AROMATICHYDROXYLATION CATALYZED BY FENTON’S REAGENT
1139
Aromatic Hydroxylation Catalyzed by Fenton’s Reagent. An Electron Paramagnetic Resonance Study. I. Furans by Takeshi Shiga Department of Physicochemical Phfisiology, Medlcal School. Osaka University, Osaka, Japan
and Akio Isomoto College of General Education, Osaka University, Toyonaka, Osaka, Japan
(Received October 1 8 , 1 9 6 8 )
Unstable intermediate free radicals of furan, 2-furoic acid, 2-furfural, and 2-furfuryl alcohol, produced by Fenton’s reagent (Fe(I1) HzOz), have been detected by electron paramagnetic resonance. The observed hyperfine structures were analyzed and molecular structures were assigned on the basis of Huckel MO calculations. Most of the free radicals were hydroxylated adducts: a t position 5 of furan, 2-furfural, and 2furfuryl alcohol, and at position 4 of furoic acid. The hydroxylation occurred at the position possessing minimal localization energy for radical attack and maximal free valence. A Ti(II1) HzOz system attacked the same position of furan derivatives as Fenton’s reagent, giving the same adducts, while the reagents abstract a hydrogen from different sites in the case of saturated hydrocarbons. The reactive species involved in these oxidation systems is therefore expected to be free radical in nature.
+
+
Introduction Hydroxylation of aromatic compounds takes place in various oxidation systems, including many biological processes.’S2 Most studies of hydroxylation have been concentrated on the product analyses of substrates, and the conclusions have been scattered. Since the final products of the reaction consist of various products resulting from many secondary reactions, even the position of initial attack cannot be determined with certainty. Therefore, it is desirable to identify an unstable intermediate during the hydroxylation reaction, before it undergoes successive reaction. Unstable aliphatic free radicals produced by Fenton’s reagent (Fe(11) H202) ,8-& as well as by a Ti (111) H202 system,6-10 were detected successfully by electron paramagnetic resonance (epr) spectroscopy. Analyzing the hyperfine structures, the molecular structure of the intermediates was determined and the nature of the reactive species involved in the oxidation systems was discussed.]l Using aromatic compounds as substrates, the hydroxylation reaction occurs, instead of hydrogen abstraction as observed with the saturated compounds. Therefore, a free radical, a hydroxylated adduct, is expected to be detectable during the reaction, and was recently observed by Dixon and Norman12J*employing the Ti (111) HzOzsystem as an oxidant,. Fenton’s reagent should also produce a detectable amount of such intermediate free radicals. The determination of their molecular structures is important for finding the relationship between the reactive site of substrates and the molecular properties, especially the quantum chemical indices. Further, these results may be significant for determining the nature of various reactive species.
+
+
+
In this paper, epr studies of the intermediate free radicals of furan derivatives are presented. The hyperfine structures of the free radicals are analyzed and the molecular structures are identified on the basis of Huckel calculations. The reaction of the Ti (111) HzOz system is also studied. Finally, the theoretical reactivity indices and the positions of the attack are compared.
