An alternative synthesis of benzobis(imidazolium) salts via a ‘one-pot’ cyclization/oxidation reaction sequence

Cyclization of N-aryl and N-alkyl 2,5-diamino-1,4-benzoquinonediimines using paraformaldehyde under acidic conditions followed by oxidation with catalytic amounts of Pd(OAc)₂ afforded their respective benzobis(imidazolium) salts in yields of 48-98%. A comparative solid-state study between a 2,5-diamino-1,4-benzoquinonediimine and its corresponding benzobis(imidazolium) dichloride was also performed.     

Phase-tunable fluorophores based upon benzobis(imidazolium) salts

A new series of highly photoluminescent benzobis(imidazolium) salts with tunable electronic and physical properties is described. Systematic structural manipulations provided a set of materials that were not only fluorescent in the condensed state but also displayed tunable glass transition temperatures ranging from -0.3 to 113 degrees C. Upon heating, these robust materials flowed while maintaining their emissive properties. Further phase tuning was demonstrated through the synthesis of two thermotropic liquid-crystalline fluorescent benzobis(imidazolium) salts.     

1,3-Di(Ferrocenylmethyl)Imidazolium and 1-Ferrocenylmethyl-3-Alkylimidazolium Salts: A High Yield and Facile Synthesis

We have developed a rapid facile synthesis of 1-ferrocenyl-3-alkyl and 1,3-di(ferrocenylmethyl)imidazolium salts. The imidazolium salts are formed in excellent yields.     

Modular fluorescent benzobis(imidazolium) salts: syntheses, photophysical analyses, and applications

A series of benzobis(imidazolium) (BBI) salts has been prepared and studied as a new class of versatile fluorescent materials. Using a high yielding, modular synthetic strategy, BBI salts with a range of functionality poised for investigating fundamental and applications-oriented characteristics, including emission wavelength tunability, solvatochromism, red-edge excitation, chemical stability, multiphoton excitation, and protein conjugation, were prepared in overall yields of 40-97%. Through structural variation, the BBIs exhibited lambda(em) ranging between 329 and 561 nm while displaying phi(f)s up to 0.91. In addition, the emission characteristics of these salts were found to exhibit strong solvent dependencies with Stokes shifts ranging from 4570 to 13 793 cm(-1), depending on the nature of the BBI core. Although red-edge effects for BBI salts with Br and BF4 counterions were found to be similar, unique characteristics were displayed by an analogue with MeSO4 anions. The stability of an amphiphilic BBI was quantified in aqueous solutions of varying pH, and >85% of the emission intensity was retained after 2 h at pH 3-9. Through multiphoton excitation experiments in aqueous solutions, a BBI salt was found to exhibit three-photon fluorescence action cross sections similar to serotonin. The application of BBI salts as fluorescent protein tags was demonstrated by conjugating bovine serum albumin to a maleimide-functionalized derivative.     

Synthesis of bis(polyfluoroalkylated)imidazolium salts as key intermediates for fluorous NHC ligands

1,3-Bis(polyfluoroalkyl)- and 1-mesityl-3-(polyfluoroalkyl)imidazolium salts differing in the length of a polyfluorinated chain and a non-fluorinated spacer were synthesized as key building blocks for fluorous NHC (N-heterocyclic carbene) ligands. A new approach using polyfluoroalkyl triflates instead of the corresponding iodides was employed allowing fine tailoring of fluorous properties, as well as of the electron density of the imidazolium ring. Using bis(polyfluoroalkylated)imidazolium salt, a fluorous analogue of the PEPPSI™ catalyst was synthesized and its structure confirmed by X-ray diffraction. The catalyst was employed in model Heck and Suzuki couplings with moderate yields, however, its recycle was not successful. Fluorophilicity of bis(polyfluoroalkylated)imidazolium salts was found to be surprisingly low compared with the analogous perfluoropolyether-based salts.     

