Benzocyclobutadiene dimer from the electrochemical reduction of a,a,a',a'-tetrabromo-o-xylene

Electrochemical reduction (c.p.e.) of α,α,α′,α′-tetrabromo-o-xylene in DMF-0.1 M Et4NClO4 at a Hg cathode, E=−1.40 V vs. SCE, yields 6a, 10b-dihydro-benzo[a] biphenylene, i.e. the rearrangement product of Diels-Alder benzocyclobutadiene dimer. Besides the dimer, a polymer and traces of tri-o-xylylene and tetra-o-xylylene, m/e=306 and m/e =408, are present. Coulometric measurements indicate a four-electron process, with the elimination of four Br−ions/molecule of the starting material. Polarographic measurements on the tetrabromide give two waves with E′1/2=−0.29 V and E″1/2=−1.22 V and a poorly resolved wave with E″1/2=−2.1V. C.p.e. of α,α,α′,α′-tetrabromo-o-xylene at −0.7 V gives 1,2-dibromo-benzocyclobutene (u.v., n.m.r.) and polymeric material. The stoichiometry of this process, involving 3 electrons/molecule of tetrabromide and the release of 3 Br− ions/ molecule, suggests two parallel routes for the reduction of α,α,α′,α′-tetrabromo-o-xylene at −0.7 V. Since the tetrabromide is an equilibrium mixture of two conformers, which do not interconvert readily, each path can be ascribed to a single conformer.