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

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.