SRN1 Substitutions

Bowman and Symons145 probed the stability of a series of radical anions involved in the SRN1 substitution for a-substituted aliphatic nitro-compounds [Me2C(X)N02] by studying with ESR at 77 K the succession of events following electron capture by Me2C(X)N02. The radical anions were more concentrated in an ether matrix than in an... 

In the case of less reactive aryl halides, a photostimulated SRN1 substitution can be applied.61... 

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]. 

Steric effects, although clearly recognized, introduce relatively small rate retardations or increases in selectivity in all these examples, probably because the transition states of all these addition reactions are rather loose ones, i.e., they occur early on the reaction coordinate when the distances between the radical and the substrates are still rather large92 93,97. An extreme example of a free radical reaction which does not response heavily to steric effects, is the SRN1 -substitution reaction of Kornblum 14 by which bonds between two quaternary carbons can be formed with great ease and in good yield, as is shown by one of many published examples114. The decisive step... 

Perfluoroalkyl iodides are well-known for their ability to act as substrates in SRN1 substitution reactions [288]. 

The most relevant results regarding the SRN1 substitutions have been extensively reviewed [1-5] consequently, in this chapter we indude only the most recent and major representative synthetic examples in the aromatic system, relating to C—C and C-heteroatom bond formation with sulfur and tin nudeophiles, and ring-dosure reactions. 

SRN1 substitution followed by a polar ring closure reaction with an ortho substituent to the leaving group. 

SRN1 substitution with substrates bearing two leaving groups and bidentate or monodentate nucleophiles, followed by a polar or catalyzed reactions. 

SRN1 substitution where a radical cyclization takes place in the propagation step (tandem cyclization-SRN1 sequence and vice versa). 

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). 

Substituent effects in reaction 1 have received little attention because of the lack of suitable methods of generating specific free radicals under basic conditions. One general process involving reaction 1 is the SRN1 substitution... 

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