NH Protons Research Articles

Fused Nickel(II) Porphyrins-Sensing of Toxic Anions and Selected Metal Ions Through Supramolecular Interactions.

Ni(II) porphyrins having fused –NH group were synthesized and characterized by various spectroscopic techniques. The fused porphyrins 1 and 2 were used to detect species of opposite polarity. 1 was used to sense toxic anions viz. cyanide and fluoride ions whereas 2 was used for detecting some selective metal ions including toxic mercury(II) ions. 1 is having acidic –NH proton, which detects anions via hydrogen bonding interactions followed by anion-induced deprotonation. On the other hand, 2 senses the metal species via weak charge transfer interactions from oxygen atom of the formyl group to the added metal ions. The limit of detection was calculated in case of 1 as 2.13 and 3.15 ppm for cyanide and fluoride ions, respectively. Similarly, the detection limit was found to be 0.930, 2.231, and 0.718 ppm for Cu(II), Fe(III), and Hg(II) ions, respectively, for probe 2. The probes were recovered and reused for several cycles.

Synthesis, characterization of vanillin based colorimetric chemosensor for sensing of fluoride ions

A novel colorimetric chemosensor (E)-4-((2-(2,4-dinitrophenyl)hydrazono)methyl)-2-methoxyphenol (VSB) was prepared, characterized using FT-IR, NMR, Mass spectrometry, UV-Visible spectroscopy and elemental analysis. VSB is documented as a colorimetric and radiometric chemosensor for particular detection of fluoride ions with the detection limit 6.1 × 10−6. The sensing mechanism of F− ion as a result of total deprotonation of OH and NH protons from VSB which is studied by UV–visible and 1H NMR titration examinations. The 1H NMR titration investigation was established the deprotonation of OH and NH protons by F− as a strong indication for fluoride ion detection. In addition, deprotonated VSB can be subjected to protonation using 0.001 M of trifluoroacetic acid as a protonating group.

Biocatalytic Strategy for Grafting Natural Lignin with Aniline.

This paper presents an enzyme biocatalytic method for grafting lignin (grafting bioprocess) with aniline, leading to an amino-derivatized polymeric product with modified properties (e.g., conductivity, acidity/basicity, thermostability and amino-functionalization). Peroxidase enzyme was used as a biocatalyst and H2O2 was used as an oxidation reagent, while the oxidative insertion of aniline into the lignin structure followed a radical mechanism specific for the peroxidase enzyme. The grafting bioprocess was tested in different configurations by varying the source of peroxidase, enzyme concentration and type of lignin. Its performance was evaluated in terms of aniline conversion calculated based on UV-vis analysis. The insertion of amine groups was checked by 1H-NMR technique, where NH protons were detected in the range of 5.01–4.99 ppm. The FTIR spectra, collected before and after the grafting bioprocess, gave evidence for the lignin modification. Finally, the abundance of grafted amine groups was correlated with the decrease of the free –OH groups (from 0.030 to 0.009 –OH groups/L for initial and grafted lignin, respectively). Additionally, the grafted lignin was characterized using conductivity measurements, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), temperature-programmed desorption (TPD-NH3/CO2) and scanning electron microscopy (SEM) analyses. The investigated properties of the developed lignopolymer demonstrated its disposability for specific industrial applications of derivatized lignin.

Asparagine and Glutamine Side-Chains and Ladders in HET-s(218-289) Amyloid Fibrils Studied by Fast Magic-Angle Spinning NMR.

