• The above calculations are in good agreement with the previous calculations by Ess et al. In addition, we explored the plausible [4+2]-ene mechanisms that were not considered before. The large computed barriers (> 31 kcal/mol at ωB97XD/def2-TZVP level of theory) are consistent with the fact that the corresponding [4+2]-ene products 5 and 6 were not observed experimentally. and the exo cycloaddition is given in the SI. The profiles at 0 GPa are from the intrinsic reaction coordinate calculations; the highpressure profiles are computed using the XP-PCM method based on the 0 GPa structures.