Alkenes neighbouring group participation

This neighbouring group participation by bromine (cf. p. 93) does not of course prove that addition to alkenes proceeds via cyclic bromonium ions, but it does mean that such species are no longer merely ad hoc assumptions, and to that extent are correspondingly more plausible as intermediates. 

Both the reactions are essentially the additions of iodine carboxylate (formed in situ) to an alkene, i.e., the reaction of an alkene with iodine and silver salt. The Prevost procedure employs iodine and silver carboxylate under dry conditions. The initially formed transiodocarboxylate (b) from a cyclic iodonium ion (a) undergoes internal displacement to a common intermediate acylium ion (c). The formation of the diester (d) with retention of configuration provides an example of neighbouring group participation. The diester on subsequent hydrolysis gives a trans-glycol. 

Reactions no. 8 [205] and no. 9 [206] of Table 5-9 show special behaviour. These reactions, which show larger medium effects, involve neighbouring group participation by the alkene or phenylthio groups in the homolytic cleavage of the O—O bond, as shown for the thermolysis of tert-butyl 2-(phenylthio)perbenzoate in Eq. (5-64) [206]. The... 

Gimstone, F.D., and B.S. Perera, The Halogenation of Some Long-Chain Hydroxy Alkenes with Special Reference to the Possibility of Neighbouring Group Participation Leading to Cyclic Ethers and Lactones, Chem. Phys. Lipids 11 43-65 (1973). 

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