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摘要
The influence of exocyclic substituents on pi-delocalization of pentafulvenes 2, heptafulvenes 3, and nonafulvenes 4 has been investigated. Pentafulvenes 2: Changes of bond lengths (induced by exocyclic substituents R-1 and R-2 of 2) are reflected by systematic changes of (3)J(H,H) (Fig. 2) as well as of (1)J(C,C) coupling constants (Fig. 4), so that linear correlations of sigma(+)(p) vs. (3)J(H, H) and (1)J(C,C) coupling constants were obtained. Plots of that type are very useful for determining the extent of pi-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 sigma(+)(p) vs. (3)J(H, H)-plots (Fig. 8) or sigma(+)(p) vs. delta(C-13)-plots (Fig. 9), although substituents R1, R2 clearly influenced (3)J(H, H)-coupling constants as well as C-13 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), (3)J(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 sigma(+)(p)-values above -0.5 to 0, strong substituent effects started to be effective, and a nearly linear approach of (3)J(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 sigma(+)(p) vs. C-13-chemical shifts around sigma(+)(p) = 0 (Fig. 9): In the range of strong electron-accepting groups (above sigma(+)(p) = 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 H-1-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 pi-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.
- 出版日期2015-6
