N-Alkyl Ammonium Resorcinarene Salts: A Versatile Family of Calixarene-Related Host Molecules

Abstract

This chapter presents a review of the recent advances in the chemistry of N-alkylammonium resorcinarenes salt receptors. The Mannich condensation between amines (primary and secondary) and resorcinarenes result in resorcinarene tetrabenzoxazines and tetra-azoxazines. Only 2 isomers out of 16 potential isomers are formed. The resorcinarene tetrabenzoxazines possess deeper cavities than the parent resorcinarenes which are suitable for binding neutral and cationic guests. In the presence of mineral acids, the six-membered oxazine ring in the resorcinarene tetrabenzoxazines is opened, resulting in N-alkylammonium resorcinarene salts (NARSs). The NARSs possess four spatially-fixed anions within a circular cation-anion seam in between the ammonium moieties. Due to similarities in structure and binding properties, the NARSs are considered as hydrogen-bonded analogues of covalent cavitands. The deeper cavities of the NARSs as compared to the corresponding resorcinarenes are suitable for a variety of neutral guests and anions. Quite interestingly, the four spatially-fixed halides in NARXs are also suitable halogen-bond acceptors leading to multiple assemblies such as deep cavity cavitands, polymeric, dimeric and capsular assemblies in the presence of organic and inorganic halogen-bond donors. With non-spherical and larger oxo-anions such as triflate, the NARS possess even larger cavities that can bind anions as well as several neutral guests at the same time. The resorcinarene tetrabenzoxazines and especially the N-alkyl ammonium resorcinarenes salts have yet a mostly unexplored potential in contemporary supramolecular chemistry as receptors for neutral molecules and anions.