CATALYSIS BY PHOSPHORUS (III) COMPOUNDS IN THE SYNTHESIS OF ARYLAMIDES OF 3-OXY-2-NAPH-THOIIC ACID

Arylamides of 3-hydroxy-2-naphthoic acid are widely used in the synthesis of dyes and pigments, as pesticides and medicines. Therefore, the development of new methods for their synthesis is an urgent task. It has been found that catalysis by phosphorus trichloride and/or phosphorous acid (2 mol %) of acylation in refluxing ortho-xylene or ortho-chlorotoluene of arylamines with 3-hydroxy-2-naphthoic acid leads to the formation of the corresponding arylamides with a high, and often almost quantitatively yield. Substituted anilines, containing electron-bearing and electron-withdrawing substituents in ortho-, meta- and para-positions, successfully enter the reaction in an air atmosphere. The catalytic reaction of 1-naphthylamine and 5-aminobenzimidazolone with 3-hydroxy-2-naphthoic acid makes it possible to obtain in high yield, respectively, 1-naphthylamide of 3-hydroxy-2-naphthoic acid and 5-(2'-hydroxy-3'-naphthoylamido)-2-benzi­midazolone only in a nitrogen atmosphere. Based on the yield of 3-hydroxy-2-naphthoic acid arylamides over a certain time in the reaction catalyzed by phosphorous acid in ortho-xylene, the average initial amidation rates were calculated at a semi-quantitative level and their dependence on pKa of the corresponding arylamines was plotted in logarithmic coordinates, according to the Bronsted equation. The graph is a curved line with two maxima and one minimum, while arylamines containing electronwithdrawing substituents (4-Br; 3-F; 3-Br; 3-Cl) are the most reactive, and the most strongly basic 4-anisidine and weakly basic 3-nitroaniline are the least reactive. Based on these data, as well as the results of the earlier interaction of substituted anilines with benzoic acid, catalyzed by polybutoxytitanates, and substituted benzoic acids and 3-hydroxy-2-naphthoic acid with aniline, catalyzed by phosphorus trichloride and phosphorous acid, under comparable conditions, a mechanism of amidation was proposed. It includes the formation in the first minutes of the reaction in situ of arylamine phosphite, which can act as a P=O–oxygennucleophilic catalyst, which forms the corresponding phosphite upon interaction with 3-hydroxy-2-naphthoic acid, which is subsequently attacked by a free arylamine molecule to obtain arylamide 3-hyd­roxy-2-naphthoic acid. In addition to the basicity of arylamine at the stage of nucleo­phi­lic attack, the reaction rate can be affected by its oxidation as such and the phosphite formed by it with atmospheric oxygen, unproductive binding of arylamine into a salt or H-complex with 3-hydroxy-2-naphthoic acid, participation of the final target product in amido-imide rearrangement with further inhibition of cata­lysis by the resulting imide form.