Influence of a remote hydroxy group in the ligand on the reactivity of a chiral hydrogenation catalyst

A kinetic study concerning the asymmetric hydrogenation of prochiral olefins by chiral diphosphine rhodium cyclooctadiene complexes based on HO-DIOPs is presented. The investigations have been carried out in order to show the influence of a remote hydroxy group on the rate of the reaction. Since in methanol under standard conditions the reactivities of all catalysts elaborated were too high, the hydrogenation experiments have been performed under reduced hydrogen pressure affording the desired reduction in rate. To avoid the induction period caused by the hydrogenation of the cyclooctadiene all precatalysts were prehydrogenated before the prochiral substrates were added. For comparison analogous Rh-DIOP complexes with the same structural framework, but without the hydroxy group in the backbone have been proven. For all hydroxy catalysts investigated a lower reactivity has been observed compared to the parent complexes, although a direct interaction between HO-group and reaction center can be presumably excluded. This assumption is supported by the X-ray structural analysis of one of the complexes which shows the remote position of the HO-group related to the metal center. (C) 1997 Elsevier Science Ltd.