摘要：

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.

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