SRNl Substitutions

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

Rossi, R.A. Rossi de, R.H. (1993) Aromatic Substitution by the SRNl Mechanism. American Chemical Society, Washington, DC. 

Thus, direct SRNl(Ar) substitution reactions allow preparing isoquinolines (Beugel-mans et al. 1984), indoles (Beugelmans Roussi 1979), and benzothiazoles (Boujlel et al. 1982) via one-pot syntheses. The photoinitiated synthesis of 2-methylindole shown in Scheme 6-31 is a representative example. 

The tandem 5-exo [67], and 6-exo-trig cyclization-SRNl [69] reactions afford 3-substi-tuted dihydrobenzofuranes and dihydronaphtho[2,l-k]furanes (28) for the former, and 4-substituted chromanes and benzo[f]chromanes (29) for the latter. In these reactions, at least one C—C bond is formed and the products obtained are feasible for further synthetic transformation (Scheme 10.53). Another approach to O-hetero-cycles is provided via SRN1 substitutions followed by a copper-catalyzed reaction (see Scheme 10.8). 

Vaillard, S.E., Postigo, A. and Rossi, R.A. (2002) Syntheses of 3-substituted 2,3-dihydrobenzofiiranes 1,2-dihydronaphtho [2,l-b]furanes, and 2,3-dihydro-l H-indoles by tandem ring dosure-SRNl reactions. Journal of Organic Chemistry, 67, 8500-8,506. 

Mechanistically most of these transformations correspond to 5N2-type substitutions chiral substrates react with inversion. In certain cases, however, azides may also be obtained under 5n1 condi-tions." " " Some di- and tri-arylmethanols for instance (Scheme 32) react with HN3 in the presence of trichloroacetic acid via the corresponding carbenium ions. Sulfuric acid seems to be inferior for these transformations." " Under highly acidic conditions the intermediate (20) obviously tends to decompose with the elimination of nitrogen (c/. Schmidt reaction "). SnI substitution is also highly probable in the reaction of a-halo ethers and sulfides with azide ions." " In some cases even SRNl-type substitution was observed for the halogen/azide exchange." ... 

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 rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

Such results indicate that, from a mechanistic point of view, it is not a matter of a simple aromatic nucleophilic substitution (in any case very difficult if not yi benzyne), but more probably of a radical substitution on an activated complex of the TCDD (SRNl mechanism) (1 ) this mechanism is favoured by the fact that the radical anion of TCDD is particularly stabilized (IJ). How this radical anion is generated from the initiator peroxide and how it reacts with the hydrogen donor (PEG or the solvent) is not at this moment clear however, interesting to note, degradation of TCDD by u.v. irradiation leads to the same intermediate compounds. 

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