What’s the Diels? Part II

The Diels Alder reaction between furan and maleic anhydride typically it done in THF. The nice thing about this solvent is that the product crystallizes in the reaction media over the course of a week and can be filtered to recover an essentially pure product. My hypothesis was that changing the solvent may influence the reaction’s preference for the exo isomer. I tried a variety of solvents but found that maleic anhydride is sparingly soluble in most solvents at room temperature. Methyl tert-butyl ether and ethyl ether were satisfactory as far as their ability to dissolve maleic anhydride, but they did not provide the variety in solvent polarity and character that I had hoped for. As an undergraduate experiment, the reaction is always done at room temperature with at least a day or two reaction time. I do not know if the reaction can be done quicker with reflux as most Diels Alder reactions are performed. Heating the reaction mixture would also increase the solubility of maleic anhydride. In the chemical literature the reaction has been run in various solvents such as dichloromethane, acetonitrile, acetone, and water. See “stereochemistry of the furan-maleic anhydride cycloaddition,”   “an NMR study of the reaction of furan with maleic anhydride and maleic acid,” and “An experimental and theoretical study of stereoselectivity of furan-maleic anhydride and furan-maleimide Diels-Alder reactions.” Evidently, the experiment needs some more method development to accommodate different solvents that can dissolve maleic anhydride but not the Diels-Alder adduct. Evaporation of the filtrate yielded a gooey mixture of the exo isomer, maleic anhydride, and furan. The reaction is reportedly highly reversible. NMR analysis of the filtrate did yield signals that could be attributed to the endo isomer but they were quite small.

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