ORGANIC TRANSFORMATIONS USING METAL BASED AND METAL-FREE CATALYTIC SYSTEMS

Abstract

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of compounds via organic transformations. The ultimate aim of a synthetic organic chemist is the development of reactions and reagents with high synthetic efficiency in an economic and environmentally acceptable manner. Transition metals and their derivatives play a dominant role in organic synthesis. In general, the types of reactions mediated by transition metals include: oxidation, reduction, carbon-carbon bond formation, carbon-heteroatom (N, O, S...) bond formation, isomerization, multicomponent reactions (MCRs) and asymmetric synthesis. In the recent years due to the increasing environmental constraints emphasis is being placed on the replacement of stoichiometric metal oxidants with clean oxidants like molecular oxygen, hydrogen peroxide and alkyl hydroperoxides. Homogeneous metal catalysts though very useful, have some drawbacks like difficult separation and recycling of expensive catalysts. Therefore immobilization of the metal complexes via functionalized ligand or by adsorption on porous supports and the use of heterogeneous catalysts in various chemical reactions are considered to be advantageous. Another important concern in the chemical reactions is the use of toxic and volatile organic solvents and replacement of these with solvent-free condition or environmentally friendly solvents like ionic liquids, water and supercritical C02 is also a fast growing area in the recent years. Though the metal catalyzed organic transformations represent the major part of organic reactions, researchers are now focusing on the development of suitable metal free catalytic pathways, as metal mediated reactions have some unavoidable drawbacks like the presence of toxic traces of heavy metals in the products and catalyst deactivation by various means. One of the major advantages of metal free organic/inorganic catalysts is their better environmental acceptance compared to transition metal catalysts. In recent years such non metal reagents have become available and are being extensively employed in oxidation and other reactions. In view of these developments the present thesis has been devoted to the development of new synthetic methodologies for various organic transformations using heterogeneous catalysts, environmentally acceptable oxidants like molecular oxygen, hydrogen peroxide and tert-buty\ hydroperoxide, and metal-free catalytic systems. Chapter 1 of the thesis consists of general introduction of the use of transition metals and metal free catalytic systems in organic transformations. This chapter describes various modes of activation of environmentally acceptable oxidants like molecular oxygen, hydrogen peroxide and /er/-butyl hydroperoxide with metals. The chapter also highlights multi-component reaction, aziridination, cyclopropanation, and various metalfree reagents which are of current interest and relevant to the present study. Chapter 2 describes vanadium catalyzed oxidation reactions by using molecular oxygen, hydrogen peroxide as oxidants and consists of two parts. The part A of the second chapter describes vanadium pentoxide catalyzed oxidation of 2-naphthols to binaphthols by using molecular oxygen as the sole oxidant (Scheme 1). This is an important synthetic transformation as l,l'-bi-2-naphthols are well known chiral inducers in synthetic organic chemistry.................