A new synthesis of in situ cyclopropanones and the observation of a thermal cyclopropanone–dienol rearrangement

A new in situ low-temperature synthesis of cyclopropanones from α,α′-dibromo ketones is described. The synthetic procedure is particularly well suited to NMR tube-scale experiments, which then easily allows one to spectroscopically characterize low-temperature solutions of these unstable molecules. Nine systems were studied, seven of which give high yields of the cyclopropanones at −78 °C or, in one case, −93 °C. The major rearrangement pathway of these cyclopropanones leads to an α,β-unsaturated ketone. The mechanism of this reaction has been studied in some detail, including deuterium isotope studies, and the direct observation of enols, silyl enol ethers, and dimers. The rearrangement reaction is postulated to involve the intermediacy of an oxyallyl, the transfer of a hydrogen from the anti-alkyl substituent of this oxyallyl to the oxyallyl oxygen atom, which leads to an enol, and then finally tautomerization of the enol to the ketone. The thermal stability of the cyclopropanones is directly related to the ease of this rearrangement and, in two cases studied here, the rearrangement takes place rapidly even at −95 °C and the cyclopropanones could not be observed. There is a remarkably strong correlation between cyclopropanone structure and reactivity, with the main structural factor being the external angles of the cyclopropanone substituents.