NOE (nuclear Overhauser effect) transfers from para-H2 enhanced magnetizations in alkene moieties at Rh(I) centers

The hydrogenation of symmetric dienes with parn-H-2 catalyzed by Rhodium complexes leads to remarkable effects in the H-1 NMR spectra of the corresponding alkene derivatives, namely, an emission peak (negative peak) in the aliphatic region ascribed to protons of the hydrogenated double bond and one or more enhanced absorption peaks. The strongest absorption peak is invariantly assigned to the two equivalent olefinic protons in the free alkene. The possibility that the observed behavior could be associated with a,reversible exchange between para-H-2 and the olefinic hydrogens has been ruled out on the basis of the lack of deuterium incorporation when the experiments are carried out with D-2. Variable magnetic field experiments have indicated that the positive peaks arise from relaxation processes, i.e., from cross-relaxation transfers (generally denoted as NOE transfers; NOE = nuclear Overhauser effect) originating from the enhanced magnetization at the hydrogenation sites in the product or, more likely, at the hydride ligands in intermediate species.