Equatorial metal enolates

The following examples show how open and closed transition states may be invoked by the choice of the reaction type. For instance, aldol-type addition normally proceeds via a closed transition state because the metal ion is shifted from the enolate oxygen to the carbonyl oxygen in an ene-like mechanism ( Zimmerman-Traxler transition state 9). The crucial interactions in the Zimmerman-Traxler transition state 16 are those between the 1,3-diaxially oriented substituents around the chair-like structure. R2 adopts the location shown, thus R3 avoids the 1,3-interaction and assumes an equatorial position. Therefore, the diastereomeric ratio depends mainly on the ( )/(Z) configuration of the enolate. Whereas (Z)-enolates 13 afford syn-config-urated enantiomers, 17 and 18, the corresponding ( )-enolates 14 lead to anti-configurated adducts 19 and 20 10. 

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

Reaction C in Fig. 4 is an aldol condensation between an achiral aldehyde and an ester enol borinate featuring a bidentate chiral substituent at the boron atom [24]. Upon enolate-boron/aldehyde-oxygen co-ordination, two chair-like TS can be formed, both featuring the aldehyde phenyl group in a pseudo-equatorial position. Preferential attack on the aldehyde Si face is determined by the spatial arrangement of the metal ligand. The almost exclusive formation of the anti diastereoisomers arises from control of the enolate geometry. 

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