摘要/Abstract

二氧化碳(CO₂)是一种理想的C1合成子. 利用其参与化学转化合成羧酸和含羰基杂环等具有高附加值的产品, 具有重要意义. 另一方面, 烯烃的双官能团化反应是有机合成化学中的一类重要反应, 可以将简单易得的烯烃快速高效地转化为结构多样性的重要化合物. 然而, 由于CO₂反应活性较低, 而且烯烃官能团化反应的选择性难以控制, CO₂参与的烯烃双官能团化反应具有较高的挑战性. 近年来, 自由基化学的蓬勃发展为该类反应的开发提供了新的策略, 实现了一些重要转化反应. 基于此, 从CO₂参与烯烃的氧-烷基化反应、碳羧基化反应、硅羧基化反应、硫羧基化反应以及双羧基化反应等反应入手, 全面总结和深入分析了最近几年CO₂参与的自由基型烯烃双官能团化反应进展; 在介绍上述进展的同时, 重点阐述了其可能经历的四类自由基化学历程. 最后对该领域的未来发展方向进行了展望, 希望为该领域的进一步发展提供一些思路.
关键词: 二氧化碳, 烯烃, 双官能团化, 自由基, 可见光催化
CO₂ is an ideal C1 source in chemical transformations. It is of great significance to utilize CO₂ in chemical conversion to synthesize high value-added compounds, including carboxylic acids and carbonyl-containing heterocycles. On the other hand, the difunctionalization of olefins is an important organic reaction, which can efficiently convert easily available olefins into important compounds with structural diversity. However, due to the low reactivity of CO₂ and the difficulty in controlling the selectivity, the difunctionalization of olefins with CO₂ is highly challenging. Recently, the significant progress of radical chemistry has provided new strategies and promoted the development of novel transformations in this field. This Perspective summarizes the recent progress of the radical-type difunctionalization of olefins with CO₂, including the oxy-alkylation, carbocarboxylation, silacarboxylation, thiocarboxylation, and dicarboxylation of alkenes with CO₂. At the same time, we also highlight the mechanism with radicals and four kinds of pathways are proposed: (1) Free radicals attack olefins to form new carbon radical intermediates. The radicals are then oxidized to form carbocations, which are further captured by carbonates or carbamates. It is also possible for direct C—O bonding reaction or sequent C—I and C—O bonds formation. (2) The new carbon radical intermediates, in-situ generated through attack of alkenes with radicals, are reduced via single electron transfer into carbanions, which could attack CO₂ to form C—C bonds. (3) CO₂ is reduced into CO₂ radical anions in the highly reductive reaction conditions. Once generated, the CO₂ radical anions might attack olefins to form carboxylate bearing more stable carbon radical intermediates (such as benzylic ones) and further form C—C bonds or carbon-heteroatom bonds. (4) Olefins are reduced via single electron transfer into alkenyl free radical anions in the highly reductive reaction conditions. These anions may attack CO₂ to form carboxylate bearing carbon radical intermediates and are further reduced to generate carbanions. Finally they may attack another CO₂ to form succinic acid derivatives. We point out the challenges and predict the future development in the field, including the more challenging substrates, more reaction types, better selectivities, and deeper mechanistic understanding.
Key words: carbon dioxide, olefin, difunctionalization, radical, visible-light photoredox catalysis


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