Polar-group-transfer reactions

The polar pathways are formally equivalent to a discrete electron-transfer step, that is, a pure SET step that is followed by a chemical step. If a hypothetical SET step is followed by coupling of a radical pair that is produced in the SET step, the overall reaction is the equivalent of a polar-group coupling reaction (Scheme 14(b)). If the coupling is accompanied by the elimination of a leaving group, a polar-group transfer reaction results (Scheme 14(a)). 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

Thus, the reaction of H3O+ with HO is a prototype example of a charge transfer or redox reaction that is also a polar-group transfer reaction. It can be resolved into three component reactions (see Scheme 16) (equations 162-164) and combined (equations 165 and 166). 

The reaction of superoxide ion (02 )> radical anion, with water also can be viewed as a polar-group transfer reaction (equation 168). The product HOO- is a radical that reacts btmolecularly to form hydrogen peroxide and dioxygen (equation 169). 

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. 

The reaction with water can be viewed as a polar-group-transfer reaction ... 

This procedure offers a convenient method for the esterification of carboxylic acids with alcohols2 and thiols2 under mild conditions. Its success depends on the high efficiency of 4-dialkylaminopyridines as nucleophilic catalysts 1n group transfer reactions. The esterification proceeds without the need of a preformed, activated carboxylic acid derivative, at room temperature, under nonacidic, mildly basic conditions. In addition to dichloromethane other aprotic solvents of comparable polarity such as diethyl ether, tetrahydrofuran, and acetonitrile can be used. The reaction can be applied to a wide variety of acids and alcohols, including polyols,2 6 a-hydroxycarboxylic acid esters,7 and even very acid labile... 

The reaction continuum for HO can be subdivided into three discrete categories that are outlined in Scheme 14 (a) displacement reactions in which the leaving group departs with an electron supplied by HO (polar-group transfer), (b) addition reactions in which a covalent bond is formed (polar-group addition), and (c) simple electron-transfer reactions in which HO acts as an electron donor (single-electron transfer). This view of the chemistry of HO also applies to the reactions of superoxide ion (O2 ) and other nucleophilic oxyanions (Table 16). 

Similar analyses are possible for the initial polar-group transfer step for CCLi ( ° red, —0.91V vs. NHE), CeCl ( ° ed, -1.26 V), and Ci2Cl,o (PCB °, ed, -E30V). Each of these substrates undergoes a net exergonic redox reaction with HO the initial step is analogous to that of equation (173). 

Scheme 17 Polar group transfer HO and O2 reactions with esters...

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. 

Most of the known reactions of HO that produce free radicals probably do not involve a direct single-electron transfer from HO in the primary step because an SET primary step is usually highly endothermic the primary step more often is an approximately thermoneutral polar reaction (polar-group transfer or polar-group coupling), with secondary reactions producing free radicals that are coupled to form stable M-OH bonds (M is a molecule or metal atom with an unpaired electron). 

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