Imino Bond Research Articles

A comprehensive investigation into the synthesis, characterization, and photocatalytic performance of modified graphene oxide via imino bond with ferrocene as a novel photocatalyst for thioamide synthesis

The study introduces a novel approach involving the amination of graphene oxide (GO) followed by the reaction with ferrocene–aldehyde, resulting in the formation of GO–NFc.

New Smart Bioactive and Biomimetic Chitosan-Based Hydrogels for Wounds Care Management.

Wound management represents a continuous challenge for health systems worldwide, considering the growing incidence of wound-related comorbidities, such as diabetes, high blood pressure, obesity, and autoimmune diseases. In this context, hydrogels are considered viable options since they mimic the skin structure and promote autolysis and growth factor synthesis. Unfortunately, hydrogels are associated with several drawbacks, such as low mechanical strength and the potential toxicity of byproducts released after crosslinking reactions. To overcome these aspects, in this study new smart chitosan (CS)-based hydrogels were developed, using oxidized chitosan (oxCS) and hyaluronic acid (oxHA) as nontoxic crosslinkers. Three active product ingredients (APIs) (fusidic acid, allantoin, and coenzyme Q10), with proven biological effects, were considered for inclusion in the 3D polymer matrix. Therefore, six API-CS-oxCS/oxHA hydrogels were obtained. The presence of dynamic imino bonds in the hydrogels' structure, which supports their self-healing and self-adapting properties, was confirmed by spectral methods. The hydrogels were characterized by SEM, swelling degree, pH, and the internal organization of the 3D matrix was studied by rheological behavior. Moreover, the cytotoxicity degree and the antimicrobial effects were also investigated. In conclusion, the developed API-CS-oxCS/oxHA hydrogels have real potential as smart materials in wound management, based on their self-healing and self-adapting properties, as well as on the benefits of APIs.

Structural modification of phenoxyimine titanium complexes and activation studies with alkylaluminum compounds

AbstractThe synthesis and characterization of three complexes of the type (N‐(3‐tert‐butylsalicylidene)‐R)2TiCl2 [R=2,6‐difluorophenyl 1, R=2,6‐dimethylphenyl 2, R=phenyl 3] is reported and compared with the highly active R=2,3,4,5,6,–pentafluorophenyl 4. The complexes were tested for ethylene polymerization when activated with different co‐catalysts, giving high catalytic activity when activated with methylaluminoxane for complexes 3 and 4. Complex 3 is the only catalyst to be inactive when activated with diethylaluminum chloride. Unexpectedly, for complexes 1 and 2, no catalytic activity is recorded when butylated hydroxytoluene in methylaluminoxane is used to remove the trimethylaluminum, while the catalytic activity for complex 3 is retained. NMR study on the activation of the four complexes in the presence of common alkyl aluminum co‐catalysts suggests the formation of Ti(III) species in the presence of tri‐iso‐butylaluminum, and also the formation of NR−CH2 (R: −Al(CH2CH(CH3)2)2) bond from the N=C−H bond of the ligand explaining the absence of catalytic activity. Density functional theory calculations using B3LYP and a combination of basis sets (6‐31+G and cc‐pVTZ) were performed on the catalyst precursors and the activated species showing insights into the frontier orbitals, localizing the LUMO on the imino bond elucidating their reactivity. Mulliken charges are also reported and an unexpected relationship between the nitrogen on the ligands and the molecular weight is observed.

Macrocyclic Iminopeptides Diversify To Better Target Proteins.

Among the many methods available for accessing conformationally diverse cyclic peptides, the derivatization of macrocyclic iminopeptides has remained notably underexplored. Now, a relevant complexity-generating method expands the repertoire of synthetic strategies exploiting the reactivity of an imino bond embedded in the cyclic peptide skeleton. Here we highlight a recent report describing the on-resin construction of a new family of macrocyclic peptide/natural product-inspired hybrids, namely "PepNats", by derivatization of cyclic iminopeptides through 1,3-cycloaddition reactions. A proof-of-concept with PepNats bearing peptide sequences that mimic protein hot loops demonstrated the potential of this strategy to create novel macrocyclic peptide ligands capable of modulating protein-protein interactions.

