Isocyanates catalytic conversion into carbodiimides

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. 

Some examples of the catalytic conversion of isocyanates into carbodiimides are shown in Table I. 

Catalytic conversion of isocyanates into carbodiimides with the aid of l,3-dimethyl-l,3,2-diazaphospholidine 2-oxides and 1,3-dimethylhexahydro-1,3,2-diazaphosphorine 2-oxides [79]. 

By Catalytic Conversion. Symmetrically substituted carbodiimides are best obtained by the catalytic conversion of two equivalents of isocyanates into a carbodiimide and carbon dioxide. This is the most convenient method of synthesis of symmetrically substituted carbodiimides, because the byproduct is carbon dioxide gas, and the yields approach quantitative. A solvent is not necessary and numerous isocyanates are readily available as starting materials. ... 

Several phosphorous compounds having a P=S bond are also catalysts for this reaction. For example, heating of phenyl isocyanate with triphenylphosphine sulfide at 160 °C affords diphenylcarbodiimide. Since triphenylphosphine sulfide was recovered unchanged, a different mechanism seems to be operative. A tricyclic P S compound, S=P[N(Me)CH2CH2l3N, also catalyzes the transformation of isocyanates to carbodiimides. Triphenylarsine oxide and triphenylantimony oxide also catalyze the conversion of isocyanates into carbodiimides. The catalytic activity of the oxides of phosphorous, arsenic and antimony are in agreement with the dipole moments of... 

Other catalysts effective in the conversion of isocyanates into carbodiimides include the naphthenates of Mn, Fe, Co, Cu and Pb, derivatives of metallic acetylacetonates, the alkoxides of titanium, zirconium and niobium and vanadium oxides or chlorides. Sterically hindered isocyanates are readily converted into carbodiimides upon heating in the presence of a catalytic amount of a strong base. For example, heating of... 

This reaction is utilized in the catalytic conversion of isocyanates into carbodiimides using phospholene oxides as the catalyst . When aliphatic diisocyanates were converted into carbodiimides using a phospholene oxide catalyst, the generated carbon dioxide was partially incorporated into six-membered ring [2+2+2] cycloadducts . ... 

This reaction was utilized in the catalytic conversion of isocyanates into carbodiimides. The order of catalytic activity is Fe(CO)4CNPh > Fe(CO)5 > Fe2(CO)9 > W(CO)6 >... 

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