Understanding the\nOrigin of Maleic Anhydride Selectivity\nduring the Oxidative Scission of Levulinic Acid

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