Single-electron shift mechanism

Reaction Classifications (Single-electron Shift Mechanism)... 

The single-electron shift mechanism appears to be general and applicable for electron, proton, atom, and group transfer reactions. The assumptions for this proposition include ... 

Reaction classifications (single-electron shift mechanism)... 

As the prototype reactions in Scheme 8-1 imply, a reaction that involves a single-electron shift may not produce observable free-radical products. Conversely, the failure to find free-radical products does not prove the absence of a single-electron-shift mechanism. Other arguments are necessary to establish the nature of polar-group-transfer and polar-coupling reactions. 

In the last few years it has been proposed that the dichotomy polar vs ET is not a real one and that the mechanism through which a substitution takes place can be viewed within a complete spectrum ranging from polar to ET29. The polar mechanism can be considered to take place by a single electron shift, in other words, a single ET accompanied by bond breaking and/or bond formation. 

Nucleophilic substitution reactions. The view that substitution or displacement reactions that involve hydroxide ion are examples of polar-group-transfer reactions (with a single-electron shift) is probably the least iconoclastic proposal. Most accept the view that many nucleophilic displacement reactions occur by a SET mechanism.22 In a number of cases free-radical intermediates have been identified, which is consistent with a discrete SET step. Only a slight extension of this concept is required to encompass all nucleophilic reactions within the categories described in Scheme 8-1. 

Pross and Shaik have examined this reaction in detail, proposing a single-electron shift model. In this model, the nucleophile transfers an electron to the electrophile as they approach each other. The reaction coordinate involves several valence bond structures such as [Nuc —R —X ] and [Nuc —R —X ] and some zwitterionic forms. The point is that a free radical may never be formed, but instead a complex between the electrophile and nucleophile is formed wherein an electron shifts between the two reactants. In essence, the mechanism of Scheme 11.8 is reduced to a single step that occurs smoothly along one reaction coordinate with no intermediates (Eq. 11.44). 

Not only the nitroaromatic species, such as IV, but also some simpler compounds, which used to be considered as typical substrates of the 8 2 reactions, can be involved in multistep radical-forming nucleophilic substitutions. Evidence has been accumulating over the last years that the nucleophilic substitution with alkyl halides occurs, at least in some instances, by the single-electron transfer mechanism. It has been suggested [29,30] that the SET and Sn2 mechanisms represent the extremes of a wide spectrum of mechanistic possibilities for substitution reactions. It has been deduced on qualitative theoretical grounds that the propensity of alkyl halide R—X to react with nucleophiles via an electron-transfer step depends crucially on the stability of the three-electron bond R—X in the initially formed radical-anion species. A more electronegative R will stabilize this bond and bring about a shift in the mechanism from the Sn2 to the SET type, which has then experimentally been shown to be a correct conclusion, see Ref. [30]. 

Fig. 1.11. Radical initiators and their mode of action (in the "arrow formalism" for showing reaction mechanisms used in organic chemistry, arrows with half-heads show where single electrons are shifted, whereas arrows with full heads show where electron pairs are shifted).

That is, the act of shifting the single electron from Y to X may occur either with or without free-radical formation. Usually, the concerted non-radicaloid process is energetically favoured. For a more detailed discussion of the various mechanisms of nucleophilic substitution reactions in aliphatic compounds and their solvent dependence, see references [14, 483, 782-785]. 

Mechanism of alkene hydroboration. The reaction occurs in a single step, in which both C-H and C-B bonds form at the same time and on the same face of the double bond. Electrostatic potential maps show that boron becomes negative in the transition state, as electrons shift from the alkene to boron, but is positive in the product. 

Coordination to the metal is not shown. The enediolate in the intermediate is intended to be below the plane of the paper, (b) Dinuclear complex of Mo and o-arabinose. Single bonds to molybdenum are drawn as broken. On the alkyl shift mechanism the C3 shifts with its bonding pair of electrons from C2 to Cl. 

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