Ab Initio Studies of Rhodium(I)−N-Alkenylamide Complexes with cis- and trans-Coordinating Phosphines: Relevance for the Mechanism of Catalytic Asymmetric Hydrogenation of Prochiral Dehydroamino Acids

A theoretical study on cationic Rh(I)−N-alkenylamide complexes is presented, which are important intermediates in the asymmetric hydrogenation affording N-acylamino acids. The assumption of different intermediates investigated is based on the inter- and intramolecular equilibrium of diastereomeric complexes in the hydrogenation mechanism. The geometry optimizations were performed at the MP2 level of theory using relativistic pseudopotentials for rhodium, followed by QCISD(T) single-point calculations. The intermediates containing cis- or trans-coordinated phosphines have been compared structurally and energetically, and the influence of solvents is discussed. In the equilibrium consisting of uncoordinated substrate and substrate complexes, all tautomeric complexes were found in the calculation and have a minimum on the potential hypersurface. However, the occurrence of enamine/imine and amide/imine-ol tautomers could be excluded for the catalytic reaction, because they implement irreversible steps (C−H ac...