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Abstract:
Density functional theory calculations are employed to study the asymmetric transfer hydrogenation of ketones catalyzed by rhodium–arene complexes containing hydroxamic acid-functionalized amino acid ligands. Firstly, the ligand–metal binding is investigated and it is shown that both the N,N and O,O binding modes Are viable. For each of these, the full free energy profile for the transfer hydrogenation is calculated according to the outer-sphere reaction mechanism. Three factors are demonstrated to influence the stereoselectivity of the process, namely the energy difference between the metal–ligand binding modes, the energy difference between the intermediate hydrogenated catalyst, and the existence of a stabilizing CH–π interaction between the Cp* ligand of the catalyst and the phenyl moiety of the substrate. Theoretical reproduction of the selectivity of a slightly modified ligand that is shown experimentally to yield the opposite enantioselectivity corroborates these results. Finally, a technical observation made is that inclusion of dispersion interactions (using the B3LYP-D2 correction or the M06 functional) proved to be very important for reproducing the enantioselectivity.
