Substitution SrnI , radical-nucleophile unimolecular

SrnI Process (Substitution, Radical-Nucleophile, Unimolecular)... 

The displacement of bromine, in the relative order 2 > 3 > 4, by an enolate or related anion under irradation, known as an SrnI process (Substitution Radical Nucleophilic, unimolecular), involves photostimulated transfer of an electron from the enolate to the heterocycle, loss of bromide to generate a pyridyl radical which then combines with a second mol of enolate, generating the radical anion of product, transfer of an electron from which sustains the chain process.The equivalent photo-catalysed displacement of bromide by hydroxide gives 3-hydroxypyridine. ... 

This chain reaction is analogous to radical chain mechanisms for nucleophilic aliphatic nucleophilic substitution that had been suggested independently by Russell and by Komblum and their co-workers. The descriptive title SrnI (substitution radical-nucleophilic unimolecular) was suggested for this reaction by analogy to the SnI mechanism for aliphatic substitution. The lUPAC notation for the SrkjI reaction is (T -t- Dm -t- An), in which the symbol T refers to an electron transfer. When the reaction was carried out in Ihe presence of solvated electrons formed by adding potassium metal to the ammonia solution, virtually no aryne (rearranged) products were observed. Instead, reaction of 95c produced only 98 (40%) and 94 (40%) but no 99, and reaction of 96c produced 99 (54%) and 94 (30%) with only a trace of 98. ... 

Besides nucleophilic and electrophilic pathways for aromatic substitutions, there are also radical pathways. With aromatic rings that are easily reduced, this is a common mechanism, because the benzene ring can delocalize the radical anion. The radical chain mechanism is referred to as SrnI, (substitution, radical-nucleophilic, unimolecular). An example is shown in Eq. 10.118. 

There is still another substitution mechanism to consider, although it is much less common. It is called SrnI (substitution, radical, nucleophilic, unimolecular) and involves a radical chain mechanism, unlike the SET mechanism just described. We have seen a radical chain substitution mechanism in Chapter 10 when we considered radical aromatic substitution (Section 10.22). 

SrnI (substitution, radical, nucleophilic, unimolecular). Section 11.6... 

I. Additions. Radicals can react with anionic species to give radical anion adducts as shown for radical 11. Such addition reactions are steps in chain reaction processes described as SrnI (unimolecular radical nucleophilic substitution) reac-... 

Since the pioneering studies of Bunnett [3], the scope of the unimolecular radical nucleophilic substitution (SrnI) reaction has increased considerably, and today this approach is well established for the formation of aryl-carbon and aryl-heteroatom bonds. The SrnI reaction is a chain process which includes radicals and radical anions as intermediates the reaction mechanism is depicted in Scheme 13.1 [1]. 

SrnI Unimolecular radical-nucleophilic aromatic substitution... 

Radical substitution reactions and their mechanisms and applications have been reviewed several times [189,190]. Thiophene participates well in radical reactions. There are reviews describing both unimolecular radical nucleophilic substitutions (SrnI) [191] and homolytic aromatic substitutions (HAS) of thiophenes [192]. The formation of thiophene radicals from peroxides, thienylamines and iodothiophenes has been discussed [192]. 

Electron transfer processes are also involved in a family of reactions which are designated by the mechanistic description SrnI. This refers to a nucleophilic substitution via a radical intermediate, involving a unimolecular decomposition of a radical anion of the substrate. There are two families of such reactions which have been... 

---