Acid-catalyzed isomerization of substituted indenes : attempted synthesis of indenylacetone from prop-2-ynylindene
Hydration of 3-prop-2-ynylindene gave a substantial quantity of 1-acetonylideneindane (III). The isomerization of the double bond from the five-membered ring of indene to the exo-position is explained in terms of an equilibrium between acetonylindene (II) and 1-acetonylideneindane (III); this isomerization was catalyzed by acids. A similar equilibrium was found in the acid-catalyzed isomerization of 3-benzylindenes C9H7·CH2Ph ⇌ C9H8CHPh.
Both steric and conformational effects were shown to influence this equilibrium very strongly.