2-Pyrones, such as coumalic acid, are promising biobased molecules that through Diels-Alder reactions can provide access to a wide range of biobased chemicals, including molecules with functionality that are not easily accessible via conventional petrochemical routes. A complete reaction network and kinetic parameters for three individual diversification routes that start from a single bicyclic lactone produced via the Diels-Alder cycloaddition of coumalic acid and ethylene were examined experimentally and probed through complementary first-principle density functional theory (DFT) calculations, in situ nuclear magnetic resonance (NMR) spectroscopy, and thin film solid-state NMR spectroscopy. These experiments provide insights into the routes for several molecular structures from bicyclic lactones by leveraging Lewis or Brønsted acid catalysts to selectively alter the reaction pathway. The bicyclic lactone bridge can be decarboxylated to access dihydrobenzenes at a substantially reduced activation barrier using γ-Al2O3 as the catalyst or selectively ring-opened via Brønsted acids to yield 1,3-diacid six membered rings. DFT computations and microkinetic modeling in combination with experimental results provide molecular insights into the catalytically active sites on γ-Al2O3 and provide a general mechanism for the catalyzed bicyclic lactone decarboxylation in polar aprotic solvents, which involves CO2 extrusion as the kinetically relevant step. Solid-state NMR spectroscopy provides direct evidence of strong binding of the bicyclic lactone to the γ-Al2O3 surface, fully consistent with DFT simulation results and experimental reaction kinetics. In addition, the role of the solvent was examined and found to be an additional means to improve reaction rates and selectively produce alternative structures from the bicyclic intermediate. The rate of the decarboxylation reaction was increased dramatically by using water as the solvent whereas methanol acted as a nucleophile and selectively induced ring-opening, showing that both pathways are operative in the absence of catalyst. Taken together, the results demonstrate an approach for selective diversification of the coumalate platform to a range of molecules.
Bibliographical noteFunding Information:
We gratefully acknowledge funding from the National Science Foundation under Award EEC-0813570 and from the Iowa State University Chemical Instrument Facility staff members and computational support from the Minnesota Supercomputing Institute (MSI) at the University of Minnesota and the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy, Office of Biological and Environmental Research at the Pacific Northwest National Laboratory. Furthermore, we would like to acknowledge co-workers in CBiRC for their support.
© 2018 American Chemical Society.
- benzoic acid
- coumalic acid diversification
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- renewable aromatics
- renewable dihydrobenzenes