The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals

Abstract

A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1+cyc, 1+acy, 2+cyc, and 2+acy, respectively. The reduction of 1+cyc and 2+cyc derived from cyclic carbenes affords the corresponding radicals 1cyc and 2cyc that are stable at room temperature. In contrast, radicals 1acy and 2acy, derived from acyclic carbenes, cannot be isolated. It is shown that 1acy is as thermodynamically stabilized as its cyclic counterpart 1cyc, but its instability is the result of β-hydrogens of the nitrogen substituent, along with the enhanced flexibility around C−N bonds, which allow for a H.-migration-elimination process. Radical 2acy is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2+acy and enolate 2−acy. This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.