GB1578713A

GB1578713A - Preparation of aromatic carbonates

Info

Publication number: GB1578713A
Application number: GB13644/78A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1976-10-12
Filing date: 1978-04-07
Publication date: 1980-11-05
Also published as: AU2826677A; DD132780A5; PL201469A1; JPS5638145B2; JPS5368746A; FR2367732A1; IN148600B; MX145820A; BR7706238A; GB1572291A; NL7711176A; FR2367732B1; AU518531B2; DE2738437A1; BE859576A

Description

(54) PREPARATION OF AROMATIC CARBONATES (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for making aromatic polycarbonates and is an improvement in and modification of the process described and claimed in our prior British Patent Application No. 41324/77 (Serial No. 1,572,291).

Our prior British Patent Specification No. 41324/77, (Serial No. 1,572,291), and the closely related British Patent Applications No. 41325/77, 41326/77 and 41328/77 (Serial Nos. 1,572,292, 1,572,293, 1,572,295) relate to the production of aromatic polycarbonates by reacting a phenol with carbon monoxide in the presence of a group VIIIB element or compound thereof.

More particularly, our prior British Patent Application No. 41324/77 (Serial No. 1,572,291) claims a process for preparing an aromatic carbonate which comprises contacting, in the presence of a dehydrating agent, a phenol, carbon monoxide, a base, a Group VIIIB element selected from ruthenium, rhodium, palladium, osmium, iridium or platinum, and an oxidant comprising an element, a compound or a complex having an oxidation potential greater than that of the said selected Group VIIIB element.

It has now been discovered that the process of British Patent Application No.

41324/77 (Serial No. 1,572,291) may be carried out particularly advantageously if a phase transfer agent (as hereinafter defined) is present during the reaction.

The present invention provides a process for preparing an aromatic carbonate which comprises contacting, in the presence of a dehydrating agent, a phenol, carbon monoxide, a base, a Group VIIIB element selected from ruthenium, rhodium, palladium, osmium, iridium and platinum an oxidant comprising an element, a compound or a complex having an oxidation potential greater than that of the said selected Group VIIIB element, and a phase transfer agent (as hereinafter defined).

Incorporation of a phase transfer agent in the reaction mixture leads to a general increase in reaction velocity and in the field of product.

The reactants and the resulting reaction products of the process according to the invention can be illustrated by the following general equations which are furnished for illustrative purposes only, since the reaction mechanisms involved in the preparation of aromatic monocarbonates (Eq. 1) and polycarbonates (Eq. 2) may be much more complex: Equation 1 2R'OH+CO+ 1/2QR'2CO3+H2O Equation 2 n+ 1 R' '(OH)2+ nCO+ l/2nO2

wherein R' is an aryl radical, R" is an arene radical, and n is a number at least equal to 1.

Any of the phenols, solvents, bases, ligands, the Group VIIIB elements, oxidants, including oxygen, redox agents, or reaction parameters relative to time, temperature and pressure disclosed in British Patent Applications No. 41324/77 (Serial No. 1,572,291) or 41327/77 (Serial No. 1,572,294) can be employed in the process according to the present invention. They will not, therefore, be described in any further detail in the present Application, and reference may be made to our above-mentioned co-pending Applications for information as to suitable materials in these categories. In general, the preferred reactants and reaction conditions disclosed in British Patent Application No. 41327/77 (Serial No. 1,572,294) are also preferred in the process of the present invention.

The process is preferably carried out under reaction conditions wherein no measurable amount of water can be detected during the course of the reaction.

