Halogenation neighbouring group participation

Neighbouring group participation by carbonyl groups in ester hydrolysis, 28, 171 Nitration, nitrosation, and halogenation, diffusion control and pre-association in, 16, 1 Nitrosation, mechanisms, 19, 381... 

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). 

Neighbouring-group participation in the halogenation of various tetramethyl bicyclo[2,2,2]oct-2-en-5,6,7,8-tetracarboxylates has been investigated. Thus, for example, bromination of (594) gave (595), whereas similar reaction of (596) afforded the dibromo-adduct (597). On the other hand, reaction of (596) with Clj or ICl gave the halogenolactone (598 X = Cl or I) mechanism are discussed. 

Just as in the case ofa-participation, the electrons of the neighbouring 7t-bond interact covalently with a carbon from which a halogen, a tosylate or other leaving group detaches, contributing thereby to the bond fission. This interaction (intramolecular nucleophilic substitution) occurs at the backside of the leaving group. 

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