Autoacceleration and inhibition: Free volume. Epoxy-amine kinetics

The epoxy-diamine cure process was studied. We found that the mechanism can be described in three steps: (i) initiation up to 20–25% of conversion. (ii) autoacceleration and (iii) inhibition. It has been observed that after the initiation there is a clear autoacceleration effect which has been explained in terms of free volume, considerations: the volume occupied by the products of reaction diminishes the available volume increasing the “local concentration” of the reactants and therefore the rate of reaction. The reaction was followed by FTIR (near infrared) as the main technique. We used a purified epoxy resin as well as phenyl glycidyl ether cured with m-xylenediamine. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1001–1016, 1998     

Enthalpy relaxation in polymethyl(α-n-alkyl)acrylates: Effect of length of alkyl chain

In this work, we have investigated by DSC the structural relaxation of amorphous polymethyl(α-n-alkyl)acrylates in which it is possible to change the length of the alkyl chain. We have evaluated the Narayanaswamy parameter, x, which controls the relative contribution of temperature and of structure to the relaxation time, the apparent activation energy, Δh*, and the nonexponentiality parameter, β, of the stretched exponential response function. The results suggest that x increases while Δh* decreases and β remains constant as the length of the side chain increases. This allows us to comment on the effect of chemical modification on the relaxation kinetics. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 583–593, 1998     

Hydrogen bonding in the inner-salt zwitterion and in two different charged forms of 5,6-bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid

5,6-Bis(2-pyridyl)­pyrazine-2,3-di­carboxyl­ic acid exists as an inner-salt zwitterion, 3-carboxy-5-(2-pyridinio)-6-(2-pyridyl)­pyrazine-2-carboxyl­ate, (Ia), C₁₆H₁₀N₄O₄. The adjacent pyridine and pyridinium rings are almost coplanar due to the presence of an intramolecular hydrogen bond involving the pyridine N atom and the NH H atom of the pyridinium group. In the crystal of (Ia), symmetry-related mol­ecules are hydrogen bonded via the carboxyl­ic acid OH group and one of the carboxyl­ate O atoms to form a polymer, which exhibits a channel-type structure. In the HCl, HClO₄ and HPF₆ salts, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium chloride 2.25-hydrate, (II), C₁₆H₁₁N₄O₄⁺·Cl⁻·2.25H₂O, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium perchlor­ate trihydrate, (IIIa), C₁₆H₁₁N₄O₄⁺·ClO₄⁻·3H₂O, and 6-car­boxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium hexa­fluoro­phosphate trihydrate, (IIIb), C₁₆H₁₁N₄O₄⁺·PF₆⁻·3H₂O, both pyridine rings are protonated. In the perchlorate form, and in the isomorphous hexa­fluoro­phosphate form, the mol­ecule possesses C₂ symmetry, with has a symmetrical intramolecular hydrogen bond involving the adjacent carboxyl­ate and carboxyl­ic acid substituents. In the crystals of the chloride and perchlorate (or hexa­fluoro­phosphate) salts, hydrogen-bonded polymers are formed which are three-dimensional and one-dimensional, respectively.     

Application of pyranoside phosphite-pyridine ligands to enantioselective metal-catalyzed allylic substitutions and conjugate 1,4-additions

A series of glucopyranoside phosphite–pyridine ligands have been applied in the metal-catalyzed allylic substitution and conjugate 1,4-addition reactions of several substrate types. We have been able to identify ligands that provided promising enantioselectivities in the Pd-catalyzed intermolecular allylic substitution of cyclic substrates (ee’s up to 86%) and desymmetrization (ee’s up to 94%) and in the Cu-catalyzed conjugate addition of challenging aliphatic enones (ee’s up to 90%).     

Studies on the chemical stability and synthetic utility of an oxazolidine linker for solid-phase chemistry

A chemical stability study on the oxazolidine linker system has been carried out using a dual-linker analytical construct within a parallel reaction scan. The study established the compatibility of the oxazolidine platform with a wide range of commonly employed synthetic reaction conditions including nucleophilic, oxidizing, and reducing conditions. The scan was further used to probe and optimize acidic conditions under which the oxazolidine could release the substrate from the solid-support and to identify reagents that could cleave while retaining other acid-labile groups. The solid-phase synthesis of a small molecular array established the utility of oxazolidine aldehyde 1 as a building block for asymmetric chemistry while exploiting the data generated by the reaction scan.     

Enantioselective synthesis of (−)-barrenazines A and B

A short enantioselective synthesis of barrenazines A and B is described. Barrenazines A and B are prepared following a common synthetic route in nine steps (19% overall yield) and eight steps (21% overall yield), respectively, from readily available 4-methoxy-3-(triisopropylsilyl)pyridine. The synthesis relies on a highly diastereoselective nucleophilic addition of a Grignard reagent to a chiral acylpyridinium salt, a radical azidation of a silyl enol ether and the assembly of the pyrazine ring by reductive dimerization of a functionalized 5-azidopiperidin-4-one.