Intermediate isocyanic acid

Urea thermolysis is usually considered a solely thermal reaction, whereas the intermediate isocyanic acid (HNCO) is stable in the gas phase but hydrolyzes on the SCR catalyst or on a dedicated hydrolysis catalyst [9]. Catalytic reactions wiU be discussed later this section is focusing on thermal decomposition. 

In aqueous alkaline solution, the isocyanate is unstable it reacts by addition of water to give the intermediate carbaminic acid 5, which subsequently decarboxy-lates to yield the amine 3. 

For reasons of safety and toxicity, urea is the preferred selective reducing agent for mobile SCR applications. Under the hydrothermal conditions in the exhaust system, urea decomposes to ammonia which reduces the nitrogen oxides on the surface of the SCR catalyst [18,19], If urea is used instead of ammonia, the DeNO chemistry involves isocyanic acid as an important intermediate which will lead to a complication of the SCR chemistry [20],... 

Different reactions pathways on Rh may explain the intermediate formation of ammonia. NH3 can be obtained via successive reaction steps between adsorbed NHX and dissociated hydrogen species [29]. Alternately, the formation of ammonia may occur via the hydrolysis of isocyanic acid (HNCO) [30]. Isocyanate species are formed by reaction between N and COads on metallic particles. Those species can diffuse onto the support leading to spectator species or alternately react with Hads yielding ultimately HNCO. Previous infrared spectroscopic investigations pointed out that isocyanate species predominantly form over rhodium-based catalysts [31]. 

The acidic nature of the reagent is important the trifluoroacetic acid liberated in the reaction catalyzes hydrolysis of the intermediate isocyanate, and also ensures that the amine which is formed is protonated and cannot react with the isocyanate to give urea by-products. The reaction can be accelerated by addition of pyridine to an observed pH of about 3.5, and is retarded by added acid or trifluoroacetate ion. In the present procedure pyridine was not employed, since the reaction in its absence proceeds with a satisfactory rate. 

Methyl-3,3 -bipyridine has been oxidized by permanganate to 3,3 -bi-pyridine-4-carboxylic acid. " 3,3 -Bipyridine carboxylic acids are easily decarboxylated and have been esterified and converted to amides, hydrazides, and acylazides. The Hofmann degradation, of the diamide of 3,3 -bipyridine-2,2 -dicarboxylic acid affords the expected 2,2 -diamino-3,3 -bipyridine, but some of the tricyclic system 108 is formed as well. A 2,2 -bis(acylazide) is converted to a similar tricyclic system with ethanol via the intermediate isocyanate, and several related reactions have been described. The simultaneous dehydration... 

Usually, amides 7 are prepared from acids 6 and amines. C. Gtirtler of Bayer MaterialScience AG in Leverkusen reports (Tetrahedron Lett. 2004,45, 2515) the develop of catalysts for the alternative condensation of an acid 6 with an isocyanate 4. It is particularly exciting that isocyanates are intermediates in the one-carbon degradation of an acid 3 to the corresponding amine. Current practice, if the protected amine were desired, is that the intermediate isocyanate 4 would be trapped with an alcohol, leading to the urethane 5. This newly-reported observation offers the alternative of ending with the amide 7, or perhaps with the sulfonamide 9. 

Amines have been prepared on insoluble supports by Hofmann degradation of amides [222] followed by hydrolysis of the intermediate isocyanates (Figure 10.5). One reagent suitable for this purpose is [bis(trifluoroacetoxy)iodo]benzene, which can be used both on cross-linked polystyrene [223] and on more hydrophilic supports such as polyacrylamides (Figure 10.6). Support-bound carboxylic acids can also be degraded via the acyl azides (Curtius degradation [224,225]) to yield isocyanates. 

In order to get information on the surface species responsible for the spectra obtained after adsorption of both cyclic nitro compounds, the adsorption of several potential intermediates has been investigated. It can be concluded from the adsorption of isocyanic acid that the absorption around 2190 cm 1 can be due to surface isocyanate species. To explain the absorption bands in the range 1600-1200 cm1, the first idea was a surface amide structure, such as was observed by Krietenbrink et al.,IS However, considering the low stability of this adsorption complex and the disappearance of the band around 1475 cm1 when going from C6HSN02 to C6D5N02 (a shift down to approximately 1395 cm1), the conclusion is arrived at that such an assignment would be incorrect. 

