Diaryl carbonate

Cychc carbonates are prepared in satisfactory quaUty for anionic polymerization by catalyzed transesterification of neopentyl glycol with diaryl carbonates, followed by tempering and depolymerization. Neopentyl carbonate (5,5-dimethyl-1,3-dioxan-2-one) (6) prepared in this manner has high purity (99.5%) and can be anionically polymerized to polycarbonates with mol wt of 35,000 (39). 

Transesterification has become a convenient method for synthesi2ing high alkyl, aryl, or alkyl aryl carbonates. Fiber- and film-forming polycarbonates are produced by transesterifying dialkyl, dicycloalkyl, or diaryl carbonates with alkyl, cycloalkyl, or aryl dihydroxy compounds (62). 

Reaction with Phenols. Phenols react with diphenyl carbonate in the presence of bases or organometaHic catalysts to produce diaryl carbonates. A specific example is the reaction of diphenyl carbonate with bisphenol A [80-05-7] to produce polycarbonate. 

Carbonates ate manufactured by essentially the same method as chloroformates except that more alcohol is required in addition to longer reaction times and higher temperatures. The products are neutralized, washed, and distilled. Corrosion-resistant equipment similar to that described for the manufacture of chloroformates is requited. Diaryl carbonates are prepared from phosgene and two equivalents of the sodium phenolates or with phenols and various... 

The reaction of cycloheptaamylose with diaryl carbonates and with diaryl methylphosphonates provides a system in which a carboxylic acid derivative can be directly compared with a structurally analogous organo-phosphorus compound (Brass and Bender, 1972). The alkaline hydrolysis of these materials proceeds in twro steps, each of which is associated with the appearance of one mole of phenol (Scheme Y). The relative rates of the two steps, however, are reversed. Whereas the alkaline hydrolysis of carbonate diesters proceeds with the release of two moles of phenol in a first-order process (kh > fca), the hydrolysis of methylphosphonate diesters proceeds with the release of only one mole of phenol to produce a relatively stable aryl methylphosphonate intermediate (fca > kb), In contrast, kinetically identical pathways are observed for the reaction of cycloheptaamylose with these different substrates—in both cases, two moles of phenol are released in a first-order process.3 Maximal catalytic rate constants for the appearance of phenol are presented in Table XI. Unlike the reaction of cycloheptaamylose with m- and with p-nitrophenyl methylphosphonate discussed earlier, the reaction of cycloheptaamylose with diaryl methylphosphonates... 

Maximal Rate Constants and Dissociation Constants of Cycloheptaamylose Complexes of Diaryl Carbonates and Methylphosphonates at 25.5° ... 

Spontaneous hydrolyses of carboxylic anhydrides, diaryl carbonates and aryl chloroformates are faster in cationic than in anionic micelles, regardless of the nature of the counteranion in the cetyltrimethylammonium micelle (Al-Lohedan et al., 1982b Bunton et al., 1984). This charge effect does not seem to be related to substrate hydrophobicity, although the extent of micellar inhibition (relative to reaction in water) is clearly dependent upon substrate hydrophobicity for anhydride hydrolyses. 

Examples of this behaviour are shown in Table 7 where k+ is related to reaction of substrate fully bound to a CTAX micelle and k to reaction in an anionic micelle of SDS. The ratio k+/k is consistently larger than unity for hydrolyses of open chain anhydrides, diaryl carbonates and aryl chloroformates. In addition hydrolysis of 4-nitrophenyl chloroformate is slightly faster in cationic micelles than in water. Spontaneous hydrolyses of N-acyl triazoles are also inhibited less by cationic micelles of CTABr than anionic micelles of SDS (Fadnavis and Engberts, 1982). 

It is reasonable to relate the values of k+fk at least qualitatively to the extents of bond making and breaking in the transition state. Bond making is all important in hydrolyses of carboxylic anhydrides, diaryl carbonates and methyl arenesulfonates. Bond breaking will be important in hydrolyses of alkyl halides and sulfonates, except for methyl derivatives, and especially so in water which can effectively solvate the leaving anion. 

Other types of ester have been studied (Fendler and Fendler, 1975 Bender and Komiyama, 1978 Szejtli, 1982), though in much less detail. Brass and Bender (1973) studied the cleavage of two diaryl carbonates and three diaryl methylphosphonates in basic buffers (Table A5.ll). For the carbonates, reacting with /1-CD, introduction of p-nitro groups increases the acceleration ratio and worsens substrate binding, so that KTs barely alters. More interesting are the results for the phosphonates in that the effects of nitro groups depend on their position and on the CD. 

Table AS. 11 Basic cleavage of diaryl carbonates and diaryl methylphosphonates by cyclodextrins."...

