Investigating the kinetics of homogeneous hydrogenation reactions using PHIPNMR spectroscopy

The combination of parahydrogen induced polarization (PHIP), kinetics and N MR spectroscopy yields a powerful analytical tool: quantitative in situ NMR spectroscopy; Two versions of PHIP NMR experiments are presented to invest igate the kinetics of homogeneously catalyzed hydrogenations. The first met hod, an experimental variation of the ROCHESTER experiment (ROCHESTER = rat es of catalytic hydrogenation estimated spectroscopically through enhanced resonances), allows one to determine the hydrogenation rate independently o f relaxation and other sources of decay, e.g., subsequent chemical reaction steps. The second method named DYPAS (dynamic PASADENA spectroscopy) uses a variable delay between the end of the hydrogen-addition period and the de tection pulse. In principle, all processes during this delay can be describ ed by a set of coupled differential equations. Their solutions can be fitte d to the experimental data by a least-squares optimization of the involved kinetic parameters. The DYPAS method can be used to determine the rates of formation as well as the rates of decomposition of stable intermediates and has been applied to the case of freshly hydrogenated and still catalyst-at tached product molecules. We provide kinetic data for the formation and dec omposition of these unusual product-catalyst complexes during the hydrogena tion of different styrene derivatives with a cationic Rh-I catalyst contain ing a chelating diphosphine ligand. The kinetic measurements indicate that the rate of formation of the catalyst-attached product increases whereas th e rate constant of its decomposition diminishes if the para position of the arene ring of styrene carries an electron-donating substituent. In the cas e of p-aminostyrene as the substrate, the detachment step turned out to be rate limiting for the catalytic cycle. With certain substituted styrenes an d cationic Rh-I complexes containing chiral chelating diphosphine ligands, two geometrically different (diastereomeric) product-catalyst adducts can b e discriminated via PHIP NMR spectroscopy. The associated alternative react ion pathways have been analyzed by applying the DYPAS method, which can als o be used to investigate the mechanism of an asymmetric hydrogenation.