Synthesis and characterisation of some molybdenum complexes and their use as epoxidation catalysts

Abstract

With the aim of inducing asymmetric epoxidation, the metal catalysed epoxidation of alkenes and substituted alkenes using tert-butyl hydroperoxide and various molybdenum catalysts have been studied. In Chapter I, the synthesis of various cis-dioxomolybdenum complexes from 2,4-pentanedione dioxomolybdenum(VI) by reaction with various racemic and chiral diol ligands and amino alcohols is described. Under more vigorous reaction conditions, cis dioxomolybdenum(VI) complexes were dimerised giving complexes with a core. The reactions of L-histidine with molybdenum hexacarbonyl and molybdenum(VI) oxide afforded the potassium salts of tricarbonyl-L-histinato and histidinatotri-oxomolybdate (VI) respectively. The complexes were characterised by 1H and 13C nmr spectroscopy. In Chapter II, kinetic data is presented for the metal catalysed epoxidation of 1-decene and 1-dodecene at 70° using various molybdenum catalysts and tert-butyl hydroperoxide. Reactions with equimolar concentrations and a ten-fold excess of olefin to tert-butyl hydroperoxide were carried out. A possible mechanism is proposed. Chapter III deals with the asymmetric epoxidation experiments carried out using the molybdenum catalysts synthesised from chiral ligands. Geraniol, 3-methyl-2-butene-l-ol and 1-methyl cyclohexene were epoxidised with varying degrees of success. The highest enantiomeric excess achieved was 85% for 3-methyl-2-butene-l-ol oxide using cis-Bis(phenyl ethane-1,2-diolato) dioxomolybdenum (VI). Epoxy alcohols were reacted with (-)-a-methoxy-a-trifluoromethyl phenyl acetyl chloride to give the corresponding esters, enabling enantiomeric excess to be calculated by nmr. Reactions carried out at different temperatures showed variations in enantiomeric excess - a higher enantiomeric excess being obtained at lower temperature.