π面ジアステレオ選択性発現機構に関する研究

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The theoretical model for pi-facial stereoselectivity of hydride redution of cyclic ketones, which was proposed last fiscal year, has been completed and the results were submitted as a letter and a full paper. This model named as Exterior Frontier Orbital Extension Model (EFOE Model) is based on the simple assumption that orbital extension with respect to both side of the pi-plane should be the origin of pi-facial stereoselectivity of carbonyl reduction. The EFOE densities, defined as an electron density of LUMO in the exterior area of a ketone, were nicely correlated with experimentally-determined activation enthalpies for alkyl-substituted cyclohexanones.These results clearly suggested that major conventional theoretical models (Cieplak Model and Felkin-Anh Model), whith simply focus on the transition tate (TS), completly neglect another, but the most important origin of pi-facial stereoselectivity-the effect of substrate LUMO.In fact, quantitative evaluation of anti-periplanar effects and torsional strain for the TS of cyclohexanone (B3LYP/6-31G**) indicated that these are only marginal with the Cieplak hyperconjugation effect greater than the Felkin-Anh effect.Importance of frontier orbital effects was found also in typical electrophilic addition to C=C bond -hydroboration of olefins. 2,3-R,R-7-methylene-bicyclo[2.2.1]heptanes(R=Me, Et, CH_2OMe, -CH_2OCH_2-, CO_2Me) were synthesized and stereoselectivity for hydroboration was determined. Surprisingly, reversed stereoselectivity was found for R=CH_2OMe and -CH_2OCH_2-, Which are electronically similar. The conventional models apparently fail to explain these puzzling results, but the initial complex and TS calculations (HF/6-31G** and NBO) indicated the interaction between HOMO of olefin and LUMO of BH_3 should dictate pi-facial stereoselectivity of hydroboration.