| Research Abstract |
The following three projects have been completed.
1. New Theoretical Model for the Prediction of π-Facial Diastereoselection.
Contrary to the claim of the conventional transition state models of π-facial diastereoselection (Felkin-Anh and Cieplak models), it is theoretically proven that the antiperiplanar effects involving the incipient bond in the transition state of nucleophilic carbonyl addition are not essential to dictate facial diastereoselection. Based on the Salem-Klopman equation and the frontier orbital theory, a new quantum chemical model for the prediction of π-facial diastereoselection has been proposed. This model, the exterior frontier orbital extension model (the EFOE Model) is found to be very effective for the prediction of facial diastereoselection of the hydride reduction of most cyclic ketones.
2. Quantitative Analysis of Weak Interaction in Organic Systems and Proteins.
Utilizing the specific property of selenium nuclear spin (1/2), the nature and the strength of weak
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interactions in hydrogen bonds involving Group 16 elements (X…H-Y (X=O, S, Se, Te and Y=O, C) ) and Se…Z (Z=N, O, F) non-bonded attractive interactions have been investigated. It is concluded that in all these cases the interactions are not electrostatic, but largely the donor-acceptor type orbital interaction. In a related project, we have detected and characterized new type of weak atomic interactions in proteins, such as C-S…O, S-S…O and C=O…S.These interactions are mainly the donor-acceptor type with strong bonding directionality.
3. Reinvestigation of the Problems in Basic Organic Chemistry
It is generally accepted that halobenzenes are deactivated because of e-withdrawing nature of halogens, in spite of the fact that fluorobenzene is more reactive than benzene in electrophilic substittution. Quantitative analysis of orbital interactions of the π-systems in halobenzenes, it is concluded that the reactivity of mono-substittuted benzenes are dependent on the energy level of the lowest occupied π-orbital (π1) and not of the frontier orbitals (π2 or π3). Halobenzenes (Cl, Br, I) are therefore deactivated because of their e-donating property. The problems of cis-effect and gaucheeffect have also been investigated by natural bond orbital (NBO) analysis. Less
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