Isocyanates, carbamic acids from

Usually, the traditional routes to the synthesis of carbamic acid esters, isocyanates and ureas start from the toxic compound phosgene [10]. Consequently, the ability to synthesize these compounds from ubiquitous, cheap and safe C02, as an alternative starting material to phosgene, represents an appealing and highly eco-friendly variation to the conventional protocols which have in the past been based on phosgenation. Moreover, it also provides an attractive answer to the current widespread requests for environmental protection [11]. 

Nitrenes, like carbenes, are immensely reactive and electrophilic, and the same Wolff-style migration takes place to give an isocyanate. The substituent R migrates from carbon to the electron-deficient nitrogen atom of the nitrene. Isocyanates are unstable to hydrolysis attack by water on the carbonyl group gives a carbamic acid which decomposes to an amine. 

Conversion of the corresponding isocyanates, resulting from the heating of the azides in benzene, with alcohols of general formula R OH, yielded the carbamic acid esters LXXVIb (150), and with amines R"R" NH, the urea derivatives LXXVIc and LXXVIIc (151, 152). 

A more detailed quantitative study of the water-o-tolyl isocyanate reaction by Shkapenko et al. 51) showed that at 80° in dioxane solution and in the presence of triethylamine or other catalysts the consumption of the isocyanate was complete within a short period when only approximately half of the theoretical amount of carbon dioxide was released. The evolution of carbon dioxide proceeded from this point on at a slow rate. It was also demonstrated that by heating the reaction mixture to 100°, 30-35% of theoretical CO2 was released, and that this portion of the CO2 was given off by the decomposition of the carbamic acid anhydride formed from the acid and a second molecule of isocyanate. Additional tests showed that 4-5% of the isocyanate formed o-tolyl ammonium-W-o-tolyl carbamate, 18.7% of the water added remained unreacted, and that a trace of the free o-tolyl amine was also present. In addition, the presence of di-o-tolyl urea was proven. 

From the hydrolysis of a polyether-based PU a diamine (or a polyamine) such as toluene diamine or diphenylmethane diamine, a polyol and carbon dioxide are formed. The resulting diamines are the precursors used for the synthesis of isocyanates [11,12,16,18]. The resulting polyol is the polyether polyol used for the initial synthesis of PU. Carbon dioxide results from the decomposition of the very unstable carbamic acid formed by the hydrolysis (20.2) ... 

The phosgene toxicity, separation of hydrochloric acid from excess phosgene, and the use of chlorinated solvents as a reaction medium are the major drawbacks of this reaction process. Extensive studies have suggested that carbamate and dicarbamate can serve as environmentally benign precursors for the synthesis of isocyanate and diisocyanate (1-5). Figure 1 illustrates nonphosgene routes for the synthesis of two important diisocyanates [i.e., 4,4 -diphenylmethane diisocyanate (MDI) and toluene diisocyanate(TDI)] via carbonate and carbamate. 

A few ISQ derivatives prepared from ISQ and TV-substituted phenylsulfonyl isocyanates or esters of TV-substituted phenylsulfonyl carbamic acid (e.g., 18) showed activity as sedatives and hypnotics and were useful as radioprotectants [22]. 

The developed solid-phase approach started with the preparation of immobilised ureas 114 obtained via acylation of amino acids linked to acid-labile Sasrin resin 104 with ortho-methoxycarbonyl aryl isocyanates or activated para-nitrophenyl carbamates 113. Alternatively, ureas of type 114 could also be generated by reaction of anthranilic esters 115 with immobilised amino acid-derived isocyanates (easily generated from tethered amino acids with triphosgene or phosgene in toluene in excess of a base such as 2,6-lutidine) or activated carbamates 116 (Scheme 4.1.23). 

The free acids are not known although their formation from isocyanic or isothiocyanic acid and one mole of a hydrogen halide can be visualized. While the esters of isocyanic acid, the isocyanates III, add one mole of hydrogen halide to afford the corresponding carbamic acid chlorides (IV), methyl isothiocyanate (V) adds hydrogen bromide to form a mixture of two isomers, to which 1-bromothioformidic acid structures VI and VII have been assigned ( ). 

The group of Jensen has designed microseparators to perform Hquid-hquid extraction and to separate liquid and gas phases. Both selfiassembled microfluidic extraction systems were used in continuous multi-step syntheses of carbamates 62 from acid chlorides 61 without isolation and storage of the intermediate azides and isocyanates (Scheme 15) (2007AGI5704). 

A wide range of thermoplastics can be converted into foams. Some of the most common materials include polyurethanes, polystyrene, and polyethylene. Polyurethanes are a popular and versatile material for the production of foams and may be foamed by either physical or chemical methods. In the physical reaction, an inert low-boiling chemical is added to the mixture, which volatilizes as a result of the heat produced from the exothermic chemical reaction to produce the polyurethane (reaction of isocyanate and diol). Chemical foaming can be done through the reaction of the isocyanate groups with water to produce carbamic acid, which decomposes to an amine and carbon dioxide gas. ... 

Another limitation stems from the fact that CO2 is an electrophile and can undergo specific interactions with Lewis bases. The most important example is the reaction of prim and secondary alkyl amines with CO2 to form carbamic acids (equation 3). This reaction is well known in conventional solvents (64) and has been us as a route to isocyanates and urethanes. The carbamic acid reacts with another equivalent of amine to form an insoluble sdt (65,66)... 

A library of dihydroxytetrahydrofurans was readily prepared from isosorbide." Epoxide 73 was produced in two steps from isosorbide (72)."" Subsequently, 73 was heated with secondary amines 68 to afford the corresponding amino alcohols 74 (Scheme 7.15). PS-lsocyanate resin was used to remove excess secondary amine 68. Treatment of alcohols 74 with isocyanates 75 gave the desired carbamates " PS-Trisamine resin removed excess isocyanate. Aqueous, acid-promoted hydrolysis of acetonides gave diols 76. 

In isocyanate chemistry many unintended side reactions can occur. The amine formed from the unstable carbamic acid can react with isocyanate, forming a urea. This secondary nitrogen urea or any other amide nitrogen may react further with another isocyanate, forming a cross-link at this new biuret functional group. The amine may also react with the anhydride on TMA, forming the amic acid. And, depending on temperature, the isocyanate may react with the solvent itself. All these side reactions and their influences on stoichiometry must be fuUy understood and controlled. 

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