Computational study of the factors controlling enantioselectivity in ruthenium(II) hydrogenation catalysts.
The reduction of prochiral ketones catalyzed by Ru(diphosphine)(diamine) complexes has been studied at the DFT-PBE level of theory. Calculations have been conducted on real size systems [trans-Ru(H)2(S, S-dpen)(S-xylbinap) + acetophenone], [trans-Ru(H)2(S, S-dpen)(S-tolbinap) + acetophenone] and [trans-Ru(H)2(S, S-dpen)(S-xylbinap) + cyclohexyl methyl ketone] with the aim of identifying the factors controlling the enantioselectivity in Ru(diphosphine)(diamine) catalysts. The high enantiomeric excess (99%) in the hydrogenation of acetophenone catalyzed by trans-Ru(H)2(S, S-dpen)(S-xylbinap) has been explained in terms of the existence of a stable intermediate along the reaction pathway associated with the (R)-alcohol. The formation of this intermediate is hindered with the competitive pathways, which consequently increases the activation energy for the hydrogen transfer acetophenone/(S)-phenylethanol reaction. For the [trans-Ru(H)2(S, S-dpen)(S-tolbinap) + acetophenone] system, the lower enantioselectivity (i.e. 80%) is rationalized by the smaller differences in the activation energy between the competitive pathways which differentiate between the two diastereomeric approaches of the prochiral ketone. The DFT-PBE results suggest that this reaction is driven to the (R)-product only by the process of binding the acetophenone to the active site of the trans-Ru(H) 2(S, S-dpen)(S-tolbinap) catalyst. For the hydrogenation of cyclohexyl methyl ketone catalyzed by trans-Ru(H)2(S, S-dpen)(S-xylbinap), the low performance in the enantioselective hydrogenation of the dialkyl ketone (i.e. 37%) is again explained by the small differences in the activation and binding energies which are the factors which could effectively differentiate between the two alkyl groups.
Di Tommaso
Devis
D
Davy Faraday Research Laboratory, Kathleen Lonsdale Building, University College of London, Gower Street, London WC1E 6BT, United Kingdom. uccaddi@ucl.ac.uk
French
Samuel A
SA
Zanotti-Gerosa
Antonio
A
Hancock
Fred
F
Palin
Erika J
EJ
Catlow
C Richard A
CR
D002384:Catalysis; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D017975:Ruthenium Compounds / Q000737:chemistry*; D013237:Stereoisomerism
D017975:Ruthenium Compounds; D006859:Hydrogen