[5]Radialene

The [n]­radialenes
are a unique family of fundamental
[n]-membered carbocyclic structures with radiating
alkenes, which have attracted significant synthetic and theoretical
attention. Whereas [3]-, [4]-, and [6]­radialenes have been prepared
and studied, all efforts to synthesize the five-membered ring compound
have thus far met with failure. Here we describe the first synthesis
of the fundamental hydrocarbon [5]­radialene, C10H10. Our approach was a departure from previous radialene syntheses
in that it utilized a low-temperature decomplexation of a stable organometallic
compound, rather than high-temperature elimination or rearrangement.
Our strategy was guided by analysis of previous radialene syntheses,
which indicated rapid decomposition in oxygen, and ab initio calculations,
which revealed an extraordinary susceptibility of [5]­radialene to
undergo Diels–Alder dimerization/polymerization. The origin
of this susceptibility was traced to a small distortion energy associated
with the formation of the transition structure geometry from the relaxed
reactant monomers and to a narrow HOMO–LUMO gap.