Datensatz

Zusammenfassung

Ligands depicting π‐acceptor properties are able to facilitate those elementary steps of a catalytic cycle that require strong Lewis acidity at the metal center. Unfortunately, commercially available π‐acceptor ligands, e.g. PF₃, P(CF₃)₃, PCl₃ are either very toxic or moisture sensitive. Therefore, the design of new π‐acceptor ligands is still a challenge in the field of ligand design. The approach outlined in this thesis is based on the introduction of positively charged substituents as a way to impart π‐acceptor properties on the resulting ligands. In addition, it circumvents the moisture sensitivity issue avoiding any labile P‐X bond in the ligand structure.
Continuing the study of the structure and reactivity of the cyclopropenium‐derived phosphines developed within our group, a second generation of strong π‐acceptor ligands was prepared by incorporating polyfluorinated aromatic substituents or by replacing the cyclopropenium moiety with the more electron‐withdrawing dihydroimidazolium unit. The superior catalytic activity of the Pt(II) complexes derived from these ligands in a hydroarylation reaction was demonstrated as well as the application of the new catalyst in the synthesis of natural products Chrysotoxene and Epimedoicarisoside A.
As an extension of this work on cationic phosphines the analogous cationic amines were synthesized and fully characterized. The solid‐state structure analyses reveal unprecedented chemical environments around the central nitrogen atom. In contrast to their phosphorus analogues, the nitrogen atom in these cations adopts a trigonal planar environment, despite the computationally calculated lone pair of electrons and negative charge at nitrogen.
Finally, in order to avoid stability issues often associated with polycationic ligands, chelating architectures, employing two electron‐rich cyclopropenimines were used for the stabilization of dicationic phosphines for the first time. A series of dications were synthesized in good yields by applying the –onium substituent transfer methodology. Despite their highly charged nature, these cations were able to coordinate Au(I) and Ag(I) metal centers and could also be oxidized to the phosphorus(V) compounds.