Synthesis of Molecular Seesaw Balances and the Evaluation of Pyridinium-π Interactions.

A series of molecular seesaw balances 1-5 have been developed to measure the relative strength of pyridinium-π (cation-π) interactions. The cycloaddition of 1-azaanthracene and o-quinodimethane under microwave irradiation afforded the efficient synthesis of 1 and 5. Introduction of substituents to the pyridine ring of balance 1 was achieved to produce 2-4 in good yields. Anion exchange of 1·MeI afforded 1·MeX with a variety of counteranions (X = Cl, Br, I, BF4, PF6, OAc). These balances adopt two distinct conformers, A and B, which are stabilized by a cation-π interaction and a π-π interaction, respectively. The conformer ratio was determined on the basis of the observed averaged 3J coupling constants for H1-C-C-H2 by comparison with the boundary JA and JB values, which were estimated by applying the Carplus-Altona equation to the dihedral angles of the optimized conformers A and B. The effects of the solvent, substituent and counteranion on the ΔG values were elucidated using these molecular balances. Thermodynamic parameters obtained from a van't Hoff plot as well as the electrostatic potential maps for both conformers A and B of the molecular balances helped us to better understand the obtained results.