Abstract
AbstractAsymmetric hydrogenations of prochiral olefines by means of chiral rhodium(I) complexes of the type [Rh(L)(PP*)]A (L = COD, [(Z,Z)‐cycloocta‐1,5‐diene], or NBD (norborna‐2;5‐diene), PP* = chiral bisphosphane forming seven‐membered chelate rings, A = anion like BF4−) are often associated with induction periods caused by partial blocking of the catalyst. NBD complexes are hydrogenated faster than the corresponding COD complexes. Catalytic hydrogenation of COD/NBD mixtures and the determination of the ratio of the Michaelis constants showed that the steady‐state concentration of the COD complex under hydrogen is higher than that of the NBD complex. However, under argon the NBD complex predominates owing to its higher thermodynamic stability compared with that of the COD complex as determined by 31P‐NMR spectoscopy. This complete reversion of the ther‐modynamically determined ratios of COD to NBD complex concentration under hydrogenation conditions was proven by means of UV/Vis spectroscopy.
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