THEORETICAL-STUDIES OF STEREOSELECTIVE ALDOL CONDENSATIONS

MNDO calculations were performed so as to shed new light on the structure-selectivity data of the aldol condensation process. Three simple cases were chosen for the calculations: the addition to benzaldehyde of the cyclopentanone boron-enolates (both the enol borate and the enol borinate) and of the butanone 2 boron-enolate (enol borate). The conformations of the starting boron-enolates were found to be s-cis for the (E)-enolate and s-trans for the (a-enolate. Different activation energies during the aldol reaction account for the different reactivity of (E)- and (2)-enol borates (E compounds are much more reactive than 2 ones). Transition-state models were designed to account for the kinetic preference shown by the aldol reaction. The calculation data were found to be in good agreement with the experimental results. Starting from (2)-enolates, the half-chair leading to the syn aldol is preferred to both the twist-boat and the half-chair leading to the anti aldol. Starting from (E)-enolates, syn aldols are kinetically preferred to anti aldols, in the absence of disturbing steric factors, as in the case of @)-enol borates. The opposite behavior ((E)-enolates give anti aldols) is encountered when bulky substituents at the metal disfavor the twist-boat leading to the syn isomer, as in the case of enol borinates.