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Ben L Feringa

Publications and source records attributed to Ben L Feringa.

At least 19 recordsLinked to original sources
PubMed2006

Copper catalyzed asymmetric synthesis of chiral allylic esters.

The complex derived from Taniaphos ligand 4 and CuBr*Me2S catalyzes the asymmetric addition of Grignard reagents to 3-bromopropenyl esters 1 to provide allylic esters 2 in high yields and high chemio-, regio-, and enantioselectivities. The work demonstrates that allylic asymmetric alkylation (AAA) can be done on substrates bearing a heteroatom at the gamma-position. The method is a practical route to chiral, nonracemic allylic alcohols. The use of functionalized substrates 1 or Grignard reagents leads to more complex products 2, which can be further manipulated as demonstrated in conversion to (S)-5-ethyl-2(5H)-furanone 6 and (S)-benzoic acid-cyclopent-2-enyl ester 7.

Journal Article↗
PubMed2006

Synthesis of enantiopure chloroalcohols by enzymatic kinetic resolution.

3-Alkenyl and heteroaryl chloroalcohols have been obtained in excellent enantiomeric excess (>99%) by enzymatic kinetic resolution using the haloalcohol dehalogenase HheC. Yields were close to the theoretical maximum for all substrates employed. Furthermore, the applicability of this methodology on multigram scale has been established.

Alcohols↗
PubMed2006

Enantioselective synthesis of beta2-amino acids using rhodium-catalyzed hydrogenation.

A series of protected beta2-dehydroamino acids has been prepared in three steps from commercially available starting materials in good yields. These were used as substrates in rhodium-catalyzed asymmetric hydrogenation applying a mixed ligand system of monodentate phosphoramidites and phosphines. Optimization of the catalyst structure was achieved by high throughput experimentation. High enantioselectivities were obtained (up to 91%) with full conversion for a number of beta-amino acids.

Alcohols↗
PubMed2006

Self-organized monolayer of meso-tetradodecylporphyrin coordinated to Au(111).

The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au(111), H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-C=N-) nitrogen atoms of the porphyrin macrocycle to Au(111).

Gold↗
PubMed2006

Catalytic asymmetric synthesis of acyclic arrays by tandem 1,4-addition-aldol reactions.

Herein, we report efficient acyclic stereocontrol in tandem 1,4-addition-aldol reactions triggered by catalytic asymmetric organometallic addition. Grignard reagents add to alpha,beta-unsaturated thioesters in a 1,4-fashion and the resulting magnesium enolates are trapped with aromatic or aliphatic aldehydes. The process provides a range of tandem products bearing three contiguous stereocenters with excellent control of relative and absolute stereochemistry. The various diastereomeric products have been fully characterized using single-crystal X-ray analysis and the origins of stereocontrol in this tandem protocol are discussed. The versatility and efficiency of this methodology are demonstrated in the first catalytic asymmetric synthesis of (-)-phaseolinic acid with 54% overall yield via a short and concise route.

Journal Article↗
PubMed2006

Synthesis, stereochemistry, and photochemical and thermal behaviour of bis-tert-butyl substituted overcrowded alkenes.

In order to study the structural limits in the design of molecular motors, a tert-butyl substituted analogue was prepared. The unexpected photochemical and thermal isomerisation processes and the stereochemistry of new overcrowded alkene are described. The bis tert-butyl substituted alkenes were synthesised in a five-step sequence with an overall yield of 7.5%. Structural assignments of the isomers based on experimental data were supported by calculations of all four isomers of the alkene. X-Ray crystal analysis showed a strongly twisted alkene (torsion angle 39 degrees ) for a less stable photochemically generated cis-isomer.

Journal Article↗
PubMed2006

Rotational reorganization of doped cholesteric liquid crystalline films.

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.

Crystallization↗
PubMed2006

Reversible three-state switching of luminescence: a new twist to electro- and photochromic behavior.

The fully reversible three-state blue/red/off emission from photo-/electrochromic substituted bis-thiaxanthylidenes is reported. The blue luminescence of the most stable (anti-folded) conformer of dimethyl- and dimethoxy-bis-thiaxanthylidene can be switched off by photochemical conversion to the meta-stable (syn-folded) conformer and switched on again by thermal reversion to the anti-folded state. The red luminescence of the bis-thiaxanthylium dication can be switched on by oxidation at approximately 1.0 and 1.2 V vs SCE of the syn- and anti-folded conformers respectively and switched off or to blue by reduction at approximately 0.35 V vs SCE.

Journal Article↗
PubMed2006

Amplification of chirality in liquid crystals.

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.

Journal Article↗
PubMed2006

On the mechanism of the copper-catalyzed enantioselective 1,4-addition of grignard reagents to alpha,beta-unsaturated carbonyl compounds.

The mechanism of the enantioselective 1,4-addition of Grignard reagents to alpha,beta-unsaturated carbonyl compounds promoted by copper complexes of chiral ferrocenyl diphosphines is explored through kinetic, spectroscopic, and electrochemical analysis. On the basis of these studies, a structure of the active catalyst is proposed. The roles of the solvent, copper halide, and the Grignard reagent have been examined. Kinetic studies support a reductive elimination as the rate-limiting step in which the chiral catalyst, the substrate, and the Grignard reagent are involved. The thermodynamic activation parameters were determined from the temperature dependence of the reaction rate. The putative active species and the catalytic cycle of the reaction are discussed.

Journal Article↗
PubMed2006

Rhodium-catalyzed addition of arylboronic acids to isatins: an entry to diversity in 3-aryl-3-hydroxyoxindoles.

[reaction: see text] A general method for the catalytic 1,2-addition of aryl and alkenyl boronic acids to isatins is described using a rhodium(I)/triphenylphosphite catalyst. The application of this transformation allows the synthesis of a variety of 3-aryl-3-hydroxyoxindole building blocks in high yields. An enantioselective version of this reaction using a rhodium(I)/phosphoramidite system is also presented.

Boronic Acids↗
PubMed2006

Raman scattering and FT-IR spectroscopic studies on dithienylethene switches--towards non-destructive optical readout.

The non-destructive readout of photochromic memory materials based on the dithienylethene unit both by IR spectroscopy and Raman scattering is explored. A representative series of C5-substituted thienyl hexahydro- and hexafluoro-cyclopentene based photochromes was investigated to explore the effect and potential usefulness of substitution for the development of multicomponent memory materials. The effect of the deposition method on the photochemistry of solid materials containing photochromic dithienylcyclopentene switches was also explored. Photoconversion in the solid state to the closed form was found to be low when starting from the open form, but, in contrast, ring opening to the open state from the closed form was found to be complete. The effect was found to be due to inner filter rather than conformational phenomena. Characteristic vibrational bands for the central dithienyl core are assigned and a comparison made of the vibrational spectroscopic properties of the perhydro- and perfluoro switches. The data enable the determination of the photoconversion achievable in the solid state as well as some assessment of the influence of the deposition method on the photoconversion. The potential of Raman spectroscopy as a method of achieving non-destructive optical readout is demonstrated through the large differences in absolute Raman scattering intensity between the open and closed states, when monitored at wavelengths which do not result in photochemical ring opening.

Journal Article↗