Syntheses and properties of di-, tri-, and tetracyclopropyltropylium and di- and tri-t-butyltropylium ions.

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The polysubstituted tropylium ions having cyclopropyl or t-butyl groups at the ring positions with the least steric hindrance, i.e. 1,3- and 1,4-dicyclopropyltropylium, 1,3- and 1,4-di-t-butyltropylium, 1,3,-5-tricyclopropyl- and 1,3,5-tri-t-butyltropylium and 1,2,4,6-tetracyclopropyltropylium ions (4), were synthesized, and their properties were compared with the known monosubstituted and unsubstituted cations. The 1H and 13C NMR spectra for the cyclopropyl derivatives and the 13C NMR spectra for the t-butyl derivatives exhibited a gradual upfield shift for the seven-membered ring signals upon consecutive substitution. Accordingly, for the t-butyl derivatives, the pKR+ values were found to linearly increase upon introduction of each substuent; for the cyclopropyl derivatives, a larger extent of the pKR+ increase was observed with each substitution, and the pKR+ of the tetrasubstituted derivative 4 amounted to 9.10 (50% aq MeCN, 25 °C), the highest value presently known for the hydrocarbon cations having the tropylium unit, although the cumulative substituent effect deviated from the linear additivity correlation as the substituent number increased. The substituent effects upon the reduction potential and the CT transition energy (pyrene as a donor) were also examined and shown to have the same tendency in general.
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