Isotope effects and the nature of selectivity in rhodium-catalyzed cyclopropanations
Isotope effects and the nature of selectivity in rhodium-catalyzed cyclopropanations
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DOI：
10.1021/ja036025q
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发表时间：
2003-12-24
影响因子：
15
通讯作者：
Singleton, DA
中科院分区：
文献类型：
--
作者：
Nowlan, DT;Gregg, TM;Singleton, DA
采用C-13动力学同位素效应和密度泛函理论计算研究了四羧酸二铑催化烯烃与未取代重氮乙酸酯、乙烯基和苯基重氮乙酸酯的环丙烷化反应机理。在Rh-2（辛酸）（4）催化下，苯乙烯与苯基重氮乙酸甲酯的环丙烷化反应在末端烯碳上表现出明显的C-13同位素效应（1.024），在内烯碳上表现出较小的同位素效应（1.003-1.004）。这与高度异步的环丙烷化过程一致。在四[（S）-N-（十二烷基苯磺酰基）脯氨酸]双铑（Rh-2（S-DOSP）（4））催化的反应中观察到非常相似的同位素效应，表明手性催化剂参与非常相似的环丙烷化过渡态几何结构。重氮乙酸乙酯的环丙烷化反应由于末端烯烃同位素效应较小（1.012-1.015）而导致较早的过渡态。密度泛函理论计算（B3 LYP）预测的反应途径涉及络合的重氮酯铑，损失N-2提供铑卡宾，和异步，但协调的环丙烷化过渡态。预测的苯基取代的铑类卡宾与苯乙烯的反应的同位素效应的误差内的实验值匹配，支持理论计算和铑类卡宾机制的准确性。计算的准确性还得到了反应势垒、立体选择性和反应趋势的出色预测的支持。讨论了这些反应中烯烃选择性和非对映选择性效应的本质，提出了Rh-2（S-DOSP）（4）催化环丙烷化反应中对映选择性的新模型。
The mechanism of the dirhodium tetracarboxylate catalyzed cyclopropanation of alkenes with both unsubstituted diazoacetates and vinyl- and phenyldiazoacetates was studied by a combination of C-13 kinetic isotope effects and density functional theory calculations. The cyclopropanation of styrene with methyl phenyldiazoacetate catalyzed by Rh-2(octanoate)(4) exhibits a substantial C-13 isotope effect (1.024) at the terminal olefinic carbon and a smaller isotope effect (1.003-1.004) at the internal olefinic carbon. This is consistent with a highly asynchronous cyclopropanation process. Very similar isotope effects were observed in a bisrhodium tetrakis[(S)-N-(dodecylbenzenesulfonyl)prolinate] (Rh-2(S-DOSP)(4) catalyzed reaction, suggesting that the chiral catalyst engages in a very similar cyclopropanation transition-state geometry. Cyclopropanation with ethyl diazoacetate was concluded to involve an earlier transition state, based on a smaller terminal olefinic isotope effect (1.012-1.015). Density functional theory calculations (B3LYP) predict a reaction pathway involving complexation of the diazoesters to rhodium, loss of N-2 to afford a rhodium carbenoid, and an asynchronous but concerted cyclopropanation transition state. The isotope effects predicted for reaction of a phenyl-substituted rhodium carbenoid with styrene match within the error of the experimental values, supporting the accuracy of the theoretical calculations and the rhodium carbenoid mechanism. The accuracy of the calculations is additionally supported by excellent predictions of reaction barriers, stereoselectivity, and reactivity trends. The nature of alkene selectivity and diastereoselectivity effects in these reactions is discussed, and a new model for enantioselectivity in Rh-2(S-DOSP)(4)-catalyzed cyclopropanations is presented.