Showing papers in "Chemical Science in 2010"

Continuous flow multi-step organic synthesis

Abstract: Using continuous flow techniques for multi-step synthesis enables multiple reaction steps to be combined into a single continuous operation. In this mini-review we discuss the current state of the art in this field and highlight recent progress and current challenges facing this emerging area.

Diamine ligands in copper -catalyzed reactions

Abstract: The utility of copper-mediated cross-coupling reactions has been significantly increased by the development of mild reaction conditions and the ability to employ catalytic amounts of copper. The use of diamine-based ligands has been important in these advances and in this review we discuss these systems, including the choice of reaction conditions and applications in the synthesis of pharmaceuticals, natural products and designed materials.

Catalytic asymmetric allylic alkylation employing heteroatom nucleophiles: a powerful method for C–X bond formation

Abstract: Metal catalyzed asymmetric allylic alkylation (AAA) reactions have been an extensively studied and fruitful area of research in organic chemistry. The use of heteroatom-centered nucleophiles in this reaction is a powerful method for asymmetric C–X (X = heteroatom) bond formation. This perspective summarizes developments and applications of metal catalyzed AAA reactions employing heteroatom nucleophiles.

Modifying MOFs: new chemistry, new materials

Abstract: The postsynthetic modification (PSM) of metal–organic frameworks (MOFs) has grown substantially in the past few years. In this minireview, recent progress in the area of PSM is highlighted, with an emphasis on several recent advancements. The scope and limitations of PSM are described and future prospects are discussed.

Hydrogen storage and carbon dioxide capture in an iron-based sodalite-type metal–organic framework (Fe-BTT) discovered via high-throughput methods

Abstract: Using high-throughput instrumentation to screen conditions, the reaction between FeCl2 and H3BTT·2HCl (BTT3− = 1,3,5-benzenetristetrazolate) in a mixture of DMF and DMSO was found to afford Fe3[(Fe4Cl)3(BTT)8]2·22DMF·32DMSO·11H2O. This compound adopts a porous three-dimensional framework structure consisting of square [Fe4Cl]7+ units linked via triangular BTT3− bridging ligands to give an anionic 3,8-net. Mossbauer spectroscopy carried out on a DMF-solvated version of the material indicated the framework to contain high-spin Fe2+ with a distribution of local environments and confirmed the presence of extra-framework iron cations. Upon soaking the compound in methanol and heating at 135 °C for 24 h under dynamic vacuum, most of the solvent is removed to yield Fe3[(Fe4Cl)3(BTT)8(MeOH)4]2 (Fe-BTT), a microporous solid with a BET surface area of 2010 m2 g−1 and open Fe2+ coordination sites. Hydrogen adsorption data collected at 77 K show a steep rise in the isotherm, associated with an initial isosteric heat of adsorption of 11.9 kJ mol−1, leading to a total storage capacity of 1.1 wt% and 8.4 g L−1 at 100 bar and 298 K. Powder neutron diffraction experiments performed at 4 K under various D2 loadings enabled identification of ten different adsorption sites, with the strongest binding site residing just 2.17(5) A from the framework Fe2+ cation. Inelastic neutron scattering spectra are consistent with the strong rotational hindering of the H2 molecules at low loadings, and further reveal the catalytic conversion of ortho-H2 to para-H2 by the paramagnetic iron centers. The exposed Fe2+ cation sites within Fe-BTT also lead to the selective adsorption of CO2 over N2, with isotherms collected at 298 K indicating uptake ratios of 30.7 and 10.8 by weight at 0.1 and 1.0 bar, respectively.

Biocompatibility and drug delivery systems

Abstract: Drug delivery technology has emerged as an important focus of biotechnological research and commercial enterprise. While much attention is justifiably focused on the design and effectiveness of drug delivery devices, the nature of their interaction with surrounding tissues – their biocompatibility – is crucial. Here we discuss biocompatibility, specifically as it relates to drug delivery systems, which differ from other biomaterial-based devices by possibly containing large quantities of drugs with their own effects on tissues.

