The Cope Rearrangement of 1,5‐Dimethylsemibullvalene‐2(4)‐d1: Experimental Evidence for Heavy‐Atom Tunneling

As an experimental test of the theoretical prediction that heavy‐atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, monodeuterated 1,5‐dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm−1 results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10−4 s−1 despite an experimental barrier of Ea=4.8 kcal mol−1 and with only a shallow temperature dependence, the results are interpreted in terms of heavy‐atom tunneling. Heavy hitter: A rare case of heavy‐atom tunneling has been observed experimentally. Monodeuterated 1,5‐dimethylsemibullvalene undergoes Cope rearrangement at temperatures as low as 3 K despite an activation barrier of 4.8 kcal mol−1. These experiments confirm an earlier theoretical prediction.