Preparation of enantiopure biimidazoline ligands and their use in asymmetric catalysis

A convenient new method for the preparation of 2,2'-biimidazolines is reported. Amino alcohols were reacted with dimethyl oxalate, and the product hydroxy amides converted into chloroamides by reaction with thionyl chloride. Treatment with PCl5, followed by diamines (ethanediamine, propane-1,3-diamine, 2,2-dimethylpropane-1,3-diamine) furnished a series of enantiopure tricyclic biimidazolines. Complexes of two of the ligands with PdCl2 were prepared and their X-ray crystal structures were determined. The biimidazolines were tested as ligands for asymmetric Pd-catalysed allylations. Moderate enantioselectivity (up to 80% ee) was found for the reaction of dimethyl malonate with diphenylallyl acetate, with the 5,7,5 fused tricyclic systems outperforming the 5,6,5 analogues. The corresponding reaction of pentenyl acetate gave lower enantioselectivity (44-57% ee), and proved very sensitive to the donor strength of the ligands, the stronger donors giving lower yields. The results provide a further demonstration of the value of the 'tunability' of imidazoline ligands.     

Synthesis and application in asymmetric catalysis of camphor-based pyridine ligands

This tutorial review deals with the synthesis and application in asymmetric catalysis of camphor-based pyridine ligands. These ligands can be roughly divided into two groups: those in which the camphor is annulated in the 2,3-positions to the beta-face of the pyridine ring and those in which the pyridine is contained as a pendant on the C2 or C3 of the camphor framework. Camphor-based pyridine ligands can also contain other donor centers located on the pyridine ring or camphor skeleton. Some of these ligands have provided interesting enantioselectivities in several asymmetric reactions, such as S(N)2' reactions, allylic oxidations, carbonyl additions with organozinc reagents and hydrogenations. This review contains a lot of chemistry on ligand synthesis and readers will find it of value and also perhaps an inspiration for the development of more active and improved versions.     

Synthesis of novel planar-chiral [2.2]paracyclophane derivatives as potential ligands for asymmetric catalysis

The synthesis of a variety of new 4,5-disubstituted [2.2]paracyclophane derivatives has been achieved employing different cross-coupling reactions. By this methodology, a heteroatom-variation of successful catalyst ligands was achieved, giving rise to a modular ligand system. The X-ray structure of 4-hydroxy-5-(1′-hydroxy-1′-phenylethyl)-[2.2]paracyclophane was determined to elucidate the configuration. Additionally, a diastereoselective synthesis of planar- and central-chiral 4-([2.2]paracyclophanyl)ethylamine was achieved, thus resulting in a planar- and central-chiral phenyl ethylamine analogue.     

Synthesis of enantiopure β2-homoalanine derivatives via rhodium catalyzed asymmetric hydrogenation

The stereoselective synthesis of chiral β²-homoalanine derivatives with 99% ee by the Rh-catalyzed enantioselective hydrogenation of prochiral 2-aminomethyl acrylates is described. The subsequent transformation to chiral 3-amino-2-methylpropanols is also demonstrated.     

A novel indium-catalysed synthesis of tetra-substituted pyridine derivatives

Indium trichloride was found to be an effective catalyst for the synthesis of various tetra-substituted pyridine derivatives via Michael addition of β-dicarbonyl compounds to α,β-unsaturated oximes and subsequent ring closure.     

MOP-phosphonites: a novel ligand class for asymmetric catalysis

Chiral phosphonite ligands (S,R(b))-5a, (S,S(b))-5b, (R,R(b))-6a and (R,S(b))-6b are introduced, comprising a MOP-type backbone with a binol-based binaphthyl group bound to the phosphorus. Their reaction with [Pd(η(3)-C(4)H(7))Cl](2) affords η(3)-methallylpalladium chloride complexes 7a/b and 8a/b which have been isolated and structurally characterised. Solid-state and solution studies indicate subtle differences in their coordination behaviour, which ultimately affects their efficacy in the asymmetric hydrosilylation of styrene.     

