Fluxional cyclopropanones. Approaching measurable equilibrium concentrations of acyclic oxyallyls

Abstract

The preparation of five new cis-2,3-di-R-cyclopropanones is described where R = tert-amyl, thexyl, 2-(2,3,3-trimethylbutyl), 1-adamantyl and 1-(7,7-dimethylnorbornyl). As structure proof for these labile compounds, the [3 + 4] furan adducts were characterized, and the cis cyclopropanones were also converted into the much more stable trans isomers for further characterization. Except for the 1-adamantyl case, all of these cis cyclopropanones exhibit dynamic NMR line-broadening at low temperature due to the reversible conversion into an unseen oxylallyl valence-bond isomer, the first time such behavior has been observed for any cyclopropanone. In the most facile example, that due to the R = 2-(2,3,3-trimethylbutyl) substituent, ΔG ^‡ was found to be only 9.8 kcal mol^–1. This barrier was solvent sensitive and data for this solvent dependence are given. These low ΔG ^‡ barriers also suggest that the oxyallyl equilibrium concentrations are approaching detectable levels.