W Tungsten

Sulfoxides are frequently used in organic synthesis as chiral auxiliaries and reagents to mediate a wide variety of chemical transformations. For example, diphenyl sulfoxide and triflic anhydride can be used to activate a wide range of glycosyl donors including hemiacetals, glycals and thioglycosides. In this way, an alcohol, enol or sulfide is converted into a good leaving group for subsequent reaction with an acceptor alcohol. However, reaction of diphenyl sulfoxide and triflic anhydride with oxathiane-based thioglycosides, and other oxathianes, leads to a different process in which the thioglycoside is oxidised to a sulfoxide. This unexpected oxidation reaction is very stereoselective and proceeds under anhydrous conditions in which the diphenyl sulfoxide acts both as oxidant and as the source of the oxygen atom. Isotopic labelling experiments support a reaction mechanism that involves the formation of oxodisulfonium (S-O-S) dication intermediates. These intermediates undergo oxygen-exchange reactions with other sulfoxides and also allow interconversion of axial and equatorial sulfoxides in oxathiane rings. The reversibility of the oxygen-exchange reaction suggests that the stereochemical outcome of the oxidation reaction may be under thermodynamic control, which potentially presents a novel strategy for the stereoselective synthesis of sulfoxides.

Sending out an SOS: The diphenyl sulfoxide/triflic anhydride combination is widely used as an activating agent for glycosylation reactions. However, oxathiane glycosyl donors are instead oxidised to sulfoxides in a stereoselective and reversible reaction. NMR spectroscopy and isotopic labelling experiments reveal the presence of oxodisulfonium (S-O-S) dication intermediates that can undergo oxygen exchange with other sulfoxides (see scheme).

Possessing high H₂ capacities and interesting dehydrogenation behavior, metal amidoborane ammoniates were prepared by reacting Ca(NH₂)₂, MgNH, and LiNH₂ with ammonia borane to form Ca(NH₂BH₃)₂⋅2 NH₃, Mg(NH₂BH₃)₂⋅NH₃, and Li(NH₂BH₃)₂⋅NH₃ (LiAB⋅NH₃). Insight into the mechanisms of amidoborane ammoniate formation and dehydrogenation was obtained by using isotopic labeling techniques. Selective ¹⁵N and ²H labeling showed that the formation of the ammoniate occurs via the transfer of one H(N) from ammonia borane to the [NH₂]⁻ unit in Ca(NH₂)₂ giving rise to NH₃ and [NH₂BH₃]⁻. Supported by theoretical calculations, it is suggested that the improved dehydrogenation properties of metal amidoborane ammoniates compared to metal amidoboranes are a result of the participation of a strong dihydrogen bond between the NH₃ molecule and [NH₂BH₃]⁻. Our study elucidates the reaction pathway involved in the synthesis and dehydrogenation of Ca(NH₂BH₃)₂⋅2 NH₃, and clarifies our understanding of the role of NH₃, that is, it is not only involved in stabilizing the structure, but also in improving the dehydrogenation properties of metal amidoboranes.

Amidoborane ammoniate formation is investigated by using isotopic labeling techniques. The formation of Ca(NH₂BH₃)₂⋅2 NH₃ is initiated by proton transfer from NH₃BH₃ to NH₂⁻ (amide), forming Ca(NH₃)²⁺ and anionic [NH₂BH₃]⁻ groups. The dehydrogenation of the ammoniate, which occurs at lower temperatures, is a result of the participation of NH₃ in the dehydrogenation process via the combination of (NH₃)H^δ+⋅⋅⋅H^δ−(NH₂BH₃⁻).

Hidden treasure: Gold nanoparticles (AuNPs) within a single protein crystal of lysozyme (see scheme, pink) are shown to efficiently catalyze the reduction of p-nitrophenol by NaBH₄, and the catalytic activity initially increased with increased size of AuNPs until the size reached 7.4 nm, after which the activity decreased with increasing size of the AuNPs. The use of chemicals to either accelerate or inhibit the activity is also demonstrated.

Ferric–hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low-spin Fe^III(OOH) complexes characterized to date, [(L₅²)Fe(OOH)]²⁺ is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single-turnover conditions. In this report we show that [(L₅²)Fe(OOH)]²⁺ decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first-order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first-order rate constants. Combining the kinetic data obtained at different temperatures and under different single-turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5-D₃]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by Fe^III(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged Fe^IV(O) and OH^. species from the Fe^III(OOH) complex have been obtained for the first time. After homolytic OO cleavage, a caged pair of oxidants [Fe^IVO+HO^.] is generated that act in unison to hydroxylate the aromatic ring: HO^. attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by Fe^IVO to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re-aromatization step. Spin-trapping experiments in the presence of 5,5-dimethyl-1-pyrroline N-oxide and GC-MS analyses of the intermediate products further support the proposed mechanism.

Oxidation by Fe^III(OOH): Investigations on a genuine non-haem Fe^III(OOH) intermediate (see figure) in the presence of either aromatic substrates or the probe substrate [1,3,5-D₃]benzene has clarified the role played by Fe^III(OOH) in aromatic oxidations. Evidence for the formation of two caged Fe^IV(O) and OH^. species from Fe^III(OOH) has been obtained for the first time. These oxidants act in unison to give phenol products with normal kinetic isotope effects and notable hydride shifts.

