Gas-phase synthesis of charged copper and silver Fischer carbenes from diazomalonates: mechanistic and conformational considerations in metal-mediated wolff rearrangements

Copper(I) and silver(I) Fischer carbenes are synthesized in the gas phase. Various diazomalonate-based compounds with an attached metal ion are introduced into the gas phase by electrospray ionization and subjected to collisional activation. Loss of N(2) generates a metastable Fischer carbene, which subsequently undergoes Wolff rearrangement and loss of CO. Further excitation leads to the loss of another CO molecule and the generation of a stable Fischer carbene. Isotopically labeled compounds are utilized to confirm the assignment of the products resulting from this process. DFT calculations are used to evaluate various mechanistic possibilities and to quantitatively assess the energetics of reactants and products. Silver(I) is shown to be more effective in facilitating Wolff rearrangement than copper(I), although both are more effective when compared to spectator charges such as sodium or a fixed quaternary nitrogen. Carbenes are not produced when copper(II), nickel(II), or a proton is used to form a quasi-molecular ion from the diazomalonate carbene precursor. Finally, trapping of the Fischer carbene by various functional groups attached through the open coordination site of the metal is investigated.     

Reactions of aziridines. Preparation and properties of 2-thiazolinium salts

Reaction of 1-substituted aziridines with thioamides in the presence of perchloric acid has provided a facile route to 2-thiazolinium salts. Thioformamide was used in this reaction to give the 2-unsubstituted 2-thiazolinium salts 3-[4-(2,6-xylyloxy)butyl]-2-thiazolinium perchlorate (11a) and 3-(2-cyelohexylbutyl)-2-thiazolinium perchlorate (IIc). A study of the rates of hydro-lytic breakdown of IIc and 3-(2-cyelohexylbutyl)-2-methyl-2-thiazolinium perchlorate (IId) showed that the 2-unsubstituted compound (IIc) was considerably less stable than the 2-methyl analog (IId) over the entire pH range. Use of 1-substituted aziridines in ring-opening reactions, previously applied only to 1-unsubstituted aziridines, has given expected products when thio-cyanate ion or thiourea was the nucleophile.     

Multinuclear NMR investigations of the oxygen, water, and hydroxyl environments in sodium hexaniobate

Solid-state (1)H, (17)O MAS NMR, (1)H-(93)Nb TRAPDOR NMR, and (1)H double quantum 2D MAS NMR experiments were used to characterize the oxygen, water, and hydroxyl environments in the monoprotonated hexaniobate material, Na(7)[HNb(6)O(19)].15H(2)O. These solid-state NMR experiments demonstrate that the proton is located on the bridging oxygen of the [Nb(6)O(19)](8-) cluster. The solid-state NMR results also show that the NbOH protons are spatially isolated from similar protons, but undergo proton exchange with the water species located in the crystal lattice. On the basis of double quantum (1)H MAS NMR measurements, it was determined that the water species in the crystal lattice have restricted motional dynamics. Two-dimensional (1)H-(17)O MAS NMR correlation experiments show that these restricted waters are preferentially associated with the bridging oxygen. Solution (17)O NMR experiments show that the hydroxyl proton is also attached to the bridging oxygen for the compound in solution. In addition, solution (17)O NMR kinetic studies for the hexaniobate allowed the measurement of relative oxygen exchange rates between the bridging, terminal, and hydroxyl oxygen and the oxygen of the solvent as a function of pH and temperature. These NMR experiments are some of the first investigations into the proton location, oxygen and proton exchange processes, and water dynamics for a base stable polyoxoniobate material, and they provide insight into the chemistry and reactivity of these materials.     