+
Materials and Methods Apparatus. A Varian V-4500 epr spectrometer with 100-kcps modulation unit was used. The design and characteristics of the continuous flow system have been described previously.sJ1 The pH of the solutions was (1) See H. 9. iMason, Advan. Enzymol., 19, 79 (1957). (2) See R. 0.C. Norman and J. R. Lindsay Smith in “Oxidases and Related Redox Systems,” Vol. I. T. E. King, H. 8. Mason, and M. Morrison, Ed., John Wiley and Sons, Inc., New York, N . Y., 1965,p 131. (3) T.Shiga, J . Phys. Chem., 69, 3805 (1965). (4) T. Shiga, A. Boukhors, and P. Douzou, i b i d . , 71, 3559 (1967). (5) T. Shiga, A. Boukhors, and P. Douzou, i b i d . , 71, 4264 (1967). (6) (a) W. T. Dixon and R . 0. 0.Norman, J . Chem. Soc., 3119 (1963);(b) W. T. Dixon, R. 0. C. Norman, and A. Buley, Ibid., 3625 (1964);(c) W. T. Dixon and R. 0. 0.Norman, ibid., 4850 (1964);(d) R. 0. 0.Norman and R . J. Pritchett, J . Chem. Sac. B , 1329 (1967);(e) A. L. Buley, R. 0. C. Norman, and R. J. Pritchett. i b i d . , 849 (1968). (7) P. Smith, J. T. Pearson, P. B . Wood, and T. C. Smith, J. Chem. Phys., 43, 1535 (1965). ( 8 ) H.Fisher, Z . Naturforsch., IPa, 866 (1964). (9) H. Yoshida and B. RBmby, J . Polymer Sei., C16, 1333 (1967). (10) J. R. Steven and J. C. Ward. Aust. J . Chem., 2 0 , 2005 (1967). (11) T. Shiga, A. Boukhors, and P. Douzou in “Recent Development of Magnetic Resonance in Biological Systems,” 8. Fujiwara and L. H. Piette, Ed., Hirokawa Publishing 00..Tokyo, 1968,p 146. (12) (a) W. T . Dixon and R . 0. 0. Norman, Proc. Chem. Soc., 97 (1963);(b) W. T. Dison and R. 0. C. Norman, J . Chem. Soc., 4857 (1964). (13) C. R. E. Jefcoate and R. 0. 0.Norman, J . Chem. Soc., B , 48 (1968). Volume 73,Number 4 April 1060
1140
TAKESHI SHIGAAND AKIO ISOMOTO
adjusted by a Hitachi F-5 pH meter. All the reactants, substrates, and salts were of reagent grade and were not purified further. Method. The solvents were 0.1 N HzS04, 0.2 M acetate buffer (pH 4 and 5 ) , and 0.1 M phosphate buffer (pH 6 and 7 ) . The solution of Fe(I1) (5 mM) and ethylenediaminetetraacetic acid (EDTA) (molar ratio of 1:l) and the solution of HzOz (0.1%) and substrates (3-5%) were mixed in the flow cell. In acid solution, EDTA was not added. Both solutions were bubbled with nitrogen gas in order to remove the dissolved oxygen. The epr spectra were recorded 10-50 msec after mixing the two solutions. The hyperfine coupling constants and g values were measured by comparing the signals with a spectrum of Fremy’s salt, which was sealed in a capillary and inserted beside the flow cell. The g values of the observed free radicals were about 2.004. The errors in the coupling constant measurements were within 5% in most cases. The 100 kcps modulation amplitude could not be reduced below 0.4 G because of a poor signal-to-noise ratio. Calculations. A NEAC 2200-500 digital computer was used for Huckel MO calculations. The Huckel parameters employed are listed in Table I. Most of
Table I: Hiickel Parameters ---Coulomb Heteroatom
-OH
1.70
=O
1.50
-0-C(OOH) -C(HO) a H z , =Hs
1.60 -0.15 -0.15 -0.20
integral-? Neighbor(C)
Bond integral
0 0.20 0.20 0.10 0.10 -0.10
0.60 1.40 0.80 0.90 0.90 2.00
hyperfine splittings (21.0, 14.8, 14.0, and 2.0 G). Two alternative structures, (I) and (11),can be assigned to this free radical. The theoretical coupling constants are
(1)
Therefore, the observed free radical is identified as the adduct (11). 2-Furoic Acid. Fenton’s reagent (at pH 7) and the Ti(II1) HzOz system (at pH 1) produce the same free radical, with an epr spectrum consisting of three proton hyperfine splittings (19.1, 10.2, and 2.2 G) (Figure 2). The calculated coupling constants for three possible structures are
+
(JY
W)
+
W)
Thus adduct IV fits the observed spectrum. %Furfural. At pH 5 and 7, Fenton’s reagent gives a spectrum shown in Figure 3a, consisting of three proton hyperfine splittings (6.3,3.2, and 1.0 G). However, a t pH 1, both reagents produce the same free radical, whose spectrum is shown in Figure 3b. The proton hyperfine coupling constants are 17.1,8.3,3.5, and 1.9 G. The calculated coupling constants for three possible structures are H
(19) (7)
CHO
(VI) these values were chosen according to the suggestion of Yonezawa, et al.,14and were slightly modified considering the inductive effect. The calculation of hydroxylated adducts was carried out for the fragment of the molecule, which was used for computing the localization energies of Wheland. The theoretical proton coupling constants ( A ) were converted from the calculated spin densities ( p ) , using the following equations: A = Q H c ~ X pwhere , Q%H = -22.5 G for the ring protons,16 and A = Q H - - ~ ~ ( o ~ ) - - (PY), p ~ where Q H - c ~ ( o ~ = ) - 40 G for a proton of hydroxylated position, -X-CH (OH) -Y(see Discussion).
