Curtin-Hammett Paradigm for Stereocontrol in Organocatalysis by Diarylprolinol Ether Catalysts
Curtin-Hammett Paradigm for Stereocontrol in Organocatalysis by Diarylprolinol Ether Catalysts
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DOI：
10.1021/ja300415t
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发表时间：
2012-04-18
影响因子：
15
通讯作者：
Blackmond, Donna G.
中科院分区：
文献类型：
--
作者：
Bures, Jordi;Armstrong, Alan;Blackmond, Donna G.
对二芳基脯氨醇醚催化的醛与硝基烯烃的共轭加成反应和醛的α-氯化反应进行了详细的机理研究，导致这样的建议，即在这些情况下的立体化学结果不是由其中立体中心由亲电试剂上的烯胺攻击形成的步骤的过渡态决定的，而是与以下的相对稳定性和反应性相关：非对映异构体中间体在催化循环的下游。动力学和热力学因素的这种组合说明了一个显着的Curtin-Hammett方案，可以导致增强或侵蚀的选择性，这将是预测的过渡态烯胺攻击亲电试剂。证据表明，这一概念可能代表了一个普遍的现象，吡咯烷基催化剂缺乏一个酸性定向质子。催化剂和反应设计的影响进行了讨论。
Detailed mechanistic study of two reactions catalyzed by diarylprolinol ether catalysts, the conjugate addition of aldehydes to nitro-olefins and the alpha-chlorination of aldehydes, leads to the proposal that the stereochemical outcome in these cases is not determined by the transition state of the step in which the stereogenic center is formed from enamine attack on the electrophile but instead is correlated with the relative stability and reactivity of diastereomeric intermediates downstream in the catalytic cycle. This combination of kinetic and thermodynamic factors illustrates a remarkable Curtin-Hammett scenario that can result in either an enhancement or an erosion of the selectivity that would be predicted by the transition state for enamine attack on the electrophile. Evidence is offered to suggest that this concept may represent a general phenomenon for pyrrolidine-based catalysts lacking an acidic directing proton. Implications for catalyst and reaction design are discussed.