Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

Abstract

Probing quantum mechanical tunneling (QMT) in chemical reactions is crucial to understanding and developing new transformations. Primary H/D kinetic isotopic effects (KIEs) beyond the semiclassical maximum values of 7‒10 (room temperature) are commonly used to assess substantial QMT contributions in one‐step hydrogen transfer reactions, because of the much greater QMT probability of protium vs. deuterium. Nevertheless, we report here the discovery of a reaction model occurring exclusively by H‐atom QMT with residual primary H/D KIEs. A 2‐hydroxyphenylnitrene, generated in N2 matrix, was found to isomerize to an imino‐ketone via sequential (domino) QMT involving anti to syn OH‐rotamerization (rate determining step) and [1,4]‐H shift reactions. These sequential QMT transformations were also observed in the OD‐deuterated sample, and unexpected primary H/D KIEs between 3 and 4 were measured at 3 to 20 K. Analogous residual primary H/D KIEs were found in the anti to syn OH‐rotamerization QMT of 2‐cyanophenol in a N2 matrix. Evidence strongly indicates that these intriguing isotope‐insensitive QMT reactivities arise due to the solvation effects of the N2 matrix medium, putatively through coupling with the moving H/D tunneling particle. Should a similar scenario be extrapolated to conventional solution conditions, then QMT may have been overlooked in many chemical reactions.