(11)
CHO
(W)
(0.3)
HO (VIm
Therefore, adduct VI11 is produced in acidic medium. Concerning the free radical produced a t pH 7, the above adducts are not compatible with the spectrum of Figure 3a, but a free radical (IX) might explain the observed spectrum.
(E)
Results
2-Furfuryl Alcohol. Both oxidation systems produce the same free radical in acidic medium (Figure 4). Fenton’s reagent gives the same free radical a t pH 5. The coupling constants are 20.5,12.9,8.3,7.5, and 1.9 G. The calculated coupling constants of the possible free
Furan. The epr spectrum of the furan free radical is shown in Figure 1. Both Fenton’s reagent (at pH 7) HzOzsystem (at pH 1) produce the and the Ti(II1) same free radical, whose spectrum consists of 4 proton
(14) T. Yonezawa, et al., “Introduction to Quantum Chemistry,” Kagaku, Kyoto, 1963, p 55. (15) H . M. McConnell and D. B. Chesnut, J . Chem. Phys., 28, 107 (1958).
+
The Journal of Physical Chemistry
1141
AROMATIC HYDROXYLATION CATALYZED BY FENTON'SREAGENT Furfural ( F e pH5,7)
radicals are
IO g a u s s
(XI)
4 k 1.0 g a u s s Iwl 3.2 6.3
(XIID
Therefore, adduct XI1 is produced. According to this assignment, a few minor peaks cannot be interpreted, and the two proton coupling constants of -CH20H are dissimilar. (See Discussion.) A t pH 7, only three peaks (separation 4.6 G) appear in the epr spectrum, for which we can give no explanation.
V
(Ee acidic) (Ti)
Furan
(Ti) H ___,
w 1.9
IO gauss
I+---+
+I+
2.0 gauss
Figure 1. Epr spectrum of furan free radical, produced by Ti(II1) H?02 system at pH 1. Conditions: microwave power 5 db; modulation 100 kcps, 0.8 G; time constant 0.3 sec; magnetic field scanning 30 G/min; temperature 18'. The marked peak (X) is the signal arising from the Ti(II1) H202 system itself.
+
+
2 - F u r o i c Acid (Fe)
H ____,
20 g a u s s
+I+
w
8.3 11.1
Figure 3. (a) Epr spectrum of 2-furfural free radical, produced by Fenton's reagent at pH 7. Conditions: microwave power 5 db; modulation 100 kcps, 0.8 G; time constant 0.3 sec; magnetic field scanning 30 G/min; temperature 20". (b) Epr spectrum of 2-furfural free radical, produced by Ti(II1) Ha02 system at pH 1. Conditions: microwave power 5 db; modulation 100 kcps, 0.5 G; time constant 0.3 sec; magnetic field scanning 30 G/min; temperature 20".