Substantially anhydrous reaction conditions are defined herein, and in the appended claims, as the practice of the process carried out in the presence of any dehydrating agent which will take up a measurable amount of any water formed as described hereinbefore by Equations 1 and 2. The dehydrating agents are preferably inert, and can be any of those known to those of ordinary skill in the art. They can be classified by any means, e.g. regenerative or non-regenerative; liquid or solid; chemical reaction, i.e. the formation of a new compound or a hydrate; physical absorption at constant or variable relative humidity; oradsorption. Preferably, the dehydrating agents employed have high capacity and/or efficiency, and preferably both, in removing moisture from the reaction medium. As employed herein, the term "capacity" means the amount of water that can be removed from a given weight of the reaction medium, and "efficiency" means the degree of dryness that can be produced by the dehydrating agent. Among the many dehydrating agents that can be employed are activated alumina, barium oxide, calcium chloride, calcium oxide, calcium sulfate, lithium chloride, and molecular sieves, e.g. dehydrating agents made from natural or synthetic crystalline alkali metal aluminosilicates of the zeolite type. Preferred dehydrating agents are natural and synthetic zeolites well known to the art, such as those described in detail in the publication Molecular Sieves, Charles K. Hersh, Reinhold Publishing Company, New York (1961). Representative natural zeolites which may be used include those in Table 3-1, page 21 of the Hersh reference.

Additional useful zeolite dehydrating agents are set forth in Organic Catalysis Over Crystalline Aluminsilicates, P. B. Venuto and P. S. Landis, Advances in Catalysis, Vol. 18, pp. 259-371 (1968). Particularly useful molecular sieves are those designated by the Linde Division of the Union Carbide Corporation as zeolite types A, X and Y, described in U.S. Patents No. 2,882,243, 3,130,007 and 3,529,033. Other zeolites are, of course, included within the scope of this invention.

In another embodiment of the process, preferably manganese or cobalt redox co-catalyst complexes are employed in addition to a drying agent. Illustrative of manganese complexes which are preferred oxidants are those commonly referred to as manganese chelates and includes those represented by the general formula LMn, wherein L is a ligand, e.g. derived from an a-diketone, p-diketone, an omegahydroxyoxime or an orthohydroxyareneoxime, including mixtures thereof, and Mn is the transition metal manganese. The manganese can be employed in any of its oxidation states, e.g. from -l to +7.

An omega-hydroxyoxime ligand, represented as "L" in the general formula LMn, can be described by the following formula:

wherein independently each Rb, Rc, Rd and Re is selected from hydrogen, acyclic and cyclic hydrocarbon radicals, and n is 0 or 1.

An ortho-hydroxyareneoxime ligand, represented as "L" in the general formula LMn, can be described by the following formula:

wherein Rf is independently selected from hydrogen and acyclic hydrocarbon radicals, Ar is an at least divalent arene radical having at least one -OH radical and at least one

radical attached directly to an ortho-position arene ring carbon atom. Methods for the preparation of manganese chelate complexes, including mixtures thereof are described in U.S. Patents No. 3,956,242, 3,965,069 and 3,972,851.

Illustrative manganese chelate complexes with omega-hydroxyoxime and ortho-hydroxyareneoxime ligands have, respectively, the following formulae:

Another and even more preferred class of manganese ligands that can be associated with LMn complexes are alpha (a)-diketones or a beta-(p)-diketone ligands, including mixtures thereof. In general, a-diketone ligands can be described by the following formula:

wherein independently each Rq is selected from hydrogen, acyclic and cyclic hydrocarbon radicals, primary amines (-NH2), secondary amines (-NHR,), tertiary amines (-NRhRl), hydroxyl radicals (-OH), oxyhydrocarbon radicals (-ORj), and halogens (F, Cl, Br or I), Rh, R, Tri being the same as either Ra as defined above, subject to the proviso that each R8 may be the same or different or may be joined together with the carbonyl groups to form a cyclic system.

In general, p-diketone ligands can be described by the following formula:

wherein independently each Rm and Rn are the same, different or may be joined together with the two carbonyl groups to form a cyclic system and are selected from the same Rg groups described in connection with Formula V above.

An illustrative a-diketone is tropolone.

and an illustrative p-diketone is acetylacetone.

Other well-known diketones include 2-acetyl- 1 3-cyclohexanedione, 2-acetyl- l-oxo-tetrahydronaphthalene, benzofuran-2-yl methyl ketone, l-benzoylacetone, 3-benzylidene-2,4-pentanedione, biacetyl, benzil, dibenzoylmethane, 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione, 2,4-pentanedione, 2,4-pentanedione, thallium(I) salt, 2,2,6,6-tetramethyl-3,5-heptanedione, thenoyltrifluoroacetone, and triacetylmethane.

