Supramolecular catalysis of the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylate by gamma-cyclodextrin

2-Anthracenecarboxylic acid (AC) makes a very stable 1:2 inclusion complex with gamma-cyclodextrin (gamma-CDx) (K(1) = 161 +/- 25 M(-1), K(2) = 38 500 +/- 3300 M(-1) at 25 degrees C). The formation of the 1:2 inclusion complex accelerated the photocyclodimerization of AC. The 1:2 inclusion could be clearly verified by UV-vis, CD, and (1)H NMR spectroscopies. Although these spectroscopies provide little information about the structural isomers of the inclusion complex, there should be several structural isomers of the 1:2 inclusion complex which have a different longitudinal orientation of the guest molecules in the cavity. The isomer distribution of the photodimerization product primarily depends on the population of these orientational isomers of the 1:2 inclusion complex in the ground state before photoreaction, because, in the lifetime of the excited singlet state, exchanging the orientation is impossible. The enantioselectivity of the photodimerization originates from the difference in the stability of the diastereomeric pair of orientational isomers of the 1:2 inclusion complex in the ground state, which are the precursors of the enantiomers of a specific chiral cyclodimer. The ee of a chiral cyclodimer 2 was 32% at 25 degrees C and was enhanced by lowering the temperature to 41% at 0 degrees C. This is the highest value reported for the asymmetric photodimerization in solution.     

Supramolecular enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by capped gamma-cyclodextrins: critical control of enantioselectivity by cap rigidity

A series of gamma-cyclodextrins (CDs) modified with capping and noncapping aromatic group(s) were synthesized to mediate the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). The complexation behavior of these gamma-CDs with AC was studied by circular dichroism, UV-vis, and NMR spectroscopy to reveal the formation of stable 1:2 host-guest complexes in all cases. The capped gamma-CD with a biphenyl group bridging the A and D glucose units was shown to confine the included AC molecules most strictly among the capped and noncapped gamma-CDs examined. Photocyclodimerization of AC mediated by capped gamma-CDs considerably improved the yield and enantiomeric excess (ee) of the head-to-head photodimer 3. The ee and the absolute configuration of syn-head-to-tail photodimer 2 critically depended on the rigidity of capping. Thus, the flexibly capped and rim-substituted gamma-CDs afforded 2 in moderate ee's of around 40%, whereas gamma-CD with a rigid biphenyl cap gave the antipodal 2 in -58% ee. Interestingly, the ee of 2 mediated by flexibly capped gamma-CDs was highly sensitive to the temperature variation as a consequence of large differential entropy changes in the enantiodifferentiation process. In contrast, the entropy effect does not appear to play a significant role in the photocyclodimerization of AC with rigidly capped gamma-CDs. The differential enthalpy and entropy changes obtained for the enantiodifferentiating photocyclodimerization mediated by native and most of the modified gamma-CDs gave an excellent enthalpy-entropy compensation plot with an exception of the biphenyl-capped gamma-CD, indicating the operation of significantly different enantiodifferentiation mechanism within the rigidly capped cyclodextrin cavity.     

Pressure and temperature-controlled enantiodifferentiating [4+4] photocyclodimerization of 2-anthracenecarboxylate mediated by secondary face- and skeleton-modified gamma-cyclodextrins

A series of secondary-face-substituted and skeleton-modified gamma-cyclodextrins (gamma-CDs) were prepared as chiral hosts for enantiodifferentiating [4+4] photocyclodimerization reactions of 2-anthracenecarboxylic acid (AC). These gamma-CD derivatives form stable ternary complexes with ACs, with altroside-bearing gamma-CDs undergoing induced-fit conformational changes upon complexation, and the photocyclodimerization of AC was, thus, dramatically accelerated. The enantiomeric excess (ee) of anti-head-to-head cyclodimer 3 was greatly enhanced in general with altroside-bearing gamma-CDs 7-9. Although mono-altro-gamma-CD 9 and 3A-azido-3A-deoxy-altro-gamma-CD 7 gave 2 in ee's smaller than those obtained with native gamma-CD, 3A-amino-3A-deoxy-altro-gamma-CD 8 yielded 2 in much higher ee's, which is likely to be ascribed to the combined effects of the less-symmetric cavity and the electrostatic interactions. The influence of temperature and high pressure on the supramolecular photochirogenic reaction has been investigated in depth. An ee as high as 71% was obtained for cyclodimer 2 in the photocyclodimerization of AC mediated by 8 at 210 MPa and -21.5 degrees C.     

