Synthesis, Physicochemical Properties, and Amine-Oxidation Reaction of Indolequinone Derivatives as Model Compounds of Novel Organic Cofactor TTQ of Amine Dehydrogenases

3,4-Disubstituted 6,7-indolequinones [1,3-dimethyl-4-(3′-methylindol-2′-yl)indole-6,7-dione (2), 3-methyl-4-phenylindole-6,7-dione (3), and 3,4-dimethyl-6,7-dione (4)] and a 3,7-disubstituted 4,5-indolequinone [3,7-dimethylindole-4,5-dione (5)] have been synthesized as models for the novel organic cofactor TTQ of bacterial amine dehydrogenases. The substituent and structural effects on the physicochemical properties of the quinones have been investigated in detail by comparing the spectroscopic data (UV-vis, IR, ¹H- and ¹³C-NMR), pK_a values of the pyrrole proton, and the two-electron redox potentials with those of model compound 1 [3-methyl-4-(3′-methylindol-2′-yl)indole-6,7-dione] previously reported (ref 5). Reactivity of each quinone in the transamination process [iminoquinone formation (k₁), rearrangement to product-imine (k₂), and aminophenol formation (k₃)] has been investigated kinetically, revealing that the substituent and structural effects on the amine-oxidation reaction are not so significant. In the aerobic catalytic oxidation of benzylamine, however, the aromatic substituents on the quinone ring play an important role to protect the quinone from the deactivation process of a Michael-type addition by the amine, making it act as an efficient turnover catalyst.