Halogen-atom and group transfer reactivity enabled by hydrogen tunneling

The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. Herein, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunnelling that uses the cyclohexadiene derivative -terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a non-canonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom/group abstraction through the reactivity of an effective H-atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable, but is rendered feasible through quantum tunnelling.