Nonstatistical dynamics in the thermal C2-C6/diels-alder cyclization of enyne-allenes: effect of topology.

The thermal C(2)-C(6) (Schmittel) cyclization of an enyne-allene with two aryl rings at the allene terminus experimentally leads to three formal Diels-Alder (DA) cycloaddition products, two of which (involving the dimethylamino phenyl unit) are shown to form in a nonstatistical process. DFT computations on the reaction paths reveal that the two aryl rings (Ph vs PhNMe2) do not interact in a dynamic manner as their minimum energy pathways (MEPs) are separated by a large barrier. The preferential formation of the more-hindered DA product 8 (ortho to the dimethylamino group) over the less-hindered product 9 (para to the dimethylamino group), despite the higher energy TS for 8, suggests the occurrence of nonstatistical dynamics in the cyclization onto the dimethylamino phenyl unit, though. Potential energy surface (PES) computations indicate that the large amount of nonstatistical dynamics (97%) arises from facile IRC dynamics (left picture) that is compared with the non-IRC dynamics of a related system (∼76%, right picture, J. Org. Chem. 2014, 79, 2368).