Asparagine and glutamine side-chains can form hydrogen-bonded ladders which contribute significantly to the stability of amyloid fibrils. We show, using the example of HET-s(218–289) fibrils, that the primary amide side-chain proton resonances can be detected in cross-polarization based solid-state NMR spectra at fast magic-angle spinning (MAS). J-coupling based experiments offer the possibility to distinguish them from backbone amide groups if the spin-echo lifetimes are long enough, which turned out to be the case for the glutamine side-chains, but not for the asparagine side-chains forming asparagine ladders. We explore the sensitivity of NMR observables to asparagine ladder formation. One of the two possible asparagine ladders in HET-s(218–289), the one comprising N226 and N262, is assigned by proton-detected 3D experiments at fast MAS and significant de-shielding of one of the NH2 proton resonances indicative of hydrogen-bond formation is observed. Small rotating-frame 15N relaxation-rate constants point to rigidified asparagine side-chains in this ladder. The proton resonances are homogeneously broadened which could indicate chemical exchange, but is presently not fully understood. The second asparagine ladder (N243 and N279) in contrast remains more flexible.

Anion-sensitive photophysics of luminescent trimetallic complexes of Fe(II), Ru(II), and Os(II) with polarized NH motifs

Anion-induced alteration of the photophysical properties of luminescent homo- and heterotrimetallic complexes comprising of FeII, RuII and OsII metals were carried out in this work. The design strategy provides two imidazole NH protons in the secondary coordination sphere of all complexes. These imidazole protons turn into acidic upon coordination with the metal centers and capable of interacting with basic anions resulting in remarkable alteration of photophysics of the complexes. By exploiting significant alteration of their photophysical properties, anion sensing aspects of the complexes were addressed here in both organic and aqueous medium via multiple optical and spectroscopic tools. The triads act as multi-channel probes for F−, CN−, AcO− and H2PO4− in MeCN without much selectivity. By contrast, they behave as chemosensor for CN− and SCN− in water. The trinuclear Ru-Fe-Ru compound, although acts as a sensor in presence of small concentration of anions, breaks down in presence of excess anions thereby limiting its practical utility. Equilibrium constants related to probe-anion interaction event were estimated in order of 106 M−1 and detection limits came down to 10−9 M. Significant alteration of excited state lifetimes of the triads validate the significance as potential lifetime-based probes for specific anions. The outcomes of different spectroscopic studies suggest the occurrence of anion-induced dissociation of the imidazole NH protons in the complexes.

On the roles of calcium and zinc ions in the formation of a catalytically active form of the metalloenzyme, l-alanyl-d-glutamate peptidase of the bacteriophage T5 (EndoT5)

Structural consequences of the binding of metal ions (regulatory Ca2+ and catalytic Zn2+) to the metalloenzyme l-alanyl-d-glutamate peptidase of the bacteriophage T5 (Endo T5) and some of its analogues containing single amino acid substitutions in the active center were analyzed by nuclear magnetic resonance (NMR), circular dichroism (CD) and calorimetry. Analyses revealed that the native EndoT5 undergoes strong structural rearrangements as a result of Zn2+ binding. This structural rearrangement resulting in the formation of an active enzyme is completed by the Ca2+ binding. In this case, the NMR spectra uncover the tautomerism of the NH protons of histidine imidazoles responsible for the Zn2+ coordination. For the EndoT5 analogues with point substitutions in the Ca2+-binding site, similar conformational rearrangements are observed upon Zn2+ binding. However, no characteristic changes in the NMR spectra associated with the Ca2+ binding were detected. The roles of the proton exchange in the process of Ca2+-induced activation of the enzymatic activity of EndoT5 is discussed.

Влияние природы растворителя на самосборку монозамещённых пиллар[5]аренов, содержащих N-(аминоалкил)амидный фрагмент

