AB-INITIO STUDIES OF RHODIUM(I)-N-ALKENYLAMIDE COMPLEXES WITH CIS-COORDINATING 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 pres ented, which are important intermediates in the asymmetric hydrogenati on affording N-acylamino acids. The assumption of different intermedia tes investigated is based on the inter- and intramolecular equilibrium of diastereomeric complexes in the hydrogenation mechanism. The geome try optimizations were performed at the MP2 level of theory using rela tivistic pseudopotentials for rhodium, followed by QCISD(T) single-poi nt calculations. The intermediates containing cis- or trans-coordinate d phosphines have been compared structurally and energetically, and th e influence of solvents is discussed. In the equilibrium consisting of uncoordinated substrate and substrate complexes, all tautomeric compl exes were found in the calculation and have a minimum on the potential hypersurface. However, the occurrence of enamine/imine and amide/imin e-ol tautomers could be excluded for the catalytic reaction, because t hey implement irreversible steps (C-H activation, deprotonation) which finish the catalytic cycle and may therefore be responsible for turno ver-limiting steps. Only the calculation of an intermediate showing he tero-pi-allyl type coordination can explain satisfactorily the crucial interconversion of the diastereomeric major/minor complexes in a nonc oordinating medium. This intermediate is favored for the rationalizati on of the intramolecular equilibrium. For trans-coordinating diphosphi ne ligands a T-shaped intermediate with only a coordinated nitrogen at om is proposed.