Association of Pyramidal Boron Lewis Superacids with Pyridines: Bending 2,4,6‐Collidine with the 10‐Sulfonium‐9‐boratriptycene

Abstract

AbstractThe association of pyridine and of 2,4,6‐trimethylpyridine (collidine) with a pyramidal triarylborane belonging to the 9‐boratriptycene family is investigated by NMR spectroscopy and by X‐ray diffraction analysis. Despite of the large size of the ortho‐disubstituted collidine Lewis base and of the large steric hindrance of the boron Lewis superacid used (9‐boratriptycene‐10‐sulfonium), the superacidity at its boron atom is enabling the formation of a B‐N bond and is resulting in a very stabe Lewis adduct. Single‐ crystal X‐ray diffraction analysis revealed an exceptional deformation of the collidine moiety with a deviation from planarity of nearly 41° between the pyridine C−N bonds and the ortho‐methyl groups C−Me bonds, which is even resulting a small twist of the collidine six‐membered heteroaromatic ring into a boat‐like conformation. Computational investigations indicate that the steric repulsions are reducing the Gibbs free energy of Lewis adduct formation by approximately 50 kJ mol−1.