Chemical Properties of Phenanthrolinequinones and the Mechanism of Amine Oxidation by o-Quinones of Medium Redox Potentials

Article abstract of DOI:10.1021/ja00351a049

The protonic acid-base properties, equilibria for hydroxide-mediated hydrations and ring contraction, and electrochemistry of the three isomeric phenanthroline-5,6-quinones <1,10 (1), 1,7 (2), and 4,7 (3)> are reported and discussed.Pyridine nitrogens peri to the o-quinone carbonyl groups increase the equilibrium constants for addition of H2O and MeOH to the latter (3>2>1).The reductions of the phenanthroline-5,6-quinones by 1-phenylethanols, hydrazine, cyclohexylamine, glycine, morpholine, and N,N-dimethylbenzylamine have been examined.In general the order of the rate of substrate oxidation is 3>2>1.Various mechanisms for amine oxidation by high-potential quinones (such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone) and low- or medium-potential quinones (such as 1, 2, and 3) are discussed, and it is concluded that the mechanisms are different.Hydrazine and N,N-dimethylbenzylamine reduce the phenanthrolinequinones to the corresponding quinols.The oxidation of N,N-dimethylbenzylamine is catalyzed by the amine-free base and hydroxide ion.The oxidations of cyclohexylamine (to cyclohexanone) and glycine provide 5-amino-6-hydroxyphenanthrolines while the oxidation of morpholine provides the aminoquinol 5.The nature of the quinone reduction products and the involvement of general base catalysis establish that the quinone plus amine redox reactions proceed via the covalent addition of amine to quinone followed by α-proton removal by general base catalysis.