C9-O-substituted derivatives of cinchona alkaloids as chiral modifiers in the Orito-reaction: Effects of structure of modifiers on sense of enantioselectivity

Abstract

The effect of the structure of C9-O-substituted cinchona alkaloids as chiral modifiers of Pt on the enantioselectivity of the Pt-alumina-catalyzed hydrogenation of ethyl pyruvate under mild experimental conditions (room temperature, hydrogen pressure of 1 bar, mainly toluene and tetrahydrofurane as solvents) was investigated. Although C9-O-substituted bulky chiral modifiers also have the open3 conformation, which is a prerequisite of high ee, the ee values observed were still low and even opposite to that expected. Based on the experimental data of a great variety of experiments (ESI-ion-trap-MS, modifier mixtures, selective hydrogenation of chiral modifiers, computational studies) the inversion can explain by tilted adsorption of chiral modifier, namely by a conformational change in the adsorbed chiral modifier and reactant 1:1 intermediate complex.