Supramolecular catalysis at work: diastereoselective synthesis of a molecular bowl with dynamic inner space

The supramolecular assistance to Pd(0)/Cu(I)-catalyzed cyclotrimerization of stannylated norbornene 7 has been investigated to give molecular bowl 1syn in a stereoselective fashion. Following a divergent strategy, racemic norbornene 7 was synthesized in satisfactory yield. Self-coupling, promoted by Pd(0)/Cu(I) catalysis acting in synergy with CsF, yielded molecular bowl 1syn in a moderate 30% yield. The reaction diastereoselectivity is affected by the concentration of Cu(I) and Cs+: increasing quantities of the cations enhanced the syn/anti ratio of the isolated cyclotrimer from statistical (1:3) to a more desirable (4.5:1) ratio, in favor of the molecular bowl 1syn. 1H NMR spectroscopic studies suggested the coordinating affinity of 1syn toward transition metals Cu(I), Ag(I), and Au(I), to account for the observed templation effect. In particular, the tridentate 1syn has been shown to bind to one Ag(I) cation in the assembly process that is driven with enthalpy (Delta H degrees = -19 +/- 2 kcal/mol, Delta S degrees = -45 eu). The complete coordination was not cooperative, and was hypothesized to be impeded with the adverse entropy. Accordingly, density functional theory (BP86) calculations of 1syn and its mono-, bis-, and tris-Ag(I) complexes suggested that the coordination of one to three silver cations is highly exothermic. The calculations also revealed that the bowl constriction is necessary for the aromatic arms to become preorganized and bind to a silver cation(s) (Delta E approximately 8 kcal/mol). Ultimately, Ag(I) has been shown to assist the diastereoselective formation of 1syn, lending support to the notion of template-directed synthesis.     

Design, synthesis, and biological evaluation of platensimycin analogues with varying degrees of molecular complexity

The molecular design, chemical synthesis, and biological evaluation of two distinct series of platensimycin analogues with varying degrees of complexity are described. The first series of compounds probes the biological importance of the benzoic acid subunit of the molecule, while the second series explores the tetracyclic cage domain. The biological data obtained reveal that, while the substituted benzoic acid domain of platensimycin is a highly conserved structural motif within the active compounds with strict functional group requirements, the cage domain of the molecule can tolerate considerable structural modifications without losing biological action. These findings refine our present understanding of the platensimycin pharmacophore and establish certain structure-activity relationships from which the next generation of designed analogues of this new antibiotic may emerge.     

Energy transfer dynamics in Re(I)-based polynuclear assemblies: a quantitative application of Förster theory

The synthesis, structure, and photophysical properties of a new family of tetranuclear FeRe 3 chromophore-quencher complexes having the general form [Fe(pyacac) 3(Re(bpy')(CO) 3) 3](OTf) 3 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy' is 4,4',5,5'-tetramethyl-2,2'-bipyridine (tmb, 1), 2,2'-bipyridine (bpy, 2), and 4,4'-diethylester-2,2'-bipyridine (deeb, 3)) are reported. Time-resolved emission data acquired in room-temperature CH 2Cl 2 solutions exhibited single exponential decay kinetics with observed lifetimes of 450 +/- 30 ps, 755 +/- 40 ps, and 2.5 +/- 0.1 ns for complexes 1, 2, and 3, respectively. The emission in each case is assigned to the decay of the Re (I)-based (3)MLCT excited state; the lifetimes are all significantly less than the corresponding AlRe 3 analogues (2250 +/- 100 ns, 560 +/- 30 ns, and 235 +/- 20 ns for 4, 5, and 6, respectively), which were also prepared and characterized. Electron transfer is found to be thermodynamically unfavorable for all three Re (I)-containing systems: this fact coupled with the absence of optical signatures for the expected charge-separated photoproducts in the time-resolved differential absorption spectra and favorable spectral overlap between the donor emission and the acceptor absorption profiles implicates dipolar energy transfer from the Re (I)-based excited state to the high-spin Fe (III) core as the dominant quenching pathway in these compounds. Details obtained from the X-ray structural data of complex 2 allowed for a quantitative application of Forster energy transfer theory by systematically calculating the separation and spatial orientation of the donor and acceptor transition moment dipoles. Deviations between the calculated and observed rate constants for energy transfer were less that a factor of 3 for all three complexes. This uncommonly high degree of precision testifies to both the appropriateness of the Forster model as applied to these systems, as well as the accuracy that can be achieved in quantifying energy transfer rates if relative dipole orientations can be accounted for explicitly.     

