Bis-4H-imidazoles-tetraazafulvalenes-2,2′-biimidazoles: three variations of one redox system

The reversibility of the two-electron reduction of tetraazafulvalenes 1 could be confirmed by employing cyclovoltammetric measurements. However, attempts to oxidize these systems electrochemically, as well as by oxidizing agents, failed. In contrast, the bis-vinylogous derivatives 2 proved to be multi-step redox systems showing two-reversible reduction as well as oxidation waves. The chemically initiated oxidation of 2 yielded the bis-4H-imidazoles 6. In the presence of an excess of the oxidizing agent they dimerized and formed the deep blue colored derivative 7a. Treatment of the phenylogous systems 8 with sodium dithionite provided a new entry to quinomethides of type 9, which can be stabilized towards oxygen by cyclization reaction to give the pigment-like bis-urea 11. Derivative 9 represents the SEM form of this four-step redox system and thus can finally be reduced to yield the tetraaminosubstituted biimidazoles 12. Based on these findings, the close correlation among bis-4H-imidazoles (OX), tetraazafulvalenes (SEM), and tetraaminosubstituted biimidazoles (RED) could be demonstrated. Due to the fact that tetraazafulvalenes constitute stable closed-shell SEM systems, their intense UV-vis absorptions can now be explained and related to their redox behavior.     

Intermolecular interactions of punicin derivatives

The 2- and 4-methyl derivates of punicin [N-(2′,5′-dihydroxyphenyl)-pyridinium chloride] were subjected to Knoevenagel reactions. Starting materials as well as reaction products were examined with respect to homo and hetero-intermolecular interactions. It was found that N-(2′,5′-dihydroxyphenyl)-2-methylpyridinium chloride forms a stable 2:1 complex to hydroquinone. Decomplexation can be accomplished by anion exchange to tetraphenylborate, or by competing complexation with p-benzoquinone. Results of three single crystal X-ray analyses as well as NMR titrations and dilution experiments are presented.     

DNA-, RNA- and self-pairing properties of a pyrrolidinyl peptide nucleic acid with a (2′R,4′S)-prolyl-(1S,2S)-2-aminocyclopentanecarboxylic acid backbone

A new pyrrolidinyl peptide nucleic acid (PNA) comprising of an alternate sequence of 4′-nucleobase-modified proline with (2′R,4′S) configuration and a (1S,2S)-2-aminocyclopentanecarboxylic acid [(2′R,4′S)-acpcPNA] backbone was synthesized and its DNA-, RNA- and self-pairing properties studied. T_m and CD studies suggested that the (2′R,4′S)-acpcPNA forms antiparallel hybrids to DNA and RNA with high sequence and direction specificity. The stability of these hybrids is comparable to those of the (2′R,4′R)-acpcPNA hybrids previously reported by our group. On the other hand, experiments with a self-complementary sequence indicated that the new (2′R,4′S)-acpcPNA forms a more stable antiparallel self-hybrid than (2′R,4′R)-acpcPNA.     

Improved quality control in gas chromatography interfaced to stable isotope ratio mass spectrometry by application of derivative chromatography

Compound-specific isotope analysis using gas chromatography interfaced to isotope ratio mass spectrometry (GC-IRMS) is a versatile technique for applications ranging from source appointment and the elucidation of biochemical pathways. When δ¹³C values are going to be determined, the sample is combusted to CO₂ and the resulting gas is analyzed relative to a standard with known stable carbon isotope ratio. With the combustion step any information on the identity of a peak is lost. Co-eluting compounds can no more be identified which can lead to significant alterations of the δ¹³C value of the analyte. For improvement of the QA/QC protocols in GC-IRMS, we used first, second, and third order derivative chromatography. The suitability of the technique was studied using mixtures of 2,3,3′,4,4′,5,5′-heptachloro-1′-methyl-1,2′-bipyrrole (Q1) and 2,2′,4,5,5′-pentachlorobiphenyl (PCB 101). By application of different GC oven programs four scenarios ranging from baseline separation to full co-elution were obtained. Derivative chromatography enabled identification of the interference of Q1 with PCB 101 even when both peaks fully co-eluted. Although the δ¹³C values could not be determined from interfered scenarios, the use of derivative spectroscopy will help to prevent acceptance of incorrect data due to co-elutions. Derivative chromatography was finally used to study the peak purity of 2,2′,3,4,4′-pentabromodiphenyl ether (BDE 85) in technical pentabromo diphenyl ether (DE-71). Already the first order derivative demonstrated that this key-BDE congener was interfered by a compound identified as 2,2′,4,4′,6,6′-hexabromodiphenyl ether (BDE 155).     

