SrnI substitution

Bowman and Symons probed the stability of a series of radical anions involved in the SrnI substitution for a-substituted aliphatic nitro-compounds [MejQXjNOj] 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... 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

The Komblum reactions at the tertiary carbon atoms also belong to the dark SrnI substitutions. Let us compare the reactions of a-cumyl chloride and 4-nitro-a-cumyl chloride with the phenylthiolate ion (Komblum 1975) (Scheme 8-8). As seen, the substitution of the arylthio moiety for chlorine at the former position of the chlorine is observed only for the 4-nitroderivative. The optically active substrate gives the racemic substitution product upon reaction with the phenylthiolate ion (Scheme 8-9). 

Study of reactions in different solvents often provides crucial mechanistic evidence. This is the case in the reaction between thiolates and a -substituted nitroalkanes when changing from dipolar aprotic solvents to protic solvents. In contrast to the above reactions in dipolar aprotic solvents, where a mixture of disulfides and SRN1 products is obtained, disulfides are exclusively formed in methanol with no SRN1 products. Early studies had shown that SrnI substitution between a-substituted nitroalkanes and anions such as nitronates and arylsulfinates takes place at a slower rate in methanol. 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row main-group or heavier nucleophiles). The best electrophiles are aryl bromides and iodides. If light is required for substitution to occur, the mechanism is almost certainly SrnI. Substitution at alkenyl C(sp2)-X bonds can also occur by an SrnI mechanism. 

Electron transfer is also the first step of the SrnI substitution mechanism (Chapter 2). In reactions that proceed by the SrnI mechanism, the electron donor is usually the nucleophile. The nucleophile may be photoexcited to give its electron more energy and make it more prone to transfer. 

Existing textbooks usually fail to show how common mechanistic steps link seemingly disparate reactions, or how seemingly similar transformations often have wildly disparate mechanisms. For example, substitutions at carbonyls and nucleophilic aromatic substitutions are usually dealt with in separate chapters in other textbooks, despite the fact that the mechanisms are essentially identical, and aromatic substitutions via diazonium ions are often dealt with in the same chapter as SrnI substitution reactions This textbook, by contrast, is organized according to mechanistic types, not according to overall transformations. This... 

In all chapters I have made a great effort to show the forest for the trees, i.e, to demonstrate how just a few concepts can unify disparate reactions. This philosophy has led to some unusual pedagogical decisions. For example, in the chapter on polar reactions under acidic conditions, protonated carbonyl compounds are depicted as carbocations in order to show how they undergo the same three fundamental reactions (addition of a nucleophile, fragmentation, and rearrangement) that other carbocations undergo. Radical anions are also drawn in an unusual manner to emphasize their reactivity in SrnI substitution reactions. 

This chain reaction is analogous to radical chain mechanisms for nucleophilic aliphatic nucleophilic substitution that had been suggested independently by Russell and by Komblum and their co-workers. The descriptive title SrnI (substitution radical-nucleophilic unimolecular) was suggested for this reaction by analogy to the SnI mechanism for aliphatic substitution. The lUPAC notation for the SrkjI reaction is (T -t- Dm -t- An), in which the symbol T refers to an electron transfer. When the reaction was carried out in Ihe presence of solvated electrons formed by adding potassium metal to the ammonia solution, virtually no aryne (rearranged) products were observed. Instead, reaction of 95c produced only 98 (40%) and 94 (40%) but no 99, and reaction of 96c produced 99 (54%) and 94 (30%) with only a trace of 98. ... 

Besides nucleophilic and electrophilic pathways for aromatic substitutions, there are also radical pathways. With aromatic rings that are easily reduced, this is a common mechanism, because the benzene ring can delocalize the radical anion. The radical chain mechanism is referred to as SrnI, (substitution, radical-nucleophilic, unimolecular). An example is shown in Eq. 10.118. 

There is still another substitution mechanism to consider, although it is much less common. It is called SrnI (substitution, radical, nucleophilic, unimolecular) and involves a radical chain mechanism, unlike the SET mechanism just described. We have seen a radical chain substitution mechanism in Chapter 10 when we considered radical aromatic substitution (Section 10.22). 

SrnI (substitution, radical, nucleophilic, unimolecular). Section 11.6... 

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