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 H-1 NMR spectra, which were analyzed by a flip mechanism; the steric effect of the three a-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.