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

Abstract

A double-decker porphyrin 1 _R bearing one chiral substituent per porphyrin ring was synthesised. The host 1 _R showed highly cooperative binding towards chiral guest 2 _R with stoichiometry of the complex (1:2) and its stepwise association constants (K ₁ and K ₂) were evaluated to be log K ₁ = 1.6 and log K ₂ = 4.1, respectively. 1 _R also recognised 2 _S cooperatively and formed into a 1:3 complex in log K ₁ = 1.9, log K ₂ = 1.2 and log K ₃ = 3.8. When we use racemic 2, 1 _R exhibits unconventional enantioselectivity only towards 2 _R 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.

Keywords:

• host–guest chemistry
• double-decker porphyrin
• enantioselective recognition
• allosteric effect

Acknowledgements

T.I. thanks the JSPS Research Fellowship for Young Scientists for financial support. This study was supported partially by KAKENHI to M.T. from the Ministry of Education, Culture, Science, Sports and Technology, Japan.

Notes

^1. Analysis of the stoichiometry for complex with 2 _S by continuous-variation plots did not indicate a significant change in the CD intensity because the concentration, [1 _R ]+[2 _S ] was too low to form the complex as shown in Figure 3(c). Moreover, this concentration cannot be raised because of the precipitate formation

^2. For the 1:3 1 _R  · (2 _S )₃ complex, the COOH–COOH interaction between the remaining pendent substituents (see also Graphical abstract) would be possible; the theoretical estimation shown in Figure 4 indicates that the CONH–CONH interaction is more likely because of steric orientation of the hydrogen bonding sites. In other words, in the molecular modelling for 1 _R  · (2 _S )₃, formation of the amide dimer is sterically more favourable than the formation of carboxylic acid dimer