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Selected AbstractsStereochemical Models for Discussing Additions to ,,, -Unsaturated Aldehydes Organocatalyzed by Diarylprolinol or Imidazolidinone Derivatives , Is There an ,(E)/(Z)-Dilemma'?HELVETICA CHIMICA ACTA, Issue 4 2010Dieter Seebach Abstract The structures of iminium salts formed from diarylprolinol or imidazolidinone derivatives and ,,, -unsaturated aldehydes have been studied by X-ray powder diffraction (Fig.,1), single-crystal X-ray analyses (Table,1), NMR spectroscopy (Tables,2 and 3, Figs.,2,7), and DFT calculations (Helv. Chim. Acta2009, 92, 1, 1225, 2010, 93, 1; Angew. Chem., Int. Ed.2009, 48, 3065). Almost all iminium salts of this type exist in solution as diastereoisomeric mixtures with (E)- and (Z)-configured +NC bond geometries. In this study, (E)/(Z) ratios ranging from 88,:,12 up to 98,:,2 (Tables,2 and 3) and (E)/(Z) interconversions (Figs.,2,7) were observed. Furthermore, the relative rates, at which the (E)- and (Z)-isomers are formed from ammonium salts and ,,, -unsaturated aldehydes, were found to differ from the (E)/(Z) equilibrium ratio in at least two cases (Figs.,4 and 5,,a, and Fig.,6,,a); more (Z)-isomer is formed kinetically than corresponding to its equilibrium fraction. Given that the enantiomeric product ratios observed in reactions mediated by organocatalysts of this type are often ,99,:,1, the (E)-iminium-ion intermediates are proposed to react with nucleophiles faster than the (Z)-isomers (Scheme,5 and Fig.,8). Possible reasons for the higher reactivity of (E)-iminium ions (Figs.,8 and 9) and for the kinetic preference of (Z)-iminium-ion formation are discussed (Scheme,4). The results of related density functional theory (DFT) calculations are also reported (Figs.,10,13 and Table,4). [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 41 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 40 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 39 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 38 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 37 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 36 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 35 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 34 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 33 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 32 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 31 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 30 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 29 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 28 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 27 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 26 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 25 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 24 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 23 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 22 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 21 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 20 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 19 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 18 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 17 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 16 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 15 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 14 2010Chem. No abstract is available for this article. [source] Top-Beiträge aus unseren Schwesterzeitschriften: Angew.ANGEWANDTE CHEMIE, Issue 13 2010Chem. No abstract is available for this article. [source] | ||||||||||