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.