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Ring current effects in carbanions
Abstract
Substituent-induced ring current effects for two related series of carbanions, 6-(4-X-phenyl)-benzo (3,4) cyclobuta (1,2) cycloheptenyl potassium (6a-c) and 7-(4-X-phenyl)-benzo (3,4) cyclobuta (1,2) cycloheptenyl potassium (7a-c), where X = OCH$\sb3$, CH$\sb3$, and H, were studied by $\sp1$H and $\sp{13}$C NMR spectroscopy. Ring-current shifts were determined by correcting the $\sp1$H shifts for charge-induced shifts calculated from the $\sp{13}$C chemical shifts. The ring current shifts for 6a-c (7a-c), on going from OCH$\sb3$ to H, are upfield (downfield) for the protons in both the benzo and cycloheptatrienyl rings. The upfield (downfield) ring-current shifts for the benzo protons are attributed to a decrease (increase) in the "second-order diamagnetic ring current" contribution from the cycloheptatrienyl ring owing to an increasing (decreasing) HOMO-LUMO energy gap, whereas the upfield (downfield) ring-current shifts for the cycloheptatrienyl ring protons are attributed to an increase (decrease) in the C$\sb{\rm 4b}$C$\sb{\rm 9a}$ $\pi$-bond order resulting from an increased electron withdrawal from the HOMO (SHOMO (second HOMO)), which is antibonding (bonding) between C$\sb{\rm 4b}$ and C$\sb{\rm 9a}$. The increased (decreased) C$\sb{\rm 4b}$C$\sb{\rm 9a}$ $\pi$-bond order increases (decreases) the "antiaromatic" character of the cycloheptatrienyl ring which increases (decreases) the paramagnetic ring current. A concurrent increase (decrease) in the cycloheptatrienyl ring folding is evidenced by changes in the vicinal coupling constants. Ring current effects for the benzocycloheptatrienyl (37) and bicyclo (5.4.1) -dodecapentaenyl (38) anions in ammonia-$d\sb3$ were determined as a function of temperature and counterion. Lowering the temperature increases the paratropicity of 37 owing to a decreased electrostatic interaction between the cation and carbanion. However, lowering the temperature decreases the paratropicity of 38 and the value of $J\sb{8,9}$, an effect that is attributed to an increased HOMO-LUMO energy gap owing to increased ring folding across the C$\sb8$C$\sb9$ and C$\sb{10}$C$\sb{11}$ bonds (where the HOMO is antibonding and the LUMO is bonding). Changing the counterion from potassium to cesium decreases the paratropicity of 37 and 38 owing to an increase in the degree of contact ion pairing for cesium relative to potassium. The decreased paratropicity of 38b (cesium salt) relative to 38a (potassium salt) is further augmented by a more folded conformation for the former as evidenced by the difference in $J\sb{8,9}$ for the two species. These results clearly demonstrate the importance of structure on ring currents in conformationally-flexible paratropic rings.
Subject Area
Organic chemistry
Recommended Citation
Facchine, Kevin Leonard, "Ring current effects in carbanions" (1987). ETD collection for University of Nebraska-Lincoln. AAI8810316.
https://digitalcommons.unl.edu/dissertations/AAI8810316