The a and A-diketones can be prepared by any method well-known to those skilled in the art including those described and referenced in Advanced Inorganic Chemistry, F. A. Cotton and G. Wilkinson, Interscience Publishers, cc. 1972, John Wiley & Sons, Inc.

Illustrative of cobalt complexes which are preferred oxidants are those commonly referred to as cobalt chelates and includes those represented by the general formula:

wherein Ar represents a divalent arene radical and R represents a divalent organic radical containing at least 2 carbon atoms. Methods for the preparation of cobalt chelate complexes including mixtures thereof are described in U.S. Patents No. 3,455,880, 3,444,133 and 3,781,382.

Generally presently preferred cobalt chelate complexes are described by the following formulae:

Since manganese and cobalt complexes can coordinate with such components as water, oxygen, alcohol, and amines, such coordination compounds are included within the context as oxidants in the practice of the invention.

In accordance with the improved process of the present invention, the reaction is carried out in the presence of an organic phase transfer agent (PTA).

Generally effective phase transfer agents include quaternary ammonium compounds, quaternary phosphonium compounds, tertiary sulfonium compounds, crown ether compounds, chelated cationic salts, and cryptates. A phase transfer agent is defined herein as any agent which is soluble in the organic phase and which enhances the transfer, maintenance or retention of a halide, and in a preferred embodiment a bromide, in the organic phase in the reaction environment.

Illustrative of well-known onium phase transfer agents are those described by C. M. Starks in J.A.C.S. 93, 195 (1971), e.g. ammonium, phosphonium and sulfonium compounds represented by the following formulae:

wherein each R is independently selected from acyclic and cyclic hydrocarbon radicals, e.g. alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, including mixtures and combinations thereof, preferably each R having from 1 to 30 carbon atoms, and more preferably from 2 to 15 carbon atoms, each X- is selected from Cl-, Br-, F-, I-, CHSO3-, CH3CO2-, CF3CO2- and OH-, and each Y= is selected from SO4=, CO3=, and C2O4=. Examples of preferred phase transfer agents include tetramethylammonium chloride, tetraethylphosphonium iodide, tripropylsulfonium bromide, triallylsulfonium acetate, tetrabutylammonium fluoride, tetracyclohexylphosphonium hydroxide, triphenylammonium bromide, tricaprylmonomethylammonium chloride, tetradodecylphosphonium trifluoroacetate, trioctadecylsulfonium sulfate, tetraeicosylammonium carbonate, tetratricosylphosphonium oxalate, and tritriacontylsulfonium methane sulfonate.

The onium compounds can be prepared by any method well-known in the art including e.g. the preparation on onium halides by the familiar addition reactions of tertiary amines, tertiary phosphines and sulfides with alkyl halides.

Examples of well-known crown ether phase transfer agents are any of those described in Aldrichimica ACTA 9, Issue &num;1 (1976) Crown Ether Chemistry: Principles and Applications, G. W. Gokel and H. D. Durst, as well as C. J. Pederson in U.S. Patent No. 3,622,577. One preferred crown ether phase transfer agent is 1 8-crown-6 having the formula:

Although 18-crown-6 is limited to oxygen, carbon and hydrogen atoms associated with the ring structure, in other crown ethers such as sulfur, nitrogen, and phosphorus, can be substituted for oxygen within the ring structure. Other wellknown crown ethers include 12-crown-4, 15-crown-5, dibenzo-18-crown-6, dicyclohexyl-18-crown-6, among many others. The crown ethers can be prepared by methods well-known in the art including among others the reaction described in U.S. Patent No. 3,622,577.

Examples of well-known chelated cationic salts are alkali or alkaline earth metal diamine halides. Specific examples include, lithium(tetramethylethylenediamine)2bromide, sodium(tetraisopropylethylenediamine)2chloride, potassium(tetrabutylethylenediamine)2fluoride, calcium(tetramethylpropylenediamine)3iodide, and magnesium(tetraethylpropylenediamine)2bromide.

