With cyanates

Isocyanic esters have only rarely been prepared by alkylation of salts of cyanic acid by alkyl halides674 675 (cf. pages 472 and 500). 

Butyl isocyanate 676 Potassium cyanate (40.5 g) is heated at ca. 90° under reflux for 15 min with butyl bromide (75 g) in anhydrous dimethylformamide (250 ml dried over calcium hydride). Then distillation gives a crude product, b.p. 105-135° (98 g isocyanate content 41 g, 82%), which on further fractionation gives material of b.p. 112-115° (37 g, 75%). 

Acyl isocyanates can be prepared from silver cyanate and acid halides. 

Because of their great sensitivity to water, acyl isocyanates must be rigidly protected therefrom. 

When they are prepared in anhydrous ethereal solution, the reaction is usually complete in 1-3 h at room temperature or after gentle warming.677 678 For preparation of benzoyl isocyanate, silver cyanate and benzoyl chloride were heated under reflux for 6 h in carbon tetrachloride.679 Older preparations, e.g., that of acetyl isocyanate,680 were made also without solvent but under cooling. 

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Specialty Isocyanates. Acyl isocyanates, extensively used in synthetic appHcations, caimot be direcdy synthesized from amides and phosgene. Reactions of acid haUdes with cyanates have been suggested. However, the dominant commercial process utilizes the reaction of carboxamides with oxalyl chloride [79-37-8]. CycHc intermediates have been observed in these reactions which generally give a high yield of the desired products (86). 

Because of this continued emphasis on adhesive bonding technology development over the years, the airframes of modem front-line aircraft such as the B-2 bomber and the F-117 and F-22 fighters are largely structurally bonded advanced composites. They tend to be comprised of materials that are more advanced (expensive) than commercial aircraft such as carbon and boron fiber reinforcements with cyanate esters, bismaleimides, polyimides or other high-temperature resin matrices and adhesives. 

Reports of the synthesis of cytosine from cyanoacetylene (or its hydrolysis product cyanoacetaldehyde) with cyanate, cyanogens or urea show that these substances react faster with nucleophilic compounds to give side products than to give the required main product. In addition, the formation of cytosine requires concentrations which are unrealistic in prebiotic environments. 

Cyanate ester monomers must be stored under dry conditions, because water can react with cyanate ester resins and deliver carbamates as undesired side products (Scheme 2) [186]. Even when the carbamates are only formed in the presence of a catalyst, this catalytic effect can be caused by traces remaining from the monomer synthesis. The formation of carbamates is critical, as they can decompose to amines and CO2. While the amine easily reacts with another cyanate ester, the CO2 can act as a blowing agent and hence leads to uncontrolled porosity during the processing. 

Alkyl or aryl aminothiatriazoles (330) in contrast react with cyanates even at — 70°C to give 5-amidino substituted 1,2,4-thiadiazoles (331) weaker electrophiles, such as isothiocyanates, thiocyanates, and cyanamide do not react with aminothiatriazoles <85JOCl295>. 

The Reissert-Henze and the Feely-Beavers-Tani reactions are considered together in this section because of their similarity. The former involves cyanation of acyloxy (formed in situ) (Scheme 113), and the latter alkoxy (Scheme 114), quaternary salts. The Reissert-Henze reaction is a facile, fairly general reaction for quinoline and isoquinoline AT-oxides (Table 19) with cyanation occurring a to the ring nitrogen. Certain substituents inhibit reaction, for example a 1-methyl group (equation 125), and others undergo replacement (Scheme 130) (81H(15)98l). Reaction of 1-methylisoquinoline 2-oxide with benzoyl chloride... 

In contrast, monosubstituted arylhydrazines react with cyanates to yield 1-semicarhazides (Eq. 9), probably as a result of the decreased basicity of the... 

Table 57 Selected Nickel(II) Complexes with Cyanate, Isothiocyanate and Isoselenocyanate Anions...

Platinum(II) halides react with cyanate salts to give cyanato complexes. Reacting the compounds Pt(NCO)4 with triphenylphosphine gives Pt(NCO)2(PPh3)2.1628 These platinum(II) complexes are N-bonded. Treatment with carbon monoxide and alcohol yields the complex (172 equation 476).1629... 

