The synthesis and catalytic epoxidation activity of a range of substituted iron porphyrins

• Main Author: Smith, Robert B.
• Published: University of Surrey 2008
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.731125

Metalloporphyrin research has been firmly established as useful for mimicking certain enzymes present in living systems, and much research has gone into using these compounds to oxidise organic substrates. However, one major drawback is that there is no clear picture of the electronic factors that influence catalyst efficiency; another is that these catalysts undergo degradation that causes a subsequent loss of activity. After reviewing the literature, it became apparent that much published research focuses on epoxidation by a small and eclectic range of tetra-substituted metalloporphyrin catalysts whose main criterion for selection is that they are readily available. This thesis seeks to remedy that by looking in detail at more finely-tuned structural and electronic variations of the substituents to iron porphyrins. In order to investigate the effects of catalyst structure on epoxidation efficiency and catalysts' stability a large number of iron porphyrins was synthesised. In general a variation of the Alder-Longo method was used as a one-pot procedure from pyrrole and the required aromatic aldehyde to yield mixtures of mono- bis-, tris- and tetrakis-meso-aryl substituted porphyrins. These were then isolated by careful column chromatography. Further porphyrins were obtained by substituent modification of these isolated ones. The newly synthesised iron porphyrins were used for a systematic study of the effect of small changes in structure on the efficiency of alkene epoxidation. The reactive alkene cyclooctene was used as substrate and the useful, but powerful, hydrogen peroxide as oxidant. The efficiency of a metalloporphyrin as an epoxidation catalyst was quantified in terms of the yield of cyclooctene oxide produced, determined using gas chromatography. The concomitant catalyst degradation was quantified by monitoring, for the the iron porphyins, the decreasing Soret band around 415nm by UV-Vis spectroscopy, and from this information determining the half life data for the majority of iron porphyrins and rate constants for some; the latter were obtained mostly using the Guggenheim kinetic method to calculate a first order rate constant kobs for degradation.