Supramolecular engineering through temperature-induced chemical modification of 2H-tetraphenylporphyrin on Ag(111): flat phenyl conformation and possible dehydrogenation reactions

Scratching the surface: Formation of a monolayer of 2H-tetraphenylporphyrins (2H-TPP) on Ag(111), either by sublimation of a multilayer in the range 525-600?K or by annealing (at the same temperature) a monolayer deposited at room temperature, induces a chemical modification of the molecules. Rotation of the phenyl rings into a flat conformation is observed and tentatively explained, by using DFT calculations, as a peculiar reaction due to molecular dehydrogenation.     

Controlled synthesis of olive-shaped Bi2S3/BiVO4 microspheres through a limited chemical conversion route and enhanced visible-light-responding photocatalytic activity

Well-defined olive-shaped Bi(2)S(3)/BiVO(4) microspheres were synthesized through a limited chemical conversion route (LCCR), where olive-shaped BiVO(4) microspheres and thioacetamide (TAA) were used as precursors and sulfur source, respectively. The as-synthesized products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission microscope (HRTEM), X-ray photoelectron spectra (XPS), UV-visible diffuse-reflectance spectroscopy (UV-vis DRS), and photoluminescence (PL) spectra in detail. Compared with pure BiVO(4) microspheres and Bi(2)S(3) nanorods, the Bi(2)S(3)/BiVO(4) products showed obviously enhanced photocatalytic activity for the degradation of rhodamine B (Rh B) in aqueous solution under visible-light irradiation (λ > 400 nm). In addition, the Bi(2)S(3)/BiVO(4) composite microspheres showed good visible-light-driven photocatalytic activity for the degradation of refractory oxytetracycline (OTC) as well. On the basis of UV-vis DRS, the calculated energy band positions, and PL spectra, the mechanism of enhanced photocatalytic activity of Bi(2)S(3)/BiVO(4) was proposed. The present study provides a new strategy to design composite materials with enhanced photocatalytic performance.     

Irradiation of imine-group VI carbene complexes in the presence of alkynes. 2. Control of product distribution

The photoreactivity of iminecarbene complexes in the presence of alkynes has been explored. Up to four different reaction paths are available depending on the alkyne and carbene complex substituents, although in each case only one type of product is isolated. 2H-Pyrrole derivatives are formed mainly from aryl alkynes. When alkyl alkynes are used, the method affords a new type of aza-dendralene product in good yields. Isoquinoline derivatives can also be formed in a two-step one-pot photochemical process when the appropriate substituents are present. Finally, indene derivatives are also available through a benzannulation reaction. To explore the underlying mechanism, we carried out computations using DFT methods. Experimental and theoretical results compare well, which allows control over the reaction and product distribution.     

Synthetic modification of manzamine A via Grubbs metathesis. Novel structures with enhanced antibacterial and antiprotozoal properties

A strategy for the structural modification of biologically important alkene-containing natural products via ring-opening olefin metathesis is described. Exposure of manzamine A 1 to the second-generation Grubbs catalyst in the presence of ethylene leads to the formation of 2 and 4. The antibacterial activity of the novel manzamine analogue 2 (IC50=0.10 nM) against Mycobacterium intracellulare is ca. 2-fold more potent than that of ciprofloxacin (IC50=0.18 nM), a drug that is frequently used against antibiotic-resistant infections.     

Substrate-selective aqueous organometallic catalysis. How size and chemical modification of cyclodextrin influence the substrate selectivity

Randomly hydroxypropylated and methylated cyclodextrins with different cavity size have been used as inverse phase transfer catalysts in a palladium catalyzed Tsuji-Trost reaction with water-insoluble alkylallylcarbonates and alkylallylurethanes as substrate. It has been shown that the molecular recognition ability of both α-CD and β-CD derivatives towards these substrates was responsible for an increase in the reaction rates and remarkable substrate selectivities between a linear and a branched structure. By contrast, the too wide cavity of γ-CD derivatives did not allow these carriers to be efficient in terms of substrate selectivity. Thus, the performances of a cyclodextrin carrier in this cleavage reaction strongly depended on the size of the cavity in which the substrate had to fill in as close as possible the available space.     

Palladium(0)-catalyzed asymmetric synthesis of 2-vinylmorpholine and 2-vinylpiperazine. Influence of the biscarbonate structure on the enantioselectivity

Palladium-catalyzed cyclization of N,N-bis(p-tolylsulfonyl)-o-phenylenediamine 2b and 2-[(2,4,6-trimethylphenyl)sulfonyl]aminophenol 2c with three allylic biscarbonates 1a–c gave quite different enantioselectivities. This indicates that the cyclization processes do not have a common intermediate, as in the case of benzene-1,2-diol.     

