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Title: Some model studies for vitamin B12 dependent enzymic reactions
Author: Atkins, Martin Philip
ISNI:       0000 0001 3431 8488
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1980
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Vitamin B12 catalyses a variety of important reactions in metabolic pathways. Impairment of vitamin B12 by man leads to deficiency diseases e.g. pernicious anaemia. Many mechanistic possibilities have been suggested to account for the transformation of organic substrates to products in these enzymic reactions ranging from purely protein mediated reactions to the existence of discrete ionic or radical intermediates. A crucial point in these rearrangements is the possibility of discrete organocorrin intermediates. To investigate this possibility and to investigate reactions of alkyl groups attached to cobalt a series of alkylcobaloximes were studied. Cobaloximes were used as models for Vitamin on account of their similarity, relative ease of synthesis, and easily interpretable Spectroscopic data. The work described in this thesis explores reactions of substituted cyclopropylmethyl- and but-3-enyl- groups attached to cobalt. By use of 13C labelling experiments and kinetic studies on the rearrangement of cyclopropylmethyl- to but-3-enylcobaloxime the mechanism of this rearrangement has been postulated to be a unimolecular reaction involving a homoallylic transition state. 1-Methylbut-3-enylcobaloxime was found to equilibrate with the 2-methylbut-3-enyl isomer, the reaction presumably proceeding through the intermediacy of methylcyclopropylmethylcobaloximes. Both cis- and trans-isomers of the postulated 2-methylcyclopropylmethyl- intermediate were synthesised and it was shown by suitable kinetic study that the cyclopropanes were, indeed, plausible intermediates. (R)- and (S)-1-methylbut-3-enylcobaloximes were found to equilibrate stereospec- ifically with (S)- and (B)-2-methylbut-3-enylcobaloximes, respectively, under catalysis by TFA - and demonstrates the first stereospecific transformation of an organic ligand attached to a metal atom. Reactions of alkyl(pyridine)cobaloximes with TFA were explored and were shown to exhibit a general trend. TFA first protonates a dimethylglyoximato ligand and a subsequent molecule removes coordinated pyridine as pyridinium trifluoroacetate. Excess of TFA causes the eventual precipitation of a red crystalline complex characterised as a novel cis-cobaloxime by single crystal X-ray diffraction study.
Supervisor: Not available Sponsor: Science Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry