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Generation and Reaction of a Phenyl-Substituted Cyclopentadienyl Cation Annelated with Two Homoadamantene Frameworks

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posted on 18.07.2008, 00:00 by Kohei Ogawa, Shinya Minegishi, Koichi Komatsu, Toshikazu Kitagawa
The generation of the title cation 2+ and its reaction under solvolytic and non-nucleophilic conditions were investigated. When the precursor chlorocyclopentadiene 5 was reacted with silica gel that contained water or with anhydrous MeOH, the corresponding 5-hydroxy- and 5-methoxycyclopentadienes (7 and 6) were produced in 68 and 81% yields, respectively. This indicates that 2+ is formed as an intermediate under solvolytic conditions and persists without any rearrangement of the homoadamantane frameworks, at least during the period before capture by the nucleophile. On the other hand, the abstraction of a chloride ion from 5 by Ag+ in the absence of a nucleophile at −78 °C resulted in the quantitative formation of allyl cation 8+, incorporated in a bicyclo[3.1.0]hexane framework, via the Wagner−Meerwein rearrangement of a homoadamantane framework. Cation 8+ was isolated as the SbF6 salt, and its structure was determined by X-ray crystallography. Quenching this cation with MeOH afforded a methyl ether 14, with a cyclopentadiene structure retained but one of the homoadamantane frameworks had undergone a structural change by a further Wagner−Meerwein rearrangement.