Monodentate Chiral Spiro Phosphoramidites: Efficient Ligands for Rhodium-Catalyzed Enantioselective Hydrogenation of Enamides**

Optically active a-arylalkylamines are an important class of compounds that are widely used in organic and pharmaceutical synthesis, and much effort has been made to develop efficient asymmetric synthetic methods for them. [1] Asymmetric catalytic hydrogenation of enamides, initiated by Kagan et al., [2] provides a direct and convenient route to chiral amine derivatives. However, many well-known chiral diphosphane ligands, such as DIOP, BINAP, and CHIRA-PHOS, which are extremely successful in the asymmetric hydrogenation of dehydroamino acid derivatives, do not give high enantioselectivity in the hydrogenation of enamides. [3, [6] Here we describe highly efficient monodentate chiral ligands 1 containing a 1,1'-spirobiindane backbone for the Rh-catalyzed asymmetric hydrogenation of a-arylethenylamine derivatives [Eq. (1)] with excellent enantioselectivities (up to 99.7 % ee). The chiral monodentate phosphoramidite ligands 1 (abbreviated SIPHOS) were conveniently synthesized in good yields from enantiomerically pure 1,1'-spirobiindane-7,7'-diol, which was easily prepared from 3-methoxybenzaldehyde by using the procedure described by Birman et al. The results in Catalysts prepared in situ from cationic Rh complexes were active in the asymmetric hydrogenation of enamide 4 a and provided a similar level of enantiocontrol, although the catalyst with a bulkier counteranion needed a longer time for completion of the reaction [5d, 5e] The influence of ligand structure on the enantioselectivity of the catalysts was also examined in the hydrogenation of 4 a. When the alkyl groups on the nitrogen atom of ligands 1 was changed from methyl ((S)-1 a) to ethyl ((S)-1 b) and isopropyl ((S)-1 c), the enantioselectivity of th