Isolation of a pentadienyl-type radical featuring a central secondary carbon

Stable tertiary R3C· carbon radicals have been known since Gomberg’s pioneering discovery of the triphenylmethyl radical more than a century ago. In stark contrast, secondary R2CH· and primary RCH2· carbon radicals are elusive species only observed spectroscopically. Herein, we describe the isolation of a crystalline pentadienyl-type radical, featuring a central secondary carbon, prepared by single-electron reduction of a bis(imino)carbene conjugate acid. The key to its stability is the presence of two N-heterocyclic imine substituents, which impart both steric protection and electronic stabilization. Density functional theory calculations confirm that the central secondary carbon atom is the principal spin carrier. Accordingly, electron paramagnetic resonance spectroscopy reveals that the hydrogen atom attached to the central carbon atom exhibits an exceptionally large hyperfine coupling constant (>10 G), which we believe is the largest recorded for an isolated organic radical. In the presence of an H· donor, hydrogen atom abstraction occurs exclusively at the central carbon to form a methylene unit. Furthermore, this radical can participate in a radical–radical cross-coupling reaction with [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl], providing an example of cross-coupling between two stable organic radicals. The isolation of secondary and primary carbon radical species is challenging, owing to their instability. Now the reduction of an acyclic bis(imino)carbene conjugate acid enables the isolation of a stable pentadienyl-type radical. In silico and in vitro probing of its properties reveal a propensity to act as a secondary carbon radical.