Recent Progress on Conversion of Carbon Dioxide into Carbamates

Carbon dioxide (CO₂) serves as one of the major greenhouse gases in the atmosphere. However, it is also abundant, non-toxic, and renewable, making it a valuable one-carbon source. Therefore, converting CO₂ into valuable chemicals holds immense significance as an effective approach towards achieving carbon neutrality. Nevertheless, due to CO₂'s thermodynamic stability and kinetic inertness, its activation and conversion present considerable challenges. Organic carbamates, both cyclic and acyclic, represent a crucial class of bioactive compounds found in various natural products, agricultural chemicals, and pharmaceutically relevant molecules. They are also widely used as essential intermediates in organic synthesis. Unfortunately, traditional methods for preparing organic carbamates often rely on highly toxic phosgene and its derivatives as raw materials, posing serious environmental and safety concerns and limiting practical applications. From a cost-effective and sustainable standpoint, substituting CO₂ for phosgene in the synthesis of organic carbamates is highly appealing. In recent decades, numerous new reactions, particularly multicomponent reactions involving CO₂ and amines, have emerged, providing efficient methods for constructing diverse and valuable carbamates. Some of these reactions can be conducted under transition-metal-free conditions, utilizing organic and inorganic bases, ionic liquids, or small organic molecules as catalysts or promoters. However, in certain cases, transition-metal catalysts, such as those based on copper, palladium, or silver, are required, especially when the reactions involve activating unsaturated hydrocarbons like alkenes and alkynes. Mechanistically, most of these methods involve in situ generation of nucleophilic CO₂-amine adducts, such as carbamate salts or carbamic acids, which then react with other electrophiles or coupling partners to yield the desired carbamates. Notably, recent advancements have led to the successful development of several elegant methods for synthesizing specific types of carbamates using electrocatalysis or photocatalysis, which are not achievable through conventional thermal catalysis. This review comprehensively summarizes the recent progress in the synthesis of organic carbamates using CO₂ and amines under various catalytic conditions, including transition metal-free conditions, transition metal-catalysis, electrocatalysis, and photocatalysis. Additionally, the review discusses the challenges and future prospects associated with converting CO₂ into organic carbamates.

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