Controlled deoxygenation of citric and amino acids towards valuable chemicals

The production of chemicals from biomass is of large interest to the sustainable chemistry community. In this domain the controlled defunctionalisation of highly functional bio-based compounds is largely undervalued. In that regard, the presented doctoral research focused on the conversion of citric and amino acids, which can be obtained in a sustainable way via fermentation of sugar-rich waste streams, such as sugar cane vinasse, or via hydrolysis of protein-rich waste streams, such as slaughter waste or press cakes from soybean, algae etc. First, the existing literature on the use of citric and amino acids as platform chemicals is reviewed and a brief introduction on their origin is given. This PhD research aimed at developing new chemocatalytic routes for the valorisation of citric and amino acids, using heterogeneous catalysts in water. Water constitutes a cheap, environmentally benign solvent that easily dissolves highly functional bio-based compounds. In the first experimental part, we developed an innovative route for the direct production of methylsuccinic acid from the cheap and widely available citric acid. Methylsuccinic acid is a promising building block for the production of e.g. biodegradable polyesters. The new reaction sequence comprised a dehydration and a decarboxylation, followed by a hydrogenation. Methylsuccinic acid yields of up to 91% could be achieved when the decarboxylation and hydrogenation rate were perfectly balanced. At 225°C, the reaction sequence only needed 40 min to reach completion and the fast decarboxylation rate was e.g. matched by using 0.5 mol% Pd/C and 8 bar H2. Also cheaper Ni catalysts could be used, although only at lower temperatures and by applying e.g. a stabilising carbon coating, a catalyst loading of 5 mol% Ni and a H2 pressure of 30 bar. In the second part, the metal-catalyzed decarboxylation of amino acids to aliphatic amines was investigated. Aliphatic amines are versatile building blocks for the production of agrochemicals, pharmaceuticals, surfactants etc. The Pd-catalyzed direct decarboxylation, already applied for the conversion of (pyro)glutamic acid to pyrrolidone, could be extended to the production of pyrrolidine from proline. To achieve high conversions with high amine selectivities, however, a careful modification of the Pd catalyst with Pb was necessary to avoid consecutive side reactions, such as dehydrogenation to pyrrole and ring opening hydrogenolysis that initiates the formation of propane. Unfortunately, this catalytic system was not applicable for the formation of primary amines from other amino acids. This is why we further on investigated the Ru-catalyzed reductive decarboxylation. This decarboxylation mechanism was shown to occur via a double hydrogenation to an amino alcohol intermediate, followed by dehydrogenation back to the amino aldehyde and subsequent decarbonylation to the desired amine. Balanced acidic aqueous conditions and H2 pressures higher than 20 bar afforded 87% isobutylamine from valine at 150°C. Moreover, this catalytic system proved to be applicable for the convergent synthesis of primary, aliphatic amines from a wide range of natural amino acids. To conclude, a critical evaluation from a chemical point of view was made to identify the most promising target compounds from citric and amino acids. The analysis based on atom economy and functionality index revealed several attractive reactions: the dehydration-decarboxylation-hydrogenation of citric acid to methylsuccinic acid, the mild hydrogenation of amino acids to chiral amino alcohols, the C‑N hydrogenolysis of N,N-dimethylglutamic acid to dimethyl glutarate etc. In addition, also broadly applicable decarboxylation methods, such as the Ru-catalyzed reductive decarboxylation, were identified as highly interesting for the valorisation of protein waste. The considerable progress in the field of controlled defunctionalisation made in this PhD research, as well as in related work, should convince the chemical community to (re)consider citric and amino acids as valid bio-based platform chemicals.status: publishe