Chemistry of macrocyclic compounds is one of the rapidly developing areas of modern organic chemistry. These macrocycles have become widespread due to the high availability of parents compounds, the possibility of synthesis of ligands based on them for highly selective recognition, ion-selective membranes, electrodes, sensors and nanocontainers for targeted drug delivery. Moreover, in the past few years, attention of researchers has again been riveted on mechanically interlocked molecules (rotaxanes and pseudorotaxans) and supramolecular polymers, which can be explained by their potential application as molecular machines and materials. Mechanically interlocked molecules are molecular architectures consisting from two or more components which mechanically bind due to their own topology. Supramolecular polymers are comprise ordered monomer units combined via non-covalent bonds (hydrogen bonds or electrostatic interactions). Thus, this manuscript presents an approach to the synthesis of monosubstituted pillar[5]arenes containing N-(aminoalkyl)amide fragments with various substituent lengths. The formation of self-inclusion complexes by synthesized macrocycles was established by one- and two-dimensional NMR spectroscopy. According to NMR data, only four carbon atoms of the alkyl fragment were included in the macrocyclic cavity regardless of the alkyl chain length. Formation of self-inclusion complexes becomes possible due to the intramolecular hydrogen bond between the NH protons and the oxygen atom of methoxyl fragment, which is confirmed by IR spectroscopy. It was shown by dynamic light scattering that the synthesized pillar[5]arenes in chloroform form aggregates with an average hydrodynamic diameter of 316-640 nm and polydispersity index from 0.18 to 0.20, while polydisperse systems are formed in dimethyl sulfoxide.

A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N,N’‐Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O2 ) to hydrogen peroxide (H2 O2 ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me2 P) and an N21,N'23-dimethylated porphyrin (syn-Me2 P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me2 P with higher turnover number (TON=250 for 30 min) than that by syn-Me2 P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me2 Iph or syn-Me2 Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O2 with isophlorins based on kinetic analysis. The ORR rate by anti-Me2 Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me2 Iph by external protons was observed. The different mechanisms in the O2 reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O2 with inner NH protons of the isophlorins.

Fluorescent 2-phenyl-2H-benzotriazole dyes with intramolecular N–H···N hydrogen bonding: Synthesis and modulation of fluorescence properties by proton-donating substituents

A series of 2-(2-phenyl)benzotriazole derivatives having amino, N-methyl amino, and acetamido groups on the ortho position of the phenyl group were synthesized to elucidate the effects of amino/amido substitutions on the molecular structures and photophysical properties. 1H NMR spectra and DFT calculations of the present dyes revealed the formation of N–H∙∙∙N intramolecular hydrogen bonds between benzotriazole and amino/amide groups in the ground state. Amino-functionalized dyes showed absorption bands that were assigned to HOMO-LUMO transition with a partial charge-transfer feature. The dye with an acetoamido group in which the acidity of NH proton was increased by the effect of electron-withdrawing acetyl groups exhibited absorption bands attributable to intramolecular hydrogen-bonded “closed” conformers in CHCl3 and non-hydrogen-bonded “open” conformers in DMSO. In comparison to 2-phenylbenzotriazole, which had no acidic proton involved in intramolecular hydrogen bonding, the present derivatives exhibited weak fluorescence emissions with quite low quantum yields, suggesting that non-radiative deactivation of ESIPT states also operated in the N–H∙∙∙N hydrogen bonded 2-phenylbenzotriazoles just as in O–H∙∙∙N intramolecular hydrogen-bonded 2-hydroxyphenylbenzotriazoles, which are known as UV absorbers. Taking a closer look at the fluorescence spectra, dyes with amino and N-methylamino groups showed faint but dual emissions from charge-transfer excited states in CHCl3 and blue-shifted single emission in DMSO. Fluorescence emissions of the acetamido-functionalized dye were unimodal in each solvent and stronger than those of amino-functionalized dyes. Thus, the fluorescence properties of these dyes depended on substitutions at amino groups. The luminescence of these derivatives was enhanced in the polymer matrix in which conformation changes and molecular motions were suppressed. Dye-doped polymer films have absorption bands in the UV range (360–400 nm) and emitted at the visible region (450–510 nm), and thus could be used as organic materials for light conversion films that convert UV light into visible light.

