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Title: Enantioselective hydrogenation of pyruvate esters over cinchona alkaloid modified platinum catalysts
Author: Jenkins, Robert Leyshon
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2007
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The enantioselective hydrogenation of pyravate esters has been investigated over cinchona alkaloid modified platinum catalysts. Three topics have been examined: (i) A1MCM-41 as a novel support for the platinum catalysed enantioselective hydrogenation of ethyl pyruvate. (ii) The enantioselective hydrogenation of pyruvate esters over cinchonidine pre-modified PtAy-AbQa catalysts in a continuous flow reactor. (iii) An investigation into the modifier concentration dependence on the sense of enantioselectivity in ethyl pyruvate hydrogenation exhibited by derivatives of hydroquinidine and hydroquinine. A series of Pt-A1MCM-41 catalysts were synthesised using both CI 6- and CI 8-alkyl chain surfactant templates with platinum loadings approaching 2 wt/%. High resolution transmission electron microscopy (HRTEM) data confirmed retention of the mesoporous structure in the active catalysts and gave information on the platinum particle size and their location within the mesopore. High resolution magic angle spinning (HRMAS) 27 Al NMR was used to study the movement of aluminium within the mesoporous structure during the different preparation stages. The resultant platinum catalysts afforded enantioselectivities comparable to that obtained with the standard reference catalyst EUROPT-1 (6.3% Pt/silica), at rates moderated by an order of magnitude by mass transfer limitations. This is the first report of an enantioselective hydrogenation occurring in a mesopore. The hydrogenation of pyruvate esters over cinchonidine pre-modified catalysts in a continuous flow reactor operating at ambient temperature and pressure (0.25 bar/g) has been investigated. With dichloromethane as a solvent sustained enantiomeric excess of > 70% was maintained at very low cinchonidine/platinum ratios and are comparable to the enantioselectivity achieved when the reaction is conducted in the same solvent at autogeneous pressures (50 bar). When dichloromethane is used as a reaction solvent conversion is seen to decrease with time on line and is considered to be the result of polymer formation on the catalyst surface. The pre-treatment of the catalyst bed with (R)-ethyl lactate prior to the introduction of methyl pyruvate is detrimental to the catalytic system with an 18 % drop in enantioselectivity, potentially the result of an unfavourable interaction between lactate and modifier. In the enantioselective hydrogenation of ethyl pyruvate over hydroquinidine-4-chlorobenzoate modified Pt/v-AbQ} the sense of enantioselectivity is a function of modifier concentration. At low concentration (S)-ethyl lactate is preferentially formed while at high concentration (K)-ethyl lactate is the preferred enantiomer with an enantiomeric swing of 40% across the modifier range tested. The opposite trend with a similar magnitude was observed with hydroquinine-4-chlorobenzoate. The effect is sensitive to the way in which the Pt/y-Al203 catalyst is prepared with reduction temperature determining if inversion of enantioselectivity was observed. The same experiments conducted over platinum silica and platinum graphite catalysts failed to demonstrate the same modifier concentration dependence on the sense of enantioselectivity.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available