Cyanates, preparation, thermally

Silyl isocyanates can readily be prepared by treatment of chlorosilanes (bromo- or iodosilanes would also work but are less readily available) with metal cyanates. With simple halosilanes the product is always the isocyanate isomer although in the case of highly sterically hindered organosilanes, normal cyanates (which thermally isomerize to isocyanates) may be formed using silver cyanate. Thus Si(NCO)4 is formed from the reaction between SiCU and KOCN in liquid SO2, AgOCN in CeHe, or Pb(OCN)2 in Similarly, Si(NCS)4 is formed in... 

Cyclohexylurea has been prepared by the reaction of cyclo-hexyl isocyanate with gaseous ammonia or ammonium hydroxide, by thermal decomposition of cyclohexyl allophanamide, by treating cyclohexylamine hydrochloride with an aqueous solution of potassium cyanate," by heating nitrosomethylurea with cyclohexylamine, and by heating an ethanolic solution of cyclohexylamine and 3,5-dimethyl-l-carbamylpyrazole. 2,6-DimethyIphenyIthiourea has been synthesized by allowing 2,6-dimethylaniline hydrochloride to react with ammonium thiocyanate. ... 

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 oxazolidin-2-ones 53 (R = H=CCH=CH2 or COEt) are obtained in a one-pot reaction of amino alcohol carbamates 52 with sodium hydroxide, followed by allyl bromide or propi-onyl chloride (94TL9533). A modified procedure for the preparation of chiral oxazolidin-2-ones 56 from a-amino acids 54, which avoids the hazardous reduction of the acids with borane and the intermediacy of water-soluble amino alcohols, is treatment of the methyl ester of the amino acid with ethyl chloro-formate to give 55, followed by reduction with sodium borohydride and thermal ring-closure of the resulting carbamate f95SC561). The 2-prop-ynylcarbamates 57 (R = Ts, Ac, Bz, Ph or allyl) cyclize to the methyleneoxazolidinones 58 under the influence of silver cyanate or copper(I) chloride/triethylamine (94BCJ2838). 

Further reaction procedures leading to alkyl cyanates cannot be discussed here. Mention has to be made, however, of the sometimes particularly useful thermal degradation of 5-alkoxy-l,2,3,4-thiatri-azoles, which are often only prepared in solution. ... 

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. ... 

Similarly, an aqueous treatment for wool uses an isocyanate blocked with a bisulfite. In this case, the by-product bisulfite salt can be removed, and possibly recycled, by washing the fabric with water at the end. The dissociation temperature decreases in the following order alcohol > lactam > phenol > ketoxime > active methylene compound. e-Caprolactam is a typical lactam, ethyl acetoacetate a typical active methylene compound. This thermal reversibility has been used to prepare a polyurethane from 1,4-butanediol, 4,4 -diphenyldiiso-cyanate, and 4,4 -isopropylidenediphenol (bisphenol A) (2.30) that can be recycled just by remolding.76... 

Su WFA (1993) Thermoplastic and thermoset main-chain liquid-crystal polymers prepared from biphenyl mesogen. J Polym Sci Polym Chem 31 3251-3256 Su WFA, Chuang CM (2002) Effects of chemical structure changes on curing reactions and thermal properties of cyanate ester-cured rigid-rod epoxy resins. J Appl Polym Sci 85 2419-2422... 

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