Influence of Ligand Sterics for [2+2] Cycloaddition with (PDI)Fe Complexes

Abstract

Within catalysis, transition-metal complexes are powerful due to their versatility and robustness, where modification of the ligand or metal can tweak catalytic activity and pathways. Redox-active ligands such as PDI are attractive as they facilitate metal-ligand cooperation and stabilize the compound. Over the years, a variety of PDI ligands have been investigated, where altering its electronics and steric bulk can increase activity or suppress undesired reaction pathways. In particular, the Chirik group has demonstrated how increasing steric bulk drives selectivity and catalytic activity in (PDI)Fe for [2+2] cycloaddition. [2+2] cycloaddition generates cyclobutane derivatives, which are building blocks for natural product and petrochemicals. Previous work in the group demonstrated that increasing steric bulk in PDI suppresses the undesired β-H elimination pathway in [2+2] cycloaddition. Since the group has not explored PDI ligands bulkier than iPrPDI , this work describes the synthesis of sBuPDI and i-PentPDI ligands and their metalation onto Fe. The reduced (PDI)Fe complexes were found to have lower catalytic activity than the parent (iPrPDI)Fe precatalyst in [2+2] cycloadditions.