Expanded Radialenes with Bicyclo[4.3.1]decatriene Units: New Precursors to Cyclo[ n ]carbons

A new method for the formation of conjugated polyynes has been developed based on both the rearrangement of vinylidenes to alkynes and the [2+1] cheletropic fragmentation of dialkynylmethylenebicyclo[4.3.1]deca‐1,3,5‐triene derivatives. A model study of the photolysis of simple dialkynylmethylenebicyclo[4.3.1]deca‐1,3,5‐trienes resulted in cheletropic fragmentation followed by 1,2‐migration to give the corresponding linear polyynes, although undesired isomerization to methylenebicyclo[5.3.0]triene derivatives took place concurrently. Expanded [3]‐, [4]‐, [5]‐, and [6]radialene derivatives with exocyclic bicyclo[4.3.1]decatriene units were prepared by oxidative coupling of the monomeric units as precursors to the corresponding cyclo[ n ]carbons, monocyclic forms of carbon clusters. The spectroscopic properties of the expanded radialenes were investigated in connection with cross conjugation of the core π system and with its perturbation by the extraannular bicyclic π system. In negative‐mode laser‐desorption time‐of‐flight (LD‐TOF) mass spectra, the expanded radialenes exhibited peaks due to the corresponding cyclo[ n ]carbon anions ( n = 18, 24, 30, and 36) formed by the stepwise loss of the aromatic indane fragments.