Inducing Propeller Chirality in Triaryl Boranes with Chiral Amines.

Abstract

Stabilization of chiral propeller conformations in triaryl compounds is challenging due to generally low racemization barriers. Nonetheless, it was recently found that chiral conformational preferences can be induced to triaryl boranes by incorporating point-chiral alkylether chains to the aryl blades and subsequently locking the structure with ammonia. A four-point interaction, meaning that the cooperative effects of Lewis-adduct formation and three hydrogen bonds, was proposed as stabilizing mechanism. Herein, it was shown that three such strong interactions suffice to introduce a preferential propeller handedness. Although DFT calculations predict no noteworthy preferences for either P- or M-chiral propellers for some of the investigated triarylborane-amine adducts that were prepared with chiral primary amines, vibrational circular dichroism (VCD) spectroscopic characterizations revealed that there is indeed a measurable excess of one propeller handedness. Furthermore, the steric demand of the amine was found to play a key role in the induction process and especially in preventing blade rotations.