Dicyclooctatetraeno[1,2:4,5]benzene dianion and tetraanion. Experimental assessment of extended paratropic vs. restricted diatropic .pi.-electron delocalization

Bicyclo[6.3.0]undecapentaenyl Anion: The Next Higher Homolog of the Indenyl Anion with Exceptionally Large Ion‐Pairing Effects on its Tropicity

“…The chemical shifts of the five‐membered ring carbon atoms (C1 and C8–C11) appear at higher field than those of the eight‐membered ring carbon atoms (C2–C7). The chemical shifts of the eight‐membered ring carbon atoms are almost the same as those of 4 ,4 which indicates that the negative charge is mainly localized on the five‐membered ring carbon atoms. The C10 carbon resonates at the highest magnetic field, which is consistent with theoretical calculations.…”

“…Thus, the effect of any paratropic ring currents associated with peripheral conjugation does not seem to be large under these conditions. On the other hand, compared with the average chemical shifts of the eight‐membered ring protons of 4 ( δ =6.09 ppm),4 the value for 1⋅ Li + in [D 8 ]THF ( δ =4.77 ppm) is shifted by 1.32 ppm to higher magnetic field. This result indicates that the eight‐membered ring protons are highly sensitive to local ring current effects.…”

“…These relatively large upfield shifts are contrary to those of benzo-COT 4. [4] The eight-membered ring protons of 18 are observed at lower magnetic field than those of 4; the aniso-tropic effect of the azulene moiety presumably accounts for the exceptionally low magnetic field resonances of the 1-and 6protons (d % 6.70 ppm). The degree of variation in the chemical shifts of the eight-membered ring protons is not as large as those of the azulene protons of 18.…”

“…The counterbalance between the partial aromatic character of the five-membered ring and the peripheral antiaromatic character should determine the overall electronic and magnetic properties of 1. [4,5] One of the other isoelectronic relatives of 1, the bicycloA C H T U N G T R E N N U N G [6.5.0]tridecahexaenyl cation 5, suffers from valence isomerization to a tricyclic isomer 6 by a symmetry-allowed pericyclic ring-closure reaction to escape antiaromatic destabilization. [6] Spectral data for 1 provide no evidence for the presence of tricyclic isomer 7.…”

“…This tendency should increase the 4 n π‐electron character of the overall molecule and perturb the electronic properties of the cyclopentadienyl anion moiety as a [4 n +2] π‐electron system. The counterbalance between the partial aromatic character of the five‐membered ring and the peripheral antiaromatic character should determine the overall electronic and magnetic properties of 1 4. 5 One of the other isoelectronic relatives of 1 , the bicyclo[6.5.0]tridecahexaenyl cation 5 , suffers from valence isomerization to a tricyclic isomer 6 by a symmetry‐allowed pericyclic ring‐closure reaction to escape antiaromatic destabilization 6.…”