engleski

The bond between inverted carbon atoms. Synthesis and chemistry of 2, 4-methano-2, 4-didehydroadamantane: a highly reactive [3.1.1]propellane

In order to provide a better insight into the reactivity and nature of the bond between inverted carbon atoms, we prepared 2, 4-methano-2, 4-didehydroadamantan(e5 ) , a prototype small propellane, and systematically examined its chemical behavior. Propellane 5 was obtained in 70% yield by pyrolysis of the dry sodium salt of the tosylhydrazone derived from 4-methylene-2-adamantanone in vacuo. It was rather stable thermally and entirely inert toward nucleophiles but highly reactive toward electrophiles and free radicals. With acetic acid, hydrogen chloride, and methanol, 5 yielded mixtures of the corresponding 2-anti-substituted 4-methyleneadamantane (7), 2-substituted 2, 4-methanoadamantane (8), and 2-(substituted-methyI) 2, 4-didehydroadamantane (9), while carbon tetrachloride, tosyl chloride, and thiophenol gave exclusively the corresponding 2, 4-disubstituted 2, 4-methanoadamantanes (loa-c) through a free radical process. The electron-transfer hydrogenation yielded 2, 4-methanoadamantane (11) as the only product. Both acids and free radicals attack the central bond producing the intermediary bridged 2, 4-methano-2-adamantyl cation and the 4-substituted 2, 4-methano-2-adamantyl free radicals, respectively. These results indicate a high electron density at the reaction site, i.e., at the back side of the inverted carbon atoms, and, consequently, a decrease in electron density between them. Such a bond appears to be a limiting form of the carbon-carbon single bond, while the other limiting form is the usual single bond between two carbon atoms in aliphatic compounds. All other carbon-carbon single bonds should necessarily lie between these two extremes.

2, 4-methano-2, 4-didehydroadamantane, [3.1.1]propellane

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