Understanding the Origin of Maleic Anhydride Selectivity during the Oxidative Scission of Levulinic Acid

Biomass-derived levulinic acid (LA) is a green platform chemical, and we have previously reported an oxidative scission pathway that selectively transforms it into maleic anhydride (MA). This reaction is curious because it requires oxidative scission of the terminal (methyl) carbon in levulinic acid, whereas gas-phase methyl ketone oxidations are typically selective toward internal (alkyl) bond scission. In order to probe the origin of this disparity, we consider trends observed during the oxidative scission of ketones, keto acids, and keto acid analogues, and we highlight influences of steric hindrances, α-carbon substitution, and the presence of a secondary carboxylic acid functionality. We further consider the role of cyclic intermediates, namely Angelica lactones, in mediating selectivity during the oxidative scission of levulinic acid. Our kinetic analysis is supported by FTIR spectroscopy, which reveals the formation of hydrogen-deficient surface intermediates prior to the onset of oxidative scission. Finally, we pair short-contact-time selectivity analysis with GCMS and NMR spectroscopy to identify a previously undisclosed reaction intermediate - protoanemonin - that forms during the oxidative scission of levulinic acid and α-Angelica lactone. We conclude that facile oxidative dehydrogenation of β-Angelica lactone to form protoanemonin is the major driving force for the high selectivity toward methyl scission during levulinic acid oxidation. We also note that protoanemonin is an intriguing polyfunctional molecule that appears well-suited to bio-based production, and we have observed that it can be synthesized in yields from 55% to 75% (albeit at low concentration presently) during periods of transient reactor operation.