Amine oxidation. Part XI. Oxidation of some substituted tertiary alkylamines and some NN-dimethylphenethylamines with potassium hexacyanoferrate(III)
Abstract
Kinetic studies on the oxidation of some diamines, amino-alcohols, amino-ethers, phenethylamines, and related tertiary amines by potassium hexacyanoferrate(III) give further information about the interactions that occur in the rate-determining step in these one-electron oxidations of tertiary amines. The over-riding influence in the acyclic diamines and aralkylamines is the electron-withdrawing inductive or field effect of the substituent which reduces the ease of oxidation. In general the closer the substituent is to the reaction centre the greater is the retardation. With the more constrained cyclic, bicyclic, and tricyclic polyamines the lone pair orbitals on the nitrogen atoms are in some instances held by the ring system in a favourable arrangement for through-bond or through-space coupling with the forming aminium radical. These interactions which are superimposed on the inductive and field effects lead to an increase in the ease of oxidation. The greater reactivity of the 1,2-amino-alcohols over their homomorphous amines is attributed to significant concentrations of the alkoxide ions in the alkaline reaction mixture. The difference in the reactivity of the amino-alcohols and their conjugate bases arises, in part from the difference in the inductive effects of the ionised and un-ionised substituents, and in part from through-bond and through-space effects.