Photochemical studies—XXI : Dimers of 3,4-(o,o'-biphenylene)cyclopentadienones: Thermal and photochemical behaviour

The 1,4-dimethyl-, 1,2,4-trimethyl and 1,2,4,7-tetramethyl substituted 3,4-(o,o'-biphenylene) cyclopentadienone-dimers (6a–6c) were prepared and found to undergo photochemically or thermally a 1,3-rearrangement to the centrosymmetrical diketones (11). Only the tetramethyl derivative (6a) undergoes at room temperature a fast, degenerate [3.3] Cope rearrangement with ΔG^≠₂₃₃ = 11.4 kcal mol.^?1 All these dimers and rearrangement products appear not to dissociate to their monomers, but react with dienophiles to give the adducts (12, 13). A stepwise mechanism involving diradical intermediates (19) is invoked. The “mixed dimer” (14) was also prepared and studied. The steric and electronic effects determining the behaviour of these compounds are discussed.