Electrochemical synthesis of amino acids by reductive amination of keto acids. II. Reduction at platinum black and palladium black electrodes

Abstract

2-Amino-2-phenylacetic acid, monoammonium glutamate, 2-aminobutyric acid, α-alanine and ammonium 5-amino-5-phenylvalerate have been prepared by electrochemical reductive amination of the corresponding keto acids (or their sodium salts) in aqueous ammonia. Platinum black, platinum black/Teflon composite on tantalum gauze and palladium black were used as electrodes. The yields ranged from 40% for α-alanine and ammonium 5-amino-5-phenylvalerate to 83% for 2-amino-2-phenylacetic acid, at ammonia concentrations from 4.7 to 15.5 M. The corresponding reduction potentials were all between -0.82 and -0.90 V relative to a saturated calomel electrode. Decreasing the ammonia concentration decreased both the yield and the current efficiency for reductive amination of 2-oxo-2-phenylacetic acid.     2,5-Diphenylimidazol-4-ol was prepared and isolated from a concentrated solution of 2-oxo-2-phenylacetic acid in strong ammonia. Small amounts of this compound probably cause the yellow coloration observed during electrolysis of 2-oxo-2-phenylacetic acid.