Origin of enantioselectivity in asymmetric hydrovinylations catalyzed by phosphoramidite nickel catalysts:: An experimentally supported density functional study

The mechanism of the nickel-catalyzed asymmetric hydrovinylation of styrene in the presence of chiral phosphoramidite ligands was investigated by means of DFT calculations including anion and solvent effects. The theoretical studies are supported by crystal structure analysis and NMR investigations of the ligand. The results obtained indicate that the performance of the catalytically active system is largely affected by a hemilabile coordination mode of one phenyl ring of the phosphoramidite ligand allowing only for two chemically plausible orientations of styrene coordination in the active nickel hydride catalyst. These two diastereomers are the starting points of two reaction paths for hydride transfer to styrene. One path is clearly preferred over the other, as can be deduced from the significantly lower activation energy (+17.57 versus +26.92 kcal/mol) corresponding to the enantiodiscriminating step of the reaction. The displacement of the hemilabily coordinated part of the ligand by ethylene initiates the subsequent reaction paths for C-C bond formation, yielding the Sand the R-configurated reaction products, respectively. The activation energies show that C-C bond formation does not proceed enantioselectively. Calculations in the presence of anions showed BF4--containing systems to be less active than systems with BARF(-) (tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate). The results obtained are in accordance with and help to rationalize experimental data.