The mononuclear MoO2 2+ complexes with stereochemically rigid 4-aminobenzhydrazide based ligands

A range of dioxidomolybdenum(VI) complexes has displayed very high catalytic activities and exquisite selectivity. The dioxidomolybdenum(VI) mononuclear complexes with 4-aminobenzoylhydrazone ligands (salicylaldehyde (H2L1), 3- methoxysalicylaldehyde (H2L2), or 4- methoxysalicylaldehyde 4-aminobenzoylhydrazone (H2L3) being of the type cis-[MoO2L1-3(D)] ; D = MeOH, EtOH, H2O, dmf] have been prepared and structurally characterized. The crystal structure analysis of mononuclear complexes (Fig.1.) reveals distorted octahedral Mo(VI) coordination by ONO donor atoms from dianionic tridentate 4- aminobenzoylhydrazone ligands, two oxido oxygen atoms from the MoO22+ moiety and the oxygen atom from coordinated alcohol molecules (D = MeOH, EtOH).The molybdenum oxido groups of the MoO22+ moiety show the expected mutual cis configuration and are located trans to the imine nitrogen atom and coordinated alcohol molecule. The bond distances range around Mo follows trend: Mo-Ot < Mo-O(phenolate) < Mo- O(hydrazone part) < Mo-N< MoO(ROH ; R=Me, Et). The bicyclic system of two chelate rings is slightly folded along the axis of molybdenum- imino bond and by the offset of metal centre apart from chelate planes towards the terminal oxygen atom. The most deviation from chelate rings coplanarity is shown by the phenolate oxygen atom due to its participation in intermolecular interactions. The complexes were tested for catalytic epoxidation of cyclooctene under eco-friendly reaction conditions by using aqueous tert-butyl hydroperoxide (TBHP) as an oxidant. Due to trans effect of oxido oxygen atom, Mo-O(D) bond is the longest bond distance within molybdenum coordination sphere and it expected to be the point of maximum catalytic reactivity of the complexes. It is previously shown that pentacoordinated molybdenum systems activate oxidizing agent TBHP through adduct formation assisted by the O–H…O hydrogen bond. Another explanation of the substituent influence is p-π conjunctive effect with methoxy group due to increasing electron density of the phenyl ring.

Reduction of α,ω-Dihalohexanes with Electrogenerated Nickel(I) Salen

A recent study has described the electrocatalytic reduction of 1,6-dibromohexane at a silver electrode.1 To mitigate the costs of silver electrodes, glassy carbon electrodes can be used in the presence of a solution-phase catalyst. [[2,2′-[1,2-Ethanediylbis-(nitrilomethylidyne)]bis[phenolato]-N,N′,O,O′]nickel(I), further referred to as nickel(I) salen, has been used as an electrogenerated catalyst in tandem with glassy carbon electrodes due to its ability to reductively cleave carbon—halogen bonds in organic compounds. Previous studies at glassy carbon electrodes with nickel(I) salen or at silver electrodes have primarily focused on reduction of monohalogenated compounds or linear hydrocarbons substituted at both primary positions with mixed halogens. Catalytic reduction of such straight-chain alkyl halides with electrogenerated nickel(I) salen yielded both straight-chain monomeric and dimeric products in high yields.2 In this study, electrogenerated nickel(I) salen was used to reduce 1,6-dibromohexane, 1,6-diiodohexane, and 1-bromo-6-iodohexane catalytically. These substrates were electrolyzed in the presenece of 0.10 M tetramethylammonium tetrafluoroborate (TMABF4) in N,N-dimethylformamide at a glassy carbon electrode. Cyclic voltammetry indicated that direct reduction of 1,6-dibromohexane and 1,6-diiodohexane occurs respectively at –1.64 and –1.56 V vs. a cadmium mercury amalgam reference electrode (Cd(Hg)) with a potential of –0.76 V vs. an aqueous saturated calomel electrode. In contrast, catalytic cathodic peaks are at –0.92 and –0.97 V vs. Cd(Hg) for 1,6-dibromohexane and 1,6-diiodohexane, respectively, indicating that nickel(I) salen does indeed reduce these substrates electrocatalytically. Controlled-potential (bulk) electrolyses of the halogenated substrates led to minor conversion to cyclohexane and halogenated dimers with significant amounts of starting material remaining in solution. This drastic decrease in yield compared to previous studies is hypothesized to be the result of alkylation of the imino bond on the salen ligand.2,3 Though nickel(II) salen can undergo a reversible, one-electron reduction to form nickel(I) salen, reduction of nickel(II) salen can also create a ligand-reduced radical at the imino bond, thus resulting in alkylation of both imino bonds on the salen ligand during electrolysis.3 This dialkylation is responsible for the decreased yield of reaction products and was confirmed with the aid of liquid chromatography—mass spectrometry. A mechanism is proposed with radical intermediates which result in dialkylated nickel(II) salen. Martin, E. T.; Strawsine, L. M.; Mubarak, M. S.; Peters, D. G., Direct Reduction of 1,2- and 1,6-Dibromohexane at Silver Cathodes in Dimethylformamide. Electrochim. Acta 2015, 186, 369–376.Mubarak, M. S.; Peters, D. G., Homogeneous Catalytic Reduction of α,ω-Dihaloalkanes with Electrogenerated Nickel(I) Salen. J. Electroanal. Chem. 1995, 388, 195–198.Martin, E. T.; McGuire, C. M.; Peters, D. G., Catalytic Reduction of Organic Halides by Electrogenerated Nickel(I) Salen. Electrochem. Soc. Interface 2016, 25, 41–45. Figure 1

