Simple and efficient synthesis of bromine-substituted 1,3-dienes and 1,3,5-cycloheptatriene by vacuum pyrolysis of gem-dibromocyclopropanes

To establish whether the results obtained by the gas phase pyrolysis of 6,6-dihalobicyclo[3.1.0]hexanes by HeI photoelectron spectroscopy using a high power CW CO2 laser as a directed heat source can be achieved on a preparatory scale using a modified apparatus, we carried out the gas phase pyrolysis of a few representative gem-dibromocyclopropanes such as 1,1-dibromo-2,2,3,3-tetramethylcyclopropane (1), 1,1-dibromo-2,2-dimethyl-cyclopropane (2), 1,1-dibromo-cis-2,3-dimethylcyclopropane (3), 1,1-dibromo-trans-2,3-dimethylcyclopropane (4), 6,6-dibromobicyclo[3.1.0]hexane (5) and 7,7-dibromobicyclo[4.1.0]heptane (6). Except 7,7-dibromobicyclo[4.1.0]heptane, that gave 1,3,5-cycloheptatriene in 72% yield at 525°C, 1, 2, 3, 4 and 5 readily lose HBr at 400–560°C in the gas phase to produce β-bromo-1,3-dienes in high chemical yields and purity. The dienes are potentially useful starting substrates for the Diels–Alder reactions.