Are There Only Fold Catastrophes in the Diels–Alder Reaction Between Ethylene and 1,3-Butadiene?

This work revisits the topological characterization of the Diels–Alder reaction between 1,3-butadiene and ethylene. In contrast to the currently accepted rationalization, we here provide strong evidence in support of a representation in terms of seven structural stability domains separated by a sequence of 10 elementary catastrophes, but all are only of the fold type (F and F^†), that is, C₄H₆ + C₂H₄:1–7–[FF]­F­[F^†F^†]­[F^†F^†]­[FF]­F^†–0: C₆H₁₀. Such an unexpected finding provides fundamental new insights opening simplifying perspectives concerning the rationalization of the CC bond formation in pericyclic reactions in terms of the simplest Thom’s elementary catastrophe, namely, the one–(state) variable, one–(control) parameter function.