Stereoselective fluorination is known to rigidify the ring structure of l-proline, as a result of a combination of electrostatic and hyperconjugative effects associated with the C–F bond. This is a potential strategy for enhancing the enantioselectivity of proline-catalysed reactions. In this study, cis- and trans-4-fluoroprolines were investigated as catalysts in five different organic transformations, including examples of both enamine and iminium ion catalysis. Some significant differences in enantioselectivity were observed between the cis- and trans-isomers of the fluorinated catalysts, confirming that the ring pucker is important. However, no substantial improvements were observed relative to the parent catalyst, l-proline.
众所周知,立体选择性氟化反应会使 l-脯氨酸的环状结构僵化,这是 C-F 键的静电效应和超结合效应共同作用的结果。这是提高脯氨酸催化反应对映体选择性的一种潜在策略。本研究将顺式和反式-4-氟脯氨酸作为催化剂,对五种不同的有机转化进行了研究,其中包括烯胺和亚胺离子催化的实例。在氟化催化剂的顺式和反式异构体之间观察到了对映体选择性的一些明显差异,这证实了环皱褶的重要性。不过,与母催化剂 l-脯氨酸相比,并没有观察到实质性的改进。