Cyanation mechanisms

Fig. 4a Cyanation mechanism at a Fe2S3 carbonyl center (from Refs 20 and 24).

In aqueous solution at 100° the change is reversible and equilibrium is reached when 95 per cent, of the ammonium cyanate has changed into urea. Urea is less soluble in water than is ammonium sulphate, hence if the solution is evaporated, urea commences to separate, the equilibrium is disturbed, more ammonium cyanate is converted into urea to maintain the equilibrium and evfflitually the change into urea becomes almost complete. The urea is isolated from the residue by extraction with boiling methyl or ethyl alcohol. The mechanism of the reaction which is generally accepted involves the dissociation of the ammonium cyanate into ammonia and cyanic acid, and the addition of ammonia to the latter ... 

The mechanism of the reaction involves the intermediate formation of an amine cyanate (see previous Section) in aqueous solution urea behaves as an equifi-brium mixture with ammonium cyanate ... 

Ethyl carbamate, C2HyN02, is developed naturally during the fermentation of alcohoHc beverages. It also appears in foods such as bread and yogurt. Since ethyl carbamate is not easily distilled, its formation most likely involves a distillable precursor. The mechanism of ethyl carbamate formation probably involves cyanate produced from the oxidation of cyanide or from urea-based compounds in the beer. Cyanate reacts with alcohol to form ethyl carbamate as follows ... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

Stewart and Van der Linden also examined the incorporation of into the cyanate from labelled permanganate. The percentage of transfer varied with alkalinity and the authors believe that significant oxygen-transfer occurs in the second-order reaction, but not in the complex reaction. Accordingly the mechanism for the second-order reaction is proposed to be... 

Acid hydrolysis of cyanates is still commonly used, following a first-stage cyanide oxidation process. At pH 2 the reaction proceeds rapidly, while at pH 7 cyanate may remain stable for weeks.24 This treatment process requires specially designed reactors to assure that HCN is properly vented and controlled. The hydrolysis mechanisms are as follows22 ... 

The results of most model studies for Ni-mediated urea degradation reported to date are consistent with a cyanate intermediate. While this differs from the most likely mechanism of urease activity as deduced from protein crystallography, there is still no definitive evidence ruling out a transient Ni-bound cyanate intermediate for the enzyme. 

Fig. 4.8 Suggested mechanisms for the synthesis of pyrimidines from cyanoacetaldehyde (CAA) and cyanate, urea or guanidine (Cleaves et al., 2007)...

It is worth noting that the partide sizes of samples prepared by the urea hydrolysis method are larger than other LDHs. Such a finding can be rationalized by considering the slow hydrolysis of urea [61], since it can be classified as a very weak Bronsted base (pfCb= 13.8). Its hydrolysis mechanism consists of the formation of ammonium cyanate as the rate-determining step, followed by fast hydrolysis of the cyanate to ammonium carbonate ... 

The rate of production of urea, (NH CO, from ammonium cyanate increases by a factor of 4 when the concentration of ammonium cyanate is doubled. Show whether this is accounted for by the following mechanism ... 

The selectivity of the hydrogen transfer is excellent When employing a catalyst with deuterium at the a-positions of the isopropoxide ligands (17), complete retention of the deuterium was observed. A computational study using the density functional theory comparing the six-membered transition state (as in Scheme 20.3, the direct transfer mechanism) with the hydride mechanism (Scheme 20.3, the hydride mechanism) supported the experimental results obtained [36]. A similar mechanism has been proposed for the MPV alkynylations [37] and cyanations [38]. 

The experiments described above indicated amino acids were oxidatively deaminated in liver and their a-amino groups converted to urea. A start on investigations of the mechanism of urea biosynthesis was made by Schultzen and Nenki (1869) who concluded that amino acids gave rise to cyanate which might combine with ammonia from proteins to produce urea. Von Knieren (1873) demonstrated that when he drank an ammonium chloride solution, or gave it to a dog, there was an increase in the formation of urea, without any rise in urinary ammonia. His results were consistent with the cyanate theory but did not eliminate the possibility that urea arose from ammonium carbonate which could be dehydrated to urea ... 

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. 

The proposed mechanism is as follows initial cyanation of the acyl silane followed by a [1,2]-Brook rearrangement yields acyl anion equivalent XIV (Scheme 4). Subsequent attack by the acyl anion equivalent XV to the aldehyde leads to... 

A 1-1. three-necked round-bottomed flask fitted with a mechanical stirrer, a thermometer, and a calcium chloride drying tube is immersed in an ice-salt bath. To the flask are added 38 g. (0.25 mole) of silver cyanate (Note 1), 34.7 g. of 75% 1,2-... 

There is a primary alcohol-to-aldehyde step in the synthesis of (+)-batzelladine A, and it was suggested that the oxidation of the primary alcohol (1) with TRAP/ NMO/PMS/CH Cl proceeds through an iminium-Ru alkoxide complex (2), rearranging as in (3)-(4) to give the aldehyde (5) (Fig. 1.13) [101] (a similar mechanism was proposed for the Ru-catalysed oxidative cyanation of tertiary amines [403] cf. 5.1.3.4, Fig. 5.3). 

Spirocychc oxazohnes 311 are produced in 37-67% yield from acylisothio-cyanates 309 and an excess of diphenyldiazomethane 310. ° The proposed mechanism for product formation is indicated in Scheme 8.86. The reaction has also been demonstrated for 2-diazofluorene. There are no reports for aliphatic diazo compounds. 

A similar mechanism could explain the reaction of benzhydroxamyl chloride and potassium cyanate which leads to 3-phenyl-5-hydroxy-oxadiazole (56). 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

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