Oxazoline ligands, enantiopure

Another type of reaction that has been developed in order to resolve nitrogen-containing compounds is the Diels-Alder cycloaddition. As a rare example, Sibi et al. [235] have recently reported the KR of pyrazohdinones that could not be obtained in high enantioselectivities from direct methods on the basis of a Diels-Alder cycloaddition with cyclopentadiene catalysed by a combination of Cu(OTf)2 with a chiral aminoindanol-derived bisoxazohne hgand. The recovered almost enantiopure (98% ee) substrate was isolated with a selectivity factor of 34. Excellent results were also reported by Fu et al. [236] for the KR of azomethine imines via copper-catalysed [3-1-2] cycloadditions with aUcynes performed in the presence of a chiral phosphaferrocene-oxazoline ligand, which furnished the chiral recovered substrates with high selectivity factors of up to 76, as shown in Scheme 3.15. 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

Several new ligands containing the oxazoline nucleus were synthesized in enantiopure form. Compounds of general structure 165 were obtained from L-serine or L-threonine and found application as catalysts for the zinc addition to aldehydes <03TA3292> or were derived from P-amino alcohols and used in diethylzinc addition to A -(diphenylphosphinoyl) imines <03JOC4322>. Also, compound 166 was derived from a commercially available amino acid and afforded good selectivity in allylic alkylation <03TL6469>. 

The ligands, which are enantiopure oxazolines, have an asymmetric carbon situated close to the metal centre. The presence of the substituents around the metal leads to steric hindrance, which even in the case of the dinuclear palladium derivatives prevents the... 

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