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Ben L Feringa

Publications and source records attributed to Ben L Feringa.

At least 73 records · Page 4Linked to original sources
PubMed2005

Cyclohexane bis-urea compounds for the gelation of water and aqueous solutions.

A new class of efficient hydrogelators has been developed by a simple modification of the peripheral substituents of cyclohexane bis-urea organogelators with hydrophilic hydroxy or amino functionalities. These bis-urea hydrogelators were synthesised in two or three steps using an alternative procedure to the common isocyanate method. Gelation was obtained with organic solvents, water and strongly basic aqueous solutions like 25% ammonia. Hydrogelation was found to depend on a delicate balance between the hydrophobicity of the alkyl chains, hydrophilicity of the terminal substituents and the enantiomeric purity of the compound. The hydrogels consisted of a network of fibers, in which all urea groups are involved in intermolecular hydrogen bonding. Most likely, gelation is driven by hydrophobic interactions of the methylene units, whereas hydrogen bond formation between the urea groups provides the necessary anisotropy of the aggregation and the high thermal stability of the gels.

Cyclohexanes↗
PubMed2005

PipPhos and MorfPhos: privileged monodentate phosphoramidite ligands for rhodium-catalyzed asymmetric hydrogenation.

A library of 20 monodentate phosphoramidite ligands has been prepared and applied in rhodium-catalyzed asymmetric hydrogenation. This resulted in the identification of two ligands, PipPhos and MorfPhos, that afford excellent and in several cases unprecedented enantioselectivities in the hydrogenation of N-acyldehydroamino acid esters, dimethyl itaconate, acyclic N-acylenamides, and cyclic N-acylenamides. In addition, a method for the parallel enantioselectivity determination of eight acylated amines is presented.

Journal Article↗
PubMed2004

Highly enantioselective conjugate additions of potassium organotrifluoroborates to enones by use of monodentate phosphoramidite ligands.

The use of phosphoramidite ligands in the rhodium-catalyzed asymmetric conjugate addition of potassium organotrifluoroborates to various enones in the absence of water is described. A systematic search for effective catalysts has been performed by use of high-throughput screening methods. Initially, we have screened reaction conditions, catalyst precursors, and focused ligand libraries. In the next stage we have used the monodentate ligand combination approach, and finally we have made a library of 96 different phosphoramidites by parallel synthesis in the robot (instant ligand libraries) and have tested these in the vinylation of cyclohexenone (up to 88% enantiomeric excess, ee) and 4-phenyl-3-buten-2-one (up to 42% ee). Arylation of cyclohexenone by use of potassium phenyltrifluoroborate gave 3-phenylcyclohexanone with 99% ee.

Journal Article↗
PubMed2004

Entrapment and release of quinoline derivatives using a hydrogel of a low molecular weight gelator.

Gels of low molecular weight gelators (LMWGs) are self-assembled, thermoreversible, viscoelastic materials which can also be rendered sensitive to light, pH or chemical substances by simple chemical modifications. In addition, the ability of some of these LMWGs to gelate water (hydrogelators) makes these gels interesting, new materials for drug delivery applications. In this paper, for the first time, a release study from LMWG gels is presented. This study concerns the release of small (model) drug molecules: 8-aminoquinoline (AQ) and 2-hydroxyquinoline (HQ), from gels of N,N'-dibenzoyl-L-cystine (DBC). DBC forms stable, clear gels in water, 150 mM NaCl solution and PBS (phosphate-buffered saline at pH 7.4) with gel-to-sol transition temperatures (Tgs) of 40-120 degrees C, depending on the gelator concentration (from 2.23 to 22.3 mM). The release of HQ from DBC gels was found to be approximately seven times faster than that of AQ and the initial release of the latter follows the kinetics of gel degradation. These observations indicate that AQ is preferentially retained in the gel, presumably as a result of stronger interactions with the gelator molecules (i.e. DBC-COO-+ H3N-AQ). These results indicate the potential of LMWG gels as delivery vehicles for small drug molecules and also show that the release profiles for such systems can be fine-tuned by the correct choice of gelator-drug combination.

Aminoquinolines↗
PubMed2004

Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation.

Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.

Alkenes↗
PubMed2004

Copper-catalyzed asymmetric conjugate addition of Grignard reagents to cyclic enones.

It is no longer necessary to use dialkylzinc reagents to obtain enantioselectivities >95% in the copper-catalyzed asymmetric conjugate addition of organometallic compounds to cyclic enones. We now report how this can be accomplished by using inexpensive and readily available Grignard reagents. Screening of bidentate ligands provided outstanding results with copper complexes of commercially available chiral ferrocenyl-based diphosphines, in particular TaniaPhos and JosiPhos derivatives. These catalysts tolerate a range of Grignard reagents and different cyclic enones as substrates, leading to high regioselectivities and unprecedented enantioselectivities. Moreover, the reactions are successful with moderate catalyst loading (5 mol %) under mild conditions and in the absence of additives.

Journal Article↗