Applications of chiral C3-symmetric molecules

Throughout history symmetry and chirality have inspired artists and scientists alike. Given that rotational axes are the only elements of symmetry compatible with chirality, it is not surprising that C2- and C3-symmetrical molecules have attracted considerable attention. In recent years, the aesthetic appeal of C2-symmetrical molecules has been translated into many widely-used applications some of which are of commercial importance by its exploitation in the area of asymmetric catalysis. In contrast, exploitation of the arguably greater aesthetic appeal of C3-symmetric molecules is still in its infancy. This review, which surveys the applications of chiral C3-symmetrical molecules in the areas of asymmetric catalysis, molecular recognition and nanoarchitecture, has been designed with a view to identifying some of the most promising areas of application of these very beautiful molecules.     

Reactions of indoles with nitrogen dioxide and nitrous acid in an aprotic solvent

The reaction of 2-phenyl- and 1-methyl-2-phenylindole with nitrogen dioxide or with nitrous acid (NaNO2-CH3COOH) in benzene leads mainly to the formation of the isonitroso and 3-nitroso indole derivatives, respectively. When reacted with nitrous acid, 1-methyl-2-phenylindole gives also the corresponding azo-bis-indole in good yields. The reaction of indole with nitrogen dioxide leads to 2-(indol-3-yl)-3H-indol-3-one as the main product together with small amounts of 2-(indol-3-yl)-3H-indol-3-oxime; whereas the major product obtained when the same indole is reacted with nitrous acid is represented by 2-(indol-3-yl)-3H-indol-3-oxime. The reaction of 3-alkyl substituted indoles with nitrogen dioxide is rather complex and results in the formation of different nitro indoles, whereas nitrosation is observed when nitrous acid is used. Crystal structures of 2-(indol-3-yl)-3H-indol-3-one and of 4-nitro-N-acetyltryptamine have been determined by X-ray analysis.     

Structural flexibility and role of vicinal 2-thienyl rings in 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)2Th2Pyz], its palladium(II) complex [(CN)2Th2Pyz(PdCl2)2], and the related pentametallic pyrazinoporphyrazines [(PdCl2)4Th8TPyzPzM] (M = Mg(II)(H2O), Zn(II))

The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)(2)Th(2)Pyz], and its Pd(II) derivative, [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, formed by reaction of [(CN)(2)Th(2)Pyz] with [(C(6)H(5)CN)(2)PdCl(2)] were characterized by X-ray, UV-visible, (1)H and (13)C NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)(2)Th(2)Pyz] shows the presence of one thienyl ring positioned orthogonal to the rest of the molecule, with the two vicinal thienyl rings lying orthogonal to each other in a rare arrangement. NMR studies of [(CN)(2)Th(2)Pyz] in the solid state and in solutions of dimethylformamide or dimethyl sulfoxide confirm a nonequivalence of the thienyl rings in the solid state and also in solution. EXAFS results indicate that two distinct Pd(II) coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, with identical Pd-N(pyz) (2.03(3) ?) and Pd-Cl (2.36(3) ?) bond lengths but with different Pd-S1 (2.25(4) ?) and Pd-S2 (3.21(5) ?) bond distances in an overall asymmetric molecular framework. Density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical studies also provide information about the structure and spectral behavior of the precursor and its metalated Pd(II) derivative. (1)H/(13)C NMR and UV-visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl(2) with [Th(8)TPyzPzM] where Th(8)TPyzPz = tetrakis-2,3-[5,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg(II)(H(2)O) or Zn(II). Spectroscopic data on the newly synthesized [(PdCl(2))(4)Th(8)TPyzPzM] compounds suggest that the binding of PdCl(2) involves coordination sites of the type S(2(th))PdCl(2) with the two thienyl rings of each di(2-thienyl)pyrazino fragment bound to Pd(II) in an equivalent manner ("th-th" coordination). This is similar to what was found for the corresponding octapyridinated analogues ("py-py" coordination).     

