Synthesis of picolines and other aza-aromatics from arylamines by isomerization-rearrangement and from dinitriles by hydrogenation-cyclization reactions

Abstract

New routes for the syntheses of α- and β-picoline and other valuable aza-aromatics with applications in the pharmaceutical and agricultural industry are described. Aniline and other arylamines, like toluidines, naphthylamines and m-phenylenediamine, all react to the corresponding o-methylsubstituted aza-aromatics when exposed to high NH 3 pressure and elevated temperature in the presence of acid catalysts. Zeolites show the best performance around 10 MPa and 600 K. The results can be explained if it is assumed that the reaction starts with addition of NH 3 to the arylamine, followed by ring opening to an enamino-imine intermediate through a reverse aldol reaction, ring closure and NH 3 elimination. The required high NH 3 pressure is explained by the need to add NH 3 to the aromatic ring, and the high temperature by the need to desorb NH 3 from the acid sites. β-Picoline was synthesized in a one-stage gas-phase process from 2-methylglutaronitrile, a by-product in the manufacture of adiponitrile, over silica-supported Pd and Pt catalysts. Initial reaction rates of both catalysts were extremely high and pore and film diffusion limitations were observed. Activity and selectivity of the Pd catalyst decreased with time on stream, because of intermolecular reactions of reaction intermediates. Therefore, a one-stage process is only viable when the conversion can be kept at 100%. Pt catalysts had a very high activity, but their selectivity was limited by hydrogenolysis reactions.