Chiral dipeptide mimics possessing a fluoroolefin moiety: a relevant tool for conformational and medicinal studies

The replacement of the amide bond in a peptide backbone is a promising strategy in peptidomimetic drug research. Over the various amide bond surrogates, the fluoroolefin moiety has been successfully developed as an effective mimic. Today, fluorine-containing compounds account for a large proportion of new active molecules in life sciences. The synthesis of fluoroolefin peptide mimics is not a trivial task and innovative approaches often need to be addressed, in particular for the stereocontrol of the double bond configuration and the chiral centres adjacent to the fluoroalkene. These fluorinated peptidomimetics have been synthesised and evaluated as metabolically stable and/or conformationally constrained analogs of enzyme inhibitors, and as tools for probing the function, structure, and binding process of receptors.     

Generation, characterization, and electrochemical behavior of the palladium-hydride cluster [Pd3(dppm)3(mu3-CO)(mu3-H)]+ (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd(3)(dppm)(3)(mu(3)-CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+). This new palladium-hydride cluster has been characterised by (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO(2) (-) and [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd(3)(dppm)(3)(mu(3)-CO)](2+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd--Pd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd--H bond length remains relatively the same.     

New insights into the initial steps of the formation of SBA-15 materials: an in situ small angle neutron scattering investigation

Time-resolved in situ SANS investigations have provided direct experimental evidence for the three initial steps in the formation of the SBA-15 mesoporous material: an induction period is followed by a shape transformation of the micelles from spherical to cylindrical ones followed by the precipitation of a two-dimensional hexagonal phase.     

Cyclometallated iridium(III) complexes with substituted 1,10-phenanthrolines: a new class of highly active organometallic second order NLO-phores with excellent transparency with respect to second harmonic emission

[Ir(ppy)(2)(5-R-1,10-phen)][PF(6)] (ppy = cyclometallated 2-phenylpyridine, phen = phenanthroline, R = H, Me, NMe(2), NO(2)) and [Ir(ppy)(2)(4-R',7-R'-1,10-phen)][PF(6)] (R' = Me, Ph) complexes are characterized by one of the highest second order NLO response (measured by the EFISH technique) reported for a metal complex, mainly due (as suggested by a theoretical SOS-TDDFT investigation) to MLCT processes from the ppy-Ir based moiety acting as donor push system to pi* orbitals of phen, acting as an acceptor pull system; the good transparency to the second harmonic emission renders these NLO-phores appealing as building blocks for molecular materials with second harmonic generation.     

Accurate heteronuclear J-coupling measurements in dilute spin systems using the multiple-quantum filtered J-resolved experiment

A new solid-state MAS NMR experiment is proposed to accurately measure heteronuclear (19)F-(207)Pb J-coupling constants, even though these couplings are not visible on high speed (19)F 1D MAS spectra; in particular, we demonstrate that the J-resolved experiment combined with scalar multiple-quantum filtering considerably improves the resolution of J-multiplet patterns for dilute spin systems.     

Structure-reactivity relationships in SHOP-type complexes: tunable catalysts for the oligomerisation and polymerisation of ethylene

For over 30 years complexes with the general formula [NiPh(P,O)L] (L = tertiary phosphine; P,O = chelating phosphanylenolato ligand) have been used as highly efficient oligomerisation catalysts suitable for the production of linear alpha-olefins. The same complexes, which are usually referred to as SHOP-type catalysts (SHOP = Shell Higher Olefin Process) can also be used as ethylene polymerisation catalysts, provided they are treated with a phosphine scavenger that selectively removes the tertiary phosphine ligand (L). This Perspective examines the impact of various parameters (influence of the substituents, backbone size, solvent, use of co-catalysts, etc.) on the catalytic outcome of the complexes. Overall, this review shows that the selectivity and activity of the catalyst may be tuned efficiently through directed modification of the P,O chelator.     

alpha-TEPHOS: a cyclodextrin-derived tetraphosphine for multiple metal binding

The tetraphosphine 6(A),6(B),6(D),6(E)-tetradeoxy-6(A),6(B),6(D),6(E)-tetra(diphenylphosphinyl)-2(A),2(B),2(C),2(D),2(E),2(F),3(A),3(B),3(C),3(D),3(E),3(F),6(C),6(F)-tetradeca-O-methyl-alpha-cyclodextrin (alpha-TEPHOS) has been prepared in high yield by reacting 6(A),6(B),6(D),6(E)-tetra-O-methylsulfonyl-2(A),2(B),2(C),2(D), 2(E),2(F),3(A),3(B),3(C),3(D),3(E),3(F),6(C),6(F)-tetradeca-O-methyl-alpha-cyclodextrin with excess PPh(2)Li. The product was purified in its BH(3)-protected form. alpha-TEPHOS is the first optically active tetraphosphine in which the four phosphine units are tethered to a cavity-shaped scaffold. When reacted with [AuCl(tetrahydrothiophene)], alpha-TEPHOS led to the corresponding tetragold complex [(alpha-TEPHOS)(AuCl)(4)] in which the four gold atoms are all located close to the primary face of the cyclodextrin. Reaction with [PdCl(o-C(6)H(4)CH(2)NMe(2))](2) gave the C(2)-symmetrical complex [(alpha-TEPHOS){PdCl(o-C(6)H(4)CH(2)NMe(2))}(4)]. Treatment of the ligand with two equiv. of [Rh(norbornadiene)(tetrahydrofuran)(2)]BF(4) afforded selectively the bimetallic complex [(alpha-TEPHOS){Rh(norbornadiene)}(2)](BF(4))(2) in which each metal centre is coordinated to two phosphorus atoms belonging to adjacent glucose units.     

Enantioselective absorption of chirally doped liquid crystalline networks studied by the use of an electronic microbalance

A polydomain cholesteric elastomer was obtained by cross-linking a nematic side-chain polysiloxane in the presence of a chiral dopant. After extraction of the chiral dopant, sorption experiments were performed, by the use of an electronic microbalance, in the presence of each enantiomer of a chiral amine molecule. The sorption kinetics corresponds to a Fickian diffusion behavior. They allowed us to determine the diffusion coefficients and to show that the doped polymer has a more pronounced affinity toward one of the enantiomers.     

Alkyne creation in aliphatic polymers: influence of side groups

In this article, we focus on the influence of side groups on terminal alkyne creation, in aliphatic polymers submitted to swift heavy ions, under vacuum. Terminal alkyne creation was compared in polyethylene and in substituted polymers. We selected two classes of side groups: alkyl groups, which differed in their length (polypropylene and polybutene) and allyl groups, which were linear (EPDMh) or cyclic (EPDMn). Irradiated samples were analyzed using Fourier transform IR spectroscopy, in the transmission mode, at room temperature. Alkynes are created when the electronic stopping power, (dE/dx)e, exceeds a threshold value. This threshold value was moderately influenced by the presence of an alkyl side chain but was clearly reduced in the presence of C=C bonds on the side chain. Nevertheless, in all-saturated polymers, below the (dE/dx)e threshold, terminal alkyne formation is observed after a latent dose, rather independently of the side-chain length and directly related to the formation of radiation-induced double bonds prior to alkyne formation. Under S ion irradiation, the radiation chemical yield G0 value obtained in EPDMh was 4 times the value of G0 in PE. This effect is understandable only if important energy transfers, from the backbone to C=C double bonds, are considered.