Molecular mechanisms during amplification of chirality in organometallic systems : in situ studies by X-ray absorption spectroscopy

摘要

The alkylation of pyrimidyl aldehyde by diisopropylzinc has received immense attention over the last decade. This is mainly because the reaction which was discovered by Soai and coworkers is capable of achieving a homochiral product from an essentially achiral precursor. The strong amplification of the enantiomeric excess occurs because of a transition state complex which is responsible for autocatalysis. Clarifying the structural nature of the organometallic species involved in the reaction is vital for understanding the mechanism of the chiral amplification process. Known mechanistic details are patchy and based on studies that address molecular level details by NMR, computational chemistry, calorimetric and kinetic studies. The studies reported in this thesis for the first time directly addressed the nature of the intermediate by structural analysis with X-ray Absorption Fine Structure (XAFS) Spectroscopy at the Zn K-edge. These measurements provide bond distances, local coordination numbers and the geometry of ligands in the local environment around the Zn centres. First, the molecular level origin of the solvent dependencies in the asymmetric amplification by the Soai process is elucidated. A rationale for the behaviour of dialkylzinc compounds in polar and non-polar solvents is reported. Structural causes for often observed chirality depletion in polar solvents are elucidated. Further studies then examined the nature of the products formed by the reaction of various chiral ligands and dialkylzinc compounds. Different chiral ligands are examined and the complex structure was determined.Finally, first results of in situ studies of the reaction progress in continuous flow channel cells are reported.