The thermal C2–C6 (Schmittel)/ene cyclization of enyne–allenes – crossing the boundary between classical and nonstatistical kinetics

The thermal C2–C6/ene cyclization of enyne–allenes has become an exciting venue for both mechanistic and synthetic studies. While at first most efforts were aimed at establishing the diradical nature of the thermal process and to derive therefrom efficient protocols for synthetic schemes, recent evidence has disclosed the C2–C6 cyclization as a unique reaction heavily influenced by nonstatistical dynamic effects. Depending on the substituents at the enyne–allene, one can readily identify transitions between classical and nonstatistical behaviors on the basis of experimental data.