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Abstract:

Nitrogen-based functionalities, in particular amines and amides, are present in a broad range of both commodity and fine chemicals. They are generally obtained by modification of nitriles, which are the result of nitrogen incorporation into olefins. Nitrile production is essential in chemical industry, as is exemplified by large-scale processes such as ammoxidation of propylene into acrylonitrile or hydrocyanation of butadiene into adiponitrile. However, the necessity of toxic reagents (e.g. HCN) or the harsh process conditions are strong incentives to explore alternative production routes. Nitrogen-containing biomass constituents, in particular the protein fraction, provide an excellent renewable resource for that purpose. Protein-rich waste streams are abundantly available from agro-industry and biofuel production and generally contain 20-40 wt% of proteins.[1] Recently, it has been shown that the amino acid constituents, obtained upon hydrolytic depolymerization, can be transformed smoothly and selectively into - often functionalized - nitriles via oxidative decarboxylation mediated by hypobromite species.[2] Instead of using halogenated organic reagents[3] or hypohalite oxidants,[4] catalytic amounts of a bromide salt are oxidized in situ using H2O2 and a heterogeneous catalyst. Here, we present a more benign strategy for oxidative decarboxylation of amino acids into nitriles, which allows using oxygen as terminal oxidant. Heterogeneous catalysts, based on higher valent ruthenium species immobilized on a solid support, e.g. Ru(OH)x/Al2O3, are already known to facilitate aerobic oxidation of primary and secondary amines.[5] This was proven to be a powerful system for nitrile and imine synthesis, but now their remarkable activity in amino acid oxidation has been elucidated too. The reaction is run in water with high conversion and good nitrile selectivity; the major amide byproduct is due to ruthenium-catalyzed nitrile hydrolysis,[6] which is inevitable under these conditions. This was illustrated for amino acids with an aliphatic side chain, such as alanine, valine, leucine and isoleucine, as well as compounds with a functionalized side chain, such as glutamic acid (Scheme 1). The latter one is the most abundant amino acid constituent in plant biomass and the nitrile product is a precursor for acrylonitrile[4] and succinonitrile.[7] In conclusion, this strikingly green oxidative transformation allows producing bio-based nitriles in the absence of any halide source and allows to close the N-loop. Scheme 1. Ruthenium-catalyzed aerobic oxidative decarboxylation of leucine and glutamic acid. [1] T.M. Lammens, M.C.R. Franssen, E.L. Scott, J.P.M. Sanders, Biomass Bioenerg. 2012, 44, 168-181. [2] L. Claes, R. Matthessen, I. Rombouts, I. Stassen, T. De Baerdemaeker, D. Depla, J.A. Delcour, B. Lagrain, D.E. De Vos, ChemSusChem, accepted. [3] G. Laval, B.T. Golding, Synlett 2003, 4, 542-546. [4] J. Le Nôtre, E.L. Scott, M.C.R. Franssen, J.P.M. Sanders, Green Chem. 2011, 13, 807-809. [5] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2003, 42, 1480-1483. [6] K. Yamaguchi, M. Matsushita, N. Mizuno, Angew. Chem. Int. Ed. 2004, 43, 1576-1580. [7] T.M. Lammens, J. Le Nôtre, M.C.R. Franssen, E.L. Scott, J.P.M. Sanders, ChemSusChem 2011, 4, 785-791.