Molecular structure of substituted phenylamine alpha-OMe- and alpha-OH-p-benzoquinone derivatives. Synthesis and correlation of spectroscopic, electrochemical, and theoretical parameters

Thirteen C-6 para-substituted anilinebenzoquinones derived from perezone (P Z) (2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-1,4-benzoquinone) were pr epared to analyze the effect of the substituents on quinone electronic prop erties. The effect of a hydrogen bond between the alpha -hydroxy and carbon yl C-4-O-4 groups was determined in perezone derivatives by substituting el ectron-donor and electron-acceptor groups such as -OMe, -Me, -Br, and -CN a nd comparing the -OH (APZs) and -OMe (APZms) derivatives. Reduction potenti als of these compounds were measured using cyclic voltammetry in anhydrous acetonitrile. The typical behavior of quinones, with or without a-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 react ion of quinone (HQ) to hydroquinone (HQH(2)), and to a second electron tran sfer, attributed to the reversible reduction of perezonate (Q(-)) formed du ring the self-protonation process. This reaction is favored by the acidity of the alpha -OH located at the quinone ring. To control the coupled chemic al reaction, we considered both methylation of the -OH group (APZms) and ad dition of a strong base, tetramethylammonium phenolate (Me4N+C6H5O-), to co mpletely deprotonate the APZs. Methylation led to recovery of reversible, b i-electronic behavior (Q/Q(.-) and Q(.-)/Q(2-)), indicating the nonacidic p roperties of the NH group. The addition of a strong base resulted in reduct ion 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 q uinone with the para-substituted ring, the UV-vis spectra of these compound s showed the existence of intramolecular electronic transfer from the respe ctive aniline to the quinone moiety. C-13 NMR chemical shifts of the quinon e atoms provided additional evidence for this electron transfer. These find ings were also supported by linear variation in cathodic peak potentials (E -pc) vs Hammett a, constants associated with the different electrochemical transformations: Q/Q(.-), Q(.-)/Q(2-) for APZms or HQ/HQH(2) and Q(-)/Q(.2- ) for APZs. The electronic properties of model anilinebenzoquinones were de termined 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 wh ich the degree of conjugation of the lone nitrogen pair with the quinone sy stem depends on the magnitude of the electronic effect of the substituents of the aniline ring. Natural charges show that C-1 is more positive than C- 4 although the LUMO orbital is located at C4. Hence, if the natural charge distribution in the molecule controls the first electron addition, this sho uld occur at carbon atom C-1. If the process is controlled by the LUMO orbi tals, however, electron addition would first occur at C-4. For the APZms se ries susceptibility of the first reduction wave to the substitution effect (rho (pi) = 147 mV) is lower than that of the second reduction wave (rho (p i) = 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 plac e at C-1.