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A new C 2 -symmetric diphosphine ligand 8a was readily prepared in one step by treatment of the synthetic precursor of VALAP, 7 , with phthaloyl chloride. Remarkably high levels of asymmetric induction, over 99% ee, were achieved using 8a in palladium-catalyzed asymmetric allylic transformations of sterically less demanding cyclohexenyl substrates 9 . The ligand system was easily extended to the development of analogous chiral auxiliaries 8b , c by the identical procedure but using isophthaloyl chloride and succinyl chloride. However, the ligands 8b , c exhibited much lower catalytic activity. In contrast to the asymmetric allylic substitutions, 8c demonstrated significant improvement of enantioselectivity, up to 64% ee, in rhodium-catalyzed asymmetric hydrosilylations of acetophenone 11a compared to 35% ee using 8a . In this way, the versatility of the present ligand system played an important role in variations of substrates and reaction types. |
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