A new class of C-2-symmetric diphosphine ligands derived from valine: remarkably diverse behavior in catalytic asymmetric transformations

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 chlorid e. Remarkably high levels of asymmetric induction, over 99% ee, were achiev ed using 8a in palladium-catalyzed asymmetric allylic transformations of st erically less demanding cyclohexenyl substrates 9. The ligand system was ea sily extended to the development of analogous chiral auxiliaries 8b,c by th e identical procedure but using isophthaloyl chloride and succinyl chloride . However, the ligands 8b,c exhibited much lower catalytic activity. In con trast to the asymmetric allylic substitutions, Sc demonstrated significant improvement of enantioselectivity, up to 64% ee, in rhodium-catalyzed asymm etric 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. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.