Oxidative Ammonolysis of Dialkylpyridines

Abstract

The cluster approach was used to simulate the chemisorption of dialkylperidines on the surface of a vanadium oxide catalyst, involving active centers of various nature, and to assess the enthalpy of proton abstraction from the alkyl substituents. Under oxidative ammonolysis conditions, the order of the transformation of the substituents in unsymmetrical dialkylpyridines into the cyano group is determined by the enthalpy of their deprotonation.Oxidative ammonolysis of alkyl derivatives of benzene and pyridine is known as the most rational method of synthesis of nitriles of aromatic and pyridinecarboxylic acids, which are valuable intermediate products in the production of medicinals, polymers, and plant protectants [1].Kinetic studies and semiempirical calculations of the properties of alkylbenzenes and monomethylpyridines in the gas phase and in conditions simulating their chemisorption on the surface of an oxide catalyst made it possible to rank the compounds by reactivity and to find out that the rate constants of the transformation of alkyl groups into the cyano group correlate with the enthalpy of proton abstraction from the a-carbon atoms of the substituents [234]. On this basis, it can be suggested that initial reaction stages involve heterolytic C3H bond fission in alkyl groups. A similar mechanism has been suggested for propylene and lower paraffins [5].As judged from published data for oxidative ammonolysis of unsymmetrical dialkylpyridines on oxide catalysts, the 2-and 4-alkyl groups are more reactive than those in the 3 and 5 positions. For this reason, for example, the primary product of the transformation of 2,3-lutidine on a V3Sn3Fe oxide catalyst is 3-methylpyridine-2-carbonitrile. The 3-Me group reacts later, when the 2-CN group formed undergoes elimination. As a result, pyridine-2,3-dicarbonitrile is lacking among the reaction products, and the major product is nicotinonitrile [6]. The oxidative ammonolysis of 2,5-lutidine and 2-methyl-5-ethylpyridine on the same catalyst initially involves the 2-Me groups to form 5-methyl-and 5-ethylpicolinonitriles. Under the conditions used, transformations of the 5-alkyl groups in the subsequent stage were not attended with elimination of the 2-CN group; as a result, both 2,5-lutidine and 2-methyl-5-ethylpyridine gave up to 73% of pyridine-2,5-dicarbonitrile [7]. A kinetic study of the oxidative ammonolysis of 2-methyl-5-ethylpyridine on a V3Ti oxide catalyst showed that the ethyl substituent is much less reactive: Neither 2-methyl-5-vinylpyridine nor 2-methylpyridine-5-carbonitrile were found among the reaction products [8]. Compelling evidence for the reactivity effect of the position of the substituent in the pyridine ring was provided by the results of the oxidative ammonolysis of 2,3-, 2,5-, and 3,4-lutidines on a Cr 2 O 3 (5%)/g-Al 2 O 3 catalyst: At a low conversion (~25%) of the starting compounds, 3-methylpyridine-2-carbonitrile, 5-methylpyridine-2-carbonitrile, and 3-methylpyridine-4-carbonitrile, respectively, were obtained with a selectivity of hi...

Preparation of Phthalimide and Kinetics of Vapour Phase Ammoxidation of o-Xylene on V-Sb-Bi-Cr/g-Al2O3 Oxide Catalyst-III

“…The possibility of intramolecular interactions at intermediate stages determines the specificity of the kinetic laws. Thus, increasing the concentration of ammonia in initial reaction mixture, usually leads to increased selectivity in dinitrile formation and decreased selectivity in imide formation on some vanadium contacts [7,[9][10][11]. Dinitrile begins to prevail among the reaction products at high partial pressures of ammonia, wherein the formation of an imide directly from o-xylene do not happens (Fig.…”

Oxidative ammonolysis of 3,4-dimethylpyridine on the vanadium oxide catalysts

“…4,5 Oxidative ammonolysis of dialkylpyridines of the unsymmetrical structure showed that alkyl groups at positions 2 and 4 are more reactive than in positions 3 and 5. 6 Therefore, the primary product of the conversion of 2,3-dimethylpyridine on the V-Sn-Fe oxide catalyst is 2-cyano-3-methylpyridine.…”

“…It was shown that the sequence of transformation of methyl substituents of dialkylpyridines into cyano group was determined by the enthalpy value of their deprotonation. 6 The main goal of this work was an experimental and quantum chemical study of the reactivity of methyl groups of 3,4-dimethylpyridine in oxidative ammonolysis. The tasks of the work included studying the effect of oxide-promoters, SnO 2 and ZrO 2 , on the activity of binary vanadium oxide catalysts.…”