Constitutional Isomers of Pentahydroxy-Functionalized Pillar[5]arenes: Synthesis, Characterization, and Crystal Structures

Abstract

We herein report the preparation of constitutional isomers of pentahydroxy-functionalized pillar[5]arenes via the deprotection of their benzylated derivatives by catalytic hydrogenation. The structures of the obtained isomers were then established using single crystal X-ray diffraction. We also found that the yield distribution of the different constitutional isomers was dependent on the nature of the substitution, as revealed by HPLC analysis of the crude mixture. Finally, further characterization of the separated constitutional isomers indicated that they possess different melting points, NMR spectra, crystal structures, and stacking patterns in the solid state.

The Trouble with Five: New Synthetic Strategies toward C5 -Symmetric Pillar[5]arenes and Beyond

“…52 Initially, allyloxy-functionalized rim-differentiated P [5] was synthesized 51 for subsequent selective removal of allylic groups; however, C 5 -symmetric pentahydroxy P [5] was first successfully produced by hydrogenolysis of the corresponding pentabenzyloxy derivative by Al-Azemi and coworkers. 50 Their reported 1% yield, owing to statistical cyclization and nontrivial isolation, prevented potential development of this compound. This is where our preoriented synthetic protocol came to the rescue.…”

“…Cao and Meier demonstrated that the four constitutional isomers were obtained in amounts that were close to the statistical 5:5:5:1 ratio, in which the rim-differentiated P [5] is the least abundant product, accounting only for one-sixteenth of all P [5] isomers. 39a Even though the work of Huang 39b and Al-Azemi 50 showed that the probability of obtaining these isomers can deviate from the idealized binomial distribution, the relatively low concentration of rim-differentiated P [5]s in the crude product still led to nontrivial purification processes, which often involved preparative high-performance liquid chromatography.…”

Single-crystal X-ray diffraction study of a host–guest system comprising monofunctionalized-hydroxy pillar[5]arene and 1-octanamine

“…Chemical modification of the pillararene system could be achieved in two ways, namely cyclization of appropriately functionalized monomers or functionalization of preformed pillararenes (Al-Azemi et al, 2017). In the former, co-cyclization of pre-functionalized monomers in an appropriate feed ratio could be employed to generate pillararenes having the desired functionalities in terms of numbers and positions.…”

Encapsulated dichloroethane-mediated interlocked supramolecular polymeric assembly of A1/A2-dihydroxy-octyloxy pillar[5]arene 1,2-dichloroethane monosolvate

“…Supramolecular polymers constructed by reversible noncovalent interactions such as hydrogen bonds, metal-ligand interactions, host-guest interactions,interactions and van der Waals forces have gained considerable interest for their intriguing properties of recycling and responsiveness to external stimuli (Raghupathi et al, 2014;Takashima et al, 2017). Pillararenes are unique three-dimensional macrocyclic compounds which possess symmetric rigid structures and are easy to functionalize with various substituents (Ogoshi et al, 2008;Al-Azemi et al, 2017). They exhibit outstanding abilities to selectively bind different kinds of guest molecules and thus are excellent host molecules for guest encapsulation and molecular recognition.…”