Formation of uretonimine
In the presence of excess isocyanate, a carbodiimide can react reversibly to form uretonimine . The uretonimine adduct is of practical importance within polyurethane technology because it results in a low-viscosity room temperature liquid . The modification is accomplished by initially forming the carbodiimide with catalyst
Figure 3.17 Catalyzed dimerization of isocyanate to form carbodiimide.
FIGURE 3.18 Phospholene catalyzed formation of carbodiimide [69, 7G].
Figure 3.19 Proposed trimerization of carbodiimide occurring during high temperature decomposition.
at approximately 100 °C. The phospholene catalyst is subsequently neutralized (with an organic acid chloride for instance). The uretonimine forms upon cooling (Fig. 3.21). Subsequent heating to approximately 200 °C dissociates the uretonimine back to carbodiimide and isocyanate. At 200 °C, the dissociation/formation rate constant ratio has been reported to be 1.63 .
Figure 3.20 Proposed reaction of carbodiimide to form six-member ring during high temperature decomposition.
Figure 3.21 Reaction of carbodiimide and isocyanate to form the industrially important uretonimine.
Figure 3.22 Reaction of aliphatic isocyanate and carboxylic acid to form the intermediate anhydride and the final amide.
Formation of Amides
The reaction of isocyanates and carboxylic acids has been reported in the literature, but its practical application has not yet been appreciated [76-80]. Commodity aromatic isocyanates such as MDI and TDI do not form amides with carboxylic acids, and instead reportedly form urea bonds. Aromatic isocyanates with strong electron-withdrawing groups such as cyano-, nitro-, and trifluoromethyl can result in the formation of amides in reaction with carboxylic acids. Alternatively, aliphatic isocyanates react readily with carboxylic acids in solvent to form amides according to Figure 3.22. In some cases, the anhydride is isolated, and in others, the anhydride proceeds immediately to form amide. The reaction of carboxylic acid with a dimethylpyrazole blocked isocyanate, and a magnesium triflate catalyst reportedly results in 100% amide yield in the presence of chloroform. While no mechanism for the isocyanate/carboxylic acid reaction has been reported, it has been determined that the carbon dioxide carbon originates from the isocyanate carbon.