+
M 14.0 Ic--;y 14.8 21.0
I X
gauss
Ic-u 3.5
2.2 gauss 10.2 19.1
Figure 2. Epr spectrum of 2-furoic acid free radical, produced by Fenton's reagent a t pH 7. Conditions: microwave power 5 db; modulation 100 kcps, 0.5 G; time constant 0.3 sec; magnetic field scanning 30 G/min; temperature 18".
Discussion Structure of Adducts. The structures of the free radicals cannot be uniquely assigned from the observed spectra, so that the observed coupling constants are compared with the estimated values of Huckel MO calculations. Since the substrates contain heteroatoms and substituents, the total number of electrons in the five-membered ring becomes ambiguous. Therefore, all the possible electronic structures of free radicals are calculated and compared with observed ring proton coupling constants; then the best fitting structures are chosen, keeping consistency among the four molecules studied. Finally, the following numbers of electrons are introduced into the ?r system: oxygen of position 1 gives 2 electrons, -COOH and -CHO groups donate 2 electrons, while each -CHZOH group contributes 4 electrons. The other possible combinations do not explain the observed coupling constants. These choices of electron numbers, i e . , of occupied energy levels, lead to good agreement between observed coupling constants ( A ) and calculated spin densities on the carbon atoms ( p ) . As can be seen in Figure 5, Volume 73,Number 4 April 1969
TAKESHI SHIGAAND AKIO ISOMOTO
1142
Table 11: Reactive Indices of Furans Localization Energy (Units of p) Furan L(E)
L(N L(N)
----Furoic
acid-
-
4
5
3
4
5
2.454 2.828 3.242
2.172 8.830 2.288
2.518 1.606 0.695
2.512 1.588 0.664
2.914 1.707 0.500
Furfural--
-----Furfuryl
alcohol--
3
4
5
3
4
B
2.067 1.855 1.643
1.987 1.143 2.299
1.860 1.721 1.582
2.454 2.812 3.160
3.553 2.893 2.233
2.310 2.571 2.832
0.37 1.06
0.54 1.32
0.43 1.16
0.38 1.04
0.55 1.18
Free Valence and Charge Density Fr qr
0.66 1.08
0.39 1.01
0.69 1.12
0.47 1.55
0.53 1.36
A = 27.4~ fits well. When the coupling constants are small, a slight disagreement exists, because the present calculation does not take the negative spin density into account. Although the Q parameter of the protons atta,ched to the hydroxylated position, -X-CH (OH)-Y-, is unknown, A = 4O(px p y ) is used as a first approximation, assuming that the dihedral angle between the CH bond and the neighboring pe orbital is 30' [thus (BO B cos2e) 40 GI, and disregarding the difference in orbital conformation of carbon and oxygen. Though the estimated coupling constants agree with the observed values, the exact molecular conformation is not certain. The proton coupling constant of substituted -CHO can also be well predicted by Q = -22.5 G from the spin density of the carbon a,tom of aldehyde.
+
+
+
M
Y
The structure of the 2-furfuryl alcohol radical is peculiar; the two proton coupling constants of -CH20H differ from each other, suggesting a hindrance of free rotation around the C2-CIt axis. Correspondingly, the observed proton coupling constants (7.5 and 8.3 G) of substituents can be approximated by A = 28pc,, indicating that this coupling would be of hyperconjugative nature. Also, it was necessary to introduce four electrons from the substituent to the ?r system. These complications suggest that the alcoholic OH may be in some manner hydrogen-bonded to the ring oxygen, giving mole electrons to the ring, and causing steric rotational hindrance of the substituted group. However, we do not intend to enter into this problem, since more evidence with similar molecules and further refinements of calculations seem t o be needed. Reactivity Indices and Reactive Sites. The structures of intermediate free radicals give more precise infonnation about the reactive sites of substrates. In Table 11,
1.9 gauss
7.5
F----M
30 g a u s s
0.44 1.54
I(--------w
8.3 12.9
6
X 20.5
10 Figure 4. Epr spectrum of 2-furfuryl alcohol free radical, produced by Ti(II1) HzOs system a t pH 1. Conditions: microwave power 5 db; modulation 100 kcps, 0.6 G; time constant 0.3 sec; magnetic field scanning 30 G/min; temperature 20". The two extreme peaks are the signals arising from the Ti(II1) H,Oa system itself.
