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Air-Persistent Monomeric (Amino)(carboxy) Radicals Derived from Cyclic (Alkyl)(Amino) Carbenes

Mahoney, Janell K., Martin, David, Thomas, Fabrice, Moore, Curtis E., Rheingold, Arnold L., Bertrand, Guy
Journal of the American Chemical Society 2015 v.137 no.23 pp. 7519-7525
endothermy, free radicals, half life
A series of monomeric (amino)(carboxy) radicals featuring carbonyl substituents with increasing electron-withdrawing properties (3a, phenyl; 3b, 3,5-bis(trifluoromethyl)phenyl; 3c, perfluorophenyl; 3d, heptafluoropropyl; 3e, 2H-pyrroliumyl) were synthesized in two or three steps from stable cyclic (alky)(amino)carbenes (CAACs). Although (amino)(carboxy) radicals had been previously considered as highly air-sensitive, some of these compounds feature half-lives of hours (3d), and even days (3c and 3e) in well-aerated solutions. DFT calculations show that (amino)(carboxy) radicals evolve from C-centered radical to ambidentate C,O-radicals when increasing the electron-withdrawing properties of the carbonyl substituent. This is paralleled with a destabilization of the peroxide resulting from the addition of dioxygen to the radical. This latter reaction is even predicted to be endothermic for substituents with Hammett constant σₚ > 0.2.