Ruthenium complexes containing chiral N-donor ligands as catalysts in acetophenone hydrogen transfer -: New amino effect on enantioselectivity

New p-cymene ruthenium species containing chiral amino alcohols (1-3), primary (4-7) and secondary (8, 9) amino-oxazolines, were tested as catalysts in the hydrogen transfer of acetophenone, using 2-propanol as the hydrogen source. A remarkable effect on the enantioselectivity, but also on the activity, was observed depending on the amino-type oxazoline, Ru/8 and Ru/9 being low active and nonselective catalytic systems, in contrast to their primary counterpart Ru/5. Complexes containing amino-oxazolines (10-12) were prepared and fully characterized, both in solution and in solid state. The X-ray structure was determined for (S-Ru,R-C)-10. The diastereomeric ratios observed for complexes 10 and 11 were determined by H-1 NMR and confirmed by means of structural modeling (semi-empirical PM3(tm) level). DFT theoretical calculations for the transition states involved in the hydrogen transfer process proved the important differences in their relative populations, which could justify the enantioselectivity divergences observed between primary and secondary amino-oxazoline ruthenium systems. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).