Ludmila Birladeanu

Stereochemistry of the methylenecyclopropane rearrangement—II: Exclusion of the concerted 45° pivot mechanisms

Abstract Mechanistic options for the thermal rearrangement of methylenecyclopropane are narrowed further by experiments with the four racemic diastereoisomeric 2-cyano-3-methyl-ethylidenecyclopropanes. The explicit assumption being made that these structural perturbations have neither weakened nor invalidated the mechanistic restrictions imposed by previously published work, it has been concluded that concerted birotational mechanisms involving transition states such as the 45° parallel twisted allylic (Woodward-Hoffmann, Mobius controlled) or the 45° perpendicularly twisted allylic (Berson-Salem subjacent orbital controlled) can be excluded from the presently acceptable set of structurally compatible, chirality-retaining, configuration-inverting mechanisms. A competitive rearrangement of the corresponding carboxylic esters to 1-methyl-2-ethyl-4-methoxyfuran makes these molecules unsuitable for study of the methylenecyclopropane rearrangement. Assignment of stereochemistry to the cyano compounds has been accomplished by application of base-catalyzed equilibration of the cyano center and photochemical iodine-catalyzed equilibration between geometrical isomers.