Iridium‐Catalyzed Asymmetric Hydrogenation of Unfunctionalized Tetrasubstituted Olefins

Abstract

Iridium complexes with chiral N,P ligands have emerged as a new class of highly efficient catalysts for asymmetric hydrogenation with an application range that is largely complementary to rhodium and ruthenium diphosphane complexes. [1][2][3] They have been used successfully for the hydrogenation of a wide range of functionalized and unfunctionalized di-and trisubstituted olefins. Unlike Rh and Ru diphosphane complexes, they do not require the presence of a coordinating group near the C=C bond, so even purely alkylsubstituted olefins can be hydrogenated with high enantioselectivity.[3] However, to date no practical catalysts are known for the asymmetric hydrogenation of unfunctionalized tetrasubstituted olefins. Although Buchwald and co-workers have obtained high enantioselectivities in the hydrogenation of tetrasubstituted aryl alkenes using a chiral zirconocene complex, [4] high catalyst loadings, long reaction times, and high sensitivity of the catalyst have prevented widespread use of this method. Ir catalysts based on chiral phosphanyl oxazoline (phox) ligands 1, [5] on the other hand, showed high activity in the hydrogenation of tetrasubstituted aryl alkenes such as 5, but enantiomeric excesses were at best moderate (Table 1). [1a, 6] We have recently found that Ir complexes based on the structurally simple and readily accessible phosphanyl oxazolines 4 are much better suited for such reactions.[7] A subsequent study with a wide range of ligands 4 and phosphinite oxazoline ligands of types 2 and 3 revealed that 5 and other tetrasubstituted olefins can be hydrogenated with high enantioselectivity at remarkably low catalyst loadings. Herein, we report the results of this study, which has led to highly efficient and practical Ir catalysts for the asymmetric hydrogenation of tetrasubstituted unfunctionalized olefins.Although the phosphanylmethyloxazoline 4 k was reported by Sprinz and Helmchen [8] many years ago, ligands of this type have not, to date, received much attention. [9] Using a modified synthetic route starting from two sets of secondary phosphanes and chloromethyloxazoline com-

Asymmetric Hydrogenation of Olefins Using Chiral Crabtree-type Catalysts: Scope and Limitations

“…The Ir-PHOX ligands have been successfully applied in the hydrogenation of tricyclic ring olefins S34 (Table 3, entry 10). 110 These results opened up the asymmetric reduction of this substrate class to the use of other Ir-N,P catalysts and some of the ligands used in the reduction of trisubstituted olefins have also been tested. In this context, the previously mentioned phosphine-benzoxazine ligands 8 ( Figure 4) provided low conversions (up to 63%) and enantioselectivities (up to 31%)…”

“…110 However, 20a provided excellent enantioselectivities (ee's up to 95%) for a limited range of tetrasubstituted dihydronaphthalenes (Table 3, entries 13 and 17).…”

Gas chromatographic enantioseparation of unfunctionalized chiral alkanes: A challenge in separation science (overview, state of the art, and perspectives)

“…The enantioseparations described in this work permitted the screening of the enantiomeric excess (ee) in the asymmetric hydrogenation 60 of alkene precursors by chiral iridium catalysts (Pfaltz, unpublished results). The described enantioseparation system is also now under scrutiny for the hyphenation of enantioselective gas chromatography and proton nuclear magnetic resonance spectroscopy.…”

Transition Metal‐Catalyzed Homogeneous Asymmetric Hydrogenation

“…Hydrogenation of a hindered tetrasubstituted olefi n was accomplished by an Ir complex to give a product with 97% ee and a complete conversion (Eq. 7.25 ) [209,229] . A cyclic analogue olefi n was hydrogenated with a related Ir catalyst to give excellent diastereo -and enantioselectivities (Eq.…”