Captodative effect benzylic radicals

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

Merenyi R, Janousek Z, Viehe HG (1986) Studies on the captodative effect. Entropy/enthalpy compensation as solvent effect in radical forming reactions. A relative radical stabilisation scale. In Viehe HG (ed) Substituent effects in radical chemistry. Reidel, Dordrecht, pp 301-324 Merga G, Schuchmann H-P, Rao BSM, von Sonntag C (1996) The oxidation of benzyl radicals by Fe(CN)63. J Chem Soc Perkin Trans 2 551-556... 

Intermolecular radical reactions have also been reported, including selective radical-radical coupling reactions. These reactions involve the formation of a-amino acid radicals (stabilized by the captodative effect), which can couple to, for example, benzyl radicals to form phenylalanine derivatives (Scheme 21). The benzyl radicals are generated by hydrogen-atom abstraction from toluenes using alkoxyl radicals derived from peroxides and/or aromatic ketone sensitizers. 

Ruggero Curd of the Universita di Bari has reported Tetrahedron Lett. 2007, 48, 3575) the stereospecific hydroxylation of 1,3-dimethyl cyclohexane 4 to the diol 6. Yasuyuki Kita of Osaka University has developed Organic Lett. 2007, 9, 3129) conditions for specific benzylic oxidation, converting 7 into 8 with high diastereocontrol. Larry E. Overman of the University of Cahfomia, Irvine has established Organic Lett 2007,9, 5267) that by using a slow H-atom donor, it was possible to effect intramolecular H abstraction, leading, by oxidation of the intermediate captodatively-stabilized radical, from 9 to the acetate 10. 

---