Direct observation of CuI/CuIII redox steps relevant to Ullmann-type coupling reactions

Abstract: A series of aryl–copper(III)-halide complexes have been synthesized and characterized by NMR and UV-visible spectroscopy, cyclic voltammetry and X-ray crystallography. These complexes closely resemble elusive intermediates often invoked in catalytic reactions, such as Ullmann–Goldberg cross-coupling reactions, and their preparation has enabled direct observation and preliminary characterization of aryl halide reductive elimination from CuIII and oxidative addition to CuI centers. In situ spectroscopic studies (1H NMR, UV-visible) of a Cu-catalyzed C–N coupling reaction provides definitive evidence for the involvement of an aryl-copper(III)-halide intermediate in the catalytic mechanism. These results provide the first direct observation of the CuI/CuIII redox steps relevant to Ullmann-type coupling reactions.

Pd-catalyzed ortho-arylation of phenylacetamides, benzamides, and anilides with simple arenes using sodium persulfate

Abstract: We report a mild and efficient Pd-catalyzed ortho-arylation of phenylacetamides, benzamides, and anilides with a range of simple arenes using sodium persulfate (Na2S2O8). This green strategy generates biaryl C–C bonds from two unactivated sp2 hybridized C–H bonds. Electron-rich and electron-neutral arenes underwent oxidative arylation under our optimized reaction conditions. In substrates bearing two reactive ortho C–H bonds, selective diarylation via quadruple C–H bond functionalization was possible. The same reaction conditions were extended to an intramolecular cross-coupling for preparing lactams. The synthesis of relevant trifluoroacetate-bridged bimetallic Pd complexes derived from anilides and their stoichiometric reactivity were investigated.

Oxygen reduction reactivity of cobalt(II) hangman porphyrins

Abstract: Cobalt(II) hangman porphyrins are delivered from easily available starting materials, in two steps, in good yields, and with abbreviated reaction times. Selected compounds from a library of Co(II) hangman porphyrins immobilized on multiwall carbon nanotubes establish that the four-electron four-proton catalytic reduction of oxygen to water in aqueous solution can be achieved at the single cobalt center of the hangman platform. Reaction trends within the library reveal that the selective reduction of O2 to H2O occurs at electron deficient hangman porphyrin platforms possessing a distal group that is capable of proton transfer.

Detection limits in photothermal microscopy

Abstract: We show how to push the detection limits in photothermal microscopy towards weaker single absorbers, by a systematic optimization of signal and noise sources. In particular, we (i) maximize the power of the probe laser beam, (ii) select optimal optical and thermal properties for the medium embedding the absorber, and (iii) thermally isolate the absorber from the glass substrate. These different experimental conditions are optimized in turn with single immobilized gold nanoparticles. We demonstrate the detection of a dissipated power of 3 nW with a signal-to-noise ratio of 8, and an integration time of 10 ms. This corresponds to a less than 0.1 K surface temperature rise for a 20 nm-diameter gold nanosphere (0.4 K for 5 nm). As an example of the achieved detection sensitivity, we show simultaneous photothermal and fluorescence detection of individual 20 nm fluorescent beads, each containing about 20 Nile red dye molecules.

Liquid phase separations by crystalline microporous coordination polymers

Abstract: Crystalline microporous coordination polymers (MCPs) are highly ordered, porous materials that have recently seen increasing attention in the literature. Whereas gas phase separations using MCPs have been extensively studied and reviewed, studies on applications in the liquid phase have lagged behind. This review details the work that has previously been reported on liquid phase separations using MCPs. Both enantioselective separations and separations of complex mixtures have been achieved using either adsorptive selectivities or size exclusion effects. Molecules that have been adsorbed include those as small as water to large organic dyes. In many cases, MCPs outperform their zeolite and activated carbon counterparts both kinetically and in efficiency of separation. The future outlook for the field is discussed in the context of current challenges in separations technologies.

Alkylgold complexes by the intramolecular aminoauration of unactivated alkenes

Abstract: Alkylgold(I) complexes were formed from the gold(I)-promoted intramolecular addition of various amine nucleophiles to alkenes. These experiments provide the first direct experimental evidence for the elementary step of gold-promoted nucleophilic addition to an alkene. Deuterium-labeling studies and X-ray crystal structures provide support for a mechanism involving anti-addition of the nucleophile to a gold-activated alkene, which is verified by DFT analysis of the mechanism. Ligand studies indicate that the rate of aminoauration can be drastically increased by use of electron-poor arylphosphines, which are also shown to be favored in ligand exchange experiments. Attempts at protodeauration lead only to recovery of the starting olefins, though the gold can be removed under reducing conditions to provide the purported hydroamination products.