Parallel synthesis of tri- and tetrasubstituted ureas from carbamoyl imidazolium salts

A method for producing tri- and tetrasubstituted ureas from carbamoyl imidazolium salts is presented. Carbamoyl imidazolium salts are prepared from the reaction of N,N carbonyldiimidazole (CDI) with secondary amines, followed by alkylation with iodomethane. These stable salts can be stored for extended periods and are effective electrophilic carbamoylation reagents. Primary and secondary amines add to carbamoyl imidazolium salts at room temperature to give tri- and tetrasubstituted ureas in excellent yields. This reaction was used to synthesize ureas using both liquid-liquid extraction and solid-phase extraction (cation exchange) purification techniques. Liquid-liquid extraction affords the product ureas more cleanly than cationic exchange. A series of urea compounds were synthesized using parallel synthesis techniques in high yields and with suitable purity for routine in vitro biological tests. These studies validate the utility of carbamoyl imidazolium salts as useful building blocks for combinatorial library synthesis.     

Novel (N‐heterocyclic carbene)Pd(pyridine)Br2 complexes for carbonylative Sonogashira coupling reactions: Catalytic efficiency and scope for arylalkynes,alkylalkynes and dialkynes

New N,N′‐substituted imidazolium salts and their corresponding dibromidopyridine–palladium(II) complexes were successfully synthesized and characterized. Reactions of palladium bromide with the newly synthesized N,N′‐substituted imidazolium bromides ( 2a and 2b ) in pyridine afforded the corresponding new N‐heterocyclic carbene pyridine palladium(II) complexes ( 3a and 3b ) in high yields. Their single‐crystal X‐ray structures show a distorted square planar geometry with the carbene and pyridine ligands in trans position. Both complexes show a high catalytic activity in carbonylative Sonogashira coupling reactions of aryl iodides and aryl diiodides with arylalkynes, alkylalkynes and dialkynes.     

Rh-N-heterocyclic carbene (NHC) complex-catalyzed addition of phenylboronic acid to N-sulfonyl and N-phosphinoyl aldimines

Rh-N-heterocyclic carbene (NHC) complexes were generated in situ from imidazolium salts, [RhCl(cod)](2) and t-BuOK in dioxane. In the presence of a catalytic amount of Rh-NHC complexes, the addition reaction of phenylboronic acid to N-sulfonylarylimines and N-phosphinyolarylimines gave the corresponding amines in high yields.     

Coumarin‐tethered (benz)imidazolium salts and their silver(I) N‐heterocyclic carbene complexes: Synthesis,characterization, crystal structure and antibacterial studies

A series of new sterically modulated chlorocoumarin‐substituted (benz)imidazolium salts and their bis‐N‐heterocyclic carbene silver(I) complexes were prepared and characterized. The complexes were prepared in good yields following the in situ deprotonation method by treating azolium salts with silver(I) oxide in the dark. All the compounds were characterized using various spectroscopic and analytical methods. Additionally, one of the benzimidazolium salts was characterized using single‐crystal X‐ray diffraction technique. In this salt, intermolecular π–π stacking interactions operate between benzimidazole as well as coumarin heterocyclic systems with adjacent molecules. In the preliminary antibacterial studies, the silver complexes were found more active than the corresponding salts against a panel of bacterial strains. Interestingly, the complexes displayed improved antibacterial efficacy against Escherichia coli strain, comparable with that of the standard drug ampicillin.     

A new synthesis of 6-(alkyl)amino-3-aryl(alkyl)-1,2,4,5-tetrazines

A new method for the preparation of 6-(alkyl)amino-3-aryl(alkyl)-1,2,4,5-tetrazines is described. Dissolving 3-aryl(alkyl)-1,2,4,5-tetrazines in liquid ammonia or a primary aliphatic amine at ?35° to ?40°, followed by addition of potassium permanganate gives the title compounds in reasonable to excellent yields.     

N-heterocyclic carbene ligands bearing hydrophilic and/or hydrophobic chains: Rh(I) and Pd(II) complexes and their catalytic activity

A series of novel imidazolium salts bearing hydrophilic tetraethylene glycol (TEG) and/or hydrophobic long-chain alkyl (n-C12) functionalities, which are precursors for desired N-heterocyclic carbene (NHC) ligands, were synthesized and characterized. Rh(I)-NHC complexes were prepared in good yields by the silver carbene transfer method with NHC-Ag species derived from the imidazolium salts. The molecular structure of the Rh(I)-NHC complex having n-C12 chains has been determined by a single-crystal X-ray diffraction study and the complex is found to possess extended alkyl chains with anti conformations in the solid state. Hydrosilylation with the Rh(I) complexes and Suzuki-Miyaura coupling reactions with the Pd(II) complexes with these NHC ligands were carried out. In the latter case, the TEG moiety enhances catalytic activity considerably.     