Hydrogen-bonded rotamers of 2′,4′,6′-trihydroxy-3′-formyldihydrochalcone, an intermediate in the synthesis of a dihydrochalcone from Leptospermum recurvum

Synthesis of 2′,4′,6′-trihydroxy-3′-methyldihydrochalcone, isolated as a natural product for the first time (ex Leptospermum recurvum), proceeds through 2′,4′,6′-trihydroxy-3′-formyldihydrochalcone. Two stable rotamers of this formyl derivative have been identified and the problems associated with NMR assignments of this type of compound have been attributed to conformational exchange. Rotamer ratios from molecular modelling and ab initio calculations agree well with those obtained from low temperature NMR studies. There is also excellent correlation between experimental NMR chemical shifts for the hydrogen-bonded hydroxyl protons with those derived from ab initio calculations. This formyl dihydrochalcone showed promising bioactivity in antiviral and antimicrobial assays.     

An efficient synthesis of novel spiro[[8H]indeno[2,1-b]-thiophene-8,9′-fluorene] building block for blue light-emitting materials

We have developed efficient synthetic routes to obtain a novel building block spiro[[8H]indeno[2,1-b]thiophene-8,9′-fluorene] (SITF), a monothiophene-containing spirobifluorene analogue, and constructed blue light-emitting materials, including 2′,7′-bis([1,1′-biphenyl]-4-yl)-spiro[indeno[2,1-b]thiophene-8,9′-fluorene] (BBP-SITF) and 2′,7′-bis(9,9′-spirobifluoren-2-yl)spiro[[8H]indeno[2,1-b]-thiophene-8,9′-fluorene] (BSBF-SITF). BSBF-SITF has shown to be a stable blue light-emitting material with high PL quantum efficiency (89%) and unique regioselective feature at the C2 of thiophene, which indicate that BSBF-SITF will be useful for constructing complicated optoelectronic systems.     

Inclusion crystals of 2,2′,7,7′,9,9′-hexahalo-9,9′-bisfluorenyl derivatives: a new family of polyhalo aryl hosts

The preparation and inclusion properties of the new halo aryl hosts, 2,2′,7,7′,9,9′-hexahalo-9,9′-bisfluorenyl derivatives 5-7, are described. The host compounds 5-7 having four halogen atoms on the aromatic rings form stable inclusion crystals with many guest compounds, whereas the parent compound 4 does not. The X-ray structures of the host 4 and representative inclusion compounds of hosts 5-7 were determined, allowing rationalization of several of the experimental observations.     

Thermal rearrangement of spiro[naphthalene(naphthopyranofurazan)]oxides to spiro[naphthalene(phenalenofurazan)oxides. A probable furazan oxide triggered tandem isomerisation process

Thermal rearrangement of (±)-spiro{naphthalene-1(2H),4′-(naphtho-[2′,1′:2,3]pyrano[4,5-c]furazan)}-2-one-11′-oxides in DMF or acetic anhydride at 140 °C gave an isomeric mixture of (±)-spiro{naphthalene-1(2H),1′-(5′-hydroxyphenalene[1,2-c]furazan)}-2-one-2′-oxides and 4′-oxides. The rearranged structure was confirmed from X-ray analysis and was consistent with the through space NOE data. The rearrangement is suggested to be an overall tandem isomerization process. Using variable temperature ¹H NMR spectroscopy the lower limit for the isomerisation barrier for a pair of tautomers was calculated to be 22 kcal mol⁻¹ at 423 K. The isomerisation equilibrium for a pair of isomers was studied by variable temperature ¹H NMR. The lower limit for the isomerisation barrier was calculated to be 22 kcal mol⁻¹ at 423 K. This low value may be indicative of the difficulty encountered in separating the isomers by chromatography. Semi-empirical AM1 and molecular mechanics calculations suggest that the (±)-spiro{naphthalene-1(2H),1′-(5′-hydroxyphenalene[1,2-c]furazan)}-2-one-2′-oxides are more stable than their 4′-oxide counterparts, in accordance to the X-ray structure. The lower population of the 4′-oxide isomers relative to that of the 2′-oxide isomers was explained in terms of an unfavourable intramolecular steric interaction found in the low energy structure of the former.     

