Hydrogenolysis, hydrogenation and nanoencapsulation in cascade reactions

• Main Author: Yap, Aaron J .
• Format: Thesis
• Language: English
• Published: The University of Sydney 2012
• Subjects: Hydrogenation; Microencapsulation; Catalysts; Antarc*; Antarctica
• Online Access: https://hdl.handle.net/2123/29153

Catalytic nanoreactors, prepared by the Layer-by-Layer encapsulation of zeolite H-Beta, were used in the dynamic kinetic resolution (DKR) of 1-phenylethanol and 1-indanol. Yields for both alcohols were clearly above 50%, with ee’s over 85%, indicating successful protection of the pH-sensitive enzyme, candida antarctica lipase B, from the acidic zeolite. Attention proceeded to the DKR of amines, which can involve the undesired hydrogenolysis reaction during the racemisation step. Thus, the phenomenon of hydrogenolysis was investigated. The hydrogenolysis of various amines, imines, and nitriles with an aromatic ring adjacent to the a-carbon was achieved under relatively mild conditions over Pd/C in high yield. The relationship between the various reactions involved in hydrogenolysis and thus hydrogenation of nitriles was clarified, which lead to the proposal of a mechanism based on homogeneous analogues. The mechanism provided an explanation for the ease of hydrogenolysis of substrates with an aromatic ring adjacent to the (it-carbon: the carbometallated intermediate is more stable when the aromatic ring is in that position. DFT calculations not only supported this explanation, but also indicated that the affinity of the aromatic ring to the Pd/C surface is also responsible for the ease of hydrogenolysis. A kinetic model was then proposed to explain why the relatively mild conditions enabled continual hydrogenolysis to occur. The high catalyst loadings, combined with a limited rate of H2 dissolution into the solvent, were responsible, as the hydrogenation reaction (unlike the hydrogenolysis) remained diffusion limited. Furthermore, at very low catalyst loadings, benzylamine was found to poison the catalyst and prevent continued hydrogenolysis. The knowledge gained on hydrogenolysis was then applied to the hydrogenolysis of secondary amines. It was found that the larger the difference between the relative rates of hydrogenolysis of the secondary amine’s constituent primary amines, the more selective the cleavage of ...