title: Two Reaction Mechanisms via Iminium Ion Intermediates: The Different Reactivities of Diphenylprolinol Silyl Ether and Trifluoromethyl-Substituted Diarylprolinol Silyl Ether.
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.
authors.affiliation: Department of Applied Chemistry, Yokohama National University, 79-5 Tokiwadai, Hodogayaku, Yokohama 240-8501 (Japan).
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Gotoh
authors.affiliation: Research Institute for Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565 (Japan).
authors.forename: Tadafumi
authors.initials: T
authors.lastname: Uchimaru
authors.affiliation: 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. yhayashi@m.tohoku.ac.jp.
authors.forename: Yujiro
authors.initials: Y
authors.lastname: Hayashi
keywords: asymmetric synthesis; iminium ions; organocatalysis; reaction mechanisms; silyl ethers