In general these reactions do not interefere with cyanation. Reaction 7 is a catalytic process (14) and is strongly favored by electron-releasing substituents on the aromatic halide. In fact in the case of p-aminochloro-benzene, formation of the phosphonium salt competes with the cyanation process. 

Reactions with Cyanate and with Diazonium-lH-tetrazole... 

The prebiotic synthesis of the pyrimidine cytosine involves cyanoac-etylene, which is synthesized in good yield by sparking mixtures of CH4 + N2. Cyanoacetylene reacts with cyanate to give cytosine,29... 

In the case of amine nucleophiles, the products from the reaction with co-ordinated cyanates are carbamates or ureas (Fig. 4-34), and this provides a particularly convenient method for the preparation of carbamate complexes. An example of this behaviour is seen in the reaction of 3,5-dimethylpyrazole with cyanate in the presence of copper(n) salts (Fig. 4-35). Similar reactions are observed with co-ordinated thiocyanates and other heterocumulene s. 

Treat the sample with Na2S203 to destroy oxidizing substances (such as Cl2 residual chlorine) that may react with cyanate. Add EDTA to complex metal ions which may be present in the sample and which may interfere by forming colored complexes with Nessler reagent. 

In 1849, Wurtz discovered the formation of aliphatic isocyanates when he reacted organic sulfates with cyanates. This represents the first recorded experimental work leading ultimately to the preparation of urethanes ... 

Pellizzari and Roncagliolo2 prepared this compound by reaction of carbohydrazide in dilute acetic acid with 2 mols of potassium cyanate and also by condensation of carbohydrazide- A-carboxamide hydrochloride with 1 mol of cyanate. The former procedure does not give a pure product, probably because of the preferential formation of the less soluble carbohydrazide-A-carboxamide. The latter reacts wjth hydrochloric acid to form the hydrochloride, which is water-soluble. Prior condensation of carbohydrazide to form the A-carboxamide is advantageous, since the hydrochloride of this compound will undergo further reaction with cyanate to give the desired product by precipitation from water. When the procedure outlined below is followed, it is not necessary to recrystallize the carbohydrazide-A, A -dicarboxamide unless an extremely pure material is desired. 

More complex, but still feasible, is the synthesis of pyrimidine bases from simple prebiotic substrates, although the reported yields of these reactions are relatively low. In this context, two main prebiotic precursors have been identified cyanoethine and a primary product of its hydrolysis, cyanoacetaldehyde. These compounds contain a preformed C-C bond which is incorporated in the C5-C6 position of the pyrimidine ring. In 1968 Ferris and co-workers reported that the reaction of cyanoethine with cyanate at 30 °C yields cytosine and, after its hydrolysis, uracil in acceptable yield [27]. trans-Cyanovinylurea was recovered as a key intermediate for this transformation. However, this reaction requires relatively high concentrations of cyanate (>0.1 mol/1), unlikely to occur in aqueous media due to its rapid degradation to carbon dioxide and ammonia. Cyanoethine also reacts with cyanate and yields cytosine and uracil at elevated temperatures. In this reaction urea or guanidine (also considered as prebiotic organic compounds) can easily replace cyanate (Figure 8.8) [26]. 

Complexes with cyanate were prepared [143] by reaction of anhydrous lanthanide chlorides with an excess of tetraethylammonium cyanate in absolute ethanol. The complex has the formula [(C2H5)4N]3Ln(NCO)6, where Ln = Eu, Gd, Dy, Ho, Er, and Yb and they are indicated to be six-coordinate and cyanate is N-bonded as evidenced by IR spectra [147]. Mixed complexes [(C2H5)4N]3[Ln(NCO)3X3], where Ln = Dy, Er and Yb and X = C1, NCS have been synthesized and they appear to be of six-coordinate [144]. Mixed complexes of the type [(C2H5)4N]3[Ln(NC0)3(N03)3], where Ln = La, Pr, Nd, Sm, Eu, Gd, Er and Yb have been synthesized and the IR spectra shows N-bonded cyanate, bidentate nitrate and the overall coordination number amounting to nine [145]. 

Table 12 Anionic Species with Cyanate, Thiocyanate and Selenocyanate...

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