Synthesis and Dynamic Stereochemistry of Tris(2-methyl-1-azulenyl)methyl Cation and the Corresponding Methane Derivative. Evidence That the Conjugative Effect Largely Contributes to the Transition State of the Ring Flipping

The stable carbocation, tris(2-methyl-1-azulenyl)methyl hexafluorophosphate (4·PF6−) was prepared by a hydride-abstraction reaction of the corresponding methane derivative, tris(2-methyl-1-azulenyl)methane (5). The dynamic stereochemistry of 4 and 5 was studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism; the steric effect of the three 2-methyl groups was also investigated by comparing it with that of the 3,3′,3″-trimethyl analogue, tris(3-methyl-1-azulenyl)methyl hexafluorophosphate (1b·PF6−). The threshold rotation mechanism for 4 was a two-ring flip, in contrast to a one-ring flip for 1b, and the activation energies for 4 (78.0 and 73.4 kJ mol−1) were higher than those for 1b due to increased crowding in the transition state for the rotation. Although the activation energies for 5 (49.9 and 43.2 kJ mol−1) were lower than those for 1b, the mechanism for 5 was also a two-ring flip. The mechanism was variable between one- and two-ring flip processes due to a steric effect of the three 2-methyl groups. These results indicate that a conjugative interaction between the central cation and the three azulene rings largely contribute to the transition state of the ring flipping as well as to the ground state.