Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C68/00—Preparation of esters of carbonic or haloformic acids
  • C07C68/02—Preparation of esters of carbonic or haloformic acids from phosgene or haloformates

Definitions

• This invention relates to a method of making ester-substituted diaryl carbonates and in particular to a method of making bis methyl salicyl carbonate.
• Ester-substituted diaryl carbonates such as bis-methyl salicyl carbonate have proven to be useful starting materials in the preparation of polycarbonates via the melt reaction of a diaryl carbonate with aromatic dihydroxy compounds. See for example, US Patent No. 4,323,668 in which rates of polymerization of bis-methyl salicyl carbonate with bisphenol A were shown to be higher than the corresponding rates of polymerization of bisphenol A with an unsubstituted diaryl carbonate, diphenyl carbonate. Notwithstanding the simplicity of its structure there are few reported preparations of ester-substituted diaryl carbonates.
• a classical preparation of diaryl carbonates involves the reaction of a hydroxy aromatic compound such as phenol with phosgene gas in a two phase reaction system comprising water, an acid acceptor such as sodium hydroxide and a solvent such as methylene chloride or chloroform.
• Typical interfacial conditions used to prepare diphenyl carbonate (DPC) utilize water and methylene chloride phases, sodium hydroxide as a pH control measure and triethylamine as a catalyst. Under such conditions it is possible to convert phenol to DPC in essentially quantitative yield.
• application of these same conditions to methyl salicylate results in only modest conversion of this ester-substituted phenol to the corresponding diaryl carbonate. Even the use of as much as 20 percent excess phosgene does not result in conversion of more than 70 to 75% of methyl salicylate to the bis methyl salicyl carbonate.
• the present invention relates to a method of preparing ester-substituted diaryl carbonates, said method having a contact time, said method comprising contacting an ester-substituted phenol with phosgene and a phase transfer catalyst in the presence of an organic solvent and an aqueous phase wherein the aqueous phase is maintained at a pH of at least about 9.3 throughout the contact time, said phosgene being used in an amount corresponding to between about 0.95 and about 1.20 molar equivalents based on said ester-substituted phenol.
• a means whereby at least 90% of the ester-substituted phenol is converted into product ester-substituted diaryl carbonate.
• the present invention further relates to a high yield method of preparing bis-methyl salicyl carbonate, a valuable starting material for use in the melt polymerization of bisphenols to afford polycarbonates.
• polycarbonate refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
• melt polycarbonate refers to a polycarbonate made by a process comprising the transesterifi cation of a diaryl carbonate with a bisphenol.
• Catalytically effective amount refers to the amount of the catalyst at which catalytic performance is exhibited.
• reaction time is used interchangeably with reaction time
• alkyl radical refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic.
• the array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen.
• alkyl radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
• aromatic radical refers to a radical having a valence of at least one comprising at least one aromatic group.
• aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl.
• the term includes groups containing both aromatic and aliphatic components, for example a benzyl group.
• cycloalkyl radical refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic.
• the array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen.
• cycloalkyl radicals include cyclcopropyl, cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.
• ester-substituted phenols such as methyl salicylate are efficiently converted to ester-substituted diaryl carbonates such as bis-methyl salicyl carbonate under mild reaction conditions while minimizing the use of excess phosgene.
• the present invention provides a method for the efficient preparation of an ester-substituted diaryl carbonate having structure I
• R 1 is independently at each occurrence C ⁇ - C 0 alkyl radical, C 4 -C 20 cycloalkyl radical or C 4 -C 20 aromatic radical
• R is independently at each occurrence a hydrogen atom, halogen atom, cyano group, nitro group
• ester-substituted diaryl carbonates which may be prepared using the method of the present invention include bis-methyl salicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicyl carbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate, bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate and the like.
• bis-methyl salicyl carbonate is preferred for use in melt polycarbonate synthesis due to its lower molecular weight and higher vapor pressure.
