Synthesis and rearrangement of alkylaryl- and aryl-substituted dihydrosemibullvalenes by thermolysis of 7,8-fused cyclo-octa-1,3,5-triene derivatives

Abstract

The thermal cycloaddition of a cyclobuteno-dienophile (1) with cyclopentadienones has been systematically investigated; the stereoisomeric adducts give convenient access by decarbonylation to a variety of tetra-aryl-, methyltriphenyl-, triphenyl-, tri-t-butyl-, and dimethyldiphenyl-cyclo-octa-1,3,5-triene derivatives as the products of electrocyclic ring-opening of the valence-tautomeric primary products of cheletropic bridge-extrusion, viz. bicyclo[4.2.0]octadienes. These compounds provide useful models for investigation of equilibria between the electrocyclic valence tautomers, the scope and mechanism for thermal cross-cyclisations in, for example, unsymmetrically substituted cyclo-octatrienes, and the thermal vinyl–cyclopropane (1,3-allylic shift) isomerisation and/or H-atom transfer disproportionation of the resulting arylated and alkylaryldihydrosemibullvalenes. The results best accord with diradical pathways for cyclo-octatriene–dihydrosemibullvalene conversions and subsequent rearrangements. Useful ¹³C and ¹H n.m.r. structure correlations and new examples of cyclopentadienones are also reported.