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.