Thiosemicarbazone-based bifunctional chemosensors for simultaneous detection of inorganic cations and fluoride anion

Two thiosemicarbazone-based bifunctional chemosensors (L1 and L2) have been synthesized and characterized by various spectroscopic techniques. Sensing behavior of the chemosensors were initially tested by colorimetric signalling, followed by absorption, emission, NMR and CV techniques, and found to behave as bifunctional molecules. These bifunctional chemosensors recognize selected cations (Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) and F¯ anion among the studied cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Hg2+ and Pb2+) and anions (F¯, Cl¯, Br¯, I¯ and HSO4¯). Both the sensor molecules displayed remarkable colorimetric and fluorimetric responses in the presence of cations and fluoride anion with a rapid response through color change of the solution. Nitrogen and thione donors of the receptor molecules are liable for cation binding, whereas –NH protons of thiosemicarbazone moiety was found to be responsible for selective fluoride ion sensing. Binding ability of the chemosensors toward cations and F¯ anion were determined with spectrophotometric data by calibration plot, and the binding stoichiometry of the complexes was studied by Job’s plot. Electrochemical measurements showed that the addition of anion to the chemosensors pretreated with cations resulted in greater changes in reduction potentials compared to the addition of anion to the receptors in the absence of cations. Computational studies using density functional theory were also carried out, whereupon comparison of the interactions of copper cation and fluoride anion with the chemosensors explains the selective detection towards respective ions.

Pyrene based chemosensor for carbon dioxide gas – Meticulous investigations and digital image based RGB analysis

A new pyrene Schiff base (KB-4) was synthesized and demonstrated as simple colorimetric chemosensor for carbon dioxide (CO2) gas activated by fluoride anion. The chemical structure of KB-4 was validated through pivotal characterization methods. A combined quantum chemical calculations and spectroscopic techniques were adopted to understand the photophysical properties of KB-4. The sensitivity of KB-4 chemosensor towards various anions is investigated and showed excellent selectivity towards F− anion with a distinct color change. The mechanism of F− anion sensing is due to the complete deprotonation of –NH proton in KB-4 as revealed by 1H NMR titration studies as well as DFT calculations. Subsequently, the in-situ deprotonated KB-4 is employed for CO2 gas sensing. Interestingly, deprotonated KB-4 is responded instantaneously to CO2 gas by exhibiting significant colorimetric and fluorimetric changes with a detection limit of 0.4 ml. Further, digital image based RGB analysis using smartphone is introduced for detection of CO2 gas. The digital images of CO2 gas induced fluorimetric changes are captured using smartphone and the RGB analysis of the images are processed using MATLAB. The intensity variation in the color components as an indicative measure for detection of CO2 gas is demonstrated.

In vitro characterization of the serotonin biosynthesis pathway by CEST MRI.

The diagnosis of monoamine-related psychiatric disorders is based on the phenomenological evaluation of symptoms and behavior by trained clinicians. The CEST technique can be sensitive to monoamines such as serotonin. This study quantifies the CEST properties of the compounds in the serotonin biosynthesis pathway with the goal of developing noninvasive techniques aimed at advancing the diagnostic assessment of serotonin dysfunction. Saturation transfer-weighted images of L-tryptophan, 5-hydroxytryptophan, serotonin, 5-hydroxyindoleacetic acid, and melatonin phantoms were acquired over a range of saturation amplitudes and frequency offsets along with observed T1 , T2 , and B1 efficiency maps at physiological temperature and pH of 5.5, 6.7, and 7.4. The CEST and MT data were fitted to a three-pool Bloch-McConnell model of exchange to estimate the model parameters. At apH of 5.5, tryptophan, 5-hydroxytryptophan and serotonin exhibited significant CEST contrast at resonance frequency offset, Δω between 2.64 ppm and 2.71 ppm, and magnetization transfer ratio asymmetry amplitudes up to 20% per 30 mM. At apH of 7.4, all molecules exhibited significant CEST contrast between 5.11 ppm and 5.47 ppm, and magnetization transfer ratio asymmetry amplitudes up to 9.5%per 30 mM. At apH of 6.7, all studied compounds except melatonin exhibited a CEST peak from each of the preceding two pHs. At apH of 5.5, tryptophan, 5-hydroxytryptophan, and serotonin CEST contrast originates from the side chain, whereas at apH of 7.4, CEST contrast is due to the chemical exchange between water and the NH proton on the indole ring. The data in this study could be used to inform future investigations aimed at detecting and measuring in vivo serotonin.

