Enantioselective recognition of dicarboxylic acid guests based on an allosteric effect of a chiral double-decker porphyrin which changes the stoichiometry upon the guest binding

A double-decker porphyrin 1R bearing one chiral substituent per porphyrin ring was synthesised. The host 1R showed highly cooperative binding towards chiral guest 2R with stoichiometry of the complex (1:2) and its stepwise association constants (K1 and K2) were evaluated to be log K1 = 1.6 and log K2 = 4.1, respectively. 1R also recognised 2S cooperatively and formed into a 1:3 complex in log K1 =1.9, log K2 = 1.2 and log K3 = 3.8. When we use racemic 2, 1R exhibits unconventional enantioselectivity only towards 2R in a specific concentration region. It not only showed a chiral screening but also changed the stoichiometry depending on the guest chirality. These high enantioselectivity and stoichiometric modulation are effected by proper incorporation of the structural information of two enantiomers into the host and utilisation of multiple equilibrium characteristic of positive allosterism.