Distinguishing the early and late transition states and exploring the validity of σ → σ*#,→ σ*, and σ → π* < inf > C=O < /inf > concepts in diastereoselection from NBO analysis

Abstract

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→C=O interactions, and they benefit from σ → σ# interactions that control π-selectivity. The evidence in favor of Anh-Felkin's σ# → σ* hypothesis is weak. The electron-withdrawing σ C-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 σC-F was held anti to ó# 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. © 2006 American Chemical Society.