The Enantioselective Dehydrogenation of Racemic Secondary Alcohols Catalyzed by Dimeric Ruthenium(II) Chiral Diphosphine Complexes

Abstract

Abstract The asymmetric dehydrogenation of racemic alcohols by an effective Ru2Cl4((−)-diop)3 (diop=2,3-O-isopropylidene-1,4-bis(diphenylphosphino)butane) was investigated with and without unsaturated hydrogen acceptors at 120–195 °C, with particular reference to the enantioselective factors and the reaction mechanism. The magnitude of enantioselectivity was substantially affected by the structures of the alcohols and of the hydrogen acceptors and by the reaction temperature; the hydrogen acceptors (R1CH=CHR2: R1\eweqH and R2\eweqH) contributed to the enhancement of the selectivity through the newly formed asymmetric centers which result from their coordination to the chiral Ru(II) complex. The activation parameters (ΔH\eweq and ΔS\eweq) obtained from the linear Arrhenius dependence of the rate constants of each enantiomer (R- or S-alcohol) established an isokinetic relationship, and the defference in the parameters of the each enantiomer (ΔΔH\eweq and ΔΔS\eweq) also showed a satisfactorily linear correlationship between them. It is deduced from the present study that the enantioselection of R- and S-alcohols occurs at different coordination distances toward the chiral Ru(II) complex between the enantiomers under asymmetric circumstances.