Experimental and Theoretical Investigations into the Stabilization of Captodative (Amino)(Carboxy) Radicals
Remarkable advances in the field of radical chemistry have been made since the seminal isolation of tri(phenyl)methyl radical by Gomberg in 1900. However, carbon centered radicals are still primarily considered as observable, but non-isolable species. Two key factors have inhibited the isolation of these species, namely their inclination to dimerize due to a high C-C bond dissociation energy, and their proclivity to react with oxygen to form peroxides and other derivatives. Herein, we will demonstrate stable monomeric C-centered (amino)(carboxy) radicals, and even related bi- and tri-radical species, can be successfully synthesized by utilizing stable carbenes as building blocks. We will also establish that simple modifications to the acyl R-substituent of these paramagnetic species can lead to highly air persistent variants. Indeed bulky and electron-withdrawing acyl substituents lead to ambidentate C,O-radicals with life-times ranging from several hours to days in the most favorable case. We will then highlight the importance of subtle steric factors, which not only affect the susceptibility of these systems toward undesired reactions, but also control the intrinsic stability of the paramagnetic species, in a comparative study on (amino)(carboxy) radicals constructed from cyclic and acyclic carbenes. Lastly, we will discuss a di-iminium with a flexible linking acyl moiety which undergoes spontaneous intramolecular cyclization upon a two-electron reduction, which surprisingly exhibits two requirements of a molecular switch, namely bistability and reversibility.