Anion pi-radicals of fused-ring oxazoles.
Aromatic hydrocarbons and heterocyclics have been the subject of much electrochemical research. This is principally because quantum mechanics and its treatments may be correlated with data directly available from electrochemical experimentation. The first electrochemical reductions of polycyclic aromatic hydrocarbons were conducted by Wawzonek and Latinen (1942) and Wawzonek and Fan (1946). These pioneering experiments, however, yielded limited data concerning the reduction mechanism. Later workers, principally Hoijtink et al. (1954), proposed using an aprotic environment, thus eliminating several solvent interference problems. Hoijtink's investigations showed that, in solvents with a relatively low proton availability, polycyclic aromatic hydrocarbons undergo a one-electron reduction to form an anion pi-radical. This process is analogous to reduction of organic species by alkali metals in solvents such as tetrahydrofuran and dimethoxyethane (Hoijtink et al., 1956; Jaqur-Grodzinski et al., 1965).
Data from subsequent electrochemical reduction experiments in aprotic solvent supporting-electrolyte systems revealed that the anion pi-radicals were stable, and that their formations were reversible and diffusion-controlled (Gough and Powers, 1966). Aromatic polycyclics undergo further reduction at negative potentials beyond that required to form the [pi]-anion, but the characteristics of these processes are complicated (Hoijtink et al., 1956; Aten et al., 1959; Aten and Hoijtink, 1959). The second reduction current is typically about half a volt more negative, one-electron and irreversible or quasi-reversible in nature (Aten et al., 1959; Aten and Hoijtink, 1959).
Most higher potential current waves show slower rates of electron transfer. These rates are affected by the potential range where the transfer takes place. The irreversibility may be attributed to the fact that the dianions are sufficiently reactive to abstract protons from the solvent. The discussions of this paper will be limited only to the first wave or low-potential reduction of fused ring oxazoles.
Work described herein is concerned specifically with the electrochemical reduction of the fused-ring oxazole compounds: 2-phenylbenzoxazole (2-PBO), 2-phenylnaphthoxazole (2-PNO), 2-naphthylbenzoxazole (2-NBO), and 2-naphthylnaphthoxazole (2-NNO) (Fig. 1). Interest in these substances stemmed from research reported previously on 2,5-substituted oxazoles by Smith et al. (1972) and Greig and Rogers (1970). The 2,5-substituted oxazoles undergo a reduction characteristic of the general aromatic model described above by Smith et al. (1972) and Greig and Rogers (1970).
[FIGURE 1 OMITTED]
Smith et al. (1972) showed that substituted oxazoles could accept an electron into the pi-system and form stable anion pi-radicals if the oxazole ring is 2,5-substituted by aromatic rings. For the aromatic substituted oxazole to form a conjugated system, all the structural components must be coplanar. This is a matter of probability because the substituents of this molecule may exhibit rotation about a sigma bond. A fused system, however, incorporates the heterocycle directly, ensuring conjugation. The effect of extending the conjugated system by means of fusing a large aromatic structure to the 4- and 5- positions of the oxazole ring has not been considered. The effects of this type of addition in terms of thermodynamic and kinetic considerations are shown to predictable only to a limited degree.
A standard three-electrode cell consisting of working, counter and reference electrodes was employed. The data presented in this study were collected with a saturated calomel reference electrode (s. c. e.), a platinum mesh counter electrode, a platinum disc working electrode for cyclic voltammetry, and a dropping mercury working electrode for polarographic experiments. A Princeton Applied Research Model 273 Programmable Potentiostat was employed.
Benzoxazole was obtained commercially (Aldrich). The remaining compounds, 2-PNO, 2-NBO and 2-NNO were synthesized according to known procedures and recrystallized until sharp melting points were obtained in agreement with the literature values (Somayajula and Subba Rao, 1964; Tanimoto et al., 1967; Kaempfen, 1972; Pushkina and Postovskii, 1964; Fries et al., 1935).