Phosphinoferrocenylaminophosphines as novel and practical ligands for asymmetric catalysis

[structure: see text] A new series of ligands with a novel phosphine-aminophosphine ligation design as depicted in structure 1 has been prepared on a ferrocenylethyl backbone. These BoPhoz ligands of structure 2 have afforded exceedingly high activity and enantioselectivity in the rhodium-catalyzed asymmetric hydrogenation of dehydro-alpha-amino acid derivatives, itaconic acids, and alpha-ketoesters. These air-stable ligands are readily prepared from cost-effective and non-pyrophoric intermediates.     

Synthesis of novel cinchona-amino acid hybrid organocatalysts for asymmetric catalysis

Three novel subclasses of cinchonidine derivatives coupled to diverse amino acids were prepared in very good overall yield and tested in a benchmark organocatalytic aldol reaction, between acetone and aromatic aldehydes. These subclasses are a family of amino acid-cinchonidine (subclass A), N-formamides-cinchonidine (subclass B) and dipeptide-cinchonidine (subclass C) hybrids. Our main goal, besides obtaining very good yields and enantioselectivities, was to understand the influence of the amino acid side chain residues on the enantioselectivity of the asymmetric aldol reactions. Different amino acid tethered cinchonidine hybrids were compared and their catalytic behaviour was evaluated, allowing good enantioselectivities to be achieved, 92% ee in one case. Other reactions such as Biginelli, Michael addition and ketimine hydrosilylation reactions were screened with these ligands, but the outcome was less successful.     

Synthesis of chiral cyclohexane-backbone P,N-ligands derived from pyridine and their applications in asymmetric catalysis

P,N-ligands trans-4 and cis-5 with a cyclohexane backbone were easily synthesized. The key step was chiral resolution of (±)-trans-2-pyridylcyclohexanols with DBTA. Enantiopure trans isomer was subjected to Mitsunobu reaction and deprotection to give the corresponding cis isomer. These ligands have been successfully used in asymmetric hydrogenation of arylalkenes with up to 93% ee using 5 and 90% ee using 4 and asymmetric allylic alkylations with up to 95% ee using 5 and 93% ee using 4. Trans and cis P,N-ligands 4 and 5 all gave the product with the same configuration. It was suggested that the absolute configuration of the product was controlled by the configuration of the stereogenic pyridyl-bearing carbon of the ligands.     

Free-energy correlation for catalysis of outer sphere electron-transfer reactions by noncoordinated pyridine derivatives

Recent experimental data concerning the rate constants and their free energy of the outer sphere electron-transfer reactions, as catalyzed by noncoordinated pyridine derivatives in aqueous solution, are examined for possible correlation. For the electron transfer reactions between V or Eu and the bipyridyl derivatives, such as N,N'-dimethyl-4,4′-bipyridyl (paraquat) or diquat, the data are correlated quite well by the Marcus equation. The electron exchange rate constant, 5 × 10⁷M^?1·s^?1, for an organic radical ion and its parent molecule obtained from semiquinones or their related compounds can be applied to these pyridine derivatives. However, in some cases such as electron transfer from paraquat or diquat cation radical to Co(en), positive departures from the Marcus model are observed. These positive departures are interpreted in terms of interaction between the molecular orbitals of electron donors and electron acceptors in the transition state.     

Synthesis and liquid crystalline properties of novel pyridine derivatives

The treatment of 4‐hydroxypyridine with cholestery p‐(ω‐bromoalkyloxy)benzoates in N,N‐dimethylformamide containing K₂CO₃ gave cholestery p‐[ω‐(4‐pyridyloxy)alkyloxy]benzoates, which exhibited liquid crystalline properties     

Synthesis of novel chiral phosphinocyrhetrenyloxazoline ligands and their application in asymmetric catalysis

Several novel planar chiral phosphinocyrhetrenyloxazolines have been synthesized, and their catalytic activities have been evaluated in a variety of asymmetric catalytic reactions. Preferable effects as compared to their ferrocenyl analogues have been observed in asymmetric allylic amination and asymmetric hydrosilylation, and up to 97% ee and 72% ee were reached, respectively. The Lewis basicity of the phosphorus on the ferrocene and the cyrhetrene, which contributes to their different behavior in catalysis, has been deduced by 31P NMR spectroscopy analysis, as indicated by 1J(77Se-31P) in the corresponding phosphine selenides.     

