Minor Enantiomer Recycling-Effect of Two Reinforcing Catalysts on Product Yield and Enantiomeric Excess

Kinetic modeling of a recycling procedure in which the minor product enantiomer from an enantioselective catalytic reaction is selectively retransformed to starting material by a second chiral catalyst demonstrates that the enantiomeric excess of the product is not affected by the relative amounts of the two catalysts, but that the yield increases when the amount of the catalyst for the product‐forming reaction is increased. The yield, but not the enantiomeric excess, is also affected by the initial substrate concentration. The recycling process is compared to sequential processes in which either the second catalyst is added after completion of the first reaction or in which the two catalysts are added simultaneously. In the sequential processes, high enantioselectivity can be obtained at the expense of product yield, whereas under recycling conditions both high enantiomeric excess and high yield can be achieved. Experimental data from a recycling procedure providing qualitative support for results from kinetic modeling are presented. The art of combining catalysts: The undesired enantiomer produced from an achiral substrate molecule is selectively converted back to starting material, which thereby receives a second chance in the catalytic reaction. The final yield of the product is influenced by the relative rates of the forward and backward reactions and by the substrate concentration, whereas the final enantiomeric excess is a function of only the selectivities of the two steps.