Iridicycle‐Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism

The mechanism of imine reduction by formic acid with a single‐site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover‐limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion‐pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer‐sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26–28 and 7–8 kcal mol^?1, respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol^?1 for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover‐limiting hydride formation step by hydrogen‐bonding to the formate anion and thereby stabilising the ion pair.     

Release and report: a new photolabile caging system with a two-photon fluorescence reporting function

A new efficient photocaging system with a fluorescence reporting function has been developed. The photolabile latch is based on adducts of C-nucleophiles with aromatic ketones, such as thioxanthones and xanthones. The system is designed to quantify the release of biological effectors and to monitor their spatial distribution and localization by single- and two-photon fluorescence microscopy. In the armed state the ketone's conjugation is disrupted by nucleophilic addition, resulting in a blue shift of the absorption maxima and a dramatic decrease in fluorescence intensity. The mechanism of the photoinduced uncaging involves homolytic C-C bond fragmentation followed by radical disproportionation, regenerating the carbonyl moiety and restoring fluorescence. The uncaging can be initiated via either a one- or two-photon process, offering a new powerful tool for molecular life sciences. The synthesis and uncaging of dendrimer- and polymeric bead-based model systems are described.     

A mixed-valence bis(diarylamino)stilbene: crystal structure and comparison of electronic coupling with biphenyl and tolane analogues

The E-4,4'-bis[di(p-anisyl)amino]stilbene cation is a class-III mixed-valence species with electronic coupling comparable to that in its biphenyl-bridged analogue, whereas its tolane-bridged analogue belongs to class II.     

Reversible Fixation of Ethylene on a SmII Calix-Pyrrole Complex

Reversible ethylene fixation in lanthanide chemistry is demonstrated by the Sm^II derivatives [R₈-calix-pyrrole)(Et₂O)Sm{Li(thf)₂}{Li(μ₃-OCH=CH₂)}] (R=Et, {−(CH₂)₅−}_0.5), which react with ethylene to afford the corresponding dinuclear complexes (see picture).     

Intramolecular alkyne hydroalkoxylation and hydroamination catalyzed by iridium hydrides

[reaction: see text] Iridium(III) hydrides prove to be air-stable active catalysts for intramolecular hydroalkoxylation and hydroamination of internal alkynes with proximate nucleophiles. The cyclization follows highly selective 6-endo-dig regiochemistry when regioselectivity is an issue.     

In situ atomic force microscopy investigation of the [100] face of KH2PO4 in the presence of Fe(III), Al(III), and Cr(III)

Here we report the effects of dissolved metal complexes of Fe(III), Al(III), and Cr(III) on the step velocities of the [100] face of KH2PO4 (KDP) as observed with atomic force microscopy. The dependence of step velocity on supersaturation (sigma) exhibits a dead zone that scales with adsorbate concentration. The observed dependence varies with the metal complex. From these data, we derive values for the characteristic adsorption time (tau) for the Al(III) and Cr(III) step-pinning adsorbates as being on the order of several hundred microseconds as compared to 10-100 s for the corresponding Fe(III) step-pinning adsorbates. The values of tau are strikingly different than rates of ligand exchange but are associated with the adsorbate-induced morphology of the surface, including elementary steps that bunch into macrosteps and supersteps. The stoichiometry of the adsorbate species is assumed to be M(HxPO4)x, where M = Fe(III), Al(III), or Cr(III). KDP crystals grown in the presence of the dissolved metals were analyzed using laser ablation inductively coupled plasma mass spectroscopy. The data revealed sectoral zoning on the [100] face, with the concentrations of the incorporated adsorbates in the sector with slower moving elementary steps being 1.7-2.0 times greater than those measured on the sector with fast moving elementary steps.     

Low-temperature synthesis of tetraalkylureas from secondary amines and carbon dioxide

The reaction of dialkylamines with CO(2) giving tetraalkylureas can be performed at 60 degrees C. The reaction requires CCl(4), is weakly promoted by DMAN or PPh(3), and is not promoted by a Pd catalyst. A two-step procedure, in which dialkylammonium dialkylcarbamate is produced in situ and then reacted with CCl(4) and free dialkylamine, gave greater yields of urea than a simple single-stage procedure.     

Structurally diverse and readily tunable photocrosslinked chondroitin sulfate based copolymers

Our group has previously reported on successful biofunctionalization of poly(ethylene glycol diacrylate) (PEGDA) gels using chondroitin sulfate (CS) and improving moduli of methacrylated‐CS (MCS) gels using short PEGDA comonomers. Here, we focused on understanding the composition‐property relationship of MCS‐PEGDA copolymers. By changing concentration, composition, and medium's ionic strength the gels were modified to show a diverse range of properties. Photopolymerized copolymers with >4:1 ratio of one component had compressive moduli of up to 24 times higher and up to 17 times lower swelling degree (q) than those of MCS alone. The increased moduli and lowered q were consistent with the hypothesis that PEGDA improves kinetic chain growth by overcoming the steric hindrances of the macromer. The swelling and moduli of the gels were tuned by changing the ratio of the comonomers. The swelling and moduli of the gels were lowered with presence of salt in solution while the fracture strain increased. These changes were hypothesized to be the result of transition of CS chain conformation from highly extended and non‐Gaussian to less extended and Gaussian distribution. The complete understanding of MCS‐PEGDA composition–property relationship provides a general strategy to tune the moduli or q of polysaccharide‐based hydrogels while avoiding undesirable phase separation. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1070–1079