The Mechanism of Carbon Dioxide Catalysis in the Hydrogen Peroxide N-Oxidation of Amines

Abstract

The reactivity of the peroxymonocarbonate ion, HCO4- (an active oxidant derived from the equilibrium reaction of hydrogen peroxide and bicarbonate), has been investigated in the oxidation of aliphatic amines. Tertiary aliphatic amines are oxidized to the corresponding N-oxides in high yields, while secondary amines give corresponding nitrones. A closely related mechanism for the H2O2 oxidation of tertiary amines catalyzed by CO2 (under 1 atm) and H2O2 at 25 degrees C is proposed. The rate laws for the oxidation of N-methylmorpholine (1) to N-methylmorpholine N-oxide and N,N-dimethylbenzylamine (2) to N,N-dimethylbenzylamine N-oxide have been obtained. The second-order rate constants for the oxidation by HCO4- are k1 .016 M(-1) s(-1) for 1 in water and k1=0.042 M(-1) s(-1) for 2 in water/acetone (5:1). The second-order rate constants for tertiary amine oxidations by HCO4- are over 400-fold greater than those for H2O2 alone. Activation parameters for oxidation of 1 by HCO4- in water are reported (DeltaH=36+/-2 kJ mol(-1) and DeltaS=-154+/-7 J mol(-1) K(-1)). The BAP (NH4HCO3-activated peroxide) or CO2/H2O2 oxidation reagents are simple and economical methods for the preparation of tertiary amine N-oxides. The reactions proceed to completion, do not require extraction, and afford the pure N-oxides in excellent yields in aqueous media.