Mesolytic reactions

Other Elements - Though most of the focus on photoinitiated SET processes remains on the mesolytic reactions of radical cations, organoselenium radical anions, produced by SET sensitisation by 1,5-dimethoxynaphthalene, show synthetic promise as intermediates leading to unimolecular group transfer radical sequences. Thus, irradiation of (525) in the presence of 1,5-dimethoxynaphthalene and ascorbic acid as sacrificial electron donor in aqueous acetonitrile... 

Similarly, the reaction of photoexcited 9,10-dicyanoanthracene (DCA) with a benzylstannane yields the contact ion pair in which the cation radical undergoes rapid mesolytic cleavage of the C—Sn bond to afford benzyl radical and tributyltin cation (which then adds to DCA- )77 (Scheme 14). When such unimolecular processes are faster than the energy-wasting back electron transfer (/cbet) within the contact ion pair, the D/A reactions occur rapidly despite unfavorable driving forces for electron transfer. 

If fcf > feBET- the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Different kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the C-Sn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NO+] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an effective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [D+, A- ] by rapid formation of a bond between the cation radical and anion radical (which effectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59]. 

Fig, 8. A free-energy relationship for mesolytic cleavage of C-C bonds in 7t-radical ions of bicumene derivatives [78, 99, 102]. The solid line represents a hypothetical reaction with no overhead (km = (kbJ /h)exp( — AG J(R T)), i.e. one with only the thermodynamic barrier). The broken lines are the best-fit lines to parabolic (Marcus-type) and hyperbolic (Weller-type) functions. 

Several classes of such fragmentation reactions have been studied in detail in order to understand the effect of substituents, the stereoelectronic requirements, and the effects of the medium and of nucleophiles. The mesolytic fragmentation of bibenzyls with the reduction of the radical and nucleophilic trapping of the cation is the main process for stabilized fragments, typical examples being cumyl or benzhydryl (see Equation 4.12). The relative stability of radical and cation makes the cleavage selective for Ar Ar. " ... 

Maslak, P. and Chapman, W.H., Mesolytic scission of C-C bonds as a probe for photoinduced electron transfer reactions of quin ones, /. Org. Chem., 61, 2647, 1996. 

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