Diels–Alder Reactions of Symmetrically 1,4‐Disubstituted Dienes: Theoretical Study on the Influence of the Configuration of the Double Bonds on the Regio‐ and Endoselectivity

It was experimentally shown in Diels‐Alder reactions that symmetrically 1,4‐disubstituted dienes exhibit high regio‐ and endoselectivity induced by the ( E , Z ) configuration of the double bonds. In order to understand the origin of this selectivity, the transition states associated with the reaction between a series of such dienes (R = OMe, CH 3 , NH 2 , F, CN) on substituted ethylene (R′ = CO 2 Me, CN, F, OMe) were determined by DFT calculations. If the regioselectivity confirmed by a single‐point MP2 calculation is predicted to be good in most cases, its direction remains difficult to predict; the R′ group of the dienophile can orient itself toward either the ( Z ) or the ( E ) side of the diene. Neither primary nor secondary frontier orbital interactions appear to be able to rationalize these results. They are more likely explained by a delicate balance between steric and electrostatic interactions. This hypothesis is further supported by the calculated reactivity of the ( E , E ) and ( Z , Z ) corresponding dienes. In any case, a standard DFT calculation, confirmed by a single‐point MP2 step, is likely to provide a reliable guideline for a synthesis strategy that allows the control of up to four stereogenic centres in a single reaction step.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)