- Radical-nucleophilic aromatic substitution
Radical-nucleophilic aromatic substitution or SRN1 in
organic chemistry is a type ofsubstitution reaction in which a certainsubstituent on anaromatic compound is replaced by anucleophile through an intermediaryfree radical species:The substituent X is a
halide and nucleophiles can besodium amide , analkoxide or a carbon nucleophiles such as anenolate . ["Phenomenon of radical anion fragmentation in the course of aromatic SRN reactions" Roberto A. RossiAcc. Chem. Res. ; 1982; 15(6) pp 164 - 170; DOI|10.1021/ar00078a001.] In contrast to regularnucleophilic aromatic substitution ,deactivating group s on the arene are not required.This reaction type was discovered in 1970 by Bunnett and Kim ["Evidence for a radical mechanism of aromatic "nucleophilic" substitution" Joseph F. Bunnett and Jhong Kook Kim
J. Am. Chem. Soc. ; 1970; 92(25) pp 7463 - 7464. (DOI|10.1021/ja00728a037)] and the abbreviation SRN1 stands for substitution radical-nucleophilic unimolecular as it shares properties with an aliphaticSN1 reaction . An example of this reaction type is theSandmeyer reaction .Reaction mechanism
In this
radical substitution the aryl halide 1 accepts an electron from anradical initiator forming aradical anion 2. This intermediate collapses into an aryl radical 3 and a halide anion. The aryl radical reacts with the nucleophile 4 to a new radical anion 5 which goes on to form the substituted product by transferring its electron to new aryl halide in thechain propagation . Alternatively the phenyl radical can abstract any loose proton from 7 forming the arene 8 in achain termination reaction.The involvement of a radical intermediate in a new type of
nucleophilic aromatic substitution was invoked when the product distribution was compared between a certain aromatic chloride and an aromatic iodide in reaction with potassium amide. The chloride reaction proceeds through a classicalaryne intermediate:The
isomer s 1a and 1b form the same aryne 2 which continues to react to theaniline s 3a and 3b in a 1 to 1.5 ratio. Clear-cutcine-substitution would give a 1:1 ratio but additional steric and electronic factors come into play as well.Replacing
chlorine byiodine in the1,2,4-trimethylbenzene moiety drastically changes the product distribution:It now resembles
ipso-substitution with 1a forming preferentially 3a and 1b forming 3b.Radical scavenger s suppress ipso-substitution in favor of cine-substitution and the addition of potassium metal as an electron donor andradical initiator does exactly the opposite ["Alkali metal promoted aromatic "nucleophilic" substitution" Joseph F. Bunnett and Jhong Kook Kim "J. Am. Chem. Soc. " 1970, "92", 7464 - 7466. (DOI|10.1021/ja00728a038)] .References
ee also
*
Birch reduction
*Nucleophilic aromatic substitution
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