Ancillary Ligand and Base Influences on Nickel-Catalyzed Coupling of CO2 and Ethylene to Acrylate

Abstract

The coupling of CO2 and ethylene to produce acrylates has been an area of increasing interest in recent years following a number of studies which have empirically improved catalytic turnover. Notably, the incorporation of moderately Brønsted and Lewis basic sodium phenoxide salts, as well as zinc dust, and Lewis acidic lithium salts were found to facilitate acrylate formation in batch catalysis. Despite these advances, there has been limited investigation into the effect of the catalyst ancillary ligand and phenoxide base structure on catalytic performance. Here, a collection of 1,2-bis(dialkylphosphino)benzene and related diphosphine ligands were used to show that the influence of steric environs has a marked effect on turnover. Ancillary diphosphine ligands featuring at least two smaller alkyl substituents are needed for strong activity, while the oft-used benzene annulation of the diphosphine does not appear to be determinant in achieving high turnover values. Additionally, the investigation of a collection of substituted sodium phenoxide bases suggests that a subtle balance of basicity and steric factors must be satisfied to obtain optimal catalytic performance. These trends appear to result from competitive, deleterious nucleophilic reactions between base and CO2 to produce carbonate and the need to maintain sufficient basicity and access to the metal coordination sphere to drive the endergonic CO2–ethylene coupling reaction.