Isocyanates from urea

Without additional reagents Ureas from isocyanates s. 17, 403... 

Without additional reagents N-(Alkylthiocarbonyl)ureas from isocyanates and thioboric acid esters... 

Furoxans and Zl -isoxazolines from prim, nitroparaffins Sym. ureas from isocyanates... 

According to the literature (6) the formation of urea from amine and isocyanate proceeds much faster than the formation of urethane or the hydrolysis of Isocyanate. 

The major problems with the substitution of the reducing agent ammonia for urea are on the one hand the homogeneous mixing of urea and exhaust gas and on the other hand the limited residence time in SCR systems for the different decomposition steps, i.e. the evaporation of water from the droplet, the thermolysis of urea to isocyanic acid and the following hydrolysis to ammonia [18]. 

Moreover, triuret, ammeline, ammelide, melamine and other products may be formed from isocyanic acid, biuret and combinations of them. If urea is heated up very fast, these reactions are suppressed and the decomposition into ammonia and isocyanic acid is the preferred reaction. Due to the high reactivity of isocyanic acid, its primary formation may subsequently lead to the formation of the aforementioned compounds of higher molecular weight. In order to avoid the formation of by-products, the heating-up must be carried out fast. Only then ammonia and isocyanic acid are obtained as sole products. In any case, local undercooling of the gas duct should be avoided and rapid dilution of the thermolysis products in the exhaust gas has to be ensured in order to avoid locally high concentrations of reactive compounds. 

Examples of non-urethane linkages derived from isocyanates a) urea, b) urea, c) biuret, d) amide, and e) allophanate... 

The history of organic synthesis is generally traced back to Wohler s synthesis of the natural product urea from ammonium isocyanate in 1828. This laid to rest the vis vitalis (vital force) theory, which maintained that a substance produced by a living organism could not be produced synthetically. The discovery had monumental significance, because it showed that, in principle, aU organic compounds are amenable to synthesis in the laboratory. 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

Another reaction taking place is the formation of a disub-stituted urea from the reaction of the preceeding amine and isocyanate ... 

From carbodiimides The aminated polymers (from haloalkylpoystyrenes and potassium phtalimide) were converted into urea with isocyanate and dehydrated to carbodiimides 20 addition of tetramethylguanidine gave the type 5 N-hexalkylbiguanides 9 (Figure 14). 

The diimide (37) reacts with CO to yield an isocyanate complex (38) (86), having a characteristic infrared absorption at 1280 cm-1 (88). This remarkable reaction suggests several catalytic schemes which might be useful for the synthesis of organic compounds containing the —N—C(O)— unit. This latter complex (38) could be viewed as the key intermediate in any catalytic cycle for the synthesis of isocyanates or urea from CO and NH3 (or N2 and H2).2 However, in our work we could not... 

The utility of urca.s and thioureas as substrates for making imidazoles is limited by the fact that the imidazole 2-substituent can only be an oxygen or sulfur function. Synthetic methods involving ureas and thioureas will also be discussed in Section 4.1, but some cyclizations of suitably functionalized species fall under the present heading. Appropriately substituted ureas and thioureas can be made from isocyanates and primary amines [36-38], from isocyanates and hydrazines [39] or thiocyanates and hydrazines [40], from or-aminonitrilcs and carbon dioxide [41] and by heating l,3,4-oxadiazol-2-oncs with amino acids [42]. Some of the substrates prepared in these ways, though, lead ultimately to reduced imidazoles such as hydantoins. Cyclizations arc usually acid catalysed, but they can also be thermal [43]. 

Elemental selenium is an excellent catalyst for fhe carbonylation of a variety of nucleophiles such as amines, alcohols, water, and carbon nucleophiles in the presence of oxygen under mild conditions [114]. In fhe synfhesis of ureas from primary amines and CO, for example (Scheme 15.42), the turnover number reaches ca. 10", giving ureas in almost quantitative yields. Successful applications of this Se/CO system include not only synfhetic reactions with a variety of carbamates, carbonates, and sulfur- or selenium-containing compounds but also metallurgical refining of selenium and several important processes as exemplified by isocyanate synfhesis, fhe water-gas shift reaction, and separation of carbon monoxide [115]. 

Formation of ureas from alcohols by reaction with isocyanates ... 

Recent work has considerably extended the synthesis of isocyanates from ureas, making isocyanates available that cannot be obtained by direct reaction of phosgene and amines.633... 

Very good yields are almost always obtained by starting with nitro-urea.1154-1157 In these cases the N02NH group is exchanged almost exclusively, so that this method is especially suitable for preparation of monosubstituted ureas and is in many cases superior even to the route from isocyanates.1155... 

Simple addition, in which both fragments of the NH molecule enter the new compound as in the preparation of alkanolamines from alkylene oxides, aminonitriles from unsaturated nitriles, and ureas and thioureas from isocyanates and isothiocyanates. 

Similarly, excess amines could be extracted with acidic ion exchange resins as exemplified by the same group in the synthesis of ureas starting from isocyanates and amines. ... 

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