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 pK(R+) values (10.3-12.7). The dynamic stereochemistries of 5b and 6b were studied by temperature-dependent H-1 NMR spectra, which were analyzed by a flip mechanism. While the H-1 NMR signals of 6b did not resolve satisfactorily, the low-temperature H-1 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.