1 Organic Chemistry, Department of Chemistry, Technical University of Denmark2 Department of Chemistry, Technical University of Denmark
The subject of this thesis is selectivity in homogeneous asymmetric transition metalcatalyzed reactions. Four different reactions within organic chemistry have been studied by kinetic measurements, computational chemistry (modelling) or both of them in parallel. A Hammett study was performed on the Heck reaction between phenyl triflate and a series of para-substituted styrenes. A linear correlation was obtained only for substitution in the α- position, where the developing positive charge is in direct conjugation with the group in the para-position. The palladium-catalyzed allylic alkylation has been the subject of several smaller projects. With bidentate nitrogen-based ligands experimental selectivities were used as a benchmark for a computational study. When a large enough part of the experimental system was included, the selectivity could be predicted with reasonable accuracy. When using PPh3 as ligand, the effect of chloride has been the subject of both experimental and computational studies. The effect on the regioisomeric distributions resulting from allylic alkylation of a branched starting material was investigated experimentally and could be rationalized computationally. A thorough computational study succeeded in explaining the observed results, although other significant results were also obtained during this study. Finally, an intramolecular reaction was studied computationally, and the rate increase observed under phase transfer catalysis conditions could be related directly to the absence of Na+. This explanation was supported by new experimental investigations. A kinetic study was conducted on the Mn(salen)-catalyzed asymmetric epoxidation reaction employing a series of methyl-substituted styrenes. The observed reactivities and selectivities could be rationalized by the existence of two different approach vectors, which are both favoured for these substrates. In addition, the study could serve as a testing ground for the development of new theoretical methods. The reactivity and selectivity in the osmium-catalyzed asymmetric dihydroxylation was studied using a series of twelve “substrate-probes”, which were designed and synthesized specifically for this purpose. Both the stoichiometric reaction with OsO4 in toluene and the more environmentally benign catalytic reaction in a two-phase system were studied. The obtained experimental results were in good agreement with predictions based on computational modelling of the transition states. Visualization of the determined transition states allowed for the construction of the new mnemonic device for prediction of absolute configuration, which also included a mapping of the important features onto an overlaid transition state.