PROCESS FOR THE PREPARATION OF HALOGENATED (2,2)-PARACYCLOPHANES

Abstract: A process for the preparation of halogenated (2,2)-paracyc­ lophanes by Hofmann elimination in an alkaline medium from p-­ methylbenzyltrimethylammonium hydroxide halogen-substituted in the nucleus.

Description: The present invention relates to a process for the preparation of halogenated derivatives of (2,2) paracyclophane and to mixtures of the resultant halogenated (2,2)-paracyclophanes having the desired substitution degree and a high purity degree. The halogenated derivatives of (2,2)-paracyclophane having general formula : in which X is a halogen and in particular chlorine, and n is an integer from 1 to 4, and in particular dichloro-(2,2)-paracyclophane and tetrachloro-(2,2)-paracyclophane, are well known in literature and are generally utilized as intermediates in the preparation of the corresponding chlorinated poly-p-xylylene. Said halogenated derivatives of (2,2)-paracyclophane, and in particular the chlorinated derivatives of said compound, are usefully employed in the preparation of films in the field of the conformal coating by application according to the technique based on vacuum vapour deposition and polymerization in situ. For this kind of application, as is well known, the halogenated starting products must comply with some characteristics, namely : a) they must be pure, i.e. they must consist of compounds having all the desired substitution degree, in order to obtain a satisfactory homogeneity of the films, and b) the isomeric composition of the fractions, at the same substitution degree, must be constant, in order to have the same applicative conditions and the same chemical-physical characteristics of the obtainable films. The method generally followed for the preparation of the halogenated derivatives of (2,2)-paracyclophane is that of subjecting to halogenation the previously formed (2,2)-paracyclophane, which is obtained by carrying out the Hofmann elimination from p.methylbenzyltrimethylammonium hydroxide, of formula : By this halogenation process carried out on previously formed (2,2)-paracyclophane it is not possible to prevent the generation of appreciable amounts of undesirable products. In particular, in the case of a dichlorination process, there are obtained not only the desired dichlorinated isomers of (2,2)-paracyclophane containing one chlorine atom on each ring, but also the undesired isomers having two chlorine atoms on one ring only and those with only one chlorine atom and/or with three chlorine atoms. These by-products lead, during the polymerization step, to the formation of copolymers of p-xylylene, chloro-p-xylylene and dichloro-p-xylylene having a high non-homogeneity in the chlorine distribution on the rings of the polymeric chains. Furthermore, from these isomer mixtures it is impossible, by means of the conventional separation techniques, to obtain only the di-halogenated isomers having an equal number of halogen atoms on each ring. It has now been found by the Applicant that halogenated isomers of (2,2)-paracyclophane, having the same total halogen content and the same number of halogen atoms in the individual rings,are obtainable by Hofmann elimination,in an alkaline aqueous solution,from p-methylbenzyltrimethylammonium hydroxide halogen-substituted in the nucleus, of formula : in which X is a halogen atom, in particular chlorine, and n is an integer from 1 to 4, in which the concentration of alkaline hydroxide in the aqueous solution is maintained lower than 40% by weight and preferably constant during the elimination reaction. It is specifically pointed out that the alkaline solution concentration is an indispensable parameter for obtaining, with industrially acceptable yields, i.e. higher than 25% by moles, the halogenated isomers of (2,2)-paracyclophane having the same number of halogen atoms in the individual rings and the same total halogen content. In fact, if Hofmann elimination is conducted in an alkaline medium at a higher concentration, poly-halogen-p-xylylenes are essentially formed, as is described in British patent No. 807,196. The halogen-substituted p-methylbenzyltrimethylammonium hydroxide of formula (III) can be prepared starting from the corresponding halide according to any known process; thus, for example, it can be formed