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首页> 外文期刊>Chemistry: A European journal >On the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions
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On the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions

机译：手性锂-类胡萝卜素促进环丙烷化反应的机理和立体化学

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An investigation into the mechanism and stereochernistry of chiral lithium-carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene- transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the m.ethylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the ste- reoselectivity observed in experiments. Both the directing group and the configuration at the C-1 centre were found to play a key role in the stereochemistry. Carbenoid I has a chiral C-1 centre of R configuration. The Lewis base group directs the cyclization of carbenoid I to form a single product. In contrast, the Lewis base group cannot direct the cyclization of carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C-1 centre in carbenoid 2. This relationship of the stereochemistry to the chiral character of the carbenoid has implications for the design of new efficient carbenoid reagents for stereoselective cyclopropanation.
机译：利用密度泛函理论（DFT）方法研究了手性锂-类胡萝卜素促进的环丙烷化反应的机理和立体化学。先前的工作表明，这种类型的环丙烷化反应可以通过亚甲基转移机理与碳金属化机理之间的竞争来进行。在本文中，证明了手性类胡萝卜素1和2促进的分子内环丙烷化反应是通过亚甲基转移机理进行的。发现碳金属化机理具有更高的反应势垒，并且似乎不与乙烯转移机理竞争。刘易斯碱基团没有充分增强碳金属化途径以与亚甲基转移途径竞争。目前的计算结果与这些环丙烷化反应的实验观察结果一致。研究了通过亚甲基转移机制控制分子内环丙烷化反应的立体化学的因素，以帮助阐明实验中观察到的立体选择性的起源。发现在C-1中心的导向基团和构型均在立体化学中起关键作用。类胡萝卜素I具有R构型的手性C-1中心。刘易斯碱基团指导类胡萝卜素I的环化形成单一产物。相比之下，由于类胡萝卜素2中手性C-1中心的S构型，Lewis碱基团无法指导类胡萝卜素2的环化以提供立体选择产物。用于立体选择性环丙烷化的新型高效类胡萝卜素试剂的设计。

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