Cyclodextrin complexation of a stilbene and the self-assembly of a simple molecular device

(E)-4-tert-Butyl-4'-oxystilbene, 1(-), is thermally stable as the (E)-1(-) isomer but may be photoisomerized to the (Z)-1(-) isomer as shown by UV-vis and (1)H NMR studies in aqueous solution. When (E)-1(-) is complexed by alphaCD two inclusion isomers (includomers) form in which alphaCD assumes either of the two possible orientations about the axis of (E)-1(-) in alphaCD.(E)-1(-) for which (1)H NMR studies yield the parameters: k(1)(298 K)= 12.3 +/- 0.6 s(-1), DeltaH(1)(++)= 94.3 +/- 4.7 kJ mol(-1), DeltaS1(++)= 92.0 +/- 5.0 J K(-1) mol(-1), and k(2)(298 K)= 10.7 +/- 0.5 s(-1), DeltaH(2)(++)= 93.1 +/- 4.7 kJ mol(-1), DeltaS2(++)= 87.3 +/- 5.0 J K(-1) mol(-1) for the minor and major includomers, respectively. The betaCD.(E)-1(-) complex either forms a single includomer or its includomers interchange at the fast exchange limit of the (1)H NMR timescale. Complexation of 1(-) by N-(6(A)-deoxy- alpha-cyclodextrin-6(A)-yl)-N'-(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, results in the binary complexes 2.(E)-1(-) in which both CD component annuli are occupied by (E)-1(-) and which exists exclusively in darkness and 2.(Z)-1(-) in which only one CD component is occupied by (Z)-1(-) and exists exclusively in daylight at lambda > or = 300 nm. Irradiation of solutions of the binary complexes at 300 and 355 nm results in photostationary states dominated by 2.(E)-1(-) and 2.(Z)-1(-), respectively. In the presence of 4-methylbenzoate, 4(-), 2.(Z)-1(-) forms the ternary complex 2.(Z)-1(-).4(-) where 4(-) occupies the second CD annulus. Interconversion occurs between 2.(Z)-1(-).4(-) and 2.(E)-1(-)+4(-) under the same conditions as for the binary complexes alone. Similar interactions occur in the presence of 4-methylphenolate and 4-methylphenylsulfonate. The two isomers of each of these systems represent different states of a molecular device, as do the analogous binary complexes of N,N-bis(6(A)-deoxy- beta-cyclodextrin-6(A)-yl)urea, 3, [3.(E)-1(-) and 3.(Z)-1(-), where the latter also forms a ternary complex with 4(-).     

The influence of anisotropic membrane inclusions on curvature elastic properties of lipid membranes

A membrane inclusion can be defined as a complex of protein or peptide and the surrounding significantly distorted lipids. We suggest a theoretical model that allows for the estimation of the influence of membrane inclusions on the curvature elastic properties of lipid membranes. Our treatment includes anisotropic inclusions whose energetics depends on their in-plane orientation within the membrane. On the basis of continuum elasticity theory, we calculate the inclusion-membrane interaction energy that reflects the protein or peptide-induced short-ranged elastic deformation of a bent lipid layer. A numerical estimate of the corresponding interaction constants indicates the ability of inclusions to sense membrane bending and to accumulate at regions of favorable curvature, matching the effective shape of the inclusions. Strongly anisotropic inclusions interact favorably with lipid layers that adopt saddlelike curvature; such structures may be stabilized energetically. We explore this possibility for the case of vesicle budding where we consider a shape sequence of closed, axisymmetric vesicles that form a (saddle-curvature adopting) membrane neck. It appears that not only isotropic but also strongly anisotropic inclusions can significantly contribute to the budding energetics, a finding that we discuss in terms of recent experiments.     

Sulfur-mediated radical cyclisation reactions on solid support

Two methods for effecting radical cyclisation reactions of solid-supported 1,6-dienes are described. Additions of thiophenol and p-tolyl benzeneselenosulfonate have each been achieved with a concomitant 5-exo-trig radical cyclisation leading to the formation of highly functionalised cyclopentanes.     

Evaluating the third and fourth derivatives of spectral data

The problem of differentiating spectral data to yield the third and fourth derivatives is converted into one of solving an integral equation of the first kind. This equation is solved by Tikhonov regularization. The method of General Cross Validation is used to guide the choice of the regularization parameter that keeps noise amplification under control. The performance of this route to third and fourth derivative spectra is demonstrated by applying it to a number of published spectra. A computational problem associated with General Cross Validation has been identified.     

