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IV. Nonchain Reactions: Radical Formation by Electron Transfer

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  • A. Transition-Metal Complexes as Electron Donors

    As is indicated in Scheme 3, radical forma­tion in a nonchain reaction often occurs by dis­sociative electron transfer from a transition-metal complex (e.g., Cp2TiCl) to a halogenated car­bo­hydrate. An example of such a reaction is shown in eq 38, where electron donation by Cp2TiCl enables a pyranos-1-yl radical to be formed from the glycosyl bromide 1; this radical then adds to methyl vinyl ketone.181 Other reactions of this type, all of which are nonchain and involve pyran­os-1-yl radicals generated from glycosyl bromides, are listed in Table 6.


    Although the reactions shown in Table 6 are nonchain in nature and, thus, do not require the typical addition of a catalytic amount of an initiator, they can be made catalytic in the trans­ition-metal complex, if the metal ion in the complex is returned to its original oxidation state quickly after electron transfer. An example of a reaction taking place by such a process is found in Scheme 27, where Ni(I) is proposed to be continuously formed by reaction of Ni(II) with man­ganese metal.25


    Cobalt is another transition metal capable of forming carbohydrate radi­cals by electron trans­fer.183–187 An overall reaction showing electron donation by a cobalt complex is given in eq 39, but the transfer actually takes place in two, distinct steps (equations 40 and 41). Because many cobalt complexes of carbo­hy­drates are stable enough to be isolated, the radical forming step for reactions of such compounds is carbon–cobalt bond homolysis (eq 41). Eq 42 describes a reaction in which a pyranos-1-yl radical, produced by C–Co bond homolysis, adds to a molecule of styrene.184



    B. Transition-Metal Complexes as Electron Acceptors

    Reaction of a CH-acidic compound such as CH2(CO2CH3)2 with (NH4)2Ce(NO3)6 [or Mn(OAc)3] transfers an electron to the transition-metal complex to produce ·CH(CO2CH3)2, an electron-deficient radical (Scheme 4).188–192 This radical then adds to an electron-rich double bond, such as that found in a typical glycal (eq 43).189 A similar glycal addition takes place with the electrophilic radical CH3NO2·, which is formed by electron transfer from CH3NO2- to (NH4)2Ce(NO3)6.193



    Roger W. Binkley (Cleveland State University) and Edith R. Binkley (Cleveland Heights-University Heights school system)

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