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

Abstract

Abstract Heck coupling reactions of methyl acrylate with various aryl bromides have been investigated using a Pd/TPP catalyst in toluene under pressurized CO 2 conditions up to 13 MPa. Although CO 2 is not a reactant, the pressurization of the reaction liquid phase with CO 2 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 CO 2 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 CO 2 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 CO 2 pressure. To examine the factors responsible for the effects of CO 2 pressurization, the phase behavior and the molecular interactions with dense phase CO 2 have also been studied by visual observation and in situ high pressure FT‐IR spectroscopy. In addition, impact of CO 2 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 CO 2 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 CO 2 molecules under pressurized conditions.