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Catalysis of 6π Electrocyclizations & Catalytic Disproportionation of Lignin Model Compounds

  • Author(s): Bishop, Lee
  • Advisor(s): Bergman, Robert G
  • et al.
Abstract

Part 1 - Chapter 1. The goal of the catalysis of 6π electrocyclizations is introduced in the context of known examples of the catalysis of pericyclic reactions. Recent examples of catalytic electrocyclizations and the computational and experimental precedents used to guide our approach are reviewed.

Part 1 - Chapter 2. The acid-catalyzed and thermal cyclization of an isolable vinyl ortho-quinone methide was investigated through DFT calculations and experimental kinetic analysis. We propose that both reactions proceed through rate-limiting exo-alkylidene bond isomerization followed by faster oxa-6π electrocyclization. The vinyl ortho-quinone methide was found to be highly basic, allowing for quantitative protonation with weak acids. In addition, we identified a new mode of Diels-Alder dimerization of vinyl ortho-quinone methides.

Part 1 - Chapter 3. Density functional theory calculations were performed on the coordination of a Lewis acid to a Lewis basic ester substituent at all positions of a hexatriene molecule and the subsequent 6π electrocyclizations of these molecules. These calculations suggested catalysis of the 6π electrocyclization of 2-carbomethoxy-substituted triene substrates is possible. The electrocyclization of hexatriene substrates with a variety of other Lewis basic substituents in the 2-position were modeled in an analogous fashion.

Part 1 - Chapter 4. A 2-carboethoxy-substituted triene substrate was synthesized and catalysis of its electrocyclization using Me2AlCl was demonstrated. Evidence is provided suggesting this reaction proceeds through rapid, reversible, exothermic formation of a catalyst-substrate complex, which then undergoes rate-limiting 6π electrocyclization.

Part 1 - Chapter 5. It was demonstrated that ester, ketone, and amide functionalities are useful Lewis-basic docking groups for the catalysis of 6π electrocyclizations. Catalysis using aldehyde moieties as docking groups was unsuccessful, most likely due to the high reactivity of the aldehyde functional group towards intramolecular nucleophilic attack, as demonstrated by the formation of a novel triene dimer. In one clear example, it was demonstrated that the hexatriene structure must be entropically biased towards electrocyclization in order for catalysis to be successful.

Part 1 - Chapter 6. Moderate levels of enantioselectivity in catalytic 6π electrocyclizations using scandium pyridine-bis-oxazoline catalyst systems were achieved. We have also discovered a catalytic photochemical electrocyclic ring-opening and kinetic resolution of a cyclohexadiene.

Part 2 - Chapter 1. The goal of the depolymerization of lignin via catalytic disproportionation was introduced in the context of the development of a liquid fuel source from this biomass input. The structure, methods of isolation, and methods of industrial and environmental degradation of lignin are introduced. Initial results of the catalytic disproportionation of a lignin model compound of the glycerol-β-aryl ether linkage of lignin as well as literature examples of the types of transfer hydrogenation and carbon-oxygen bond cleavage reactions employed in this system are shown.

Part 2 - Chapter 2. Disproportionation of a 1,3-diol model compound was investigated in order to understand and optimize the retro-aldol cleavage process observed in the disproportionation of a glycerol-β-aryl ether model compound. Evidence is provided suggesting the disproportionation of the 1,3-diol model compound proceeds through rate-limiting retro-aldol cleavage. Also active processes in this reaction are dehydration, carbonyl and olefin hydrogenation, dehydrogenation of formaldehyde, and a water-gas-shift reaction. Early metal and aluminum alkoxides and aryloxides were employed as potent co-catalysts in this disproportionation reaction.

Part 2 - Chapter 3. A number of model systems were employed to investigate the carbon-oxygen bond cleavage processes that are observed in the disproportionation of a glycerol-β-aryl ether model compound. These studies suggest this reaction proceeds through ruthenium-enolate intermediates. Attempts at characterizing the catalyst resting state of the carbon-oxygen bond cleavage reaction via both NMR and IR analysis were unsuccessful. Experiments were performed which suggest the carbon-oxygen bond cleavage of a 2-phenoxy-1-phenylpropenone substrate proceeds through a novel mechanism.

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