Reactivity and Selectivity Studies in Iridium-Catalyzed Asymmetric Hydrogenation

Sammanfattning: Transition-metal catalyzed stereoselective hydrogenation constitutes an indispensable method for the preparation of chiral molecules. The work in this thesis describes the development of efficient catalytic asymmetric hydrogenations with a focus on the elucidation of reactivity patterns and chemoselectivity in hydrogenations. Iridium catalysis plays a central role in the forthcoming developments that were often driven by mechanistic observations. First, the influence of the alkene geometry on the stereoselective outcome of the hydrogenation was studied. Olefins having a chelating group on the prochiral terminus were found to undergo an enantioconvergent hydrogenation and alkanes with high enantiopurity were yielded even when the reaction started from a geometric mixture. Combined theoretical and experimental studies suggested that the convergency arose from the ability of the catalyst to hydrogenate these substrates via a chelating mechanism. Next, the importance of the conformation of a conjugated alkene with respect to a carbonyl group was studied. It was found that alkenes in the (s)-cis conformation experienced a large rate acceleration compared analogues in the (s)-trans conformation. This insight was used to develop a novel type of regiodiscrimination in the monohydrogenation of dienes. In addition, further reduction for the installation of multiple stereogenic centers at once was also demonstrated. The thesis was continued with a study of the effect of additive in the hydrogenation of enones. Ultimately, with the use of benzamide, a single catalyst that otherwise forms the chiral ketone as the dominant product in the absence of benzamide produced solely the saturated alcohol with two contiguous stereogenic centers. The role of benzamide was proposed to extend the lifetime of the active state of the catalyst by a reversible cyclometallation preventing the irreversible trimerization, deactivating the catalyst. Finally, a dual catalytic iridium approach was undertaken to hydrogenate enones to saturated alcohols in a stereocontrolled manner. A new family of catalysts able to hydrogenate ketones under pH neutral conditions in non-coordinative solvents and that also utilized molecular hydrogen as the reductant was developed. These catalysts operated independently of one another once used concomitant with an iridium complex suitable for the hydrogenation of alkenes and each accounted for the reduction of only one π-bond.