Synthesis of organic carbamates as polyurethane raw materials from CO2: the quest for metal alkoxides as regenerable reagents

Abstract

It is well known that the utilization of carbon dioxide (CO₂) for chemical materials is attracting research attention from the viewpoint of the carbon cycle. To contribute to the reduction of CO₂ emission through such CO₂ utilization reactions and counteract global climate change, the target compounds should be core chemical products that are distributed in large quantities and used for a long time. One such synthetic target is isocyanates that are used as raw materials for the production of polyurethanes, which are versatile polymeric materials with a service life of approximately 10 years. However, since direct synthesis of isocyanate from CO₂ is quite difficult due to equilibrium constraints, a method via the use of its alcohol adduct, organic carbamate, as a precursor has been proposed. In this Perspective, we present regenerative metal alkoxide reactants, such as tin alkoxide, titanium alkoxide, and alkoxysilane, as environmentally benign reactants for the synthesis of organic carbamates from CO₂. We also present a practical and environmentally friendly method for the highly efficient synthesis of various organic carbamates, including industrially important diisocyanate precursors, from 1 atm CO₂ using alkoxysilanes. Furthermore, prospects for the practical application of these carbamate synthesis reactions are also discussed.