Selective oxidation of unactivated C-H bonds by supramolecular control

Efficient methods for dioxirane-based selective C-H bond oxidation by supramolecular control in H(2)O have been developed. With β-cyclodextrin as the supramolecular host, site-selective oxidation of the terminal over the internal tertiary C-H bond of 3,7-dimethyloctyl esters 3a-c was achieved. In addition, β-cyclodextrin selectively enhanced the C-H bond oxidation of cumene in a mixture of cumene and ethyl benzene in H(2)O. Through (1)H NMR studies, the selectivity in C-H bond oxidation could be attributed to the inclusion complex formation between β-cyclodextrin and the substrates.     

Functional monomers and polymers 159 synthesis and photochemical reactions of polymers containing thymine photodimer units in the main chain

Polyamides containing thymine photodimer units in the main chain were synthesized, and their photolysis by ultraviolet irradiation below 260 nm were studied in film state. Photodimers of thymine derivatives were obtained by photochemical reaction of the carboxylic acid derivatives of thymine in aqueous solution irradiated above 270 nm. An attempt was made to resolve the isomers of the photodimers, and the two kinds of cis isomers [cis–syn(head to head), and cis–anti(head to tail)] were isolated successfully. The polyamides were prepared by condensation of the photodimers with diamine using an activated ester method. The photodissociation of the thymine photodimer in the polymer main chain caused the breakage of the polymer chains, leading to the production of oligomers and dimer compounds containing thymine bases at the ends of the molecule. The dissociation rate of the polymer did not depend on the kind of the thymine photodimer which was in the main chain of the polymer.     

Xenon biosensor amplification via dendrimer-cage supramolecular constructs

Polyamidoamine dendrimers were synthesized with a single biotin moiety and used with cryptophane-A cages to form supramolecular biosensor constructs. These new biosensors amplified the NMR signals obtained from polarized xenon 8 times more than the original Xe biosensor.     

Electrostatic bubbles and supramolecular assistance of photosensitization by carboxylated Ru(II) complexes

The paper examines the supramolecular effects at play during photosensitization by carboxylated Ru(II) sensitizers, both by experiment and by modeling. Experimentally, twelve Ru(II) complexes of pyrazolylpyridine and polypyridine ligands, including two benchmark complexes and two new species, were assessed as photosensitizers by measurement of the kinetics of methyl viologen cation radical (MV(*)(+)) generation through an oxidative, photoinduced electron transfer (PET) to methyl viologen (MV(2+)) under continuous irradiation in the presence of a sacrificial reductant. All complexes, luminescent or not, produced measurable amounts of MV(*)(+) in CH(3)CN. The assessment protocol was found to be useful with sensitizers of widely varying excited-state lifetimes (tau) as well as being easier and faster than conventional approaches. The seven sensitizers bearing peripheral COOH groups were found to be significantly more active than their non-carboxylated analogues, which is consistent with ionization of the COOH groups and electrostatic promotion of PET. Only the luminescent complexes were active in aqueous solvents, where tau appears to be the dominant effector. The benefits are exemplified by the singly carboxylated [Ru(H1)(bpy)(2)](2+) (H1 is 1-(4-carboxyphenyl)-3-(2-pyridyl)-4,5,6,7-tetrahydroindazole), a weakly luminescent sensitizer that was less active in aqueous solvents than [Ru(bpy)(3)](2+) (bpy is 2,2'-bipyridine), but which became the better sensitizer in CH(3)CN. Computationally, electrostatic field and dissociation energy calculations demonstrated that even a single peripheral COO(-) substituent suffices to provide supramolecular assistance: it defines a spheric "bubble" of electrostatically attractive space that is sufficiently large to allow the supramolecular preassociation of MV(2+), which provides an entropic advantage to PET that reduces the importance of tau in organic solvent. Calculations also show that the PET is electrostatically favored over its reverse (BET) even with cationic sensitizers because the "bubble" contracts after PET while the bulk medium becomes more repulsive, and favorable cation exchanges can occur to effect post-PET dissociation. Two peripheral COO(-) groups can define a two-point binding site for MV(2+) in an attractive sector of space that contracts to a kidney-shaped "bubble" after PET. This enables unimolecular PET while the reverse reaction remains bimolecular. The resultant benefits are illustrated with [Ru(Na1)(2)(bpy)](2+), a very weakly luminescent sensitizer that was totally inactive in H(2)O but appreciably active in CH(3)CN, despite the need to displace Na(+) in order to derive any electrostatic benefit. The Marcus free energies of activation for PET and BET corroborate the benefits of carboxylation, solvent, and other factors and correlated with the experimental rate constants.     

