Dissociation nucleofugal

The propagation steps in the SrnI sequence are the dissociative step (equation 2), the associative step (equation 3), and the single-electron transfer step (equation 4). These processes are discussed extensively in Chapter 8 of ref. 18. The nature of the nucleofuge is critical in that poor nucleofuges retard the dissociation of the intermediate radical anion, ArX-- (equation 2), and prevent the reaction from proceeding at a reasonable rate or stop it completely (see Section 2.2.2.3). 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

Hayami, J., Hihara, N., Kaji, A. Sn2 reactions in dipolar aprotic solvents. IX. An estimation of nucleophilicities and nucleofugicities of anionic nucleophiles studied in the reversible Finkelstein reactions of benzyl derivatives in acetonitrile - dissociative character of the reaction as studied by the nucleofugicity approach. Chem. Lett. 1979, 413-414. 

With further increasing leaving ability of Y, the reaction becomes dissociative and becomes a vinylic SnI (SnVI) reaction involving a vinyl cation as intermediate. SnV 1 reactions have been extensively studied, both with substrates giving stabilized vinyl cations and/or with substrates with a good nucleofuge such as triflate (trifiuoromethanesulfonate, TfO ) and are the subjects of several reviews." Their stereochemical consequences are discussed in Section 2. 

With this terminology we may not only describe a fully concerted, one-step Sn2 reaction (AnDn), and a stepwise SnI reaction involving intermediate ions that diffuse apart (Dn -H An), but we may also concisely represent a stepwise reaction involving a transient ion pair (E An). The lUPAC nomenclature system can also be used to describe other substitution reactions. Among them are the SnI (substitution nucleophilic unimolecular with rearrangement) reaction, equation 1.8, which is denoted by lUPAC as an (1/Dn -I- 3/An) reaction. The numbers before the slash symbols indicate atoms involved in the dissociation and association steps. Thus, 1/Dn means that the nucleofuge dissociates from one atom (1), while the 3/An term means that the nucleophile associates at an allylic position (3). 

Early evidence for ion pairing was reported by Winstein from the reaction of a,a-dimethylallyl chloride (14) in acetic acid with added acetate. Although the major product of the reaction was the expected acetate, there was rapid isomerization of 14 to the isomeric Y,y-dimethylallyl chloride (15, equation 8.24). Formation of 15 can be explained by a mechanism in which the nucleofuge. Cl , competes as a nucleophile with solvent and rebonds to the allyl cation at the less hindered position. The rate of formation of 15 was found not to be a function of the concentration of added chloride ion, however. This finding is significant because the rate of the solvolysis should be decreased by addition of chloride ion if the mechanism involves dissociation of 14 to free ions (see equation 8.16). It appears, therefore, that the chloride ion in 15 must originate in the same molecule and not from the bulk medium. This process was characterized as internal return of a chloride ion held as part of intimate ion pair (16) within a solvent shell. Similar conclusions were obtained from... 

With this in mind, the Welton group reasoned that the same approach could be taken to the study of the effects of ionic liquids on the rates of reactions. They used the Kamlet-Taft polarity scales to develop Linear Solvation Energy Relationships (LSERs) to describe the effects of solvents on the reaction kinetics of various 8 2 nucleophilic substitution reactions f Schemes 10.1-10.4). 8 2 reactions occur in a concerted step in which the nucleophile replaces the nucleofuge or leaving group as it dissociates from the subsmate. Their reaction profiles have the form of that in Figure 10.1. 

The nucleofugal (with respect to the ith atom) dissociation of a bond between the ith and Jth atoms and between the ith atom and a virtual atom is represented by the operators and respectively. The corresponding association is represented by the operators... 

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