Synthesis of Amides and Amidines by Reaction of Carboxylic Acids and Amines in the Presence of Polyphophoric Acid Trimethylsilyl Ester (PPSE)

The reaction between amines and carboxylic aicds in the presence of polyphosphoric acid trimethylsilyl ester (PPSE) has been investigated from the view point of the synthesis of amides and amidines. Benzanilide was selectively prepared from benzoic acid and aniline in the presence of PPSE and pyridine at 100 °C, whereas a mixture of benzanilide and N,N′-diphenylbenzamidine was obtained without the use of pyridine. The reaction at 160 °C almost exclusively gave N,N′-diphenylbenzamidine. Various symmetrical amidines were obtained from combinations of carboxylic acids and aromatic amines in high yields by simply heating at 160 °C with four molar amounts of PPSE. The reactions of aliphatic and aromatic amines with N-monosubstituted and N,N-disubstituted amides also afforded corresponding unsymmetrical amidines under the same conditions. The reaction mechanism is discussed in some detail.