Abstract
Based on the simple assumption that π-facial diastereoselection should be originated from the difference in the magnitude of the reaction driving force over the two faces of the π-plane, a new theoretical model is proposed for the prediction of faical diastereoselection. According to the Salem-Klopman equation, two quantities - steric effects (1st term) and the spatial extension of frontier orbital of a substrate ketone (3rd term) - are quantitatively evaluated as the π-plane-divided accessible space (PDAS) and the exterior frontier orbital extension density (EFOE density). The latter is obtained by integrating the frontier orbital (LUMO of a ketone) extension over one side of the π-plane in the exterior area of the van der Waals surface of the ketone under the specific criteria that the driving force vector located on the hydride should be maximally directed toward the carbonyl carbon. Numerous cases of facial diastereoselectivity of hydride reduction including ketones with polar substituents, heterocyclic systems, symmetric bi- or tricyclic ketones, imines, iminium ion are successfully explained by these two new simple quantities. These results as well as quantitative evaluation of transition effects (mainly the antiperiplanar hyperconjugative stabilization involving the incipient bond) strongly suggest that the orbital interaction between reactants generated in the initial stage rather than transition state effects must be the essential process of diastereoselection.
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