A suspension of 4 g of trans-2-(2,5-dimethoxy-4-methylphenyl)-cyclopropanecarboxylic acid in an equal volume of H20, was treated with sufficient acetone to effect complete solution. This was cooled to 0 °C and there was added, first, 2.0 g triethylamine in 35 mL acetone, followed by the slow addition of 2.5 g ethyl chloroformate in 10 mL acetone. This was stirred for 0.5 h, and then there was added a solution of 1.7 g NaN i in 6 mL If20, dropwise. After 1 h stirring at 0 °C, the mixture was quenched by pouring into H20 at 0 °C. The separated oil was extracted with EtzO, and extracts dried with anhydrous MgS04. Removal of the solvent under vacuum gave a residue of the azide, which was dissolved in 10 mL anhydrous toluene. This solution was heated on the steam bath until the nitrogen evolution was complete, and the removal of the solvent under vacuum gave a residue of crude isocyanate as an amber oil. This intermediate isocyanate was... 

RHF methodology, used to investigate the cycloaddition reaction of isocyanic acid with methylenimine, confirmed a two-step mechanism via a cis intermediate for the equimolar reaction.22 The 2 + 2-cycloaddition of chlorosulfonyl and trichloroacetyl... 

The use of the toxic and hazardous hydrazoic acid is avoided by generating it in situ by adding sodium azide gradually to the carboxylic acid in the presence of concentrated sulphuric acid and chloroform (eg.. 3,5-dinitroaniline, Expt 6.54). The reaction involves the hydrolysis of an intermediate isocyanate (RNCO), which is formed by a mechanistic pathway analogous to that involved in the Hofmann reaction. 

An ab initio study of the 2 + 2-cycloadditions of allene to isocyanic acid and ketene to vinylimine found the reactions to be concerted and mostly asynchronous.28,29 The diastereoselective 2 + 2-cycloaddition of dichloroketene with a chiral enol ether (26) produced the cyclobutanone (27), which leads to a key intermediate (28) in (g) the total synthesis of the natural alkaloid (-)-Swainsonine (29) (Scheme 8).30 The... 

Several other synthetic sequences have been developed that lead to the production of potentially useful intermediates for the total synthesis of galanthamine-type alkaloids. For example, the 4-arylbutyric acid 348 has been converted to the tetrahydrobenzazepine 349 by a modified Curtius reaction followed by cycliza-tion of the intermediate isocyanate with polyphosphoric acid (168). N-Methyla-tion of 349 and photooxidation of the resulting tertiary lactam in the presence of NBS gave 350. 

Methyl isocyanate and all isocyanic acid esters are an interesting and highly reactive class of organic compounds, since the isocyanate group (-NC0) reacts readily with a wide variety of compounds as well as with itself to form dimers, trimers, ureas, and carbodi-imides. Methyl isocyanate (MIC) is an intermediate in the preparation of carbamate pesticides and conceivably could be applied to the production of special heterocyclic polymers and derivatives. 

Cellulose reacts with isocyanates in anhydrous pyridine or with urea and substituted ureas at relatively high temperature to yield carbamates. The optimum carbamation reaction of microcrystalline cellulose with urea in a dry solid mixture has been studied [51]. In addition, a preferentially C6-modified cellulose carbamate derivative has been obtained [52]. Heating of cellulose with thiourea at 180°C yielded cellulose thiocarbamate [53]. Heat treatment of cellulose isocyanate products has been utilized for the production of urethanes [54]. When ceUuIose was treated with phenylisocyanate at 100 C in DMF in the presence of dibutyltin dilaurate and triethylenediamine, celiuiose bisphenylcarbamate was formed [55]. Treatment of cellulose with urea at temperatures at or above the latter s melting point (where urea decomposes into isocyanic acid and ammonia) has been employed for the production of cellulose carbamates fibers [56]. The advantages and disadvantages of using urea as an intermediate for production of fiber have been discussed [57]. 

Carboxylic acid azides give rise to three different reactions under different conditions. Azide coupling (equation 9) was the earliest method in peptide synthesis and is still one of the most important in fragment condensation and preparation of cyclic peptides due to its almost complete lack of racemization. At elevated temperatures a frequent side reaction is the Curtius rearrangement. Trapping of the intermediate isocyanate with amines (equation 10) gives urea derivatives and with carboxylic acids rearranged amides are obtained (equation 11). ... 

Schmidt rearrangement of tricyclo[2.2.1.0 - ]heptane-2-carboxylic acid (21) leads to the corresponding urethane (22) in 61% yield upon treatment with hydrazoic acid followed by methanolysis of the intermediate isocyanate. ... 

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