Abstract
The polybutoxytitanates catalysis of aniline acylation by orthosubstituted benzoic acids leads to the production of substituted benzanilides. Catalytic rate constants of the second order reaction (the first with respect to aniline and ortho-substituted benzoic acid; boiling ortho=xylene, 145°C) correlate well according to the Hammett and Bronsted equations with straight line segments with ρ=1.93 and α=0.66, in contrast to the reaction of aniline with meta- and parasubstituted benzoic acids and substituted anilines with benzoic acid. This dependence drops out 2=nitrobenzoic and 1=naphthoic acids, which have relatively low reactivity and the greatest steric hindrances both for nucleophilic attack by aniline and for possible coordination with catalytically active centers of the corresponding ortho-substituted titanium polybutoxybenzoates formed in situ.
Based on these data, the previously proposed mechanism of bifunctional catalysis due to titanium polybutoxybenzoates and their complexes with meta- and parasubstitutedbenzanilides was supplemented by the possibility of the steric inhibition of reaction by the most bulky substituents and chelate structures formation of orthosubstituted benzoic acids and their anilides with individual titanium atoms of the catalyst, as well as the simultaneous H-bonding of the amino group hydrogen atoms of aniline, which leads to its activation to a nucleophilic attack, with a carbonyl group and an orthopositioned substituent of the orthobenzoate ligand in the coordination sphere of titanium. Taking into account such chelation and steric barriers, as well as inhibition of acid catalysis due to the formation of the imide form of anilides, containing electron-withdrawing substituents, the equations for the rate constants of the catalytic reaction of ortho-substituted benzoic acids with aniline are derived, corresponding to the experimentally obtained Hammett dependence.
Publisher
V.I. Vernadsky Institute of General and Inorganic Chemistry
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