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Studies for controlling rate and enantioselectivity of the asymmetric hydrogenation with Rhdiphosphine catalysts by cooperative attractive interactions within the framework of catalyst–substrate complexes are represented. In strong contrast to a Rh(I)[threo-1,4-bis(diphenylphosphino)butane-2,3-diol] catalyst an extremely fast reaction took place applying the complex of the analogue erythro ligand. This is likely due to a strong intramolecular hydrogen bond between the vicinal HO groups impeding the hemilabile coordination of one of the hydroxy groups on the metal center during the hydrogenation. When a substrate with strong hydrogen bond acceptor properties such as (E)-methyl 3-dimethoxyphosphorylbut-2-enoate was hydrogenated even the threo catalyst exhibited a fast reaction. The product was obtained in 83% ee. In contrast, when in the ligand both HO groups were replaced by MeO groups only poor conversion and 24% ee were achieved. While with the complex bearing HO groups in methanolic solution the corresponding diastereomeric catalyst substrate complexes were dominant, analogue MeO groups bearing catalyst substrate complexes could not be detected by NMR. For the latter, only the Rh(I)–solvent complex was found revealing the high importance of the additional functional groups on this equilibrium.

In cationic Rh[( R , R )-1,4-bis(diphenylphosphino)butane-2,3-diol] complexes the interaction of the hemilabile coordinating hydroxy groups with the metal is strongly dependent upon the nature of the counter-ligand and the solvent. As a result of the `arm-off, arm-on' mechanism the conformation of the chirality inducing backbone is changed. Spectroscopic and catalytic investigations demonstrate that the η 3 -coordination mode of the tetradentate ligand is responsible for the deceleration of the asymmetric hydrogenation. By the assistance of the second HO-group the Rh–O interaction can be suspended.

An easily accessible device for the study of reactions with gases in the liquid phase, consisting of a 10 mm NMR tube with a gas inlet, is presented. This arrangement serves for reaction monitoring and mechanistic studies by acquiring NMR spectra (1H, 13C, 31P) with a resolution that allows one to observe and characterize intermediates that cannot be isolated as pure substances. The quantity of the gas to be consumed is not limited. Examples from organometallic chemistry are presented: the formation of a zirconacyclopentene from a zirconium–alkyne complex and ethylene and investigations on rhodium-catalysed olefin hydrogenation. The qualitative and quantitative evaluation of the measurements and application to reaction kinetics are discussed. An uptake of 0.1 ml of gas per minute at normal pressure (isobaric) is possible without a marked influence of diffusion (stationary conditions). © 1997 John Wiley & Sons, Ltd.

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.

The synthesis of chiral bisphosphines bearing a hydroxyl group in a rigid backbone is described. These compounds which are analogues of the prominent ligand DIOP formed definite rhodium complexes. IR spectra provided evidence that the hydroxyl groups do not participate in the complexation to the metal or to one of the acetal oxygen atoms. The complexes have proven successful in the asymmetric hydrogenation of prochiral functionalized olefins in different solvents. In methanol only small effects could be detected in comparison to parent complexes which do not carry a hydroxyl group. However, when hydrogenations were performed in methylene chloride or toluene significant differences in the enantioselectivities (by up to 17 %ee) were observed, especially in the reaction with itaconic acid or its dimethyl ester, respectively. In the latter cases the effect is contingent on the spatial orientation of the hydroxyl group in the catalyst.