Electrosteric Activation by using Ion-Tagged Prolines: A Combined Experimental and Computational Investigation

We have recently proposed the empirical concept of electrosteric activation to explain the improved catalytic performances observed for a series of ion-tagged catalysts compared to the parent tag-free structures. Here, the results of a combined experimental and computational investigation on the asymmetric aldol reaction between cyclohexanone and benzaldehyde, catalyzed by a family of tag-free and ionic-tagged prolines, are presented. Whereas diastereo- and enantioselectivities remain very high in all cases examined, the ion-tagged catalyst cis-4-(2-(3-methyl-imidazol-3-ium-1-yl)acetoxy)-proline bistriflimide, cis-7, displays a remarkably high activity compared to its tagged trans analogue and to the tag-free catalysts cis and trans-4-(2-phenylacetoxy)-proline 8. A computational investigation of ion-tagged and tag-free model systems shows that the transition state involving cis-7 is stabilized by a complex interplay of hydrogen bonds (in particular, those involving the counter ion oxygen atoms and the hydrogen atoms of the ionic tag), π-stacking interactions involving the aldehyde phenyl ring, and similar π interactions between the proline carboxyl group and the imidazole ring. The overall effect of these interactions accounts for the observed enhanced activity.