Bulletin of the Korean Chemical Society

Korean Chemical Society (대한화학회)
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$^{13}C-^{1}H$ Coupling Constant as a Criterion for the Presence of $\pi$ Bridging in Substituted 9-Benzonorbornenyl Cation

  • Published : 1993.10.20
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Abstract

The discrete structure of substituted 9-benzonorbornenyl cation 3a and 3c was studied using the empirical ${\Delta}$J equation which was developed by Kelly and coworker$^5$. The ${\Delta}$J values of substituted 9-benzonorbornenyl cations were obtained from p-methyl-6,7-dimethyl benzonorbornen-9-yl (3a) and 9-methyl-6,7-dimethyl benzonorbonen-9-yl (3c) cations under stable ion conditions, and were compared with those of the corresponding ketone analog; these cations were generated by dissolving the corresponding carbinols in superacid at -120$^{\circ}$C and the nmr spectra taken at -60$^{\circ}$C~-90$^{\circ}$C. The ${\Delta}$J values are 8.7 Hz for the bridgehead carbons in cation 3c and 3.1 Hz for cation 3b. The ${\Delta}$J values at C5,8 in fused benzene ring are 14.3 Hz for cation 3c and 8.7 Hz for cation 3a. The excellent correlation of the ${\Delta}$J values with 1$^9F$ chemical shifts of p-fluorophenyl-6,7-dimethylbenzonorbornenyl cation (3d) indicate that ${\Delta}$J value is a reliable probe to charge density at adjacent cationic carbon. These NMR parameters strongly support that the symmetrically ${\pi}$-bridged nonclassical structure (type 2) of substituted 9-benzonorbornenyl cations in stable ion conditions.

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References

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