Asymmetric and Organometallic Catalysis Mediated by a Chiral Supramolecular Host
- Author(s): Brown, Casey Jameson;
- Advisor(s): Raymond, Kenneth N.;
- Bergman, Robert G.
- et al.
Chapter 1. An overview of supramolecular catalysis is presented with attention to the diverse strategies used in developing these materials. Different strategies for the design and preparation of chiral supramolecular structures are described along with their applications to enantioselective binding and catalysis. The intrinsically chiral M4L612- supramolecular assembly developed by the Raymond group is introduced, and its catalytic scope is reviewed.
Chapter 2. The development of an asymmetric variant of the aza-Cope transformation is presented. Application of the resolved Ga4L612- assembly to this catalytic transformation gives modest but significant enantioselectivity (up to 64% ee) with a varied scope of allyl enammonium cations. This represents the first reported example of asymmetric supramolecular catalysis with substantial enantioselectivity and catalytic turnover. Use of a relatively unreactive substrate allowed characterization of a host-substrate complex by X-ray diffraction, giving insight into the means of enantiodifferentiation by the chiral host. Additionally, the structure of the M4L612- assembly was modified by substituting other trivalent cations (Fe3+, 3+) into the M4L612- host, modulating the enantioselectivity of the rearrangement. Efforts to emulate allosteric regulation using externally associated chiral cations are also described.
Chapter 3. The engineering of a high-turnover supramolecular catalyst containing a cationic ruthenium complex is described. The RuCp(PMe3)(MeCN)2+ cation, an allyl alcohol isomerization catalyst, was incorporated into the Ga4L612- assembly. While encapsulated, the organometallic complex experiences only mild rate attenuations, while maintaining its remarkably high activity. This hybrid catalyst is capable of more than 1000 turnovers, eclipsing other known supramolecular catalysts. The kinetics of catalysis are also analyzed, revealing inhibition both by acetonitrile and the allyl alcohol substrates. The protection offered by encapsulation in the assembly also allowed the preparation of the larger RuCp*(PMe3)(MeCN)2+ complex, which is both stable and catalytically active. Preliminary investigations of this complex are described.
Chapter 4. Cationic gold(I) catalysts encapsulated inside assembly 14L6 assembly, R3PAuX (X = Cl, Br) complexes have been found to lose their halide ligand and reside in the nanovessel's cavity as the free cation. This host-guest complex is catalytically active in both the hydroalkoxylation and hydroamination of allenes, generating cyclic, chiral ethers and amines. The rate of catalysis for the bound gold cation is observed to be higher than that for the bulk-solution gold species by about an order of magnitude. Efforts to incorporate this gold catalyst into the resolved Ga4L612-assembly are detailed, though this family of organometallic catalysts has thus far been incompatible with the resolved Ga4L612- assembly.