Cyanate ions, reactions

When the reagent is the thiocyanate ion, S-alkylation is an important side reaction (10-43), but the cyanate ion practically always gives exclusive N-alkylation. ° Primary alkyl halides have been converted to isocyanates by treatment with sodium nitrocyanamide (NaNCNN02) and m-chloroperoxybenzoic acid, followed by heating of the initially produced RN(N02)CN. ° When alkyl halides are treated with NCO in the presence of ethanol, carbamates can be prepared directly (see 16-7). ° Acyl halides give the corresponding acyl isocyanates and isothiocyanates. For the formation of isocyanides, see 10-111. 

A phase-transfer catalysed nucleophilic displacement reaction on chloro-acetanilides by cyanate ions, followed by ring-closure (Scheme 5.10), provides a simple and viable synthesis of hydantoins [41], The formation of the hydantoins is inhibited by substituents in the orf/to-position of the aryl ring, but the addition of potassium iodide, or tetra-n-butylammonium iodide, generally increases the overall rate of formation of the cyclic compounds, presumably by facilitating the initial nucleophilic substitution step. 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

No reaction of excited 1-nitroazulene with hydroxide and cyanate ions or p nidine could be detected... 

Again, a hnear relationship of and [CNS] i shows a bimolecular reaction between the excited triplet state of 1 and the nucleophile to take place. The triplet lifetime of la is 4.7 X 10 s in water and 1.2xl0 8s in aqueous 10 2M solutions of potassium cyanide as determined from quenching studies The nitro group in 7 a is likewise replaced photochemically by methoxide and cyanate ions. 

The structures of lithium and sodium cyanates and isocyanates and their related ion-pair 5n2 reactions have been examined by using quantum mechanics at the Hartree-Fock (HF)/6-31G V/HF/6-31G level. (The cyanate ion is NCO the isocyanate ion is CNO. ) The isocyanate ion pairs are the most stable monomeric forms the lowest energy dimers are planar eight-membered rings. For the ionic 5 n2 reaction of cyanate ion with MeF or MeCl, methyl isocyanate is the predicted major product. Predictions about the 5ivr2 reactions of the ion pairs were also made. 

A few examples will show the importance of these reactions in synthetic organic chemistry. If X is an alkoxide ion, equation (1) describes the Williamson ether synthesis (2). If X is a cyanide ion, (1) describes a standard nitrile preparation (3). If X is an aromatic nucleus, and the departure of the leaving group is facilitated by A1C13, then (1) describes the Friedel-Crafts alkylation (4). With X as the cyanate ion, (1) describes the preparation of isocyanates (5) and if it is ammonia, (1) describes a common method of preparation of amines (6). In addition, nucleophilic transmethylation is a very... 

Complete and balance an equation for the reaction in which cyanate ion (CNO ) is oxidized to CO2 and N03 by Mn04, in which two elements in the same chemical species are oxidized. (Hint The carbon to nitrogen ratio is set by the formula of CNO. )... 

Lister suggested that the rate-determining step is a bimolecular reaction between hypochlorous acid and cyanate ions. The subsequent steps are very uncertain. From measurements in the temperature range 50-75 °C, the activation energy of the rate determining step is 15.1 kcal.mole and the -factor is 3.2x10 1. mole sec" ... 

Halomercaptans are converted to thietanes by treatment with a as exemplified by the formation of 3-hydroxythietane 21 from 3-chloro-l-mercapto-2-propanol 20 4-thiocyano-2-pentanol 22 is converted to a mixture of cis- and trans-2,4-dimethylthietane 23 by treatment with sodium hydride. The latter reaction involves displacement of cyanate ion by the mercaptide ion. 2,2,4-Trimethylthietane was also prepared by this method. 

Treatment of alkyl halides with alkali metal or silver cyanate is of no preparative value for the synthesis of cyanates. - - As cyanate ions do represent ambident nucleophiles. Holm and Wentrap tried to reach cyanates under well-defined SnI conditions (Scheme 28). But even treatment of isopropyl iodide with silver cyanate in nitromethane provided only a very low yield of a 1 1 mixture of isopropyl cyanate and the corresponding isocyanate, and from a reaction of silver cyanate with triphenylchloro-methane they isolated only triphenylmethyl isocyanate, although the cyanate may have been an intermediate in this transformation." ... 

The CE for the cyanide destruction process is obviously determined by the extent of the parallel reaction (7), a factor which becomes much more important as the cyanide levels drops to below 100 ppm [88]. Complete mineralization of the CN-by reaction (8) removes all traces of the offending species, but at the expense of considerably more charge consumption. It may be reasonable to stop the reaction at the CNO- stage, something which is quite feasible since reaction (8) occurs with more difficulty than (6) [89] at a potential of almost 0.5 V more anodic [87]. At a PH < 10, the cyanate ion hydrolyzes on its own according to the following reaction ... 

Treatment of Electroplating Wastewater. Toxic cyanide ions are the major pollutant in electroplating wastewater. They must be removed before discharge of the wastewater. As shown in Eqs. (3) and (4), ozone can oxidize free cyanide ion rapidly to less toxic cyanate ion, which then slowly hydrolyzes to nitrogen and ammonia. The reaction equations are as follows " ... 

Houdson RL, Moore MH. Reaction of nitriles in ices relevant to Titan, comets and interstellar medium formation of cyanate ion, ketenimines and isonitriles. Icarus 2004, in press. 

Mild oxidizing agents convert [CN] to cyanogen (equation 13.72) but with more powerful oxidants such as PbO or neutral [Mn04], cyanate ion, 13.34, is formed (reaction 13.79). Potassium cyanate reverts to the cyanide on heating (equation 13.80). 

The fulminate ion, [CNO] , is an isomer of the cyanate ion. Fulminate salts can be reduced to cyanides but cannot be prepared by oxidation of them. The free acid readily polymerizes but is stable for short periods in Et20 at low temperature. Metal fulminates are highly explosive mercury(II) fulminate may be prepared by reaction 13.82 and is a dangerous detonator. 

An interesting synthetic approach was reported in 1994. In this reaction, A-fluoropyridinium salts (137) were treated with nitriles in the presence of cyanate ion, affording pyridotriazines (138) together with the amides (139) (Equation (23)) <94TL207>. A carbene, derived from (137), was proposed as intermediate. 

A synthesis of the novel 2-substituted pyrido[l,2-a]-l,3,S-triazin-4-ones (12) has been achieved by reacting iV-fluoropyridinium salts (11) with cyanate ion and carbonitiiles. The evidence indicates that the reaction proceeds via a carbene intermediate [94TL207]. 

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