Aromatic substitution by the SRNI mechanism

R. A. Rossi and R. H. de Rossi, Aromatic Substitution by the SrnI Mechanism , American Chemical Society, Washington, DC, 1983. 

The second general reaction which proceeds by an SrnI mechanistic pattern involves aryl halides. Aryl halides react with a variety of potential electron donors to give nucleophilic aromatic substitution by a chain mechanism of the SrnI class. Many of the reactions are initiated photochemically and most have been conducted in liquid ammonia solutions. 

The fact that reactions in both (a) and (b) require light suggests that a radical mechanism is involved for each of them. This rules out a simple Si.,2 substitution in part a or a nucleophilic aromatic substitution by an addition-elimination reaction in part b. When substitution occurs under basic conditions in the presence of light, the most likely mechanism is SrnI-... 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row or heavier nucleophiles). The best electrophiles for the SrnI mechanism are able to delocalize the odd electron in the radical anion (aromatic leaving groups, carbonyl compounds), can make a stable radical (3° alkyl halides), and have a weak R-X (Br, I) bond. Tosylates and other pseudohalides are very poor SrnI electrophiles. If light is required for substitution to occur, the mechanism is almost certainly SrnI. 

There has been a major review of substitution by the radical-chain 5rn1 mechanism. It has been shown that reaction by the SrnI pathway of the enolate anions of 2- and 3-acetyl-l-methylpyrrole may yield a-substituted acetylpyrroles. The dichotomy of reactions of halonitrobenzenes with nucleophiles has been nicely summarized major pathways include reduction via radical pathways and. SnAt substitution of halogen. EPR spectroscopy has been used to detect radical species produced in the reactions of some aromatic nitro compounds with nucleophiles however, whether these species are on the substitution pathway is questionable. The reaction of some 4-substimted N,N-dimethylanilines with secondary anilines occurs on activation by thallium triacetate to yield diphenylamine derivatives radical cation intermediates are proposed. ... 

Copper(I) salts such as CuCN and ROCu undergo aromatic substitution reactions very readily with ordinary aryl halides. The mechanism has not been established with certainty. One reasonable possibility is an SrnI mechanism. Another reasonable possibility involves oxidative addition of Ar X to N=C Cu(I) to give a Cu(III) complex, followed by reductive elimination of Ar-CN to give CuX. 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rules out certtdn mechanisms and suggests others. For example, under acidic conditions, aromatic substitution reactions take place by either one of two mechanisms electrophilic addition-elimination or SnI substitution of aryldiazonium ions. Under basic conditions, they take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnI. If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

Existing textbooks usually fail to show how common mechanistic steps link seemingly disparate reactions, or how seemingly similar transformations often have wildly disparate mechanisms. For example, substitutions at carbonyls and nucleophilic aromatic substitutions are usually dealt with in separate chapters in other textbooks, despite the fact that the mechanisms are essentially identical, and aromatic substitutions via diazonium ions are often dealt with in the same chapter as SrnI substitution reactions This textbook, by contrast, is organized according to mechanistic types, not according to overall transformations. This... 

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