Mechanism of homogeneously and heterogeneously catalysed Meerwein-Ponndorf-Verley-Oppenauer reactions for the racemisation of secondary alcohols

The mechanism of hydrogen transfer from alcohols to ketones, catalysed by lanthanide(III) isopropoxides or zeolite Beta has been studied. For the lanthanide catalysed reactions, (S)-1-phenyl-(1-(2)H(1))ethanol and acetophenone were used as case studies to determine the reaction pathway for the hydrogen transfer. Upon complete racemisation all deuterium was present at the 1-position, indicating that the reaction exclusively takes place via a carbon-to-carbon hydrogen transfer. Zeolite Beta with different Si/Al ratios was applied in the racemisation of (S)-1-phenylethanol. In this case the racemisation does not proceed via an oxidation/reduction pathway but via elimination of the hydroxy group and its re-addition. This mechanism, however, is not characteristic for all racemisation reactions with zeolite Beta. When 4-tert-butyl cyclohexanone is reduced with this catalyst, a classical MPV reaction takes place exclusively. This demonstrates that zeolite Beta has a substrate dependent reaction pathway.     

Stereochemical variations on the colchicine motif. Peracid oxidation of thiocolchicone. Synthesis, conformation and inhibition of microtubule assembly

When 7-oxodesacetamidothiocolchicine (1) was treated with various peroxides in order to afford a Baeyer-Villiger rearrangement, a complex mixture of products was formed, which included the sulfoxide, (2) and sulfone, (3). When peracetic acid was used two additional products were formed; a C-ring lactone (4) and a ring-contracted allocolchicine derivative (5). The sulfoxide (2) was semi-preparatively resolved into enantiomers by chromatography on microcrystalline triacetylcellulose. Rotational barriers around the A-C pivot bond of, and were determined by dynamic 1H NMR analysis. The compounds, and exhibit moderate inhibition of tubulin polymerization, according to in vitro turbidity studies, whereas was inactive.     

Cyclodextrin-surfactant coassembly depends on the cyclodextrin ability to crystallize

Full equilibrium phase diagrams are presented for two ternary systems composed of the cationic surfactant dodecyltrimethylammonium bromide (DTAB), water (D(2)O), and a cyclodextrin, either β-cyclodextrin (β-CD) or (2-hydroypropyl)-β-cyclodextrin (2HPβCD). (2)H NMR, SAXS, WAXS, and visual examination were used to determine the phase boundaries and characterize the nature of the phases formed. Additionally, diffusion (1)H NMR was used to investigate parts of the diagrams. The water solubility of 2HPβCD is 80% (w/w), whereas it is only 1.85% (w/w) for β-CD. Solubility increases for both species upon complexation with DTAB; while the increase is minute for 2HPβCD, it is dramatic for β-CD. Both systems displayed an isotropic liquid solution (L(1)) one-phase region, the extension of which differs extensively between the two systems. Additionally, the DTAB:2HPβCD:water system also comprised a normal hexagonal (H(1)) area, which was not found for the DTAB:β-CD:water system. In the DTAB:β-CD:water system, on the other hand, we found cocrystallization of DTAB and β-CD. From this work we conclude that DTAB and CD molecules form 1:1 inclusion complexes with high affinities. Moreover, we observed indications of an association of 2HPβCD to DTAB micelles in the isotropic solution phase, which was not the case for β-CD and DTAB micelles. This is, to our knowledge, the first complete phase diagrams of surfactant-CD mixtures; as a novel feature it includes the observation of cocrystallization at high concentrations.     

Structural transitions induced by shear flow and temperature variation in a nonionic surfactant/water system

In this study, we investigate structural transitions of tetraethylene glycol monohexadecyl ether (C(16)E(4)) in D(2)O as a function of shear flow and temperature. Via a combination of rheology, rheo-small-angle neutron scattering and rheo-small-angle light scattering, we probe the structural evolution of the system with respect to shear and temperature. Multi-lamellar vesicles, planar lamellae, and a sponge phase were found to compete as a function of shear rate and temperature, with the sponge phase involving the formation of a new transient lamellar phase with a larger spacing, coexisting with the preceding lamellar phase within a narrow temperature-time range. The shear flow behavior of C(16)E(4) is also found to deviate from other nonionic surfactants with shorter alkyl chains (C(10)E(3) and C(12)E(4)), resembling to the C(16)E(7) case, of longer chain.     

Selective Dimerization of Lewis‐Acid/Base‐Stabilized Phosphanylalanes

The reaction of [{(CO)₅W}PRH₂] (R=H, Ph) with H₃Al ? NR₃ (R=Et, Me) leads to the formation of four‐membered heterocyclic compounds [({(CO)₅W}P(H)AlH ? NEt₃)₂] and [({(CO)₅W}PhPAlH ? NMe₃)₂]. Upon dissolving the solid compounds, fast equilibria between the isomers are observed on the NMR timescale. Further insight into the stability and reactivity of the isomers was gained by applying theoretical methods. DFT calculations predict that hydrogen elimination in the case of [({(CO)₅W}PhPAlH ? NMe₃)₂] may be reversible.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.