This invention relates to a novel carbonylation process for the preparation of both aromatic carboxylic esters and an iodine-containing compound from which the iodine values can be economically recovered. The carbonylation is conducted in the presence of an ether and a catalytic amount of iridium. The carbonylation of aromatic halides in the presence of various Group VIII metal catalysts to obtain aromatic carboxylic acids and esters is well known in the art. For example, U.S. Patent 3,988,358 discloses the palladium-catalyzed carbonylation of aromatic halides in the presence of an alcohol and a tertiary amine to produce the corresponding carboxylic acid ester. Nakayama and Mizoroki [Bull. Che . Soc. Japan _2 (1969) 1124] disclose the nickel-catalyzed carbonylation of aromatic halides in the presence of an alcohol and potassium acetate to produce the corresponding acid ester.

While it is known that aromatic iodides can be carbonylated, the use of these materials has been discouraged by the cost associated with the difficulty of recovering the iodine values. For example, the use of basic materials in the carbonylation of aromatic halides, such as tri n-butyl amine in U.S. Patent 3,988,358, results in the formation of halide salts from which the halide values can be reclaimed only through uneconomical procedures involving severe chemical treatments. In U.S. Patent 2.565,462, Prichard and Tabet disclose the carbonylation of aromatic halides to aromatic carboxylic esters in the presence of alcohols, ethers, and phenols using nickel

tetracarbonyl. However, only noncatalytic quantities of iron, nickel, and cobalt are used as promoters ^• under reaction conditions of both temperature and pressure that are much more severe than is shown by our invention. We have discovered a process which not only results in the carbonylation of aromatic iodides to aromatic carboxylic esters with low acid content in excellent yields and at excellent rates of conversion but also a process which results in production of alkyl iodides from which the iodides values can be economically recovered. In this invention, the carbonylation is conducted in the presence of an ether and a catalytic amount of a iridium catalyst under aromatic carboxylic ester and alkyl iodide-forming conditions of temperature and pressure.

The advantage afforded by our invention over the prior art is three-fold. First, the iridium-based catalyst has not been disclosed or recognized in the prior art to be an efficient carbonylation catalyst for aryl halides. Second, the iodine values in the alkyl iodide.may be readily recovered by simply flashing the relatively volatile alkyl iodide from the mixture resulting from the carbonylation reaction. This can be accomplished either in the carbonylation reactor or, more preferably, in a pressure reduction vessel to which the mixture resulting from the carbonylation reaction is fed. Third, the object in feeding organic ethers is to minimize the amount of water in the carbonylation reactor which will reduce the acid content of the ester product. The ratio of aromatic esters to acids produced in the present invention is dependent on the concentration of water present in the carbonylation reactor. The capability of producing aromatic

carboxylic esters with low acid content is both novel and useful. The low acid content allows for simpler and less expensive production and purification schemes and eliminates the need for an esterification step when esters are the desired product. The aromatic iodides which may be used in our process may be onoiodo or polyiodo, e.g., di-, tri- and tetra-iodo aromatic compounds. The aromatic nucleus or moiety can contain from 6 to 18 carbon atoms, preferably 6 to 10 carbon atoms and may be carbocyclic aromatic such as benzene, biphenyl, terphenyl, naphthalene, anthracene, etc., or heterocyclic aromatic such as pyridine, thiophene. pyrrole, indole, etc. In addition to one or more iodine atoms, the aromatic moiety may be substituted by various εubstituents substantially inert under the conditions employed in our process. Examples of such substituents include alkyl of up to about 12 carbon atoms such as methyl, ethyl, iεobutyl, hexyl, 2-ethylhexyl. nonyl, decyl, dodecyl, etc.: cycloalkyl of 5 to 12 carbon atoms such as cyclopentyl, cyclohexyl, 4-butylcyclohexyl, etc.; halogen such as chloro and bromo; alkoxycarbonyl of from 2 to 8 carbon atoms such as methoxycarbonyl, ethoxycarbonyl, butoxycarbonyl. hexyloxycarbonyl, etc.; carboxyl; cyano; alkenyl of 2 to 12 carbon atoms such as vinyl allyl, etc.; formyl; alkanoyl of 2 to 8 carbon atoms such as acetyl, propionyl, butyryl, hexanoyl, etc.; alkanoylamido of 2 to 8 carbon atoms such as acetamido butylamido, etc.; aroylamino such as benza ido; and alkylsulfonamide such as methanesulfona ide hexanesulfonamide. etc.

