Arylation of Click Triazoles with Diaryliodonium Salts

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.

Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]2 to give the corresponding [Pd(η3-allyl)Cl(mNHO)] coordination complexes. 13C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH3-mNHO adduct by reacting the triazolium salts with AuCl(SMe2) and BH3·THF, respectively. The BH3-mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.

Review of triazole scaffolds for treatment and diagnosis of Alzheimer's disease

Triazole scaffolds, a series of 5-membered heterocycles, are well known for their high efficacy, low toxicity, and superior pharmacokinetics. Alzheimer's disease (AD) is the first neurodegenerative disorder with complex pathological mechanisms. Triazole, as an aromatic group with three nitrogen atoms, forms polar and non-polar interactions with diverse key residues in the receptor-ligand binding procedure, and has been widely used in the molecular design in the development of anti-AD agents. Moreover, considering the simple synthesis approaches, triazole scaffolds are commonly used to link two pharmacodynamic groups in one chemical molecule, forming multi-target directed ligands (MTDLs). Furthermore, the click reaction between azide- and cyano-modified enzyme and ligand provides feasibility for the new modulator discovery, compound tissue distribution evaluation, enzyme localization, and pharmacological mechanism study, promoting the diagnosis of AD course.

Synthesis methods of 1,2,3-/1,2,4-triazoles: A review

Triazole, comprising three nitrogen atoms and two carbon atoms, is divided into two isomers 1,2,3-triazole and 1,2,4-triazole. Compounds containing a triazole are one of the significant heterocycles that exhibit broad biological activities, such as antimicrobial, analgesic, anti-inflammatory, anticonvulsant, antineoplastic, antimalarial, antiviral, antiproliferative, and anticancer activities. A great quantity of drugs with a triazole structure has been developed and proved, for example, ketoconazole and fluconazole. Given the importance of the triazole scaffold, its synthesis has attracted much attention. This review summarizes the synthetic methods of triazole compounds from various nitrogen sources in the past 20 years.

Copper-Promoted One-Pot Sandmeyer-Type Reaction for the Synthesis of N-Aryltriazoles

Herein, we report the copper-catalyzed one-pot Sandmeyer-type reaction of aromatic amines with triazoles to afford N-aryl-1,2,3-triazoles. Diazonium salts, formed from aromatic amines and tert-butyl nitrite in the presence of fluoroboric acid, reacted with triazoles in a copper-catalyzed Sandmeyer-type reaction. The reaction proceeded under mild conditions to afford N-aryltriazoles in moderate to good yields. This method is amenable to a wide range of aromatic amines and triazoles and shows diverse functional group tolerance. Inhibition of the reaction upon the addition of free radical scavengers suggested a radical pathway, in which the aryl radical, copper, and triazole formed a complex that underwent reductive elimination to give aryltriazole compounds; this is consistent with the mechanism underlying the Sandmeyer reaction. Thus, we demonstrate a new effective strategy for the construction of C-N bonds via Sandmeyer-type reactions and a valuable alternative approach for the synthesis of aryltriazole derivatives.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Mechanochemical Synthesis of Tripodal Tris[4-(1,2,3-triazol-5-ylidene)methyl]amine Mesoionic Carbene Ligands and Their Complexation with Silver(I)

The conjugate acids of 1,2,3-triazolylidene mesoionic carbenes can be prepared in a straightforward fashion by alkylation of 1-substituted 1,2,3-triazoles. However, this becomes a much more challenging proposition when other nucleophilic centers are present, which has curtailed the development of ligands containing multiple 1,2,3-triazolylidene donors. Herein, methylation of a series of tris[(1-aryl-1,2,3-triazol-4-yl)methyl]amines possessing both electron-rich and electron-deficient aromatic substituents, using Me₃OBF₄, is shown to proceed with much higher chemoselectivity under mechanochemical conditions than when conducted in solution. This provides a means to reliably access a series of tricationic tris[4-(1,2,3-triazolium)methyl]amines in good yields. DFT calculations suggest that a potential reason for this change in regioselectivity is the difference between the background dielectric of the DCM solution versus the solid state, which is predicted to have a large effect on the relative thermodynamic driving force for alkylation of the tertiary amine center versus the triazole rings. Homoleptic silver complexes of the triazolylidene ligands derived therefrom, of formulas [Ag₃(1a-d)₂](X)₃ (X^- = BF₄^-, TfO^-), have been isolated and fully characterized. In the case of the ligand bearing the smallest aryl substituents, 1b, argentophilic interactions yield a triangular Ag₃ core. The [Ag₃(1a-d)₂](X)₃ silver salts are viable agents for transmetalation to other transition metals, demonstrated here for cobalt. In the case of 1a, the complex [Co^II(1a)(NCMe)](OTf)₂ was obtained. Therein, the bulky mesityl substituents enforce a tetrahedral geometry, in which only the triazolylidene donors of 1a coordinate (i.e., it acts as a tridentate ligand). Transmetalation of the less sterically encumbered ligand 1b yields six-coordinate cobalt(III) complexes, [Co^III(1b)(Cl)(NCMe)](OTf)₂ and [Co^III(1b)(NCMe)₂](OTf)₃, in which the ligand coordinates in a tetradentate fashion. These are the first examples of tris(1,2,3-triazolylidene) ligands containing an additional coordinating heteroatom and, more generally, of tetradentate 1,2,3-triazolylidene ligands. Crucially, we believe that the divergent chemoselectivity under mechanochemical conditions (vs conventional solution-based chemistry) demonstrated herein offers a pathway by which other challenging synthetic targets, including further multidentate carbene ligands, can be prepared in superior yields.