Molecular Structure of Substituted Phenylamine α-OMe- and α-OH-p-Benzoquinone Derivatives. Synthesis and Correlation of Spectroscopic, Electrochemical, and Theoretical Parameters

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Abstract
Thirteen C6 para-substituted anilinebenzoquinones derived from perezone (PZ) (2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-1,4-benzoquinone) were prepared to analyze the effect of the substituents on quinone electronic properties. The effect of a hydrogen bond between the α-hydroxy and carbonyl C4−O4 groups was determined in perezone derivatives by substituting electron-donor and electron-acceptor groups such as −OMe, −Me, −Br, and −CN and comparing the −OH (APZs) and −OMe (APZms) derivatives. Reduction potentials of these compounds were measured using cyclic voltammetry in anhydrous acetonitrile. The typical behavior of quinones, with or without α-phenolic protons, in an aprotic medium was not observed for APZs due to the presence of coupled, self-protonation reactions. The self-protonation process gives rise to an initial wave, corresponding to the irreversible reduction reaction of quinone (HQ) to hydroquinone (HQH2), and to a second electron transfer, attributed to the reversible reduction of perezonate (Q-) formed during the self-protonation process. This reaction is favored by the acidity of the α-OH located at the quinone ring. To control the coupled chemical reaction, we considered both methylation of the −OH group (APZms) and addition of a strong base, tetramethylammonium phenolate (Me4N+C6H5O-), to completely deprotonate the APZs. Methylation led to recovery of reversible, bi-electronic behavior (Q/Q•- and Q•-/Q2-), indicating the nonacidic properties of the NH group. The addition of a strong base resulted in reduction of perezonate (Q-) obtained from the acid−base reaction of APZs with Me4N+C6H5O- to produce the dianion radical (Q•2-). Although the nitrogen atom interferes with direct conjugation between both rings by binding the quinone with the para-substituted ring, the UV−vis spectra of these compounds showed the existence of intramolecular electronic transfer from the respective aniline to the quinone moiety. 13C NMR chemical shifts of the quinone atoms provided additional evidence for this electron transfer. These findings were also supported by linear variation in cathodic peak potentials (Epc) vs Hammett σp constants associated with the different electrochemical transformations: Q/Q•-, Q•-/Q2- for APZms or HQ/HQH2 and Q-/Q•2- for APZs. The electronic properties of model anilinebenzoquinones were determined at a B3LYP/6-31G(d,p) level of theory within the framework of the density functional theory. Our theoretical calculations predicted that all the compounds are floppy molecules with a low rotational C−N barrier, in which the degree of conjugation of the lone nitrogen pair with the quinone system depends on the magnitude of the electronic effect of the substituents of the aniline ring. Natural charges show that C1 is more positive than C4 although the LUMO orbital is located at C4. Hence, if the natural charge distribution in the molecule controls the first electron addition, this should occur at carbon atom C1. If the process is controlled by the LUMO orbitals, however, electron addition would first occur at C4. For the APZms series susceptibility of the first reduction wave to the substitution effect (ρπ = 147 mV) is lower than that of the second reduction wave (ρπ = 156 mV). Thus, the first, one-electron transfer in the quinone system is controlled by the natural charge distribution of the molecule and therefore takes place at C1.

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