Mass spectrometric screening of chiral catalysts and catalyst mixtures

The use of quasi-enantiomeric substrates and ESI-MS as an analytical tool has made it possible to determine the intrinsic enantioselectivity of chiral catalysts by monitoring catalytic intermediates. In this way, potential problems of methods based on product analysis, which may be caused by catalytically active impurities, partial dissociation of a chiral ligand-metal complex, or a non-catalytic background reaction can be avoided. ESI-MS-based screening is fast, reliable, and operationally simple, as it does not require work-up or purification steps. Moreover, mixtures of catalysts with different molecular masses can be screened simultaneously, which is not possible with methods relying on product analysis. In this way catalyst libraries prepared in one batch by combinatorial methods can be screened without the need to synthesize and purify the catalysts individually. This screening method was successfully applied to Pd-catalyzed allylic substitutions and metal-catalyzed and organocatalytic Diels-Alder reactions.