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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...
Upon gas-phase oxidative ammonolysis on vanadium oxide catalysts 4-nitro-, 2-halo, 2-and 4-hydroxy-, and 4-methoxy-and 4-phenoxytoluenes much less selectively convert into the corresponding substituted benzonitriles than their derivatives with the methyl group substituted by chloromethyl, methoxymethyl, or alkoxycarbonyl groups. This fact is explained in terms of different mechanisms of formation of the cyano group: The methyl group converts via deprotonation to form a carbanion, whereas the heteroatomic groups, via the energetically more favorable carbocation formation.Oxidative ammonolysis of methyl-substituted aromatic compounds is the most practical synthetic approach to nitriles of aromatic acids [1,2]. The understanding of the mechanism of initial stages of oxidative ammonolysis of alkylbenzenes has been refined with the accumulation of knowledge of organic CH acids [3,4], elaboration of the theory of adsorption and catalysis on the surface of transition metal oxides [5], and development of physical methods for experimental investigation of surface compounds formed in the course of catalytic reactions [6]. At present there are strong reasons to believe that the activation of alkylaromatic compounds under conditions of their heterogeneous catalytic oxidation and oxidative ammonolysis occurs via reaction of the C @! 3H @+ bond polarized by conjugation with the aromatic ring with the surface M @+ 3O @! acid3base pair of the oxide catalyst [739]. This reaction results in heterolytic cleavage of the alkyl C3H bond with intermediate carbanion formation. Therewith, the nucleophilic oxide anion (Lewis base) acts as proton acceptor and the nucleophilic metal cation (Lewis acid) stabilizes the carbanion. Evidence for this mechanism was provided by quantum-chemical calculations and the experimentally established correlation between the rates of selective oxidation products and the concentration and strength of basic centers [8]. We found a linear correlation between the deprotonation enthalpy of alkyl substituents and the rate constants of their conversion into the cyano group [10,11], which, too, argues in favor of the heterolytic mechanism.The aim of the present work was to find out the reasons for the higher, compared with the methyl group, reactivity and selectivity of the conversion into the cyano group of such heteroatomic substituents in the aromatic ring as chloromethyl, methoxymethyl, and alkoxycarbonyl. We performed quantum-chemical calculations of the electronic structure of the starting compounds and probable intermediate products (radicals, carbanions, and carbocations), as well as simulation of the reaction of the starting compounds with active surface centers of the oxide catalyst in terms of the cluster approach.Both electron-acceptor and electron-donor substituents in the aromatic ring of toluene, probably do not change the mechanism of conversion of the methyl group into C=N under oxidative ammonolysis conditions, but accelerates this stage, which is probably explained by decreased deprotonation enth...
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