Oxidative cleavage of ribofuranose 5-(α-hydroxyphosphonates): a route to erythrofuranose-based nucleoside phosphonic acids

We report here an oxidative cleavage of (5R)- and (5S)-ribofuranosyl-5-C-phosphonate derivatives with periodate anion under both strong acidic and neutral conditions. In both cases, only (5R)-configured compound underwent the expected oxidation reaction and afforded the desired (4R)-erythrofuranosylphosphonate, whereas the second epimer, (5S)-ribofuranosyl-5-C-phosphonate did not provide the corresponding (4S)-erythrofuranosylphosphonate derivative. This different behavior of epimers toward oxidative cleavage is an important phenomenon. The obtained (4R)-erythrofuranosylphosphonate was used for the preparation of phosphonate mimic of adenosine 5′-phosphate via classical nucleosidation reaction. Condensation of the protected shortened AMP analogue with adenosine derivatives, however, provided only the 2′,5′-linked ApA analogue. Study on hybridization of the modified 2′-5′ ApA with polyU revealed its ability to form stable triplex-like complex, similar to natural 2′-5′ r(ApA) and 3′-5′ r(ApA). NMR spectroscopy study showed that the erythrofuranose part of the phosphonate nucleotide unit of modified 2′-5′ ApA was predominantly in the C2′-endo conformation, which is characteristic for B-DNA.     

Reaction of 3/2-formylindoles with TOSMIC: formation of indolyloxazoles and stable indolyl primary enamines

3-Formylindole and its 1-substituted and 1,5-disubstituted derivatives react with TOSMIC in presence of potassium carbonate in methanol under reflux to furnish 5-(3′-indolyl)oxazoles, new stable E-2-(3′-indolyl)-2-tosylethenamines and two diastereomers of N-[2-(3′-indolyl)-1,2-dimethoxy]ethylformamides. In contrast, 2-formylskatole furnishes N-(1-tosyl-2-skatolyl)ethenylformamide.     

Dynamic kinetic resolution of secondary alcohols with a readily available ruthenium-based racemization catalyst

An easy to handle and stable racemization catalyst for secondary alcohols is obtained by an in situ mixture of readily available [Ru(cymene)Cl₂]₂ with chelating aliphatic diamines. Optimization of the reaction revealed that N,N,N′,N′-tetramethyl-1,3-propanediamine as ligand racemizes aromatic alcohols completely within 5 h. This easy to handle and stable catalytic system is combined with a lipase-catalyzed resolution to provide an efficient dynamic kinetic resolution of secondary alcohols.     

A new molecular precursor route for the synthesis of Bi-Y, Y-Nb and Bi-doped Y-Nb oxides at moderate temperatures

Yttrium-based multimetallic oxides containing bismuth and/or niobium were prepared by a method starting from pre-isolated stable water-soluble precursors which are complexes with the ethylenediaminetetraacetate ligand (edta). The cubic Bi_1−xY_xO_1.5 (x=0.22, 0.25 and 0.3) and Y₃NbO₇ oxides were obtained in a pure form in a range of moderate temperatures (600-650 °C). This preparation method also allowed to stabilize at room temperature, without quenching, the tetragonal YNbO₄ oxide in a distorted form (T′-phase) by calcining the precursor at 800 °C. When heated up to 1000 °C, this metastable T′-phase transforms into the metastable “high-temperature” T oxide, which converts on cooling down to room temperature into the thermodynamically stable monoclinic M oxide. Doping the YNbO₄ oxide with Bi³⁺ cations (0.5% and 1% Bi with respect to total Bi+Y amount) led at 800 °C to a mixture of the T′-phase and the thermodynamically stable monoclinic one. At 900 °C, the almost pure monoclinic structure was obtained.     