In the process of the invention, any amount of dehydrating agent can be employed. Those skilled in the art can be determined, by means of routine experimentation, the optimum amounts of any particular drying agent which can be used. For example, those skilled in the art can readily estimate the optimum amounts of molecular sieve required for selective absorption of water by routine reference to Linde Company, molecular Types 3A and 4A "Water Data Sheets" published and distributed by Union Carbide Corporation.

In a still more preferred embodiment, the invention is carried out in a reaction environment which contains at least one member of each of the following groups (A), (B) and (C): (A) a base comprising any elemental alkali or alkaline earth metal base, including organic or inorganic basic compounds thereof, e.g. lithium, sodium, potassium, calcium, or barium hydroxide; sodium, lithium or barium carbonate, sodium acetate, sodium benzoate, or sodium methoxide, including mixtures thereof. Presently preferred are strong alkali metal hydroxide bases, e.g. sodium hydroxide, and potassium hydroxide, because of their efficacy and economics in this embodiment, (B) Any phase transfer agent. Presently preferred are onium halides where at least one, and more preferably each, of the R groups of formula XIII contain at least 4 carbon atoms, and each anion X- is a halide, especially preferred being chloride or bromide, and more especially preferred being bromide. Examples include tetrabutylammonium bromide, and tetrabutylphosphonium bromide.

(C) A manganese redox co-catalyst of any a-diketone or p-diketone, or mixtures thereof, preferably, because of their efficacy, manganese complexes associated with acetyl-acetone, e.g. manganese(II)-bis(acetoacetonate).

In another preferred embodiment enhanced reaction rates are generally obtained wherein at least one of the groups defined in paragraphs (A), (B) and (C) set out above are employed, subject to the proviso that the phase transfer agent contains a halide counter-ion, especially where the counter-ion is a bromide ion, and further subject to the proviso that the phase transfer agent having a halide associated therewith is present in an amount in excess of the molar amounts of the base.

Any amount of phase transfer agent can be employed. In addition, effective mole ratios of phase transfer agents to "the Group VIIIB element" are within the range of from 0.00001:1 to 1000:1, but may be even higher, preferably from 0.05:1 to 100:1 and more preferably from 10:1 to 20:1.

In order that those skilled in the art may better understand the invention, the following Examples are given which are illustrative of the best mode of this invention. In the Examples, unless otherwise specified, all parts are by weight and the reaction products were verified by infrared spectrum, C-13 nuclear magnetic resonance and mass spectrometry.

COMPARISON EXAMPLE Preparation of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate using pcumylphenol, carbon monoxide, 2,2,6,6,N-penta-methylpiperidine, palladium(II)dibromide, manganese(II)bis-(acetylacetonate) and a molecular sieve.

A reaction vessel was charged with 2.12 g. (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.075 g (0.0030 moles) of manganese(II) acetylacetonate, 0.23 g (0.0015 moles) of 2,2,6,6,N pentamethylpiperidine, 30 ml of methylene chloride and 2.0 g of a 200"C vacuoactivated Linde (Registered Trade Mark) Union Carbide 3A molecular sieve.

Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 1.10 g (49% yield) of 4,4' - (a,a - dimethylbenzyl) diphenylcarbonate. After 55 hours, there was obtained a reaction product containing 1.69 g (75% yield) of the aromatic carbonate.

EXAMPLE 1 Preparation of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate using p- cumylphenol, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, manganese(II) bis(acetyl-acetonate), a Type 3A molecular sieve, and tetrabutylammonium bromide as a phase transfer agent.

A reaction vessel was charged with 2.12 g (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.075 g (0.00030 moles) of manganese(II) bis(acetyl-acetonate), 0.515 g (0.0016 moles) of tetrabutylammonium bromide, 0.716 g (0.0013 moles) of a 25% aqueous sodium hydroxide solution, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuoactivated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 18 hours. Liquid chromatography indicated the presence of 1.35 g (60% yield) of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate. After 42 hours, there was obtained a reaction product containing 2.01 g (89% yield) of the aromatic carbonate.

