There is a widespread perception that the high level of endo 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 endo:exo ratios close to 1:1. Maleonitrile, butenolide, α-methylene γ-butyrolactone, and N-methylmaleimide behave differently, as does methyl vinyl ketone under Lewis acid catalysis. CBS-QB3 calculations incorporating solvent and temperature parameters give endo:exo product ratios that are in near quantitative agreement with these and earlier experimental findings. This work challenges the preconception of innate endo-selectivity by providing the first experimental evidence that the simplest Diels-Alder reactions are not endo-selective. Trends in behaviour are traced to steric and electronic effects in Diels-Alder transition structures, giving new insights into these fundamental processes.
CITATION STYLE
Lording, W. J., Fallon, T., Sherburn, M. S., & Paddon-Row, M. N. (2020). The simplest Diels-Alder reactions are not: Endo -selective. Chemical Science, 11(43), 11915–11926. https://doi.org/10.1039/d0sc04553e
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