Formation, properties, and reactions of the 1,2:3,4:5,6-Tris(bicyclo[2.2.2]octeno)tropylium ion

Article abstract of DOI:10.1016/S0040-4020(01)96150-2

The titled cation 4 has been synthesized and its properties and chemical reactivities investigated in detail. The cation 4 was shown to be formed in high yields not only by hydride abstraction from but also by the action of halogenoacetic acids upon the cyclohepatriene derivative 6. Based on the results of UV-Vis and NMR spectroscopies together with iodometric analysis, this reaction was shown to proceed via protonation of 6 followed by dehydrogenation with molecular oxygen. The cation 4 was also shown to be formed by the action of rather weak Bronsted acids such as H₃PO₄, H₂CO₃, H₃BO₃, C₆H₅OH, and CH₃NO₂ upon the 7-methoxycycloheptatriene 10. These facts are in agreement with the extraordinarily high thermodynamic stability of 4 as is shown by its pK(R)+ value, 13.0 (50% aqueous CH₃CN, 25°C). The cation 4 does not show any reactivity towards the nucleophiles with the pK(a) of conjugate acid smaller than 10.3 except for the case of a slow reaction with CN^- (pK(a)(NuH) 9.21). Also 4 does not react with the carbanions R^- with pK(a)(RH) below 8.3, such as the tris(7H-dibenzo[c,g]fluorenylidenemethyl)methyl anion (Kuhn's anion, C₆₇H₃₉^-) and the 9-cyanofluorenyl anion, and a new hydrocarbon salt, 4·C₆₇H₃₉^-, was isolated. In contrast, the reaction of 4 with the carbanions R^- with pK(a)(RH) larger than 11, such as dicyanomethyl, α-cyano-p-nitrobenzyl, 1,1-dicyanoethyl, and dibenzo[c,g]fluorenyl anions, afforded the covalent compounds 4-R. Whereas the compound 4-R (as represented by the case of R=dibenzo[c,g]fluorenyl) only regenerated the cation 4 upon treatment with the trityl cation, the 7-methylcycloheptatriene 13 afforded the fully substituted cation 14, which was found to be rather destabilized (pK(R)+ 12.4) as compared with the hexasubstituted cation 4.