Electrocyclic reactions. Part IV. Some reactions of 2,4-dibromo-1,5-diphenylpenta-1,4-dien-3-one (αα′-dibromodibenzylideneacetone)

Abstract

Treatment of αα′-dibromodibenzylideneacetone (I) with hot hydriodic acid and red phosphorus has been claimed to yield 60% of cis-1,2-diphenylcyclopentane; in fact, the product consists of trans-3,4-diphenylcyclopentanone (24%), which is not reduced by the reagent, 3,4-diphenylcyclopent-2-enone (5%), which is reduced by the reagent to a mixture of cis-1,2-diphenylcyclopentane and a little trans-1,2-diphenylcyclopentane, and a polymeric hydrocarbon of unknown structure (ca. 25%). Treatment of the dibromo-ketone (I) with cold concentrated sulphuric acid furnishes 3-bromo-2-hydroxy-trans-4,5-diphenylcyclopent-2-enone (10%) and trans-2,5-di-bromo-3,4-diphenylcyclopent-2-enone (85%). The primary cyclic products appear to arise by thermal ground state concerted conrotatory [2 + 2] electrocyclic reactions.