Factors affecting the reversibility of the electrophilic step in olefin bromination. The case of 5H-Dibenzo[a,d]cycloheptene.

The tendency of ion-pair intermediates generated by treatment of trans-10-bromo-10,11-dihydro-11-hydroxy-5H-dibenzo[a,d]cycloheptene (5) with HBr to release Br2 giving 5H-dibenzo[a,d]cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion-pair collapse to the trans-dibromide (7) and Br2 release to olefin 1, as compared with the 3:7 ratio found with the 5H-dibenz[b,f]azepine-5-carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion-pair formation in the bromination of 1. The structural parameters obtained by x-ray diffraction of the dibromide 7 show for this compound an angle strain at C-10 and C-11 analogous to that found in trans-10,11-dibromo-10,11-dihydro-5H-dibenz[b,f]azepine-5-carbonyl chloride. Olefin 1 adds Br2 in 1,2-dichloroethane at 25-degrees-C according to a third-order rate law, with k3 = 30(3) M-2 s-1. About 10% of 5-bromo-5H-dibenzo[a,d]cycloheptene, arising from collapse of a dibenzo[a,d]tropylium ion intermediate (9), is formed in this reaction besides the expected trans-dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H-dibenzo[a,d]cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and Br2 with respect to the corresponding symmetrically bridged bromonium ions.