LAUSR

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. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-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 noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or 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 tunneling. Description Carbon–hydrogen bonds through tunneling Much of organic chemistry revolves around bonding carbon to atoms other than hydrogen. Occasionally, it is necessary to restore a carbon–hydrogen bond, and the methods can be surprisingly cumbersome, often involving tin reagents. Constantin et al. report a photoinitiated method to transform carbon–halogen and analogous carbon–heteroatom bonds into carbon–hydrogen bonds using a cyclohexadiene derivative that undergoes aromatization. Kinetic studies support a quantum mechanical tunneling mechanism for the transfer of hydrogen atom equivalents. —JSY Cyclohexadiene derivatives deliver hydrogen atom equivalents through a tunneling mechanism.