Quantification of nonstatistical dynamics in an intramolecular Diels-Alder cyclization without trajectory computation.

Experimental and computational (DFT) investigations reveal that enyne-allenes with an aryl group as probe at the allene terminus follow a dynamic non-IRC Diels-Alder cyclization pathway. Starting from two separate C(2)-C(6) (Schmittel) transition states (TS), two distinct reaction paths originate that share a common diradical intermediate, however, without mixing! Because the momentum of the initial TS is transmitted into product formation, we suggest a simple protocol without trajectory computations to estimate the fraction of molecules that follow nonstatistical dynamics: It was calculated from the partitioning at the TSs, as derived from DFT computations, and the experimental ratio. The thus-determined percentage of dynamically reacting molecules only slightly depends on the depth of the intermediate well but rather on ΔΔG(‡) of the initial and the follow-up transition states.