Reactions of isocyanates with nitrogen compounds

Although the addition of nitrogen compounds to the C=N bond of isocyanates offers a number of synthetic possibilities (see page 406), formal replacement of the isocyanate-oxygen by nitrogen has only recently become important and then only for one case. This case is the conversion of isocyanates into carbodiimides  

Aromatic isocyanates, particularly those containing electron-attracting substituents, react noticeably more easily than aliphatic isocyanates. In the former case it often suffices to leave the components for some hours at room temperature in a water-pump vacuum, whereas for aliphatic compounds boiling under reflux is usually necessary and sometimes use of a higher-boiling solvent such as decalin. The best catalysts are cyclic phosphine oxides of type (1) (phospholene oxides)862 and cyclic phosphonic diamides of type (2) (1,3,2-diazaphospholidine oxides).863 The best results were obtained with compound (1, R = C2H5) (for its preparation see Campbell et a/.862) the phenyl compound, although more readily prepared,864 is somewhat less reactive. The amounts of catalyst needed are 0.1% for (1) or 0.5% for (2). 

Detailed directions for preparation of diphenylcarbodliinide are given in Organic Syntheses.865 

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