ABSTRACT The study of hydrogen bonding organocatalysis is rapidly expanding. Much research has been directed at making catalysts more active and selective, with less attention on fundamental design strategies. This study systematically increases steric hindrance at the active site of pH switchable urea organocatalysts. Incorporating strong intramolecular hydrogen bonds from protonated pyridines to oxygen stabilizes the active conformation of these ureas thus reducing the entropic penalty that results from substrate binding. The effect of increasing steric hindrance was studied by single crystal X-ray diffraction and by kinetics experiments of a benchmark reaction. Graphical Abstract

中文翻译：

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pH 值可切换的取代（2-吡啶鎓）脲有机催化剂的空间效应：溶液和固相研究

摘要 氢键有机催化的研究正在迅速扩大。许多研究旨在使催化剂更具活性和选择性，而对基本设计策略的关注较少。该研究系统地增加了 pH 可切换尿素有机催化剂活性位点的空间位阻。结合从质子化吡啶到氧的强分子内氢键可以稳定这些尿素的活性构象，从而减少底物结合导致的熵损失。通过单晶 X 射线衍射和基准反应的动力学实验研究了增加空间位阻的影响。图形概要