16430-63-0Relevant articles and documents
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
Ito, Shunji,Morita, Noboru,Asao, Toyonobu
, p. 1409 - 1436 (2007/10/02)
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 pKR+ 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 pKR+ 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.