Evidence for the existence of nitrile selenides, potential 1,3-dipolarophiles in cycloaddition reactions, has been provided by direct spectroscopic methods. The parent nitrile selenide, selenofulminic acid (HCNSe), and its methyl and cyano derivatives have been photolytically generated in an inert solid argon matrix from 1,2,5-selenadiazoles by 280, 254, and 313 nm UV irradiation, respectively, and studied by ultraviolet spectroscopy and mid-infrared spectroscopy. Ground-state geometries have been obtained from quantum-chemical calculations at the CCSD(T)/aug-cc-pVTZ level. Nitrile selenides are predicted to be linear with a relatively weak NSe bond.

Nitrile selenides (XCNSe, X=H, CH₃, CN) were photochemically generated in a low-temperature solid argon matrix and studied by UV and IR spectroscopies and quantum chemical calculations.

A unique class of oligothiophene-based organogelator bearing two crown ethers at both ends was synthesized. This compound could gelatinize several organic solvents, forming one-dimensional fibrous aggregates. From the observation of circular dichroism, it was confirmed that the helical handedness of the fibrous assembly is controllable by the chirality of 1,2bisammonium guests, thus suggesting that one guest molecule bridges two gelator molecules through the crown–ammonium interaction. Interestingly, we have found that such chirality is created by thermal gelation, whereas it disappears by thixotropic gelation. The new finding implies that the present organogel system is applicable as a reversible switching memory device, featuring memory creation by a heat mode and memory erasing by a mechanical mode.

Chiral memory switching: A unique class of functional organogelator based on crown ether appended oligothiophene was created, the chirality of which was influenced by the interaction with chiral ammonium guests. Stimuli-induced circular dichroism with controlled handedness of aggregates was observed. The reversibility of thixotropic/thermal behaviors can be applied to chirality switching memory devices (see scheme).

We demonstrate that the electronic structure of mesoporous silicon is affected by adsorption of nitro-based explosive molecules in a compound-selective manner. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon using soft X-ray spectroscopy. The Si atoms strongly interact with adsorbed molecules to form SiO and SiN bonds, as evident from the large shifts in emission energy present in the Si L_2,3 X-ray emission spectroscopy (XES) measurements. Furthermore, we find that the energy gap (band gap) of mesoporous silicon changes depending on the adsorbant, as estimated from the Si L_2,3 XES and 2p X-ray absorption spectroscopy (XAS) measurements. Our ab initio molecular dynamics calculations of model compounds suggest that these changes are due to spontaneous breaking of the nitro groups upon contacting surface Si atoms. This compound-selective change in electronic structure may provide a powerful tool for the detection and identification of trace quantities of airborne explosive molecules.

Compound-selective changes in the electronic structure of mesoporous silicon have been observed upon adsorption of nitro-based explosive molecules. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon by using soft X-ray spectroscopy (see figure).

The large tendency of catechol rings to adsorb on surfaces has been studied by STM experiments with molecular resolution combined with molecular-dynamics simulations. The strong adhesion is due to interactions with the surface and solvent effects. Moreover, the thermodynamic control over the differential adsorption of 1 and the nonanoic solvent molecules has been used to induce a new temperature-induced switchable interconversion. Two different phases that differ in their crystal packing and the presence of solvent molecules coexist upon an increase or decrease in the temperature. These results open new insight into the behavior of catechol molecules on surfaces and 2D molecular suprastructures.

Recognized strong adhesion and organization of catechols on surfaces has been used as a means to study the main parameters that control molecular self-assembly processes on surfaces, namely, the energetics (molecule/molecule, molecule/surface interactions) and thermodynamics (solvent effects) (see figure). This knowledge is used to establish temperature-induced switchable 2D supramolecular structures

Closed-shell contacts between two copper(I) ions are expected to be repulsive. However, such contacts are quite frequent and are well documented. Crystallographic characterization of such contacts in unsupported and bridged multinuclear copper(I) complexes has repeatedly invited debates on the existence of cuprophilicity. Recent developments in the application of Bader’s theory of atoms-in-molecules (AIM) to systems in which weak hydrogen bonds are involved suggests that the copper(I)–copper(I) contacts would benefit from a similar analysis. Thus the nature of electron-density distributions in copper(I) dimers that are unsupported, and those that are bridged, have been examined. A comparison of complexes that are dimers of symmetrical monomers and those that are dimers of two copper(I) monomers with different coordination spheres has also been made. AIM analysis shows that a bond critical point (BCP) between two Cu atoms is present in most cases. The nature of the BCP in terms of the electron density, ρ, and its Laplacian is quite similar to the nature of critical points observed in hydrogen bonds in the same systems. The ρ is inversely correlated to CuCu distance. It is higher in asymmetrical systems than what is observed in corresponding symmetrical systems. By examining the ratio of the local electron potential-energy density (V_c) to the kinetic energy density (G_c), |V_c|/G_c at the critical point suggests that these interactions are not perfectly ionic but have some shared nature. Thus an analysis of critical points by using AIM theory points to the presence of an attractive metallophilic interaction similar to other well-documented weak interactions like hydrogen bonding.

Mission critical: Bond critical points (BCP) in dimeric copper(I) complexes have been analyzed. The electron density and Laplacian in the BCPs are similar to those of the BCP in hydrogen bonds. The critical points show some shared character and are not completely ionic in nature (see figure). Electron density at the BCP is higher in asymmetrical dimers than in comparable symmetrical dimers.

Annular aggregation: Atomic force microscopy and the recently developed microsecond force spectroscopy are used to identify a new annular structure of human Islet amyloid polypeptide on negatively charged tantalum oxide, a substrate that has the same surface charge as cell membranes (see figure). Finally, an accumulation model is proposed to explain how annular aggregation is initiated and developed.