Regioselectivity halogen atom attack

When halogenation of alkenes is carried out in aqueous solvent, a vicinal halohydrm is obtained. The reaction is regioselective, and follows the Markovnikov rule. The halide adds to the less substituted carbon atom via a bridged halonium ion intermediate, and the hydroxyl adds to the more substituted carbon atom. The reaction mechanism is similar to the halogenation of alkenes, except that instead of the halide nucleophile, the water attacks as a nucleophile. 

When a hetero atom, such as N, O, S,259 or a halogen,260 is present in a molecule containing an aromatic ring or a double bond, lithiation is usually quite regioselective.261 The lithium usually bonds with the sp2 carbon closest to the hetero atom, probably because the attacking species coordinates with the hetero atom.262 In the case of aromatic rings this means attack at the ortho position.263 Two examples are... 

As the azocinoindole 40 constitutes the tricyclic substructure of the indole alkaloid apparicine , we attempted to improve the cyclization yield. Satisfactorily, the regioselectivity was completely switched to the 8-endo mode when the alkene acceptor was substituted at the internal position by a bromine atom. Thus, cyclization of selenoester 43 led to the desired target 40 as the only reaction product in 75% yield. Clearly, the bromine atom not only sterically prevented the competitive 1-exo attack, but also benefited the cyclization by activation of the double bond. It should be noted that similar halogen-controlled 8-endo cyclizations are known in the literature, but involving amidyl-type radicals <06OL2647>. 

The formation of S-halogeno-sulphonium salts from the halogen and a sulphide is in competition with an a-halogenation-oxidation reaction which leads to sulphonyl chlorides when water is present. The initial attack at sulphur may be followed by a Pummerer rearrangement to the adjacent carbon atom, and in the case of unsymmetrical sulphides the regioselectivity is determined by the relative electron density at the a-carbon atoms. ... 

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