Carbodiimides, formation

Carbodiimide formation is markedly accelerated when phosphine oxides (R PO) or phosphates are used as catalysts. Intermediates with P—NR bonds have been postulated as iatermediates ia these reactioas (59,60). 

Garbodiimide Formation. Carbodiimide formation has commercial significance in the manufacture of Hquid MDI. Heating of MDI in the presence of catalytic amounts of phosphine oxides or alkyl phosphates leads to partial conversion of isocyanate into carbodiimide (95). The carbodiimide (39) species reacts with excess isocyanate to form a 2 + 2cycloaddition product. The presence of this product in MDI leads to a melting point depression and thus a mixture which is Hquid at room temperature. 

The formation of carbodiimide from 2 molecules of MDI is shown in Equation 1. Carbodiimide formation is accompanied by the evolution of carbon dioxide, and this reaction is believed to be the primary source of pressure upon the thermal decomposition of MDI. 

Further, isodithiobiurets of the tj ArNH-CS N C(SAlk)NAIks (lacking the necessary hydrogen atoms for carbodiimide formation) are not convertible into biguanides (141). 

Alkyl-5-picrylimino-l,2,3,4-thiatriazolines are exceptionally thermostable. The 4-benzyl and 4-isopropyl derivative did not thermolyze at 90 °C in chlorobenzene or toluene decomposition started only after prolonged heating at 105 °C and was complete after 3-5 days. The resulting mixtures showed no IR spectroscopic evidence of carbodiimide formation <90JHC1059>. 

In the case of aromatic isocyanates, electron-withdrawing groups increase the rate of carbodiimide formation in proportion to their electron-withdrawing power. Electron-releasing groups tend to inhibit the reaction. p-Nitrophenyl isocyanate reacts almost explosively to yield the carbodiimide when catalyzed [5]. o-Chlorophenyl isocyanate reacts seven times as fast as the o-methyl analog [33]. 

Carbodiimide formation. Carbodiimide formation has commercial significance in the manufacture of liquid MDI. Healing of MDI in Ihe... 

The structure of the starting isocyanate determines the rate of carbodiimide formation. Ahphatic isocyanates react slower than aromatic isocyanates. 

The recent trend to abolish chlorofluorocarbon (CFC) blowing agents in rigid foam manufacture has prompted a new look on PMDI derived foams containing partial carbodiimide structure. Since carbon dioxide, generated in carbodiimide formation, can be used... 

Several types of additives have been found which reduce or eliminate the effects of various catalysts in polyurethane formation. When the catalyst is a simple base, an acid or acid precursor such as an acid chloride will reduce the rate of reaction due to neutralization of the basic catalyst. This is often most desirable, since many of the bases stronger than tertiary amines promote side reactions such as trimerization, carbodiimide formation and allophanate formation. The beneficial effect of acid retarders in a variety of systems was illustrated by Heiss et al. [151], as well as by others. 

N-N Bond scission and synthesis of N-heterocycles 86UK1785. Carbodiimides, formation of N-heterocycles from 82UK848 81AG855. Catalytic synthesis of N-heterocycles 83KGS1587, 83YGK604. Condensed N-heterocycles, syntheses using modified Japp-Klingemann... 

An interesting and practical illustration of this behavior is the case of MDI that starts out by being a solid (melting point 37°C) and thus is not easy to handle on a large, industrial scale. However, a small degree of carbodiimide formation depresses the melting point to give the much more convenient liquid MDI. 

Carbodiimide formation. Deoxygenative dimerization of isocyanate esters occurs on heating with [CpFe(CO)2]2 in xylene. ... 

Sodium periodate. 15, 294 Carbodiimides. Format tion with Nal04 in DMF 1 lenonoamides. 

Carbodiimides. Formation of carbodiimides from selenoureas is observed on reaction with NaI04 in DMF. The selenoureas are obtained from nitrile oxides and se-lenonoamides. 

With aryl isocyanates in general pentavalent phosphorus compounds could act as carbodiimide formation catalysts. In addition trivalent phosphorus compounds such as triethyl phosphine are also efficient carbodiimide catalysts [8b]. 

These rules were confirmed by combined quantitative multivariate SAR and che-modynamic studies in the subclass of N-(pyrid-3-yl)thioureas and -carbodiimides (Fig. 28.1.4 Y equals N). These studies support the SAR concept that, for thiourea analogs of diafenthiuron, the rate and efficiency of carbodiimide formation and its photolytic and chemical stability governs the overall biological potency [50, 51]. 

Polyisocyanates have no commercial application, and the conditions for their formation should be avoided when planning other urethane chemical reactions. An important chemical reaction of isocyanates, which proceeds at high temperature without catalysts, is carbodiimide formation. CO2 is generated in the process ... 

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