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

The tendency of ion‐pair intermediates generated by treatment of trans ‐10‐bromo‐10, 11‐dihydro‐11‐hydroxy‐5 H ‐dibenzo [ a , d ] cycloheptene (5) with HBr to release Br 2 giving 5 H ‐dibenzo [ a , d ,] cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion‐pair collapse to the trans ‐dibromide (7) and Br 2 release to olefin 1, as compared with the 3:7 ratio found with the 5 H ‐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‐5 H ‐dibenz [ b , f ,] azepine‐5‐carbonyl chloride. Olefin 1 adds Br 2 in 1,2‐dichloroethane at 25°C according to a third‐order rate law, with k 3 = 30(3) M −2 s −1 . About 10% of 5‐bromo‐5 H ,‐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 Br 2 , is presented. the results obtained with the 5 H ‐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 Br 2 with respect to the corresponding symmetrically bridged bromonium ions.