A new rigid cinchona modified (α-IQ) platinum catalyst for the enantioselective hydrogenation of activated ketones: Data to the origin of enantioselection

The enantioselective hydrogenations of ethyl pyruvate (EP), methyl benzoylformate (MBF), ketopantolactone (KPL) and pyruvaldehyde dimethylacetal (PADA) were studied on Pt–alumina catalyst modified by a new modifier namely α-isoquinine (α-IQ) with rigid conformation and for comparison by quinine (Q) in toluene and in acetic acid. The effects of modifier concentration, mixtures of modifiers, hydrogenation of α-IQ and theoretical calculations were examined on the interpretation of features of reactions. Using the Engelhard 4759 catalyst under mild experimental conditions (room temperature, 1 bar hydrogen pressure) the ees were lower in the case of α-IQ than for Q. The inversion of enantioselectivity observed in the case of the previously studied β-isocinchonine (β-ICN) containing C8(R) and C9(S) atoms in toluene as solvent failed to occur in the presence of α-IQ containing C8(S) and C9(R) atoms. Indirect experimental evidence for structure of adsorbed chiral modifier was supported by studies on hydrogenation and relative adsorption strength of α-IQ as well as by theoretical calculations. The significant enantioselectivity changes along the series of otherwise structurally related modifiers (Q, α-IQ and β-ICN) were compared with the ab initio computed geometrical features. The results revealed that besides the effects disclosed up to now in the literature, the orientational angle of the N-lone pair in the quinuclidine moiety relative to the quinoline also influences the structure and the adsorption mode of the intermediate responsible for the enantioselection.

Graphical abstractThe results revealed that the orientational angle of the N-lone pair in the quinuclidine moiety relative to the quinoline plain has a strong effect on the enantioselection. Figure optionsDownload full-size imageDownload as PowerPoint slide