Accurate measurement of the relative bond energies of CO and H2O ligands in Fe+ mono- and bis-ligated complexes

The systems Fe(H(2)O)(n) (+)/CO[bond]H(2)O and Fe(CO)(n) (+)/CO[bond]H(2)O (n = 1 and 2) were investigated in a triple cell Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Using mixtures of CO with a very small amount of water, the ligand exchange equilibrium was reached, allowing experimental determination of the relevant equilibrium constants and free energies of reaction. Quantum chemical calculations at the B3LYP level of theory on the reactant and product species allowed us to determine the entropic terms and to derive the relative bond energies of CO and H(2)O in the mono- and bis-ligated complexes. For n = 1, H(2)O is more strongly bound to Fe(+) than CO by 4.1 +/- 1.6 kJ x mol(-1) at 298 K. For n = 2, at the same temperature, H(2)O is more strongly bound than CO to (H(2)O)Fe(+) by 7.6 +/- 1.6 kJ x mol(-1), and to (CO)Fe(+) by more than 20.1 kJ x mol(-1).     

Asymmetric intramolecular [2 + 2] photocycloadditions: alpha- and beta-hydroxy acids as chiral tether groups

Chiral alpha- and beta-hydroxy acids such as (S)-lactic acid, (S)-phenyllactic acid, (S)-mandelic acid, or (3R)-3-hydroxybutyric acid have been used as tether groups for intramolecular and diastereoselective [2 + 2] photocycloaddition of 3-oxocyclohexene carboxylic acid derivatives. Total regiocontrol toward the straight adduct and high diastereoselectivities (up to 94%) were observed in the case of butenyl lactate 11. After separation of the two diastereoisomers, cleavage of the chiral tether under basic conditions afforded cyclobutane lactones in good yield and enantiomeric pure form. An X-ray structure has been recorded that confirmed the relative and absolute configuration of the three contiguous stereogenic centers assigned according to CD spectra.     

Micelle-vesicle transition of fatty acid based ion-pair surfactants: interfacial evidence and influence of the ammonium counterion structure.

We describe the synthesis and the physicochemical study of new ion-pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface-tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show a phase transition, with two plateaux in the plot of surface tension versus log(c) (c=concentration). Such behaviour is related to the structure of the counterion, the alkyl chain length and the temperature. Pulsed gradient spin echo NMR spectroscopy experiments were performed to demonstrate the existence of micelles on the first plateau and to confirm the phase transition. The existence of vesicles on the second plateau of the surface tension was proved by CryoTEM observation and dynamic light scattering (DLS) measurements. Mainly, according to the structure of the counterion, there is either a strong association and a positioning along the chain leading to vesicles or a less strong association leading to external positioning and the formation of micelles at low concentrations or vesicles at higher concentrations.     

The redox state influences the interaction of ubiquinones with phospholipid bilayers

Differential scanning calorimetry (DSC) has been used to study the interaction of ubiquinones (UQn) and their reduced forms, ubiquinols (UQnH₂), with dipalmitoylphosphatidylcholine (DPPC) in multilamellar vesicles. The influence of the redox state has been investigated by comparing the effect of two ubiquinones with four (UQ4) and ten isoprene units (UQ10). In the presence of increasing amounts of UQ4 or UQ4H₂, concomitant shift of the gel to liquid crystalline phase transition towards lower temperatures and vanishing of the pre-transition are observed. Short-chain homologues are thus inserted parallel to phospholipid chains, their quinone ring close to the polar headgroups of the DPPC, with a larger degree of penetration for the reduced form. In addition, broadening and skewing of the main transition peak claim for a lateral self-organization in highly concentrated regions of UQ4 and UQ4H₂, with a redox state influence on the distribution in size and shape of lipid domains. The lipid thermotropic behavior is not affected by the presence of UQ10 which remain homogeneously dispersed within the midplane of the phospholipid bilayer, while effect of reduced UQ10H₂ argues for a different organization.     

Expedient synthesis of conjugated triynes via alkyne metathesis

The first synthesis of conjugated triynes by molybdenum-catalysed alkyne metathesis is reported. Strategic to the success of this approach is the utilization of sterically-hindered diynes that allowed for the site-selective alkyne metathesis to produce the desired conjugated triyne products. The steric hindrance of the alkyne moiety was found to be crucial in preventing the formation of diyne byproducts. This novel synthetic strategy was amenable to self- and cross-metathesis providing straightforward access to the corresponding symmetrical and dissymmetrical triynes with high selectivity.

