A study of Mn(II)/Mn(III) oxidation catalysts in alkene epoxidation and chromophore bleaching

Robinson-Miller, Adam (2015) A study of Mn(II)/Mn(III) oxidation catalysts in alkene epoxidation and chromophore bleaching. Doctoral thesis, Northumbria University.

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Abstract

My original contribution to knowledge is presented in this work, in the new application of a previously synthesised manganese (III) complex as an effective and selective oxidation catalyst. Development of a series of conditions in which epoxidation reactions can successfully be undertaken was established with both hydrogen peroxide and n-tetrabutyl ammonium oxone. Epoxidations undertaken with hydrogen peroxide were commensurate with a competitive catalase like reaction. Establishment of the factors that drive the catalase type of oxidative reaction with the Mn(DMHP)2.Cl complex, including Michaelis Menten kinetic parameters and the effects of temperature and pH, allowed for a protocol to be set up that can favour epoxidation over catalase-like dismutation of hydrogen peroxide. To coincide with epoxidation work on hydrogen peroxide, a DFT study on the proposed reaction intermediates is presented with a focus on the likely active catalytic species. The role of imidazole was probed and showed that coordination of imidazole is slightly unfavoured and will be in equilibrium in solution, which coincided with experimental observations that imidazole was required in a slight excess for successful epoxidation. This work further demonstrated that in acidic conditions an oxomanganese(V) species is highly favoured and corresponds excellently to what is observed experimentally where very good conversion to epoxide of cyclooctene is achieved at pH 4.7. Work utilising n-tetrabutyl ammonium oxone as oxidant exhibited good to excellent conversions, and a trend was observed of high selectivity to strained cyclic alkenes. This was further probed by a joint study using a series of strained substrates that were synthesised and computational chemistry to determine the strains over the alkene bond. Collaborative work with the national mass spectrometry service allowed for identification of the intermediates of the reaction my mass spectrometry coupled with a UV/Vis spectroscopy study. Identification of an oxomanganese(V) species is presented in mass spectrometry as the active intermediate in epoxidations using n-tetrabutyl ammonium oxone . A study as to the effects of additive bases using cyclic voltammetry demonstrated a significant electronic effect on the oxidation potential of the catalyst as the source of the inductive effect, identification by mass spectrometry and UV/Vis showed direct evidence of axial ligation of the additive bases.

Using an industrial manganese (II) catalyst in the bleaching of red stains (β-carotene and crocin), a protocol for bleaching was established, using UV/Vis as a method of analysis. Synthesis of a series of chelants which are used to stabilise the catalyst in the formulation was undertaken. Testing of the chelants in the bleaching system demonstrated their utility in the wash system across a pH range. The utility of surfactant when using chelants with the catalyst was established to give excellent bleaching and stability of the catalyst.

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