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action, the diphenylprolinol silyl ether catalyst is preferred. Be-
cause the generation of iminium ions is promoted by acid,
acid accelerates the reaction. However, in the Michael-type re-
action (type A), acid also reduces the concentration of anionic
nucleophiles by protonation. Thus, the appropriate selection of
acid is advisable. In general, a diphenylprolinol silyl ether is the
superior catalyst in type A reactions, whereas a trifluoromethyl-
substituted diarylprolinol silyl ether is preferred in type B reac-
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[14] DFT and HF calculations were performed by using Gaussian 09, Revision
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “Advanced Molecular Transforma-
tions by Organocatalysts by The Ministry of Education, Culture,
Sports, Science and Technology, Japan (23105010). We thank
Prof. Dieter Seebach at ETH-Zurich for significant discussions
and the Tsukuba Advanced Computing Center (TACC) for pro-
viding computational facilities.
Keywords: asymmetric synthesis
·
iminium ions
·
[15] We have already reported the results of the conformational analysis of
the iminium cation 7’ in our preceding paper.[7] In the present work we
have found that the conformational features of the iminium cation 8’
are intrinsically the same as those of 7’. In the major conformers of 7’
and 8’, the CÀC single bond between the C=N and C=C double bonds
and the exocyclic CÀC single bond adopt s-trans and sc-exo conforma-
tions, respectively. The relation between these conformational features
and the enantiomeric differentiation mechanism is schematically repre-
sented as type A or type B in Figure 2 and has been discussed in detail
in the preceding paper.[7] The two conformers of the iminium ion 7’ in
Table 6 will give rise to the same enantiomer of the Michael adduct,
and the five conformers of the iminium ion 8’ will similarly give rise to
the same enantiomer of the cycloaddition product (see text).
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