关键词: 过渡金属催化, C—H键活化, 环化反应, 氢化还原, 氟化反应

In recent years, organic reactions involving difluorocyclopropenes have attracted the attention of organic chemists and have made great progress. The reactions mainly include: (1) cyclization: a) transition-metal catalyzed C—H bond activation cyclization with directing groups; b) cyclization reactions with non-directing groups; (2) hydrogenation reduction; (3) fluorination as “F” source reagents. In this paper, the synthesis methods and applications of difluorocyclopropenes in past 10 years are summarized. The conversion reactions of difluorocyclopropenes are emphasized. Additionally, difluorocyclopropenes have aroused considerable interests both from a structural standpoint and their participation in various ring-opening reactions. Given the increasing application of cyclopropyl skeleton in the development of drugs and unarguable importance of fluorinated compounds in medicinal chemistry and agrochemistry, it is no doubt that difluorocyclopropenes are encountered into bioactive molecular and at present lie among “emerging fluorinated motifs”. Although the synthesis and application of structurally diverse difluorocyclopropenes have been witnessed in the past decade, the most widely used methods for the preparation of these compounds include difluoromethylenation of alkynes and difluoromethylation of heteroatom nucleophiles (such as NaF, NaI, ⁿBuN₄X, etc.) with a difluorocarbene reagent, which can be generated from various precursors (such as TMSCF₃, TMSCF₂X, Ph₃P^＋CF₂CO₂^-, TFDA, etc.). For the transition metal-catalyzed cyclization of difluorocyclopropenes, some common metal salts (such as rhodium, ruthenium, copper, palladium, silver) are used as catalysts. Moreover, the metallic hydrogen (M—H) reduction strategy is a simple and efficient method for the hydrogenation reduction of difluorocyclopropenes leading to difluorocyclopropanes, and the asymmetric hydrogenation reduction of difluorocyclopropenes can be achieved in the presence of chiral ligands. In fluorination reactions, difluorocyclopropenes have some advantages that cannot be achieved by traditional fluorination reagents for the direct fluorination and functionalization of hydroxyl groups (such as fluorination of polyhydroxyl alcohols). Of course, the biggest disadvantage of difluorocyclopropene as a fluorine source lies in its poor atomic economy, which has been criticized. Despite the remarkable achievements in the reactions of difluorocyclopropenes, there are still many issues that need to be addressed. For instance, difluorocyclopropenes are rarely applied in traditional radical reactions, photocatalysis, electrocatalysis and flow chemistry. Hopefully, difluorocyclopropenes can gradually appear in photo- and electro-catalyzed radical chemistry, and the related asymmetric reactions will also get more attention and development in the near future.

Key words: transition metal-catalyzed, C—H bond activation, cyclization, hydrogenation reduction, fluorination