in situ by reacting an alkaline hydroxide with the corresponding halide, or it can be pre-formed by eluting an aqueous solution of the halide through a basic ion-exchange resin column, or by treatment with silver oxide. The p-methylbenzyltrimethylammonium halide containing one or more halogen atoms in the nucleus is known in literature and can be prepared by reaction of trimethylamine with p-methylbenzyl chloride halogenated in the nucleus, as is described in British patent No. 807,196, according to the scheme : According to the invention, the Hofmann elimination is conducted on halogen-substituted p-methylbenzyltrimethylammonium hydroxide of formula (III), at a temperature from 50° to 130°C and for a time comprised between 0.5 and 20 hours, in the presence of an aqueous solution of an alkaline hydroxide, such as potassium hydroxide or sodium hydroxide, the concentration thereof being maintained below 40% by weight and preferably from 25 to 35%. Furthermore it is preferred to maintain constant such concentration during the elimination step. Various measures can be taken to this purpose; generally it is preferable to remove the water in excess by distillation. Reaction times from 0.5 to 10 hours and temperatures ranging from 60° to 120°C are preferred. The Hofmann elimination can be also advantageously conducted in the presence of an inert organic solvent, such as e.g. toluene, xylene, benzene and dimethylsulfoxide In the case of dihalogenation of (2,2)-paracyclophane, the process of the present invention permits to obtain, with industrially acceptable yields, i.e. higher than 25% by moles, a mixture of dihalogen-substituted isomers having a halogen on each ring and a purity degree higher than 99%, and in particular free from mono- and/or tri-halogen derivatives. In particular, through the process according to the present invention, when the Hofmann elimination is accomplished from the chloro-derivative of p-methylbenzyltrimethylammonium hydroxide, it is possible to obtain an isomer mixture (mixture A) having the following composition : 30-50% by weight of 5,12-dichloro(2,2)-paracyclophane; 10-30% by weight of 5,11-dichloro(2,2)-paracyclophane; 20-40% by weight of 5,14-dichloro(2,2)-paracyclophane, and 5 -15% by weight of 5,13-dichloro(2,2)-paracyclophane, wherein the sum of the isomers is equal to 100 and the dichloro-derivatives content is higher than 99%. It is expressly pointed out that 5,13-dichloro(2,2)-paracyclophane is not obtainable in appreciable amounts through the conventional chlorination processes of (2,2)-paracyclophane. From this mixture (A) it is possible to prepare, by means of a proper separation technique, for example fractionated crystallization, various mixtures (B) and (C) having a different isomeric composition, such as : Mixture B 40-65% by weight of 5,12-dichloro(2,2)-paracyclophane, 30-55% by weight of 5,14-dichloro(2,2)-paracyclophane, 1 -20% by weight of 5,13-dichloro(2,2)-paracyclophane, and 0 -10% by weight of 5,11-dichloro(2,2)-paracyclophane; in which the sum of the isomers is equal to 100 and the dichloro-derivatives content is higher than 99%. Mixture C 65-99% by weight of 5,11-dlchloro(2,2)-paracyclophane and 1-35% by weight of 5,13-dichloro(2,2)-paracyclophane. Furthermore, since these mixtures, which are applicated by means of the vapour vacuum deposition technique, have different chemical-physical characteristics, they are particularly suitable for specific fields of use. For example, isomeric mixture C permits to obtain films endowed with a high dielectric strength as is required in the ferrite coating. It is expressly pointed out that said mixtures A, B and C are not obtainable through the processes of the art which comprise a direct halogenation on the preformed (2,2)-paracyclophane. The present invention is further clarified by the following examples, examples 2 and 3 do not illustrate the invention as claimed. In the following examples, all parts, percentages and ratios are by weight, unless otherwise indicated. Example 1 Into a 1 liter reactor, equipped with a stirrer, a thermometer and a condenser, there were charged, at room temperature : 50 g of chloro-p-methylbenzyltrimethylammonium chloride of formula : 87 g of water, 43 g of NaOH and 500 ml of xylene. The reaction mass was brought to 110°C in about 3 hours and it was maintained at the boiling temperature for further 5 hours. In the course of boiling, water was distilled while maintaining the concentration of NaOH in the aqueous solution at about 33% by weight. The reaction mass was filtered at 80°C thus obtaining, after washing with xylene and water and drying, about 15.1 g of poly-chloro-p-xylene. The filtrate consisted of an alkaline aqueous phase and of a xylene solution, which were separated. The xylene solution was repeatedly washed with water up to the full neutrality of the washing water, and subsequently it was subjected to distillation in order to remove the xylene. 