Spectral grade N, N-dimethylformamide distilled from anhydrous copper sulfate was used as a solvent. Tetra-N-propylammonium perchlorate recrystallized from 20 percent acetonitrile-water solution was used as a supporting electrolyte in all experiments. All solutions analyzed were prepared to be 0.1 molar in supporting electrolyte and 1 mM in reductant.
All polarograms and cyclic voltammograms were recorded after establishing current-voltage baselines in solutions absent the reductant. Indications of chemi-adsorption were not observed for the oxazoles on mercury or platinum.
Experimentally determined polarographic [E.sub.1/2] values of the four compounds studied appear in Table 1. Each of the four compounds examined exhibits a first wave that is diffusion-controlled (Table 2). Additionally, the waves are reversible and one-electron in nature (Table 2). These diagnoses are made based on polarographic and cyclic voltammetric data using well-known criteria (Bard and Faulkner, 1980). These criteria are based on the polarographic diffusion current [i.sub.d], currents at defined voltages on the rising portion of the polarographic wave, i, and the adjusted mercury head above the dropping mercury electrode, h. Cyclic voltammetric analyses are based on anodic and cathodic peak currents, [i.sub.pa] and [i.sub.pc] respectively, and cathodic peak potentials, [E.sub.pc].
The electrode mechanism accepted for the reduction of an aromatic molecule via a protonated pi-anion was first postulated by Peover (1967).
R + [e.sup.-] [??] [R.sup.-]
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
RH* + HQ [right arrow] R[H.sup.-]
R[H.sup.-] + HQ [right arrow] R[H.sub.2] + [Q.sup.-]
A series of polarographic and cyclic voltammetric experiments scanning the oxazoles in progressively higher hydroquinone (HQ) concentrations was conducted. For all compounds, addition of HQ results in growth of the first wave at the expense of the second, accompanied by an anodic shift of both (Fig. 2). A plot of the proportional growth in the diffusion current of the first wave versus HQ concentration yields n-apparent values listed in Table 1. These values relate to the number of electrons transferred per molecule and the number of bonds saturated during the complete electrochemical reduction of a molecule (Smith et al., 1972). The 2-NNO shows an n-apparent of two, consistent with a limiting two electron transfer or one bond saturated per molecule at limiting HQ concentrations. The 2-PBO exhibits a limiting n-apparent of four electrons per molecule. The n-apparent values of 2-NBO and 2-PNO are between 2-PBO and 2-NNO, and are not integer values or a multiple of two. This is explained by the probable existence of a mixture of reduction products at high proton donor concentrations, each representing differing degrees of saturation.
Cyclic voltammetric scans of the compounds in the presence of HQ were used to determine internally consistent pseudo first-order rate constants for the protonation of the pi-radical. Protonation has a significant effect on the cyclic voltammogram appearance of the compounds studied (Fig. 3). The anodic current decreases at increasing concentrations of HQ. This behavior indicates that protonation of the anion radical initiates an irreversible chemical process (Nicholson and Shain, 1965). Theoretical derivations by Nicholson and Shain provide a straightforward technique for estimation of the rate constant, [k.sub.f] for protonation of the initial anion radical (Nicholson and Shain, 1965). Ratios of the anodic peak current ([i.sub.pa]) and cathodic peak current ([i.sub.pc]), measured on scans of increasing HQ concentration at different sweep rates, were calculated and plotted according to Nicholson and Shain (1965). The resulting [k.sub.f] values are listed in Table 1. Significant trends are shown to exist among the [k.sub.f] constants. They indicate for example, that the 2-PBO pi-radical reacts approximately 3X[10.sup.2] times faster than 2-NNO, showing that the smaller pi-radical is significantly more reactive than other oxazoles in the structurally related series.
