The simplest Diels-Alder reactions are not -selective.

There is a widespread perception that the high level of selectivity witnessed in many Diels-Alder reactions is an intrinsic feature of the transformation. In contrast to expectations based upon this existing belief, the first experimental Diels-Alder reactions of a novel, deuterium-labeled 1,3-butadiene with commonly used mono-substituted alkenic dienophiles (acrolein, methyl vinyl ketone, acrylic acid, methyl acrylate, acrylamide and acrylonitrile) reveal kinetic  :  ratios close to 1 : 1. Maleonitrile, butenolide, α-methylene γ-butyrolactone, and -methylmaleimide behave differently, as does methyl vinyl ketone under Lewis acid catalysis. CBS-QB3 calculations incorporating solvent and temperature parameters give  :  product ratios that are in near quantitative agreement with these and earlier experimental findings. This work challenges the preconception of innate -selectivity by providing the first experimental evidence that the simplest Diels-Alder reactions are not -selective. Trends in behaviour are traced to steric and electronic effects in Diels-Alder transition structures, giving new insights into these fundamental processes.