• an ester-substituted phenol is contacted with phosgene in an amount equivalent to from about 0.95 to about 1.20, preferably about 1.0 to about 1.1 and even more preferably about 1.01 to about 1.05 moles of phosgene per mole of ester-substituted phenol, said contact taking place in a in a two phase system comprising water and a water-immiscible solvent, an acid acceptor, a phase transfer catalyst, and optionally a tertiary a ine catalyst, the ester-substituted phenol being contacted with said phosgene for a contact time of sufficient length to allow the conversion of at least 90% of the ester-substituted phenol into the product ester-substituted diaryl carbonate 1.
• the ester-substituted phenol is at least one compound selected from among phenols having structure II
• R 1 and R 2 are defined as in structure I and b is an integer 0-4.
• ester-substituted phenols which may serve as starting materials for the method of the present invention include methyl salicylate, ethyl salicylate, propyl salicylate, butyl salicylate, benzyl salicylate, methyl 4- chlorosalicylate and the like. Typically, methyl salicylate is preferred.
• the two phase system is comprised of an aqueous phase and an organic phase.
• the pH of the aqueous phase is controlled throughout the reaction by the addition of aqueous base.
• Suitable bases include alkali metal hydroxides such as sodium hydroxide and lithium hydroxide and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide.
• An aqueous solution of sodium hydroxide containing from about 5 to about 50 percent by weight NaOH is preferred. Care must be taken in order to maintain a pH of the aqueous phase of at least about 9.3 during the contact time because rates of ester-substituted diaryl carbonate formation drop dramatically at lower pH.
• the pH of the aqueous phase is maintained at between about 9.3 and about 12, preferably between about 10.3 and about 12.
• the organic phase is at least one solvent said solvent being immiscible with water.
• the organic phase may comprise a halogenated or a non-halogenated solvent.
• halogenated solvents suitable for use in the method of the present invention are methylene chloride and chloroform.
• non- halogenated solvents suitable for use in the method of the present invention are toluene and ethyl acetate.
• the amount of solvent used is such that there is sufficient solvent to dissolve the ester-substituted phenol.
• a solution of the ester- substituted phenol in the water immiscible solvent contains between about 5 and about 50 weight percent ester-substituted phenol.
• the volume of the aqueous phase is roughly equal to the volume of the organic phase at the outset of the reaction.
• ester-substituted phenol may take place at below ambient temperature, ambient temperature or above ambient temperature.
• ester-substituted phenol is contacted with phosgene at a temperature of between about 0°C and about 50°C, preferably between about 10°C and about 40°C.
• the contact between the ester-substituted phenol and phosgene is of a sufficient length of time such that greater than 90% preferably greater than 95% and still more preferably greater than 98% of the starting ester-substituted phenol is converted to product ester-substituted diaryl carbonate and is referred to as the reaction time.
• the reaction time is in a range between about 5 and about 60 minutes.
• phosgene is added to a solution of the ester-substituted phenol reaction times are limited by the rate of phosgene addition.
• phase transfer catalyst dramatically improves the conversion of ester-substituted phenols to product diaryl carbonates when said phenols are contacted with phosgene in a two phase reaction system comprising aqueous and organic phases in which the pH of the aqueous is controlled by the addition of an aqueous base such as aqueous sodium hydroxide.
• Suitable phase transfer catalysts are widely available and include quaternary ammonium salts of aliphatic amines, quaternary ammonium salts of aromatic amines, quaternary phosphonium salts, sulfonium salts, polyethers and the like.
• Quaternary ammonium salts of aliphatic amines are illustrated by methyl tributyl ammonium chloride, tetramethyl ammonium chloride and the like.
• Quaternary ammonium salts of aromatic amines are illustrated by N-benzyl pyridinium chloride, N-benzyl 4-N',N'-dimethylamino pyridinium chloride and the like.
• Quaternary ammonium salts include hexaalkyl guanidinium compounds such as hexaethyl guanidinium chloride.
