Electrophilic reactions intermediate sources

Asymmetric deprotonation of a prochiral compound having a sufficiently acidic C-H bond can be performed by a lithium amide generated from an enantio-pure secondary amine or by an organolithium reagent in the presence of a chiral tertiary amine [557, 559]. A chiral mixed aggregate is usually formed [77, 81, 974], and the reaction of this intermediate with electrophiles (including proton sources) can lead to a predominant enantiomer. 

You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

Ha.logena.tlon, 3-Chloroindole can be obtained by chlorination with either hypochlorite ion or with sulfuryl chloride. In the former case the reaction proceeds through a 1-chloroindole intermediate (13). 3-Chloroindole [16863-96-0] is quite unstable to acidic aqueous solution, in which it is hydroly2ed to oxindole. 3-Bromoindole [1484-27-1] has been obtained from indole using pytidinium tribromide as the source of electrophilic bromine. Indole reacts with iodine to give 3-iodoindole [26340-47-6]. Both the 3-bromo and 3-iodo compounds are susceptible to hydrolysis in acid but are relatively stable in base. 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Oxidation of the heterocycles with common reagents such as MCPBA, sodium periodate or hydrogen peroxide cleanly affords the sulfoxides and sulfones, and it is clear that the sulfur atom is the principal centre of reaction for electrophiles. While the sulfone is a quite inert functionality, the sulfoxides may be reduced to the sulfides with phosphorus pen-tasulfide as for the tetrahydro systems (78CJC1423). Positive halogen sources likewise react at sulfur, and the intermediate sulfonium halide rearranges, usually by 1,2-shift to the a-halo product. 

Reaction of the ra-ene-5-yne carboxylic acid 977 with biscollidine iodine(l) hexafluorophosphate leads to exclusive formation of the C(6)-( )-iodovinyl tetrahydropyran-2-one 979. The product arises via attack of the acid onto the iodinium ion intermediate 978 (Scheme 258) <2004TL4503>. A similar iodolactonization of hex-5-enoic acid can be induced by Oxone -potassium iodide to furnish 6-(iodomethyl)tetrahydropyran-2-one in excellent yield (Equation 380) <2004SL368>. Similarly, treatment of 5-hexynoic acid with a polymer bound source of electrophilic iodide affords the tetrahydropyran-2-one 980 bearing a diiodo-substituted exocyclic double bond (Equation 381) <1999OL2101>. 

The intermediate in both reactions is a cation but the first (from cyclohexene) adds an anion while the second (from benzene) loses a proton so that the aromatic system can be restored. Notice also that neutral bromine reacts with the alkene but the cationic AICI3 complex is needed for benzene. Another way to produce a more electrophilic source of bromine is to use a pyridine catalyst. Pyridine attacks the bromine molecule producing a cationic bromine compound. 

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