Specific examples of the aromatic iodide reactants include iodobenzene, 1,3- and 1,4-diiodobenzene, 1,3,5-triiodobenzene, 4-iodotoluene, 4-iodophenol, 4-iodoaniεole,

4-iodoacetophenone , 4,4'-diiodobiphenyl, 4-chloroiodobenzene. 3-bromoiodobenzene and 2.6- and 2.7-diiodonaphthalene. Our process is particularly useful for the preparation of benzenedicarboxylic and naphthalenedicarboxylic esters with low acid content and thus the preferred reactants are diiodobenzenes, especially 1,3- and 1.4-diiodobenzene, and diiodonaphthaleneε, especially 2,6- and 2,7-diiodonaphthalene. The aromatic iodide reactants are known compounds and/or can be prepared according to published procedures. For example, T. Hudlicky et al.. The Chemistry of Halides. Pseudohalideε and Azides. Supplement D, Part 2, 1142-1158, the diεcloεure of which iε incorporated herein by reference in itε entirety diεcloεeε a number of such processes. Another process described in J. Chem. Soc. 150 (1952) compriseε treating an aromatic compound, such as benzene, with iodine in the presence of silver sulfate dissolved in concentrated sulfuric acid.

The ether used in the procesε of thiε invention, which iε preferably dimethyl ether, reεultε in the formation of methyl carboxylate eεterε, which may be used in transeεterification reactionε, and produces methyl iodide which is the most volatile of the alkyl iodides. However, other ethers containing up to 12 carbon atoms, preferably up to 4 carbon atoms, may be employed if desired. Examples of other suitable ethers include diethyl ether, dipropyl ether, dibutyl ether, dipentyl ether, dihexyl ether, diheptyl ether, dioctyl ether, didecyl ether, dibenzyl ether, dioxane, anisole, or mixed dialkyl ethers. Mixture of these ethers may also be employed. For each mole equivalent of aromatic ester produced, one mole of

ether is required.

The process provided by our invention can also be carried out in the presence of an organic co-solvent such aε aliphatic, alicyclic and aromatic hydrocarbons, and halogenated hydrocarbons. Examples of such solvents include benzene, toluene, the xylenes, hexane, heptane, chlorobenzene, ethylene dichloride, methychloroform, naphthalene, etc. However, the uεe of a co-εolvent is not critical to the practice of this invention. Water or potential eεterifying agents such as alcohols and their carboxylate esterε may alεo be preεent in the reaction mixture depending upon the deεired eεter to acid ratio.

The iridiu catalyεt can be provided to the reaction medium aε any of a number of iridium εalts or complexeε that are capable of providing iridium in a εolution form in the reaction. Illustrative εourceε of iridium are iridum trichloride, iridium tribromide. iridium triiodide, iridium acetylacetonate. iridium dioxide, and dodecacarbonyltetrairidium and their phosphine and halogen substituted analogs. The amount of iridium is not significant as long as enough is present to catalyze the reaction. Preferably, the catalyst is present in a concentration of 10 to 0.01 mole percent, preferably 1.0 to 0.1 mole percent baεed on the moles of aromatic iodide reactant. Therefore, the total reaction medium haε a catalyεt concentration of 10,000 ppm to 10 ppm with preferred catalyst concentrations of 1,000 to 100 ppm.

The carbonylation reaction iε conducted in the presence of carbon monoxide, which is employed in amounts such that the total reaction pressure iε suitable for the formation of both the aromatic carboxylic ester and the alkyl iodide. The carbon

monoxide employed may be eεsentially pure or it may contain other gases such as carbon dioxide, hydrogen, methane and other compounds produced by synthesis gas plants. Normally, the carbon monoxide will be at least 90, preferably at least 95, percent pure.

The proceεε of the preεent invention can be conducted at temperatures and pressures suitable for formation of both the aromatic carboxylic eεter and alkyl iodide. The temperatureε and presεureε are interdependent and can vary conεiderably. Normally,

2 the pressure will be at least 7 kg/cm . While the proceεs can be carried out at pressures as high aε

2 703 kg/cm , the cost of utilities and equipment required for such high pressure operation may not be commercially justified. Thus, the pressure normally will be in the range of 21 to 281 kg/cm 2,

2 preferably 52 to 105 kg/cm . A particularly

2 preferred pressure is 70 kg/cm . While temperatures as low as 125°C and higher than 225°C may be uεed, our procesε normally iε carried out between 150° to 275°C. The preferred temperature range iε 180° to 250°C. A particularly preferred temperature iε 220°C.

The relative amounts of carbon monoxide, ether and aromatic iodide used in our proceεs can be varied substantially and are. in general, not critical.

