Preparation of diesters of hexenedioic acid
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the preparation of diesters of hexene-1,6-dioic acid. These diesters can be hydrogenated to the corresponding diesters of adipic acid or adipates, which can in turn be hydrolyzed to produce adipic acid. (Adipic acid is one of the basic raw materials for nylon 66; for this reason alone, any novel process for the ultimate synthesis of such diacid and/or derivative thereof is of fundamental worth). 
     The present invention more especially relates to the preparation of diesters of 3-hexene-1,6-dioic acid by reacting carbon monoxide and an alcohol with at least one butene compound containing two acyloxy substituents in the presence of a catalyst based on palladium. 
     2. Description of the Prior Art 
     The preparation of a monoester of an alcohol and of a butenoic acid by reacting carbon monoxide and an alcohol with a monoester of an allyl alcohol, in the presence of a palladium-based catalyst, is described in U.S. Pat. No. 3,367,961. Ethyl vinylacetate can thus be prepared from allyl acetate, carbon monoxide and ethanol in the presence of palladium chloride and of palladium deposited on charcoal. 
     In U.S. Pat. No. 4,611,082 it is also indicated that the carbonylation of a solution of 1,4-diacetoxy-2-butene in a polar and nonbasic aprotic solvent selected from among the nitriles, bis(2-methoxy)-2-butene, bis(2-methoxyethyl) ether and methylene chloride, at 80° to 140° C. in the presence of a transition metal halide, is not observed in practice and that, in the presence of an alcohol, the rates increase and are comparable with those observed in the case of the carbonylation of 2-butene-1,4-diol. Concerning said latter substrate, it is also indicated that it does not enable satisfactory yields of linear carbonylation products to be expected under the aforementioned conditions and, in this context, substrates substituted by alkoxy groups in the 1,4-position are the preferred. 
     Moreover, 1,4-diacetoxy-2-butene is easily prepared by the acetoxylation of butadiene. It would, therefore, be highly desirable to provide this art with a process making it possible to produce, with a high efficiency, the diesters of 3-hexene-1,6-dioic acid from 1,4-diacetoxy-2-butene, for example, and, more generally, from butenes disubstituted by acyloxy groups. 
     SUMMARY OF THE INVENTION 
     Accordingly, a major object of the present invention is the provision of a novel process for the preparation of diesters of 3-hexenedioic acid by reacting carbon monoxide and an alcohol with at least one butene compound containing two acyloxy substituents, in the presence of a catalyst based on palladium, and characteristically wherein the reaction is also conducted in the presence of a quaternary onium halide of an element from Group VB of the Periodic Table selected from among nitrogen and phosphorus, said element being tetracoordinated to carbon atoms, with the proviso that the nitrogen may be coordinated to two pentavalent phosphorus atoms, and the halide anion is either the chloride or bromide. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     More particularly according to the present invention, it has now surprisingly and unexpectedly been determined that the subject dicarbonylation reaction may be carried out under conditions of pressure and temperature which are acceptable on an industrial scale, with an appreciable selectivity for a linear dicarbonylated final product, with the proportions of monocarbonylated product and of branched dicarbonylated products being minimal. 
     The process according to the invention can be represented by the following reaction scheme, beginning with a 2-butene compound containing two acyloxy groups in the 1,4-positions: ##STR1## in which R is a linear or branched chain alkyl radical containing from 1 to 12 carbon atoms, optionally substituted by a phenyl group, or an aryl radical containing from 6 to 10 carbon atoms, optionally substituted by one or two alkyl radicals containing from 1 to 4 carbon atoms, with the proviso that such alkyl or aryl radicals may contain from 1 to 3 substituents selected from among fluorine and chlorine atoms and dialkylamino and N,N-dialkylamido radicals, the alkyl moieties of which containing not more than 4 carbon atoms; and R&#39;, which may be identical to or different from R, is a linear or branched chain alkyl radical containing from 1 to 12 carbon atoms, optionally substituted by a phenyl group, or an aryl radical containing from 6 to 10 carbon atoms, optionally substituted by one or two alkyl radicals containing from 1 to 4 carbon atoms, or a substituted such alkyl or aryl radical containing from 1 to 3 substituents selected from among fluorine, bromine and chlorine atoms. 