Boron‐Containing Probes for Non‐optical High‐Resolution Imaging of Biological Samples

Boron has been employed in materials science as a marker for imaging specific structures by electron energy loss spectroscopy (EELS) or secondary ion mass spectrometry (SIMS). It has a strong potential in biological analyses as well; however, the specific coupling of a sufficient number of boron atoms to a biological structure has proven challenging. Herein, we synthesize tags containing closo‐1,2‐dicarbadodecaborane, coupled to soluble peptides, which were integrated in specific proteins by click chemistry in mammalian cells and were also coupled to nanobodies for use in immunocytochemistry experiments. The tags were fully functional in biological samples, as demonstrated by nanoSIMS imaging of cell cultures. The boron signal revealed the protein of interest, while other SIMS channels were used for imaging different positive ions, such as the cellular metal ions. This allows, for the first time, the simultaneous imaging of such ions with a protein of interest and will enable new biological applications in the SIMS field.     

Molecular dynamics of liquid acetone determined by depolarized Rayleigh and low-frequency Raman scattering spectroscopy

Slow to ultrafast dynamics of liquid acetone at variable temperature was investigated by depolarized Rayleigh and low-frequency Raman scattering spectroscopy, in the region 0-200 cm(-1). A detailed analysis was performed on the spectra and corresponding time responses, and a consistent view of the molecular dynamics of this dipolar solvent was obtained. The effects of temperature on the spectra were interpreted, and distinct dynamical processes identified. At very low frequencies, or long time scales, acetone dynamics is characterized by a slow diffusive reorientation obeying the Stokes-Einstein-Debye hydrodynamic theory only in the limit of subslip boundary conditions. An alternative model based on the microviscosity concept proved to be able to reproduce this correlation time and its temperature dependence. A comparative analysis of collective and single-molecule reorientational times, these latter estimated from intramolecular Raman spectra, led to an orientational correlation parameter g(2) of unity, which denotes a statistical disorder of molecular polarizability tensors. A fast local restructuring process is putatively responsible for an additional contribution at subpicosecond time scales often referred to as intermediate response in other molecular liquids. The high frequency portion of the dynamical susceptibility showed the signature of librational intermolecular motions, giving rise to an ultrafast decay of the time correlation function of polarizability anisotropy. The overall approach, which provided valuable information on dynamics, structure and molecular interactions of neat acetone, will be applied to acetone electrolytic solutions.     

Reactivity of copper(II)-alkylperoxo complexes

Copper(II) complexes 1a and 1b, supported by tridentate ligand bpa [bis(2-pyridylmethyl)amine] and tetradentate ligand tpa [tris(2-pyridylmethyl)amine], respectively, react with cumene hydroperoxide (CmOOH) in the presence of triethylamine in CH(3)CN to provide the corresponding copper(II) cumylperoxo complexes 2a and 2b, the formation of which has been confirmed by resonance Raman and ESI-MS analyses using (18)O-labeled CmOOH. UV-vis and ESR spectra as well as DFT calculations indicate that 2a has a 5-coordinate square-pyramidal structure involving CmOO(-) at an equatorial position and one solvent molecule at an axial position at low temperature (-90 °C), whereas a 4-coordinate square-planar structure that has lost the axial solvent ligand is predominant at higher temperatures (above 0 °C). Complex 2b, on the other hand, has a typical trigonal bipyramidal structure with the tripodal tetradentate tpa ligand, where the cumylperoxo ligand occupies an axial position. Both cumylperoxo copper(II) complexes 2a and 2b are fairly stable at ambient temperature, but decompose at a higher temperature (60 °C) in CH(3)CN. Detailed product analyses and DFT studies indicate that the self-decomposition involves O-O bond homolytic cleavage of the peroxo moiety; concomitant hydrogen-atom abstraction from the solvent is partially involved. In the presence of 1,4-cyclohexadiene (CHD), the cumylperoxo complexes react smoothly at 30 °C to give benzene as one product. Detailed product analyses and DFT studies indicate that reaction with CHD involves concerted O-O bond homolytic cleavage and hydrogen-atom abstraction from the substrate, with the oxygen atom directly bonded to the copper(II) ion (proximal oxygen) involved in the C-H bond activation step.     

