The Great Escape? Investigations of Tunneling Contributions to the Valence Tautomerizations of Benzene Chalcogenides and Heteroepins

ABSTRACT Valence tautomerizations are prototypical pericyclic rearrangements with broad relevance in (bio)organic chemistry that can exhibit significant contributions from quantum mechanical tunneling (QMT), as demonstrated previously e.g., for the oxepin—benzene oxide equilibrium. Such contributions were evaluated computationally for the valence tautomerizations of heteroepins C 6 H 6 X (X = S, Se, and Te) and thiepin oxides (X = SO or SO 2 ). Ring contraction of the heteroepins becomes increasingly exothermic from X = O to X = Te; however, as an exception to the Bell–Evans–Polanyi principle, the associated activation barriers also increase. Analysis of the intrinsic reaction coordinates reveals that the critical C1–C6 distance in the heteroepins strongly widens within this series, resulting in increasingly wider and higher barriers despite the growing thermodynamic driving force. Consequently, estimated tunneling probabilities decrease dramatically across the series, ruling out tunneling as a significant contributor to the experimentally observed instability of thiepin, selenepin, and tellurepin. In contrast, the valence tautomerizations of thiepin‐ S ‐oxide and thiepin‐ S,S ‐dioxide follow conventional Bell–Evans–Polanyi behavior. The equatorial conformer of thiepin‐ S ‐oxide is expected to not exhibit any appreciable tunneling reactivity, while its axial conformer is projected to undergo rapid ring contraction by heavy‐atom tunneling. The bicyclic tautomer of C 6 H 6 SO 2 is predicted to ring‐expand to the less strained thiepin‐ S,S ‐dioxide with a half‐life suitable for matrix isolation spectroscopy experiments. Overall, this study of the potential role of QMT in the valence tautomerizations of heteroepins and thiepin oxides provides broader insights into how characteristic molecular features influence the intrinsic barrier width of a reaction.