Catalytic Reduction of α,ω-Dihalohexanes By Electrogenerated Nickel (I) Salen in Dimethylformamide

A previous study has described that [[2,2′-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]-N,N′,O,O′]nickel(I), also known as nickel(I) salen, can be electrogenerated from nickel(II) salen. Through the use of controlled–potential electrolysis, nickel(I) salen can catalytically reduce α,ω-dihaloalkanes. Early work focused on reduction of α,ω-dihaloalkanes substituted by mixed halogens, and revealed that this reduction could yield both straight-chain monomeric and dimeric products.1 This research project involves the catalytic reduction of 5.0 and 10.0 mM solutions of 1,6-dibromohexane and 10.0 mM 1,6-diiodohexane by nickel(I) salen electrogenerated at a glassy carbon cathode in dimethylformamide containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4) as a supporting electrolyte. A point of major interest is to separate, identify, and quantitate the products that arise from this catalytic process. Cyclic voltammetry indicates that the reduction peaks for 1,6-dibromohexane are –1.64 V for direct reduction and –0.92 V in the presence of nickel(I) salen; all potentials are reported versus a cadmium mercury amalgam reference electrode with a potential of –0.76 V versus an aqueous saturated calomel electrode. Reduction potentials for 1,6-diiodohexane are –1.56 V and –0.97 V, respectively. Bulk electrolysis of nickel(II) salen in the presence of these substrates has been performed, and the various products were separated and identified with the aid of gas chromatography–mass spectrometry and electrospray ionization–mass spectrometry. The results revealed that there were small amounts of cyclohexane and dihalogenated dimers with a significant amount of starting material left after electrolysis. This low yield is assumed to be from the alkylation of the imino bond on the salen ligand on the basis of previous studies; dihaloalkylated salen species were identified with the aid of liquid chromatography–mass spectrometry.1,2 Reference: Mubarak, M. S.; Peters, D. G., Homogeneous Catalytic Reduction of α,ω-Dihaloalkanes with Electrogenerated Nickel(I) Salen. Electroanal. Chem. 1995, 388, 195–198.Martin, E. T.; McGuire, C. M.; Peters, D. G., Catalytic Reduction of Organic Halides by Electrogenerated Nickel(I) Salen. Soc. Interface 2016, 25, 41–45.

Catalytic reduction of 1-bromodecane and 1-iododecane by electrogenerated, structurally modified nickel(I) salen

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been used to characterize the behavior of a structurally modified nickel(I) salen species, that can be electrogenerated at carbon cathodes in dimethylformamide (DMF) containing 0.10 M tetra‑n‑butylammonium tetrafluoroborate (TBABF4) to catalyze the reductions of 1‑bromodecane (1) and 1‑iododecane (2). This nickel(I) complex possesses a methyl protecting group on each imino (CN) bond, as well as a N‑methyl‑N‑phenylaminomethyl moiety on each aryl group. In its active form, the nickel(I) species can reduce 1 or 2 at a more positive potential than is needed to reduce either substrate directly at a carbon cathode. Bulk electrolysis of the parent nickel(II) complex in the presence of 1 and 2 results in a mixture of n‑eicosane, n‑decane, 1‑decene, and N,N‑dimethylundecanamide, that were identified and quantitated with the aid of gas chromatography–mass spectrometry (GC–MS) and gas chromatography (GC). A mechanistic scheme is proposed to account for the formation of the various products. Robustness of the new nickel(I) catalyst was also investigated and was compared to that encountered with unmodified nickel(I) salen.