13.1 g of an oily mixture were obtained. The mixture was made to flow through a liquid-liquid chromatography column, consisting of 200 g of SiO₂ and containing hexane as an eluent. From the eluted hexane solution there were obtained, after removal of the solvent, 8.8 g (molar yield: about 30%) of a mixture (A) of isomers of dihalogenated (2,2)-paracyclophane of general formula : which, on gas-chromatographic analysis, revealed the following isomeric composition : 21% by weight of 5,11-dichloro-(2,2)-paracyclophane, 39% by weight of 5,12-dichloro-(2,2)paracyclophane, 10% by weight of 5,13-dichloro-(2,2)-paracyclophane, and 30% by weight of 5,14-dichloro-(2,2)-paracyclophane. The structure of the individual isomers was determined by NMR analysis. The purity degree, measured by gas-chromatography, was higher than 99%. Example 2 8.8 g of the isomer mixture obtained in example 1 were hot treated with 100 ml of hexane to boiling. After cooling, the mixture was filtered and the solid portion was dried, so obtaining 1.87 g of a mixture (C) which , on gas-chromatographic analysis, revealed the following isomeric composition : 85.7% by weight of 5,11-dichloro-(2,2)-paracyclophane, and 14.3% by weight of 5,13-dichloro-(2,2)-paracyclophane. The purity degree was higher than 99%. From the filtrate there were obtained, by distillation of the solvent, 6.93 g of a mixture (B) which, on gas-chromatographic analysis, revealed the following isomeric composition : 3.6 % by weight of 5,11-dichloro-(2,2)-paracyclophane, 49.5 % by weight of 5,12-dichloro-(2,2)-paracyclophane, 8.8 % by weight of 5,13-dichloro-(2,2)-paracyclophane, and 38.1 % by weight of 5,14-dichloro-(2,2)-paracyclophane. The purity degree was higher than 99%. Example 3 Example 1 was repeated till obtaining 13.1 g of an oily mixture. This mixture was treated with 100 ml of hexane to boiling temperature. After cooling, the solid portion was filtered,so obtaining a dichloro-(2,2)-paracyclophane mixture in an amount and with an isomeric distribution comparable with those of mixture (C) of example 2. The filtrate was eluted with hexane in a SiO₂ column following the same operative modalities of example 1. Recovered was a mixture of dichloro-(2,2)-paracyclophanes in an amount and with an isomeric distribution comparable with those of mixture (B) of example 2.

Claims: A process for preparing halogenated isomers of (2,2)-paracyclophane by Hofmann elimination, in an alkaline aqueous solution, from p -methylbenzyltrimethylammonium hydroxide halogen-substituted in the nucleus, of formula : in which X is a halogen atom, in particular chlorine, and n is an integer from 1 to 4, in which the concentration of the alkaline hydroxide in the aqueous solution is maintained lower than 40% by weight and preferably constant, during Hofmann elimination reaction. The process of claim 1, wherein the concentration of the alkaline hydroxide ranges from 25 to 35% by weight. The process of claim 1 or 2, wherein the concentration of the alkaline hydroxide is maintained constant in the course of Hofmann elimination process. The process of any of the preceding claims, wherein p-methylbenzyltrimethylammonium hydroxide halogen-substituted in the nucleus having general formula (III) is formed in situ by reaction of an alkaline hydroxide on the corresponding halide. The process of any of the preceding claims, wherein Hofmann elimination is conducted at a temperature from 50° to 130°C and for a time of 0.5-20 hours, preferably of 0.5-10 hours.

Inventors: BERETTA, MAURIZIO AUGUSTO, MALACRIDA, ALESSANDRO, PREGAGLIA, GIAN FRANCO
Applicants: MONTEDISON S.P.A.
Application year: 1986
Main classification: C07C25
IPC codes: C07C25, C07C17, C23C14, C07C67