A comprehensive computational examination of transannular DielsAlder reactions of unsubstituted C14 trienes Barriers, template effects, and the CurtinHammett principle

A combination of MM3-level molecular mechanics calculations and PM3-level semiempirical molecular orbital calculations has been employed, in conjunction with an algorithm for the comprehensive conformational analysis of cyclic compounds, to obtain 1202 unique 1,3,9-cyclotetradecatriene conformations, distributed over the six possible geometrical isomers, and 70 unique transannular Diels–Alder transition structures leading to the six possible stereoisomeric tricyclic olefins. A kinetic analysis that takes into account all minima of a given geometrical isomer and all transition structures leading to the same tricyclic product leads to a free energy of activation that is almost the same as the free energy difference between the lowest minimum and the lowest transition structure (the Curtin–Hammett principle). A substantial template effect, mainly entropic in origin, is found when the transannular reactions are compared to the Diels–Alder reactions of the cognate 2,4-hexatrienes with the 2-butenes. Although t...