Ermitas Alcalde

Imidazolium ionic liquids: A simple anion exchange protocol

An efficient and simple protocol was developed to obtain quantitative iodide or bromide exchange for a broad range of anions in imidazolium ionic liquids. Selected anions were loaded in an anion exchange resin using two different procedures and were then used to provide a pure convenient ion pair.

Indene-based scaffolds. 2. An indole-indene switch: discovery of novel indenylsulfonamides as 5-HT6 serotonin receptor agonists.

Scaffold selection involving an indole-to-indene core change led to the discovery of a series of indenylsulfonamides that act as 5-HT6 serotonin receptor agonists. The variety of the targeted ligands and their synthetic complexity required multistep synthetic approaches. The novel indenylsulfonamides exhibited variable binding affinities for the 5-HT6 receptor, and the in vitro primary binding profiles of the preferred compounds revealed them to be 5-HT6 receptor agonists with Ki values > or =4.5 nM. The structural changes responsible for enhancing the affinities indicated a directing effect modulated by the nature of the indene core, the substitution at the aminoethyl side chain, and especially by the aryl(heteroaryl)sulfonyl group on the indene 5-position. A representative of the family, the N-(inden-5-yl)imidazothiazole-5-sulfonamide (43), exhibited a high affinity and functioned as a potent full agonist for the 5-HT6 receptor (Ki = 4.5 nM, EC50 = 0.9 nM, Emax = 98%).

Bis(imidazolium)−Calix[4]arene Receptors for Anion Binding†

Efficient access to the bis(imidazolyl)calixarene 2 and dicationic bis(imidazolium) salts 1a,b x 2X directly bonded to the upper rim of calixarene structure has been reported. Anion binding properties of the new receptors were studied by 1H NMR spectroscopic methods. Bis(N-butylimidazolium) dication 1a exhibited the best recognition properties toward carboxylate anions with a 1:1 receptor-anion binding stoichiometry, whereas the presence of a bulky group such as isopropyl (1b) increased the difficulty of both imidazolium moieties to be able to support the association with the same single anion.

Heterocyclic Betaines: Pyridinium (Imidazolium) Azolate Inner Salts with Several Interannular Linkages1

Publisher Summary This chapter illustrates an ensemble of highly dipolar heterocyclic compounds within heterocyclic betaines, molecules with a betaine character and their crucial immediate precursors. It is noted that in heterocyclic betaines the positive charge is located on a pyridinium ring and the negative charge on an azolium ring. Both fundamental and practical interests in heterocyclic betaines are mainly due to their dipolar character that has a dominant influence on their chemistry. Both the classification and generation of heterocyclic systems, essentially brings heterocyclic compounds into relation with aromatic ones. Heterocycles are related to aromatic compounds in two simple ways— namely, by replacing sp 2 carbon atom by a pyridine-like nitrogen atom phenanthrene leads to phenanthridine, or by replacing two adjacent sp 2 carbons atoms and an aromatic C (sp 2 )–C (sp 2 ) bond by a heteroatom. The concept of aromaticity for heterocyclic compounds has been the subject of extensive research. It is also observed that if the reference compounds are unusual structures, such as sesquifulvalene and its vinylogues, the opportunities for developing the new compounds can be great.

A direct synthetic approach to novel quadrupolar [14] azolophanes

Abstract A convergent ‘3+1’ synthesis allowed a simple entrance to the first examples of [1 4 ] meta azolophanes 1 and [1 4 ]( meta-ortho ) 2 azolophanes 2 built up from heterocyclic betaine subunits, illustrating a prototype of phanes constructed by both highly π-excessive and highly π-deficient heteroaromatic moieties linked in a 1,3-alternating fashion.

Imidazolium-Based Dicationic Cyclophanes. Solid-State Aggregates with Unconventional (C–H)+···Cl− Hydrogen Bonding Revealed by X-ray Diffraction

The first single-crystal X-ray crystallographic diffraction analysis of a dicationic heterophane showed a non-classic (C–H)+···Cl− hydrogen bond between the imidazolium rings and halide anions and the formation of unconventional charged assisted hydrogen bonds, which were the non-covalent forces driving the anion interactions shown by the dications 4·2X. Here is reported the halide-templated controlled synthesis and chemical response in basic media of 4·2X. Their structural properties were examined at the gas phase by electrospray ionization mass spectrometry in the negative-ion mode and in the solid-state by X-ray crystallography. Thus, the negative-ion ESI-MS response showed that the formation of non-covalent self-aggregates of macrocyclic dications is a consequence of hydrogen-bonded complexes with halide anions. Notably, X-ray diffraction of dication 4a·2Cl·2H2O provides evidence for the H-bonding network, which has a crucial role in crystal packing. The solid-state aggregates showed that chloride anions and water molecules formed channels among dications 4a+.

Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.

A simple halide-to-anion exchange method for heteroaromatic salts and ionic liquids.

A broad and simple method permitted halide ions in quaternary heteroaromatic and ammonium salts to be exchanged for a variety of anions using an anion exchange resin (A− form) in non-aqueous media. The anion loading of the AER (OH− form) was examined using two different anion sources, acids or ammonium salts, and changing the polarity of the solvents. The AER (A− form) method in organic solvents was then applied to several quaternary heteroaromatic salts and ILs, and the anion exchange proceeded in excellent to quantitative yields, concomitantly removing halide impurities. Relying on the hydrophobicity of the targeted ion pair for the counteranion swap, organic solvents with variable polarity were used, such as CH3OH, CH3CN and the dipolar nonhydroxylic solvent mixture CH3CN:CH2Cl2 (3:7) and the anion exchange was equally successful with both lipophilic cations and anions.

Hydrogen bonded driven anion binding by dicationic [14]imidazoliophanes

Imidazolium units represent the main structural motifs for the formation of unconventional C–H···Cl– hydrogen bonds which become the noncovalent forces driving the anion interactions exibited by dicationic [14]imidazoliophanes in the solid state, while C–H···O hydrogen bonds with water play a crucial role in governing the conformation of the solid–state aggregates, as well as in solution, wherein NMR studies also reveal the importance of hydrogen bonds in controlling the tendency to anion binding.

Non-classical [14]metaheterophanes containing betaine units. Synthesis, NMR spectroscopy and X-ray crystallography

The N-diphenylmethyl derivative 4 is the immediate precursor of the N-substituted dipolar [14]metaheterophane 3, and allows the convenient synthesis of the unsubstituted macrocycle 5, which can be transformed into the quadrupolar or dipolar heterophanes 1 and 2; the molecular structures of 1 and 2 were confirmed by single-crystal X-ray analysis.