Counter-ligand and solvent dependent oxygen-metal interactions of hemilabile coordinating hydroxy groups in chiral diphosphine rhodium(I) hydrogenation catalysts

In cationic Rh[(R,R)-1,4-bis(diphenylphosphino)butane-2,3-diol] complexes t he interaction of the hemilabile coordinating hydroxy groups with the metal is strongly dependent upon the nature of the counter-ligand and the solven t. As a result of the 'arm-off, arm-on' mechanism the conformation of the c hirality inducing backbone is changed. Spectroscopic and catalytic investig ations demonstrate that the eta(3)-coordination mode of the tetradentate li gand is responsible for the deceleration of the asymmetric hydrogenation. B y the assistance of the second HO-group the Rh-O interaction can be suspend ed. (C) 1999 Elsevier Science Ltd. All rights reserved.