Formation of Highly Stable 1,2-Dicarbonyl Organic Radicals from Cyclic (Alkyl)(amino)carbenes

The synthesis of air-persistent organic radicals (APORs) is challenging because of the presence of highly reactive species in air. It was found that N-heterocyclic carbenes (NHCs) are effective in stabilizing radical centers because of their bulky substituents and low-lying empty orbitals, thereby leading to the development of several APORs. Herein, we report the synthesis of two air-stable organic radicals derived from oxalyl chloride and cyclic (alkyl)(amino)carbene (cAAC), which included the unexpected formation of a known (amino)(carboxy) radical cation ([2]BF4) and a 1,2-dicarbonyl radical cation ([3]BF4). Most importantly, the reduced 3-oxetanone compound (4) was discovered as a new product originating from [3]BF4, which differs from the generation of NHC-based 1,2-dicarbonyl radicals. The highly strained 3-oxetanone 4 was obtained by a single-electron reduction of [3]BF4 with a mild reducing agent, chloride salt, and vice versa with a AgBF4 oxidant. Both [2]BF4 and [3]BF4 retained their intrinsic blue color and exhibited high air stability. Furthermore, [3]BF4 produced a temperature-dependent electron paramagnetic resonance spectrum.