Syntheses of Azulene Analogues of Triphenylmethyl Cation: Extremely Stable Hydrocarbon Carbocations and the First Example of a One-Ring Flip as the Threshold Rotation Mechanism for Molecular Propellers

Article abstract of DOI:10.1246/bcsj.68.1409

A series of azulene analogues of triphenylmethyl cation (tri(1-azulenyl)methyl, di(1-azulenyl)phenylmethyl, and (1-azulenyl)diphenylmethyl hexafluorophosphates) were synthesized by hydride abstraction from the corresponding methane derivatives with DDQ.In order to examine the effect of substituents on the cations, and to enhance their stabilities, a series of cations bearing 3-methyl, 3-methoxycarbonyl, 3,6-di-t-butyl, or 3-t-butyl, 6-t-butyl groups on each of the azulene rings were also synthesized.Their pK_R+ values showed that the stabilities of these cations dramatically increase with the number of azulene rings.Tris(3,6-di-t-butyl-1-azulenyl)methyl cation showed the highest pK_R+ value (14.3) ever observed.The high stabilities of these cations were attributed to a large conjugative effect between the central cation and the azulene ring(s).The dynamic stereochemistry of these cations was also studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism.Low-temperature NMR studies indicated that tri(1-azulenyl)methyl cations exist in two types of propeller conformations (symmetrical and unsymmetrical propellers, which have C₃(A<*>) and C₁(B<*>) symmetries, respectively), and at higher temperature the NMR reflect the rapid isomerization.The lower ativation energy of the process B-><*>(or<*>->A), compared with that of B-><*>, indicates that the threshold rotation mechanism for the cation is a one-ring flip.This is the first example of a molecular propeller with a threshold rotation mechanism comprising a one-ring flip.