SYNTHETIC ATTEMPTS TOWARDS AROMATIC NONAFULVENES

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 1a) 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 1d with R(1) = R(2) = NMe(2)) are strongly influenced by temperature and solvent polarity due to an equilibrium 1 reversible arrow 1(+/-) between the nonpolar olefinic 1 and dipolar planarized 1(+/-). So far, Type-D nonafulvenes occurring exclusively in the dipolar form 1(+/-) 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.