[FIGURE 2 OMITTED]
SUMMARY AND CONCLUSIONS
The data presented herein lead to several interesting conclusions concerning the electrochemistry of fused ring oxazoles. The size of the pi-system (both fused and substituted is in general a determining factor when considering the reactivity of the protonated anion. The relatively low [E.sub.1/2] value and relatively small [k.sub.f] of 2-NNO demonstrate that the anion radical of the larger aromatic system is formed from lower-lying, anti-bounding [pi]-orbitals and that it is less reactive. A greater degree of electron delocalization is consistent with this observation. The compound 2-PBO possesses the greatest [E.sub.1/2] and [k.sub.f] values. These conclusions are readily predicted by molecular orbital theory (Smith, 1974).
[FIGURE 3 OMITTED]
Another significant observation is that substitution at the 2-position on the oxazole ring exhibits a considerable effect on pi-anion formation energy and reactivity toward the proton donor hydroquinone (Table 1). Relying simply on delocalization theory, one would predict that 2-NBO would be more reactive than 2-PNO inasmuch as the naphthoxazole possesses a higher degree of delocalization, having a larger planar conjugated pi-system and lower pi* energy levels. This was not observed to be the case. The influence of the hetero-atoms on the compound's pi-system must be taken into account when explaining reactivity. Aromatic hydrocarbon pi-electron densities are uniformly or symmetrically distributed over the molecule. This does not hold true for heterocyclic aromatics. The higher charge density of the hetero-atoms skews the pi-system. This effect appears to be of great importance in an oxazole system. The relative effect of an aromatic substituent at the 2-position is as significant as the nature of a fused ring (Table 1). This is confirmed by the thermodynamic and kinetic values [E.sub.1/2] and [k.sub.f], respectively. The 2-PNO has a higher [E.sub.1/2] value and [k.sub.f] value than 2-NBO (Table 1). Even its n-apparent value suggests greater reactivity by a higher degree of saturation during reduction. The 2-NBO's [E.sub.1/2] and [k.sub.f] are closer to those of 2-NNO than to those of 2-PNO as seen in Table 1.
Carbon-13 NMR spectra and semi-empirical quantum mechanical calculations provide additional evidence confirming the relative importance of substitution at the 2-position. The 2-carbon is significantly deshielded by the adjacent oxygen and nitrogen atoms (Belen'kii et al., 1985). This phenomenon, coupled with expected distortions in the bonding and nonbonding orbitals, influences the stability and reactivity of the pi-radicals. The evidence presented emphasizes the great importance of the 2-carbon substitution in determining the character of oxazole and fused ring oxazole anion radical behavior.
TABLE 1. Polarographic and cyclic voltammetric data. [k.sub.f] Compound [E.sub.1/2] (V) n-apparent (min.[.sup.-1]) 2-phenylbenzoxazole (2-PBO) -2.03 4 600 2-phenylnaphthoxazole (2-PNO) -1.82 3.0 30 2-naphthylbenzoxazole (2-NBO) -1.76 2.5 8 2-naphthylnaphthoxazole (2-NNO) -1.63 2.0 2 TABLE 2. Polarographic and cyclic voltammetric data. Cyclic voltammetric data Compound Polarographic data [i.sub.pa]/[i.sub.pc] [i.sub.d]/[h.sup.1/2] log [[i.sub.d]/ ([E.sub.pc]- (ma/[cm.sup.1/2]) ([i.sub.d]-i)] [E.sub.pc/2]) (mV) 2-PBO 0.66 (a) 60.3 0.99 60 2-PNO 0.54 (a) 60.8 0.99 62 2-NBO 0.39 (a) 66.0 0.99 62 2-NNO 0.29 (a) 64.8 0.93 66 (a) constant ratio.
The financial support of the Robert A. Welch Foundation is gratefully acknowledged. Hughlett served as a Welch Foundation Undergraduate Scholar during the course of the study.
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J. W. ROGERS, E. H. SUND, AND R. K. HUGHLETT
Department of Chemistry, Midwestern State University, 3400 Taft Blvd., Wichita Falls, Texas 76308
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|Author:||Rogers, J.W.; Sund, E.H.; Hughlett, R.K.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 1991|
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