NMR-Spectroscopic Investigations of Potentially Planarized Nonafulvenes

¹H- and ¹³C-NMR spectra of nonafulvene 1e and nonafulvenes 2 and 3 have been assigned, high-resolution ¹H-NMR spectra of 2 (600 MHz, Fig. 3) and of 3 (400 MHz, Fig. 2) have been analyzed, and the data are compared with those of other nonafulvenes (Tables 1–6). Generally speaking, according to their spectroscopic behavior, four classes of nonafulvenes (A–D) may be distinguished (Fig. 1). The investigation shows that compounds 1e and 3 belong to class A, being characterized by ¹H-chemical shifts around 6 ppm, strongly alternating ³J(H,H) and ¹³C chemical shifts in the range of 123 to 130 ppm, thus existing in the olefinic form with a non-planar nine-membered ring. On the other hand, 2 is the first nonafulvene of class D, being characterized by ¹H chemical shifts in the aromatic range, large ³J(H,H) values of the same size, and ¹³C chemical shifts around 110 ppm. Since NMR parameters are virtually not influenced by temperature (?50° to 50°) or solvents, it is concluded that 2 exclusively exists in the dipolar structure 2 ^± with a planarized nine-membered ring. According to Fig. 4, these classes (and their spectroscopic data) are linked by 10,10-bis(dimethylamino)nonafulvene ( 1c ; and its temperature-dependent NMR parameters): for 1c , a temperature-dependent equilibrium 1c?1c ^± had earlier been established.