However, it is preferable to have at least εtoichiometric amounts preεent relative to the aromatic iodide of complete converεion iε deεired. When a polyiodo aromatic compound iε used as the reactant in our carbonylation procesε, the products obtained include both aromatic polycarboxylic esters and partially carbonylated products such aε iodoaromatic carboxylic eεters. The latter compounds

are uεeful aε inter ediateε in the preparation of derivatives of aromatic carboxylic esters, for example, by displacement reactions whereby the iodo subεtituent iε replaced with other radicalε. The difunctional eεterε, such aε dimethyl 2,6-naphthalenedicarboxylate, can be reacted with diols to produce high molecular weight polyeεterε suitable for molding plaεticε. Uεeful articles can be molded from these plastics, such aε by injection molding. The relative amounts of partially or totally carbonylated products is highly dependent on the period of time that the reactant resides under carbonylation conditions.

The alkyl iodides prepared according to the procesε of our invention may be uεed in other chemical procesεes such as in the preparation of carboxylic acids and carboxylic anhydrides according to known carbonylation procedures. Alternatively, the alkyl iodide can be oxidatively decomposed at elevated temperature to produce a gaseous mixture of iodine, carbon dioxide, and water from which the iodine can be recovered. Alternatively, the alkyl iodides may be thermally decomposed to iodine and an alkane, or hydrogenated to hydrogen iodide and methane. our procesε is carried out at a PKa of less than 5. Therefore, there are no significant amounts of basic materials which preferentially combine with hydrogen ioxide and interface with the formation of an alkyl iodide. Examples of such bases which are not present in significant amounts in our proceεε include amineε, particularly tertiary amineε, and hydroxides, alkoxides and weak acid εaltε, e.g., carboxylateε of the alkali and alkaline earth metals. Our invention iε further illuεtrated by the following examples. In the procedures utilized in

the examples, the materials employed except dimethyl ether are loaded into a 300 mL autoclave constructed of Hastelloy B2 alloy which is designed to operate in a rocking mode. The autoclave is presεurized with

2 14 kg/cm carbon monoxide gas preεεure at room temperature and then the gas is vented and the autoclave is sealed. In Examples 1-5, the autoclave is charged with the desired amount of dimethyl ether and then presεurized to a total pressure of 21

2 kg/cm with carbon monoxide gas at ambient temperature and heated and rocked until reaction temperature was reached, at which time additional carbon monoxide gas iε added to increaεe the autoclave internal preεεure to the predetermined value. Reactor presεure iε maintained by adding carbon monoxide at the same rate at which it iε consumed by the reactants. The carbon monoxide used is essentially pure. When the predetermined reaction time is completed, the autoclave is cooled by a stream of cold air to approximately 25°C. After the gas is vented from the autoclave, the crude product iε isolated by filtration and analyzed by gas chromatoqraphic methods. The percent conversion iε the mole percent of iodo-group converted to carboxylic acid or eεter. The resultε are εhown below.

Example No

Iodoaromatic 2.6-diiodonaph- 2, 6-diiodonaph- Wt (g) thalene thalene

30.0 30.0

Catalyst IrCl 3' 3H ₂ 0 IrCl. 3H ₂ 0 Wt (g) 0.51 0.50

Ether Dimethyl Ether Dimethyl Ether

Vol (mL) 42.0 42.0

Co-Solvent 1-Methylnaρh- 1-Methylnaph- Wt (g) thalene thalene 100.5 100.7

Time (hour)

Pressure 105 70

(kg/cm )

Temp. (°C) 200 220 % Conversion 38.1 84.0

Example Co

Iodoaromatic 2,6-diiodonaph- 2,6-diiodonaph- Wt (g) thalene thalene 30.0 30.0

Catalyst IrCl .3H o IrCl„.3H.,0 3 2 3 2 Wt (g) 0.51 0.50

Ether Dimethyl Ether Dimethyl Ether

Vol (mL) 42.0 42.0

Co-Solvent 1-Methylnaph- 1-Methylnaph- Wt (g) thalene thalene 100.5 100.9

Time (hour)

Pressure 105 105

(kg/cm )

Temp. (°C) 220 240 % Conversion 79.0 99.1

Example No

Iodoaromatic 2,6-diiodonaph- Wt (g) thalene 30.0

Catalyεt IrCl ₃ .3H ₂ 0 Wt (g) 0.51

Ether Diethyl Ether

Vol (mL) 42.0

Co-Solvent 1-Methylnaphthalene Wt (g) 100.1

Time

(hour)

Pressure 105

(kg/cm )

Temp. (°C) 220

% Conversion 94.6

While the invention haε been deεcribed in detail with particular reference to preferred embodiments thereof, it will be underεtood that variations and modifications can be effected within the εpirit and εcope of the invention.