Netherlands' Catalysis and Chemistry Conference edition:15 location:Noordwijkerhout, the Netherlands date:10-12 March 2014
Oxidative decarboxylation of α-amino acids is generally mediated by hypobromite (‘Br+’) species, which are produced from organic reagents like N-bromosuccinimide or by NaOCl induced oxidation of NaBr. However, large amounts of (in)organic waste products are generated. On the other hand, it has been shown that the enzyme vanadium chloroperoxidase is able to perform the oxidative decarboxylation of L-glutamic acid and L-phenylalanine with hydrogen peroxide (H2O2). Based on previous work on olefin epoxidation, a tungstate-exchanged layered double hydroxide (LDH) has been developed as a heterogeneous mimic of the enzyme’s active site: electrostatically immobilized tungstate ions are transformed in situ by H2O2 into peroxotungstate species, which in turn oxidize bromide ions to hypobromite species. A catalytic system containing LDH and NH4Br in an organic solvent or even water allows selective oxidative decarboxylation of α-amino acids into nitriles, whereas in enzyme-catalyzed reactions selectivity is often reduced due to aldehyde formation. H2O2 is added in a controlled way to increase oxidant efficiency. In this way, phenylalanine can be transformed into phenylacetonitrile with 88% yield using 8 equivalents of H2O2. The catalyst material is stable under reaction conditions and recycling is easy.
The substrate scope has been extended to other α-amino acids. Alanine, valine, leucine, isoleucine and norleucine were converted into aliphatic nitriles with 88-100% yield. Moreover, the system is compatible with alcohol and amide moieties. For carboxylic acids, amines and guanidines, neutralization with respectively NaOH or HBr prior to decarboxylation is recommended; nitrile yields are often > 90%. Methionine has been transformed in two steps into its nitrile-sulfone derivative.