Chemical Selectivities Disguised by Mass Diffusion. VIII. Influence of pH-gradients on the product distribution in mixing-disguised azo coupling reactions. 9th Communication on the selectivity of chemical processes

In mixing-disguised azo coupling reactions the protons which are released during the electrophilic substitution steps cause a pH-gradient in the reaction zone. Since, owing to the acid-base pre-equilibria, these pH-gradients also determine the local concentrations of the reactants in the reaction zone, they also have an influence on the measured product distribution of such fast reactions. In the present work, the product distribution of the azo coupling of 1-naphthol with 4-sulfophenyldiazonium ion was experimentally measured and the results were compared with the distributions predicted from our mixing-reaction model developed previously [1]. Furthermore, some experimental evidence for the existence of isoselectivity points are presented, at which a change of the initial pH-value has no influence on the product distribution.     

Layer‐by‐Layer Assemblies of Catechol‐Functionalized TiO2 Nanoparticles and Porphyrins through Electrostatic Interactions

In the current work, we present the successful functionalization and stabilization of P‐25 TiO₂ nanoparticles by means of N1,N7‐bis(3‐(4‐tert‐butyl‐pyridium‐methyl)phenyl)‐4‐(3‐(3‐(4‐tert‐butyl‐pyridinium‐methyl)phenylamino)‐3‐oxopropyl)‐4‐(3,4‐dihydroxybenzamido)heptanediamide tribromide ( 1 ). The design of the latter is aimed at nanoparticle functionalization and stabilization with organic building blocks. On one hand, 1 features a catechol anchor to enable its covalent grafting onto the TiO₂ surface, and on the other hand, positively charged pyridine groups at its periphery to prevent TiO₂ agglomeration through electrostatic repulsion. The success of functionalization and stabilization was corroborated by thermogravimetric analysis, dynamic light‐scattering, and zeta potential measurements. As a complement to this, the formation of layer‐by‐layer assemblies, which are governed by electrostatic interactions, by alternate deposition of functionalized TiO₂ nanoparticles and two negatively charged porphyrin derivatives, that is, 5,10,15,20‐(phenoxyacetic acid)‐porphyrin ( 2 ) and 5,10,15,20‐(4‐(2‐ethoxycarbonyl)‐4‐(2‐phenoxyacetamido)heptanedioic acid)‐porphyrin ( 3 ), is documented. To this end, the layer‐by‐layer deposition is monitored by UV/Vis spectroscopy, scanning electron microscopy, ellipsometry, and profilometry techniques. The resulting assemblies are utilized for the construction and testing of novel solar cells. From stable and repeatable photocurrents generated during several “on‐off” cycles of illumination, we derive monochromatic incident photo‐to‐current conversion efficiencies of around 3 %.     

Trapping of hydride forming elements within miniature electrothermal devices. Part 2. Investigation of collection of arsenic and selenium hydrides on a surface and in a cavity of a graphite rod

The interaction of arsenic and selenium hydrides with bare and modified graphite was investigated by atomic absorption spectrometry and by radiotracer technique using ⁷⁵Se radionuclide in a laboratory made brass cylindrical chamber equipped with a vertical quartz tube torch for supporting miniature hydrogen diffusion flame atomizer. Strong interaction was observed at elevated temperatures above 800 °C. In contrast to the very often-reported data for conventional graphite tube atomizers, this high temperature interaction was also accompanied by a pronounced trapping of analytes at elevated temperatures close to 1100–1200 °C when modified graphite was used. Comparing modifiers tested (Ir, Pt and Rh), iridium appeared the only useful permanent modifier. Among various graphite-rod traps designed, the most efficient trapping of analytes was achieved in a graphite cavity. The net selenium trapping efficiencies of approximately 53% and 70% were found by radiotracer technique for the iridium-treated graphite surface and the iridium-treated graphite cavity, respectively. In contrast to the molybdenum surface, bare graphite did not exhibit any significant trapping effect. Trapping isotherms obtained at different temperatures displayed non-linear course in the range up to the upper limit of the analytical relevance of 100 ng of an analyte, indicating a limited trapping capacity of the modified graphite surface and the same trapping mechanism at low and elevated temperatures applied (300–1300 °C). Radiography experiments with ⁷⁵Se radiotracer showed that a major part of selenium was collected within the small cavity of the graphite rod and that selenium was also deposited after the trapping and vaporization steps in the trap chamber and on the quartz tube wall of the burner. Complementary experiments performed with the conventional transversally heated graphite tube and with bare and thermally shielded injection capillaries for hydride introduction, showed that the pronounced trapping effect could not be observed at elevated temperatures in conventional systems equipped with the bare capillary. The losses of analytes in the non-shielded bare introduction capillary exposed to the heat decrease the transport efficiency of hydrides into the graphite tube, and consequently they cause reduction of the overall trapping efficiency at elevated temperatures.     

