![]() ![]() A combination of the above two mentioned electron transfer processed affords the three-membered metallacycle complex IV. In this case, a more electron-deficient metal centre is favoured for the electron transfer from oxygen atom to transition metal. Meanwhile, complex III should be more stable, since CO 2 acts as a bidentate ligand with two oxygen atoms. ![]() Adduct II (end-on type) seems less plausible due to the weak interaction between the lone pair of only one oxygen atom and the metal centre. Electron-rich metal centres are more feasible to form these types of complexes via electron transfer from metal centre to carbon atom. More specifically, complex I with an M-C bond is sometimes termed as a metallacarboxylate. If a metal centre reacts with one molecule of CO 2, there are five different chelating modes possible. As a result, a series of transition metal CO 2 complexes are known. In its ground state, carbon dioxide possesses two equivalent C-O bonds that could both coordinate to a transition metal centre. 1) (2) different reaction modes for CO 2 activation (main focus of this review) and (3) potential new applications of CO 2 valorization.Īs shown above, CO 2 has multiple reactive sites: the carbon atom is an electrophilic Lewis acid centre and the oxygen atoms act as weak nucleophilic Lewis base. While classical methods for CO 2 hydrogenation have been reviewed elsewhere and will not be included here 5, 6, 7, the following subjects will be addressed: (1) novel transformations using carbon dioxide (briefly summarized in Fig. A special focus is given on the reaction modes for the CO 2 activation and its application as C1 building block in organic synthesis. This review will describe the most recent advances made in the area of CO 2 valorization-turning CO 2 into a useful chemical feedstock-under mild conditions. To create C-C bonds with CO 2, the use of carbon nucleophiles is specifically limited to strong nucleophilic organolithiums and Grignard reagents, as well as phenolates.Īlong with the rapid development of organometallic chemistry and catalysis (see Box 1), various types of efficient CO 2 transformations in organic synthesis have been discovered in the past decades, greatly improving its efficiency and applicability. In particular, the catalytic coupling of CO 2 with energy-rich substrates, such as epoxides and aziridines, to generate polycarbonates/polycarbamates and/or cyclic carbonates/carbamates has drawn significant attention over the past decades. Consequently, most of the known studies used highly reactive substrates and/or severe reaction conditions to activate CO 2, limiting the application of such methods. However, activation and utilization of CO 2 is still problematic due to the fact that it is the most oxidized form of carbon, which is also thermodynamically stable and/or kinetically inert in certain desired transformations. As a result, valorization of CO 2 is currently receiving considerable and ever increasing attention by the scientific community 2, 3, 4. Although the extensive use of carbon dioxide for chemical production cannot solve this problem alone, CO 2 is a useful one-carbon (C1) building block in organic synthesis due to its abundance, availability, nontoxicity and recyclability. Obviously, there is an urgent need to control CO 2 emissions and develop efficient carbon capture systems. This increase in CO 2 concentration is largely due to the combustion of fossil fuels, which are required to meet the world’s energy demand 1. Since pre-industrial times, the amount of CO 2 has steadily increased and nowadays CO 2 is a component of greenhouse gases, which are primarily responsible for the rise in atmospheric temperature and probably abnormal changes in the global climate. In addition to biomass, CO 2 offers the possibility to create a renewable carbon economy. Until now, the vast majority of carbon resources are based on crude oil, natural gas and coal. However, when dealing with the feedstock of the chemical industry, the level of sustainability is still far from satisfactory. A more sensible resource management is the prerequisite for the sustainable development of future generations. ![]()
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