Modified surface chemistry of activated carbons

In this study, immersion calorimetry was used to characterise different samples of commercial granular activated carbon (GAC) which undergo oxidation with HNO₃ (GACOxN) and thermal treatments to modify its superficial group contents, as well as to determine the textural characteristics of the materials through nitrogen adsorption at 77 K and its superficial chemistry by Boehm titration and zero point of charge. Correlations between the immersion enthalpies and the results of the other techniques of characterisation were established. The immersion enthalpies in dichloromethane obtained were greater, which were found to be between ?88.36 and ?155.6 J g^?1, in contrast to those in carbon tetrachloride, which were found to be between ?50.21 and ?94.29 J g^?1. The dependence of the immersion enthalpies in water on the contents of total acidity and basicity surface groups was also established, and a good correlation between the accessible surface area determined by calorimetric technique and the BET area was found.     

Optimization of the HPLC enantioseparation of 3,3′‐dibromo‐5,5′‐disubstituted‐4,4′‐bipyridines using immobilized polysaccharide‐based chiral stationary phases

The HPLC enantioseparation of nine atropisomeric 3,3′,5,5′‐tetrasubstituted‐4,4′‐bipyridines was performed in normal and polar organic (PO) phase modes using two immobilized polysaccharide‐based chiral columns, namely, Chiralpak IA and Chiralpak IC. The separation of all racemic analytes, the effect of the chiral selector, and mobile phase (MP) composition on enantioseparation and the enantiomer elution order (EEO) were studied. The beneficial effect of nonstandard solvents, such as tetrahydrofuran (THF), dichloromethane (DCM), and methyl t‐butyl ether on enantioseparation was investigated. All selected 4,4′‐bipyridines were successfully enantioseparated on Chiralpak IA under normal or PO MPs with separation factors from 1.14 to 1.70 and resolutions from 1.3 to 6.5. Two bipyridines were enantioseparated at the multimilligram level on Chiralpak IA. Differently, Chiralpak IC was less versatile toward the considered class of compounds and only five bipyridines out of nine could be efficiently separated. In particular, on these columns, the ternary mixture n‐heptane/THF/DCM (90:5:5) as MP had a positive effect on enantioseparation. An interesting phenomenon of reversal of the EEO depending on the composition of the MP for the 3,3′‐dibromo‐5,5′‐bis‐(E)‐phenylethenyl‐4,4′‐bipyridine along with an exceptional enantioseparation for the 3,3′‐dibromo‐5,5′‐bis‐ferrocenylethynyl‐4,4′‐bipyridine (α = 8.33, R_s = 30.6) were observed on Chiralpak IC.     

Fungistatic activity of Zanthoxylum rhoifolium Lam. bark extracts against fungal plant pathogens and investigation on mechanism of action in Botrytis cinerea

Plant-derived compounds are emerging as an alternative choice to synthetic fungicides. Chloroform–methanol extract, obtained from the bark of Zanthoxylum rhoifolium, a member of Rutaceae, showed a fungistatic effect on Botrytis cinerea, Sclerotinia sclerotiorum, Alternaria alternata, Colletotrichum gloeosporioides and Clonostachys rosea, when added to the growth medium at different concentrations. A fraction obtained by gel separation and containing the alkaloid O-Methylcapaurine showed significant fungistatic effect against B. cinerea and S. sclerotiorum, two of the most destructive phytopathogenic fungi. The underlying mechanism of such an inhibition was further investigated in B. cinerea, a fungus highly prone to develop fungicide resistance, by analysing the expression levels of a set of genes (BcatrB, P450, CYP51 and TOR). O-Methylcapaurine inhibited the expression of all the analysed genes. In particular, the expression of BcatrB gene, encoding a membrane drug transporter involved in the resistance to a wide range of xenobiotic compounds, was strongly inhibited (91%).     

β-Phenylproline: the high β-turn forming propensity of proline combined with an aromatic side chain

The conformational propensities of the proline analogue bearing a phenyl substituent attached to the β carbon, in either a cis or a trans configuration relative to the carbonyl group, have been investigated. The behaviour of cis- and trans(βPh)Pro has been compared with that of proline in homochiral and heterochiral dipeptide sequences. NMR and IR studies as well as X-ray diffraction analysis provide evidence that the β-phenyl substituent does not disrupt the tendency of proline to occupy the i+1 position of a β-turn. The puckering of the pyrrolidine ring is significantly affected by the presence of the aromatic substituent, which tends to occupy positions that minimize steric repulsions. As a consequence, this substituent adopts specific well-defined orientations, which are more restricted for the cis derivative. Interactions between this aromatic group and that in the adjacent phenylalanine residue may be responsible for some of the conformational differences observed among the different peptides studied.