Benzene SrnI reactions

Many reactions leading to heterosubstituted-arylphosphonic diesters have been described which fall within the general Scheme 6 of sequential photostimulated SrnI processes, where Ar is a benzene, naphthalene, or pyridine nucleus. Starting with the substituted (2-, 3-, and 4-) arylamine (124) pC = I, Br or (EtO)2P(0)], in reaction with KOP(OEt)2 [Nu(l)], and replacement of the amino group in (125) by a second replaceable group by classical methods, allows a second SrnI reaction to be then carried out on (126) with ArSH, KOP(OEt)2, C5H4NSH, C4H3N2SH, etc. [Nu(2)]." ... 

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions).7S2 R may be alkyl or aryl. As in 0-35, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson reaction (0-12), yields are improved by phase-transfer catalysis.753 Instead of RS ions, thiols themselves can be used, if the reaction is run in benzene in the presence of DBU (p. 1023).754 Neopentyl bromide was converted to Me3CCH2SPh in good yield by treatment with PhS in liquid NH3 at -33°C under the influence of light.755 This probably takes place by an SrnI mechanism (see p. 648). Vinylic sulfides can be prepared by treating vinylic bromides with PhS in the presence of a nickel complex,756 and with R3SnPh in the presence of Pd(PPh3)4.757 R can be tertiary if an alcohol is the substrate, e.g,758... 

Photo-stimulated reactions of neopentyl iodide with several carbanionic nucleophiles have been studied in which inhibition experiments with the TEMPO radical trap suggest the reaction occurs via an SrnI mechanism.76 Comparison of 22 nucleophiles in then. Srn 1 reactions with iodobenzene by Fe(II)- and photo-induction has revealed that both are enhanced by high electron-donation ability of the nucleophile. The radical anion Phl is a key intermediate.77 The SET reactions of perfluoroalkyl iodides have been reviewed.78 Flash photolysis of H2O2 was used to generate HO and 0 radicals which were reacted with a, a. z-trifluorotolucnc (TFT) and 4-fluorotoluene (4FT) and the rate constants calculated.79 The diminished reactivity of TFT towards HO or O with respect to toluene or benzene was consistent with radical addition to the aromatic ring, whilst the reactivity of 4FT was of the same order as electron-deficient toluenes, which favour H abstraction from the aliphatic side-chain. 

The mechanism of the reaction depicted in Scheme 4-6 differs from the SN1 or SN2 mechanism in that it involves the stage of one-electron oxidation reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of the fluorenyl radical. The latter is unsaturated at position 9, near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthio anion. That leads to the anion radical of the substitution product. Further electron exchange produces the substrate anion radical and the final product in its neutral state. The reaction takes place and consists of radical (R) nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol of SRN1. Reactions of SrnI type may have both branch-chain and nonchain character. 

Just as in aryl halides, the halogen can be replaced by hydrogen and by a metal, or be involved in transition metal-catalyzed processes (covered in Section 3.2.3.11.2). Three of the mechanisms of such nucleophilic substitutions are familiar from benzene chemistry via arynes, SrnI processes, and Pd(0)-catalyzed sequences. However, of the two further mechanisms of nucleophilic replacement, the ANRORC (Addition of Micleophile, Ring Opening, Ring Closure) is unique to heterocycles, and Sae reactions occur only with strongly activated benzenoid systems. 

Examples are the reaction of the nitronate anion 4.14 with p-nitrobcnzyl chloride 4.15, and the reaction of the pinacolone enolate 4.16 with bromo-benzene 4.17. The former might have been a straightforward SN2 reaction, but actually takes the SrnI pathway because the nitro groups make the electron transfer exceptionally easy. The latter cannot take place by a conventional Sk2 reaction, because aryl (and vinyl) halides are not susceptible to direct displacement, and the SrnI pathway overcomes this difficulty. 

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