Cage-Shaped Borate Catalyst Recognizes Aromatic Aldehydes

A boron-based organocatalyst with an enzymelike π-binding pocket has been designed to catch and hold aromatic aldehydes to facilitate Diels-Alder cycloaddition reactions. The research team led by Makoto Yasuda and Akio Baba of Japan’s Osaka University believes this is the first reported example of the molecular recognition of an aromatic aldehyde over an aliphatic aldehyde in a catalytic manner. Yasuda, Baba, and coworkers previously designed cage-shaped triphenylborate Lewis acids, such as B(OC₆H₄)₃CH, that allow them to increase Lewis acidity over planar, open-shaped borate catalysts. The triphenylborate has a shallow binding pocket and reacts nearly equally with aromatic and aliphatic aldehydes, which the team showed in competitive Diels-Alder reactions pitting benzaldehyde against butanal. The researchers have now taken the borate catalysts to a different level by adding aryl groups to the phenyl rings to form B(OC₆H₃aryl)₃CH analogs, such as the naphthyl example shown (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201200346). The additional aryl rings create a deeper pocket and a larger π-aromatic framework for the catalyst to selectively bind aromatic aldehydes. The naphthyl catalyst favors p-cyanobenzaldehyde over butanal at a ratio of up to 27.5:1.