     The process according to the present invention requires the use of at least one butene compound disubstituted by acyloxy groups. By &#34;acyloxy group&#34; is intended a group of the formula RCOO- in which R is as defined above; &#34;disubstituted butenes&#34; are compounds of 2-butene which are substituted in positions 1 and 4 and compounds of 1-butene which are substituted in positions 3 and 4. Mixtures of 2-butene compounds disubstituted by different acyloxy groups; mixtures of 1-butene compounds disubstituted by different acyloxy groups; and mixtures of disubstituted 2-butene compounds and 1-butene compounds, can also be used as starting materials according to the present invention. 
     Indeed, it has now surprisingly been found that the selectivity of the subject process for a linear diester is substantially the same whether beginning with a 2-butene compound disubstituted by acyloxy groups in positions 1 and 4 or beginning with a 1-butene compound disubstituted by acyloxy groups in positions 3 and 4. 
     Exemplary of such butene compounds disubstituted by acyloxy groups are diacetoxybutenes, dipropionyloxybutenes, dibutyryloxybutenes and dibenzoyloxybutenes. 
     1,4-Diacetoxy-2-butene, 3,4-diacetoxy-1-butene, and mixtures thereof, are preferred starting materials according to the present invention. 
     The process according to the present invention also requires the use of an alcohol of formula R&#39;OH, in which R&#39; is as defined above. 
     Exemplary such alcohols are methanol, ethanol, isopropanol, n-propanol, tert-butanol, n-hexanol, 2-ethylhexanol, 1-nonanol, dodecanol, phenylethanol, phenol and trifluoroethanol. 
     An alkanol in which the radical R&#39; contains at most 12 carbon atoms and preferably 4 carbon atoms is advantageously employed. 
     The amount of alkanol used according to this invention is not critical and may vary over wide limits. 
     To satisfactorily carry out the subject reaction, the molar ratio of alcohol to disubstituted butene will range from 1 to 100 and preferably from 1 to 50. 
     The process according to the present invention is conducted in the presence of a palladium-based catalyst. 
     Although the precise nature of the catalytically active species in the reaction under consideration has not been absolutely determined, it has been found that various palladium compounds and metallic palladium are useful precursors for carrying out the process of this invention. 
     Exemplary sources of palladium which can be used to carry out the process of the invention, the following are representative: 
     (i) metallic palladium deposited, if appropriate, onto a support such as charcoal, alumina or silica; 
     (ii) PdCl 2 , Pd(OAc) 2  ; 
     (iii) palladium salts or π-allyl complexes, in which the anion coordinated to the Pd cation is selected from among the following anions: carboxylates such as the formate, acetate, propionate or benzoate, acetylacetonate, and halides such as Cl -  and Br -  and preferably Cl - . 
     The precise amount of catalyst to be used, which may vary over wide limits, will primarily depend on a compromise between the desired efficiency and the catalyst cost and the other reaction conditions. In general, good results are obtained employing a substituted butene/palladium molar ratio of from 10 to 50. A ratio on the order of 200 can be used, the reaction being slower, and a ratio on the order of 2 or less, and such ratios present obstacles which are only economic in nature. 
     One of the critical features of the process of this invention is that the reaction is conducted in the added presence of a quaternary onium chloride (or bromide) of an element of Group VB of the Periodic Table that is either nitrogen or phosphorus, said element being tetracoordinated to carbon atoms, with the proviso, moreover, that the nitrogen may be coordinated to two pentavalent phosphorus atoms. 
     By &#34;quaternary onium cation in which the element of Group VB is tetracoordinated to carbon atoms&#34; are intended cations formed from nitrogen or phosphorus and from four identical or different monovalent hydrocarbon groups, in which the free valency is borne by a carbon atom, each group being bonded to the above-mentioned element via such free valency, with the proviso that any two of these groups may together form a simple divalent radical. 