Site-specific photocatalytic splitting of methanol on TiO2(110)

Abstract: Clean hydrogen production is highly desirable for future energy needs, making the understanding of molecular-level phenomena underlying photocatalytic hydrogen production both fundamentally and practically important. Water splitting on pure TiO2 is inefficient, however, adding sacrificial methanol could significantly enhance the photocatalyzed H2 production. Therefore, understanding the photochemistry of methanol on TiO2 at the molecular level could provide important insights to its photocatalytic activity. Here, we report the first clear evidence of photocatalyzed splitting of methanol on TiO2 derived from time-dependent two-photon photoemission (TD-2PPE) results in combination with scanning tunneling microscopy (STM). STM tip induced molecular manipulation before and after UV light irradiation clearly reveals photocatalytic bond cleavage, which occurs only at Ti4+ surface sites. TD-2PPE reveals that the kinetics of methanol photodissociation is clearly not of single exponential, an important characteristic of this intrinsically heterogeneous photoreaction.

Asymmetric Brønsted acid catalysis in aqueous solution

Abstract: A biologically inspired reaction design leads to the development of the first highly enantioselective Bronsted acid catalysed reaction in aqueous solution.

Bimetallic mechanism operating in the copolymerization of propylene oxide with carbon dioxide catalyzed by cobalt–salen complexes

Abstract: Dinuclear Co–salen complexes (R,R)-(S,S)-1–4 were synthesized and compared with their mononuclear counterpart rac-5 to suggest that a bimetallic mechanism is in charge for the alternating copolymerization of propylene oxide with CO2 in the absence of onium salt while a monometallic mechanism is more probable in its presence

Highly uniform SERS substrates formed by wrinkle-confined drying of gold colloids

Abstract: SERS substrates were fabricated through self-assembly of gold nanoparticles upon solution-drying in a periodic confining structure. The technique leads to uniform, parallel linear nanoparticle arrays with the precise arrangement defined through the dimensions of the particles and the grooves, which provide high and uniform SERS enhancement over extended areas.

Membrane protein biosensing with plasmonic nanopore arrays and pore-spanning lipid membranes

Abstract: Integration of solid-state biosensors and lipid bilayer membranes is important for membrane protein research and drug discovery. In these sensors, it is critical that the solid-state sensing material does not have adverse effects on the conformation or functionality of membrane-bound molecules. In this work, pore-spanning lipid membranes are formed over an array of periodic nanopores in free-standing gold films for surface plasmon resonance (SPR) kinetic binding assays. The ability to perform kinetic assays with a transmembrane protein is demonstrated with α-hemolysin (α-HL). The incorporation of α-HL into the membrane followed by specific antibody binding (anti-α-HL) red-shifts the plasmon resonance of the gold nanopore array, which is optically monitored in real time. Subsequent fluorescence imaging reveals that the antibodies primarily bind in nanopore regions, indicating that α-HL incorporation preferentially occurs into areas of pore-spanning lipid membranes.

N-heterocyclic carbenes which readily add ammonia, carbon monoxide and other small molecules,

Abstract: N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent σ-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N → Ccarbene π-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide—reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr2)2, adds CO. This reaction affords a β-lactam by a subsequent intramolecular process involving a C–H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

Acyclic indole and carbazole-based sulfate receptors

Abstract: The anion complexation properties of a series of acyclic receptors consisting of diindolylurea groups appended with amide, amidoindole or amidocarbazole groups have been studied. The receptors selectively bind and encapsulate sulfate via either six or eight hydrogen bonds. Receptors containing eight hydrogen bond donors perturb the pKa of bound dihydrogen phosphate and bicarbonate to the extent that they are deprotonated by free anion in solution.

Highly enantio-, regio- and diastereo-selective one-pot [2 + 3]-cycloaddition reaction via isomerization of 3-butynoates to allenoates ,

Abstract: Phosphine-catalyzed one-pot isomerization and [2 + 3]-cycloaddition of 3-butynoates with electron-deficient olefins affords highly functionalized cyclopentenes with both good yields and excellent selectivities of up to 99%.

