Functionalizing Pillar[n]arenes

Abstract

Macrocyclic chemistry has relied on the dominance of some key cavitands, including cyclodextrins, calixarenes, cyclophanes, and cucurbiturils, to advance the field of host-guest science. Very few of the many other cavitands introduced by chemists during these past few decades have been developed to near the extent of these four key players. A relatively new family of macrocycles that are becoming increasingly dominant in the field of macrocyclic chemistry are the pillar[n]arenes composed of n hydroquinone rings connected in their 2- and 5-positions by methylene bridges. This substitution pattern creates a cylindrical or pillar-like structure that has identical upper and lower rims. The preparation of pillar[n]arenes is facile, with pillar[5]- through pillar[7]arene being readily accessible and the larger macrocycles (n = 8-14) being accessible in diminishing yields. The rigid pillar[n]arene cavities are highly π-electron-rich on account of the n activated aromatic faces pointing toward their centers, allowing the cavities to interact strongly with a range of π-electron-deficient guests including pyridiniums, alkylammoniums, and imidazoliums. The substitution pattern of pillar[n]arenes bestows chirality onto the macrocycle in the form of n chiral planes. The absolute configuration of the chiral planes in pillar[n]arenes can be either fixed or rapidly undergoing inversion. The future of pillar[n]arenes is going to be dependent on their ability to fulfill specific applications. Chemical modification of the parent pillar[n]arenes lets us create functionalized hosts with anticipated chemical or physical properties. The featured potential applications of pillar[n]arenes to date are far reaching and include novel hosts with relevance to nanotechnology, materials science, and medicine. Pillar[n]arenes have an overwhelming advantage over other hosts since the number of ways available to incorporate handles into their structures are diverse and easy to implement. In this Account, we describe the routes to chemically modified pillar[n]arenes by discussing the chemistry of their functionalization: monofunctionalization, difunctionalization, rim differentiation, perfunctionalization, and phenylene substitution. We assess the synthetic complications of employing these functionalization procedures and survey the potential applications and novel properties that arise with these functionalized pillar[n]arenes. We also highlight the challenges and the synthetic approaches that have yet to be fully explored for the selective chemical modification of these hosts. Finally, we examine a related class of macrocycles and consider their future applications. We trust that this Account will stimulate the development of new methods for functionalizing these novel hosts to realize pillar[n]arene-containing compounds capable of finding applications.

Pillar[n]arenes and Other Cavitands: Aspects of Complex Thermodynamics

“…[11, 19,22,24,25] Their cavity sizes are 4.7 and 5.9 Å, respectively, which are similar to those of the α-and β-CDs. [11,20b,23b] The cavities of PA[n]s are π-electron rich due to the hydroquinone moieties, and thus they attract electron poor molecules into their cavities.…”

“…This novel class of cavitands, also represented as pillar[n]arenes (PA[n]s or pillarenes), has a robust, symmetric shape, with a similar pore size at each end, unlike the pores of the CDs but similar to those of the cucurbit[n]urils (CB[n]s). [15][16][17] [18] It is also worth noting that the functional groups on PA[n]s are more accessible for the purpose of rim functionalization compared to the CDs and the CB[n]s. Recent efforts made by a number of research groups have enabled the preparation of a diverse range of functionalized PA[n]s. [18,19] PA [5] is composed of 5 hydroquinone units bridged by methylene groups, which link the 2 and 5-positions of the 5-hydroquinone units. [11a,20,21] The first step in its preparation involves the reaction of 1,4-dimethoxybenzene with paraformaldehyde in the presence of boron trifluoride diethyl etherate (BF 3 •OEt 2 ) to form dimethoxy pillar [5]arene.…”

pH-Responsive Host–Guest Complexation in Pillar[6]arene-Containing Polyelectrolyte Multilayer Films

“…Since then, this novel type of host has attracted great attention on account of the ease of chemical modification, resulting from the reactive substituents located at the upper and lower rim, respectively. Due to the selective addressability of the reactive groups, a series of homologues (n = 5-15) bearing a large variety of properties are available to date, although the synthesis of higher homologues still suffers from low yields [20,21]. The π-electron-rich cavity combined with the opportunity of structure-tailoring motivated scientists to investigate the potential of P[n]s in the field of host-guest chemistry.…”

Supramolecular FRET modulation by pseudorotaxane formation of a ditopic stilbazolium dye and carboxylato-pillar[5]arene

“…It was hoped that the results on such a model system can be helpful in the application of solvatochromic FRET-based sensors in complex matrices of biological origin. [19], they have soon become one of the most popular targets in host-guest chemical studies [20][21][22][23][24][25][26][27]. Especially, the discovery of water soluble derivatives (WPns) have further extended their applications in various biological systems.…”