Generation of a cyclopentenyl cation labelled with a single deuterium at C‐4. Rate studies of the scrambling reaction

The kinetics of the hydrogen scrambling reaction in the cyclopentenyl cation 1 were studied. The pure 4‐deutero derivative of 1 was prepared using superacid conditions and the activation energy Δ G ‡ for the rearrangement of this label to the C‐1(3) position of 1 was determined using low‐temperature 2 H NMR spectroscopy. In addition, the sequential rearrangement of the deuterium label from C‐4(5) → C‐1(3) → C‐2 was noted in a qualitative fashion. Based on a statistical analysis, the activation free energy Δ G ‡ for a single hydrogen (deuterium) migration leading to the 4‐cyclopentenyl cation intermediate was determined to be 18.3 kcal mol −1 (−20.8 °C), in good agreement with the less direct determination provided by an earlier NMR line‐broadening and spin‐saturation transfer experiment. Copyright © 2003 John Wiley & Sons, Ltd.