Spectroscopic and TDDFT investigation of highly selective fluoride sensors by substituted acyl hydrazones.

In this work, we report the synthesis of two receptors for fluoride ions based on acyl hydrazone, such as N'-[(1Z)-1-(4-fluorophenyl)ethylidene]benzohydrazide (R1) and N'-[(1Z)-1-(2-hydroxyphenyl)ethylidene]benzohydrazide (R2). The receptors R1 and R2 were synthesized from the corresponding ketones and benzoic acid hydrazide and characterized spectroscopically by UV-visible, IR and 1HNMR techniques. The response of R1 and R2 towards different anions was studied colourimetrically in acetonitrile. The receptors exhibited a specific response towards fluoride ions. Further studies of 1:1 composition of receptors, R1/R2:fluoride ions by different spectroscopic techniques such as UV-Visible, IR and 1HNMR spectroscopy indicated the participation of -NH proton of the receptors in the sensing action through the hydrogen bonding. To understand the mechanism, Time-Dependent Density Functional Theory (TD-DFT) studies were done using the CAM-B3LYP/6311G++ (3df,2p) with Grimme's D3BJ empirical dispersion basis set. The studies supported the role of hydrogen bonding interaction of -NH and-OH protons of the receptors with the fluoride ions.

Facile Supramolecular Engineering of Porphyrin cis Tautomers: The Case of β-Octabromo-meso-tetraarylporphyrins.

A porphyrin cis tautomer, where the two central NH protons are on adjacent pyrrole rings, has long been invoked as an intermediate in porphyrin tautomerism. Only recently, however, has such a species been isolated and structurally characterized. Thus, single-crystal X-ray structure determinations of two highly saddled free-base porphyrins, β-heptakis(trifluoromethyl)-meso-tetrakis(p-fluorophenyl)porphyrin, H2[(CF3)7TFPP], and β-octaiodo-5,10,15,20-tetrakis(4′-trifluoromethylphenyl)porphyrin, H2[I8TCF3PP], unambiguously revealed cis tautomeric structures, each stabilized as a termolecular complex with a pair of ROH (R = CH3 or H) molecules that form hydrogen-bonded N–H···O–H···N straps connecting the central NH groups with the antipodal unprotonated nitrogens. The unusual substitution patterns of these two porphyrins, however, have left open the question how readily such supramolecular assemblies might be engineered, which prompted us to examine the much more synthetically accessible β-octabromo-meso-tetraphenylporphyrins. Herein, single-crystal X-ray structures were obtained for two such compounds, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4′-trifluoromethylphenyl)porphyrin, H2[Br8TCF3PP], and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4′-fluorophenyl)porphyrin, H2[Br8TFPP], and although the central hydrogens could not all be located unambiguously, the electron density could be convincingly modeled as porphyrin cis tautomers, existing in each case as a bis-methanol adduct. In addition, a perusal of the Cambridge Structural Database suggests that there may well be additional examples of porphyrin cis tautomers that have not been recognized as such. We are therefore increasingly confident that porphyrin cis tautomers are readily accessible via supramolecular engineering, involving the simple stratagem of crystallizing a strongly saddled porphyrin from a solvent system containing an amphiprotic species such as water or an alcohol.

