Electrophiles, with reactive halogen

Organomercury compounds are weak nucleophiles and react only with very reactive electrophiles. They readily undergo electrophilic substitution by halogens. 

The scope of vinyl metals as sources of nucleophilic vinyl groups is very great. As well as the expected electrophiles such as halogens, alkyl and acyl halides, aldehydes and ketones, unsaturated carbonyl compounds and epoxides, they also combine with aryl and alkenyl halides with palladium catalysis. The usual stereochemical course is retention at the vinyl group. It is necessary to decide whether the vinyl metal is reactive enough or whether it must first be transformed into an ate complex. Since most of these vinyl metals can be converted into each other with retention, this is an unusually versatile group of reagents. 

In many ways the chemistry of indole is that of a reactive pyrrole ring with a relatively unreactive benzene ring standing on one side—electrophilic substitution almost always occurs on the pyrrole ring, for example. But indole and pyrrole differ in one important respect. In indole, electrophilic substitution is preferred in the 3-position with almost all reagents whereas it occurs in the 2-position with pyrrole. Halogenation, nitration, sulfonation, Friedel—Crafts acylation, and alkylation all occur cleanly at that position. 

The reactivity of the 3- and 5-azolones centres mainly on their ability to react with electrophiles such as halogens (giving 4,4-dihalo-derivatives with excess reagent), or to nitrate, or undergo Vilsmeier formylation the example shown below is the formylation of Antipyrine , once used as an analgesic. Many dyestuffs have been synthesised via coupling of aryldiazonium cations with 5-pyrazolones at C-4 - tartrazine is such an example. 

Carbenes are uncharged reactive intermediates of general formula CR2, in which carbon atom has a sextet of electrons. Depending on the nature of substituents R, they can behave as electrophilic or nucleophilic species, able to enter a variety of synthetically useful reactions. Perhaps the most important reaction of electrophilic carbenes is addition to C=C double bond giving cyclopropanes. Many electrophilic carbenes can be generated via the base-induced a-elimination process—abstraction of a proton from a carbon atom, connected with a halogen, followed by departure of the halogen anion from the initially formed a-halocarbanion (eq. 101). 

Reactivity was enhanced remarkably with the more electrophilic alkenyldichlorob-oranes [60]. Moreover, amination with azides gave the corresponding amines in good yields. With a halogen atom in the appropriate position, ring closure of intermediate (o-halogenoalkylamines can be realized by treatment with base to afford bicyclic pyrrolidines and piperidines with a trans jtmction (Scheme 9.27) [61]. 

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