Impact of Carbon Dioxide Pressurization on Liquid Phase Organic Reactions: A Case Study on Heck and Diels–Alder Reactions

Abstract

AbstractHeck coupling reactions of methyl acrylate with various aryl bromides have been investigated using a Pd/TPP catalyst in toluene under pressurized CO2 conditions up to 13 MPa. Although CO2 is not a reactant, the pressurization of the reaction liquid phase with CO2 has positive and negative impacts on the rate of Heck coupling depending on the structures of the substrates examined. In the case of either 2‐bromoacetophenone or 2‐bromocinnamate, the conversion has a maximum at a CO2 pressure of about 3 MPa; for the former, it is much larger by a factor of 3 compared with that under ambient pressure. For 2‐bromobenzene, in contrast, the conversion is minimized at a similar CO2 pressure, being half compared with that at ambient pressure. In the other substrates, including the other isomers of these three aryl bromides, the conversion simply decreases or does not change so much with the CO2 pressure. To examine the factors responsible for the effects of CO2 pressurization, the phase behavior and the molecular interactions with dense phase CO2 have also been studied by visual observation and in situ high pressure FT‐IR spectroscopy. In addition, impact of CO2 pressurization was also studied for the Diels–Alder reactions of isoprene with a few dienophiles like methyl acrylate, methyl vinyl ketone, and acrolein in the same solvent, toluene, but a heterogeneous silica‐alumina catalyst was used (the reaction system was liquid‐solid biphasic). When the CO2 pressure is raised, the conversion monotonously decreases for the three dienophiles; however, the product selectivity changes with the pressure, in particular for acrolein. The FT‐IR spectroscopic measurements suggest that its reactivity is altered by interactions with CO2 molecules under pressurized conditions.