Thermally induced cyclopropene-carbene rearrangements: An overview

The thermal ring opening of cyclopropenes provides a template for the examination of the core reactions of vinylidenes and vinylcarbenes and, to a rather lesser extent, of cyclopropylidenes. The introduction of a range of substituents can dramatically reduce the temperature at which ring opening occurs. Thus, simple 3,3-dialkoxycyclopropenes ring-open at about 80°C, and the addition of selected substituents leads to reaction even at 0°C, while 1,2-dihalocyclopropenes react at 0-20°C. Alkyl- or aryl-substituted 3-arylcyclopropenes generally rearrange at temperatures of 170-200°C and lead to indenes by apparent cyclization of a vinyl-carbene intermediate, which in some cases can be trapped in intramolecular processes. The corresponding 1,2-dichloro-3-arylcloropenes undergo a similar cyclization and cannot be isolated under the conditions of their generation at below ambient temperature.