Mesolytic cleavage

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. 

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

MESO FORM (or, MESO ISOMER) MESOLYTIC CLEAVAGE Metabolic activity of an individual organism, BASAL RATE... 

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. 

Table 8. Unimolecular rate constants for mesolytic cleavage of 4MBC+ and 4DMABC+ in CH2Cl2-MeOH at room temperature.

Mesolytic cleavage in radical anions of 1,2-diphenylethane derivatives... 

The unimolecular rate constants for C-C bond cleavage of 1 measured by ESR in DMSO are collected in Table 15, together with the corresponding activation free energies for mesolytic cleavage, AG m [300]. 

Table 15. Unimolecular rate constants [kf) and activation free energies (AG ni) for mesolytic cleavage of 4-nitrodiphenylethane derivative radical anions 1 (X = H), in DMSO at T = 27 °C.

Table 17. Unimolecular rate constants kf) for mesolytic cleavage of 4 , in MeCN aXT = 27 °C.

Maslak, P. and Narvaez, J. N., Mesolytic cleavage of carbon-carbon bonds a comparison of homolytic and heterolytic processes with similar substrates, Angew. Chem. Int. Ed. Engl, 29, 283, 1990. 

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