Nonreductive Catalytic Cyclizations of Carbon Dioxide into Synthons for Fine-Chemical, Pharmaceutical, and Polymer Applications

Conspectus Nature enables the transformation of carbon dioxide and water into glucose and oxygen via photosynthesis representing a crucial chemical process for life on earth. This carbon fixation process also helps to balance the global carbon cycle controlling the uptake of carbon from the atmosphere. Ever since the industrial revolution, anthropogenic carbon emissions have increased at such a pace resulting in an imbalance of the carbon cycle and, as a consequence, effects known as global warming and the occurrence of more extreme weather patterns. The “excess” of carbon dioxide is commonly perceived as waste, with little to no value to humanity. Thus, ideally technologies able to transform this waste carbon into value-added compounds and materials would be economically of interest to various communities. The principal technology to complement the chemical diversification that Nature can provide for carbon dioxide conversion is catalysis. Catalysis offers an approach to activate substrates in different ways building on diverse reactivity principles and contemplating on the possibility to activate CO2 directly or the coreactant. This includes the use of Brønsted or Lewis bases, transition metals, photo- and organo-catalysis, offering us a wide variety of activation modes. In this Account, we feature our extensive efforts in the area of catalytic carbon dioxide conversion with a particular emphasis on the activation of cyclic ethers and epoxy alcohols/amines used as partnering reagents. These two substrate classes require different chemical environments and thus the selection of the catalyst, additive and the reaction conditions (solvent, temperature, pressure) are crucial to control the process outcomes in terms of chemo- and stereoselectivity, and activity. The modular character of these coreagents will be highlighted together with the specific design features of the catalytic process, as to offer not only an overall good performance but also using cheap, attractive and easy-to-construct catalysts that mediate the straightforward activation of epoxides/oxetanes, and/or epoxy alcohols/amines. The latter category of precursors embeds two distinct functional groups (an oxirane flanked by either an alcohol or amine) and thus provides more ample opportunities for catalytic activation. Besides the different substrate categories, our focus was initially on improving the catalytic activity under metal and metal-free catalysis, delineating the relationship between the structure, functionality and substitution degree of cyclic ethers and how these features affect the catalytic coupling with carbon dioxide. As soon as we realized the limiting nature of these coupling reactions, especially in terms of the steric requirements, we turned our attention to epoxy alcohols/amines as alternative precursors. These complementary precursors provided new entries to otherwise difficult or impossible to prepare heterocycles with high (stereochemical) control. Moreover, the manifold by which these transformations take place were studied in detail and revealed the unique intermediacy of a long-thought elusive hemicarbonate species, which helped to advance a whole range of additional CO2 conversions. Closely associated with these findings are the development of new synthetic procedures for larger-ring carbonates and carbamates (ring size ≥ 6) that provide synthons of value for fine chemical/pharmaceutical and polymer applications. Over the past decade, our research on nonreductive catalytic carbon dioxide valorization has resulted in access to a significantly larger area of chemical space with a main focus on the creation of CO2-derived heterocycles. This chemistry should be both inspiring and synthetically useful for various communities, including those developing sustainable carbon recycling methods, homogeneous catalysis and alternative polymer development, going thus beyond the preparation of the heterocycles themselves.