Design of chiral boronate-substituted acrylanilides.: Self-activation and boron-transmitted 1,8-stereoinduction in [4+2] cycloaddition

The [4+2] cycloaddition of ortho-boronoanilide dienophile 4 with cyclopentadiene was found to proceed faster than both its para isomer 8 and the unsubstituted derivative 6, thereby confirming that self-activation by internal coordination is operative in the case of 4. Chiral boronic esters derivatives 9–13 provided a small level of remote 1,8-stereoinduction transmitted through a putative tetrahedral stereogenic boronate complex. These results show that dialkoxyboronic esters can operate as weak, internal Lewis acids and activate carbonyl-containing functionalities in cycloaddition reactions.     

Metallation reactions XXVII.: Metallation of (methylthio)anilines

The metallation reactions of (methylthio)anilines with organolithium reagents and with the butyllithium–potassium tert-butoxide superbasic mixture are here described. The results show that the para isomer when treated with butyllithium gave a mixture of products with no selectivity. Using tert-butyllithium or superbases we obtained the substitution of the thiomethyl hydrogen. Moreover, superbase allowed to prepare the disubstituted product with the new groups in the thiomethyl and in ortho to this group. On the other side, both ortho and meta isomers were lithiated at the thiomethyl carbon by butyllithium and the other reagents. Starting from the unalkylated amine we prepared through three successive one-pot monometallations N,N-disubstituted amines with equal or different groups and bearing an alkylthio chain as long as wanted.     

Substituent position effect on the properties of new unsymmetrical isomeric diarylethenes having a chlorine atom

Three new unsymmetrical photochromic diarylethenes bearing a chlorine atom at para-, meta- and ortho-position of one terminal benzene ring, namely {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(4-chlorophenyl)-3-thienyl]}perfluorocyclopentene (para 1o), {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-chlorophenyl)-3-thienyl]}perfluorocyclopentene (meta 2o), and {1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(2-chlorophenyl) -3-thienyl]}perfluorocyclopentene (ortho 3o), have been synthesized. The substituent position effect on their properties, including photochromism, fluorescence both in solution and in PMMA amorphous film and their electrochemical properties were investigated in detail. The results elucidated that the chlorine atom and its substituent position had a remarkable effect on the absorption characteristics, photochromic reactivity, fluorescence, as well as the electrochemical performances of these diarylethene compounds. For diarylethenes 1, 2 and 3, the cycloreversion quantum yields were gradually increased when the chlorine atom was attached to the para-, meta- and ortho-positions of the one terminal benzene rings; but their molar absorption coefficients both of their open-ring and closed-ring isomers were remarkably decreased. The fluorescent properties of para-substituted diarylethene embedded in poly(methyl methacylate) (PMMA) amorphous film showed good fluorescent switches (quenched to 28%). Furthermore, the cyclic voltammograms experiments elucidated that the electrochemical properties of these diarylethene derivatives were also remarkably dependent on the chlorine atom position effect, which may be attributed to the different electron-inducing ability and steric effect when the chlorine atom was substituted on the different position of the terminal benzene ring.     

Three photon annihilations of positrons and positronium in solids with two detectors in coincidence

The two-detector coincidence positron annihilation system at Washington State University was used to observe three-photon annihilation events of positrons. The decay of ortho-positronium (o-Ps) can be distinguished for para-Ps decay and positron two photon annihilations by the unique difference energy distribution. The apparatus was used to detect rare three-photon annihilations of positrons in metals.     

A general method for coulometric titration of alkylphenols with bromine

A general method for coulometric titration of alkylphenols with anodically generated bromine is described. The reaction is carried out in a water-acetic acid medium and the reactivity is governed by varying the water content and the concentration of bromide ion and by the addition of pyridine. In that way all types of alkylphenols can be titrated quantitatively. For phenols containing more than one free ortho and para position, the titration can also be carried out either to the monobromination stage or to the full bromination stage. The mean relative error is ± 1·2% for monobromination and ±1·5% for full bromination. A method for rapid determination of the number of free ortho and para positions in alkylphenols by using two coulometric titrations is also described.     

