Single electron shift

For a discussion of single electron shifts in two-electron transfer processes see Refs [4d] and... 

Sn2 reactions are single electron shift processes (Pross and Shaik, 1983). 

Let us now discuss these in turn. The first conclusion, a rather controversial one, is that an SN2 reaction really only involves a single electron shift. This possibility has been proposed both in the past and recently (Bank and Noyd, 1973 Chanon and Tobe, 1982), but appears to have been largely dismissed. The CM analysis, however, lends considerable weight to this view, since the only electronic change that is necessary to convert reactant configuration [20] to product configuration [21] is a single electron shift (77)... 

The fundamental mechanistic difference between the SN2 and electron transfer processes is whether after the single electron shift takes place, an intermediate is formed or not. Any factor capable of delaying N—R coupling (77) after the single electron shift may lead to the actual generation of radical intermediates. Let us explore what these factors may be. 

In summary, we see that both SN2 and SRN1 processes involve a single electron shift. In the SN2 case the electron shift occurs synchronously with electron coupling. In the SRN1 case the electron shift leads to the formation of radical intermediates. 

In the section on nucleophilic substitution processes we saw that the SN2 process is simply a single electron shift coupled with bond reorganization. The question we wish to consider in this section is how the CM model treats standard electron transfer reactions such as (89) and how the model relates to the Marcus theory, now universally accepted for treating such processes (Marcus, 1964, 1977). 

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. 

To avoid any misinterpretations concerned with the digit style of numbers, the decimal point is used throughout the book instead of a comma (i.e. computer notation 1.03 instead of 1,03, except for some graphical representations). In representative molecular structures, spin-paired non-bonding electrons around an atom of a molecule are represented (if necessary) by a bold line —in accord with commonly used leivis-structures. Single electrons are represented by a dot A full arrow (-> ) indicates shifts of electron pairs, whereas single electron shifts are... 

Pross, A., 1985, The single electron shift as a fundamental process in organic chemistry the relationship between polar and electron-transfer pathways Acc. Chem. Res. 18 212n 219. 

The reactivity of hydroxide ion (and that of other oxyanions) is interpreted in terms of two unifying principles (a) the redox potential of the YO /YO- (Y = H, R, HO, RO, and O) couple (in a specific reaction) is controlled by the solvation energy of the YO anion and the bond energy of the R-OY product (RX - - YO R-OY - - X ), and (b) the nucleophilic displacement and addition reactions of YO occur via an inner-sphere single-electron shift. The electron is the ultimate base and one-electron reductant which, upon introduction into a solvent, is transiently solvated before it is leveled (reacts) to give the conjugate base (anion reductant) of the solvent. Thus, in water the hydrated electron... 

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 ... 

Scheme 14 Single-electron shift (equivalent to the transfer of an electron from O to X)...

Consistent with the idea that the single-electron shift is a fundamental process is the notion that the electron distribution (or partial charge) on atoms near the reaction site changes during the course of the reaction. This is so obvious as to seem trivial, but bears repeating because our thinking often is misdirected by the assignment of oxidation numbers (or... 

Scheme 15 Redoc energetics for single-electron shift reactions...

The reactions of HO and 02 - with alkyl halides exhibit the same general pattern (Scheme 16), with second-order kinetics and inversion of configuration. Radicals are not detected in the reactions with hydroxide ion, which indicates that there probably is not a discrete SET step, but rather that the transfer of the entering and leaving groups is synchronous with a single-electron shift. 

Several examples of reduction by H0 of transition metal complexes are known (see Table I7).3. i64,i7s-i78 reaction of Au+ with H0 in MeCN is believed to be a prototype of reactions that involve a single-electron shift and the formation of a metal atom/hydroxyl radical bond (equation 175). 

For further reading on this concept of the single-electron shift, see Pross, A. The Single Electron Shift as a Fundamental Process in Organic Chemistry The Relationship between Polar and Electron-Transfer Pathways. Acc. Chem. Res. 1985, 18, 212-219. 

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

Scheme 8-1 Single-Electron Shift (Equivalent to the Transfer of an Electron from OtoX)...

Scheme 8-2 Redox Energetics for Single-Electron-Shift Reactions...

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. 

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. 

That mJz 109 reacts rapidly with methanol whereas the structurally related ion mJz 110 is unreactive (except in proton transfer) may appear to be surprising. However, mJz 110 is a radical cation, and after analysis of the exchange process in terms of single electron shifts according to the method of Pross (30), group coupling to form a bond to phosphorus is favorable only with mJz 109. 

D,A react to form D+-A. The difference is that in the polar pathway two odd electrons on D +and A brought about by the single-electron shift, are paired into a single bond. In the case of the SET process, no such interaction occurs. 

Any factor that inhibits or disallows coupling of the two odd electrons after the single-electron shift will encourage SET over a polar nucleophilic pathway. This statement forms the basis for understanding the competition between these two routes. The following factors will have a bearing on the polar-SET competition (2). 

Radical Delocalization. If the two radical centers on D and A are extensively delocalized, coupling is inhibited and a SET pathway is encouraged. This actually represents a special case of D-A bond strength, because the coupling of delocalized radicals leads to weak bonds. Numerous examples of all of these predictions exist (2) these examples make the foregoing analysis a most useful one. We see therefore that only by viewing the polar process as a single-electron shift does the relationship between the polar and SET pathways become clear. 

The direct attack of a nucleophile, N, on a cation, R+, involves a single electron shift (3, 38). The reaction may be described by the avoided crossing of DA and D+A curves as indicated in Figure la. A single-electron shift from N to R+ in the R+ N pair generates the R N+ radical pair, which can collapse to form an R-N a bond. The case of a radical cation is, however, quite different. An electron shift from Nu to a radical cation A + merely regenerates A, the parent hydrocarbon therefore, a simple nucleophilic... 

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