Thermal rearrangement of 9,9-dibromobicyclo[6,1,0]non-4-ene and related compounds

Abstract

9,9-Dibromobicyclo[6,1,0]non-4-ene (IV) isomerizes in hot quinoline solution to give 1,9-dibromocyclonona-1,5-diene (V) or (VI). The rearrangement proceeds under unexpectedly mild conditions and without the loss of the elements of hydrogen bromide. The product, which is probably the cis, cis-isomer (V), has a notably unreactive allylic bromo-substituent; it is quantitatively converted into cyclonona-1,2,6-triene (VIII) by treatment with Zn–MeOH, LiMe–Et₂O, and also, surprisingly, with LiAlH₄–Et₂O.

9,9-Dichlorobicyclo[6,1,0]non-4-ene (XXIII) likewise isomerizesin hot lepidine solution to give 1,9-di-chlorocyclonona-1,5-diene (XXIV) or its trans,cis-isomer). Although endo-9-chlorobicyclo[6,1,0]non-4-ene (XIIb) is comparatively stable to heat, the exo-isomer (XIIIb) rearranges at 165–170° to give a mixture of the epimeric cis-1,2-divinyl-3-chlorocyclopentanes [(XIV and (XV)]. It is possible that trans, cis-3-chlorocyclonona-1,6-diene (XVI) is an intermediate in the rearrangement of (XIIIb).