Two Reaction Mechanisms via Iminium Ion Intermediates: The Different Reactivities of Diphenylprolinol Silyl Ether and Trifluoromethyl‐Substituted Diarylprolinol Silyl Ether |
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Authors: | Prof. Dr. Hiroaki Gotoh Dr. Tadafumi Uchimaru Prof. Dr. Yujiro Hayashi |
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Affiliation: | 1. Department of Applied Chemistry, Yokohama National University, 79‐5 Tokiwadai, Hodogayaku, Yokohama 240‐8501 (Japan);2. Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305‐8565 (Japan);3. Department of Chemistry, Graduate School of Science, Tohoku University, 6–3 Aramaki‐Aza Aoba, Aoba‐ku, Sendai 980‐8578 (Japan) http://www.ykbsc‐chem.com |
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Abstract: | The reactions of α,β‐unsaturated aldehydes with cyclopentadiene in the presence of diarylprolinol silyl ethers as catalyst proceed via iminium cations as intermediates, and can be divided into two types; one involving a Michael‐type reaction (type A) and one involving a cycloaddition (type B). Diphenylprolinol silyl ethers and trifluoromethyl‐substituted diarylprolinol silyl ethers, which are widely used proline‐type organocatalysts, have been investigated in this study. As the LUMO of the iminium ion derived from trifluoromethyl‐substituted diarylprolinol silyl ether is lower in energy than that derived from diphenylprolinol silyl ether, as supported by ab initio calculations, the trifluoromethyl‐substituted catalyst is more reactive in a type B reaction. The iminium ion from an α,β‐unsaturated aldehyde is generated more quickly with diphenylprolinol silyl ether than with the trifluoromethyl‐substituted diarylprolinol silyl ether. When the generation of the iminium ion is the rate‐determining step, the diphenylprolinol silyl ether catalyst is the more reactive. Because acid accelerates the generation of iminium ions and reduces the generation of anionic nucleophiles in the Michael‐type reaction (type A), it is necessary to select the appropriate acid for specific reactions. In general, diphenylprolinol silyl ether is a superior catalyst for type A reactions, whereas the trifluoromethyl‐substituted diarylprolinol silyl ether catalyst is preferred for type B reactions. |
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Keywords: | asymmetric synthesis iminium ions organocatalysis reaction mechanisms silyl ethers |
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