Racemic and Meso Crystal Structures of an Axial-Chiral Spirobi-(dinaphthoazepin)ium Salt: Emergence of an S4-Symmetric Molecule

Philipp Honegger,Michael Widhalm,Alexander Prado-Roller,Natalie Gajic

doi:10.3390/sym13081365

Abstract

To date, only a few instances of S4-symmetric organic molecules exist. In principle, spirobi-(dinaphthoazepin)ium cations can achieve this highly symmetric point group. Heating racemic 2,2′-bis(bromomethyl)-1,1′binaphthyl with aqueous ammonia afforded a mixture of rac- and meso-3,3′,5,5′-tetrahydro-4,4′-spirobi[dinaphtho[2,1-c:1′,2′-e]azepin]-4-ium bromide which was separated by fractional crystallisation. Both stereoisomers were characterised spectroscopically, and their crystal structures were determined and compared. The rac crystal structure differs significantly from the known enantiopure one. The meso molecules display a near-perfect S4 symmetry. Upon treatment with KOtBu, both isomers undergo Stevens rearrangement.

Highlights

Dinaphthoazepinium compounds (Figure 1) have found widespread application as chiral phase transfer catalysts [1,2]
Heating racemic 2,2 -bis(bromomethyl)-1,1 binaphthyl with aqueous ammonia afforded a mixture of rac- and meso3,3,5,5 -tetrahydro-4,4 -spirobi[dinaphtho[2,1-c:1,2 -e]azepin]-4-ium bromide which was separated by fractional crystallisation
The situation is less obvious in case C, where the conformation of the biphenyl moiety is controlled by the binaphthyl chirality

Summary

Introduction

Dinaphthoazepinium compounds (Figure 1) have found widespread application as chiral phase transfer catalysts [1,2]. Of particular interest is the formation of N-spiro compounds where a dibenzo- with a dinaphthoazepine (C) or two dinaphthoazepines (D) are merged, resulting in a conformatively stable ammonium ion [3,4,5,6,7,8]. In cases A and B, a product with C2 symmetry is formed. The same is true for D if connecting binaphthyls with the same chirality. The situation is less obvious in case C, where the conformation of the biphenyl moiety is controlled by the binaphthyl chirality. If combining identical homochiral units in case D (R1 = H), a spiro compound with D2 symmetry is obtained as confirmed by X-ray analysis; from binaphthyl halves with opposite chirality, an achiral species with S4 symmetry may result

Methods

Results

Conclusion