SYNTHETIC APPLICATION OF CERTAIN METAL CHELATES AS CATALYSTS IN OXIDATION REACTIONS

Abstract

The thesis entitled "Synthetic Application of Certain Metal Chelates as Catalysts in Oxidation Reactions" deals mainly with a study of oxidation of different substrates effected by molecular oxygen in the presence of transition metal complexes. The substrates ranged from the simplest hydrogen sulphide as its sodium salt to its organic derivatives, viz. thiols, both alkyl and aryl, substituted phenols, which are the oxygen analogues ofthiols, through alkyl and aryl sulphides. The subject matter ofthe thesis has been divided into five chapters. Each chapter presents the relevant literature survey and the concerned bibliography. Experimental work followed by results and discussion are described in chapters 2 through 5. Chapter-1 gives an over view of the transition metal catalyzed oxidations using dioxygen. The chapter briefly traces the development of the two disciplines, organic synthesis and catalysis and describes the high atom utilization in oxidation by molecular oxygen as against stoichiometric oxidants, the nature of molecular oxygen or dioxygen, need for its activation, methods of activation, and the merger of the two disciplines as exemplified by the homogeneous catalysis involving the transition metal complexes in oxidation. In this the chemist's goal has been mimicking the biological oxidative processes that take place under the mildest conditions in the most efficient manner. Chapter-2 deals with the oxidation of hydrogen sulphide to elemental sulphur. The importance of one of the latest processes of treating H2S by aqueous Fe(III)-chelates has been discussed. Fe(III)- chelate is reduced to its corresponding Fe(II)-chelate while H2S is oxidized to sulphur in an alkaline solution. Fe(II)-chelate is reoxidized by passing oxygen through the solution thus enabling the process to become catalytic. A need for the development of a simple technique for the estimation of sulphide ion in solution was seriously felt. Accordingly, a very convenient electrochemical method involving the potential measurements was developed for measuring the sulphide ion concentration at ppm levels. An advantage of this method lies in the fact that it is possible to monitor the Fe(III)/Fe(II) ratio at any given instant, which is very vital for optimising the process conditions in the development of the technology. The method has been published in Fresenius Journal of Analytical Chemistry, 348, 742- 744(1994). This is a case of redox reaction by one-electron transfer involving the metal chelate. Oxidation: 2Fe(III)-EDTA +H2S > 2Fe(II)-EDTA +S°+2H+ Regeneration : 2 Fe(II)-EDTA + l/202 + H20 > 2 Fe(III)-EDTA + 20H" An obvious extension of this experience with the Fe(III)-EDTA was considered to be its application to the oxidation of alkyl and aryl thiols by molecular oxygen. The corresponding disulphides were obtained in quantitative yields under mild conditions. Thus an efficient synthetic procedure has been developed for the oxidation of thiols to disulphides. This study is described in Chapter-3. A survey of the available synthetic procedures has been presented at the beginning of this chapter. Fe(III)-EDTA + RS" > Fe(II)-EDTA + RS* 2 RS* > RSSR Phenols are the oxygen analogues of thiols. Chapter-4 describes our studies of oxidation of 2,6-di-/-butylphenol (1) and several substituted hydroquinones with molecular oxygen using the Fe(III)-EDTA catalytic system, under ambient conditions, in aqueous methanol solution. Oxidation of 2,6-di-/-butylphenol (1) by a variety of oxidants, mostly stoichiometric, has been reviewed. Research Ill activity at times was directed to obtaining selectively one of the two products of oxidation, viz 2,6-di-/- butyl-l,4-benzoquinone (2) and 3,3',5,5'-tetra-/-butyl-4,4'-diphenoquinone (3). In our case the diphenoquinone (3) could be obtained quantitatively under ambient conditions using 10 mol. % of Fe(III)-EDTA by reaction of 2-6-di-/-butylphenol (1) with molecular oxygen. Under these conditions the oxidation proceeds by a one-electron transfer process. Similar reaction by molecular oxygen with a series of substituted hydroquinones resulted in the corresponding benzoquinones in excellent yields. The methodology could find application in the effluent treatment in eliminating the toxicity of the phenolic contaminants; iron is an environmentally acceptable metal. It also offers an economical and environment-friendly route to such synthetic transformations. These results have been published in Tetrahedron Letters, 35, 5083-84(1994). ..............