Metallocycle synthesis accelerated by high pressure

Reaction of cis-[FeH₂(dmpe)₂](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC₂CHPh)(dmpe)₂] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa.     

The variation of carrier gas viscosities with temperature

After describing simplified equations exspressing the temparature dependency of the viscosity of carrier gases (helium, nitrogen and hydrogen ) relative to a base value, absolute relationships based on the kinetic theory of gases are discussed. Comparative data obtained using various calculation methods are given and are compared to measured values. Based on the kinetic relationshipsm, of viscosity. Finally, the influence of pressure on the viscosity is also briefly discussed. As a supplement, Viscosity data are tabulated for the three gases in the range of 0°C to 400°C in increments of 2 K, calculated using the kinetic relationships.     

Ab initio excited-state dynamics of the photoactive yellow protein chromophore

The photoisomerization mechanism of the neutral form of the photoactive yellow protein (PYP) chromophore is investigated using ab initio quantum chemistry and first-principles nonadiabatic molecular dynamics (ab initio multiple spawning or AIMS). We identify the nature of the two lowest-lying excited states, characterize the short-time behavior of molecules excited directly to S2, and explain the origin of the experimentally observed wavelength-dependent photoisomerization quantum yield.     

Development of an effective chiral auxiliary for hydroxyalkyl radicals

The development of an effective chiral auxiliary for hydroxyalkyl radicals is delineated. Both the 2-tetrahydropyranyl (THP) and tri-O-benzyl-2-deoxy-alpha-D-glucopyranosyl (GLU) auxiliaries resulted in diastereoselective radical additions to methyl acrylate at -78 degrees C (ds = 6/1 and 11/1, respectively). The developing stereochemistry at the radical center was completely under auxiliary control. Correlation experiments showed that the D-GLU auxiliary led to attack on the radical Si-face. The selectivity of these radical additions dropped-off considerably when the more reactive 2-nitropropene trap was employed. Computational studies suggested that the observed facial selectivity was due primarily to entropic factors in the transition state but that a smaller temperature-dependent enthalpic contribution was also involved. It was hypothesized that incorporation of a quaternary center at C-6 (THP numbering) would restore the facial selectivity with more reactive radical traps by restricting the orientations available to the incoming alkene. In the event, the trans-6-tert-butyltetrahydropyranyl (tBu-THP) auxiliary resulted in very good diastereoselection with 2-nitropropene (ds = 35/1 at -78 degrees C, 15/1 at 0 degrees C, and 8/1 at RT) as did the tri-O-benzyl-6,6-dimethyl-2-alpha-D-deoxyglucopyranosyl (diMe-GLU) auxiliary during additions to ethyl alpha-trifluoroacetoxyacrylate (ds = 10/1 at 0 degrees C). A protocol for recovery of the sugar-derived chiral auxiliaries was also established. This work sets the stage for the development of a novel approach to 1, 3, 5.(2n + 1) polyols based on iterative radical homologation as well as the application of these pyranosidic auxiliaries to other synthetically important reactions.     

Amino(oligo)thiophene-based environmentally sensitive biomembrane chromophores

There is a growing need for cellular imaging with fluorescent probes that emit at longer wavelengths to minimize the effects of absorption, autofluorescence, and scattering from biological tissue. In this paper a series of new environmentally sensitive hemicyanine dyes featuring amino(oligo)thiophene donors have been synthesized via aldol condensation between a 4-methylpyridinium salt and various amino(oligo)thiophene carboxaldehydes, which were, in turn, obtained from amination of bromo(oligo)thiophene carboxaldehyde. Side chains on these fluorophores impart a strong affinity for biological membranes. Compared with benzene analogues, these thiophene fluorophores show significant red shift in the absorption and emission spectra, offering compact red and near-infrared emitting fluorophores. More importantly, both the fluorescence quantum yields and the emission peaks are very sensitive to various environmental factors such as solvent polarity or viscosity, membrane potential, and membrane composition. These chromophores also exhibit strong nonlinear optical properties, including two-photon fluorescence and second harmonic generation, which are themselves environmentally sensitive. The combination of long wavelength fluorescence and nonlinear optical properties make these chromophores very suitable for applications that require sensing or imaging deep inside tissues.     

Electrochemical and structural coupling of the naphthoquinone amino acid

As a prelude to engineering artificial energy conversion proteins emulating biology, we examine the inclusion of a synthetic naphthoquinone amino acid in a characterized host-guest protein and determine the effects of its quinone and hydroquinone forms on the helix-coil distribution.     

New detection modality for label-free quantification of DNA in biological samples via superparamagnetic bead aggregation

Combining DNA and superparamagnetic beads in a rotating magnetic field produces multiparticle aggregates that are visually striking, enabling label-free optical detection and quantification of DNA at levels in the picogram per microliter range. DNA in biological samples can be quantified directly by simple analysis of optical images of microfluidic wells placed on a magnetic stirrer without prior DNA purification. Aggregation results from DNA/bead interactions driven either by the presence of a chaotrope (a nonspecific trigger for aggregation) or by hybridization with oligonucleotides on functionalized beads (sequence-specific). This paper demonstrates quantification of DNA with sensitivity comparable to that of the best currently available fluorometric assays. The robustness and sensitivity of the method enable a wide range of applications, illustrated here by counting eukaryotic cells. Using widely available and inexpensive benchtop hardware, the approach provides a highly accessible low-tech microscale alternative to more expensive DNA detection and cell counting techniques.     

Solvothermal depolymerization and recrystallization of imine-linked two-dimensional covalent organic frameworks

Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N₂ porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by in situ wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature ¹H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (TAPB-PDA COF), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (TAPB-DMTA COF) forms initially with improved crystallinity compared to TAPB-PDA COF and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.

Conditions for which imine-linked 2D COF polymerizations are temperature-sensitive are identified that enable a dissolution/repolymerization process akin to molecular recrystallization.     

THE CHEMISTRY OF HETEROARYLPHOSPHORUS COMPOUNDS. PART XII.1 THE DIFFERENTIAL-PULSE POLAROGRAPHIC REDUCTION OF HETEROARYL-AND HETEROARYLMETHYLPHOSPHONIUM SALTS

Abstract

The polarographic reduction of series of heteroaryl- and heteroarylmethylphosphonium salts has been studied using a differential pulse polarographic technique, this having a number of advantages over classical dc polarography. For the salts Ar₃P⁺CH₂PhBr^-, the observed order of ease of reduction correlates with the electron-withdrawing ability of the heteroaryl group (i.e. Ar = 2-furyl > 2-thienyl > phenyl > 1-methylpyrrol-2-yl). In contrast, for the salts Ph₃P⁺CH₂ArBr^- (Ar = 2-furyl, 2-thienyl, phenyl, 1-methylpyrrol-2-yl, 3-thienyl and 3-furyl), the ease of reduction correlates best with the order of stability of the forming carbanions as established in earlier studies of the alkaline hydrolysis of these salts.