Exogenous‐Base‐Free Palladacycle‐Catalyzed Highly Enantioselective Arylation of Imines with Arylboroxines

Enantiomerically pure benzylic amines are important for the development of new drugs. A readily accessible planar‐chiral ferrocene‐derived palladacycle is shown to be a highly efficient catalyst for the formation of N‐substituted benzylic stereocenters; this catalyst accelerates the 1,2‐addition of arylboroxines to aromatic and aliphatic imines with exceptional levels of enantioselectivity. Using aldimines an exogenous base was not necessary for the activation of the boroxines, when acetate was used as an anionic ligand. Common problems such as aryl–aryl homocouplings and imine hydrolysis were fully overcome, the latter even in the absence of molecular sieves.     

Voices from the Front Lines: Exemplary Science Teachers on Education Reform

The purpose of this study is to gain insight into the experiences that nationally award‐winning, exemplary science teachers have had over their career and examine the alignment of their responses with calls for K‐12 science education reform from a selection of prominent commissioned government reports since 1980. From an assessment of the alignment of exemplary teachers, calls for reform have had a limited effect and highlight the weakness of using national reports as a wide‐scale, nationalized approach to science education reform. Findings are focused on seven different areas of teacher development: classroom issues, teaching scientific inquiry, use of technology, preservice experiences, professional development of in‐service teachers, vertical articulation, and science education reform over time. Among other issues, the teachers indicated one of the biggest barriers to inquiry teaching is the pressure to conform to high‐stakes testing and the lack of examples of inquiry teaching during teacher education experiences.     

The Effect of Nature of Science Metacognitive Prompts on Science Students’ Content and Nature of Science Knowledge,Metacognition, and Self‐Regulatory Efficacy

The purpose of the present explanatory mixed‐method design is to examine the effectiveness of a developmental intervention, Embedded Metacognitive Prompts based on Nature of Science (EMPNOS) to teach the nature of science using metacognitive prompts embedded in an inquiry unit. Eighty‐three (N = 83) eighth‐grade students from four classrooms were randomly assigned to an experimental and a comparison group. All participants were asked to respond to a number of tests (content and nature of science knowledge) and surveys (metacognition and self‐regulatory efficacy). Participants were also interviewed. It was hypothesized that the experimental group would outperform the comparison group in all measures. Partial support for the hypotheses was found. Specifically, results showed significant gains in content knowledge and nature of science knowledge of the experimental group over the comparison group. Qualitative findings revealed that students in the comparison group reported scientific thinking in similar terms as the scientific method, while the experimental group reported that scientists were creative and had to explain events using evidence, which is more closely aligned to the aspects of the nature of science. EMPNOS may have implications as a useful classroom tool in guiding students to check their thinking for alignment to the nature of science.     

Hydration dependent studies of highly aligned multilayer lipid membranes by neutron scattering

We investigated molecular motions on a picosecond timescale of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes as a function of hydration by using elastic and quasielastic neutron scattering. Two different hydrations corresponding to approximately nine and twelve water molecules per lipid were studied, the latter being the fully hydrated state. In our study, we focused on head group motions by using chain deuterated lipids. Information on in-plane and out-of-plane motions could be extracted by using solid supported DMPC multilayers. Our studies confirm and complete former investigations by Ko?nig et al. [J. Phys. II (France) 2, 1589 (1992)] and Rheinsta?dter et al. [Phys. Rev. Lett. 101, 248106 (2008)] who described the dynamics of lipid membranes, but did not explore the influence of hydration on the head group dynamics as presented here. From the elastic data, a clear shift of the main phase transition from the P(β) ripple phase to the L(α) liquid phase was observed. Decreasing water content moves the transition temperature to higher temperatures. The quasielastic data permit a closer investigation of the different types of head group motion of the two samples. Two different models are needed to fit the elastic incoherent structure factor and corresponding radii were calculated. The presented data show the strong influence hydration has on the head group mobility of DMPC.     

New Examples for the Unexpected Stability of the 10π‐Electron Hückel Arene [Si6]10–

The compounds Ba₄Ag₂Si₆, Eu₄Ag₂Si₆, and Ca₄Ag₂Si₆, prepared from the elements at 1273 K (the components in inner corundum crucibles are enclosed in sealed quartz ampoules), are brittle semiconductors with silvery luster. They react slowly with acids liberating hydrogen. Ba₄Ag₂[Si₆] and Eu₄Ag₂[Si₆] crystallize like Ba₄Li₂[Si₆] (space group Fddd (No. 70); a = 8.613 Å, b = 14.927 Å, c = 19.639 Å, and a = 8.420 Å, b = 14.585 Å, c = 17.864 Å, respectively), whereas Ca₄Ag₂[Si₆] represents a new structure type (space group Fmmm (No. 69); a = 8.315 Å, b = 14.391 Å, c = 8.646 Å). The three compounds are Zintl phases with the formal charges M²⁺, Ag⁺ and [Si₆]^10–. The mean bond lengths d(Si–Si) = 2.335–2.381 Å in the 10π‐Hückel arene [Si₆]^10– as well as d(Ag–Si) = 2.464–2.595 Å vary with the size of the M²⁺ cations. The chemical bonding was analyzed in terms of the Electron Localization Function (ELF) and compared with the bonding in related systems (Ce₄Co₂Si₆).     

Light Alters the NH3 vs N2H4 Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH2) Intermediate

Whereas synthetically catalyzed nitrogen reduction (N₂R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N₂H₄) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P₃^B) Fe catalyst, P₃^BFe⁺, significantly alters its product profile to increase N₂H₄ versus NH₃; P₃^BFe⁺ is otherwise known to be highly selective for NH₃. We posit a key terminal hydrazido intermediate, P₃^BFe=NNH₂, as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH₃, a low-lying triplet excited state leads to reactivity at N_α and formation of N₂H₄. Associated electrochemical and spectroscopic studies establish that N₂H₄ lies along a unique product pathway; NH₃ is not produced from N₂H₄. Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.     

Ground-state equilibrium thermodynamics and switching kinetics of bistable [2]rotaxanes switched in solution, polymer gels, and molecular electronic devices

We report on the kinetics and ground-state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable [2]rotaxanes in acetonitrile solution, polymer electrolyte gels, and molecular-switch tunnel junctions (MSTJs). For all rotaxanes a pi-electron-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF4+ and RTTF4+) contain tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT4+ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF4+), the TTF unit is replaced by a pi-extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT4+ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT4+ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT4+ ring to the DNP site. Reduction back to TTF0 (BPTTF0) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical-switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground-state equilibrium properties of all three bistable [2]rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.