Direct experimental evidence relating to the existence of free radicals 65-67 and 68 among the photoproducts of aryl esters has not yet been obtained. On the photolysis of diaryl carbonates, no features attributable to trapped radical parts were seen.49 In the esr spectra obtained at 77°K only a single weak line at g = 2.005 due to phenoxy radical was detected.49 A much more intensive spectrum of the phenoxy radical was observed by irradiation... 

Transesterifications of aliphatic carbonate esters with glycols are catalysed by alkali metal alkoxides. No catalyst is needed for the transesterification of diaryl carbonates with aliphatic diols. Alkyl carbonate esters and p-xylylene glycol undergo transesterification reactions when certain titanium compounds are used as catalysts. The preparation of aromatic polycarbonates by transesterification is best... 

Diaryl carbonates (e. g. carbonyl diimidazol, 4-nitrophenyl carbonate) can react sequentially with carbohydrate derivatives to furnish mixed sugar carbonates (O Scheme 81f) [456]. Although normally anomeric mixtures are generated the use of a succinimidyl group, in the presence of K2CO3, was effective for the synthesis of pure 8-carbonates. 

Obviously only dimethyl carbonate is obtained in acceptable selectivities and space-time yields according to this method, although the analogous preparation of higher dialkyl carbonates and simple cyclic carbonates has also been described. Phenols do not give diaryl carbonates by CuCl-catalyzed oxidative carbonylation. 

Diaryl carbonates are made from the reaction of phosgene with two molar equivalents of the particular sodium phenolate [3]. However, the direct conversion of phenols can be achieved by reaction of phosgene in the presence of quaternary ammonium salts, or acid acceptors (such as pyridine), at ambient temperatures [2016]. The three dihydroxybenzenes... 

Reactions of the phenoxides with phosgene have also been used to produce diaryl carbonates ... 

Janatpour, M. Shafer, S.J. Process for Making Diaryl Carbonates European Patent 228,672 A2, July 15, 1987. 

In order to obtain purer products, a process based on the transesterification of a glycol and a dialkyl or diaryl carbonates (reaction 8.33) was developed. 

Aqueous cationic micelles speed and anionic micelles inhibit bi-molecular reactions of anionic nucleophiles. Both cationic and anionic micelles speed reactions of nonionic nucleophiles. Second-order rate constants in the micelles can be calculated by estimating the concentration of each reactant in the micelles, which are treated as a distinct reaction medium, that is, as a pseudophase. These second-order rate constants are similar to those in water except for aromatic nucleophilic substitution by azide ion, which is much faster than predicted. Ionic micelles generally inhibit spontaneous hydrolyses. But a charge effect also occurs, and for hydrolyses of anhydrides, diaryl carbonates, chloroformates, and acyl and sulfonyl chlorides and SN hydrolyses, reactions are faster in cationic than in anionic micelles if bond making is dominant. This behavior is also observed in water addition to carbocations. If bond breaking is dominant, the reaction is faster in anionic micelles. Zwitterionic sulfobetaine and cationic micelles behave similarly. 

Bond making is clearly dominant in spontaneous hydolyses of carboxylic anhydrides and diaryl carbonates and here k+/k > 1 (Table IV). These values of k+/k are not related in any obvious way to the reactivity or hydrophobicities of the substrates, although hydrophobicity seems to affect the overall micellar inhibition, probably because the more hydrophobic substrates penetrate the micelles and are shielded from water molecules. 

Hydrolyses of acyl halides are sometimes described in terms of the Sn1-Sn2 duality of the mechanism, or variants of it (56, 57), but these descriptions are unsatisfactory because they neglect the possibility of rehybridization of the carbonyl group in the course of reaction. Strongly electron withdrawing substituents favor nucleophilic addition by water to acyl centers, with assistance by a second water molecule acting as a general base (56-60), and good evidence for this mechanism exists in hydrolyses of carboxylic anhydrides and diaryl carbonates. This addition step should be followed by very rapid conversion of an anionic covalent intermediate into products, and the intermediate should have only a transient existence, at most, in polar, nucleophilic solvents. 

Some novel procedures have been introduced for the synthesis of diaryl carbonates, substances of interest in polymer chemistry. Unsymmetrical compounds in the series have been obtained by the slow addition over 5 hours at OX of... 

As already mentioned, water removal in the oxidative carbonylation of phenols to make diaryl carbonates is a necessary process since it greatly enhances the productivity of the reaction and thus reduces reactor cost per unit mass of product. The use of 3 or 4 A molecular sieves, effectively removing water in laboratory-scale runs [8d], is impractical at an industrial scale. 

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