Reversibility of Bromonium Ion Formation and Its Effect on Olefin Reactivity in Electrophilic Bromination. New Evidence from the 5H-Dibenzazepine System

Article abstract of DOI:10.1021/ja00210a039

It has been shown that the bromonium-bromide ion-pair intermediate generated from the reaction of trans-10-bromo-10,11-dihydro-11-hydroxy-5H-dibenzazepine-5-carbonyl chloride (3) with HBr in 1,2-dichloroethane, chloroform, or carbon tetrachloride can either collapse to trans-10,11-dibromo-10,11-dihydro-5H-dibenz/b,f/azepine-5-carbonyl chloride (4) or release molecular Br2 to give 5H-dibenzazepine-5-carbonyl chloride (1).The ratio of these two products changes from ca. 7:3 in 1,2-dichloroethane to ca. 7:3 in carbon tetrachloride.Olefin 1 is similarly obtained in 50 percent yield when bromohydrin 3 is reacted with BF3.Et2O in chloroform in the presence of resorcinol, which acts as a Br⁺ scavenger and is transformed into its 4-bromo derivative. 5H-Dibenzazepine-5-carbonyl chloride adds Br2 in 1,2-dichloroethane at 50 deg C according to a third-order rate law with a rate constant (K₃=1.9(0.1)E-2 M_-2s_-1) that is 4 orders of magniture lower with respect to than of the acyclic analogue cis-stilbene and gives only the trans-dibromide 4.The latter is debrominated easily to olefin 1 at 25 deg C in dimethylformamide without the addition of any specific reagent or slowly in acetonitrile in the presence of cis-stilbene as a Br2 scaverger, through the same bromonium-bromide ion-couple intermediate involved in the reaction of bromohydrin 3 with HBr.The easy reversion of this intermediate back to olefin and Br2 has been rationalized on the basis of the structural parameters obtained by X-ray diffraction of dibromide 4, showing a highly strained C(10)-C(11)-C(11a) internal angle of 121 deg and strongly unequivalent bromine atoms, with a rather longer Br-C(11) bond.