Definition of the uptake mechanism and sub- cellular localisation profile of emissive lanthanide complexes as cellular optical probes

Abstract: A series of experiments has been undertaken in order to gain a greater understanding of the cellular uptake and localisation behaviour of emissive lanthanide complexes as cellular stains or probes. Out of a large number of structurally related complexes characterised recently, a set of seven representative examples has been examined in detail, containing either tetraazatriphenylene or azaxanthone-based sensitising chromophores. Intracellular localisation profiles and cellular uptake and egress behaviour have been studied by microscopy and flow cytometry. Typically, the maximum intracellular concentration was of the order of 0.4 mM, or about 109 complexes per cell. The complexes studied were generally not toxic and did not perturb the mitochondrial membrane potential. A common uptake mechanism of macropinocytosis has been identified. A generalisation of trends in behaviour, and structure–activity relationships is presented, and the implications for future probe design discussed.

Substituent effects in ditetrel alkyne analogues: multiple vs. single bonded isomers

Abstract: The synthesis and characterization of a series of digermynes and distannynes stabilized by terphenyl ligands are described. The ligands are based on the Ar′ (Ar′ = C6H3-2,6(C6H3-2,6-iPr2)2) or Ar* (Ar* = C6H3-2,6(C6H2-2,4,6-iPr3)2) platforms which were modified at the meta or para positions of their central aryl rings to yield 4-X-Ar′ (4-X-Ar′ = 4-X-C6H2-2,6(C6H3-2,6-iPr2)2, X = H, F, Cl, OMe, tBu, SiMe3, GeMe3) and 3,5-iPr2-Ar′ or Ar* and 3,5-iPr2-Ar*. The compounds were synthesized by reduction of the terphenyl germanium(II) or tin(II) halide precursors with a variety of reducing agents. The precursors were obtained by the reaction of one equivalent of the lithium terphenyl with GeCl2 dioxane or SnCl2. For germanium, their X-ray crystal structures showed them to be either Ge–Ge bonded dimers with trans-pyramidal geometries or V-shaped monomers. In contrast, the terphenyl tin halides had no tin–tin bonding but existed either as halide bridged dimers or V-shaped monomers. Reduction with a variety of reducing agents afforded the digermynes ArGeGeAr (Ar = 4-Cl-Ar′, 4-SiMe3-Ar′ or 3,5-iPr2-Ar*) or the distannynes ArSnSnAr (Ar = 4-F-Ar′, 4-Cl-Ar′, 4-MeO-Ar′, 4-tBu-Ar′, 4-SiMe3-Ar′, 4-GeMe3-Ar′, 3,5-iPr2-Ar′, 3,5-iPr2-Ar*), which were characterized structurally and spectroscopically. The digermynes display planar trans-bent core geometries with Ge–Ge distances near 2.26 A and bending angles near 128° consistent with Ge–Ge multiple bonding. In contrast, the distannynes had either multiple bonded geometries with Sn–Sn distances that averaged 2.65 A and an average bending angle near 123.8°, or single bonded geometries with a Sn–Sn bond length near 3.06 A and a bending angle near 98°. The 3,5-iPr2-Ar*SnSnAr*-3,5-iPr2 species had an intermediate structure with a longer multiple bond near 2.73 A and a variable torsion angle (14–28°) between the tin coordination planes. Mossbauer data for the multiple and single bonded species displayed similar isomer shifts but had different quadrupole splittings.

Ligand-assisted nickel-catalysed sp3–sp3 homocoupling of unactivated alkyl bromides and its application to the active template synthesis of rotaxanes

Abstract: An efficient, mild and operationally simple Ni-catalysed sp3-carbon-to-sp3-carbon homocoupling of unactivated alkyl bromides has been developed and utilised in the active metal template synthesis of an alkyl chain axle [2]rotaxane. The key to the transformation is the use of tridentate nitrogen-donor-atom (terpy or pybox derived) ligands which inhibit competing β-hydride elimination of alkyl-Ni intermediates.

DNA fluorocode: A single molecule, optical map of DNA with nanometre resolution

Abstract: We present a new method for single-molecule optical DNA mapping using an exceptionally dense, yet sequence-specific coverage of DNA with a fluorescent probe. The method employs a DNA methyltransferase enzyme to direct the DNA labelling, followed by molecular combing of the DNA onto a polymer-coated surface and subsequent sub-diffraction limit localization of the fluorophores. The result is a ‘DNA fluorocode’; a simple description of the DNA sequence, with a maximum achievable resolution of less than 20 bases, which can be read and analyzed like a barcode. We demonstrate the generation of a fluorocode for genomic DNA from the lambda bacteriophage using a DNA methyltransferase, M.HhaI, to direct fluorescent labels to four-base sequences reading 5′-GCGC-3′. A consensus fluorocode that allows the study of the DNA sequence at the level of an individual labelling site can be generated from a handful of molecules.

