Carbodiimides isocyanate salts

Review. New synthetic reactions based on the onium salts of aza-arenes have been reviewed (75 references). The reactions discussed involve activation of carboxylic acids or alcohols with 2-haIopyridinium, benzoxazolium, benzothiazolium, and pyridinium salts to afford 2-acyloxy or 2-alkoxy intermediates, which can be transformed into esters, amides, thiol esters, (macrocyclic) lactones, acid fluorides, olefins, allenes, carbodiimides, isocyanates, isothiocyanates, and nitriles under appropriate conditions. 

The 2 1 Cycloadducts 61 are also obtained in the reaction of isocyanate salts with carbodiimides . 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Reaction of isocyanates with the silylamine salt 90 affords the carbodiimide 91. ... 

Carbodiimides undergo cyclooligomerization reactions. In this regard they are similar to isocyanates, the mono imides of carbon dioxide. For example, aliphatic carbodiimides undergo rapid dimerization catalyzed by tetrafluoroboric acid at room temperature to give salts of the cyclodimers 183. Neutralization with dilute sodium hydroxide, or better filtration through basic AI2O3, afford l,3-dialkyl-2,4-bisalkylimino-l,3-diazetidines 184. ... 

Trimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 2,4,6-trisubstituted 1,3,5-triazines (see Section 6.12.9.5). The use of lanthanide trifluoromethanesulfonate and ammonia as cocatalysts is claimed as a big improvement. The trisaminal of 2,4,6-triformyl-l,3,5-triazine is also useful for further derivatization to unusual structures (see Section 6.12.7.1). Treatment of a 1 1 pyridine/conc. ammonia solution of an aromatic aldehyde with excess Fremy s salt is another development. Separation of the amide coproduct was claimed to be easy. The synthesis fails with aliphatic aldehydes (see Section 6.12.9.5.4). Aminolysis of 2,4,6-triaryl-1,3,5-oxadiazinium salts gives symmetrical 1,3,5-triazines but the reactions are limited in that electron-withdrawing groups in the aromatic rings lead to instability and difficulty in separation of products (see Section 6.12.10.4). 

Isoxazolium salts Trialkyloxonium salt Amidinium salts Ketenimines Diazomethane K-methyldiazotate Hexamethylenetetramine Azo compounds T riazenes p-Nitrobenzene-diazonium sulfate Sulfanilic acid, diazotized HCONH2, CHsC0NH2 Dimethylacylamines HC0N(CHs)2, CHsC0N(CHs)2 1-Benzyl-1,4-dihydro-nicotinamide N,N -Diethylnicotin-amide C0(NH )2, (CHs)2NCON(CHs)2 Phenyl isocyanate Carbodiimides Dicyclohexylcarbodi-imide, Di-p-tolyl-Antipyrine Imidazole 1-Acylimidazole N,N -Carbonyldi( azoles ) N, N -Carbonyldi-imidazole... 

Heterocumulenes, such as isocyanates and carbodiimides also react with l-aza-2-azoniaallenes across the C=N bond to give [3+2] cycloadducts. For example, aliphatic and aromatic isocyanates add at -20 °C to l-aza-2-azoniaallene salts 50 to give 4,5-dihydro-5-oxo-3/f-l,2,4-triazolium salts 51 or the rearranged triazolium salts 52. ... 

The 1,3-dipolar character of triazaallenium salts, from now on referred to as l,3-diaza-2-azoniaallene salts, is evidenced by the many [3+2] cycloaddition reactions these types of compounds can participate in. The l,3-diaza-2-azoniaallene salts are generated in situ and trapped with suitable dipolarophiles. For example, l,3-diaza-2-azoniaallene salts undergo stereospecific [3+2] cycloaddition reactions with alkynes and olefins. However, they fail to react with isocyanates, isothiocyanates and azo compounds. Also, [3+2] cycloadditions to carbodiimides and cyanamides are observed. In contrast, nitriles fail to react. 1,3-Diaza-2-azoniaallene salts are obtained in the oxidation of 1,3-disubstituted triazenes with t-butyl hypochlorite. The resultant Al-chlorotriazenes react with antimony pentachloride to form the salts as reactive intermediates. Above —25°C, l,3-diaza-2-azoniaaUene salts disproportionate into diazonium salts and azo compounds. 

In recent years, many of the processes initiated by Synhydride (soluble in nonpolar media) and aliphatic or aromatic (di)isocyanates [82], carbodiimides [84] or N-lauroyl-e-caprolactam [83] have been studied, although the first initiation system was composed of the sodium salt of CL and CO2 [85]. 

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