Bromonium ions, bromination without

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

The bromonium ion may be generated by other methods than direct bromination solvolytic reaction of trans-2-bromo-[(4-bromophenyl)sulphonyl]cyclohexane (36) (and cyclopentane) forms (reaction 7) the bromonium ion94 (37). If Br- is present in the reaction mixture, the generation of Br2 (and olefin) is observed (Scheme 14). This confirms the reversibility of the bromonium ion formation in the usual bromination pathway. When other olefin scavengers are present, a formal Br+ transfer is observed94. This may occur without the formation of Bit. 

Instead of a bromonium ion, in certain cases an isomeric acyclic and sufficiently stable cation can occur as an intermediate of the bromine addition to alkenes. This holds true for bromine-containing benzyl cations. Therefore, the bromine addition to /J-methyl styrene shown in Figures 3.5 and 3.6 takes place without stereocontrol. 

We drew the conclusion that, at 0.2 mM, the concentration is too dilute for the Br transfer depicted in panel D to occur. Each alkene was therefore reacting independently, and without the intmsion of nondegenerate Br" transfer, the reactimis occurring according to their rate constants [41, 42]. As noted above. Brown [46, 48] had previously observed cyclic bromonium ion transfer with highly hindered alkenes, notably adamantylideneadamantane[s]. But Fig. 3 makes it evident that unhindered alkenes are also candidates, and thus bromonium ion transfer appears to be a hitherto unrecognized phenomenon in the bromination of alkenes [47,49]. 

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