Biomimetic asymmetric catalysis

Enzymes are the core for biological transformations in nature. Their structures and functions have drawn enormous attention from biologists as well as chemists since last century. The large demand of bioactive molecules and the pursuit of efficiency and greenness of synthesis have spurred the rapid development of biomimetic chemistry in the past several decades. Biomimetic asymmetric catalysis, mimicking the structures and functions of enzymes, has been recognized as one of the most promising sy...     

Novel nitrogen ligands based on imidazole derivatives and their application in asymmetric catalysis

Recently prepared chiral amines have been used in the preparation of novel tridentate ligands based on an imidazole ring with an additional (hetero)ring. The synthesis was carried out by the reaction of chiral amines with suitable aldehydes (2-phenylimidazole-4-carbaldehyde, 2-hydroxybenzaldehyde or pyridine-2-carbaldehyde) under reductive conditions (H₂/Pd or NaBH₄). All ligands prepared showed strong hydrogen bonds in d₆-DMSO solution, which resulted in hindered imidazole tautomerism. The observed hindered tautomerism was studied by ¹H NMR spectroscopy. The structures of the prepared ligands were also confirmed by APCI mass spectroscopy. Both chiral amines and tridentate compounds have been applied as ligands in copper (II)-catalyzed nitroaldol reactions (Henry reaction). Various reaction conditions for the Henry reaction have been studied (influence of temperature, molar ratio, solvent or copper (II) precursors). The compounds prepared with the two imidazole rings showed fast reaction times and a reversal in enantioselectivity compared to other chiral amines.     

Application of novel phosphine oxazoline ligands in asymmetric allylations of 4-acyloxy-2-pentene derivatives

A library of novel phosphine oxazoline ligands 2 was tested in the asymmetric allylation of 4-acyloxy-2-pentenes 1. High throughput screening techniques were employed to accelerate this process. The data accumulated allowed the extent of asymmetric induction to be correlated with the ligand substituent R and ligand-to-metal ratios.     

Resolution of racemic 2-aminocyclohexanol derivatives and their application as ligands in asymmetric catalysis

A preparatively easy and efficient protocol for the resolution of racemic 2-aminocyclohexanol derivatives is described, delivering both enantiomers with >99% enantiomeric excess (ee) by sequential use of (R)- and (S)-mandelic acid. A simple aqueous workup procedure permits the isolation of the amino alcohols in analytically pure form and the almost quantitative recovery of mandelic acid. Debenzylation of enantiopure trans-2-(N-benzyl)amino-1-cyclohexanol by hydrogenation and subsequent derivatization give access to a broad variety of diversely substituted derivatives. Furthermore, the corresponding cis isomers are readily available. Applications of these optically active aminocyclohexanols in catalyzed asymmetric phenyl transfer reactions to benzaldehydes and transfer hydrogenations of aryl ketones lead to products with up to 96% ee.     

Dendrimeric phosphines in asymmetric catalysis

Dendrimers are hyperbranched nanosized and precisely defined molecules, attracting increasing attention each year due to their numerous properties in catalysis, materials science, and biology. This tutorial review concerns the use of dendrimers as catalysts and focuses more precisely on their properties as enantioselective catalysts. Emphasis is put on chiral phosphine complexes constituting the core or the end groups of various types of dendrimers. The effect of the location of the catalytic entities, the effect of the size (generation) and the nature of the dendritic skeleton on the enantiomeric excesses are discussed.     

双中心协同催化作用在不对称合成中的应用

介绍了双中心协同催化作用的概念以及A型和B型两类催化剂。综述了A型催化 剂在不对称硝基－类羟醛反应、迈克尔加成反应、亚胺和醛的氢膦酰化反应、曼尼 希反应、不对称醛酮羟醛反应以及环氧开环反应中的应用；同时简要介绍了B型催 化剂在不对称合成中的应用。