An investigation of the catalytic potential of mono- and dicationic imidazolium N-heterocyclic carbenes in the benzoin condensation

The catalytic potential of imidazolium salts in the benzoin condensation was investigated. Various aromatic aldehydes were tested in the benzoin condensation under the optimised protocol to afford α-hydroxyketones using N-heterocyclic carbenes derived from mono- and dicationic imidazolium salts. The products were obtained in good yields within short reaction times. Dicationic imidazolium salts with a long aliphatic chain between the imidazole rings were found to be more effective pre-catalysts for the benzoin condensation in comparison to the corresponding monocationic salts having the same aliphatic chain length.     

Benzobis(imidazolium)-cucurbit[8]uril complexes for binding and sensing aromatic compounds in aqueous solution

The utilities of benzobis(imidazolium) salts (BBIs) as stable and fluorescent components of supramolecular assemblies involving the macrocyclic host, cucurbit[8]uril (CB[8]), are described. CB[8] has the unusual ability to bind tightly and selectively to two different guests in aqueous media, typically methyl viologen (MV) as the first guest, followed by an indole, naphthalene, or catechol-containing second guest. Based on similar size, shape, and charge, tetramethyl benzobis(imidazolium) (MBBI) was identified as a potential alternative to MV that would increase the repertoire of guests for cucurbit[8]uril. Isothermal titration calorimetry (ITC) studies showed that MBBI binds to CB[8] in a 1:1 ratio with an equilibrium association constant (K(a)) value of 5.7×10(5) M(-1), and that the resulting MBBI·CB[8] complex binds to a series of aromatic second guests with K(a) values ranging from 10(3) to 10(5) M(-1). These complexation phenomena were supported by mass spectrometry, which confirmed complex formation, and a series of NMR studies that showed the expected upfield perturbation of aromatic peaks and of the MBBI methyl peaks. Surprisingly, the binding behavior of MBBI is strikingly similar to that of MV, and yet MBBI offers a number of substantial advantages for many applications, including intrinsic fluorescence, high chemical stability, and broad synthetic tunability. Indeed, the intense fluorescence emission of the MBBI·CB[8] complex was quenched upon binding to the second guests, thus demonstrating the utility of MBBI as a component for optical sensing. Building on these favorable properties, the MBBI·CB[8] system was successfully applied to the sequence-selective recognition of peptides as well as the controlled disassembly of polymer aggregates in water. These results broaden the available guests for the cucurbit[n]uril family and demonstrate potentially new applications.     

Synthesis and anticancer properties of gold(I) and silver(I) N-heterocyclic carbene complexes

Bis(1,3-dimethylimidazol-2-ylidene)silver(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)silver(I) nitrate were prepared by reacting the corresponding imidazolium nitrate salts with silver oxide. Bis(1,3-dimethylimidazol-2-ylidene)gold(I) nitrate and bis(4,5-dichloro-1,3-dimethylimidazol-2-ylidene)gold(I) nitrate salts were prepared via transmetallation of their silver precursors with chloro dimethylsulfide gold. The anticancer properties were determined using NCI-H460 lung cancer cells. Efficacy was established by comparison of the gold and silver compounds with cisplatin.     

A prototype of benzobis(imidazolium)-embedded conjugated polyelectrolyte: Synthesis by direct C‒H arylation and fluorescent responses to anions

We report the convenient synthesis of a benzobis(imidazolium)-embedded conjugated polyelectrolyte pBBI by a Cu-catalyzed direct C?H arylation of a cationic benzobis(imidazolium) monomer with a diiodide comonomer. pBBI shows weak fluorescence in solution due to rotation of the repeat units in the conjugated backbone, and enhanced fluorescence when electrostatically interacting with a variety of anions to form aggregates. Specially, pBBI responds to the bisulfite anion with intensified unique deep-blue fluorescence easily discriminated by naked eye.     