Selective supramolecular bindings for stepwise signal output

The design and construction of functional supramolecular systems capable of digital information processing is very important. In this work, we synthesized two fluorescent guest compounds (OVN and MVN) for supramolecular bindings at different sites with cucurbit[7]uril (CB7) and α-cyclodextrin (α-CD). The selective host–guest formation was employed to control the systems’ fluorescent signal for different-mode performances. The structural characterizations were conducted by ¹H NMR, HRMS, and 2D NOESY ¹H NMR spectroscopy. Specifically, the selective supramolecular bindings of CB7 and α-CD will cause a stepwise variation of the fluorescent signal output in OVN guest system. This signal can also be stepwise reset by introduction of different competing substances. On the other hand, supramolecular bindings can only lead to a binary signal change in MVN guest system.     

Signal amplification and detection via a supramolecular allosteric catalyst

The design of a supramolecular allosteric catalyst system for catalytic signal amplification and detection is presented. The catalyst was switched "on" by the introduction of an analyte that also behaves as an allosteric activator. Concentrations of Cl- ions as low as 800 nM were catalytically amplified and detected. The signal was transduced via a pH-sensitive fluorescent probe and observed visually using a laboratory, handheld UV lamp and by spectrophotometry. Furthermore, the allosteric effect was quantified using gas chromatography for a range of Cl- concentrations. This three-part detection scheme involving analyte binding, allosteric catalyst activation, and signal transduction represents a new approach to small-molecule detection.     

Selective aldehyde reductions in neutral water catalysed by encapsulation in a supramolecular cage

The enhancement of reactivity inside supramolecular coordination cages has many analogies to the mode of action of enzymes, and continues to inspire the design of new catalysts for a range of reactions. However, despite being a near-ubiquitous class of reactions in organic chemistry, enhancement of the reduction of carbonyls to their corresponding alcohols remains very much underexplored in supramolecular coordination cages. Herein, we show that encapsulation of small aromatic aldehydes inside a supramolecular coordination cage allows the reduction of these aldehydes with the mild reducing agent sodium cyanoborohydride to proceed with high selectivity (ketones and esters are not reduced) and in good yields. In the absence of the cage, low pH conditions are essential for any appreciable conversion of the aldehydes to the alcohols. In contrast, the specific microenvironment inside the cage allows this reaction to proceed in bulk solution that is pH-neutral, or even basic. We propose that the cage acts to stabilise the protonated oxocarbenium ion reaction intermediates (enhancing aldehyde reactivity) whilst simultaneously favouring the encapsulation and reduction of smaller aldehydes (which fit more easily inside the cage). Such dual action (enhancement of reactivity and size-selectivity) is reminiscent of the mode of operation of natural enzymes and highlights the tremendous promise of cage architectures as selective catalysts.

Herein, we use a supramolecular coordination cage as a catalyst for the reduction of aldehydes to the corresponding alcohols using a weak hydride donor in neutral water, with a mode of action reminiscent of natural enzymes.     