Unraveling the spatial distribution of immunoglobulins, enzymes, and polyelectrolytes within layer-by-layer self-assembled multilayers. Ellipsometric studies

The ellipsometric characterization of a layer-by-layer electrostatically self-assembled multilayer of polyphenol oxidase and alkaline phosphatase with the polycation poly(dimethyldiallylammonium chloride) built on an immunologic layer formed by immunoglobulin G (IgG) and glucose oxidase-conjugated anti-IgG (IgG-GOD) on glassy carbon is reported. The step-by-step evolution of the psi-Delta ellipsometric angles was followed during film growth. Two optical models, named the three-layer film model and reorganization film model, were employed and found suitable for ellipsometric data interpretation. A comparative analysis of film optical properties, film thickness, and ellipsometric mass assessed from both models is also presented.     

An experimental and theoretical study of the product distribution of the reaction CH2 (X 3B1) + NO

Measurements of the product branching ratios of the reaction CH2 (X 3B1) + NO (1) are presented together with calculations of the thermal rate constant and branching ratios using unimolecular rate theory. The reaction was investigated experimentally at room temperature using FTIR spectroscopy. The yields of the main products HCNO and HCN were found to be gamma HCNO = 0.89 +/- 0.06, gamma HCN = 0.11 +/- 0.06. Other minor products could be rationalized by numerical simulations of the reaction system taking into account possible consecutive reactions. The potential energy surface for the reaction was characterized by quantum chemical calculations using ab initio and density functional methods. The proposed reaction pathways connecting reactants to products were explored by multi-channel unimolecular rate theory calculations to determine the CH2 (X) + NO capture rate constant and the rate constants for the different product channels as a function of temperature. The calculated capture rate constant of k = 2.3 x 10(13) cm3 mol-1 s-1 is in good agreement with experimental values at room temperature. Collisional stabilization of the initial H2CNO recombination complex was predicted to be negligible up to pressures of > 1 bar. For ambient pressures and temperatures up to 2000 K, HCNO + H were calculated as the dominating products, with gamma HCNO approximately 0.94 in agreement with the experiments. The channel to HCN + OH was calculated with 0.015 < or = gamma HCN < or = 0.05, only slightly below the experimental value.     

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.     

Nonempirical analysis of unusual chemical bonds. II. AlH2BH4 and AlH2C3H5

Ab initio computations have been performed on the compounds AlH₂BH₄ and AlH₂C₃H₅ in order to achieve a better understanding of the peculiar characteristics of the metal to ligand bond. Our results suggest a bidentate coordination for the tetrahydroborate complex and an η³ coordination for the allyl complex. Due to the nonrigidity of the BH and allyl complexes, possible interconversion paths between different coordination modes have also been analyzed.     

Mechanism of Azo Coupling Reactions XXXIII. pH-dependence and micromixing effects on the product distribution of couplings with 6-amino-4-hydroxy-2-naphthalenesulfonic acid. Evidence for N-coupling of a naphthylamine derivative

Azo coupling of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (1) with 3-trifluoromethyl- and 4-nitrobenzenediazonium ion in relative highly concentrated aqueous alkaline solutions gave ratios of aminoazo to hydroxyazo compounds which are much higher than expected on the basis of the acid-base pre-equilibria of 1 . These product ratios are disguised by effects of micromixing. In dilute solution (≤10^?2 mol/l) product ratios and kinetics both correspond to the theory of acid-base pre-equilibria. A bisazo dye 10 was formed as a by-product in couplings with 3-trifluoromethylbenzediazonium ion, as expected for reactions with micromixing effects. In the reaction with benzenediazonium ions, the products of azo coupling of diazotized 1 with 1 (compound 8 ) and of the monoazo compound 8 with benzenediazonium ion (compound 9 ) were found. It is likely that diazotized 1 is formed by N-coupling of 1 with benzenediazonium ion, tautomeric rearrangement and protonation of the triazene to diazotized 1 and aniline. This seems to be the first case of N-coupling of a naphthylamine which was assumed to be capable of C-coupling only.