NUCLEAR SINGLET-TRIPLET MIXING DURING HYDROGENATIONS WITH PARAHYDROGEN - AN IN-SITU NMR METHOD TO INVESTIGATE CATALYTIC REACTION-MECHANISMS AND THEIR KINETICS .2. HOMOGENEOUS HYDROGENATION OF 1,4-DIHYDRO-1,4-EPOXYNAPHTHALENE USING DIFFERENT RHODIUM CATALYSTS

Depending on the catalyst, parahydrogen gives rise to different polarization patterns in the H-1-NMR spectra of the reaction products during homogeneously catalyzed hydrogenations due to the PASADENA/PHIP phenomenon. A detailed analysis of these patterns reveals mechanistic and kinetic information about the catalytic reaction pathway. This behavior is exemplified using either Wilkinson's or a cationic rhodium bis-(phosphinite) catalyst to mediate the homogeneous hydrogenation of 1,4-dihydro-1,4-epoxynaphthalene to 1,4-epoxytetralin as the universal reaction product. The different polarization patterns in the NMR spectra of an identical final product are attributed to a different level of admixture of nuclear triplet character to the initial pure singlet state of the parahydrogen nuclei along the individual reaction pathway. Computer calculations allow to elucidate mechanistic details and establish a time scale to estimate the dwell time of the hydrogen nuclei in magnetically inequivalent positions in intermediate hydrogen complexes or hydrides.