Conformation of aromatic rings in isolable atropisomers of 2-arylindoline derivatives and kinetic evidences for π–π interaction

The equilibrium constants [K=anti/syn] of a pair of atropisomers due to restricted rotation about Csp3–Csp2 bond for [2-(2-hydroxynaphthalen-1-yl)-3,3-dimethylindolin-1-yl](4-substituted phenyl)methanone were determined in some solvents. The presence of the effective π–π interaction was demonstrated by the correlation between the equilibrium constants (K) and the substituent effect of the phenyl groups (σp), suggesting that the ‘neutral–type’ interaction is operative.