Close Amide NH···F Hydrogen Bonding Interactions in 1,8-Disubstituted Naphthalenes

In this note, we present a series of N-(8-fluoronaphthalen-1-yl)benzamide derivatives designed to maximize amide-NH···F hydrogen bond interactions therein. A combination of IR and NMR spectroscopy indicates a linear correlation between the high energy shift in NH stretching frequency and the electron withdrawing nature of the substituent, consistent with the trend predicted by DFT calculations. Additionally, a limiting case of hydrogen bonding is observed when the benzamide derivatives are replaced with trifluoroacetamide, causing an additional red shift of 44 cm-1 in the NH stretching frequency. Most importantly, 1H-19F coupling constants in this series are among the largest measured for amide-NH···F interactions. X-ray crystallography reveals face-to-face alignment of naphthalene rings in these derivatives resulting in part from the NH···F hydrogen bonds. This motif also dictates the formation of sheets composed of stacked naphthalene rings in the crystal structure as opposed to unfluorinated analogues wherein NH···OC hydrogen-bonding interactions force benzamide and naphthalene rings to engage in T-shaped π-π interactions instead. Additionally, the NH proton in the trifluoroacetamide derivative engages in extended H-bond interactions in its crystal structure.

Rationally designed imidazole derivative as colorimetric and fluorometric sensor for selective, qualitative and quantitative cyanide ion detection in real time samples.

A new imidazole derivative of 1,2-diaminoanthraquinone and fluorene-2-carboxaldehyde was designed as a sensor B2 to selectively detect the cyanide (CN-) ion through colorimetric and/or fluorometric methods. The photochemical characterizations of sensor B2 were tested using absorption and emission spectral studies in CH3CN-H2O (8:2) semi-aqueous medium. An excited state proton transfer process (ESIPT) was proved by theoretical and spectral studies. The colorimetric and fluorescence detection limit of CN- ion was found to be 5.3×10-6M and 4.11×10-8M, respectively. 1H NMR titration, electrochemical and DFT studies were supported the removal of -NH proton from B2. In order to utilize this sensor in real-time applications, we developed a test cassette which is coated with sensor B2 detected the presence of CN- ion in the food sample with endogenous cyanide ion.

Site-Selective N-Methylation of 5,15-Diazaporphyrins: Reactive Cationic Porphyrinoids that Provide Isoporphyrin Analogues.

N-Alkylation significantly changes the electronic and optical properties, as well as the reactivity of nitrogen-containing π-conjugated molecules. In this study, it is found that treating 5,15-diazaporphyrins with methyl triflate selectively affords the corresponding N-methyl-5,15-diazaporphyrinium cations in good yield. N-Methylation substantially alters the electronic properties and reactivity of diazaporphyrins. The electron-accepting properties of the N-methyl-5,15-diazaporphyrinium cations are enhanced due to their lowered LUMO level. Stabilization of the LUMO energy enables regio- and stereoselective Diels-Alder reactions of the cationic diazaporphyrin with cyclopentadiene. N-Methylation also enhances the acidity of the inner NH protons, and thus, allows facile deprotonation to provide nitrogen-substituted isoporphyrin analogues with only one NH group in the central cavity.

Experimental and theoretical exploration of photophysics and trans-cis photoisomerization of styrylbenzene conjugated terpyridine complexes of Ru(II): Strong effect of deprotonation from second coordination sphere

We report herein synthesis, characterization, photophysics, and photo-isomerization behaviors of a new class of heteroleptic Ru(II) complexes incorporating styrylbenzene conjugated terpyridine and 2,6–bis(benzimidazole–2–yl)pyridine ligands. All the complexes display moderately strong luminescence at room temperature with appreciably long lifetime varying in the range between 7.2 and 62.2 ns, depending upon the electronic nature of substituent as well as the solvent. The styrylbenzene moiety induces trans to cis isomerization resulting in remarkable changes in absorption and both steady state and time-resolved luminescence spectral characteristics upon irradiating with UV light. Reversal of cis to trans isomerization were also achieved on interacting with visible light as well as UV light of specific wavelength. Removal of two benzimidazole NH protons from the secondary coordination sphere of the complexes were made possible upon treatment with strong base accompanying with substantial modulation of their absorption and emission spectral properties. Interestingly, rate constant as well as quantum yield of photoisomerization dramatically increases upon deprotonation of the imidazole NH protons. Computational studies employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were also executed for understanding the electronic charge distribution within the complex backbone and appropriate assignment of the observed spectral bands. In addition, observed spectral trend upon NH deprotonation as well as deprotonation-induced photoisomerization processes were also compared with the calculated results.

