Ligands phosphinoaryl oxazoline

The highest enantiomeric excess in the palladium(0)-assisted enantiosclcctive alkylation of racemic 3-cycloalkenyl acetate has so far been observed with the chiral ligands 2, 3a and 3b60-61 (Table 17). The addition of tetrahexylammonium bromide dramatically increases enantioselec-tivity in the case of ligand 2, as does changing the solvent from tetrahydrofuran to dichloro-methane. which is believed to enhance the formation of dimeric ionic salts of the nucleophile. In contrast, additives such as tetraalkylammonium salts or crown ethers diminish the enantiomeric excess in reactions catalyzed by the phosphinoaryl oxazoline ligands 3a and 3b. bearing a chiral phosphorus on the aryl moiety. 

Further studies, exploring the scope of these catalysts and the mechanism of enantioselection, are in progress. Considering the promising results obtained so far, it will be interesting to test phosphinoaryl-oxazolines as ligands in other metal-catalyzed reactions. 

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