Synthese und Pyrolyse einer Triafulven‐Vorstufe

Synthesis and Pyrolysis of a Triafulvene Precursor 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‐barrelene ( 11→12 ; 44%), halogen‐Li exchange followed by methylation ( 12→14 , 100%) and HBr elimination ( 14→3 , 62%) ( 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 behavior of 3 show that RDA reaction, although occurring 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 55% ( Scheme 6 ). On the other hand, gas‐phase pyrolysis of 3 at 400° mainly produces rearranged 28 (56%; Scheme 7 ). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 ( Scheme 8 ).