     To satisfactorily carry out the process of the invention, the quaternary onium salt advantageously has a quaternary onium cation corresponding to one of the formulae (I) to (IV) below: ##STR2## in which A is nitrogen or phosphorus; R 1 , R 2 , R 3  and R 4 , which may be identical or different, are each a linear or branched chain alkyl radical containing from 1 to 16 carbon atoms, optionally substituted by a phenyl, hydroxyl, halo, nitro, alkoxy or alkoxycarbonyl group, a linear or branched chain alkenyl radical containing from 2 to 12 carbon atoms, preferably from 4 to 8 carbon atoms, an aryl radical containing from 6 to 10 carbon atoms, optionally substituted by one or more alkyl radicals containing from 1 to 4 carbon atoms, or alkoxy, alkoxycarbonyl or halo radicals, with the proviso that two of such radicals R 1  to R 4  may together form a single linear or branched chain alkylene, alkenylene or alkadienylene radical containing from 3 to 6 carbon atoms; R 5 , R 6 , R 7  and R 8 , which may be identical or different, are each a linear or branched chain alkyl radical containing from 1 to 4 carbon atoms, with the provisos that the radicals R 7  and R 8  may together form a single alkylene radical containing from 3 to 6 carbon atoms, and that the radicals R 6  and R 7  or R 6  and R 8  may together form a single alkylene, alkenylene or alkadienylene radical containing 4 carbon atoms and forming a nitrogenous heterocyclic ring with N; R 9  is a linear or branched chain alkyl radical containing from 1 to 4 carbon atoms or a phenyl radical; R 10  is a linear or branched chain alkyl radical containing from 1 to 4 carbon atoms, identical to or different from R 9 , or a linear or branched chain alkenyl radical containing from 2 to 12 carbon atoms, preferably from 4 to 8 carbon atoms; D is an integer greater than or equal to 1 and less than or equal to 10 and preferably less than or equal to 6; and R 11  is an aryl radical containing from 6 to 10 carbon atoms, optionally substituted by one or more alkyl groups containing from 1 to 4 carbon atoms, or alkoxy, alkoxycarbonyl or halo groups. 
     Exemplary of the quaternary onium cations corresponding to formula (I), the following are representative: 
     tetramethylammonium 
     triethylmethylammonium 
     tributylmethylammonium 
     trimethyl(n-propyl)ammonium 
     tetraethylammonium 
     tetrabutylammonium 
     dodecyltrimethylammonium 
     methyltrioctylammonium 
     heptyltributylammonium 
     tetrapropylammonium 
     tetrapentylammonium 
     tetrahexylammonium 
     tetraheptylammonium 
     tetraoctylammonium 
     tetradecylammonium 
     butyltripropylammonium 
     methyltributylammonium 
     pentyltributylammonium 
     methyldiethylpropylammonium 
     ethyldimethylpropylammonium 
     tetradodecylammonium 
     tetraoctadecylammonium 
     hexadecyltrimethylammonium 
     benzyltrimethylammonium 
     benzyldimethylpropylammonium 
     benzyldimethyloctylammonium 
     benzyltributylammonium 
     benzyltriethylammonium 
     phenyltrimethylammonium 
     benzyldimethyltetradecylammonium 
     benzyldimethylhexadecylammonium 
     dimethyldiphenylammonium 
     methyltriphenylammonium 
     buten-2-yltriethylammonium 
     N,N-dimethyltetramethyleneammonium 
     N,N-diethyltetramethyleneammonium 
     tetramethylphosphonium 
     tetrabutylphosphonium 
     ethyltrimethylphosphonium 
     trimethylpentylphosphonium 
     octyltrimethylphosphonium 
     dodecyltrimethylphosphonium 
     trimethylphenylphosphonium 
     diethyldimethylphosphonium 
     dicyclohexyldimethylphosphonium 
     dimethyldiphenylphosphonium 
     cyclohexyltrimethylphosphonium 
     triethylmethylphosphonium 
     methyltri(isopropyl)phosphonium 
     methyltri(n-propyl)phosphonium 
     methyltri(n-butyl)phosphonium 
     methyltri(2-methylpropyl)phosphonium 
     methyltricyclohexylphosphonium 
     methyltriphenylphosphonium 
     methyltribenzylphosphonium 
     methyltri(4-methylphenyl)phosphonium 
     methyltrixylylphosphonium 
     diethylmethylphenylphosphonium 
     dibenzylmethylphenylphosphonium 
     ethyltriphenylphosphonium 
     tetraethylphosphonium 
     ethyltri(n-propyl)phosphonium 
     triethylpentylphosphonium 
     hexadecyltributylphosphonium 
     ethyltriphenylphosphonium 
     n-butyltri(n-propyl)phosphonium 
     butyltriphenylphosphonium 
     benzyltriphenylphosphonium 
     (β-phenylethyl)dimethylphenylphosphonium 
     tetraphenylphosphonium 
     triphenyl(4-methylphenyl)phosphonium 
     tetrakis(hydroxymethyl)phosphonium 
     tetrakis(2-hydroxyethyl)phosphonium. 
