Catalysis through molecular association

Abstract

The thesis entitled "Catalysis through Molecular Association" is divided into four chapters. The objective of the work described here was to develop a new method of ortho-carbonation of phenol in water, at room temperature and atmospheric pressure. It essentially involved the development of an enzyme model capable of satisfying the following points: First, it should associate with phenol in a way as to station one of its nucleophilic centres near to the ortho position of phenol. Second, the concerned nucleophilic centre should capture a carbon dioxide molecule and transfer the latter to phenol to bring about ortho-carbonation, leading to salicylic acid.

Chapter I The "General Introduction" presents a review of the important physico-chemical aspects of the catalytic action of enzyme models, as related to the behaviour of enzymes. Concepts such as proximity effect, orbital steering, covalent catalysis, charge transfer interactions, hydrophobic association, etc., are discussed. Some novel findings in the field of enzyme models, as well as some biomimetic reactions of possible synthetic utility, are considered. The project is introduced through a brief discussion on Kolbe-Schmitt reaction.

The chapter, "The Strategy of Carbonation and Syntheses of the Model Compounds", gives an account of how the carbonation of phenol, using model compounds for capture and transfer of carbon dioxide, was planned. The compounds passing the tests for a model compound were synthesized. This list included compounds like indazolone (I), sym-bis(triphenylmethyl)hydrazine (II), triphenylmethylhydrazine (III), zinc-imidazole complex (IV), benzimidazole (V), ortho-aminobenzyl alcohol (VI), 4-hydroxy-1,2,3,4-tetrahydroquinoline (VII), etc. The solubility of a model compound in water was an important factor. Only o-aminobenzyl alcohol was quite soluble in water; the others were sparingly soluble.

Chapter II The magnitude suggested that the complex formation was thermodynamically favourable. Supporting the previous point, the values of free energy of association (?G) were moderate: K(25°C) = 4.46 × 10³, ?G(25°C) = –6.341 kcal/mol K(45°C) = 4.826 × 10³, ?G(45°C) = –6.816 kcal/mol The free energy values arrived at were comparable with those reported before for phenyl-phenyl association.