Ringerweiterung in organischen Molekeln unter Elektronenbeschuss: Die massenspektrometrische Fragmentierung von 2,3‐dicarbalkoxy‐spiro‐cyclopropan‐1, 9′‐fluorenen

Abstract

AbstractRing Expansion of a three‐ to a five‐membered cyclic structure in conjunction with loss of a carbalkoxy radical from the molecular ions is postulated for a series of 2,3‐dicarbal‐koxy‐spiro‐cyclopropane‐1,9′‐fluorenes. Initial rupture of the strained spiro‐cyclopropane system with destruction of the distinct molecular geometry is inferred from virtually identical spectral patterns of, cis‐ and trans‐isomers, whereas reclosure of the ‘open’ intermediate to a cyclopentadiene ring can be deduced from the secondary decomposition of the resulting (M–COOR) fragments. These latter ions exhibit loss of the second carbalkoxy group together with a hydrogen atom of unusual positional origin (1′ or its equivalent 8′) to form a pair of neutral species, ROH and CO. Since the ejection of hydrogen from aromatic positions can be taken as evidence of rearomatisation in the course of substitution, this origin is liable to mark the site of the new ring junction and thus to reflect the size of the enlarged cyclic moiety.