Aryl ethers, cyanation

Within the past several years, improvements in the toughening of high-temperature epoxies and other reactive thermosets, such as cyanate esters and bismaleimides, have been accomplished through the incorporation of engineering thermoplastics. Additions of poly(arylene ether ketone) or PEK and poly(aryl ether sulfone) or PES have been found to improve fracture toughness. Direct addition of these thermoplastics generally improves fracture toughness but results in decreased tensile properties and reduced chemical resistance. 

Cyanation of aryl ethers, also a carbon-carbon coupling reaction, proceeds in MeCN-Et4NCN with fair to excellent yields to methoxybenzonitriles. Here a methoxy group is replaced by cyanide [67]. 

Aryl, heteroaryl, and alkenyl cyanides are prepared by the reaction of halides[656-658] or triflates[659,660] with KCN or LiCN in DMF, HMPA, and THF. Addition of crown ethers[661] and alumina[662] promotes efficient aryl and alkenyl cyanation. lodobenzene is converted into benzonitrile (794) by the reaction of trimethylsiiyl cyanide in EtiN as a solvent. No reaction takes place with aryl bromides and chlorides[663]. The reaction was employed in an estradiol synthesis. The 3-hydroxy group in 796 was derived from the iodide 795 by converting it into a cyano group[664]. 

Subsequently, it was shown that the reaction was catalyzed by base (48M(79)106). Dicyandiamide reacts with nitriles (the most valuable method), amidines, cyanamides, ammonia, cyanates, thiocyanates, carboxylic acids and anhydrides to yield 1,3,5-triazines (Table 13). This synthetic route has been reviewed thoroughly several times (59HC(13)1, p. 219,61MI22000, p. 650, 73ZC408). The base-catalyzed reaction of dicyandiamide with alkyl or aryl nitriles (Scheme 65) proceeds via the imino ether anion and the rate determining step is solvent dependent. In DMSO the formation of the imino ether is rate determining, but in 2-methoxyethanol the reaction between the anion and dicyandiamide controls the rate (66T157). 

The first synthesis of ethyl cyanate48 from 5-ethoxythiatriazole49 was briefly mentioned in the previous review of thiatriazoles.1 The thiatriazole decomposes smoothly in ethereal solution at 20° with formation of nitrogen, sulfur, and ethyl cyanate. Although several other methods are now available for the preparation of both alkyl and aryl cyanates the thiatriazole method prevails as the most generally applicable for the thermally and chemically sensitive alkyl cyanates, since the only by-products are inert—sulfur and nitrogen. Thus a number of 5-alkoxy,50-54 5-aryloxy,55-57 and 5-aralkoxythiatriazoles12,53 have been prepared. 

The aromatic cyanates are of much more importance. If phenols are treated with cyano halides in such a way as to definitely prevent an excess of the corresponding phenolate, aryl cyanates can be isolated in up to quantitative yields. To achieve this, triethylamine is slowly added to an equimolar mixture of the phenol and the cyano halide in a nonprotic solvent, preferably acetone or n-pentane/diethyl ether (equation 27).Heteioaromatic hydroxy compounds can be treated in the same way. The method fails, however, if various electron-attracting substituents are present, as for instance with 2,4-dinitrophenol or polyhalophenols. ... 

Martin et al report investigations on the kinetics and the mechanism of aryl cyanate formation frx>m cyanogen bromide and phenols in the presence of different tertiary aliphatic amines. According to Vo-winkel," the use of diethyl ether as the solvent at -10 C raises the yields and various patents describe how the amines can be replaced by sodium hydroxide. There are even reports on work in aqueous solution under phase transfer conditions. With all synthetic variants it is crucial to avoid a surplus of the phenolate as only under these conditions can the often quite fast formation of imino carbonates be prevented (equation 28). 

Of the higher aza-compounds, only derivatives of l,2,3,4-thiatriazole are well defined, but even here alkyl derivatives decompose at or below 0 °C, though 5-aryl and amino derivatives are generally fairly stable. Many other derivatives are, however, dangerously explosive, for example the 5-chloro and 5-thiolate derivatives. The controlled decomposition of 5-aIkoxy-l,2,3,4-thiatriazoles (for example the 5-ethoxy derivative in ether at 20 °C) has been recommended as the best preparation of pure alkyl cyanates thermal decomposition of 5-aryl compounds gives the corresponding nitrile. ... 

When at least 2 equivalents of an aryl cyanate 9 are treated with one equivalent of ammonia in anhydrous ethereal solution, iminobiscarboximidates 10 can be isolated361,362 (cf. Houben-Weyl, Vol. E4, p 1073). Compounds 10 can be converted to 1,3,5-triazines by reaction with either a further equivalent of aryl cyanate, or with formylating reagents such as triethyl orthoformate.362 A direct synthesis of 1,3,5-triazines 11 from aryl cyanates and ammonium carbonate in aqueous solution has also been described.363... 

Because of their predictable behavior and reactivity, thioacyl isocyanates comprise the bulk of this work, and extensive studies of their [4 + 2] reactions with olefins,83 enamines,84 enol ethers,843 thioacyl isocyanates,85 imines,85 1 86 carbodiimides,84387 isocyanates,843 azirines,88 /3-enaminoke-tones,89 dianils,86d azines,90 hydrazones,91 imidazoline-4,5-diones,92 aryl cyanates,93 disubstituted cyanamides,93 aldehydes,94 ketones,94 ketenes,94 alkyl or aryl iminodithiocarbonates,95 and the carbon-carbon double bond of ketenimines96 have been detailed. In an extensive comparative study of the [4 + 2] cycloaddition reactions of thioacyl isocyanates, the heterocu-mulenes bearing strong electron-withdrawing substituents were found to be more stable and less prone to participate in cycloaddition reactions.84 Representative examples are summarized in Scheme 9-IV. 

Aryl cyanate esters such as bisphenol A dicyanate can be processed much like epoxy resins for many applications. This may be implied by comparing the monomer structure of bisphenol A diglycidyl ether to bisphenol A dicyanate. 

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