A combined experimental and theoretical study of 1-phenylpropane-1,2-dione hydrogenation over heterogeneous cinchonidine-modified Pt catalyst

The enantioselective hydrogenation of vicinal diketones over cinchonidine-modified Pt resulted in enantiomeric excess of structurally similar (R)-enantiomers. Furthermore, the kinetic resolution was caused due to faster reaction of (S)-hydroxyketone further to diols, resulting in an increase of ee. The diastereoselectivities in diols were similar. The (R,S) or (S,R) diols were always the main products whereas considerably less (R,R) of (S,S) were formed. For the first time in 1-phenylpropane-1,2-dione (A) hydrogenation enantiomeric excesses of both C1O1 and C2O2 group have been reported. The ee1-OH and ee2-OH were 50 and 25%, respectively, at 50% conversion of A. Based on batch and continuous reactor experiments it could be concluded that the source of enantioselectivity is an increased formation rate of (R)-enantiomer and decreased formation rate of (S)-enantiomer. Theoretical calculations revealed that in the substrate-modifier diastereomeric complex the reactant forms a nonplanar s-cis conformation and bonds to the protonated cinchonidine either via a bifurcated hydrogen bond or with two hydrogen bonds where the OH group is involved also. Optimized diastereomeric complexes were equal in energy. The calculated proton affinity of CD was high, 1000 kJ mol−1, indicating that protonation is feasible under typical experimental conditions.