Computational evidence for heavy-atom tunneling in the bergman cyclization of a 10-membered-ring enediyne

DFT and CASSCF calculations for the cyclization of (3Z)-cyclodec-3-en-1,5-diyne were carried out to investigate heavy-atom tunneling. At 37 °C, tunneling was computed to enhance the rate by 38-40% over the transition-state theory rate. Intramolecular (12)C/(13)C kinetic isotope effects were predicted to be substantial, with a steep temperature dependence. These results are discussed in relation to recent experimental findings that show heavy-atom tunneling at moderate temperatures. The calculations point to the possibility of a simple computational test for the likelihood of heavy-atom tunneling using standard quantum-chemical information.