Lanthanide catalysts for the hetero Diels–Alder reaction: effect of ligand structure and acidity

A series of yttrium, ytterbium, and lanthanum hexa-or heptacoordinate complexes were prepared and their catalytic activities tested in the hetero Diels–Alder reaction between crotonaldehyde and ethyl vinyl ether. It was found that a pKa below 7 of the ligand was necessary but not sufficient for catalytic activity. It was determined that the ligand should possess a perfluoromethyl-β-diketonate functionality. Other factors such as bite angle and hinging motion may also play a determining role.