Search

Natural bond orbital (NBO) analysis of several early TSs does not support the σ→σ*# hypothesis. The σ→π*CO interaction controls the carbonyl pyramidalization that, in turn, controls the π-selectivity of a nucleophilic addition. In contrast, late TSs are devoid of σ→π*CO interactions, and they benefit from σ→σ*# interactions that control at-selectivity. The evidence in favor of Anh-Felkin's σ#→σ* hypothesis is weak. The electron-withdrawing σCF in the 2-fluoropropanal-LiCN TS did not align anti to the incipient bond even though there was complete conformational freedom. The initial guess for the TS in which σCσF was held anti to o# optimized to what had lost the said geometrical relationship. Furthermore, in the TS for axial addition of LiCN to 2-ax-F-cyclohexanone, the net σ→σ*# interaction was considerably larger than the net σ#→σ* interaction. The relative TS energies require that the equatorial addition of LiCN to 2-ax-F-cyclohexanone be favored over the axial addition in good compliance with the available experimental results. [ABSTRACT FROM AUTHOR]