Ylides: Stabilization of Novel, Low Valent Carbon-Based Ligands with Applications in Catalysis
- Author(s): Asay, Matt
- Advisor(s): Bertrand, Guy
- et al.
The ylide has found practical applications in synthesis for many years, especially in the creation of carbon-carbon double bonds by way of the Wittig reaction. However, this work is concerned with new applications of ylides, especially the unique stabilization powers of phosphorus and nitrogen ylides. While the species reported in this dissertation are fundamentally interesting they are also directed toward synthetically useful applications. The first such species reported herein is the first phosphorus-sulfur bis-ylide, an analog of the well know carbodiposphorane, and has coordination behavior that is similar. This type of species could be a valuable reagent to do tandem olefination and epoxidation reactions on aldehydes or ketones.
A major part of this work is directed at using ylides to create stable carbon based ligands for homogeneous catalysis. A stable cyclic(amino)[(bis)ylide] carbene is the first example. The carbenic carbon itself is part of a formal nitrogen ylide, which is further stabilized by one anionic carbon of a phosphorus bis-ylide. The other anionic carbon remains free to coordinate transition metals creating very stable bidentate complexes.
The largest portion of this dissertation is focused on the synthesis and ligand behavior of the first stable, fully characterized cyclic vinylidenephosphorane. Once again the special properties of ylides, specifically phosphorus ylides are on display as only one phosphorus stabilizes the low valent carbon center. This new species is a very strong donor for late transition metals, stronger than phosphines and carbenes. As such it is an ideal ligand for numerous catalytic reactions. Two new catalysts using this ligand are made and successfully used in olefin metathesis and hydroamination reactions, respectively.
The final portion focuses on the use of N-Heterocyclic Carbenes (NHCs) to stabilize a propadienylidene, a valence isomer of cyclopropenylidene. While the free species alluded synthesis several adducts are stable including a silver complex that can be used in transmetallation reaction.