1H‐ and 13C‐NMR Investigations of Heptafulvenes

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Abstract
1H‐ and 13C‐NMR spectra of a series of 8‐R1‐substituted as well as of 8,8‐R1, R2‐disubstituted heptafulvenes, varying from inversely polarized (3l) to unpolar (3h) and polar heptafulvenes with electron‐withdrawing groups (3d, e, f), have been analyzed and compared with those of methoxytropylium salt 5a. The results concerning 3J (H,H) values and 13C‐chemical shifts are shown in Figs. 5 and 6. It turns out that all the NMR parameters are strongly influenced by substituents R1, R2, but contrary to planar pentafulvenes, no linear correlations of the NMR parameter vs. Hammett substituent constant σ+ are obtained in the series 3l → 3d. 3J coupling constants J(2,3)/J(4,5) and J(3,4) are not much influenced by substituent changes in the series 3l → 3h, but are approaching in the row 3h → 3d. Similarly, signals of the 13C‐atoms undergo a moderate shift to higher frequencies in the row 3l → 3h, but are strongly influenced by M groups, whereby the sensitivity is decreasing in the series C(7) > C(2)/C(5) > C(3)/C(4) > C(1)/C(6). These results are essentially explained by a boat conformation of inversely polarized heptafulvenes of the type 3l and an increasing planarization of the ring on going to polar heptafulvenes of type 3d.