Determination of azidothymidine and its degradation product thymine in pharmaceutical dosage forms by HPLC and HPTLC densitometry

Summary HPTLC densitometry and HPLC are considered for the determination of azidothymidine and its degradation product thymine in pharmaceutical dosage forms. In HPTLC the substances were separated on silica gel with fluorescence indicator in methanol-chloroform (1090) and methanol-chloroform (1585) systems. Absorbance measurement (detection of reflectance) of the separated substances was carried outin situ at 268 nm using four-level calibration (external standard, linear regression function) in the concentration range of 25–100 ng thymine/spot and using single-level calibration (external standard) at the concentration of 100 ng azidothymidine/spot. HPLC was carried out using RP-18 stationary phase and methanol+aqueous 0.03 mol/l KH₂PO₄ (18+82, v/v) as the mobile phase. The temperature was 50°C and the detection wavelength 266 nm. The detection limit of thymidine was 0.05%. The concentration range for azidothymidine was 0.5–1.5 mg/ml and for thymine 1–40 g/ml (for an injection volume of 10 l). The results were evaluated by linear regression analysis.     

Mirror symmetry breaking and chiral amplification in foldamer-based supramolecular helical aggregates

Spontaneous asymmetric generation of supramolecular chiral fibers was observed in the folding induced self-assembly of a lock-washer shaped foldamer. A secondary nucleation growth mechanism is proposed to explain the observed chiral amplification or deracemization of these supramolecular fibers.     

Activation and amplification of the third-order NLO and luminescent responses of a precursor cluster by a supramolecular approach

Solvothermal reactions of [Et(4)N][Tp*WS(3)(CuCl)(3)] (1) (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) with CuCN and KCu(CN)(2) afforded two [Tp*WS(3)Cu(3)]-based coordination polymers [Tp*WS(3)Cu(3)(μ(3)-DMF){Cu(CN)(3)}](2) (2) and K[Tp*WS(3)Cu(3)(μ(3)-DMF){Cu(2)(CN)(4.5)}](2) (3). The third-order NLO and PL responses of 1 were activated and greatly amplified through its assembly via the [Cu(CN)(3)](2-) and [Cu(4)(CN)(9)](5-) species in 2 and 3.     

Selective pyrolysis of Organosolv lignin over zeolites with product analysis by TG-FTIR

Organosolv lignin has been selected to investigate the thermal behavior of lignin over zeolites by using a thermogravimetric analyzer coupled with a Fourier-transform infrared spectrometer (TG-FTIR). The chemical structure of this lignin has been determined by ¹H NMR to obtain the distribution of main functional groups such as methoxyl groups and free aliphatic and phenolic hydroxyl groups. All three zeolite catalysts tested, HZSM-5, H-β, and USY, exerted significant influences on the dehydration reaction in the initial stage, the deoxygenation reaction of oxygenated compounds such as methanol and phenols, and the char-forming process during lignin pyrolysis in the range 30–800 °C. The dehydration reaction was enhanced in the order USY > HZSM-5 > H-β, while char formation was suppressed in the reverse order. The presence of HZSM-5 and H-β catalyzed the conversion of both oxygenated compounds and chars into the low-molecular-weight gases CO, CO₂, and methane. The addition of USY clearly aided decomposition of the oxygenated compounds, but had little effect on the char degradation.     

Electroreduction of bicyclic heteroaromatics. Selective product formation

The electroreductions of 1-methylindole and several benzofurans were investigated and compared with the corresponding alkali-metal/liquid ammonia reductions. The cathodic reductions were performed using a simple apparatus with aqueous THF as the solvent and were found to be regioselective. Over-reduction and other side reactions were avoided. The possible mechanism of the electroreductions is discussed.     

Ferrocene-bis(thymine/uracil) conjugates: base pairing directed, spacer dependent self-assembly and supramolecular packing

X-ray crystallographic studies of methylene linked Ferrocene-bis(thymine/uracil) conjugates Fc(T:T)(M) and Fc(U:U)(M) reveal base dependent 2-D supramolecular assemblies generated via wobble self-pairing for bis-thymine and reverse wobble self-pairing for bis-uracil conjugates, differing in architecture from the corresponding butylene spacer linked conjugates.