Synthesis of Aromatic Carbamates from CO₂. Implications for the Polyurethane Industry

Polyurethanes are ubiquitous polymers that have wide ranging applications and are present in many of the products we use on a daily basis. An entire industry has developed over many years to exploit the inherent versatility of these materials, and major capital investments continue to be made in the production of the chemical components. For the isocyanates, conversion of primary amines by reaction with phosgene has become established as the normal synthesis method, but research aimed at avoiding use of this toxic chemical continues. Replacement of phosgene with CO₂ to make intermediate carbamate polymers is one alternative. This chapter will consider the potential of this chemistry by providing a brief introduction to the polyurethane industry and then the current and potential use of CO₂ in polyurethane production. Thereafter, various alternative routes to carbamates are discussed, followed by a description of the state-of-the-art technology for the synthesis of aromatic carbamates from aniline, CO₂, and an alcohol. The thermodynamics of this reaction requires removal of the coproduced water to increase carbamate yields, and the investigation herein has utilized ionic liquids (ILs) as potential water-sequestering cosolvents. This empirical study has utilized a high-throughput batch-scale reactor to screen a large number of ILs and potential catalysts. Three catalyst/IL combinations, which provide higher selectivity for carbamate than for the in situ-produced precursor diphenylurea, have been identified. One of these combinations has been scaled up and provides the first example of one-pot synthesis of an aromatic carbamate from aniline, CO₂, and an alcohol.