Methylenecyclopropene


3-Methylenecyclopropene, also called methylenecyclopropene or triafulvene, is a hydrocarbon with chemical formula 44. It is a colourless gas that polymerizes readily as a liquid or in solution but is stable as a gas. This highly strained and reactive molecule was synthesized and characterized for the first time in 1984, and has been the subject of considerable experimental and theoretical interest. It is an example of a cross-conjugated alkene, being composed of cyclopropene with an exocyclic double bond attached.

Description

Methylenecyclopropene is the smallest of the fulvenes.
The structure of methylenecyclopropene has two interacting double bonds, which represents the simplest transversally conjugated π-bonding system. It is fundamentally not an alternant hydrocarbon. The value of its dipole moment can be calculated by the Hückel method.
Its study has involved the use of isotopic isomers.

Reactivity

Most fulvenes are typical non-aromatic in nature, having properties closer to alkenes. In the case of tria- and pentafulvene, the possibility of dipole forms of resonance suggests an aromatic character to the cyclic structure; furthermore, as opposed to pentafulvene, one of the triafulvene resonance structures has a negative charge on the exocyclic carbon.
Similarly to heptafulvene, triafulvene polymerizes easily at −20°C and is stabilized by electron-accepting groups bonded to the exocyclic carbon atom.