Synthesis and Dynamic Stereochemistry of Di-1-azulenyl(1- and 2-naphthyl)methyl Hexafluorophosphates. Clear Evidence of the One-Ring Flip Mechanism of a Molecular Propeller by Controlling the Flipping Ring

In order to control the flipping ring of a molecular propeller and to obtain clear evidence of a one-ring flip mechanism, stable carbocations, di-1-azulenyl(1-naphthyl and 2-naphthyl)methyl cations, bis(3-methyl-1-azulenyl)(1-naphthyl (5b) and 2-naphthyl (6b))methyl cations, and bis(3,6-di-t-butyl-1-azulenyl)(1-naphthyl and 2-naphthyl)methyl cations were synthesized by hydride abstraction of the corresponding hydrocarbons. These cations showed extreme stabilities with high pKR+ values (10.3—12.7). The dynamic stereochemistries of 5b and 6b were studied by temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism. While the 1H NMR signals of 6b did not resolve satisfactorily, the low-temperature 1H NMR spectra of 5b showed that the flipping ring of the molecular propeller was controlled by a one-ring flip of the 1-naphthyl group. These analyses afforded clear evidence of the one-ring flip mechanisms of (1-azulenyl)methyl cations.