EPOXIDATION OF UNSATURATED COMPOUNDS USING TRANSITION METAL COMPLEXES AS CATALYST

Abstract

The oxides of olefins have immensely contributed to the field of scientific and commercial world during the last two decades. Epoxidation processes have evinced a lot of interest due to their varied applications in the field of synthetic, industrial and biological chemistry. The vast utility of epoxides in organic, chemistry is obvious, as these compounds can be exploited under various conditions, to yield useful products. Epoxides are cyclic ethers with three-membered rings which due to strain show high reactivity. Thus ct-epoxides are very reactive. The opening of this ring affords a very convenient route for forming C-C (or C-X, where X = N.O.S, Se or TeJ <r bonds. Epoxides are used as intermediates in the synthesis of biologically important compounds such as a-tochopherol, prostaglandins, pepdermide, lepiochlorin and pheromones etc. Epoxides are also widely used in paper, leather, fur, metal-coating, fertilizers, detergents, cosmetics and a host of several other industries. Literature survey reveals that the metal catalysed epoxidation of olefins have extensively been studied. Though vanadium complexes are found to be very good catalyst in epoxidation vactions, they have not been used much in the epoxidation of chalcones with oxidants like t-butylhydroperoxide. Epoxidations catalysed by immobilized catalysts to produce stereoselective products has a vast scope of studies. Hence it was thought worthwhile to prepare immobilized oxo-vanadium complexes and to test their activities towards epoxidation of certain a,/3-unsaturated compounds. 1,3-diphenyl-2-propen-l-ones, commonly known as chalcones are important intermediates in the biosynthesis of flavanones. Also, epoxides of chalcones are found to possess anti-bacterial and anti-fungal activities. The solubility of chalcones in organic phases enables us to try several phase transfer catalysts for their epoxidation. Cheaper oxidants like, hydrogen peroxide, bleaching powder t-alkyl hydroperoxides with metal complexes of iron and vanadium as catalysts have also been used in the epoxidation of chalcones. Results of our studies have been summarized in the form of the thesis. Work presented in thesis is divided in five chapters. In order to obtain an update knowledge on the work carried out on epoxidation of olefinic compounds, an extensive survey of literature has been presented in the first chapter, which also includes the historical developments and significance of the epoxidation processes, general methods for the synthesis of epoxides, different mechanistic aspects involved and their characterization. The second chapter summarizes the experimental part including the methodology, preparation of various transition metal complexes, namely, oxo-bis (pentene-2,4, dionato) vanadium (IV); [VO(acac)2],tri-sodium-dioxo (ethylenediamine-N,N, N'-N'-tetraacetate) vanadium(V) ; Na3(V02EDTA), N,N'-ethylene bis(salicylideneiminto) iron (III) oxo-N,N'-ethylene bis(salicylideneiminto) iron (III) ; (Fe-Salen)20 and the synthesis of a series of substituted 1,3-diphenyl-propen-l-ones by condensing various acetophenones with ii appropriately substituted benzaldehydes. IR, UV, and ^NMR spectral data for the identification of various chalcones and their epoxides are also presented in this chapter. The third chapter deals with the kinetic studies on epoxidation of maleic, fumaric and crotonic acids carried out with resin supported VO(acac)2 / HgO system. Results show increase in epoxidation rate over the corresponding unsupported VO(acac) /HO system. Epoxidation rate is found to follow the order, crotonic acid > fumaric acid > maleic acid. Esters of these acids fail to undergo epoxidation by resin supported V0(acac)2 HO system. This may be due to the blocking of carboxyclic group in esters which makes the availability of proton difficult which is involved in the reaction mechanism. Based on the results a tentative reaction mechanism is also proposed. Fourth chapter comprises the results of kinetics of epoxidation of different substituted chalcones with Na [VO EDTA] /t-BuOOH and (Fe-salen) 0 / t-BuOOH systems. Effect on the epoxidation rate due to substituents in the phenyl rings of chalcones and the effect of solvent, temperature, pH and catalyst concentration has also been investigated. Values of activation energies are also reported and a tentative free radical mechanism for the epoxidation is proposed. Epoxidation studies on certain water insoluble chalcones with aqueous oxidising agents such as HO and bleaching powder usi 2 2 mg phase transfer catalysts ; namely, tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB) and tetrabutylammonium bromide (TBAB) were carried out and the results are incorporated in fifth and 111 the last chapter. The effect of substituent groups in chalcones on the yields of their epoxides is explained on the basis of electron donating and electron withdrawing behaviour of substituted groups present in chalcones. It is concluded from the studies that the order of efficiency of phase-transfer catalysts used is TBAB > TEAB > TMAB. It may be attributed to steric nature of alkyl groups present in the catalyst. Tentative mechanisms are also proposed to explain the epxidation reactions.