Synthesis of Adenine Nucleosides by Transglycosylation using Two Sequential Nucleoside Phosphorylase‐Based Bioreactors with On‐Line Reaction Monitoring by using HPLC

AbstractUridine phosphorylase from Clostridium perfringens (CpUP, EC 2.4.2.3) was immobilized covalently in an aminopropylsilica monolithic column (25 mm×4.6 mm) upon functionalization with glutaraldehyde. Imino bonds that result from the reaction between the enzyme and the support were reduced chemically to afford a 66 % yield (13 mg) determined spectrophotometrically. The CpUP immobilized enzyme reactor (IMER) was connected to a silica particle‐based IMER that contained a purine nucleoside phosphorylase from Aeromonas hydrophila (AhPNP, EC 2.4.2.1), which was developed previously and used successfully for the fast synthesis of some purine ribonucleosides by a “one‐enzyme” transglycosylation. CpUP‐IMER and AhPNP‐IMER were connected to a HPLC system by a six‐way switching valve. In this set‐up, the synthesis of 2′‐deoxyadenosine (dAdo, 8), adenosine (Ado, 9), and arabinosyladenine (araA, 10) by a “two‐enzyme” transglycosylation is coupled directly to on‐line reaction monitoring. Under the optimized transglycosylation conditions (2:1 ratio sugar donor/base acceptor; 10 mm phosphate buffer; pH 7.25; temperature 37 °C, flow rate 0.1 mL min−1), defined by a 2(5‐2)III experimental design, the conversion of dAdo and Ado was approximately 90 %, and araA was synthesized in 20 % yield.

Benzimidazole functionalised Schiff bases: Novel pH sensitive fluorescence turn-on chromoionophores for ion-selective optodes

Abstract Benzimidazole-based Schiff bases 1–3, designed as D-π-A chromophores, are presented as novel pH responsive chromo- and fluoroionophores for ion-selective optodes. Tuning of their ICT character and photophysical properties has been achieved by introduction of an electron donating N,N-diethyl amino group and strong electron withdrawing groups −CN and NO2 on the benzimidazole acceptor moiety. The core molecular structure also contains a salicylidene moiety that potentially introduces photoinduced proton transfer reactivity. In general, pH chemosensors based on Schiff bases are relatively rare due to the common problem of protonation induced hydrolysis of the imino bond in water. The photophysical and sensing properties of 1–3 are characterised in solutions and thin polymer films, and the corresponding apparent pKa and pKa* values are determined. In ethanol/water solutions, Schiff bases 1–3 show very weak fluorescence with reversible naked-eye visible colour change upon protonation (apparent pKa ∼ 4). Upon immobilisation in plasticised PVC films the photophysical properties and pH responses of 1–3 significantly change resulting in reversible fluorimetric turn-on and turn-off pH sensitivity with apparent pKa values in the physiologically relevant pH range (apparent pKa ∼ 6-7). A reversible spectroscopic response to pH is achieved because protonation of the immobilised benzimidazole Schiff bases occurs on the stable benzimidazole moiety (electron acceptor), whilst the imino bond remains preserved. An application for these novel fluoroionophores is demonstrated with a fluorescence turn-on potassium selective optode, based on an ion-exchange sensing mechanism.