Thermal oxidative stability and effect of water on gas transport and mechanical properties in PA6‐EVOH films

The thermal oxidative stability and the effect of water on gas transport and mechanical properties of blends of polyamide 6 (PA6) with ethylene‐co‐vinyl alcohol (EVOH) and EVOH modified with carboxyl groups (EVOH‐COOH) have been investigated. The presence of EVOH reduces water vapor and oxygen gas permeability of polyamide, as well as small amounts of EVOH‐COOH further improve barrier properties, especially to oxygen. This has been explained in terms of improved interactions of the blend constituents in the amorphous phase, due to ionic linkages between the polyamide amino groups and the carboxyls of modified EVOH. The permeation to gases was found to increase with the amount of sorbed water. The morphology of the samples was found to have an effect on barrier properties, as the presence of EVOH causes the PA6 α crystalline form to increase, lowering the permeability to oxygen and water vapor. Mechanical properties are strongly affected by water sorption, as tensile modulus and strength decrease with increasing water content. Chemiluminescence (CL), infrared spectroscopy (FTIR), and tensile test were employed in order to assess the correlation between chemical composition and the thermal oxidative stability of the films aged at 110 °C in air. CL experiments suggest that the presence of EVOH and EVOH‐COOH efficiently inhibits the formation of peroxidized species during the processing, and increases the thermal oxidative stability of the films. Infrared spectroscopy showed a build‐up of carbonyl absorption in the range 1700–1780 cm^?1, due to the formation of oxidation products, which is greater in the case of the pure polymer. Tensile tests on films revealed a reduction in ductility as a result of ageing for neat PA6, whereas in comparison the blends exhibit a far better long‐term stability. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 840–849, 2007     

Thermal and photochemical Wallach rearrangement of azoxybenzene in zeolite cages

Acidic zeolites HY and CaY catalyse the Wallach rearrangement of azoxybenzene (I) leading mainly to the formation of para-hydroxyazobenzene (II) and ortho-hydroxyazobenzene (III). In this transformation, increasing the loading level of I results in the formation of a larger amount of para-isomer. As observed in isotropic media, photochemical Wallach rearrangement in the presence of various cation-exchanged faujasites results in the predominant formation of the ortho-isomer from the S₁ state and this rules out any appreciable heavy atom effect.     

Spectroscopic investigation of chalcone-analogous ferrocenes ortho-substituted in the aromatic ring II. Ferrocenyl---CO---CH=CH---aryl-type compounds

Chalcone-analogous ferrocenes of the type Fc---CO---CH=CH---Ar, ortho-substituted in the aromatic ring were prepared. Their conformations and electron distributions were investigated by cyclic voltammetry, IR, ¹H and ¹³C NMR spectroscopy. The spectroscopic properties measured for this series were compared with those of para-subsituted analogues and with data for structural isomers synthesized and studied earlier and having the aryl and ferrocenyl groups bonded to the enone moiety in reversed positions (Fc---CH=CH---CO---Ar). In contrast with the isomers investigated earlier, the new series are coplanar and the S-cis → S-trans conformational equilibria are shifted in favour of the former.     

Chloromethylphenylcarbamate derivatives of cellulose as chiral stationary phases for high-performance liquid chromatography

A new class of eight chloromethylphenylcarbamate derivatives of cellulose was prepared by introducing both an electron-donating methyl group and an electron-withdrawing chloro group on to the phenyl moieties and their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography. The superiority of these derivatives over dichloro- and dimethylphenylcarbamates of cellulose as CSPs was demonstrated for some racemic compounds. The elution order and enantioselectivity were greatly dependent on the positions of the substituents. Meta- and para-disubstituted derivatives showed higher chiral recognition than ortho- and meta- or para- disubstituted derivatives. The correlation between the chemical shifts of the N---H protons of the carbamate moieties and the enantiomer-resolving abilities of the derivatives is discussed. Some of the derivatives were effective CSPs in both normal- and reversed- phase conditions and could efficiently separate some chiral drug enantiomers.     

