SrnI process, initiation step

Unactivated aryl halides will undergo nucleophilic displacement via electron transfer in the initial step the so-called SrnI mechanism. It is now clear that in the case of heteroaromatic compounds, nucleophilic substitution by the SRN process often competes with the additionelimination pathway. SRN reactions are radical chain processes and are usually photochemically promoted. For example, ketone 913 is formed by the SRN1 pathway from 2-chloroquinoxaline 912. 

This is called the SrnI mechanism," and many other examples are known (see 13-3, 13-4,13-6,13-12). The lUPAC designation is T+Dn+An." Note that the last step of the mechanism produces ArT radical ions, so the process is a chain mechanism (see p. 895)." An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH2 in NH3. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded." Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar- of Ft from the solvent NH3. Besides initiation by solvated electrons," " SrnI reactions have been initiated photochemically," electrochemically," and even thermally." ... 

Two further quite different cyclization processes each have an initial intermolecular Srn 1 reaction followed by a second in which the ring closure is made. The reactions of o-dihalobenzenes (24) and (25) with the dithiolate (26) to give (27) have already been mentioned (see equation 28), but the yield of analogous product formed between (26) and 1 -brotno-2-iodonaphthalene is only 24% and use of 1,2-eth-anedithiolate with o-diiodobenzene gives only 13% of benzo-l,4-dithiane.98 A rather convenient one-step synthesis of [m.m]-mera-cyclophanediones (m = 3-8) in most acceptable yields is obtained on tandem SrnI reactions of the ketones (45 equation 50).165... 

---