Mechanistic aspects of dihydrogen activation and transfer during asymmetric hydrogenation in supercritical carbon dioxide.

A new "CO2-philic" chiral rhodium diphosphinite complex was synthesized and applied as catalyst precursor in the asymmetric hydrogenation of dimethyl itaconate in scCO2, scC2H6 and various liquid organic solvents. Deuterium labeling studies and parahydrogen-induced polarization (PHIP) NMR experiments were used to provide the first detailed mechanistic insight into the activation and transfer of the dihydrogen molecule during hydrogenation in scCO2. Chemical interactions between CO2 and reactive intermediates of the catalytic pathway could be excluded as possible explanations for the experimentally verified difference in the catalytic behavior in scCO2 and hexane. Copyright 2000 Wiley-Liss, Inc.