EXAMPLE 2 Preparation of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate using p- cumylphenol, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, manganese(II) bis(acetylacetonate), a Type 3A molecular sieve, and tetrabutylphosphonium bromide as a phase transfer agent.

A reaction vessel was charged with 2.12 g (0.10 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.075 g (0.00030 moles) of manganese(II) bis(acetylacetonate), 0.542 g (0.0016 moles) of tetrabutylphosponium bromide, 0.216 g (0.0013 moles) of a 25% aqueous solution of sodium hydroxide, 30 ml of methylene chloride, and 4.0 g of 200"C vacuoactivated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 1.22 g (54% yield) of 4,4'-(a,a-dimethylbenzyl)diphenylcarbonate.

EXAMPLE 3 Preparation of 4,4' - (a,- dimethylbenzyl)diphenylcarbonate using pcumylphenol, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, 3-fluoro-cobalt-salen, a Type 3A molecular sieve and a phase transfer agent, (tetrabutylammonium bromide).

A reaction vessel was charged with 2.12 g (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.109 g (0.00030 moles) of 3fluoro-cobalt-salen, 0.515 g (0.0016 moles) of tetrabutylammonium bromide, 0.216 g (0.0013 moles) of a 25% aqueous sodium hydroxide solution, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 0.247 g (11% yield) of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate.

EXAMPLE 4 Preparation of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate using pcumylphenol, carbon monoxide, aqueous potassium hydroxide, palladium(II) dibromide, manganese(II) bis(acetyl-acetonate), a Type 3A molecular sieve, and a phase transfer agent (18-crown-6).

A reaction vessel was charged with 2.12 g (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.075 g (0.00030 moles) of manganese(II) acetylacetonate, 0.34 g (0.0013 moles) of 18-crown-6, 0.35 g (0.0013 moles) of a 25% aqueous solution of potassium hydroxide, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 0.70 g (31% yield) of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate. After 64 hours, reaction product contained 1.58 g (70% yield) of the aromatic carbonate.

EXAMPLE 5 Preparation of a polycarbonate of bisphenol-A by contacting bis(4 - hydroxyphenyl)propane 2,2, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, manganese(II) bis(acetylacetonate), a Type 3A molecular sieve, and a phase transfer agent, (tetrabutylammonium bromide).

A reaction vessel was charged with 3.42 g (0.015 mole) of bisphenol-A, 0.08 g (0.00030 moles) of palladium(II) dibromide, 0.225 g (0.00090 moles) of manganese(II) acetylacetonate, 1.93 g (0.0060 moles) of tetrabutylammonium bromide, 0.38 g (0.0048 moles) of a 50% aqueous solution of sodium hydroxide, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 96 hours. Liquid chromatography indicated the presence of polycarbonate, My=609, My=824 WHAT WE CLAIM IS: 1. A process for preparing an aromatic carbonate, which comprises contacting, in the presence of a dehydrating agent a phenol, carbon monoxide, a base, a Group VIIIB element selected from ruthenium, rhodium, palladium, osmium, iridium and platinum, an oxidant comprising an element, a compound or a complex having an oxidation potential greater than that of the said selected Group VIIIB element, and a phase transfer agent (as hereinbefore defined).

2. A process as claimed in claim 1 wherein the oxidant is a manganese or cobalt complex.

3. A process as claimed in claim 2 wherein the oxidant is a manganese chelate of the formula L Mn

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

manganese(II) bis(acetylacetonate), 0.542 g (0.0016 moles) of tetrabutylphosponium bromide, 0.216 g (0.0013 moles) of a 25% aqueous solution of sodium hydroxide, 30 ml of methylene chloride, and 4.0 g of 200"C vacuoactivated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 1.22 g (54% yield) of 4,4'-(a,a-dimethylbenzyl)diphenylcarbonate.

