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Molecular Structure of Substituted Phenylamine α-OMe- and α-OH-p-Benzoquinone Derivatives. Synthesis and Correlation of Spectroscopic, Electrochemical, and Theoretical Parameters
journal contribution
posted on 2001-11-08, 00:00 authored by M. Aguilar-Martínez, J. A. Bautista-Martínez, N. Macías-Ruvalcaba, I. González, E. Tovar, T. Marín del Alizal, O. Collera, G. CuevasThirteen 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.