• Quaternary phosphonium salts are illustrated by tetrabutyl phosphonium acetate and the like.
• Sulfonium salts are illustrated by trimethyl sulfonium chloride and the like.
• Polyethers are illustrated by polyethylene glycol and crown ethers such as 18-crown 6 and the like.
• R -R are independently a C
• Suitable anions X include hydroxide, halide. carboxylate, sulfonate, sulfate, carbonate and bicarbonate.
• X ⁇ is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structure III are properly balanced.
• R 3 -R 6 in structure III are each methyl groups and X ⁇ is carbonate, it is understood that X " represents V2 (CO -2 ).
• Quaternary ammonium compounds having structure III and which are suitable for use as phase transfer catalysts according to the method of the present invention are illustrated by methyl tributyl ammonium chloride, tetrabutyl ammonium chloride and decyl trimethyl ammonium chloride.
• phase transfer catalyst employed is in a range between about 0.1 and about 2, preferably between about 0.25 and about 1.0 mole percent catalyst per mole of ester-substituted phenol employed.
• a tertiary amine is also included as a co-catalyst for the formation of ester-substituted diaryl carbonates.
• the tertiary amine has been found to accelerate the formation of ester-substituted diary carbonate product and to act to minimize the presence of the intermediate ester- substituted phenyl chloroformate in the product.
• the optional use of a tertiary amine added after phosgene addition has been completed has been found useful in reaction systems in which the chloroformate intermediates tend to persist.
• phosgene addition to a two phase reaction system comprising a water immiscible organic solvent, water, an acid acceptor, an ester-substituted phenol and a phase transfer catalyst under the conditions of the present invention may at times result in the a product mixture comprising ester-substituted diaryl carbonate and the intermediate ester-substituted phenyl chloroformate.
• the amount of ester-substituted phenyl chloroformate is low, less than 1 mole percent based upon the total number of moles of phenol employed but its presence in the product is undesirable.
• a small amount of a tertiary amine added following the phosgenation step provides a means of eliminating residual chloro formates present in the product mixture.
• the amount of tertiary amine co-catalyst used is in a range between about 0.01 mole and about 1 mole percent based upon the total number of moles of ester -substituted phenol employed.
• Tertiary amines suitable for use as co-catalysts according to the method of the present invention are illustrated by tri ethyl amine, diispropyl ethyl amine, tributyl amine, and 1 ,4-diazabicyclooctane.
• Comparative Examples 2-5 were carried out under essentially identical conditions using 20 mole percent excess phosgene except that the pH of the aqueous phase was varied between 10.3 and 7.3. Initial starting concentrations for Comparative Examples 1 -5 was 31 percent solids.
• Examples 1-7 which illustrate the method of the present invention.
• Examples 2-7 were carried out essentially identically to Example 1 with the following exceptions.
• Examples 2-7 each employed a small amount of triethylamine as a co-catatlyst. In Examples 3 and 4 the triethylamine was added prior to phosgenation whereas in Examples 2, 5, 6 and 7 the triethylamine was added after the completion of phosgenation.
• Examples 1-4 were run at a concentration equivalent to that shown for Comparative Examples 2-5 of Table 1.
• Example 1 -4 and Comparative Examples 1 -5 were such that, assuming 100% conversion of methyl salicylate or phenol to product BMSC or DPC, the weight of the product diaryl carbonate would represent 31 percent by weight of the methylene chloride employed at the outset of the reaction. This is designated 31 percent solids.
• Examples 5, 6 and 7 were run at slightly higher concentrations 37.3, 45.4 and 70 percent solids respectively. At concentrations of about 45 percent solids and higher the product BMSC was observed to precipitate from the reaction mixture and additional methylene chloride was added for work up and HPLC analysis.
• Examples 1 -6 were run at ambient temperature. In Example 7 the reaction mixture was immersed in an ice bath during the reaction.

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