Multicomponent Synthesis of Unsymmetrical Unsaturated N‐Heterocyclic Carbene Precursors and Their Related Transition‐Metal Complexes

A low‐cost, modular, and easily scalable multicomponent procedure affording access in good yields and excellent selectivity (up to 93 %) to a wide range of (a)chiral unsymmetrical 1‐aryl‐3‐cycloalkyl‐imidazolium salts is disclosed. Electronic and steric properties of the corresponding unsymmetrical unsaturated N‐heterocyclic carbene (U₂‐NHC) ligands were evaluated and evidenced strong electron donor ability, high steric discrimination, and modular steric demand.     

Planar chiral imidazolium salts based on [2.2]paracyclophane in the asymmetric rhodium-catalyzed 1,2-addition of arylboronic acids to aldehydes

A series of planar chiral imidazolium salts derived from [2.2]paracyclophane have been synthesized and characterized. By using these imidazolium salts as carbene precursors, the Rh-catalyzed 1,2-addition of arylboronic acids to aldehydes proceeded readily with low catalyst loadings (0.03–0.3 mol %) and gave a variety of chiral diarylmethanols in excellent yields and moderate enantioselectivities.     

Continuous Synthesis of Peralkylated Imidazoles and their Transformation into Ionic Liquids with Improved (Electro)Chemical Stabilities

A versatile and efficient method to synthesize tetrasubstituted imidazoles via a one‐pot modified Debus–Radziszewski reaction and their subsequent transformation into the corresponding imidazolium ionic liquids is reported. The tetrasubstituted imidazoles were also synthesized by means of a continuous flow process. This straightforward synthetic procedure allows for a fast and selective synthesis of tetrasubstituted imidazoles on a large scale. The completely substituted imidazolium dicyanamide and bis(trifluoromethylsulfonyl)imide salts were obtained via a metathesis reaction of the imidazolium iodide salts. The melting points and viscosities are of the same order of magnitude as for their non‐substituted analogues. In addition to the superior chemical stability of these novel ionic liquids, which allows them to be applied in strong alkaline media, the improved thermal and electrochemical stabilities of these compounds compared with conventional imidazolium ionic liquids is also demonstrated by thermogravimetrical analysis (TGA) and cyclic voltammetry (CV). Although increased substitution of the ionic liquids does not further increase thermal stability, a definite increase in cathodic stability is observable.     

Synthesis of di(Imdazolium) and di(Pyrazolium) Salts as Precursors for N-heterocyclic Dicarbene Complexes

Alpha,omega-bis(pyrazol-1-yl)alkanes and alpha,omega-bis(imidazol-1-yl)alkanes with spacers consisting of four to ten methylene groups have been prepared from pyrazole, 3,5-dimethylpyrazole or imidazole and corresponding dibromoalkanes in a superbasic medium KOH-DMSO. The proposed method of synthesis allowed the preparation of new flexible bidentate ligands without the need to use toxic solvents and tedious workup procedures. Bis(pyrazol-1-yl)alkanes were further functionalized for their use as precursors for “non-classical” mesoionic N-heterocyclic carbene ligands. One the first step, iodine atoms were introduced to positions 4 of pyrazole rings by oxidative iodination using I₂-HIO₃ system. On the next step, nitrogen atoms in positions 2 of pyrazole rings were alkylated using several agents. Reaction with methyliodide unexpectedly led to the formation of only mono-alkylated products even after 7 days of refluxing in a neat alkyliodide. Methylation by trimethyloxonium tetrafluoroborate or methyltriflate led to dimethylated products in high yields. Bis(imidazol-1-yl)alkanes were easily alkylated by methyliodide to give di(imidazolium) salts – precursors to “classic” N-heterocyclic dicarbenes.     

Cross—Coupling of Aryl Iodides with Malononitrile Catalyzed by Palladium N—Heterocyclic Carbene Complex System

Eight N-heterocyclic carbenes (NHC),generated in situ from their imidazolium salts,as ligands of palladium complexes were used for the catalytic coupling of iodobenzene with malononi-trile anion,It was found that 1,3-bis(2,4,6-trimethylphenyl)-imidazolium chloride (IMesHCl)-Pd2(dba)3 catalytic system has the highest activity to obtain phenyl malononitrile among the imidazolium salts.The substituted iodoarenes reacted with malononitrile anions by using the catalytic system to give cross-coupling products in yields from 50% to 96%.