EXAMPLE 3 Preparation of 4,4' - (a,- dimethylbenzyl)diphenylcarbonate using pcumylphenol, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, 3-fluoro-cobalt-salen, a Type 3A molecular sieve and a phase transfer agent, (tetrabutylammonium bromide).

A reaction vessel was charged with 2.12 g (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.109 g (0.00030 moles) of 3fluoro-cobalt-salen, 0.515 g (0.0016 moles) of tetrabutylammonium bromide, 0.216 g (0.0013 moles) of a 25% aqueous sodium hydroxide solution, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 0.247 g (11% yield) of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate.

EXAMPLE 4 Preparation of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate using pcumylphenol, carbon monoxide, aqueous potassium hydroxide, palladium(II) dibromide, manganese(II) bis(acetyl-acetonate), a Type 3A molecular sieve, and a phase transfer agent (18-crown-6).

A reaction vessel was charged with 2.12 g (0.010 mole) of p-cumylphenol, 0.026 g (0.00010 moles) of palladium(II) di-bromide, 0.075 g (0.00030 moles) of manganese(II) acetylacetonate, 0.34 g (0.0013 moles) of 18-crown-6, 0.35 g (0.0013 moles) of a 25% aqueous solution of potassium hydroxide, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 16 hours. Liquid chromatography indicated the presence of 0.70 g (31% yield) of 4,4' - (a,a - dimethylbenzyl)diphenylcarbonate. After 64 hours, reaction product contained 1.58 g (70% yield) of the aromatic carbonate.

EXAMPLE 5 Preparation of a polycarbonate of bisphenol-A by contacting bis(4 - hydroxyphenyl)propane 2,2, carbon monoxide, aqueous sodium hydroxide, palladium(II) dibromide, manganese(II) bis(acetylacetonate), a Type 3A molecular sieve, and a phase transfer agent, (tetrabutylammonium bromide).

A reaction vessel was charged with 3.42 g (0.015 mole) of bisphenol-A, 0.08 g (0.00030 moles) of palladium(II) dibromide, 0.225 g (0.00090 moles) of manganese(II) acetylacetonate, 1.93 g (0.0060 moles) of tetrabutylammonium bromide, 0.38 g (0.0048 moles) of a 50% aqueous solution of sodium hydroxide, 30 ml of methylene chloride, and 4.0 g of a 200"C vacuo-activated Linde Union Carbide Type 3A molecular sieve. Carbon monoxide and air were bubbled slowly through the reaction vessel mixture at room temperature for 96 hours. Liquid chromatography indicated the presence of polycarbonate, My=609, My=824 WHAT WE CLAIM IS: 1. A process for preparing an aromatic carbonate, which comprises contacting, in the presence of a dehydrating agent a phenol, carbon monoxide, a base, a Group VIIIB element selected from ruthenium, rhodium, palladium, osmium, iridium and platinum, an oxidant comprising an element, a compound or a complex having an oxidation potential greater than that of the said selected Group VIIIB element, and a phase transfer agent (as hereinbefore defined).
2. A process as claimed in claim 1 wherein the oxidant is a manganese or cobalt complex.
3. A process as claimed in claim 2 wherein the oxidant is a manganese chelate of the formula L Mn

where L. is a ligand derived from an a-diketone, a p-diketone, an omega hydroxyoxime or an ortho-hydroxyareneoxime.
4. A process as claimed in claim 2 or 3 wherein the oxidant is manganese(II) bis(acetylacetonate).
5. A process as claimed in any preceding claim wherein the phase transfer agent is a quaternary ammonium compound, a quaternary phosphonium compound, a crown ether compound, a chelated cationic salt or a cryptate.
6. A process as claimed in any preceding claim wherein reaction is carried out in a medium including at least one member of the groups (A), (B) and (C) in which (A) is an alkali metal or alkaline earth metal base, (B) is any phase transfer agent, and (C) is a manganese redox co-catalyst of an a- or ,B-diketone.
7. A process as claimed in claim 1 and substantially as hereinbefore described with reference to any of Examples 1 to 5.
8. Aromatic carbonates when prepared by a process as claimed in any of the preceding claims.