|Budget Amount *help
¥7,800,000 (Direct Cost : ¥7,800,000)
Fiscal Year 1996 : ¥2,000,000 (Direct Cost : ¥2,000,000)
Fiscal Year 1995 : ¥5,800,000 (Direct Cost : ¥5,800,000)
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 effec
ts 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.
Natural Bond Orbital解析による定量的結果から,この系におけるヒドロホウ素化反応の面選択性はフロンティア軌道軌道相互作用(基質のHOMOとBH_3のLUMO)に支配されていることがわかった.反応初期に生じる錯体の安定性(この大きさが試薬の大きさを考慮したフロンティア軌道相互作用の大きさで決まる;この定量的評価も我々が初めてである)で面選択性が決まり,遷移招待における超共役,骨格ひずみなどは副次効果であると結論された.フロンティア軌道理論の奥の深さを実証する結果となった. Less