Abstract
While CO2 attracts intense current interest as the primary chemical motor leading to the calamity of climate change, the primary driver for the development of CO2 as a C1 feedstock is its sustainability. Despite the thermodynamic challenges involved, CO2 is the preferred reagent over other C1 sources such as phosgene, COCl2, and CO, and, because of these limitations, fundamental catalytic studies for enhancing the reactivity of CO2 are needed for advancing chemical sustainability. These issues are particularly relevant where CO2 can be used as a replacement for extremely toxic phosgene, which produces two equivalents of Cl-containing waste when used as a carbonylating agent. An industrially applicable target reaction for the replacement of phosgene by CO2 is the production of an aromatic carbamate from CO2, aniline, and an alcohol, as potential isocyanate precursors for the polyurethane industry. The only known CO2-based catalysis relevant to this goal is the production of diphenylurea (DPU) from aniline and CO2, which takes place with moderate yields in ionic liquids (ILs) and CsCO3 as catalyst [1]. Other CO2-based reactions show improved yields in the presence of ILs [2], perhaps because of the enhanced solubility of CO2 in ILs as opposed to conventional solvents [3], or due to the ability of ILs to sequester co-produced H2O and therefore shift equilibria toward the desired products. While IL properties can be tuned by judicious choice of cation and anion, the large number of potential anions and cations makes any a priori IL choice haphazard. In order to investigate the potential of ILs in CO2-utilization reactions, and to study a large variety of ILs systematically, we have initiated a program to screen with high-throughput technology a large parameter space of ILs and catalysts for the production of aromatic carbamates.Our studies have revealed a number of new catalyst/IL systems for the synthesis of diphenylurea from CO2 and aniline, and furthermore, the first