RESUMO
The configuration of 1,2,3,4-tetrabromocyclohexane (mp 90 °C), 5a, has been established as 1,2t,3t,4c by an X-ray crystallographic determination. The structures of two of three dibromocyclohexenes (3a, 3b, and 3c) have been reassigned on the basis of NMR evidence and an X-ray crystallographic determination of the structure of 3b (trans-3,6-dibromocyclohexene). The structures of the two major isomers of 3,4,5,6-tetrabromocyclohexane-1,2-diol obtained by bromination of cis-cyclohexa-3,5-diene-1,2-diol also were established by X-ray crystallographic determinations of their monobenzoate esters. Studies of their formation indicate that the mechanism of bromination of 1,3-cyclohexadiene and cis-cyclohexa-3,5-diene-1,2-diol are similar. Addition of 1 equiv of bromine occurs rapidly by anti 1,2-addition, which is followed by rearrangements to form products of conjugation addition. A second equivalent of bromine adds to afford mostly the 1,2t,3t,4c-tetrabromo compounds at -70 °C and, with cyclohexadiene, the 1,2t,3c,4t-tetrabromo compound at higher temperature.