Electrocyclic reactions. Part VIII. Some reactions of trans,trans- and cis,trans-2-bromo-1,5-diphenylpenta-1,4-dien-3-one (α-bromodibenzylideneacetone)

Abstract

Treatment of trans,trans-2-bromo-1,5-diphenylpenta-1,4-dien-3-one and its cis,trans-isomer with hot 10% hydriodic acid–acetic acid gave (by reduction) 1,5-diphenylpentan-3-one and (by cyclisation)trans-3,4-diphenyl-cyclopentanone; since the cis,trans-isomer rearranges to the trans,trans-isomer in the presence of a trace of iodine in refluxing carbon tetrachloride, such rearrangement evidently occurs prior to cyclisation. The trans,trans-isomer was little changed by concentrated sulphuric acid at 0 °C, but at 25 °C gave 2-bromo-3,4-diphenylcyclopent-2-enone (40%), trans-2-bromo-3,4-diphenylcyclopent-4-enone (18%), and 5-hydroxy-2,3-diphenylcyclopent-2-enone [Vorlander's ketol](1%). The cis,trans-isomer reacted readily with concentrated sulphuric acid at 0 °C to give the above-mentioned three compounds in different proportions (23, 13, and 2%), together with the known dimer of 3,4-diphenylcyclopenta-2,4-dienone (8%), and cis-2-bromo-3,4-diphenylcyclopent-4-enone (38%). The formation of these products is discussed in terms of thermal symmetry-allowed conrotatory [_π2_s+_π2_a] electrocyclic reactions of the appropriate cations.