Catalytic defunctionalization of amino acids from protein waste to value-added nitriles and amines

Amino acids are a potentially interesting feedstock for conversion in chemical industry. They are abundantly available from bio-based resources: they are produced efficiently by fermentation, starting from cheap carbon and nitrogen feedstocks, as is the case for lysine or sodium glutamate; or they can be recovered from protein waste, which is available after processing of agricultural crops in the food industry (e.g. waste from sugar beet or cane), or after production of fuel from biomass. Efficient methods for selective defunctionalization of amino acids are desired. For instance, decarboxylation results in either monofunctional nitrogenous compounds, or in certain cases, in difunctionalized molecules; they allow to close the nitrogen cycle. Separation of amino acids can be tough, as their functional groups often impart an ionic character, but particularly decarboxylation can result in much easier separation and isolation of the desired products. We will present new catalytic routes and mechanisms that result in efficient decarboxylation. Oxidative decarboxylation of amino acids to nitriles can be performed using heterogeneous catalysts and either H2O2 or dioxygen as the terminal oxidant. Using H2O2 and a supported tungstate catalyst, bromides are in situ oxidized to bromonium species; in this way catalytic amounts of bromide suffice to convert the -CH(NH2)COOH group. Nitrile selectivity is excellent, and the system is compatible with amide, alcohol, and in particular carboxylic acid, amine, and guanidine functional groups after appropriate neutralization. The system can also successfully be applied to mixtures of amino acids obtained by acidic hydrolysis of protein waste. In a complementary system, the reaction is performed using air as the oxidant and an ionic Ru catalyst, either homogeneous RuCl3 at pH 7, or immobilized Ru(OH)x on alumina. The system excels by its greenness (water solvent, O2 oxidant, high selectivity) but its substrate scope is slightly narrower than when using H2O2. Nevertheless, glutamate, the most abundant amino acid in proteins from plant biomass, is converted with 70% yield. In an alternative approach, we also explore the non-oxidative decarboxylation to (di)amines. One of the few catalyst types that is effective for this reaction are the unsaturated ketone organocatalysts. While their use has been claimed in patents, their mechanisms of operation have been seldomly studied. We will show how they can be used for decarboxylation of amino acids in concentrations as low as 1 mol%, and how the judicious choice of acid-base co-catalysts sheds light on their mechanisms of operation. For instance, isophorone-catalyzed decarboxylation of lysine allows to obtain the special 1,5-diaminopentane in good to excellent yields.status: publishe