Asymmetric Organocatalysis by Daniela Kampen, Corinna M. Reisinger, Benjamin List (auth.),
By Daniela Kampen, Corinna M. Reisinger, Benjamin List (auth.), Benjamin List (eds.)
Kerstin Etzenbach-Effers, Albrecht Berkessel: Non-Covalent Organocatalysis in response to Hydrogen Bonding: Elucidation of response Paths through Computational Methods.- Petri M. Pihko, Inkeri Majander, and Anniina Erkkilä: Enamine Catalysis.- Jennifer L. Moore, Tomislav Rovis: Lewis Base Catalysts 6: Carbene Catalysts.- Amal Ting, Jennifer M. Goss, Nolan T. McDougal, and Scott E. Schaus: Brønsted Base Catalysts.- O. Andrea Wong, Yian Shi: Chiral Ketone and Iminium Catalysts for Olefin Epoxidation.- Alan C. Spivey, Stellios Arseniyadis: Amine, Alcohol and Phosphine Catalysts for Acyl move Reactions.- John B. Brazier, Nicholas C.O. Tomkinson: Lewis Base Catalysts 2 Secondary and first Amine Catalysts for Iminium Catalysis.- Oksana Sereda, Sobia Tabassum, and René Wilhelm: Lewis Acid Organocatalysts.- Daniela Kampen , Corinna M. Reisinger , and Benjamin record: Chiral Bronsted Acids for C Organocatalysis.-
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The catalyst salts of stronger inorganic acids as well as sulfonic acids failed to promote the reaction. By NMR studies, the authors were able to observe that the formation of iminium species between the catalyst salt and substrate correlated to high reaction efficiency. The iminium ion likely equilibrates with the reactive enamine and thus enhances the reaction rate. Additionally, the acid cocatalyst may stabilize the iminium ion of the formed product and push it towards hydrolysis instead of retroaldolization which might compromise the reaction selectivity.
Figure 17 shows the overall dependence of the activation parameters on the number of HFIP molecules involved. Interestingly, the activation enthalpy of the epoxidation decreases steadily from zero to fourth order in HFIP. As expected, the activation entropy −TDS‡ shows a continuous increase with increasing numbers of specifically coordinated HFIP molecules. Due to the increasing entropic contribution, the value of DG‡ approaches saturation when three or four HFIP molecules are involved. For methanol, however, no influence of explicit coordination of the solvent on the activation parameters of oxygen transfer could be found, so it seems to 22 K.
While a strong acid is beneficial in the first iminium forming step, its conjugate base has only a weak ability to remove the a proton. On the other hand, a relatively strong counter base would favor the formation of the enamine, but the initial formation of the iminium ion would be somewhat compromised. Hence both general acid and base cocatalysis has to be considered in enamine formation in addition to the choice of the catalytic amine species. 38 P. M. Pihko et al. Hine has demonstrated that simple amino acids, such as glycine and b-alanine, are not capable of intramolecular deprotonation in the reaction with isobutyraldehyde-2-d (Scheme 8) .