     Exemplary of the cations corresponding to formula (II), the following are representative: 
     N-methylpyridinium 
     N-ethylpyridinium 
     N-hexadecylpyridinium 
     N-methylpicolinium. 
     Exemplary of cations corresponding to formula (III), the following are representative: 
     1,2-bis(trimethylammonium)ethane 
     1,3-bis(trimethylammonium)propane 
     1,4-bis(trimethylammonium)butane 
     1,3-bis(trimethylammonium)butane. 
     Exemplary of the cations corresponding to formula (IV), the following are representative: 
     bis(triphenylphosphine)iminium 
     bis(tritolylphosphine)iminium. 
     One or more quaternary onium chloride(s) is (are) preferably used. 
     The onium cations which advantageously are used are those corresponding to the above formula (I) in which A is phosphorus, and R 1 , R 2 , R 3  and R 4 , which may be identical or different, are each a linear or branched chain alkyl radical containing from 1 to 8 carbon atoms, or a phenyl or 4-methylphenyl radical. 
     A tetraalkylphosphonium chloride is preferably employed. 
     Tetrabutylphosphonium chloride is commercially available and particularly effective. Thus, it is especially preferred. 
     It has also been found that the beneficial effect contributed by the presence in the carbonylation medium of a quaternary onium salt as defined above is perceptible starting with an onium cation/palladium molar ratio of 0.5; however, a particularly advantageous effect has been found when said ratio ranges from 1 to 50, although a higher ratio is not detrimental to the reaction. 
     Also for satisfactorily carrying out the carbonylation reaction according to this invention, it is preferable, when selecting the onium cation/palladium ratio, to take account of the palladium concentration in the mixture and especially of the disubstituted butene/palladium molar ratio. Thus, the higher the disubstituted butene/palladium ratio, the more advantageous it is to employ a high onium cation/palladium ratio. 
     The reaction can generally be conducted in liquid phase at a temperature of from 50° to 150° C., preferably from 80° to 130° C., under a carbon monoxide pressure of from 20 to 250 bar and preferably from 90 to 180 bar. Inert gases, such as nitrogen, argon or carbon dioxide, may be present together with the carbon monoxide. 
     The reaction can, of course, be conducted in the presence of solvents or diluents which are exogenous to the reaction mixture, such as aromatic hydrocarbons, ketones, nitriles, esters, or amides of carboxylic acids. 
     When the reaction is complete or the intended reaction time has elapsed, the desired diester is recovered by any appropriate means, for example by distillation. 
    
    
     In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that same are intended only as illustrative and in nowise limitative. In said examples to follow, the yields are expressed in relation to the diacetoxybutene charged. 
     EXAMPLE 1 
     The following materials were introduced into an autoclave made of Hastelloy B2: 
     30 cm 3  (740 mmol) of methanol, 23 mmol of 1,4-diacetoxy-2-butene, 1.25 mmol of PdCl 2  and 5.0 g (17 mmol) of tetrabutylphosphonium chloride. The reaction mixture was maintained at 100° C. for 21 hours, under stirring, at a constant pressure of 165 bar, and with a pure carbon monoxide feed. 
     Analysis by gas phase chromatography, using the internal reference quantitative technique, made it possible to ascertain: 
     (a) a complete degree of conversion of diacetoxybutene; 
     (b) a 65% yield of dimethyl 3-hexenedioate. 
