Four Pillar[5]arene Constitutional Isomers: Synthesis, Crystal Structures, and Host‐Guest Complexation of Their Derivatives with Paraquat in Water

Abstract

A pillar [5]arene prepared from monomer 1 has four constitutional isomers. Although these constitutional isomers have the same molecular formula, their other properties that we have investigated are more or less different. They have different melting points, different NMR spectra and different crystal structures. Furthermore, the corresponding four anionic constitutional pillar [5]arene isomers derived from these four constitutional isomers showed different binding abilities with paraquat G in water. The association constants of 3a⊃G, 3b⊃G, 3c⊃G and 3d⊃G were determined to be (3. , respectively.

A pillar[5]arene-based covalent organic framework with pre-encoded selective host–guest recognition

“…[24][25][26] Although interesting molecular recognition properties of pillar[n]arenes in the solution phase have been demonstrated, they generally exhibit poor guest binding capability in the solid state due to their random packing. [27][28][29][30][31][32] The opening channels are largely blocked, and the intrinsic cavities are buried inside the solid matrix and not accessible to the guest species. 33,34 Meanwhile, the supramolecular structures assembled via noncovalent interactions are vulnerable and tend to collapse upon guest removal.…”

Functionalization at Will of Rim-Differentiated Pillar[5]arenes

“…Cyclization of asymmetrically substituted 1,4-dialkoxybenzene monomers is feasible, but generates the expected C 5 -symmetric isomer in low selectivity and poor yields (≤5%). 32−36 Recently, this statistical process was recently greatly improved by employing the so-called preoriented strategy 37,38 starting from asymmetrically substituted 2,5-dialkoxybenzyl alcohols, thereby optimizing the syntheses of C 5 -symmetric isomers with selectivity higher than 50% and isolated yields up to 20%. This development paved the way for more widespread applications and novel chemistries of rim-differentiated P[5] platforms, which have already been used as amphiphilic self-assemblies bearing hydrophilic and hydrophobic groups on opposite rims 39−43 and as promising candidates for multivalent surface grafting.…”

“…Most common P[5]­s are per-functionalized, with 10 identical substituents, which are appealing for their high symmetry and facile synthesis. , However, a wealth of additional structural complexity and functionality can be added by alternative functionalization schemes, including monosubstitution, , A1/A2 disubstitution, − phenylene substitution, , methylene bridge functionalization, , and rim differentiation. , The latter approach, which led to tiara-P[5]­s, have received relatively less attention, since there is no straightforward pathway capable of converting easily available D 5 -symmetric per-functionalized P[5]­s into their corresponding C 5 -symmetric rim-differentiated ones. Cyclization of asymmetrically substituted 1,4-dialkoxybenzene monomers is feasible, but generates the expected C 5 -symmetric isomer in low selectivity and poor yields (≤5%). − Recently, this statistical process was recently greatly improved by employing the so-called preoriented strategy , starting from asymmetrically substituted 2,5-dialkoxybenzyl alcohols, thereby optimizing the syntheses of C 5 -symmetric isomers with selectivity higher than 50% and isolated yields up to 20%. This development paved the way for more widespread applications and novel chemistries of rim-differentiated P[5] platforms, which have already been used as amphiphilic self-assemblies bearing hydrophilic and hydrophobic groups on opposite rims − and as promising candidates for multivalent surface grafting. , Although the cyclization step has been made more efficient, this preoriented protocol requires the synthesis of the corresponding dialkoxybenzyl alcohol monomer for each target compound, and the subsequent purification by column chromatography can be nontrivial.…”

Rim-Differentiated C5-Symmetric Tiara-Pillar[5]arenes

“…The syntheses of P[5]s with lower symmetry, such as mono- 14 and (A1/A2)-disubstituted 15 ones, can be still relatively easily achieved through cocyclization of different types of 1,4-dialkoxybenzene monomers, or partial demethylation of the 1,4-dimethoxybenzene moieties in the P[5] scaffold. In contrast, selective syntheses of oligo-substituted P[5]s, in particular the so-called “rim-differentiated” C 5 -symmetric pillar[5]arenes, 16 or briefly tiara-pillar[5]arenes (T-P[5]s), in which the two rims of the P[5] macrocycles are decorated differently, have remained elusive. This is due to the statistical nature of the cyclization process of 1,4-dialkoxybenzenes with different alkoxy groups, which yields the four constitutional isomers in a ratio of 5:5:5:1, with only the latter being the T-P[5].…”

“…Among the hundreds of publications on pillararenes that have appeared in the past decade, only a handful of them have addressed 16 , 17 , 20 , 21 the syntheses and applications of the T-P[5]s. Generally, penta-functionalized P[5]s can be prepared ( Figure 1 a) from the Lewis acid-catalyzed cyclization of asymmetrically functionalized 1,4-dialkyoxybenzenes ( M 1 ) with paraformaldehyde 16b or 1,4-dialkoxy-2,5-bis(ethoxymethyl)benzenes ( M 2 ), 16a forming two key intermediates ( M A / M B ) via Friedel–Crafts alkylation/dealkylation. The subsequent oligomerization processes involving M A and M B lead to two types of isomeric dimers D syn and D anti , depending on how the two 1,4-alkoxylated benzene rings and the methylene bridges are positioned relative to each other.…”