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At least 1,279 records · Page 71Linked to original sources
PubMed2006

Water absorption and desorption from the dipole ordered polymer poly(methylvinylidene cyanide).

From thermal desorption studies, we find evidence that absorbed water in the bulk of poly(methylvinylidene cyanide) is more weakly bound than is the case for copolymer films of poly(vinylidenefluoride-trifluoroethylene). Ultraviolet laser enhanced thermal desorption of absorbed water exhibits little light polarization dependence for poly(methylvinylidene cyanide) in contrast to absorbed water in copolymer films of poly(vinylidenefluoride-trifluoroethylene). The implications of these differences are discussed.

Absorption↗
PubMed2006

Biomimetic mineralization of hydroxyapatite crystals on the copolymers of vinylphosphonic acid and 4-vinilyimidazole.

In the present work, copolymers of vinylphosphonic acid and 4-vinilyimidazole (poly(4-VIm-co-VPA)) were found to be substrates favoring the precipitation of nanohydroxyapatite (HAP) crystals from stable supersaturated solutions at pH 7.4 and 37 degrees C. Deposition kinetics were studied by the constant composition technique. The rates of crystallization both on HAP seed crystals as reference and on the copolymer in powder form were investigated at constant supersaturation conditions. The rates of HAP crystal growth on the polymeric substrate were found to depend on the amount of seed material and on the phosphate content of the copolymer.

Biomimetics↗
PubMed2006

H-bonding-directed self-assembly of synthetic copolymers containing nucleobases: organization and colloidal fusion in a noncompetitive solvent.

The self-organization of random copolymers composed of a nucleobase monomer (either 1-(4-vinylbenzyl)thymine or 9-(4-vinylbenzyl)adenine) and dodecyl methacrylate (DMA) was studied in dilute chloroform solutions. The balance between the molar fractions of the nucleobase monomer (leading to intermolecular H-bonding) and DMA (soluble moiety in chloroform) in the polymer chains was found to be the parameter that principally influences the self-organization. DMA-rich copolymers are molecularly soluble in chloroform, whereas nucleobase-rich copolymers are insoluble in this solvent. Copolymers possessing an equimolar comonomer composition self-assemble into micrometer-sized particles physically cross-linked by intermolecular H-bonds (either thymine-thymine or adenine-adenine interactions, depending on the studied copolymer). Nevertheless, when mixed together, thymine- and adenine-based colloids fuse into thermodynamically stable microspheres cross linked by adenine-thymine interactions.

Benzene↗
PubMed2007

Rapid proton-coupled electron-transfer of hydroquinone through phenylenevinylene bridges.

We describe the synthesis of two oligo(phenylene vinylene)s (OPVs) with a hydroquinone moiety and a thiol anchor group: 4-(2',5'-dihydroxystyryl)benzyl thioacetate and 4-[4'-(2' ',5' '-dihydroxystyryl)styryl]benzyl thioacetate. Monolayers on gold of these molecules were examined by electrochemical techniques to determine the electron transfer kinetics of the hydroquinone functionality (H2Q) through these delocalized tethers ("molecular wires") as a function of pH. Between pH 4 and 9, rate constants were ca. 100-fold faster than for the same H2Q functionality confined to the surface via alkane tethers. Also, in this same pH range rate constants were independent of the length of the OPV bridge. These new electroactive molecules in which the hydroquinone functionality is wired to the gold surface by means of OPV tethers should be useful platforms for constructing bioelectronic devices such as biosensors, biofuel cells, and biophotovoltaic cells with a fast response time.

Chemistry, Physical↗
PubMed2000

Synthesis of unnatural amino acids via Suzuki cross-coupling of enantiopure vinyloxazolidine derivatives.

[formula: see text] (R and S)-alpha-Amino alcohols and alpha-amino acids, including 4-methoxyhomophenylalanine, with a variety of unnatural side chains have been synthesized via palladium-catalyzed cross-coupling Suzuki reactions. The key building blocks 1 and 2, synthesized from the common achiral precursor 2-butene-1,4-diol, were made enantiopure utilizing a Pseudomonas cepacia lipase-catalyzed kinetic resolution. The optimal conditions for the Suzuki cross-coupling and the subsequent oxidations of the resultant alpha-amino alcohols are described.

Amino Acids↗
PubMed2000

A regioselective Ru-catalyzed alkene-alkyne coupling.

[reaction: see text] The reaction of silylalkynes and terminal alkenes proceeds with complete control of regioselectivity by the silyl substituent to give geometrically defined vinylsilanes. Since terminal alkynes normally give mixtures, protodesilylation of these adducts then constitutes a regioselective addition of terminal alkynes to terminal alkenes.

