Bis-oxazoline ligands

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

Recently Desimoni et used the same bis(oxazoline) ligand in the magnesium(II) catalysed Diels-Alder reaction of the N-acyloxazolidinone depicted in Scheme 3.4. In dichloromethane a modest preference was observed for the formation of the S-enantiomer. Interestingly, upon addition of two equivalents of water, the R-enantiomer was obtained in excess. This remarkable observation was interpreted in terms of a change from tetrahedral to octahedral coordination upon the introduction of the strongly coordinating water molecules. 

Since Evans s initial report, several chiral Lewis acids with copper as the central metal have been reported. Davies et al. and Ghosh et al. independently developed a bis(oxazoline) ligand prepared from aminoindanol, and applied the copper complex of this ligand to the asymmetric Diels-Alder reaction. Davies varied the link between the two oxazolines and found that cyclopropyl is the best connector (see catalyst 26), giving the cycloadduct of acryloyloxazolidinone and cyclopentadiene in high optical purity (98.4% ee) [35] (Scheme 1.45). Ghosh et al., on the other hand, obtained the same cycloadduct in 99% ee by the use of unsubstituted ligand (see catalyst 27) [36] (Scheme 1.46, Table 1.19). 

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Numerous articles have been devoted to the synthesis of structurally modified bis(oxazoline) ligands and to their ability to promote enantios-elective Diels-Alder transformations. For example, Davies et al. [74] synthesized and tested several Evans-type auxiliaries, i.e., bis(oxazolines) or pyridine-bis(oxazolines), bearing various sterically-hindering substituents. The best results were obtained according to the conditions presented in Scheme 26, and afforded the endo diastereomer with 95% ee by using ligand 58 (Scheme 28). 

Non-covalently Immobilized Catalysts Based on Chiral Bis(oxazoline) Ligands... 

Only one example of the functionalization of a bis(oxazoline) ligand with fluorinated chains has been described to date [44], but the use of these functionalized hgands does not meet our description of a supported catalyst... 

Apart from the cyclopropanation reaction, only one example has been published of the application of ionic liquids as reaction media for enantio-selective catalysis with bis(oxazoline) ligands. In this case, the complex 6b-ZnCl2 was used as a catalyst for the Diels-Alder reaction between cyclopen-tadiene and N-crotonyloxazolidin-2-one in dibutyUmidazoUiun tetrafluorob-orate (Scheme 9) [48]. Compared with the same process in CH2CI2, the reaction was faster and both the endofexo selectivity and the enantioselectivity in the endo product were excellent. However, experiments aimed at recovering the catalysts were not carried out. 

Metal complexes of chiral bis(oxazoline) ligands, in most cases Cu(II) complexes, have been supported by cationic exchange on inorganic, organic, and composite anionic solids. 

Fig. 22 Chiral bis(oxazoline) ligands used for immobilization in zeolites...

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

Scheme 1.54 Test reaction with dibenzothiophene-bis(oxazolines) ligands.

A new application of bis(oxazolines) ligands was reported by Reiser et al. who obtained some excellent results, such as that depicted in Scheme 2.14 for the 1,4-addition of ZnEt2 to 2-cyclohexenone. These authors used a bimetallic complex in which the substrate was locked in a two-point binding mode via a zinc atom and a copper atom. 

Scheme 2.14 Cu-catalysed 1,4-addition of ZnEt2 to 2-cyclohexenone with thioether-bis(oxazolines) ligand.

A number of nonsulfur-containing bis(oxazolines) ligands with numerous structural diversities have been designed by several groups and successfully... 

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