Relation between the substituent effect and aromaticity in imidazole derivatives: A comparative study

Synthesis and characterization of 2-(anthracene-9-yl)-4,5-diphenyl-1H-imidazole derivatives as environmentally sensitive fluorophores

2-(Anthracene-9-yl)-4,5-diphenyl-1H-imidazole (ADPI) provides an intriguing molecular platform for developing organic fluorophores with diverse properties and fluorescence performances. However, derivatives of ADPI have not yet been well explored and extensive studies are warranted. To shed more light on this, we have synthesized a series of π-extended ADPIs through a concise synthetic route involving an efficient cross-condensation reaction followed by Pd-catalyzed Suzuki cross-coupling. The obtained compounds were subjected to X-ray single crystallographic analysis to understand their molecular conformational and solid-state packing properties. Furthermore, UV-Vis absorption and fluorescence spectroscopic analyses were conducted. Our experimental results have disclosed interesting solvatofluorochromic properties of these compounds which are useful for solvent polarity-sensitive applications. The presence of an amphoteric imidazolyl group in the ADPI derivatives also renders them sensitive fluorescence responses to strong protic acids (e.g., trifluoroacetic acid) as well as fluoride anion. It transpires that the fluorescence changes are dependent on the functional groups attached to the ADPI core, suggesting a bottom-up molecular tuning approach for development of fluorophores and chemosensors with diverse functions.

Substituent effects and electron delocalization in five-membered N-heterocycles

Five-membered N-heterocycles are principal constituents of many compounds of vital importance in various fields of chemistry, biochemistry or pharmaceutical chemistry. For this reason, unequivocal identification of structural factors determining electron donating/withdrawing properties of specific groups attached to the heterocyclic moiety becomes an utmost need together with elucidation of the substitution-induced changes in cyclic and noncyclic electron delocalization. Thus, quantum-chemical calculations were performed for pyrrole, imidazole, pyrazole, 1,2,3- and 1,2,4-triazole, and their C-substituted mono-derivatives (X = NO2, CN, Br, Cl, F, SH, OH, NH2). The obtained dataset contains information on substituent properties (cSAR - charge of the substituent active region method), delocalization (EDDB - electron density of delocalized bonds) and geometry. It follows that the positions of endocyclic N atoms relative to the substituent influence in the most profound manner its properties. N atoms in ortho positions significantly boost the electron-donation and weaken the electron-withdrawal by induction. Another factor is the resonance charge transfer from the substituents to N atoms, and then inductive interactions with further (non-ortho) N atoms. While substituent constants correctly describe the changes of their properties (including those attached to the heterocycles), a testimony to Hammett's genius, quantum chemical models must be used to quantify the exact properties. In most heterocycles, electron-donating substituents hinder the cyclic delocalization, except 4-pyrazole. The applied recent EDDB method allows to study this phenomenon in detail. It follows that changes in aromaticity originate from the π-electronic effects of substituents on the ring bonds, changing the localization and delocalization of particular bonds in a correlated manner.

Luminescent probes for hypochlorous acid in vitro and in vivo

HClO/ClO^- is the most effective antibacterial active oxygen in neutrophils. However, its excessive existence often leads to the destruction of human physiological mechanisms. In recent years, the developed luminescent probes for the detection of HClO/ClO^- are not only conducive to improve the sensitivity and selectivity of HClO/ClO^- detection, but also play a crucial role in understanding the biological functions of HClO/ClO^-. In addition, luminescent probe-based biological imaging for HClO/ClO^- at sub-cellular resolution has become a powerful tool for biopathology and medical diagnostic research. This article reviews a variety of luminescent probes for the detection of HClO/ClO^-in vitro and in vivo with different design principles and mechanisms, including fluorescence, phosphorescence, and chemiluminescence. The photophysical/chemical properties and biological applications of these luminescent probes were outlined. Finally, we summarized the merits and demerits of the developed luminescent probes and discussed their challenges and future development trends. It is hoped that this review can provide some inspiration for the development of luminescent probe-based strategies and to promote the further research of biomedical luminescent probes for HClO/ClO^-.

One-Step Synthesis of 2,5-Diaminoimidazoles and Total Synthesis of Methylglyoxal-Derived Imidazolium Crosslink (MODIC)

Here we describe a general method for the synthesis of 2,5-diaminoimidazoles, which involves a thermal reaction between α-aminoketones and substituted guanylhydrazines without the need for additives. As one of the few known ways to access the 2,5-diaminoimidazole motif, our method greatly expands the number of reported diaminoimidazoles and further supports our previous observations that these compounds spontaneously adopt the non-aromatic 4(H) tautomer. The reaction works successfully on both cyclic and acyclic amino ketone starting materials, as well as a range of substituted guanylhydrazines. Following optimization, the method was applied to the efficient synthesis of the advanced glycation end product (AGE) methylglyoxal-derived imidazolium crosslink (MODIC). We expect that this method will enable rapid access to a variety of biologically important 2,5-diaminoimidazole-containing products.

Concise total synthesis of glucosepane

Glucosepane is a structurally complex protein posttranslational modification that is believed to exist in all living organisms. Research in humans suggests that glucosepane plays a critical role in the pathophysiology of both diabetes and human aging, yet comprehensive biological investigations of this metabolite have been hindered by a scarcity of chemically homogeneous material available for study. Here we report the total synthesis of glucosepane, enabled by the development of a one-pot method for preparation of the nonaromatic 4H-imidazole tautomer in the core. Our synthesis is concise (eight steps starting from commercial materials), convergent, high-yielding (12% overall), and enantioselective. We expect that these results will prove useful in the art and practice of heterocyclic chemistry and beneficial for the study of glucosepane and its role in human health and disease.

On the non-additivity of the substituent effect in ortho-, meta- and para-homo-disubstituted benzenes

The non-additivity of the substituent effect in para-, meta-, and ortho- homo-disubstituted benzenes on π-valence orbitals is smaller than that on σ-ones. The former increases while the latter decreases with π-electron-donating character of the substituent which demonstrates the role of hyperconjugation in the substituent effect.