Excited Donor–Acceptor Complexes in the Polymerization of Vinyl Monomers

Butyl methacrylate was found to affect the composition of radical intermediates formed in the photoreduction of benzophenone with triethylamine. In the presence of the monomer, the yield of free radicals decreased and the yield of complexes of the geminate radical pair increased. This was explained by the formation of excited ternary complexes resulted from the interaction of the excited triplet state of benzophenone with the ground-state complex of butyl methacrylate and triethylamine. The substituent effect in benzophenone on the stability of the radical complex was studied. The reaction rate constant for the decay of the radical complex was correlated with the Hammett ⁰ _c constant that determines the mesomeric effect of the substituent.     

A Laser Flash Photolysis Study on Fenofibric Acid

Fenofibric acid (FA) is a photosensitizing drug used in the treatment of hyperlipidemia. This compound follows two different photodegradation pathways: the free acid exhibits the typical benzophenone photoreactivity, while its sodium salt undergoes photodecarboxylation via a triplet biradical, that undergoes intramolecular electron transfer to form a carbanion, or cyclization to give an intramolecular light-absorbing transient (LAT). The obtained photoproducts are explained as the result of pro-tonation of the carbanion, ring opening of the LAT with rearrangement or oxygen trapping of any of the triplet intermediates. The above mechanism is supported by direct detection of the triplet state of FA and two long-lived intermediates in laser flash photolysis experiments. The triplet lifetime of the carboxylate form in methanol is 0.06 μ.s; by contrast, in the case of the free acid, it is 10 times longer. The benzophenone moiety is clearly the key chromophore involved in the photobehavior of FA.     

Coulometric titration of deactivated phenols with bromine

A number of deactivated phenols containing fluorine, chlorine or bromine, formyi, acetyl, carboxyl or nitro groups have been titrated with anodically generated bromine. The reaction was carried out in a water-acetic acid-pyridine medium and the reactivity was controlled by varying the water and pyridine content and the concentration of bromide ion. Hydrogen in all free positions ortho and para to the phenolic hydroxyl group is generally exchanged for bromine, but in certain instances a partial bromination is possible. The method as developed is widely applicable for deactivated phenols. Only certain ortho-substituted phenols could not be quantitatively titrated. The mean relative error for the phenols titrated was ± 1·2%.     

A study of photochemistry of flavins in pyridine and with a donor

Abstract— Both anaerobic and aerobic photolysis of riboflavin in pyridine yielded several photoproducts, analogous to the photochemical reaction in aqueous solution. Lumiflavin was also photoreduced in pyridine (an electron donor) without decomposition of the isoall-oxazine ring Differences in the reactivity of excited singlet and triplet states with respect to formation of photoproducts have been confirmed in pyridine. In the photoreduction of riboflavin by N, N'-dimethyl-N-benzylethylene diamine, an initial rate with a higher relative quantum yield than that of the reaction at a later stage was observed both in water and pyridine. Photolysis in D₂O with the donor showed no significant solvent isotope effect. These results strongly suggest that the photolysis of riboflavin in water does not involve the water molecule in the primary photochemical act. A detailed mechanism of flavin photoreaction in water to account for solvent oxygen incorporation into a photoproduct (benzaldehyde) from N, N'-dimethyl-N-benzylethylene diamine has been proposed without involving photochemical splitting of water based on our results and molecular orbital computations. Preliminary results on the kinetics of free radical decay were obtained using an ESR spectrometer and the significance of the results are discussed.     

Identification of products formed during UV irradiation of tamoxifen and their use for fluorescence detection in high-performance liquid chromatography

During the UV irradiation of tamoxifen, isomerization of the trans to the cis isomer takes place and consequently corresponding highly fluorescent phenanthrene derivatives are formed. Their formation can be used for the sensitive and selective detection of tamoxifen in high-performance liquid chromatography (HPLC). The structure of photoproducts was identified by 1H NMR spectroscopy, HPLC, gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry. Owing to the variety of products formed and the higher selectivity and fluorescence response, on-line postcolumn photocyclization is preferred to the precolumn mode. A chromatographic system for the separation of isomers and photoproducts is suggested.     

