Asymmetric ion-pairing catalysis of the reversible cyclization of 2'-hydroxychalcone to flavanone: Asymmetric catalysis of an equilibrating reaction

The asymmetric catalytic cyclization of the simple 2'-hydroxychalcone (1) to flavanone (2), a model for the chalcone isomerase reaction, has been realized as a catalytic asymmetric ion-pairing process with chiral quaternary ammonium salts (e.g., 9-anthracenylmethlycinchoninium chloride; 9-Am-CN-Cl) and NaH as small-molecule co-catalyst. In toluene/CHCl ₃ solution, the process reaches an intrinsic enantioselectivity of up to S = 14.4 (er = 93.5:6.5). The reversible reaction proceeds in two steps: A fast initial reaction approaches a quasi-equilibrium with K _R/S = 4.5, followed by a second, slow racemization phase approaching K _rac = 9. A simple mechanistic model featuring a living ion-pairing catalysis with full reversibility is proposed. Deuterium transfer from co-solvent CDCl ₃ to product 2 and isolation of a Michael conjugate formed from 2 and 1 demonstrate the intermediacy of flavanone enolate ion pairs. A kinetic model shows good agreement with the experimentally observed, peculiar, time-dependent evolution of the species concentrations and the enantiomeric excess of 2. The reaction is a chemical model of the chalcone isomerase enzymatic reaction. Furthermore, it is an ideal model for studying the characteristic behavior of reversible asymmetric catalyses close to their equilibria.