Stereochemical Models for Discussing Additions to α , β ‐Unsaturated Aldehydes Organocatalyzed by Diarylprolinol or Imidazolidinone Derivatives – Is There an ‘( E )/( Z )‐Dilemma’ ?

The structures of iminium salts formed from diarylprolinol or imidazolidinone derivatives and α , β ‐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. Acta 2009 , 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 α , β ‐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 ).