| Abstract
| - Optically active C2-symmetric tetraphosphine 4 was prepared via the phosphine-borane methodology.Its dirhodium complex 5 was structurally characterized and probed as a catalyst in asymmetrichydrogenations of representative prochiral substrates, demonstrating high activity and good to excellentenantioselectivities. A mechanistic study revealed that 5 can be cleanly and stereoselectively convertedto the tetrahydride species 6a, which is stable up to 0 °C and at higher temperatures slowly decomposeswithout 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 thesubstrate coordinated only via the amidocarbonyl group, whereas the double bond of the substrate remainednoncoordinated. Raising the temperature to −40 °C resulted in irreversible isomerization of complex 7to 8, which differs from 7 only by the spatial arrangement of ligands. Migratory insertion proceedingsimultaneously with the isomerization of 7 to 8 yields the trihydride complex 9, which is an analogue ofthe monohydride intermediates described previously. When the reaction of 6a with MAC was carriedout in the presence of an excess of MAC, the released dirhodium complex was captured by the substrateto give the catalyst−substrate complex 10, which was characterized by multinuclear NMR. SubstrateMAC 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 theassociation step of the Rh-catalyzed asymmetric hydrogenation.
- A new chiral tetraphosphine dirhodium complex was prepared, characterized structurally, and probed as a catalyst in asymmetric hydrogenation. A mechanistic study revealed quantitative formation of a tetrahydride complex, which was reacted with methyl (Z)-acetamidocinnamate to give a series of detectable intermediates.
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