Femtosecond transient absorption, nanosecond time-resolved resonance Raman, and density functional theory study of fenofibric acid in acetonitrile and isopropyl alcohol solvents

Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropyl alcohol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments. The singlet excite state ((1)FA) (nπ*) with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA ((3)FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* (3)FA species does not decay obviously within 3000 ps. In the ns-TR(3) experiments, the nπ* (3)FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* (3)FA species seems to have a much higher hydrogen abstraction reactivity so that (3)FA decays fast and generates a FA ketyl radical like species. In isopropyl alcohol solvent, the nπ* (3)FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropyl alcohol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropyl alcohol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR(3) spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the ns-TR(3) spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.     

Synthetic utility of 4-bromo-2,3,5,6-tetrafluoropyridine

The regiochemistry of nucleophilic substitution of 4-bromo-2,3,5,6-tetrafluoropyridine has been investigated. Efficient, regioselective reactions occur with alkylamine, benzylamine and alkoxide nucleophiles, yielding products where substitution occurs ortho to the ring nitrogen. The resulting 2-substituted-4-bromo-3,5,6-trifluoropyridines can be functionalised further, either by a second regioselective nucleophilic displacement or palladium catalysed elaboration at the 4-position. Reactions with aromatic N-nucleophiles yield mixtures of ortho- and para-substituted products.     

Calorimetric study of the ionization process for 2,5-dinitrophenol in water—dimethylsulfoxide mixtures at 25°C

The ionization and solution enthalpies of 2,5-dinitrophenol were measured calorimetrically at 25°C in water—DMSO mixtures ranging from 0.1 to 0.8 mole fraction of DMSO.

The effect of the nitro group in the ortho position seems to prevail over that of the nitro group in the meta position.

The greater acidity of dinitrophenol with respect to the mononitro isomers is explained on the basis of the interference of the nitro group in the metaposition on the interactions between the nitro group in the ortho position and the hydroxyl group or phenolate oxygen.     

Carboranedithiols: building blocks for self-assembled monolayers on copper surfaces

Two different positional isomers of 1,2-dicarba-closo-dodecaboranedithiols, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (1) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.     

The preparation of, and electrochemical studies on, some substituted aryl dithiadiazolylium salts and dithiadiazolyl radicals

A series of aryl-substituted 1,2,3,5-dithiadiazolylium cations (I) and 1,3,2,4-dithiadiazolylium cations (II) were prepared as their hexafluoroarsenate(V) salts using standard methods. Electrochemical studies on I and II showed reversible one-electron reductions. The half-wave reduction potentials for a series of meta-substituted derivatives of both I and II exhibited a linear free energy relationship with the Hammett parameter, σ_m. The small value of the reaction constant, , for both meta and para-derivatives indicates that electronic effects are small and in the case of the ortho-derivatives of II, steric effects dominate the redox process. Reduction of the 1,2,3,5-dithiadiazolylium cations, as their chloride salts, yielded the corresponding dithiadiazolyl radicals (III).     

Liquid-phase fluorination of aromatic compounds by elemental fluorine

The fluorination of aromatic compounds (benzene, toluene, phenol and benzoic acid) by elemental fluorine diluted with nitrogen has been investigated in various solvents (Freon 11, chloroform, methanol, trifluoroacetic acid, 2,2,2-trifluoroethanol, water) in order to define the influence of the experimental conditions on the reaction. Experiments have been carried out by varying the temperature, the substrate concentration in solution, the molar ratio of fluorine to substrate, and the concentration of fluorine in the fluorine/nitrogen mixture. In all cases, the effects on the yield of fluorinated products were studied. Monofluorinated compounds were mainly found in the reaction mixture, the isomers formed being in accord with the mechanism for electrophilic substitution. The highest yield of monofluorinated products was obtained with polar solvents and the following order was found: CFCl₃ < CHCl₃ < CH₃OH < CF₃CH₂OH < CF₃COOH. Interesting results were also found using particular additives (for instance, KOH or C₄F₉SO₃Na in methanol) or water as the solvent. A direct relationship was observed between the yield of monofluorinated compounds and the molar ratio of fluorine to substrate, which has to be less than one in order to obtain high yields. In contrast, low selectivity, expressed as the yield ratio of ortho to para (or meta) isomers, was found.