Complete energy profile of a chiral propeller compound: Tris-(2′-methylbenzimidazol-1′-yl) Methane (TMBM). Chromatographic resolution on triacetyl cellulose, x-ray structures of the racemic and one enantiomer, and dynamic NMR study

Abstract

Tris-(2′-methylbenzimidazol-1 ′-y) methane (TMBM) presents an interesting example of propeller-like chirality, which is discussed according to Mislow's and Dunitz's descriptions. Fortunately, the two most stable isomers (the three methyl groups “up”, i . e . on the same side of the methine proton, and two methyl groups “up” and the third one “down”) were present in the solid state, thus allowing the determination of their molecular structure by x-ray crystallography. The activation barrier which separates both isomers (9.8 kcal.mol −1) was determined by dynamic 1H n.m.r., whereas that corresponding to enantiomers (28.5 kcal. mol −1) was determined kinetically by racemization, after pure enantiomers were resolved by chromatography on microcrystalline triacetyl cellulose. Both the racemic TMBM and its enantiomers crystallize with a larger number of water molecules, six and seven respectively, forming cyclic structures.