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.