EPOXIDATION OF ^-UNSATURATED CARBONYLS USING NOVEL CATALYTIC SYSTEMS

Abstract

The commercial and synthetic importance of epoxides made the area of epoxide chemistry an active field of research. Epoxides are cyclic ether O with three membered ring ^>c —cS • In IUPAC nomenclature epoxides are called oxiranes'. Due to steric strain, oxirane ring is very reactive and hence oxiranes have been used as intermediates in the synthesis of ct-plasticizers and synthetic resin adhesives. The vast utility of epoxides in organic chemistry is obvious as these compounds can be exploited under various conditions to yield useful products. Epoxides have recently been employed as intermediates in the synthesis of biologically important compounds like, a-tocopherol, prostaglandins, pederamides etc. Epoxides play a central role in industry and are widely used in textile, pharmaceuticals, cosmetics, food and feed, leather and mineral oil industries. Ethyleneoxide, propyleneoxide and their adducts are used as polishes, fertilizers and detergents. Epoxides are also reported to have been used in coating for marble and semiconductors. A number of epoxynitriles are cyclized to obtain intermediates for the synthesis of compounds like grandisol. Epoxides when reacted with strong base such as lithiumdiethylamide furnish ketones, amino alcohols and transanular product alongwith allylic alcohols in the case of medium ring compounds. In the light of the above, epoxides are thought to be of great importance in the chemical world. Catalysis by metal complexes, plays an (i) important role in the control of oxidation of alkanes, olefins and aromatic hydrocarbons to useful products. Transition metal catalysed oxidation of hydrocarbons is of synthetic as well as biochemical interest. Though the role of metal ions in catalyzing chemical reactions affording products in higher yield has long been established, use of metal complexes anchored on immobilised supports as catalyst has received much attention these days. Metal complexes as such or anchored on solid supports through direct anchoring or adsorption processes work as efficient catalysts. Metal ions anchored on montmorillonite have been found excellent catalyst in the oligomerisation of activated nucleotides. During recent years, various transition metal complexes have been incorporated into polymer resins and used as catalysts. A resin supported catalytic system has some special advantages over its homogeneous counterpart by virtue of its insolubility in the reaction mixture and reusability. Secondly, use of immobilised catalyst helps preventing contamination by traces of the catalyst in reaction products which is often detrimental to product stability. One of the main reasons for employing anchored catalysts in epoxidation is their ability to give a required level of stereoselectivity. Ionic iron(III) tetraarylporphyrins are readily adsorbed onto cross-linked polystyrene ion-exchange resins. These catalysts are effective in the epoxidation of cyclohexene and (Z)- cyclooctene. Epoxidation of cyclooctene has been catalysed by Robust sulphonated manganese and iron porphyrins immobilised on cationic ion exchanger. Comparative studies between the polymer supported and the homogeneous systems suggest extra stability for the polymer anchored (ii) I catalytically active species. Linden and Farona observed remarkable increase in the catalytic activity of resin-bound vanadyl ion compared to those on other systems. The homogeneous catalyst, VO(acac)2 promoted the yield of cyclohexene oxide in the range of 10-12%. Vanadyl ion anchored to a polystyrene support through acetylacetonate or ethylenediamine ligands gave yields of cyclohexene oxide upto 26%, roughly two-fold increase over the homogeneous system. However, with vanadyl ion immobilised on the sulphonated ion-exchange resin yields upto 74% cyclohexene oxide were realized. During the recent years, the epoxidation of alkenes using polymer supported catalysts is gaining importance. Resin supported tungstate catalysed epoxidation of maleic acid with hydrogen peroxide and its kinetics is reported by Allan and Neogi. Bhaduri et al. studied kinetics of cyclohexene epoxidation using polymer supported oxo-vanadium complex. The kinetics of epoxidation of crotonic acid and citraconic acid by hydrogen peroxide in the presence of sodium tungstate have been studied in our laboratory. Kinetic studies have also been carried out on the epoxidation of l,3-diphenyl-2-propen-1-one with t-butylhydroperoxide using Na3[V02(EDTA)] as a catalyst. The results showed that the order of epoxidation is one each with respect to chalcone, t-butylhydroperoxide and catalyst. The epoxidation of