Via Radical Coupling

The second method is to generate the quinone methide via radical coupling reactions, analogously to the way they are produced via radical coupling during lignification (Fig. 12.4c) orto exploit radical disproportionation reactions (Fig. 12.4d). 

Termination is principally via radical coupling forming hexabutylditin, or to a lesser degree via the coupling of ketyl radicals. In the case of the mr ketones a different mechanism is proposed. The rate of abstraction of H from the tributyltinhydride by benzylic radicals is slower than the corresponding abstraction by alkyl radicals. Since the rate at which the tributyltin radical will add to aromatic carbonyls is similar to the addition rate to aliphatic carbonyls, the dominant radical species for the tttt systems is the ketyl radical. The primary termination process involves the coupling of the predominant radical species resulting in pinacol formation. 

HOOH in acetonitrile yields stoichiometric fluxes of HOO. (equation 97). Reduction of O2 in MeCN at a platinum electrode in the presence of excess protons yields adsorbed Pt(HOO.) (equation 98) which forms [HOOOOH] via radical coupling at the surface and homolytically dissociates to O2 and HOOH. ... 

Intramolecular reactions are much faster than intermolecular reactions, and the anchimeric assistance, measured by the ratio of the rate constant of the unimolecular reaction to that of the analogous bimolecular reaction, depends strongly on the size of the ring that should be formed in the unimolecular process. The ratio of these constants can reach values of up to 10 mol/L. Thus, intramolecular trapping confirms the presence of radicals as short-lived intermediates but does not prove that chain growth proceeds via radical coupling. 

The electrochemical reduction of group 14 metal halides to form metal-metal bonds is rather common. This type of reaction proceeds either via radical coupling or via nucleophile attack of metal anions on metal halides, depending on the reaction conditions such as cathode potentials. 

Scheme 22. Preparation of highly functionalized cyclopentanes via radical coupling of two 71-systems...

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