Synthesis and Pyrolysis of a Triafulvene Precursor

Article abstract of DOI:10.1002/hlca.19930760522

In view of retro-Diels-Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three-step synthesis by dibromocarbene addition at dibenzo-barralene (11-->12; 44percent), halogen-Li exchange followed by methylation (12-->14, 100percent) and HBr elimination (14-->3, 62percent) Scheme 3).Reactivity of the so far unknown bridged 1,1-dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and "carbene dimerizations" (Scheme 4).First experiments with respect to the thermal behaviour of 3 show that RDA reaction, although occuring in most cases, is not the predominant pathway.When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55percent (Scheme 6).On the other hand, gas-phase pyrolysis of 3 at 400 deg mainly produces rearranged 28 (56percent; Scheme 7).It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).