Transient products in the photoreduction of benzophenone derivatives in poly(ethylene-vinyl alcohol) film

The formation, stability, and reactivity with oxygen of the so-called light-absorbing transient (LAT) photoproducts formed on photoreduction of benzophenone and several benzophenone derivatives in poly(ethylene-vinyl alcohol) (EVOH) films were investigated. The maximum total yield of LAT photoproducts on photoreduction of benzophenone was estimated to be approximately 50%, which was found to comprise around 75% of the para-LAT isomer with the remainder being attributed to the ortho-LAT isomer. The concentrations of these LAT isomers were found to decay over a period of weeks at room temperature in the absence of oxygen, with the decay of the para-LAT isomer being substantially slower than the ortho-LAT isomer. Furthermore, the decay rate of the para-LAT isomer was found to increase markedly in the presence of oxygen. Mechanisms for the decay of the LAT isomers that are consistent with available data are postulated. The presence of alkyl, alkoxy, or aryloxy moieties on the phenyl rings of benzophenone appears to have little effect on the efficiency of formation, stability, and reactivity with oxygen of the LAT photoproducts in EVOH films, for the benzophenone derivatives studied in this work.     

Synthesis of homoleptic Re(I) complexes of isocyano-carboranes and the X-ray structure of hexakis(p-carboran-1-yl-isonitrile)Re(I)

Two homoleptic Re(I) complexes of ortho and para-carborane isocyanide ligands were prepared as the first examples of a new class of metal-based BNCT and BNCS agents. The target compounds were prepared in low yield through the reaction of [Re₂(O₂CPh)₄Cl₂] and [Re₂(OAc)₄Cl₂] with 3-isocyano-1,2-dicarba-closo-dodecaborane and a para-carborane azetidine derivative respectively. The desired product from the latter reaction was characterized crystallographically and is only the second reported molecular structure of a homoleptic Re(I) isonitrile complex.     

Selective double Suzuki cross-coupling reactions. Synthesis of unsymmetrical diaryl (or heteroaryl) methanes

Using a double Suzuki cross-coupling reaction ortho- or para-bromobenzyl bromides are easily transformed into unsymmetrical diaryl (or heteroaryl) methanes     

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.     

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.     

A rationalization of orientation and reactivity in the nucleophilic replacement reactions of aromatic polyhalo-compounds

C₆F₅X compounds react with nucleophiles at the position para to X in some cases (X = H, CH₃, NMe₂, SMe, CF₃), meta in others (X = NH₂, O⁻) and occasionally at both meta and para in comparable amounts (X = OMe, NHME); significant amounts of ortho-replacement also occur (X = Cl, OMe). These orientations, and those in polychloro-aromatics and polyfluoro-polynuclear aromatics, can be rationalized by considering the relative stabilities of the Wheland-type intermediates involved (I, II, III), provided the halogens are taken as electron-repelling in π-systems. Reactivities can be rationalized if ground state stabilities are considered as well. A number of predictions have been made on the basis of these arguments.     

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).     

Diastereoisomer separation and absolute configuration of (ortho- and (meta-MeC6H4CO2Me)Cr(CO)2L (L = (+)-neomenthyl-PPh2)

Replacement of a carbonyl ligand by (+)-neomenthylPPh₂ provides a method for diastereoisomer separation of the methyl esters of (ortho- and (para-toluic acid)Cr(CO)₃. The absolute planar configuration have been assigned.     

Computational studies on glyceraldehyde and glycine Maillard reaction-II

Density-functional theory (BLYP/6-31G*) and semiempirical theory was used to study compounds related to a strained cyclic hexaalkyne that was postulated by Staab et al. as a trimeric intermediate in the copper-mediated oxidative coupling of 2,2′-diethynylbiphenyl. Among the products of this reaction is an ortho-arene cyclyne. It was found that the hexaalkyne→ortho-arene cyclyne rearrangement is quite exothermic (ca. 130 kcal/mol). Of the semiempirical methods tested, the AM1 results were closer than PM3 or MNDO to BLYP/6-31G*. For the intermediate, two conformations were found: the expected D₃-symmetric conformation and an unexpected C₂-symmetric conformation. Structures and energies of hexamethyl- and dodecamethyl-substituted analogues of the intermediate and product were also calculated at the AM1 level, in order to test whether steric effects might stabilize the intermediate, relative to the product. It was found that even very sterically crowded products were much more stable than the relevant intermediates. This suggested that the large exothermicity of the hexaalkyne→ortho-arene cyclyne rearrangement might be used to drive formation of interesting strained products. An example used to illustrate this is the rearrangement of a binaphthyl analogue of Staab's biphenyl intermediate. BLYP and AM1 calculations suggest that the rearrangement of this intermediate would be exothermic by ca. 105–110 kcal/mol, despite strain induced by formation of three [5]helicene moieties in the product.     

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.     

Big macrocyclic assemblies of carboranes (big MACs): synthesis and crystal structure of a macrocyclic assembly of four carboranes containing alternate ortho- and meta-carborane icosahedra linked by para-phenylene units

The macrocyclic compound, [1,2-C₂B₁₀H₁₀-1,4-C₆H₄-1,7-C₂B₁₀H₁₀-1,4-C₆H₄]₂ (5)—a novel cyclooctaphane, was prepared by condensation of the C,C′-dicopper(I) derivative of meta-carborane with 1,2-bis(4-iodophenyl)-ortho-carborane. The X-ray crystal structure of 5·C₆H₆·6C₆H₁₂ was determined at 150 K, revealing an extremely loose packing mode. Molecule 5 has a crystallographic C_s and local C_2v symmetry; the macrocycle adopts a butterfly (dihedral angle 143°) conformation with the ortho-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes. Multinuclear NMR spectra suggest that molecule 5 in solution inverts rapidly via the planar D_2h geometry, which (from ab initio HF/6-31G* calculations) is only 1 kcal mol⁻¹ higher in energy than the C_2v one. An attempt to prepare an even larger macrocycle, comprising three para-carborane and three ortho-carborane units linked by six para-phenylene units, was unsuccessful.     

