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 preparat ory scale using a modified apparatus, we carried out the gas phase pyrolysi s of a few representative gem-dibromocyclopropanes such as 1,1-dibromo-2,2, 3,3-tetramethylcycloprop (1), 1,1-dibromo-2,2-dimethyl-cyclopropane (2), 1, 1-dibromo-cis-2,3-dimethylcyclopropane (3), 1,1-diblomo-trans-2,3-dimethylc yclopropane (4), 6,6-dibromobicydo[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 degreesC, 1, 2, 3, 4 and 5 readily l ose HBr at 400-560 degreesC in the gas phase to produce beta -bromo-1,3-die nes in high chemical yields and purity. The dienes are potentially useful s tarting substrates for the Diels-Alder reactions. (C) 2001 Published by Els evier Science Ltd.