Amide chlorides self-condensation

The liquid ammonia was then driven off, an equivalent of the ketone was added, and the mixture refluxed for completion of the aldolization. Lithium amide proved to be superior to sodium amide for these condensations. /-Butyl esters were preferred because they are readily cleaved by pyrolysis or by acid. Dehydration to the a,fi-unsaturated esters could be effected with thionyl chloride and pyridine. Hauser and Lindsay succeeded in effecting aldol condensation with ethyl acetate, which undergoes self-condensation very readily two equivalents of base were used. Sisido has reported further alkylations of /-butyl esters. 

If an ortho-substituted benzene ring is attached to the nitrogen, the self-condensation is minimized due to steric hindrance and the yield of imidoyl chlorides increases in this order CH3 < Cl < Br ( ). Likewise, lowering of the basicity of the nitrogen prevents condensation. For example, N-sulfonimidoyl chlorides of aliphatic carboxylic acids are obtained in excellent yield ( ). The carboxylic acid amides of a,j8-unsaturated acids also do not undergo tautomerization, because the formation of allenes is not favored under mild conditions... 

In many instances, however, it is possible to purify imidoyl halides by vacuum distillation. If the compounds are thermally sensitive, purification can be conducted by solution in nonpolar organic solvents, in which the polar by-products, as well as products formed by self-condensation, are less soluble, or completely insoluble. While it is often difficult to establish melting points, because recrystallization and thereby exposure to moisture results in partial conversion to the corresponding carboxylic acid amides, identification by infrared spectroscopy is a good analytical tool. The characteristic spectral feature of the imidoyl halides is the C=N double bond absorption which occurs at 1650-1689 cm in imidoyl chlorides (see Table V). 

If, for example, an a-hydrogen is attached to the carbon atom of the iminium bond, transformation into the corresponding enamine (CXXV) occurs readily, which results in subsequent reaction of CXXV with the starting iminium chloride. Since attack on nitrogen is not possible self-condensation via attack on carbon has been observed, and jS-ketocarboxylic acid amides CXXVI are obtained in good yield... 

This type of reaction was first applied to the preparation of polyamides, as discussed above, for which it is somewhat more effective in forming high molecular weight polymers, but it has now been used for the synthesis of a wide variety of aromatic polyesters, either by the self-condensation of hydroxyacids or by the co-condensation of dicarboxylic acids and difunctional phenols.A fairly wide variety of phosphorous compounds can be used as reducing or dehydrating agents in these reactions, in addition to phosphines, including phosphites, chlorophosphates, phosphates, polyphosphates and phosphazenes. In most cases, lithium chloride is added and the reaction is run in either pyridine or an amide solvent system. The reaction has also been found to be catalyzed by tertiary amine salts. 

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