Synthesis, stability and molecular structure of novel hydrocarbon cations consisting of a tropylium cation and two spiro[4.5]deca-2,4-dienes

Two novel hydrocarbon cations with a tropylium ion ring fused by two spiro[4,5]deca-2,4-dienes, 9 and 10, were synthesized by a six-step sequence starting from 1,6- and 1,5-diacetyl-1,3,5-cycloheptatrienes, respectively. The pK_R⁺ values of 9 and 10 were found to be the same at 13.2, providing a new access to highly stable cations. The density functional calculations predict that both 9 and 10 have cyclohexane rings with the chair conformation spread toward molecular edges as the most stable structure. The crystal packing and structure of 9 including dichloromethane were also elucidated by X-ray crystallographic analysis.