Radical cation/anion pairs, electrophilic oxidation reaction

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

Both nucleophilic and electrophilic reactions are known, and the reaction sequence can be directed by a suitable choice for a heteroligand [41,52], As suggested by Scheme 6.11a, the ability of the heteroligand to direct electrophilic or nucleophilic attack by the peroxocomplex can provide an important tool in oxidative reactions, where selectivity of action is required. A second mode of electrophilic reaction chemistry is available through attack of sulfur electrons at the vanadium center to give a transient anion/cation radical pair via formation of V(TV) and S-+ (Scheme 6.1 lb). 

Both, strained and unsaturated organic molecules are known to form cation radicals as a result of electron transfer to photoexdted sensitizers (excited-state oxidants). The resulting cation radical-anion radical pairs can undergo a variety of reactions, including back electron transfer, nucleophilic attack on to the cation radical, electrophilic attack on the anion radical, reduction of anion radical, and addition of anion radical to the cation radical. This concept has been nicely demonstrated by Gassman et al. [103, 104], using the photoinduced electron-transfer cydization of y,8-unsatu-rated carboxylic add 232 to y-ladones 233 and 234 as an example (see Scheme 8.65). 

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