Mechanistic Studies for the Formation of Dialkylated Nickel(II) Salen Involved in the Catalytic Reduction of (Bromomethyl)Cyclopropane

Nickel(II) salen has been widely used as the catalyst for the electrochemical reduction of organic halides (RX). The corresponding catalytic reaction mechanism were examined and proposed by various research groups.1 -5 Generally, nickel(II) salen (1) would undergo a one-electron reversible reduction to generate either the metal-reduced nickel(I) salen (2) or the ligand-reduced radical-anion (3, Scheme 1), which can subsequently transfer an electron to the organic halide substrate to produce a radical and a halide ion. Afterward, the substrate radicals can undergo different reactions such as coupling,6,7 disproportionation,6 intramolecular cyclization,8,9 abstraction of hydrogen atom from solvent, etc to afford a series of products. However, side reactions may also take place to cause the alkylation of nickel(II) salen. As the result, a significant amount of substrates could be lost9 and nickel(II) salen would be deactivated.10,11 Peters and his colleagues proposed two possible routes (Route 1 or 2, Scheme 2) involving the SN2 nucleophilic substitution and radical coupling reactions between catalyst 3 and substrates for the formation of dialkylated nickel(II) salen.3,4 Alternatively, a derivative pathway (Route 3, Scheme 2) as well as the direct radical addition to the imino bond of nickel(II) salen (Route 4, Scheme 2)12cannot be ruled out. Nevertheless, a definite reaction mechanism still awaits further research. In this study, we employed (bromomethyl)cyclopropane as the substrate for the electrochemical reduction catalyzed by nickel(II) salen. The catalytic process should lead to the formation of cyclopropylmethyl radicals, which undergo an extremely fast ring opening rearrangement to give 3-butenyl radicals at a rate constant of 8.6×107 s-1 (298 K).13 The cyclopropyl ring could be retained in SN2 nucleophilic substitution while the radical coupling would involve 3-butenyl radicals. Consequently, the dialkylation of nickel(II) salen will render different products (4-7, Scheme 3), depending upon which reaction route it takes. We carried out cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) for the initial investigations and the electrolyzed solution was subject to HPLC analysis. After purified by preparative-scale HPLC, the dialkylated nickel(II) salen was examined by ESI-MS, 1H NMR, COSY, and HECTOR NMR spectrometry. Its complete structure was resolved and found to be species 4. Thus, we concluded that Route 1 (Scheme 2) should be the plausible reaction mechanism for the dialkylation of nickel(II) salen in the catalytic reduction of organic halides. References D. M. Goken, D. G. Peters, J. A. Karty, and J. P. Reilly, J. Electroanal. Chem., 564, 123 (2004). J. A. Miranda, C. J. Wade, and R. D. Little, J. Org. Chem., 70, 8017 (2005). D. M. Goken, M. A. Ischay, D. G. Peters, J. W. Tomaszewski, J. A. Karty, J. P. Reilly, and M. S. Mubarak, J. Electrochem. Soc., 153, E71 (2006). P. W. Raess, M. S. Mubarak, M. A. Ischay, M. P. Foley, T. B. Jennermann, K. Raghavachari, and D. G. Peters, J. Electroanal. Chem., 603, 124 (2007). M. N. Nguyen, M. E. Tomasso, D. C. Easter, and C. Ji, J. Electrochem. Soc., 163, G1 (2016). M. S. Mubarak and D. G. Peters, J. Electroanal. Chem., 388, 195 (1995). A. L. Bulter and D. G. Peters, J. Electrochem. Soc., 144, 4212 (1997). M. S. Mubarak and D. G. Peters, J. Electroanal. Chem., 332, 127 (1992). D. M. Fang, D. G. Peters, and M. S. Mubarak, J. Electrochem. Soc., 148, E464 (2001). A. Gennaro, A. A. Isse, and F. Maran, J. Electroanal. Chem., 507, 124 (2001). A. A. Isse and A. Gennaro, J. Electrochem. Soc., 149, D113 (2002). H. Miyabe, Y. Yamaoka, and Y. Takemoto, J. Org. Chem., 71, 2099 (2006). V. W. Bowry, J. Lusztyk, and K. U. Ingold, J. Am. Chem. Soc., 113, 5687 (1991). Figure 1

Formation and Reactivity of a C-P-N-Sc Four-Membered Ring: H2 , O2 , CO, Phenylsilane, and Pinacolborane Activation.

