Cycloallenes, 13 - Cyclohexa-1,2,4-triene from 1-Bromocyclohexa-1,4-diene

1-Bromocyclohexa-1,4-diene (3) was prepared from trans-4,5-dibromocyclohexe ne by elimination of hydrogen bromide. Treatment of a solution of 3 in fura n or 2,5-dimethylfuran with KOtBu afforded the tetrahydroepoxynaphthalenes 4 and 5, respectively. The structure of these products is evidence for the title cycloallene (isobenzene 1) being the reactive intermediate. The compo unds 4 and 5 were dehydrogenated by DDQ to the known dihydroepoxynaphthalen es 6 and 7, respectively. These conversions unambiguously confirm the struc tures of 4 and 5. In pure styrene, 3 was not attacked by KOtBu, and only up on the addition of 18-crown-6 did a reaction occur. This reaction did not, however, furnish the known [2+2] cycloadduct 9 of styrene and 1, but, inste ad, a small amount of 1,2-diphenylethane (8) was formed. In agreement with this finding, the conjecture that 1 was deprotonated to give the phenyl ani on was confirmed by the treatment of a solution of 3 and benzophenone in TH F with KOtBu, which resulted in the formation of triphenylmethanol.