Synthetic Attempts towards ‘Aromatic’ Nonafulvenes

Abstract

According to their spectroscopic behavior, four classes of nonafulvenes may be distinguished, but, so far, only three classes have been identified. Type-A nonafulvenes (including parent la) are typically olefinic molecules with strongly alternating bond lengths and a nonplanar nine-membered ring. Type-B nonafulvenes are characterized by four pairs of equivalent ring H-atoms and ring C-atoms. Spectra of both Type-A and Type-B nonafulvenes are not dependent on temperature and solvent polarity. However, spectra of Type-C nonafulvenes (including prototype Id with R' = R2 = NMe,) are strongly influenced by temperature and solvent polarity due to an equilibrium 1 p l * between the nonpolar olefinic 1 and dipolar planarized l*. So far, Type-D nonafulvenes occurring exclusively in the dipolar form l * were unknown. Synthetic attempts towards nonafulvenes of Type D are described and problems encountered in nonafulvene syntheses are discussed. Several new cyclononatetraenes and four new nonafulvenes (or nonafulvalenes) 31, In, 3, and 5 have been synthesized. Spectroscopic evidence shows that 11,12-bis(diethylamino)nonatriafulvalene 5 is the first Type-D nonafulvene.

The Fulvalenes

“…there is a slight tendency towards cyclopentadienide character. Chai and Neuenschwander [215] have also provided the olefinic 167, the TTF analogue of 11, in 30 % yield. The synthesis (Scheme 37) involves coupling of sodium (c,c,c,t)-157 with (thiomethyl)dithiolium tetrafluoroborate followed by removal of methanethiol with triethylamine.…”

“…More than a decade later Chai and Neuenschwander [215,216] were able to isolate bis(diethylamino)trianonafulvalene (163) as pale yellow cubic crystals. The successful route, almost the same as that described above, employed sodium (c,c,c,t)-cyclononatetraenide (157) with the bis(diethylamino)chlorocyclopropenyl perchlorate as shown in Scheme 35.…”