Bromonium transfer, cyclic

There are reasons for believing that this common product mixture from each of the two alkenes—(67) and (70)—does not arise from equilibration of these starting materials before addition proper takes place. It could well be that the higher degree of TRANS stereoselectivity observed (p. 318) for addition at lower temperature, and with higher concentration of HBr, results not from the intervention of a cyclic bromonium radical (71), but from slower rotation about the central carbon-carbon bond. Relatively rapid H transfer (by the higher concentration of HBr) could then take place to the less hindered side of (69) or (73), leading to preferential TRANS additional overall. 

A mechanism that explains anti addition is one in which a bromine molecule transfers a bromine atom to the alkene to form a cyclic bromonium ion and a bromide ion, as shown in step 1 of A Mechanism for the Reaction that follows. The cyclic bromonium ion causes net anti addition, as follows. 

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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