Preparation and structural study of a novel nonlinear molecular material: the -histidinum dihydrogenarsenate orthoarsenic acid crystal

-Histidinium dihydrogenarsenate orthoarsenic acid (LHA) crystal ;

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belongs to P2₁ space group of the monoclinic system, Z=2, a=9.264(2) Å, b=8.929(2) Å, c=8.874(2) Å, β=108.61(3)°. The crystal is isomorphous to

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(LHP) crystal. The H₂AsO₄⁻ anions and H₃AsO₄ molecules are joined into layers parallel to the (100) crystallographic planes by the O–HO type hydrogen bonds. The histidinium cations occupy the space between the layers. The histidinium cation posses the carboxylic COO⁻ group. The positive charges appear in the ring (the proton is attached to the =N(3)– nitrogen) and in the ammonium group. The differential scanning calorimetric (DSC) diagram does not show any phase transition till 100 K. The infrared spectrum fully confirms the X-ray crystal structure. The LHA crystal is a good second harmonic generator (0.46 relative to KDP)     

A study of the reactivity of elemental Cr/Se/Te thin multilayers using X-ray reflectometry, in situ X-ray diffraction and X-ray absorption spectroscopy

The reactivity of [Cr/Se/Te] multilayers under annealing was investigated using X-ray reflectometry, in situ X-ray diffraction, X-ray absorption fine structure (XAFS) measurements and transmission electron microscopy. For all samples, interdiffusion was complete at temperatures between 100 and 300 °C, depending on the repeating tri-layer thickness. A crystalline phase nucleated approximately 20 °C above the temperature where interdiffusion was finished. The first crystalline phase in a binary Cr/Te sample was layered CrTe₃ nucleating at 230 °C. In ternary samples (Se:Te=0.6-1.2), the low-temperature nucleation of such a layered CrQ₃ (Q=Se, Te) phase is suppressed and instead the phase Cr₂Q₃ nucleates first. Interestingly, this phase decomposes around 500 °C into layered CrQ₃. In contrast, binary Cr/Se samples form stable amorphous alloys after interdiffusion and Cr₃Se₄ nucleates around 500 °C as the only crystalline phase. Evaluation of the XAFS data of annealed samples yield Se-Cr distances of 2.568(1) and 2.552(1) Å for Cr₂Q₃ and CrQ₃, respectively. In the latter sample, higher coordination shells around Se are seen accounting for the Se-Te contacts in the structure.     

Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4'-oxyazobenzene: UV-visible, 1H NMR and ab initio studies

alpha-Cyclodextrin, beta-cyclodextrin, N-(6(A)-deoxy-alpha-cyclodextrin-6(A)-yl)-N'6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea and N,N-bis(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea (alphaCD, betaCD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4'-oxyazobenzene, E-3(-). In aqueous solution at pH 10.0, 298.2 K and I = 0.10 mol dm(-3)(NaClO(4)) spectrophotometric UV-visible studies yield the sequential formation constants: K(11) = (2.83 +/- 0.28) x 10(5) dm(3) mol(-1) for alphaCD.E-(-), K(21) = (6.93 +/- 0.06) x 10(3) dm(3) mol(-1) for (alphaCD)(2).E-3(-), K(11) = (1.24 +/- 0.12) x 10(5) dm(3) mol(-1) for betaCD.E-(-), K(21) = (1.22 +/- 0.06) x 10(4) dm(3) mol(-1) for (betaCD)(2).E-(-), K(11) = (3.08 +/- 0.03) x 10(5) dm(3) mol(-1) for .E-3(-), K(11) = (8.05 +/- 0.63) x 10(4) dm(3) mol(-1) for .E-3(-) and K(12) = (2.42 +/- 0.53) x 10(4) dm(3) mol(-1) for .(E-3(-))(2). (1)H ROESY NMR studies show that complexation of E-3(-) in the annuli of alphaCD, betaCD, 1 and 2 occurs. A variable-temperature (1)H NMR study yields k(298 K)= 6.7 +/- 0.5 and 5.7 +/- 0.5 s(-1), DeltaH = 61.7 +/- 2.7 and 88.1 +/- 4.2 kJ mol(-1) and DeltaS = -22.2 +/- 8.7 and 65 +/- 13 J K(-1) mol(-1) for the interconversion of the dominant includomers (complexes with different orientations of alphaCD) of alphaCD.E-3(-) and (alphaCD)(2).E-3(-), respectively. The existence of E-3(-) as the sole isomer was investigated through an ab initio study.