Dissecting the Complex Recognition Interfaces of Potent Tetrazole- and Pyrrole-Based Anion Binders

Functionalization of Tetrazoles Bearing the Electrochemically Cleavable 1N-(6-Methylpyridyl-2-methyl) Protecting Group

6-Methylpyridyl-2-methyl protected tetrazoles can be C-H deprotonated using the turbo-Grignard reagent and involved in the reactions with aldehydes and ketones. The protecting group can be cleaved under reductive electrochemical conditions using Pb bronze as a cathode and Zn as a sacrificial anode.

Functionalization of 1N-Protected Tetrazoles by Deprotonation with the Turbo Grignard Reagent

1N-PMB-protected tetrazole undergoes C-H deprotonation with the turbo Grignard reagent, providing a metalated intermediate with increased stability. This can be used for the reaction with electrophiles such as aldehydes, ketones, Weinreb amides, and iodine. C-H deprotonation with the turbo Grignard reagent is compatible with the PMB-protecting group at the tetrazole, which can be cleaved using oxidative hydrogenolysis and acidic conditions. The method enables the tetrazole functionalization at the fifth position by overcoming the difficulties associated with retro [2 + 3] cycloaddition of the metalated intermediates.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F^- and H₂PO₄^- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and ¹H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.

Interaction of aryl tetrazolones with anions: proton transfer vs. hydrogen bonding

Tetrazolones 1a,b interact with HSO₄⁻, Br⁻, NO₃⁻, NCS⁻ and Cl⁻ through H-bonding, but undergo deprotonation with a more basic AcO⁻ anion.

On the recognition of chloride, bromide and nitrate anions by anthracene–squaramide conjugated compounds: a computational perspective

Anionic recognition appears in several biological processes. Here, the interaction between anthracene–squaramide conjugated compounds and Cl⁻, Br⁻ and NO₃⁻ anions has been explored using density functional theory (DFT) calculations.

Pyrrole N-H Anion Complexes

Synthetic pyrrole-based anion receptors date back to the 1990s. They have been extensively developed in the context of macrocyclic systems as expanded porphyrins and calixpyrroles, and related systems. The chemistry of open-chain pyrrolic systems is, in many respects, no less venerable. It also has more direct analogy to naturally occurring pyrrole-based anion binding motifs. However, it has not been the subject of a comprehensive review. Presented herein is a summary of efforts devoted to the creation of de novo pyrrole-based receptors, as well as the anion recognition chemistry of naturally occurring pyrrolic systems as prodigiosins and their synthetic analogues.