SRN1 reaction, electrochemical

Cleavage of a C—S bond in the initially formed anion radical has been shown to be the first chemical step in the electrochemically induced rearrangement of 5,5-diarylbenzene-1,2-dicarbothioates to 3,3-bis(arylthio)phthalides in dimethylformamide. The reaction can be effected with 0.1 F/mol and is considered a kind of internal SRN1 reaction (Praefke et al. 1980). 

Alkoxide or aryloxide anions are also reputed to be inactive in SRN1 reactions. There is, however, one example of such a reaction at an sp3 carbon the nitro-derivative of 4-nitrocumyl reacts with phenoxide and 1-methyl-2-naphthoxide ions yielding the corresponding ethers (Kornblum et al., 1967). A similar reaction has been reported for halobenzenes in t-butyl alcohol upon stimulation by sodium amalgam (Rajan and Sridaran, 1977). This reaction could not, however, be reproduced (Rossi and Pierini, 1980) and other attempts to make phenoxide ions react at sp2 carbons have been equally unsuccessful (Ciminale et al., 1978 Rossi and Bunnett, 1973 Semmelhack and Bargar, 1980). It has been found, more recently, that phenoxide ions react with a series of aryl halides under electrochemical induction, but that the coupling occurs at the p- or o-phenolic carbon rather than at the phenolic oxygen (Alam et al., 1988 Amatore et al., 1988). This is... 

Fig. 17 Indirect electrochemical induction of SRN1 reactions. Cyclic voltammetry of (a) 4-cyanopyridine (2.2 mM) in the absence (—) and presence (—) of 8.9 mM 2-chlorobenzonitrile (b) 4-cyanopyridine (6.6 mM) in the presence of 8.9 mM 2-chloro-benzonitrile and 35mM PhS. (Adapted from Amatore et al., 1984b.)...

On the other hand, analysis of the cyclic voltammetric data pertaining to the direct and indirect electrochemical induction of SRN1 reactions by means... 

Formation of heterocyclic rings may be accomplished by electrochemically induced SRN1 reactions136,137 [Eqs. (63)—(66)]. A requirement for such radical-promoted nucleophilic substitutions to proceed as a chain reaction is that the product anion-radical ArNu- be able to transfer an electron to the substrate [Eq. (65)] the oxidation potential of ArNu7 should thus be more negative than the reduction potential of ArX. 

A large number of radical reactions proceed by redox mechanisms. These all require electron transfer (ET), often termed single electron transfer (SET), between two species and electrochemical methods are very useful to determine details of the reactions (see Chapter 6). We shall consider two examples here - reduction with samarium di-iodide (Sml2) and SRN1 (substitution, radical-nucleophilic, unimolecular) reactions. The SET steps can proceed by inner-sphere or outer-sphere mechanisms as defined in Marcus theory [19,20]. 

Electron-catalyzed (or electron-stimulated) processes constitute a relatively new class of reactions of great potential synthetic interest (Zelenin and Khidekel, 1970 Linck, 1971). Foremost among these ranks the SRN1 mechanism, which is an electron-initiated radical-chain mechanism of nucleophilic substitution (21-24 X- = halide ion) (for reviews, see Kornblum, 1975 Bunnett, 1978, 1982). The initiation step (21) can be performed photochemically, electrochemically, or by adding alkali metal (Pinson and Saveant, 1978 Amatore et al., 1979 van Tilborg et al., 1977, 1978 Saveant, 1980). 

Electrochemical induction of the reaction (Pinson and Saveant, 1974), as shown to occur in a number of cases with aromatic and heteroaromatic substrates (Saveant, 1980a, 1986, 1988), also provides evidence for the SRN1 mechanism. The electrochemical approach allows, in addition, a quantitative analysis of the mechanism and reactivity problems and will be described in this connection in the next subsection. 

At this point, it can be concluded that the direct and indirect electrochemical approach of the reaction in the case of aryl halides has provided a quantitative kinetic demonstration of the SRN1 mechanism and the establishment of the nature of the side-reactions (termination steps in the chain process). In poor H-atom donor solvents, the latter involve electron-transfer reduction of the aryl radical. 

---