СATALYTIC METHOD FOR THE SYNTHESIS OF 3-HYDROXY-2-NAPHTOIC ACID ANILIDE

Arylamides of 3-hydroxy-2-naphthoic acid are widely used in the production of organic azo pigments, as medicines and pesticides. Titanium tetrachloride, tetrabutoxytitanium and polybutoxytitanates, previously used for the synthesis of the most important of them., 3-hydroxy-2-naphthoic acid anilide by boiling of the latter in aniline (184 °C), proved to be ineffective as catalysts in carrying out this reaction in ortho-xylene. The study of the reasons for this inhibition of catalysis showed that it can be associated with the interaction of Ti(4+) with the hydroxy group of 3-hydroxy-2-naphthoic acid, its oxidation, accompanied by a decrease in the effective charge of titanium. Taking into account this inhibition, in the search for new catalysts for the synthesis of 3-hydroxy-2-naphthoic acid anilide, the known literature data on the oxidizing ability (according to the values of the reaction rate constant k) with respect to phenoxyl radicals of a number of Lewis acids, including titanium tetrachloride, were used: SbCl5>TiCl4 >SnCl4>PCl5>AlCl3 k, l·mol-1·sec-1   105     157      156       42      18. This made it possible to choose phosphorus trichloride as an effective catalyst. It is shown that this compound, in an amount of only 2% mole. from 3-hydroxy-2-naphthoic acid, allows in boiling ortho-xylene (145 °C), under relatively mild conditions, compared to boiling aniline (184 °С), to obtain anilide 3-hydroxy-2-naphthoic acid (by reaction of the latter with aniline) of good quality with a practically quantitative yield. Antimony trifluoride also has similar pro­perties as a catalyst, at the level of efficiency of phosphorus trichloride. The ability of catalysts to act as an oxidizing agent should, apparently, always be taken into account during the catalytic amidation of aromatic oxycarboxylic acids at elevated temperatures.