Functional dyes: bipyridines and bipyrimidine based boradiazaindacene

The synthesis of molecules bearing pyridine, 2,2^′-bipyridine or 2,2^′-bipyrimidine donor units appended with one or two highly luminescent indacene fragments was undertaken by using two distinct pathways. The first used direct condensation of krytopyrrole with the corresponding aldehydes. The second, and more universal is based on a Pd cross-coupling reaction between an iodo functionalized BODIPY and the corresponding stable ethynyl derivatives. All compounds are strongly luminescent in solution with quantum yields as high as 70%. Interlocking of two 6,6^′-BODIPY substituted bipyridines around a single copper(I) center is observed by proton NMR and cyclic voltammetry. These synthetic routes have made a variety of functionalized dyes available for studies of their optical properties in the presence of incoming cations, of their coordination chemistry and as light emitting solid state materials.     

Effects of adjacent groups of benzimidazole on antioxidation of polybenzimidazoles

The chemical oxidative stabilities of poly(2,2-(m-phenylene)-5,5-bibenzimidazole) (PBI-ph), poly(2,5-benzimidazole) (ABPBI), poly(2,2′-hexyl-5,5′-bibenzimidazole) (PBI-hex), and poly(2,2′-imidazole-5,5′-bibenzimidazole) (PBI-imi) are studied. By means of FTIR and ¹HNMR analysis, more information about the degradation process of PBI-ph is found as: CH₂ groups are left in the residual polymers; after the N-H bond and the trisubstituted benzene ring are oxidized by oxidative free radicals, the meta-phenylene is relatively stable. Through Fenton tests, the chemical oxidative stabilities of these PBIs are compared and results show that PBI-ph is the stablest material while PBI-imi is the unstablest one. Through FTIR analysis, the structure changes to those degraded PBIs are compared. The conjugated structure formed between meta-phenyl and benzimidazole can protect the main chain of PBI-ph from the attack of oxidative free radicals. Additionally, effects of acid on PBI-ph degradation rate are evaluated and the results show that phosphoric acid can slow down the chemical oxidative degradation.     

Effect of structure on thermal behaviour of epoxy resins

The paper deals with the curing behaviour of diglycidyl ether of bisphenol-A (DGEBA) using three novel multifunctional aromatic amines having phosphine oxide and amide-acid linkages. The amines were prepared by reacting tris(3-aminophenyl)phosphine oxide (TAP) with 1,2,4,5-benzenetetracarboxylic acid anhydride (P)/4,4^′-(hexafluoroisopropylidene)diphthalic acid anhydride (F)/3,3^′,4,4^′-benzophenonetetracarboxylic acid dianhydride (B). Amide-acid linkage in these amines is converted to thermally stable imide linkage during curing reaction. Curing temperatures of DGEBA were higher with phosphorylated amines than the conventional amine 4,4^′-diamino diphenyl sulphone (D). A decrease in initial decomposition temperature and higher char yields were observed when phosphorus containing amide-acid amines were used as curing agents for DGEBA.     

Conformational analysis of new 14-membered ring diketal dilactam macrocycles: molecular mechanics, liquid and solid state NMR studies

A conformational study of new diversely substituted 14-membered diketal dilactam macrocycles was conducted by NMR spectroscopy in liquid and solid states, molecular mechanics calculations and, for one compound, a previous X-ray analysis. The results obtained by the different techniques show that the conformations depend closely on whether the molecules are chiral or achiral and on the stereochemistry of the ketal OMe groups. In achiral compounds, the most stable conformation of each compound has, in both the liquid and solid states, the two NH-CO links positioned perpendicular to the macrocycle plane, lending to the trans-7,7′-OMe macrocycles 6b and 7b a rectangular [3434]-type structure. In contrast, in chiral compounds, the most stable conformations are not the same in the liquid and solid phases. In the liquid state the conformations are set by the presence of one or two N₄-H?O_1′, N_4′-H?O₁ intramolecular hydrogen bonds that position the amide group parallel to the macrocycle plane, whereas in the solid state the amide moieties again adopt a perpendicular position which can be stabilized, when the 3-R substituent is not too bulky, by intermolecular N-H?OC bonds between parallel sheets, and exceptionally, in the cis-7,7′-OMe-3,3′-Ph compound 1c, by a π-π stacking effect between the phenyl groups.     

