Mechanistic study of asymmetric amplification in the Soai autocatalytic reaction

• Main Author: Quaranta, Michela
• Other Authors: Armstrong, Alan ; Blackmond, Donna
• Published: Imperial College London 2010
• Subjects: 547.1223
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527767

Soai’s discovery of chiral amplification in the autocatalytic alkylation of pyrimidine-5-carbaldehyde with diisopropylzinc is one of the most noteworthy findings of the last decade of the 20th century. This is the first experimental confirmation of an early theoretical rationalisation of autocatalysis as a mechanism for the evolution of biological homochirality from a racemic environment (Frank, 1953). This thesis describes kinetic and spectroscopic investigations that were conducted with the aim of better understanding the mechanism under which chiral amplification is achieved in the Soai system. The methodology used to perform the kinetic studies that are presented in this thesis focuses on the use of reaction calorimetry as in-situ tool coupled with the appropriate analytical technique for enantiomeric excess measurements. Observations of an unusual temperature effect on the reaction rate and a profound induction period are reported together with extensive kinetic investigations. Kinetic experiments were designed and carried out following Reaction Progress Kinetic Analysis methodology, which is described in detail. These experiments were carried out in order to ascertain the concentration dependence of the substrates and the reaction product, and revealed a 1.6 order in pyrimidyl aldehyde, a zero order in diisopropylzinc and a first order in the reaction product. Meticulous NMR studies of the alkoxide product at low temperature demonstrated its tendency to form tetrameric complexes, which could be either directly involved in the autocatalysis or be the precursors of the active catalytic species. Possible mechanisms that involve tetramers formation are proposed and supported by simulations carried out using COPASI simulation software. This thesis also includes a separate Chapter on the MIB mediated alkylation of benzaldehyde with diethylzinc, a system characterised by a marked nonlinear effect. Kinetic studies demonstrate how the high degree of chiral amplification comes at the expense of the reaction rate.