Thermal Rearrangements, XIII [1]. Thermal Isomerization of Exocyclic Allenes

The thermal isomerization of several exocyclic mono‐ and bis‐allenes in a high‐temperature flow system has been investigated. At 600°C vinylidenecyclobutane (10) rearranges cleanly to the allene dimer 1,2‐bismethylenecyclobutane (12) by an allene‐diene isomerization. The methylallene 16 behaves analogously (600°C), although the primary product 18 does not survive under the reaction conditions: the isolated cross‐conjugated triene 20 is a product of a 1,5‐hydrogen shift followed by an electrocylic ring‐opening reaction. Whereas vinylidenecyclopentane (27) yields aromatic products only (800°C, toluene (36), benzene (38)), the five‐ring 1,3‐bisallene 30 isomerizes to the cyclopentene derivative 43 at 650°C. Vinylidenecyclohexane (46) affords only aromatic products again (760°C, o ‐xylene (57), 36, 38) and 1,3‐bisvinylidenecyclohexane (49) isomerizes to the dienynes 60 and 61 by a series of hydrogen shifts at 550°C. On the other hand, the allene‐diene isomerization is observed again, when a C–C bond is doubly activated by an allene and methylene group as in 52 or by two allene moieties as in 53: at 500°C the triene 63 and the exocyclic tetraene 65, respectively, are obtained in good yield.