Search results for "405"
showing 10 items of 3208 documents
Recent developments and applications of the chiral Brønsted acid catalyzed allylboration of carbonyl compounds
2018
The 50-year-old allylboration reaction has seen dramatic developments since the dawn of the new century after the first catalytic asymmetric versions came into play. In the past decade alone, several methodologies capable of achieving the desired homoallylic alcohols in over 90% ee have been developed. This review focuses on the chiral Brønsted acid catalyzed allylboration reaction, covering everything from the very first examples and precedents to modern day variations and applications.1 Introduction2 Early Developments3 Synthetic Applications4 Variants5 Computational Contribution6 Conclusions
Allocation of Ambipolar Charges on an Organic Diradical with a Vinylene-Phenylenediyne Bridge.
2021
Two redox and magnetically active perchlorotriphenylmethyl (• PTM) radical units have been connected as end-capping groups to a bis(phenylene)diyne chain through vinylene linkers. Negative and positive charged species have been generated, and the influence of the bridge on their stabilization is discussed. Partial reduction of the electron-withdrawing • PTM radicals results in a class-II mixed-valence system with the negative charge located on the terminal PTM units, proving the efficiency of the conjugated chain for the electron transport between the two terminal sites. Counterintuitively, the oxidation process does not occur along the electron-rich bridge but on the vinylene units. The • …
Gene delivery agents possessing antiradical activity: self-assembling cationic amphiphilic 1,4-dihydropyridine derivatives
2013
Seventeen 1,4-dihydropyridine (1,4-DHP) amphiphiles including differently substituted pyridinium, pyrazinium, N-methyl piperidinium or N-methyl morpholinium moieties as the cationic head-group of the molecule have been designed and synthesised. 1,4-DHP amphiphiles have been earlier proposed as a promising tool for plasmid DNA (pDNA) delivery in vitro. In this work the ability of the 1,4-DHP amphiphiles to self-assemble, to bind pDNA and to transfer it into the cells as well as the cytotoxicity of 1,4-DHP amphiphiles–pDNA complexes was studied. Furthermore, antiradical activity (ARA) of the 1,4-DHP derivatives was determined. We have revealed that all new 1,4-DHP amphiphiles possessed self-a…
A Tunable, Fullerene‐Based Molecular Amplifier for Vibrational Circular Dichroism
2019
Abstract Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C60 moiety is used as an electron acceptor and local VCD amplifier for an alanine‐based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low‐lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low‐lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the…
Efficient Conversion of Light to Chemical Energy : Directional, Chiral Photoswitches with Very High Quantum Yields
2020
Abstract Photochromic systems have been used to achieve a number of engineering functions such as light energy conversion, molecular motors, pumps, actuators, and sensors. Key to practical applications is a high efficiency in the conversion of light to chemical energy, a rigid structure for the transmission of force to the environment, and directed motion during isomerization. We present a novel type of photochromic system (diindane diazocines) that converts visible light with an efficiency of 18 % to chemical energy. Quantum yields are exceptionally high with >70 % for the cis–trans isomerization and 90 % for the back‐reaction and thus higher than the biochemical system rhodopsin (64 %). T…
Through space singlet-singlet and triplet-triplet energy transfers in cofacial bisporphyrins held by the carbazoyl spacer
2007
The through space singlet-singlet and triplet-triplet energy transfers in cofacial bis(etio-porphyrins) rigidly held by the carbazoyl spacer were investigated. The studies on singlet-singlet transfer, which operates via a Förster mechanism, were performed using the zinc porphyrin and free base chromophores as energy donor and acceptor, respectively, while the investigation on triplet-triplet processes was performed using the palladium porphyrin, and the zinc porphyrin and free base chromophores as donor and acceptors, respectively. The rate for singlet-singlet transfer ( k ET ( singlet )) is unexpectedly slower than that reported for other similar, rigidly held bisporphyrins such as H 2( D…
Candida antarctica Lipase A-Based Enantiorecognition of a Highly Strained 4-Dibenzocyclooctynol (DIBO) Used for PET Imaging
2020
The enantiomers of aromatic 4-dibenzocyclooctynol (DIBO), used for radiolabeling and subsequent conjugation of biomolecules to form radioligands for positron emission tomography (PET), were separated by kinetic resolution using lipase A from Candida antarctica (CAL-A). In optimized conditions, (R)-DIBO [(R)-1, ee 95%] and its acetylated (S)-ester [(S)-2, ee 96%] were isolated. In silico docking results explained the ability of CAL-A to differentiate the enantiomers of DIBO and to accommodate various acyl donors. Anhydrous MgCl2 was used for binding water from the reaction medium and, thus, for obtaining higher conversion by preventing hydrolysis of the product (S)-2 into the starting materi…
Microwave-Assisted Esterification of Tall Oil Fatty Acids with Methanol Using Lignin-Based Solid Catalyst
2016
During alkaline pulping significant amounts of lignin, carbohydrates (mostly hemicelluloses), and extractives (tall oil soap and turpentine) are removed from wood feedstock. In this study, the catalytic esterification of fatty acids in tall oil with methanol to produce fatty acid methyl esters under microwave irradiation was performed at 100 °C for 10–60 min. A novel heterogeneous acid catalyst tested for this purpose was synthesized from the hardwood alkali lignin that was precipitated by acidification from the black liquor from soda-AQ pulping. The comparative reaction data were obtained by using other solid catalysts, Amberlyst 15, and p-toluenesulfonic acid. The results showed the highe…
Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Function…
2017
International audience; Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPL(OR) s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand _ONOO(R'2) ₍2-) , which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (Ps up to 0.91 with R'=cumyl) to very high isoselectivity (Pi…
Oxidovanadium(v) complexes with l-proline-based amino acid phenolates
2019
Abstract l -proline was used to prepare chiral, tridentate amino acid phenol proligands H2L1–4. These proligands react with vanadium precursors VO(acac)2, VOSO4∙5H2O and VO(OPr)3 in methanol to form the corresponding oxidoalkoxidovanadium( v ) complexes 1–4. The complexes crystallize from methanol, and are octahedrally coordinated with a general formula [VO(L1–4)(OMe)(MeOH)]. In solution, however, they adopt several different conformations or isomeric structures depending on the solvent.