Reaction of scandium dimethyl complex LScMe2 (L=[2-{N(DIPP)C(Ph)N}C6 H4 CH=N(DIPP)]- , DIPP=2,6-(iPr)2 C6 H3 ) (2) with bulky phosphine ArPH2 (Ar=2,6-{3,5-(Me)2 C6 H3 }2 C6 H3 ) gives an unprecedented scandium 2,3-azaphosphametallacyclobutane complex L'Sc (L'=[2-{N(DIPP)C(Ph)N}C6 H4 C(H)P(2,6-{3,5-(CH3 )2 C6 H3 }2 C6 H3 )N(DIPP)]3- ) (3) that contains a C-P-N-Sc four-membered ring. Complex 3 was characterized by X-ray crystallography, revealing a short distance between the Sc and P atoms (2.786(1) Å), but without a direct Sc-P bonding interaction. The formation of 3 involves a complete cleavage of the imino bond of the tridentate ligand L and C-P and N-P bond coupling. Complex 3 not only activates H2 under mild conditions to give a H-H bond cleavage product, but also reacts with O2 , CO, phenylsilane, and pinacolborane to produce novel products through cleavage and formation of various bonds.

Supramolecular solid-state structure, potential energy surfaces and evaluation of antiproliferative effect of 2-benzothiazolylhydrazone derivatives in vitro

The in situ condensation reaction of 2-hydrazinobenzothiazole with salicylaldehyde, 3,4-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde, 2-methoxy-1-naphthaldehyde, 4-methoxy-1-naphthaldehyde and 6-methoxy-2-naphthaldehyde produced 9 hydrazone Schiff bases (L1–L9, respectively) which were identified and characterized by elemental analysis, IR and NMR spectroscopy. The crystal and molecular structures of four Schiff bases (L1, L7–L9) have been determined by the single-crystal X-ray diffraction method confirming the imino form of L1 and the amino tautomeric form of L7–L9 compounds. Molecular structure analysis also confirmed that reported compounds are E-isomers relative to exo C = N imino bond. The Nhydrazino–H group of amino tautomers forms Nhydrazino–H···Nthiazolyl intermolecular hydrogen bonds shaping molecules into R22(8) rings, while imino tautomer of L1 forms C(4) infinite helical chains via Nthiazolyl–H···Nhydrazino type of intermolecular hydrogen bond. The methoxy group (L7–L9) further shaped these primary supramolecular synthons into different supramolecular arrangements via C–H···O, C–H···N and C–H···S intermolecular hydrogen bonds. The role of aryl substituents in the shaping and stabilization of supramolecular architectures of L1, L7–L9 is supported by quantum chemical calculations. Strong antiproliferative effects on tumor cells and cytotoxic effects on fibroblasts are shown for all ligands L1–L9 with exception of L6 and L7 that had no effect on fibroblast cells.

Electrochemical Reduction of Primary Alkyl Halides with the Aid of a Structurally Modified Nickel Salen Catalyst

Electrochemical reduction of primary alkyl halides has been extensively investigated through the use of [[2,2′-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-(N,N′,O,O′)]nickel(II) (nickel salen) as a procatalyst. In these studies, there was a proclivity for the grafting of alkyl chains onto the imino (C=N) bonds of the nickel species, which lowers the yields of the desired reduction products significantly.1,2 In the present study, we have used a structurally modified nickel(II) salen-like species possessing methyl protecting groups to improve the performance of the catalyst.3 The efficacy of this modified nickel species as a procatalyst for electrochemical reductions of 1-bromo- and 1-iododecane has been studied with the aid of cyclic voltammetry and controlled-potential (bulk) electrolysis in N,N-dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) as a supporting electrolyte. Results indicate that a majority of the reduced substrate appears as a dimeric species (n-eicosane); in addition, smaller amounts of n-decane and 1-decene are found. A mechanism is proposed for the process that involves radical intermediates. Several products corresponding to the incorporation of DMF fragments into the reduced substrate were detected. Use of this procatalyst has also been investigated to overcome solvent incorporation under similar conditions in an acetonitrile medium. References Goken, D.M.; Ischay, M. A.; Peters, D. G.; Tomaszewski, J. W.; Karty, J. A.; Reilly, J. P.; Mubarak, M. S. J. Electrochem. Soc. 2006, 153, E71–E77. Raess, P. W.; Mubarak, M. S.; Ischay, M. A.; Foley, M. P.; Jennermann, T. B.; Raghavachari, K.; Peters, D. G. J. Electroanal. Chem. 2007, 603, 124–134. Ourari, A.; Ouennoughi, A.; Aggoun, D.; Mubarak, M. S.; Pasciak, E. M.; Peters, D. G. Polyhedron 2014, 67, 59–64. Figure 1

Mechanism of Fluorescence Switching in One ESIPT-Based Al(3+) Probe.

