Mahatthananchai Jessada, Bode Jeffrey W
Chem Sci. 2012 Jan 1;3(1):192-197. doi: 10.1039/C1SC00397F.
The majority of N-heterocyclic carbene catalyzed reactions of -functionalized aldehydes, including annulations, oxidations, and redox reactions, occur more rapidly with -mesityl substituted NHCs. In many cases, no reaction occurs with NHCs lacking ortho-substituted aromatics. By careful competition studies, catalyst analogue synthesis, mechanistic investigations, and consideration of the elementary steps in NHC-catalyzed reactions of enals, we have determined that the effect of the -mesityl group is to render the initial addition of the NHC to the aldehyde irreversible, thereby accelerating the formation of the Breslow intermediate. These studies rationalize the experimentally observed catalyst preference for all classes of NHC-catalyzed reactions of aldehydes and provide a roadmap for catalyst selection and design.
大多数 N-杂环卡宾催化的 β-官能化醛的反应，包括环化反应、氧化反应和氧化还原反应，使用对-均三甲苯基取代的 NHC 时反应速度更快。在许多情况下，缺乏邻位取代芳烃的 NHC 不会发生反应。通过仔细的竞争研究、催化剂类似物合成、机理研究以及对 NHC 催化烯醛反应基本步骤的考虑，我们确定对-均三甲苯基的作用是使 NHC 与醛的初始加成不可逆，从而加速布雷斯洛中间体的形成。这些研究使实验观察到的醛的所有类型 NHC 催化反应中催化剂偏好合理化，并为催化剂的选择和设计提供了路线图。
