Heavy Atom Quantum Tunnelling in N‐Heterocyclic Pentalenes

Heavy atom quantum tunnelling was already observed in several π‐bond‐shifting reactions of antiaromatic molecules. In these, their Jahn‐Teller distortions create sharp energy profiles with small atomic trajectories, which are ideal for observing tunnelling. Herein, we studied the effect of the nitrogen in the N‐heterocyclic pentalenes, distinguishing between isothermic and exothermic reactions. The reactivity was not easily predictable, with our computations revealing a wide variability of rates, from the complete cancellation of the barrier to high isomer stability. For all these systems, quantum tunnelling emerged as the most significant mechanism at room temperature and the only valid one at cryogenic conditions. Heavy atom quantum tunnelling is the only possible mechanism for the π‐bond shifting of antiaromatic molecules at cryogenic conditions. N‐derivatives of pentalene show a rich range of tunnelling rates for this degenerate rearrangement reaction.