A recently synthesized Schiff base used as a probe for aluminum cations was studied with ab initio models. The primary reason for the lack of fluorescence in aprotic solvents was found to be the presence of an efficient conical intersection (CI) between the ground-states and the first singlet excited-states close to the Franck-Condon geometry. The excited-state pathway leading to this CI is barrierless but implies large amplitude motions, explaining why the fluorescence was observed in frozen acetonitrile matrix. Our calculations suggest that constraining the molecule by impending the rotation around the imino bond enables excited-state intramolecular proton transfer. A similar stiffening mechanism is responsible for the strong fluorescence turn-on after formation of complexes between Al(3+) cations and dehydrogenated Schiff base. Finally, the analysis of the possible fluorescence mechanisms in water indicates that the anion of 1 is the likely fluorescence source. Overall, this work allows one to disentangle the various origins of fluorescence switching in a probe.

Linker-Region Modified Derivatives of the Deoxyhypusine Synthase Inhibitor CNI-1493 Suppress HIV-1 Replication.

The inhibition of cellular factors that are involved in viral replication may be an important alternative to the commonly used strategy of targeting viral enzymes. The guanylhydrazone CNI-1493, a potent inhibitor of the deoxyhypusine synthase (DHS), prevents the activation of the cellular factor eIF-5A and thereby suppresses HIV replication and a number of other diseases. Here, we report on the design, synthesis and biological evaluation of a series of CNI-1493 analogues. The sebacoyl linker in CNI-1493 was replaced by different alkyl or aryl dicarboxylic acids. Most of the tested derivatives suppress HIV-1 replication efficiently in a dose-dependent manner without showing toxic side effects. The unexpected antiviral activity of the rigid derivatives point to a second binding mode as previously assumed for CNI-1493. Moreover, the chemical stability of CNI-1493 was analysed, showing a successive hydrolysis of the imino bonds. By molecular dynamics simulations, the behaviour of the parent CNI-1493 in solution and its interactions with DHS were investigated.

A computational study of regioselectivity in β-lactam iminothiazolidinone formation

Density functional theory calculations were performed to explain the different regioselectivity for the formation of β-lactam iminothiazolidinones. Computational results were in agreement with experimental observations that phenyl and cyclohexyl derivatives led to the thermodynamically more stable regioisomers formed by cyclization at the nitrogen atom directly attached to the β-lactam ring which was in contrast to the n-hexyl derivative where the regioisomer with the β-lactam ring attached to the imino bond is more stable instead. It was demonstrated that the different regioselectivity was the consequence of larger steric effects when bulky substituents and the leaving ethoxy group were in close contact during the cyclization step.

Iridicycle‐Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism

The mechanism of imine reduction by formic acid with a single-site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover-limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion-pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer-sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26-28 and 7-8 kcal mol(-1) , respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol(-1) for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover-limiting hydride formation step by hydrogen-bonding to the formate anion and thereby stabilising the ion pair.

Cyclic [2]Catenane Dimers, Trimers, and Tetramers

AbstractDimeric, trimeric, and tetrameric cyclic [2]catenanes have been prepared directly through one‐pot sodium‐ion‐templated dynamic imine formation from a diamine and a tetraaldehyde. NaBH4 mediated reduction of the labile imino bonds of these cyclic [2]catenane oligomers, followed by methylation of the resulting secondary amino groups enabled the isolation and characterization of oligomeric cyclic [2]catenanes as stable, covalently linked compounds.

Cyclic [2]Catenane Dimers, Trimers, and Tetramers.

Dimeric, trimeric, and tetrameric cyclic [2]catenanes have been prepared directly through one-pot sodium-ion-templated dynamic imine formation from a diamine and a tetraaldehyde. NaBH4 mediated reduction of the labile imino bonds of these cyclic [2]catenane oligomers, followed by methylation of the resulting secondary amino groups enabled the isolation and characterization of oligomeric cyclic [2]catenanes as stable, covalently linked compounds.