a,p-unsaturated ester viz., diethyl fumarate and methyl crotonate has been carried out with hydrogen peroxide in aqueous medium using phosphotungstic acid as a catalyst. The kinetics of the reaction was studied under pseudo conditions keeping ester concentration in excess. The results showed first order dependence of epoxidation rate on each of (iii) the substrates and catalyst, while, it showed zero order dependency on HO concentration. l,3-diphenyl-2-propen-l-one, commonly known as chalcone constitute an important class of organic compounds. They are found as intermediate in the biosynthesis of flavones. It works as starting material in the synthesis of isoxazole which possess antitubercular, antibacterial and antiviral activities. In view of the above, the present work is directed to the study of epoxidation of chalcone by some easy-to-handle oxidants like, H20 , t- BuOOH, and m-chloroperbenzoic acid (m-CPBA) using catalysts derived from complexes of metal ions in their anchored and unanchored forms. Several complexes of iron, molybdenum and vanadium have been prepared and were used as catalyst in the epoxidation of l,3-diphenyl-2-propen-lone using oxidatnts like, H202, t-BuOOH and m-CPBA. Some of the complexes were immobilised on appropriate solid supports and were also used as catalyst in epoxidation reaction. Epoxidation of chalcone has also been studied under phase transfer catalytic conditions. The result of the researches have been presented in the thesis which is divided into five chapters. Results are as follows : Chapter -1 presents literature survey on epoxidation reaction using metal complexes as catalyst in their immobilised and unimmobilised form. This chapter also discusses various mechanisms involved in epoxidation reactions, applications of epoxides and lastly defines the scope of the current research. (iv) Chapter-2 deals with the experimental part which includes synthesis of different substituted l,3-diphenyl-2-propen-l-one. Preparation of various transition metal complexes viz., VO(acac)2, K3[V02(EDTA)], K[VO(02)(EDTA)], K2[nicH[VO(02)]H20, [VO(PS-FSAL-OAP)DMF], [VO(PS-FSALEN)] [Mo02(PS-FSAL-OAP)DMF],[Fe(PS-FSALOAP) C1.2DMF], [Fe(PS-FSALEN)Cl.DMF] and their immobilisation on appropriate solid supports. Methods for the epoxidation of chalcones, and their reaction kinetics have been discussed. IR, UV and 'H NMR spectral data for the identification of various substrates and their products are also presented in this chapter. Chapter -3 summarizes results of kinetic studies on the epoxidation of chalcone with t-BuOOH and m-CPBA using VO(acac), as catalyst. For the sake of clarity this chapter has been divided into two parts. Chapter -3(A) describes the epoxidation kinetics of chalcone with t-BuOOH whereas the Chapter-3(B) describes the epoxidation kinetics of chalcone with m-CPBA. In both the cases VO(acac)2 was used as catalyst. Epoxidation of chalcone with both the oxidants namely, t-BuOOH and m-CPBA using VO(acac)2 as catalyst showed almost similar kinetic features. In both cases reaction was found first-order with respect to chalcone and oxidant. Reaction rate showed first and zero order dependency on the catalyst in case of epoxidation with t-BuOOH and m- CPBA, respectively. Further in both cases the effect of substituents on the phenyl ring of chalcone, and the effect of solvent and temperature have been investigated. On the basis of experimental observations suitable mechanisms (v) have been proposed. Chapter- 4 includes the results on epoxidtion of chalcone with t-BuOOH using VOS04, VO(acac)2, [VO(CH3COO)] and K3[V02(EDTA)], as such and their anchored form, [VO(PS-FSAL-OAP)DMF] [VO(PSFSALEN)], [Mo02(PS-FSAL-OAP)DMF] and [Fe(PS-FSAL-OAP)C12DMF] were also used in their immobilised form as catalyst. Results showed that epoxidation using immobilised metal complexes was faster in comparison to the homogeneous counter part. Results also indicated that electron withdrawing groups present in the phenyl ring of chalcone enhanced the epoxidation rate. The last and fifth chapter includes the results on epoxidation studies of chalcones with aqueous H202 and bleaching powder in the presence of phase transfer catalysts (PTC), viz., tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB) and tetrabutylammonium bromide (TBAB). The effect of substituent groups in chalcone on the yield of their epoxide is explained on the basis of electron donating and electron withdrawing behaviour of substituted groups present in the phenyl ring of chalcones. From these studies, it was concluded that the efficiency of PTC used follows the order TBAB>TEAB>TMAB. A tentative mechanism is also proposed to explain the epoxidation reaction.