Distinguishing the early and late transition states and exploring the validity of [sigma]->[sigma.sup.*#], [sigma.sup.#]->[sigma.sup.*], and [sigma]->[pi*.sub.C=O] concepts in diastereoselection from NBO analysis

The results from a thorough natural bond orbital analysis of several transition states (TSs) for nucleophilic additions to carbonyl function are discussed. It is demonstrated that the absence or presence of the Cieplax type [sigma]->[sigma.sup.*#] interaction depends on whether the TS in question is of early or late character.