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 scCO(2), scC(2)H(6) and various liquid organic solvents. Deute rium labeling studies and parahydrogen-induced polarization (PHIP) NMR expe riments were used to provide the first detailed mechanistic insight into th e activation and transfer of the dihydrogen molecule during hydrogenation i n scCO(2). Chemical interactions between CO2 and reactive intermediates of the catalytic pathway could be excluded as possible explanations for the ex perimentally verified difference in the catalytic behavior in scCO(2) and h exane. (C) Wiley-Liss, Inc.