Dynamic equilibrium between ion pairs of aromatic [9]annulene anion and of olefinic nonafulvene structure

1H‐ and 13C‐NMR Investigation of Nonafulvenes

“…Bearing in mind the well known influence of H-C-C bond angles on ' J coupling constants [19], a tentative argument is that this effect could counterbalance the expected decrease of 3J values due to delocalisation. Summarising, the NMR results presented here are in good agreement with an equilibrium of the type l k e l k ' as suggested by Boche [7]: in unpolar solvents and at ambient temperature, chemical shifts and coupling constants as well as substituent effects of l k are very similar to those of olefinic nonafulvenes la-h. On the other hand, the NMR spectra measured in CD,Cl, solution at -80" to -90" are consistent with a dipolar amidino-cyclononatetraenide lk'. Whereas the "C-NMR shifts suggest an increase of negative charge of the ring C-atoms, the low-field shift of ring protons seems to be due to the ring-current effect of the cyclononatetraenide unit, and the pronounced increase of 3J coupling constants of formal single bonds reflects an increasing ring flattening and 71 delocalisation.…”

“…Solvent and temperature effects of such an extent are not known so far in other cross-conjugated systems such as pentafulvenes 4 or heptafulvenes. On the other hand, similar effects have been observed by Boche et al [7] for acyl-cyclononatetraenides and have been attributed to an equilibrium between the nonafulvenolate 3 existing as contact ion-pair, and the solvent separated ion pair of the acyl-cyclononatetraenide 3*, the latter being preferred in polar solvents at low temperature. Subsequently, Boche suggested a similar equilibrium between l k and lk* [7] [8].…”

“…According to Fig. 2, some J values of 10-methoxynonafulvene (lc) are surprising in so far, that J (2,3) and J (6,7), or J (1,2) and J (7,8) are approaching each other. These deviations of a nonafulvene bearing a smaller substituent could result from a higher portion of lc" in the equilibrium of conformers 1c'i.tlc".…”

“…While vinylic coupling constants lying in the range of 11. 8-13.2 Hz are only slightly affected by substituents R (with exception of J (1,2)), J (6,7) is nearly 2 times as large as J (2,3) and J (4,s) in the spectra of nonafulvenes l b and Id. Bearing in mind the well known Karplus equation [14], the conclusion is that the dihedral angle between H-C(7), H-C(6) is comparably small, whereas the angle of the other fragments H-C(2), H-C(3) and H-C(4), H-C(5) is comparably large (near 90").…”

“…Furthermore, since there exists an 1 : 1 population of nonafulvenes 1' and I", substituent effects (e.g. on C (7,2) or J (2,3)/J (6,7)) should be smaller than for nonafulvenes lM and felt on both sides of the ring. According to ' ) Sign not determined because of first-order spectrum.…”