     EXAMPLE 2 
     The procedure of Example 1 was repeated, but with the total pressure reduced to 100 bar and the time to 6 hours. 95% of the substrate was converted, with a methyl 3-hexenedioate yield equal to 43%. 
     EXAMPLE 3 
     The procedure of Example 2 was repeated, but the methanol was replaced with a mixture of 10 cm 3  of methanol and 20 cm 3  of acetonitrile. Over 6 hours, 98% of the diacetoxybutene was converted, with an 84% yield of methyl 3-hexenedioate. 
     EXAMPLE 4 
     1,4-Diacetoxy-2-butene was isomerized to 3,4-diacetoxybutene by heating to 100° C. under a nitrogen atmosphere in a glass ampoule in the presence of H 2  PtCl 6  (approximately 3% by weight). 
     After removal of the platinum on kieselguhr, the mixture of isomers (2/1 linear/branched) was subjected to a carbonylation under the conditions of Example 1. It was found that both isomers of diacetoxybutene were converted into linear diester. 
     EXAMPLE 5 
     The procedure of Example 3 was repeated, but the acetonitrile was replaced with the same volume of methyl isobutyl ketone. 
     Over 6 hours, 92.5% of the diacetoxybutene was converted, with an 80% yield of methyl 3-hexenedioate. 
     EXAMPLE 6 
     The procedure of Example 3 was repeated, but using 10 cm 3  of 1-nonanol instead of methanol. The formation of dinonyl 3-hexenedioate and of nonyl acetoxypentenoate was observed. 
     EXAMPLE 7 
     The procedure of Example 3 was repeated, but the acetonitrile was replaced with the same volume of toluene. 
     Over 6 hours, 91% of the diacetoxybutene was converted, with an 82% yield of methyl 3-hexenedioate. 
     EXAMPLE 8 
     Example 7 was repeated at 60° C. 
     Over 6 hours, 60% of the diacetoxybutene was converted, with a 55% yield of methyl 3-hexenedioate. 
     EXAMPLE 9 
     Example 7 was repeated at 140° C. 
     Over 6 hours, 92% of the diacetoxybutene was converted, with an 85% yield of methyl 3-hexenedioate. 
     EXAMPLE 10 
     Example 7 was repeated, using only one mmol of tetrabutylphosphonium chloride. 
     Over 6 hours, 89% of the diacetoxybutene was converted, with a 13% yield of methyl 3-hexenedioate. 
     CONTROL TEST (a): 
     Example 7 was repeated, but the tetrabutylphosphonium chloride was omitted. 
     Over 6 hours, 85% of the diacetoxybutene was converted, with a 5% yield of methyl 3-hexenedioate. 
     EXAMPLE 11 
     Example 7 was repeated, with the tetrabutylphosphonium chloride being replaced with 17 mmol of tetramethylammonium chloride. 
     Over 6 hours, 70% of the diacetoxybutene was converted, with a 70% yield of methyl 3-hexenedioate. 
     EXAMPLE 12 
     Example 11 was repeated, but the tetrabutylammonium chloride was replaced with tetramethylphosphonium chloride. 
     Over 6 hours, 70% of the diacetoxybutene was converted, with a 51% yield of methyl 3-hexenedioate. 
     EXAMPLE 13 
     In the autoclave and according to the operating procedure described in Example 1, a test was carried out on a charge containing: 
     (i) 10 cm 3  of methanol; 
     (ii) 20 cm 3  of toluene; 
     (iii) 23 mmol of 1,4-diacetoxy-2-butene; 
     (iv) 17 mmol of tetrabutylphosphonium chloride; and 
     (v) 1 mmol of palladium, introduced in the form of palladium deposited onto carbon black (at a concentration of 5% of palladium). 
     Over 6 hours, at 100° C., at a constant pressure of 100 bar and with a pure carbon monoxide feed, 98% of the diacetoxybutene was converted, with a 58% yield of methyl 3-hexenedioate. 
     EXAMPLE 14 
     The procedure of Example 3 was repeated, but the acetonitrile was replaced with the same volume of ethyl acetate, and methanol with the same volume of ethanol. 
     Over 6 hours, 87% of the diacetoxybutene was converted, with an 83% yield of 3-hexenedioate. 
     While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims, including equivalents thereof.