Nonstatistical Dynamic Effects in the Thermal C2-C6 Diels-Alder Cyclization of Enyne-Allenes

The Diels-Alder (DA) reaction channel of the thermal C-2-C-6 (Schmittel) cyclization of enyne-allenes is studied computationally and experimentally evaluating the influence of temperature on product ratios. Remote substituents at the alkyne terminus influence the mechanism of the C-2-C-6/DA cyclization steering it either to a stepwise or a concerted course. Temperature independent product ratios, selectivity of product formation, and computational results obtained at (U)BLYP/6-31G(d) level unveil a mechanism that is strongly controlled by nonstatistical dynamics.