Effects of catalyst introduction methods using PAMAM dendrimers on selective electroless nickel deposition on polyelectrolyte multilayers

We studied the effects of catalyst introduction methods using poly(amidoamine) (PAMAM) dendrimers on the nickel patterning of polyelectrolyte multilayer (PEM)-coated substrates. Three different approaches to palladium catalyst introduction using microcontact printing as the patterning technique were utilized and compared. The catalyst introduction methods are (1) direct catalyst stamping, (2) directed assembly using PAMAM dendrimer stamping, and (3) catalyst encapsulation and reduction to nanoparticles within PAMAM dendrimers before stamping. After patterning, the sample surfaces were placed in an electroless bath where nickel was selectively plated onto the patterns. The patterned surfaces were characterized using optical microscopy, atomic force microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The metal plating rates on different homogeneous surfaces that simulate the patterned surfaces were measured using a quartz crystal microbalance. In addition, the effect of PEM film thickness (i.e., number of bilayers) on the selectivity of nickel patterning was investigated.     

Electrochemical polarisation and galvanic couple behaviour of the primary phase of 55% Al–Zn coating investigated using band microelectrodes (BME) and band microelectrode arrays

Electrochemical polarisation experiments have shown that anodic dissolution processes on Al–40% Zn alloys are significantly enhanced in chloride compared to sulfate-based electrolytes. The aluminium content of the alloys allowed passive behaviour to be observed in sulfate electrolyte even in the presence of zinc-rich precipitates on the surface. Electrolyte pH affected cathodic processes, which was attributed to the rate of proton reduction and the passivity of the surface. Monitoring the OCP of the alloy band during polarisation of neighbouring zinc electrodes in band microelectrode (BME) arrays showed that generation of alkaline pH at the zinc electrodes affected the OCP of the alloy when the inter-electrode spacing was 10, 50, and 200 μm. Where elements of a BME array were close enough to interact via mass transport, the overall galvanic behaviour of the cell was found to be anodic or cathodic, whereas the alloy was consistently cathodic with respect to zinc in galvanic cells at larger separations. Dedicated to the 80th birthday of Keith B. Oldham.     

Accurate magnetic exchange couplings in transition-metal complexes from constrained density-functional theory

We demonstrate an accurate method for extracting Heisenberg exchange-coupling constants (J) from density-functional theory (DFT) calculations. We note that the true uncoupled low-spin state of a given molecule should be identified with the ground state of the system subject to a constraint on the spin density of the atoms. Using an efficient optimization strategy for constrained DFT we obtain these states directly, leading to a simple, physically motivated formula for J. Our method only depends on state energies and their associated electron densities and assigns no unphysical meaning to the Kohn-Sham determinant or individual orbitals. We study several bimetallic transition-metal complexes and find that the constrained DFT approach is competitive with, if not better than, the best broken symmetry DFT results. The success of constrained DFT in these cases appears to result from a balanced elimination of self-interaction error and static correlation from the simulation.     

Synthesis of tertiary beta-hydroxy amides by enolate additions to acylsilanes

The synthesis of tertiary beta-hydroxy amides from acylsilanes, acetamides, and electrophiles is described. The addition of amide enolates to acylsilanes generates beta-silyloxy homoenolate reactivity by undergoing a 1,2-Brook rearrangement. These unique nucleophiles formed in situ can then undergo smooth addition to alkyl halides, aldehydes, and ketones. Enolates derived from amides are crucial for the success of this process since ketone enolates suffer from internal return of the beta-carbanion onto the carbonyl carbon. The use of optically active amide enolates delivers beta-hydroxy amide products with good levels of diastereoselectivity (>/=10:1).