Steven Feldgus and, C. Landis
Dec 27, 2000
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Journal
Journal of the American Chemical Society
Abstract
The potential energy surface for the [Rh((R,R)-Me-DuPHOS)]+-catalyzed asymmetric hydrogenation of a prochiral enamide, α-formamidoacrylonitrile, has been computed using a three-layer hybrid quantum mechanics/molecular mechanics method (ONIOM). The bond-breaking and bond-forming region is modeled using a nonlocal density functional method (B3LYP), whereas HF theory and molecular mechanics (UFF) are used to describe the electronic and steric impact of the outer coordination sphere of the catalyst. Intermediates and transition states were calculated along four isomeric pathways of two diastereomeric manifolds. The starting point for each manifold is a square planar catalyst−enamide complex. Binding of the re enantioface of the enamide to the catalyst generates the more stable, major diastereomer, favored by 3.6 kcal/mol over the minor diastereomer, which has the si face bound. However, the net free energy barrier for the reaction is 4.4 kcal/mol lower for the minor diastereomer than for the major diastereome...