Abstract
Direct cyclobutylation of 1,6-dihydroazulene gave the spiro hydrocarbon 6, which was subjected to hydride abstraction with a molar equivalent of trityl salt to lead to the title cation 4. Its hexafluorophosphate salt was obtained as crystals. The cation 4 in deuterated acetonitrile was stable at 0°C; however, it underwent expansion of the cyclobutane ring at elevated temperatures to give 2,3-trimethylene-1H-azulenium cation 7. On the other hand, treatment of 6 with an excess of trityl salt gave 1-(4,4,4,-triphenylbutylidene)-1H-azulenium ion ( 11). The cation 4 reacted with pyrrolidine to give the adducts at their seven-membered ring carbons, which gradually rearranged to the adduct at the 2 position under the reaction conditions. Cycloaddition of the latter adduct with dimethyl acetylenedicarboxylate is also described. The cation 4 showed distinctive chemical behavior compared with its three-membered ring homolog.
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