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

The 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 degrees 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).

Automated summary

The association between pyridine and collidine with a pyramidal triarylborane belonging to the 9-boratriptycene family is explored using NMR spectroscopy and X-ray diffraction analysis. Despite the large size and steric hindrance, the boron Lewis superacid enables the formation of a stable Lewis adduct. X-ray diffraction analysis reveals an exceptional deformation of the collidine moiety, resulting in a small twist of the collidine ring into a boat-like conformation. Computational investigations suggest that steric repulsions decrease the Gibbs free energy of Lewis adduct formation.