Substituent Effects on π‐Bond Delocalization of Fulvenes and Fulvalenes. Are Fulvenes Aromatic?

Abstract

AbstractThe influence of exocyclic substituents on π‐delocalization of pentafulvenes 2, heptafulvenes 3, and nonafulvenes 4 has been investigated.Pentafulvenes 2: Changes of bond lengths (induced by exocyclic substituents R1 and R2 of 2) are reflected by systematic changes of 3J(H,H) (Fig. 2) as well as of 1J(C,C) coupling constants (Fig. 4), so that linear correlations of σp+ vs. 3J(H,H) and 1J(C,C) coupling constants were obtained. Plots of that type are very useful for determining the extent of π‐delocalization of various pentafulvalenes 5–8 (Figs. 6 and 12). Charge density effects of pentafulvenes and pentafulvalenes were observed by substituent‐induced shifts of the ring C‐atoms (Fig. 5). Heptafulvenes 3: Contrary to planar pentafulvenes, heptafulvenes did not show any linear correlations of σp+ vs. 3J(H,H)‐plots (Fig. 8) or σp+ vs. δ(13C)‐plots (Fig. 9), although substituents R1, R2 clearly influenced 3J(H,H)‐coupling constants as well as 13C chemical shifts of the ring H‐atoms and ring C‐atoms. In the NMR spectra of ‘heptafulvenes with inverse ring polarization’ (in the lower range of Fig. 8), 3J(H,H)‐coupling constants were strongly alternating and were barely influenced by exocyclic substituents. This supported a boat conformation of the corresponding heptafulvenes. In the range of Hammett σp+‐values above −0.5 to 0, strong substituent effects started to be effective, and a nearly linear approach of 3J(H,H)‐coupling constants J(2,3)/J(4,5) and J(3,4) was observed. This meant that, as soon as heptafulvenes were planar or nearly planar, there existed similar substituent effects as for planar pentafulvenes. – A similar ‘turning point’ was observed in plots of σp+ vs. 13C‐chemical shifts around σp+=0 (Fig. 9): In the range of strong electron‐accepting groups (above σp+=1), there was a marked substituent‐induced high‐frequency shift which strongly decreased in the series C(7)>C(2)/C(5)>C(3)/C(4), while C(1)/C(6) was barely influenced. Nonafulvenes 4: Most nonafulvenes are non‐planar olefins with strongly alternating vicinal H,H‐coupling constants. This has been convincingly shown by the high‐resolution 1H‐NMR spectrum of 10‐dimethylaminononafulvene (4c, Fig. 10), which was not planar but contained a nearly planar (E)‐dienamine substructure of the segment C(7)C(8)C(9)C(10)NMe2 according to the NMR data. Only with very strong π‐donors (like two dimethylamino groups in 4b), planarization of the nine‐membered ring could be observed at low temperatures (Fig. 10). Finally, the first stable nonatriafulvalene (11,12‐bis(diethylamino)nonatriafulvalene (10)) existed in the planar dipolar form in the whole temperature range and even in unpolar solvents.