(13e) Homogeneous Catalysis Coupled with Benign Separation In Polyethylene Glycol-Organic-Carbon Dioxide Tunable Solvent Systems

In this work, we used a polyethylene glycol (PEG)-dioxane-carbon dioxide solvent system for sustainable coupling of homogeneous catalytic reactions with facile heterogeneous separations. At low molecular weights (i.e. 200-600), PEG is non-toxic and has negligible vapor pressure, making it an attractive alternative to traditional volatile organic solvents. By applying carbon dioxide pressure to the system, we can split the PEG-dioxane mixture into two phases, allowing for catalyst recycle in the PEG phase and product recovery in the gas expanded dioxane phase. These carbon dioxide tunable solvent systems enable improved reaction rates, catalyst recycle, and facile product recovery. Specifically, we have used these systems for palladium catalyzed carbon-oxygen coupling of aryl halides to produce both phenols and asymmetric aromatic ethers. These molecules are prevalent in both pharmaceutical and chemical industries, but often production requires harsh and hazardous reaction conditions. In the phenol case, carbon dioxide plays a dual role by acting as the neutralization media via in-situ generation of carbonic acid. Carbonic acid is formed from the equilibrium between water and carbon dioxide and is reversible upon venting. This benign work-up step eliminates conventional acidifications which create large quantities of waste and inactivate the catalyst for further use. Initial results show high selectivity to phenol or ether, enhanced reaction rates, and favorable product partitioning into the carbon dioxide expanded dioxane phase.