Irradiating Suspensions: Blue Light-Driven Isomerization of cis,cis - to trans,trans -Muconic Acid with High Productivities at Low Iodine Catalyst Loading

Abstract Muconic acid is an important biorenewable platform molecule that exists in several diene (i.e., cis,cis-, cis,trans-, trans,trans-) as well as lactone (i.e., muconolactone and muconodilactone) isomers. While cis,cis-muconic acid is the isomer obtained via biosynthetic as well as chemical routes, the trans,trans-isomer is preferred for cycloaddition reactions and its resistance to undesired lactonization avoids unselective reactions in further downstream transformations. Current strategies rely on isomerization of the corresponding esters, i.e., alkyl muconates, avoiding lactonization and concomitantly improving substrate solubility. Herein, we disclose a direct isomerization of cis,cis- to trans,trans-muconic acid. The reaction can be conducted as a concentrated suspension of substrate in ethanol under blue-light irradiation, employing a very low iodine catalyst loading (0.025 mol %). The method is readily scalable in classical laboratory glassware with overhead stirring (up to 200 mmol) effectively transforming the substrate suspension into product suspension, allowing straightforward product isolation by filtration. Remarkably, our findings demonstrate that in contrast to the state of the art, a homogeneous cis,cis-muconic acid solution is not required to achieve high productivities (up to 980 mM h–1).