Isocyanates, addition isocyanate

Further reaction of the active hydrogens on nitrogen in the urethane groups (3) can occur with additional isocyanate (1) at higher temperatures to cause formation of aHophanate stmctures. The active hydrogens in urea groups can also react with additional isocyanate to form disubstituted ureas which can stiU further react with isocyanate to form biurets (13). 

In addition, isocyanates may, under appropriate conditions, react with themselves to give dimers, trimers (isocyanurates) and carbodi-imides. 

Formulations should be based on stoichiometric considerations. Based on a knowledge of the hydroxyl value of the polyol the amount of isocyanate necessary to cause chain growth should be calculated. The gas evolved will depend on the water content and additional isocyanate must be incorporated corresponding to the water present. When the isocyanate used equals the theoretical amount the system is said to have a TDI index of 100. In practice a slight excess of isocyanate is used (TDI index 105-110) to ensure complete... 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

Impurities in CL have also been destroyed by oxidation with ozone22 followed by distillation. Ozonation treatment of waste CL leaves no ionic impurities. However, the most commonly used oxidizing agents are potassium permanganate, perboric acid, perborate, and potassium bromate. Treatment of CL with these oxidizing agents is carried out in a neutral medium at 40-60°C. Strongly alkaline or acidic conditions accelerate the oxidation of CL to form isocyanates. Hie undesirable oxidation reaction is fast above pH 7 because of the reaction with isocyanate to form carbamic acid salts, which shifts the equilibrium to form additional isocyanate. 

The rate-determining first step is the reaction of the isocyanate with phosphane oxide, resulting in the formation of isocyanate and CO2. Once the isocyanate is formed, it reacts with additional isocyanate to give carbodi-imide and regenerate the catalyst. A more detailed investigation of the reaction mechanism (Scheme 25) followed years later (69ACSA2697 72ACSA1777). 

Iodine isocyanate addition to olefins / 22 Halogen azide addition to olefins / 24 Via epoxides and / -mesyloxy azides / 27 By addition of alkoxyamines / 29... 

On prolonged reaction with aryl isocyanates, the pyrido [ 1,2-a]pyrimidines (262) give pyrido[l,2-a]-s-triazines (266) in poor yield.328 The suggested pathway is the addition of the aryl isocyanate to form the 3,3-disubstituted product (263), which eliminates the ketene (265) the residual part of the molecule (264) reacts with an additional mol of aryl isocyanate to give 266. The ketene (265) forms the quinoline (267), by ring closure, which gives 268 by aryl isocyanate addition. The quinoline (267 R = CH2Ph, X = H) could be isolated. 

R,3S)-p-Phenyl-isoserine methyl ester (4.35 g, 22 mM) is dissolved in dry THF (100 ml) and the flask cooled to 0°C. To the mixture is added t-butyl isocyanate (2.8 ml, 25 mM). TLC after 15 min shows some starting material left so additional isocyanate (0.5 ml) is added. TLC after 1 h shows no starting material so the solvent is concentrated under reduced pressure to give the N-(t-butylaminocarbonyl)-p-phenyl isoserine methyl ester. 

In a similar fashion, a urea hydrogen atom can react with an additional isocyanate group to produce a biuret. 

Carbodiimides can react with additional isocyanate groups to form uretone imines, which sometimes are used to modify polyisocyanates used in urethane structural adhesives. 

Isocyanates also can react with each other to produce carbodiimides with the loss of carbon dioxide. This reaction requires high temperatures unless catalyzed by specific phosphorus compounds. Formation of carbodiimides normally is not an important cross-linking mechanism in polyurethane adhesives. However, carbodiimides are sold by Dow Chemicals (Ucarlnk ), Nisshinbo Industries (Carbodilite ), and Stahl USA (XR-2569). They have been recommended as water scavengers, crosslinkers, and stabilizers for carboxyl functional polyurethanes. The carbodiimide can react with water to give a urea, which still can react with additional isocyanate to produce a biuret. 

An interesting application of the isocyanate addition is a reaction which initially yielded carbodiimide that reacted with a simple enamine to give an intermediate leading in a hetero-Diels-Alder reaction to a phenanthridine derivative13 (equation 9). 

The Addition of Isocyanates to Isocyanates (Formation of Carbodiimides) Alkylimino-de-oxo-bisubstitution... 

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. 

---