The Physico-chemical Properties of Cinchona Alkaloids Responsible for their Unique Performance in Chiral Catalysis

Abstract

The physico-chemical properties of cinchona alkaloids have been characterized in connection to their use for catalytic enantioselective conversions. Adding to the previous identification of their active site at the nitrogen atom in the quinuclidine ring and the chiral environment provided by the carbon centers of the neighboring alcohol linker, an argument is made here for the importance of the adoption of certain rotational conformations by those cinchona alkaloids in optimizing their chiral promotion. Because catalysis with cinchona alkaloids involves a liquid phase, there is a dynamic conformation isomerization process controlled by a number of factors having to do with the exact structure of the cinchona as well as with the nature of the solvent used and, in the case of heterogeneous catalysis, the presence of a solid surface. Solvents of intermediate polarity have been found to be the best for dissolving the cinchona, for establishing rapid adsorption equilibria with metal surfaces, and for promoting chiral catalysis. Protonation also leads to a dramatic change in performance, locking the cinchona molecule in a specific conformation held in place by the counter anion of the acid used, and modifying the chemical and biological activity of the system. Comparative studies with several cinchona indicate that molecular groups attached to peripheral positions also exert a great influence on the conformational and adsorption behavior of these molecules.

Electrochemical studies of irreversibly adsorbed ethyl pyruvate on Pt{hkl} and epitaxial Pd/Pt{hkl} adlayers

“…Although several models have been proposed to explain the mechanism of enantioselection under reaction conditions [9,22,[27][28][29][30][31] no one theory appears to account satisfactorily for all of the unusual experimental features of this complex surface reaction, for example; the rate acceleration associated with the modified hydrogenation reaction [32][33][34][35][36][37][38][39], its solvent dependency [40][41][42][43][44], the effect of modifier [31,41,[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] and substrate [52,[64][65][66] structure and concentration [37,39] together with catalyst pr...…”

Adsorption of 1‐(1‐Naphthyl)ethylamine from Solution onto Platinum Surfaces: Implications for the Chiral Modification of Heterogeneous Catalysts

“…This effect was certainly proven important in determining the relative uptakes of different cinchona alkaloids. [4,19,[21][22][23] However, in the present case, all compounds studied are liquid and fully soluble in the solvent (CCl 4 ).…”

Disrupting Aryl-CH···O Interactions on Pt(111) Through the Coadsorption of Trifluoroacetic Acid and 2,2,2-Trifluoroacetophenone (TFAP): Inhibition of Competing Processes in Heterogeneous Asymmetric Catalysis

“…They can function as organocatalysts [1][2][3][4] and as an active surface component in heterogeneous reactions on supported metal catalysts [5][6][7][8][9]. Since cinchona organocatalysts can be tuned, through substitution, to stereodirect and accelerate a wide range of reactions they are classed as privileged chiral catalysts [10].…”