SRN1 reaction features

The mechanistic aspects of the SRN1 reaction were discussed in Section 11.6 of Part A. The distinctive feature of the SRN1 mechanism is an electron transfer between the nucleophile and the aryl halide.181 The overall reaction is normally a chain process. 

SRN1 reactions are, in general, eased up sterically. This feature is employed for synthetic purposes. For instance, the cyanoacetate substituent was inserted in the sterically shielded position of a benzene ring (Suzuki et al. 1983) (Scheme 8-14). The reaction proceeds in hexamethylphosphorotriamide. Photoirradiation results in the formation of undesirable by-products, but initiation with cuprous iodide leads to the target substance, at more than 60% yield. 

This type of reaction is important and deserves some comment. The addition of one electron to a molecule generates an anion radical. This results in an increase in its reactivity. In particular, the bond dissociation energies in anion radicals are much smaller than those in the corresponding neutral molecules. Thus, substitution reactions proceed more easily in the case of anion radicals. The reactions of the SRN1 type are good examples of this feature. Two possible schemes (a and b) for the reaction course are listed here ... 

The addition of a nucleophile to a radical to form the radical anion of the substitution product constitutes the main feature of a SRN1 process, although the chain can be short or even nonexistent (reaction 2). For a photoinduced reaction, a quantum yield higher than 1 can be taken as evidence of a chain reaction, although a global quantum yield below 1 cannot be used as a criterion against a chain reaction [9,10]. 

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