Stable free radical polymerization of n-butyl acrylate in the presence of high-temperature initiators

Living radical polymerizations of acrylate are known to be difficult to achieve using TEMPO as a mediator. The stable free radical polymerization (SFRP) of acrylate tends to stop at low monomer conversion due to the accumulation of TEMPO in the medium as a result of unavoidable bimolecular termination. Rather than solving this problem by destroying the excess nitroxide using ascorbic acid or glyceraldehyde associated with pyridine as reported recently, high temperature initiators were used to slowly and continuously generate new radicals throughout the polymerization to consume the excess TEMPO molecules. Polymerizations of n-butyl acrylate initiated by the alkoxyamine unimer (1-benzoyloxy)-2-phenyl-2-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)ethane (BST) were performed between 130 °C and 134 °C in the presence of a series of high temperature peroxide and azo initiators. The best results in this study were obtained by the continuous addition of small amounts of di-tert-amyl peroxide throughout the polymerization. Under these conditions, the acrylate polymerizations fulfilled the criteria of a controlled polymerization process although the molecular weight distributions were slightly broad (M_w/M_n ∼ 1.5).     

Enantioselective recognition of amines with an atropisomeric 1,8-bisphenolnaphthalene

1,8-Bis[5′(2′-hydroxy-4′-methylbiphenyl)]naphthalene, 2, was prepared from 1,8-dibromonaphthalene and 4-methoxy-2-methylphenylboronic acid in four steps with 51% overall yield. The axially chiral anti-isomer of 2 is stable to racemization at room temperature and the free energy of activation for the conversion of the anti-isomer to the syn-form was determined as 110.0 kJ/mol at 77.1 °C. At submillimolar concentration, enantiopure 2 can be used as circular dichroism sensor to detect a wide range of chiral amines.     

Synergistic liquid-liquid extractive spectrophotometric determination of gold(III) using 1-(2′,4′-dinitro aminophenyl)-4,4,6-trimethyl-1,4-dihydropyrimidine-2-thiol

Synergistic liquid-liquid extractive spectrophotometric determination of gold(III) using 1-(2′,4′-dinitro aminophenyl)-4,4,6-trimethyl-1,4-dihydro pyrimidine-2-thiol [2′,4′-dinitro APTPT] has been described. Equal volumes (5 cm³) of the 2′,4′-dinitro APTPT (0.02 mol L⁻¹) in the presence of pyridine (0.5 mol L⁻¹) form an orange-red coloured ternary complex with gold(III) of molar ratio 1:1:1 at pH 1.8-2.4 with 5 min of shaking. The absorbance of coloured organic layer in 1,2-dichloroethane is measured spectrophotometrically at 445 nm against reagent blank. A pronounced synergism has been observed by the binary mixture of 2′,4′-dinitro APTPT and pyridine, which shows that the enhancement in the absorbance is observed in the presence of pyridine by the adduct formation in the organic phase. Beer's law was obeyed in the concentration range 2.5-20.0 μg mL⁻¹, with molar absorptivity and Sandell's sensitivity values of 8.7 × 10³ dm³ mol⁻¹ cm⁻¹ and 0.023 μg cm⁻² respectively. A repetition of the method was checked by finding relative standard deviation (R.S.D.) (n = 10) which was 0.17%. The composition of the gold(III)-2′,4′-dinitro APTPT-pyridine adduct was established by slope analysis, molar ratio and Job's method. The ternary complex was stable for more than 48 h. The influence of various factors such as pH, 2′,4′-dinitro APTPT concentration, solvent and pyridine on the degree of complexation has been established. A number of foreign ions tested for their interferences and use of suitable masking agents wherever necessary are tabulated, which show that selectivity of the method has been enhanced. The method is successfully employed for the determination of gold(III) in binary, synthetic mixtures and ayurvedic samples. The reliability of the method is assured by inter-comparison of experimental values, using an atomic absorption spectrometer.     

Solvent-free oxidation of alcohols by t-butyl hydroperoxide catalyzed by water-soluble copper complex

The catalytic system composed of CuCl₂ and 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt (BQC), was found to be highly efficient for the selective oxidation of secondary benzylic, allylic and propargylic alcohols to the corresponding ketones, with aqueous t-butyl hydroperoxide under phase-transfer catalysis conditions. The catalytic system is stable and can be recycled and reused several times without loss of activity.