Improved Synthesis of Aza-bis(oxazoline) Ligands

Synthesis of Chiral Allylic Esters by Using the New Recyclable Chiral Heterogeneous Oxazoline-Based Catalysts

A new class of recyclable supported chiral heterogeneous ligands has been synthesized by the reaction of functionalized mesoporous SBA-15 with aliphatic- and aromatic-substituted chiral amino oxazoline ligands. The obtained chiral heterogeneous ligands were characterized by several techniques such as Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and BET-BJH. The application of these new heterogeneous ligands in copper-catalyzed asymmetric allylic oxidation of olefins by using perester showed better yields up to 95% and better enantioselectivities up to 96% compared to the corresponding homogeneous catalysts. These findings can be considered as an important step in the advancement of green chemistry. Investigation of the recyclability of the catalysts confirmed that they were easily recovered and reused eight times without significant losses in reactivity, yield, and enantioselectivity.

Bis-(benzothiazol-2-yl)-amines and their metal amides: a structural comparison in the solid state

A series of functionalised bis-(benzothiazol-2-yl)-amine ligands and the related dimethylaluminium amides were synthesised and characterised. The solid state structural comparison reveals various coordination motifs, specific folding parameters and hydrogen bonding patterns in detail and guide the way to future Janus head ligand design.

Application of metal-based reagents and catalysts in microstructured flow devices

Over the years, organic synthesis has witnessed several improvements through the development of new chemical transformations or more efficient reagents for known processes. Likewise, technological advances, aiming at speeding up reactions and facilitating their work-up, have established themselves in academic as well as in industrial laboratories. In this Minireview, we highlight very recent developments in flow chemistry, focusing on organometallic reagents and catalysts. First, we describe reactions with homogeneous catalysts immobilized on different support materials using the concept of packed bed reactors. In the last chapter, we will discuss applications that utilize organometallic reagents.

C1-Symmetric VersusC2-Symmetric Ligands in Enantioselective Copper–Bis(oxazoline)-Catalyzed Cyclopropanation Reactions

A thorough experimental and theoretical study of the enantioselective cyclopropanation of alkenes catalyzed by chiral bis(oxazoline)- and azabis(oxazoline)-copper complexes, which comprise a new family of ligands that lack C2 symmetry, has been conducted. Surprisingly high enantioselectivities were observed with some of these ligands, which were rationalized on the basis of molecular modeling studies. The course of the asymmetric induction in connection with ligand symmetry and the implications for supported enantioselective catalysts are discussed.

Synthesis and Application of Phosphorus Dendrimer Immobilized Azabis(oxazolines)

Phosphorus dendrimer immobilized azabis(oxazoline) ligands can be efficiently synthesized up to the third generation with 48 ligand molecules being attached to the periphery using click chemistry. The so-assembled macromolecules were evaluated in copper(II)-catalyzed asymmetric benzoylations, showing good yields and enantioselectivities. Moreover, the copper(II)-catalysts could be readily recovered and reused in several cycles. The globular structure of the dendritic ligands seems to prevent interference of the triazole moieties in the catalysis, contrasting MeOPEG or polystyrene bound ligands of the same type.

Simple and Efficient Heterogeneous Copper Catalysts for Enantioselective C−H Carbene Insertion

Immobilized box-Cu complexes are able to efficiently catalyze the insertion of a carbene, from methyl phenyldiazoacetate, into C-H bonds of THF, with high enantioselectivity (up to 88% ee). The immobilization not only allows recovery and reuse of the enantioselective catalyst but also provides an improvement in the selectivities from the values obtained in solution, probably due to a confinement effect of the bidimensional support. [reaction: see text].

Highly Enantioselective Michael Additions of Indole to Benzylidene Malonate Using Simple Bis(oxazoline) Ligands: Importance of Metal/Ligand Ratio

[Structure: see text] Simple bis(oxazoline) ligands, especially azabis(oxazolines), can catalyze the copper-catalyzed addition of indoles to benzylidene malonates in up to >99% ee, provided that excess of chiral ligand is avoided. The paradigm followed in many asymmetric catalyses that an excess of chiral ligand with respect to the metal should improve enantioselectivity because a background reaction by free metal is suppressed, is not applicable here, which might call for revisiting some of the many copper(II)-bis(oxazoline)-catalyzed processes known.

Cu(II)−Aza(bisoxazoline)-Catalyzed Asymmetric Benzoylations

[reaction: see text] Racemic 1,2-diols and alpha-hydroxy carbonyl compounds can be asymmetrically benzoylated in a kinetic resolution in the presence of various Cu(II)-aza(bisoxazoline) catalysts. A novel bisbenzyl-substituted aza(bisoxazoline) ligand proved to be especially effective when immobilized on MeOPEG(5000), giving from 91 to > or = 99% ee in 37-49% yield for each of five sequential reactions.