Insights into the Diels-Alder Reaction of Furan with Maleic Anhydride from Its Prereactive Intermediate.

The prereactive intermediate in the furan-maleic anhydride cycloaddition, a classical Diels-Alder reaction, has been captured and characterized in pulsed jets by Fourier transform microwave spectroscopy for the first time. The observed species is stabilized by the π-π* interaction between the two moieties, which connects to the endo channel of the cycloaddition. The secondary interactions between the C=C and C=O in the observed isomer are accountable for its lower energy with respect to the one with the exo channel. The present study tries to fill the significant void of the experimental information on prereactive intermediates as the first stage of Diels-Alder cycloadditions, by outlining the stability of the prereactive intermediate and its accurate molecular structure and by emphasizing the role of the π-π* interaction in governing the stereochemical outcome of the reaction.