Cell surface-based differentiation of cell types and cancer states using a gold nanoparticle-GFP based sensing array

Abstract: Gold nanoparticle-green fluorescent protein (NP-GFP) based arrays have been created for rapid identification of mammalian cells on the basis of cell surface properties. Highly reproducible characteristic patterns were obtained from different cell types enabling the identification of cell types and cancer states. Using these arrays we could differentiate between isogenic normal, cancer and metastatic cell types using only ∼5000 cells.

Ultrafast studies of single semiconductor and metal nanostructures through transient absorption microscopy

Abstract: The past decade has seen tremendous progress in the development of absorption based techniques for studying single semiconductor and metal nanoparticles. These techniques complement the more common emission and Rayleigh scattering measurements. The advantages of absorption-based detection are that a wider range of materials can be examined compared to emission, and smaller particles can be studied than is possible with Rayleigh scattering. These advantages are somewhat offset by increased difficulty. Importantly, by employing absorption based detection with a pump and probe beam, transient absorption experiments can be performed on single nanostructures. This allows dynamics measurements on ultrafast (sub-ps) timescales, and is the focus of this mini-review. These experiments provide information about how processes such as electron–phonon coupling, energy dissipation, and charge carrier trapping vary between nanostructures, as well as how they depend on environment.

Heterolytic and heterotopic dissociation of hydrogen on ceria-supported gold nanoparticles. Combined inelastic neutron scattering and FT-IR spectroscopic study on the nature and reactivity of surface hydrogen species

Abstract: Inelastic neutron scattering of gold nanoparticles (3–4 nm, 0.48 wt%) supported on nanoparticulate ceria (5 nm) (Au/CeO2) shows that hydrogen treatment of Au/CeO2 gives rise to the appearance of a new band (400–600 cm−1) corresponding to bridging hydroxyl groups. This band disappears (without observing the appearance of any new band) when the sample is exposed to CO2 or O2. FT-IR spectroscopy on the same samples shows the formation of Au–H after hydrogen chemisorption. The Au–H band also disappears when the sample is subsequently treated with CO2 or O2. In the last case formation of hydroperoxide is observed. The band assignments were supported by isotopic labelling with D2 and 18O2. The above data clearly establish that upon treatment of Au/CeO2 with hydrogen at 150 °C, heterolytic bond cleavage of dihydrogen occurs leading to a proton bonded to a ceria oxygen and a hydride bonded to gold.

Temperature responsive channel uniformity impacts on highly guest-selective adsorption in a porous coordination polymer

Abstract: Selective adsorption, so called “molecular sieving”, is one of the significant functions of porous materials because it can play an important role in separation processes to obtain highly pure gases or petroleum. One of the next outstanding challenges of porous materials is the achievement of controllable “molecular sieving” in response to external environments. Here, we show a new temperature responsive 1-dimensional porous compound (CPL-11) which changes its structural uniformity in response to ambient temperature, which gives rise to changes in the effective pore size. As a result of the structural responsiveness, the seeming adsorption properties of CPL-11 are apparently opposite to conventional porous compounds expected from thermodynamic law, resulting in a highly selective adsorption for O2 compared to Ar. This unexpected adsorption behavior indicates that the adsorption properties can be controlled by changing the channel uniformity of a porous framework depending on the ambient temperature.

Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

Abstract: Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4A2 → 4T2 ligand-field absorption of Cr(acac)3. The transient kinetics were fit to a rapidly damped 164 cm−1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4T2 → 2E intersystem crossing that immediately follows 4A2 → 4T2 excitation. DFT calculations indicate that the 164 cm−1 oscillation likely corresponds to a combination of Cr–O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast time-resolved absorption measurements on Cr(t-Bu-acac)3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) – an electronically similar but more sterically encumbered molecule – which exhibits a 4T2 → 2E conversion that is more than an order of magnitude slower than that observed for Cr(acac)3. These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.

Task specific ionic liquids for the ionothermal synthesis of siliceous zeolites

Abstract: The first genuine ionothermal synthesis of siliceous zeolites MFI and TON has been accomplished by utilising the ionic liquid 1-butyl-3-methyl imidazolium bromide/hydroxide as both solvent and structure directing agent.