Design, Synthesis, Mechanistic Rationalization and Application of Asymmetric Transition-Metal Catalysts

Abstract: This thesis describes mechanistic studies, rational ligand design, and synthesis of asymmetric transition metal catalysts. The topics addressed concerned [Papers I-VII]:[I] The asymmetric addition of diethyl zinc to N-(diphenylphosphinoyl)benzalimine catalyzed by bicyclic 2-azanorbornyl-3-methanols was studied. An efficient route to both diastereomers of new bicyclic 2-azanorbornyl-3-methanols with an additional chiral center was developed, in the best case 97% ee was obtained with these ligands. The experimental results were rationalized by a computational DFT-study.[II] An aza-Diels-Alder reaction of cyclopentadiene with chiral heterocyclic imines derived from (S)-1-phenylethylamine and different heteroaromatic aldehydes was developed. The cycloaddition proved to be highly diastereoselective and offers a very rapid access to possible biologically active compounds and interesting precursors for chiral (P,N)-ligands. [III] A convenient and high-yielding method for the preparation of (R)-tolterodine, utilizing a catalytic asymmetric Me-CBS reduction was developed. Highly enantio-enriched (R)-6-methyl-4-phenyl-3,4-dihydrochromen-2-one (94% ee) was recrystallized to yield practically enantiopure material (ee >99%) and converted to (R)-tolterodine in a four-step procedure. [IV] The reaction mechanism of the iridium-phosphanooxazoline-catalyzed hydrogenation of unfunctionalized olefins has been studied by means of DFT-calculations (B3LYP) and kinetic experiments. The calculations suggest that the reaction involves an unexpected IrIII-IrV catalytic cycle facilitated by coordination of a second equivalent of dihydrogen. On the basis of the proposed catalytic cycle, calculations were performed on a full system with 88 atoms. These calculations were also used to explain the enantioselectivity displayed by the catalyst.[V and VI] A new class of chiral (P,N)-ligands for the Ir-catalyzed asymmetric hydrogenation of aryl alkenes was developed. These new ligands proved to be highly efficient and tolerate a broad range of substrates. The enantiomeric excesses are, so far, the best reported and can be rationalized using the proposed selectivity model.[VII] The complex formed between the quincorine-amine, containing both a primary and a quinuclidine amino function, and [Cp'RuCl]4 catalyzes the hydrogenation of aromatic and aliphatic ketones in up to 90% ee approx. 24-times faster than previously reported Ru-diamine complexes. The reason for the lower but opposite stereoselectivity seen with the quincoridine-amine, as compared to the quincorine-amine, was rationalized by a kinetic and computational study of the mechanism. The theoretical calculations also revealed a significantly lower activation barrier for the alcohol mediated split of dihydrogen, as compared to the non-alchol mediated process. A finding of importance also for the diphosphine/diamine mediated enantioselective hydrogenation of ketones.