SrnI mechanism

Alternatively, the SrnI mechanism can be coupled to nucleophilic additions as was the case during the formation of substituted indole 212 [182], an interesting variation on a reaction first reported by Beugelmans and Roussi [183]. 

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These reactions also appear to be chain reactions that proceed through the SrnI mechanism. DMSO is a particularly favorable solvent for this reaction, probably because... 

Instances of substitution of hindered alkyl halides by the SrnI mechanism have also been documented. Some examples are shown below. 

Vinylic halides can react by a SrnI mechanism (p. 855) in some cases. An example is the FeCl2 catalyzed reaction of l-bromo-2-phenylethene and the enolate anion of pinacolone (t-BuCOCH2 ), which gave a low yield of substitution products along with alkynes. ... 

In this book, we make the above distinction between the SET and SrnI mechanisms. However, many workers use the designation SET to refer to the SrnE the chain version of the SET, or both. 

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." ... 

In certain cases, the SrnI mechanism has been found (p. 856). When the substrate is a heterocyclic aromatic nitrogen compound, still a different mechanism [the Sn(ANRORC) mechanism], involving opening and reclosing of the aromatic ring, has been shown to take place. 

The same products are obtained (though in different proportions) when Na or K is omitted but the solution is irradiated with near-UV light.In either case other leaving groups can be used instead of halogens (e.g., NR3, SAr) and the mechanism is the SrnI mechanism. Iron(II) salts have also been used to initiate this reaction. The reaction can also take place without an added initiator Enolate ions of ketones react with Phi in the dark. " In this case, it has been suggested that initiation... 

Furthermore, Saveant et al. have shown elegant examples of electrochemi-cally induced nucleophilic substitution of perfluoroalkyl halides. The reaction mechanism is a slightly modified version of the classical SRNI mechanism in... 

FIGURE 2.39. Example of the cyclic voltammetric observation of an SrnI reaction upon addition of the nucleophile. Upper scheme ECE-DISP reduction of the substrate in the absence of nucleophile. Lower scheme SrnI mechanism. 

The second part, substitution of N2 by I, proceeds by the SrnI mechanism. 

The first clue to the existence of the SrnI mechanism came from product studies both in aliphatic and aromatic cases. It was noticed that in the reaction of benzyl and substituted benzyl chlorides with the 2-nitropropane anion, oxygen alkylation, yielding the oxime and then the aldehyde, occurs exclusively in the case of benzyl chloride and 3-nitrobenzyl chloride, whereas, with 4-nitrobenzyl chloride, the yield of aldehyde is only 6% and the carbon-alkylated (104) product is obtained in 92% yield (Kornblum, 1975). This was interpreted as the result of a competition between 8, 2 (O-alkylation) and S l (C-alkylation) reactions. In the aromatic case, it was observed that the reaction of 5- and 6-halopseudocumenes with KNHj in liquid ammonia (Kim and Bunnett, 1970) forms the 5- and 6-pseudocumi-dines in a ratio which is the same whether the starting compound is the 5- or 6-isomer in the case of the chloro- and bromo-derivatives, as expected from an aryne mechanism (Scheme 9), whereas much more non-rearranged... 

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... 

For studies of nucleophilicity in the SrnI mechanism, see Amatorc Combcllas Robvcillc Savcant Thiebault J. Am. Chem. Soc. 1986, 108. 4754, and references cited therein. 

Halothiophenes undergo photostimulated reaction with acetone enolate ion to form substitution products (76H(5j377). This is believed to occur by the radical-chain SrnI mechanism. The propagation steps are as follows ... 

A more intriguing type of competition is due to radical processes, which usually involve the substrate radical anion. These can fragment, if they carry a suitable substituent, via anion expulsion.56 The resulting aryl radical, Ar, can form the reduced product, ArH, by hydrogen abstraction57 or the product of substitution via reaction with the nucleophile according to the SrnI mechanism, discussed in Chapter 2.2 of this volume. Examples of competition between SnAt and radical processes of this type have been reported.57-59... 

R. A. Rossi and R. H. de Rossi, Aromatic Substitution by the SrnI Mechanism , American Chemical Society, Washington, DC, 1983. 

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 photolysis of aromatic species with tetranitromethane in perfluoro alcohol solvent has been studied, in which the radical cations were observed by EPR spectroscopy.284 Photo-stimulated reaction of 1- and 2-haloadamantanes and 1,2- and 1,3-dihaloadamantanes with various carbanionic nucleophiles afforded products rationalized through an SrnI mechanism.285 286 Photolysis of the cycloadduct formed between a functionalized derivative of C6o and diazomethane has been shown to afford a pah of ling-opened structures (125) and (126) via a proposed biradical intermediate (127) (Scheme 19). The UV-photolytic fragments of /-butyl iodide (T and /-Bu ) have been ionized by resonance-enhanced multiphoton ionization for TOF mass spectro-metric analysis.287 A two-dimensional position-sensitive detector provided angular distribution and translational energy data. 

The reactions of 1,3-dihaloadamantanes with various carbanions in DMSO have been studied.18 For example, potassium enolates of acetophenone and pinacolone and the anion of nitromethane react with 1,3-diiodoadamantane (19) under photo-stimulation a free-radical chain process forms a 1-iodo monosubstitution product (20) as an intermediate, which undergoes concerted fragmentation to yield derivatives of 7-methylidenebicyclo[3.3.1]nonene (21). These and other results were interpreted in terms of the Srn1 mechanism. The work has been extended to the reactions of 1- and 2-halo- and 1,2-dichloro-adamantanes, examples of the SrnI mechanism again being found.19... 

The kinetics and mechanism of the reactions of/i-nitrocumyl bromide with azide ions in DMSO have been studied.104 In contrast to the reactions of the corresponding chloride in dipolar aprotic solvents, no evidence for an SrnI mechanism was found. A similar situation obtained in a related study of the reactions of nitrite ions with p-nitrocumyl bromide.105... 

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