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.     

Planarity recognition of cationic dyes by anionic, crystalline bilayer aggregates

Steric selectivity in terms of molecular planarity of cationic dyes was investigated using anionic bilayer aggregates. Planar cationic dye (para-type stilbazolium) could be incorporated into the hydrophobic region of anionic crystalline bilayer aggregates, whereas structurally related, less planar dyes (ortho-type stilbazolium) could not be incorporated in spite of somewhat higher hydrophobicity resulting from lengthening of the N-alkyl group.     

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.     

Dynamic, electrooptical and energetic nonequivalency of NH bonds in 1:1 and 1:2 complexes of aminopyridines with proton acceptors

The influence of the –NH₂ group position in the pyridine ring on the proton donor ability of N–H groups in hydrogen bonding as well as on the spectral behaviour of stretching and bending vibrations of aminopyridines has been studied. The proton donor ability was shown to increase in the row: meta-, ortho-, and para-aminopyridines. It was established tha N–H bonds in ortho-aminopyridine were not equivalent, and the evaluation of their dynamic nonequivalence was made.

The influence of temperature on the spectral characteristics of the absorption bands of the stretching vibrations of amine groups in the free and hydrogen bonded molecules in CCl₄ has been studied (in temperature range 290–330 K), the formation constants of the complexes have been determined, enthalpy of the 1:1 complexes formation (−ΔH₁) between ortho- and meta-aminopyridines with dimethylformamide, dimethylsulphoxide and hexamethylphosphoramide has been calculated in temperature range 290–330 K. The 1:2 complexes of ortho-, meta- and para-aminopyridines with acetonitrile, tetrahydrofurane, dimethylsulphoxide, hexamethylphosphoramide were studied at the indoor temperature. Enthalpy of the 1:2 complex (−ΔH₂) was estimated on the basis of ‘intensity rule’; −ΔH₁=ΔB^1/2 assuming that parameter does not depend on the composition of a complex.

The vibrational and electrooptical tasks were solved for the free and H-bounded molecules of aminopyridines as well as its complexes of the 1:1 and 1:2 compositions. Dynamic, electrooptical and energetic nonequivalency of NH bonds of aminogroups in aminopyridines was studied quantitatively. The independent calculations of dynamic constants proved mentioned above nonequivalency of NH bonds.

Correlations between spectral characteristics of the absorption bands, geometric, dynamic and electrooptical parameters of –NH₂ group in aminopyridines in the free and hydrogen bonded molecules have been established. Those correlations allow to determine the most important molecular characteristics obtained on the basis of spectral measurements in the range of the absorption bands of the stretching vibrations of aminogroup.     

半联胺重排机理

半联胺通常是N,N’-二芳基肼在酸性条件下发生联苯胺重排反应时的副产物,包括邻半联胺和对半联胺。在某些情况下,如取代芳基的N,N’-二芳基肼重排的主要产物为邻半联胺和对半联胺。关于半联胺重排反应的机理,曾提出过π络合物过渡态机理、环己二烯正离子中间体机理和氯离子桥链的双阳离子型自由基对中间体机理,但通过动力学同位素效应、理论计算和实验结合研究确定该重排反应是构型翻转的N[1,3]σ迁移反应为关键步骤的周环反应。本文介绍了半联苯胺重排反应机理的研究历程及其代表性的实例。     

Liquid crystalline properties and photochromism of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-methoxy and -ethoxy anilines

New homologous series of N-[4-(4-n-alkoxybenzoyloxy)-2-hydroxybenzylidene]-methoxy and -ethoxyanilines were synthesized. The phase transitions of these homologues were determined using differential scanning calorimetry and an polarizing microscopy: para-substituted homologues exhibit a nematic phase, while ortho-substituted members do show no mesomorphic phase. In addition, some members of these homologous series exhibit a photochromic property in the solid state.     

The stability of columns comprising alternating triphenylene and hexaphenyltriphenylene molecules: variations in the structure of the hexaphenyltriphenylene component

Previous investigations of complementary polytopic interaction (CPI) columnar mesophases, in which the columns are built up of alternating hexaalkoxytriphenylene (HAT) and hexaphenyltriphenylene (PTP) molecules, concentrated mainly on the effect of variations in the structure of the HAT component. This investigation is concerned with the effect of variations in the structure of the PTP component and, in particular, variations in the position of an alkoxy side chain in the phenyl ring. Stable columnar mesophases are obtained when a hexyloxy substituent is placed in the meta- or para-position but not in the ortho-position. In the case of the meta- and para-substituted systems, the two-component CPI columnar phases are stable over a considerably larger temperature range than the one-component HAT systems. The evidence suggests that unfavourable PTP/PTP stacking is as much a driving force for the formation of these mixed stacks as is favourable PTP/HAT stacking, but both need to be explained in terms of the sum of atomically dispersed van der Waals and coulombic interactions. On cooling from the isotropic into the Col_h phase, the columnar phase based on a 1:1 mixture of hexakis(hexyloxy)tripenylene and the meta-hexyloxy-substituted PTP gives an unusual texture consisting of 'viking-axe'-shaped structures.     

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.