SYNTHESES OF AZULENE ANALOGS OF TRIPHENYLMETHYL CATION - EXTREMELY STABLE HYDROCARBON CARBOCATIONS AND THE FIRST EXAMPLE OF A ONE-RING FLIP AS THE THRESHOLD ROTATION MECHANISM FOR MOLECULAR PROPELLERS

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, 6-t-butyl, or 3-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 H-1 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 C-3 (<A(A)over bar>) and C-1(<B(B)over bar>) symmetries, respectively), and at higher temperature the NMR reflect the rapid isomerization. The lower activation energy of the process B-->(A) over bar(or (B) over bar-->A), compared with that of B-->(B) over bar, 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.