Hetero Diels-Alder Reactions of Masked Dienes Containing Heavy Group 15 Elements.

Treatment of N,C,N-chelated organopnictogen(I) compounds ArE (1-3) (Ar=2,6-(RN=CH)2 C6 H3 , E/R=As/Dmp (1), Sb/tBu (2), and Bi/tBu (3), where Dmp=2,6-Me2 C6 H3 ) with various electron-deficient alkynes RC≡CR' (R/R'=CO2 Me (DMAD), R/R'=H/CO2 Me, or R/R'=NC5 F4 ) affords different types of heterocyclic compounds. In these reactions, 1-3 act as hidden dienes and participate in hetero-Diels-Alder (DA) reactions, a feature that has been only rarely reported for heavier pnictogen compounds. In this way, reactions of 1 furnished the set of 1-arsanaphthalenes 4-6. The most likely mechanism involves two steps, that is, an arsa-DA reaction giving a 1-arsa-1,4-dihydro-iminonaphthalene, followed by CH→NH proton migration. In contrast, this reaction sequence terminates at the first step in the case of the antimony analogue 2, thus giving the 1-stiba-1,4-dihydro-iminonaphthalenes 7 and 8. Reactions of the bismuth congener 3 gave one of two products depending on the alkyne used. Reaction of 3 with DMAD gave 1-bisma-1,4-dihydro-iminonaphthalene 9, whilst its reaction with two equivalents of HC≡C(CO2 Me) gave bismacyclohexadiene (10). All compounds have been characterized by NMR, IR, and Raman spectroscopies and X-ray diffraction analysis. The experimental data were complemented by a computational study, including a description of the reaction profiles of the hetero-DA reactions and an assessment of the aromaticities of the heterocyclic naphthalene derivatives.

Self-Assembled Cagelike Receptor That Binds Biologically Relevant Dicarboxylic Acids via Proton-Coupled Anion Recognition.

We report here a fully organic, self-assembled dimeric receptor, constructed from acyclic naphthyridyl-polypyrrolic building blocks. The cagelike dimer is stable in the solid state, in solution, and in gas phase, as inferred from X-ray diffraction and spectroscopic analyses. This system acts as a receptor for oxalic acid, maleic acid, and malonic acid in the solid state and in THF solution. In contrast, acetic acid, propionic acid, adipic acid, and succinic acid, with pKa values > ca. 2.8, were not bound effectively within the cagelike cavity. It is speculated that oxalic acid, maleic acid, and malonic acid serve to protonate the naphthyridine moieties of the host, which then favors binding of the corresponding carboxylate anions via hydrogen-bonding to the pyrrolic NH protons. The present naphthyridine-polypyrrole dimer is stable under acidic conditions, including in the presence of 100 equiv trifluoroacetic acid (TFA), p-toluenesulfonic acid (PTSA), H2SO4, and HCl. However, disassembly may be achieved by exposure to tetrabutylammonium fluoride (TBAF). Washing with water then regenerates the cage. This process of assembly and disassembly could be repeated >20 times with little evidence of degradation. The reversible nature of the present system, coupled with its dicarboxylic acid recognition features, leads us to suggest it could have a role to play in effecting the controlled "capture" and "release" of biologically relevant dicarboxylic acids.