. 6. Process according to claim 2which comprises the 2-chlorothiobenzoyl chloride according to Formula 2, where Ri is different from, or equals R 2 both are represented by hydrogen, chlorine, bromine, alkyl, aryl and alkoxy radicals having 1 to 8 carbon atoms. 7. Process according to claim 2which comprises the 2-chlorothiobenzoyl chloride according to Formula 2 where preferably 8. Process according to claim 2which comprises the aromatic substrate represented by Formula 3 where and R5 can equals one another by pair or all three, or be different and be represented by hydrogen, chlorine, bromine, hydroxyl, alkyl, cycloalkyl, aryl, alkoxy, alkylcarbonylaUcoxy, carbonylalkyl, carbonylalkoxy, carbamido or R 3 and R 4 can form a 5, 6 or seven member ring fused to the aromatic ring. and R5 can be radicals having 1 to 10 carbon atoms. 9. Process according to claim 2which comprises preferably the aromatic substrate represented by formula 3 where R 3 = isopropyl, R 2 =R 3 =H. 10. Process according to claim Iwhich comprises the thioxanthone according to formula 1 where Ri is different from, or equals R 2 both are represented by hydrogen, chlorine, bromine, alkyl, aryl and alkoxy radicals having 1 to 8 carbon atoms. R 3 , j and R5 can equals one another by pair or all three, or be different and be represented by hydrogen, chlorine, bromine, hydroxyl, alkyl, cycloalkyl, aryl, alkoxy, alkylcarbonylalkoxy, carbonylalkyl, carbonylalkoxy, carbamido or R 3 and - t can form a 5, 6 or seven member ring fused to the aromatic ring. R 3 ,Rf and R 5 can be radicals having 1 to 10 carbon atoms. 11. Process according to claim Iwhich comprises the thioxanthone preferably to be a mixture of 2- and 4-isopropylthioxanthone. 12. Process according to claim Iwhich comprises the thioxanthone preferably to be a mixture of 2- and 4-chlorothioxanthone. 13. Process according to claim Iwhich comprises the thioxanthone preferably to be 2,4-diethylthioxanthone. 14. Process according to claim Iwhich comprises the thioxanthone preferably to be 2-chloro, 4-chloro, 2-bromo, 4-bromo, 2,7-dichloro, 2-methyl, 4-methyl, 2-isopropyl or 2-carboethoxymethyl. 15. Process according to claim Iwhich comprises the thioxanthone preferably to be include polynuclear aromatic moiety as biphenyl as naphthalene. 16. Process according to claim 2which comprises the process to be carried out in an organic solvent which are particularly known to be useful for Friedel- Craft reaction such as methylene chloride, 1,2-dichloroethane or 1 , 1 ,2,2-tetrachloroethane. 17. Process according to claim 2which comprises the process to be carried out preferably with 1,2-dichloroethane as solvent. 18. Process according to claim 2which comprises the process to be carried out by dissolving the reactants represented by Formula 2 and 3 in 1 to 30 parts of solvent of claims 16 and 17. 19. Process according to claim 2which comprises the process to be carried out by forming a slurry between the catalyst of claim 3 in 1 to 20 parts solvent of claims 16 and 17. 20. Process according to claim 2which comprises the process to be carried out by mixing the reactants represented by Formula 2 and 3 in the molar proportion 1 : 1 to 1 : 20. 21. Process according to claim 2which comprises the process to be carried out according to claim 20 where the molar proportion between reactants represented by Formula 2 and 3 specially are 1 : 1.1 to 1 : 1.2 22. Process according to claim 2which comprises the process to be carried out at a temperature from 0 to 80°C. 23. Process according to claim 2which comprises the process to be carried out at a temperature specially between 20 and 45°C. 24. Process according to claim 2which comprises the process to be carried out where the 2-chlorothiobenzoyl chloride represented by Formula 2 is purified by a vacuum distillation at a temperature from 80 - 200°C and a pressure of 0.1 to 300 mm Hg. 5. Process according to claim 2which comprises the process to be carried out where the 2-chlorothiobenzoyl chloride specifically is represented by Formula 2 where Ri = R 2 = H is purified by a vacuum distillation at a temperature from 165 to 170°C and a pressure of 10 mm Hg.

Inventors: BERG CARSTEN, BERG, CARSTEN
Applicants: BERG CARSTEN, PROM LTD, BERG, CARSTEN, PROM LIMITED
Application year: 2000
Main classification: C07D335
IPC codes: C07B61, C07D335