Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

• Published in: Angewandte Chemie International Edition 2019-11, Vol.58 (47), p.16889-16893
• Main Authors: Ben Saida, Ali, Chardon, Aurélien, Osi, Arnaud, Tumanov, Nikolay, Wouters, Johan, Adjieufack, Abel I., Champagne, Benoît, Berionni, Guillaume
• Format: Article
• Language: English
• Subjects: Acidity; Adducts; Aromatic compounds; Boron; Boron compounds; Carbon; Covalent bonds; Lewis acid; Lewis acids; Lewis base; Lewis superacid; Magnetic resonance spectroscopy; NMR; NMR spectroscopy; non-planar; Nuclear magnetic resonance; Phosphorus; triarylboranes
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201910908

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9‐boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X‐ray diffraction of the Lewis adducts of these non‐planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of a trivalent boron compounds with the Tf2N− anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using a cationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity. Introduction of a phosphorus and a boron atom at each edge of a triptycene framework, allowed access to new bifunctional Lewis acid‐base 9‐phospha‐10‐boratriptycenes featuring promising reactivity for the activation of carbon‐halogen bonds. Pyramid power: Owing to the drastic pyramidalization around the trivalent boron atom and the presence of a cationic phosphonium back‐side bridge, the Lewis acidity of the cage‐shaped 9‐bora‐10‐phosphonium triptycenes surpasses that of all trivalent boron Lewis acids generated to date.