Alkenes↗
PubMed2000

Vinylogous Mannich reactions: some theoretical studies on the origins of diastereoselectivity.

[reaction: see text] Ab initio calculations at the RHF/3-21G level were used to investigate the limiting transition states in the addition of 2-methoxyfuran to a pyrrolinium ion. Four stationary points were found on the potential energy surface with relative energies of threo Diels-Alder, 0.0 kcal/mol; erythro open, 0.9 kcal/mol; erythro Diels-Alder, 1.3 kcal/mol; and threo open, 1.8 kcal/mol.

Calorimetry↗
PubMed2001

On the inherent instability of alpha-amino alpha'-fluoro ketones. Evidence for their transformation to reactive oxyvinyliminium ion intermediates.

[figure: see text] alpha-Amino alpha'-fluoro ketones are shown to be inherently unstable intermediates. Evidence is presented that they undergo enolization toward the amino group followed by expulsion of fluoride ion, forming a proposed oxyvinyliminium ion (amino-substituted oxyallyl cation). In protic, nucleophilic media the proposed intermediate is trapped by solvent. In the presence of a reactive diene, [4 + 3] cycloadducts have been isolated. Prior observations concerning fluorinated amino ketones are discussed in light of these findings.

Cations↗
PubMed2001

Catalytic asymmetric vinylation of ketone enolates.

[see reaction]. A protocol for the catalytic asymmetric vinylation of ketone enolates has been developed. Key to the success of this process was the development of new electron-rich chiral monodentate ligands.

Catalysis↗
PubMed2001

An olefin cross-metathesis approach to vinylphosphonate-linked nucleic acids.

[reaction: see text]. The synthesis of vinylphosphonate-linked nucleotide dimers has been achieved using an olefin cross-metathesis (CM) reaction as a key step. The 1,3-dimesityl-4,5-dihydroimidazol-2-ylidine-containing catalyst 5 (Grubbs' second-generation catalyst) was found to be the superior catalyst for this transformation. Both metathesis partners were readily available using known methodology, and the vinylphosphonate-linked dimer was produced with high levels of (E)-selectivity (>20:1) in 58% yield (70% based on recovered starting material).

Nucleic Acids↗
PubMed2001

Solid-phase synthesis of isoxazoles using vinyl ethers as chameleon catches.

[reaction: see text] Regioselective 1,3-dipolar cycloadditions of supported vinyl ethers R(1)C(=CH(2))O-CH(2)-polymer, prepared by the Tebbe olefination of R(1)CO(2)-CH(2)-polymer, with ethyl cyanoformate N-oxide gave supported isoxazoline derivatives. Release from the support under mild acidic conditions gave the isoxazoles ethyl 5-R(1)-isoxazole-3-carboxylates. Alternatively, further on-resin functionalization of the R(1) substituent using Suzuki coupling reactions and release from the support under acidic conditions gave more structurally diverse isoxazoles.

Ethers↗
PubMed2001

Electrophilic cyclizations of vinylcyclopropanols to tethered aldehydes.

[reaction: see text]. The intramolecular, stereoselective addition of 1-vinylcyclopropanols to tethered aldehydes has been achieved under mild conditions. Thus, sequential application of the titanium-mediated cyclopropanation of alpha,beta-unsaturated esters and the electrophilic cyclization of the aldehyde-tethered cyclopropanol products provides the facile formation of carbocyclic rings.

Aldehydes↗
PubMed2001

Palladium-mediated regiodivergent kinetic resolution.

[reaction: see text]. The allylic substitution of racemic 5-vinyloxazolidinones with phthalimide and a chiral palladium catalyst afforded optically enriched regioisomeric products. Optimal divergence was found by employing chiral DIOP ligands in toluene. These results demonstrate the influence of chiral ligand/chiral substrate matching on the regioselectivity in a novel resolution strategy.

Indicators and Reagents↗
PubMed2002

P-chirogenic binaphthyl-substituted monophosphines: synthesis and use in enolate vinylation/arylation reactions.

[reaction: see text] New phosphine ligands possessing both axial chirality and a chirogenic phosphorus center were prepared from (R)-2-bromo-2'-N,N-(dimethylamino)-1,1'-binaphthyl (1) via a simple Li-halogen exchange protocol. The asymmetric vinylation of a ketone enolate with (R,R(P))-2-(tert-butylphenylphosphino)-2'-N,N-(dimethylamino)-1,1'-binaphthyl (2a) afforded the coupling product with good enantiomeric excess.

Models, Molecular↗