P-Chiral Tetraphosphine Dirhodium Complex as a Catalyst for Asymmetric Hydrogenation:  Synthesis, Structure, Enantioselectivity, and Mechanism. Stereoselective Formation of a Dirhodium Tetrahydride Complex and Its Reaction with Methyl (Z)-α-Acetamidocinnamate

Optically active C 2-symmetric tetraphosphine 4 was prepared via the phosphine-borane methodology. Its dirhodium complex 5 was structurally characterized and probed as a catalyst in asymmetric hydrogenations of representative prochiral substrates, demonstrating high activity and good to excellent enantioselectivities. A mechanistic study revealed that 5 can be cleanly and stereoselectively converted to the tetrahydride species 6a, which is stable up to 0 °C and at higher temperatures slowly decomposes without the loss of hydrogen. The low-temperature (−80 °C) reaction of 6a with methyl (Z)-α-acetamidocinnamate (MAC) cleanly gave the tetrahydride complex 7 containing one molecule of the substrate coordinated only via the amidocarbonyl group, whereas the double bond of the substrate remained noncoordinated. Raising the temperature to −40 °C resulted in irreversible isomerization of complex 7 to 8, which differs from 7 only by the spatial arrangement of ligands. Migratory insertion proceeding simultaneously with the isomerization of 7 to 8 yields the trihydride complex 9, which is an analogue of the monohydride intermediates described previously. When the reaction of 6a with MAC was carried out in the presence of an excess of MAC, the released dirhodium complex was captured by the substrate to give the catalyst−substrate complex 10, which was characterized by multinuclear NMR. Substrate MAC is much more loosely bound in octahedral complexes 7 and 8 than in the square planar catalyst−substrate complex 10. This finding provides experimental support for the stereoselection during the association step of the Rh-catalyzed asymmetric hydrogenation.