The first synthesis of symmetrical and dissymmetrical conjugated triynes by self- and cross-metathesis was successfully achieved thanks to the use of hindered diynes.     

Rapid assembly of the polyhydroxylated beta-amino acid constituents of microsclerodermins C, D, and E

A very short and efficient synthesis of protected derivatives of APTO and AETD, the complex polyhydroxylated beta-amino acid residues present in microsclerodermins C, D, and E, is described. The targets are obtained in only five steps, in 23% and 16% overall yields, respectively. The key transformation involves the completely diastereoselective two-carbon homologation of appropriately selected intermediate chiral sulfinimines.     

Geometric lifting of the canonical basis and semitoric degenerations of Richardson varieties

In the case, A. Berenstein and A. Zelevinsky (1996) studied the Schützenberger involution in terms of Lusztig's canonical basis. We generalize their construction and formulas for any semisimple Lie algebra. We use the geometric lifting of the canonical basis, on which an analogue of the Schützenberger involution can be given. As an application, we construct semitoric degenerations of Richardson varieties, following a method of P. Caldero (2002).

     

Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand

This work tackles the photochemistry of a series of mononuclear Cr⁰, Mo⁰ and W⁰ carbonyl complexes containing a bidentate mesoionic carbene ligand of the 1,2,3-triazol-5-ylidene type. FTIR spectroscopy, combined with density functional theory calculations, revealed a clean photo-induced reaction in organic solvents (acetonitrile, pyridine, valeronitrile) to give mainly one photoproduct with monosubstitution of a carbonyl ligand for a solvent molecule. The highest photodissociation quantum yields were reached for the Cr⁰ complex under UV irradiation (266 nm). Based on previous investigations, the kinetics of the dark reverse reactions have now been determined, with reaction times of up to several hours in pyridine. Photochemical studies in the solid state (KBr matrix, frozen solution) also showed light-induced reactivity with stabilization of the metastable intermediate with a free coordination site at very low temperature. The identified reactive species emphasizes a mechanism without ligand–sphere reorganization.     

Novel polymerizable surfactants from 1:1 mixtures of alkylcarboxylic acids and norbornene methylenamine

A new family of polymerizable surfactants was synthesized starting from a 1:1 mixture of alkylcarboxylic acids (C(10) to C(16)) and norbornene methyleneamine. The ion-paired surfactants exhibited cloud temperatures, surface activity, and critical aggregation concentrations that differed according to the chain length, with a variation indicating a strongly associated ion pair. Light-scattering measurements and electron microscopy observations confirmed the spontaneous formation of stable vesicles (90 nm < d < 370 nm). Also, NMR experiments showed the enclosing of the norbornene part inside the vesicle membrane. Moreover, the addition of sodium chloride allowed the formation of a tubular structure leading to a viscous or gel-like solution. Finally, a preliminary vinylic polymerization test proved the polymerizable character of these ion-paired surfactants by an organometallic catalysis, leading to partially polymerized vesicles.     

Spontaneous polymerization at the air-water interface: a Brewster angle microscopy study

When a dioctadecyldimethylammonium bromide (DODA) monolayer is spread onto a styrene sulfonate (SSt) aqueous solution, this monomer undergoes a spontaneous polymerization process [Fichet, O; Teyssié, D. Macromolecules 2002, 35, 5352]. However, the polymer synthesized in this monolayer cannot be investigated by classical characterization techniques. Brewster angle microscopy has thus been used as a complementary method in order to study this spontaneous polymerization. From these measurements, the threshold concentration above which the spontaneous polymerization occurs has been determined more precisely; the monomer adsorption under the DODA monolayer has been evidenced as being very fast, as supposed previously; moreover, sodium bicarbonate is confirmed as an inhibitor of the polymerization. Also, the replacement of SSt by toluene sulfonate (TSt) confirms the SSt spontaneous polymerization. Finally, the molecular weight and/or the structure of the polymer synthesized in the monolayer seems to be different from those synthesized in solution.