A new paradigm for cationic cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes

A new example of stereospecific cationic cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes is provided. Protonation of a double bond vicinal to the iron tricarbonyl diene moiety is employed to trigger the cyclization, rather than the previously reported Lewis/protic acid dehydroxylation of diastereomeric alcohols, eliminating one step of separation and avoiding some reactivity problems previously encountered for one of the alcohol diastereoisomers. [reaction: see text]     

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)]: structural and spin-state phase transitions and low-temperature magnetic ordering in a unique 2-dimensional material

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and M?ssbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the alpha-, beta-, and gamma-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature M?ssbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and M?ssbauer techniques, occurs at T(c) approximately 6.5 K.     

Hyperpolarized 129Xe NMR investigation of multifunctional organic/inorganic hybrid mesoporous silica materials

An extensive study has been made on a series of multifunctional mesoporous silica materials, prepared by introducing two different organoalkoxysilanes, namely 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) and 3-cyanopropyltriethoxysilane (CPTES) during the base-catalyzed condensation of tetraethoxysilane (TEOS), using the variable-temperature (VT) hyperpolarized (HP) 129Xe NMR technique. VT HP-129Xe NMR chemical shift measurements of adsorbed xenon revealed that surface properties as well as functionality of these AEP/CP-functionalized microparticles (MP) could be controlled by varying the AEPTMS/CPTES ratio in the starting solution during synthesis. Additional chemical shift contribution due to Xe-moiety interactions was observed for monofunctional AEP-MP and CP-MP as well as for bifunctional AEP/CP-MP samples. In particular, unlike CP-MP that has a shorter organic backbone on the silica surface, the amino groups in the AEP chain tends to interact with the silanol groups on the silica surface causing backbone bending and hence formation of secondary pores in AEP-MP, as indicated by additional shoulder peak at lower field in the room-temperature 129Xe NMR spectrum. The exchange processes of xenon in different adsorption regions were also verified by 2D EXSY HP-129Xe NMR spectroscopy. It is also found that subsequent removal of functional moieties by calcination treatment tends to result in a more severe surface roughness on the pore walls in bifunctional samples compared to monofunctional ones. The effect of hydrophobicity/hydrophilicity of the organoalkoxysilanes on the formation, pore structure and surface property of these functionalized mesoporous silica materials are also discussed.     

Diels-Alder reaction of optically active (E)-γ-keto-α,β-unsaturated p-tolylsulfoxides with cyclopentadiene

The Diels-Alder reaction of enantiomerically pure (E)-γ-keto-α,β-unsaturated p-tolylsulphoxides 3 with cyclopentadiene give four easily separable diastereomers. The effect of several Lewis acids on the reaction was studied, finding a high endo selectivity with respect to the carbonyl group and moderate π-diastereoselectivity using BF₃·Et₂O as catalyst. The reactivity of compounds 3 as well as their endo selectivity are both higher than those observed for the corresponding (E)-3-sulfinylacrylates.     

Generation of new landomycins by combinatorial biosynthetic manipulation of the LndGT4 gene of the landomycin E cluster in S. globisporus

A 3 kb DNA fragment from the Streptomyces globisporus 1912 landomycin E (LaE) biosynthetic gene cluster (lnd) was completely sequenced. Three open reading frames were identified, lndGT4, lndZ4, and lndZ5, whose probable translation products resemble a glycosyltransferase, a reductase, and a hydroxylase, respectively. Studies of generated mutants from disruption and complementation experiments involving the lndGT4 gene allowed us to determine that LndGT4 controls the terminal L-rhodinose sugar attachment during LaE biosynthesis and that LndZ4/LndZ5 are responsible for the unique C11-hydroxylation of the landomycins. Generation of the novel landomycins F, G, and H in the course of these studies provided evidence for the flexibility of lnd glycosyltransferases toward their acceptor substrates and a basis for initial structure-activity relationships within the landomycin family of antibiotics.     

Systems-based design of bi-ligand inhibitors of oxidoreductases: filling the chemical proteomic toolbox

Genomics-driven growth in the number of enzymes of unknown function has created a need for better strategies to characterize them. Since enzyme inhibitors have traditionally served this purpose, we present here an efficient systems-based inhibitor design strategy, enabled by bioinformatic and NMR structural developments. First, we parse the oxidoreductase gene family into structural subfamilies termed pharmacofamilies, which share pharmacophore features in their cofactor binding sites. Then we identify a ligand for this site and use NMR-based binding site mapping (NMR SOLVE) to determine where to extend a combinatorial library, such that diversity elements are directed into the adjacent substrate site. The cofactor mimic is reused in the library in a manner that parallels the reuse of cofactor domains in the oxidoreductase gene family. A library designed in this manner yielded specific inhibitors for multiple oxidoreductases.     

In-lens cryo-high resolution scanning electron microscopy: methodologies for molecular imaging of self-assembled organic hydrogels.

The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.     

Ab initio density functional theory: the best of both worlds?

Density functional theory (DFT), in its current local, gradient corrected, and hybrid implementations and their extensions, is approaching an impasse. To continue to progress toward the quality of results demanded by today's ab initio quantum chemistry encourages a new direction. We believe ab initio DFT is a promising route to pursue. Whereas conventional DFT cannot describe weak interactions, photoelectron spectra, or many potential energy surfaces, ab initio DFT, even in its initial, optimized effective potential, second-order many-body perturbation theory form [OEP (2)-semi canonical], is shown to do all well. In fact, we obtain accuracy that frequently exceeds MP2, being competitive with coupled-cluster theory in some cases. Furthermore, this is accomplished within a relatively fast computational procedure that scales like iterative second order. We illustrate our results with several molecular examples including Ne2,Be2,F2, and benzene.     

Analyticity of the density of states in the anderson model on the Bethe lattice

Let H=1/2+V on l²(B), whereB is the Bethe lattice andV(x),x B, are i.i.d.r.v.'s with common probability distribution. It is shown that for distributions sufficiently close to the Cauchy distribution, the density of states(E) is analytic in a strip about the real axis.