Yao-Wei Xu

Inherently Chiral Calixarenes: Synthesis, Optical Resolution, Chiral Recognition and Asymmetric Catalysis

Inherently chiral calixarenes, whose chirality is based on the absence of a planar symmetry or an inversion center in the molecules as a whole through the asymmetric array of several achiral groups upon the three-dimensional calix-skeletons, are challenging and attractive chiral molecules, because of their potential in supramolecular chemistry. The synthesis and optical resolution of all varieties of inherently chiral calixarenes are systematically discussed and classified, and their applications in chiral recognition and asymmetric catalysis are thoroughly illustrated in this review.

Chelant extraction of heavy metals from contaminated soils using new selective EDTA derivatives

Soil washing is one of the few permanent treatment alternatives for removing metal contaminants. Ethylenediaminetetraacetic acid (EDTA) and its salts can substantially increase heavy metal removal from contaminated soils and have been extensively studied for soil washing. However, EDTA has a poor utilization ratio due to its low selectivity resulting from the competition between soil major cations and trace metal ions for chelation. The present study evaluated the potential for soil washing using EDTA and three of its derivatives: CDTA (trans-1,2-cyclohexanediaminetetraacetic acid), BDTA (benzyldiaminetetraacetic acid), and PDTA (phenyldiaminetetraacetic acid), which contain a cylcohexane ring, a benzyl group, and a phenyl group, respectively. Titration results showed that PDTA had the highest stability constants for Cu(2+) and Ni(2+) and the highest overall selectivity for trace metals over major cations. Equilibrium batch experiments were conducted to evaluate the efficacy of the EDTA derivatives at extracting Cu(2+), Zn(2+), Ni(2+), Pb(2+), Ca(2+), and Fe(3+) from a contaminated soil. At pH 7.0, PDTA extracted 1.5 times more Cu(2+) than did EDTA, but only 75% as much Ca(2+). Although CDTA was a strong chelator of heavy metal ions, its overall selectivity was lower and comparable to that of EDTA. BDTA was the least effective extractant because its stability constants with heavy metals were low. PDTA is potentially a practical washing agent for soils contaminated with trace metals.

Regio- and Enantioselective Photodimerization within the Confined Space of a Homochiral Ruthenium/Palladium Heterometallic Coordination Cage

The photoinduced regio- and enantioselective coupling of naphthols and derivatives thereof is achieved in the confined chiral coordination space of a RuII metalloligand based cage. The racemic or enantiopure cages encapsulate naphthol guests, which then undergo a regiospecific 1,4-coupling, rather than the normal 1,1-coupling, to form 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinones; moderate stereochemical control is achieved with homochiral cages. The photoreactions proceed under both aerobic and anaerobic conditions but through distinct pathways that nevertheless involve the same radical intermediates. This unusual dimerization constitutes a very rare example of asymmetric induction in biaryl coupling by making use of coordination cages with dual functionality-photoredox reactivity and stereoselectivity.

Fluorescent calix[4]arene chemosensor for acidic and basic amino acids in pure aqueous media

A modified calix[4]arene based receptor 2, conjugated with four bithiophene–cyanoacrylic acid groups, not only recognizes acidic amino acids (Asp and Glu) by quenching fluorescence, but also shows highly selective sensing for basic amino acids (Lys and Arg) by turning on fluorescence in sodium phosphate buffer solution without any organic solvent. The UV-Vis spectroscopy binding analysis and ESI-MS studies indicate that the above complexes have a 1 : 1 stoichiometry. 1H NMR and molecular modeling studies suggest a supramolecular complex structure like the relationship between a lid and cup, but not a typical host-guest relationship of the cavity. A plausible mechanism involving intramolecular charge transfer (ICT) is proposed.

Synthesis of inherently chiral wide rim ABC substituted calix[6]arene derivatives

An efficient synthetic route to inherently chiral calix[6]arenes with an ABC substitution pattern at the wide rim and a mesitylenyl unit at the narrow rim in the cone conformation was developed for the first time. Based on the selective formylation and bromination of 1,3,5-bridged calix[6]arene 1, two new inherently chiral calix[6]arene derivatives 4 and 5 have been prepared with moderate yields. The 1H NMR spectra indicate that these inherently chiral compounds adopt a cone conformation and their three different groups at the wide rim can create a chiral environment inside the cavity that maybe sensed by some chiral guests, which has also been confirmed by X-ray crystal structure of 4 and molecular modelling of 5. Complexation studies show that 5 displays exceptional properties for the binding of .

A fluorescent calixarene-based dimeric capsule constructed via a MII–terpyridine interaction: cage structure, inclusion properties and drug release

Two analogues of capsule-like fluorescent cages have been constructed by dimerization of terpyridine-containing calixarene derivatives utilizing a MII–terpyridine (M = Zn and Cd) interaction. 1H NMR spectral studies show that the self-assembled molecular capsules Zn4L12 and Cd4L12 have a highly symmetrical D4h-structure. The encapsulation of the anticancer drug mercaptopurine in their cavities has been documented by NMR, ESI-TOF-MS, fluorescence switching, and molecular simulation, indicating that strong S–π and π–π interactions between drug and cage are of importance for the host–guest binding. The nanoscale cages exhibit excellent behaviors to control the release of mercaptopurine in phosphate buffered saline solution (pH = 7.4). These results further highlight the potential of self-assembled Zn4L12 cages for drug-carrier applications.

Catalytic Space Engineering of Porphyrin Metal-Organic Frameworks for Combined CO2 Capture and Conversion at a Low Concentration

Porous porphyrin metal-organic frameworks (PMOFs) provide promising platforms for studying CO2 capture and conversion (C3) owing to their versatility in photoelectric, catalytic, and redox activities and porphyrin coordination chemistry. Herein, we report the C3 application of two PMOFs by engineering the coordination space through the introduction of two catalytic metalloporphyrins doped with rhodium or iridium, Rh-PMOF-1 and Ir-PMOF-1, both of which can serve as heterogeneous catalysts for the chemical fixation of CO2 into cyclic carbonates with yields of up to 99 %. Remarkably, the catalytic reactions can effectively proceed under low CO2 concentrations and high yields of 83 % and 73 % can be obtained under 5 % CO2 in the presence of Rh-PMOF-1 and Ir-PMOF-1, respectively. The synergistic effect of the metalloporphyrin ligand and the Zr6 O8 cluster, in combination with the CO2 concentration effect from the pore space, might account for the excellent catalytic performance of Rh-PMOF-1 under low CO2 concentration. Recycling tests of Rh-PMOF-1 show negligible loss of catalytic activity after 10 runs.