Radical-Type Difunctionalization of Alkenes with CO2

Abstract

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 CO2to form C-C bonds. (3) CO2is 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 CO2to form carboxylate bearing carbon radical intermediates and are further reduced to generate carbanions. Finally they may attack another CO2to 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.