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π面ジアステレオ選択性発現機構に関する研究

π面非对映选择性表达机制研究

基本信息

批准号：
07454161
负责人：
TOMODA Shuji
金额：
$ 4.99万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1996
项目状态：
已结题

项目摘要

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.

上一财年提出的环酮氢化物还原的π面立体选择性理论模型已经完成，结果以信件和全文的形式提交。这个模型被称为外前沿轨道扩展模型（EFOE模型），它基于这样一个简单的假设：相对于π平面两侧的轨道扩展应该是羰基还原的π面立体选择性的起源。 EFOE 密度，定义为酮外部区域 LUMO 的电子密度，与实验确定的烷基取代环己酮的活化焓密切相关。这些结果清楚地表明，主要的传统理论模型（Cieplak 模型和 Felkin-Anh 模型）仅关注过渡态 (TS)，完全忽略了 pi面立体选择性的另一个但最重要的起源是底物LUMO的效应。事实上，对环己酮(B3LYP/6-31G**)的TS的反周面效应和扭转应变的定量评估表明，这些只是边缘性的，Cieplak超共轭效应大于Felkin-Anh效应。前沿轨道效应的重要性是也发现于典型的 C=C 键亲电加成 - 烯烃的硼氢化反应中。合成了2,3-R,R-7-亚甲基-双环[2.2.1]庚烷(R=Me,Et,CH_2OMe,-CH_2OCH_2-,CO_2Me)并测定了硼氢化反应的立体选择性。令人惊讶的是，发现R=CH_2OMe和-CH_2OCH_2-具有相反的立体选择性，它们在电子上是相似的。传统模型显然无法解释这些令人费解的结果，但最初的复杂和TS计算（HF/6-31G**和NBO）表明烯烃的HOMO和BH_3的LUMO之间的相互作用应该决定硼氢化的π面立体选择性。