+
+
The Journal of Phyeical Chemistry
Observed (gauss) Figure 5. The relationship between the observed ring proton coupling constant8 and the spin densities on the carbon atoms: X , furan; 0, furoic acid; 0 , furfural (adduct); 0 ,furfural (positive ion); A, furfuryl alcohol.
NOTES
1143
the localization energies of substrafes for nucleophilic, radical, and electrophilic attack, as well as the free valences and n-electron densities of each position, are shown. The occupied energy levels are so determined as t o give a good agreement with the structure of free radicals (vide supra). All the hydroxylated positions, i e . , position 5 (or 2) of furan, position 4 of furoic acid, position 5 of furfuryl, and position 5 of furfuryl alcohol, possess minimal localization energies for radical attack, and also maximal values of free valence (Table 11). Therefore, it can be concluded that reactions of both oxidation systems, Ti(II1) HzOz and Fe(I1) HzOz, involve radical species as a reactive intermediate. For consistency, four electrons were introduced from CHzOH to the ring of furfuryl alcohol. No other justification is offered a t this time. Nature of Reactive Species. The Ti(II1) HzOz system has reactive properties similar to the OH radical with u compounds, while Fenton’s reagent possesses an opposite character as an ~xidant.~-bJ~ However, concerning aromatic hydroxylation reactions, it is clearly shown that two oxidation systems behave similarly. The hydroxylated position of the intermediate free radical has been shown to be a most reactive site for radical attack of the original molecule. Similar results weie obtained with phenol derivatives.16 The chemical nature of the reactive species involved in these oxidation systems is still unknown, OH-like species are proposed for the Ti(II1) f HzOzsystem as attacking agents, because Livingston and Zeldes17have
+
+
+
shown that the intermediate free radicals obtained by photochemically generated OH radicals are similar to the intermediates produced by the Ti(II1) HzOz system. A number of kinetic studies are now being carried out using epr.lgdz2 Despite considerable efforts to clarify the nature of the reactive species involved in Fenton’s reagent since the 1930’s, no definite conclusion has yet been reached. Recently, a series of epr studies demonstrated that Fenton’s reagent possesses properties as an oxidant for saturated hydrocarbons3-6J1 different from those of Ti(II1) HzOz. Taking as an example the oxidation of alcohols, Fenton’s reagent extracts a hydrogen from the position farthest away from the alcoholic OH, while the Ti (111) HzOzsystem attacks the position nearest t o the alcoholic OH. Therefore, it was concluded that the two oxidation systems involve different reactive species in the abstraction of hydrogen from saturated compounds. However, the present study demonstrates that both oxidants have similar properties in yielding hydroxylated adducts from furans. Further studies on aromatic hydroxylation are now in progress.
+
+
+
(16) T.Shiga, in preparation. (17) R. Livingston and N. Zeldes, J. Chem. Phgs., 44, 1245 (1966). (18) L. H. Piette and G . Bulow, Preprints, 9,2-c, p c-9 American Chemical Society, 1967. (19) F. Sicilio, R . E. Florin, and L. A. Wall, J. Phys. Chem., 70, 47 (1967). (20) Y. 9. Chiang, J. Craddock, D . Mickewich, and J. Turkevich, i b i d . , 70, 3509 (1967). (21) H. Fischer, Ber. Bunsenges. Phys. Chem., 71, 685 (1967). (22) K . Takakura and B. R%nby,J. Phys. Chem., 72, 164 (1968).
NOTES
Effect of Density and Electron Scavengers
in Nitrous Oxide Radiolysis’*
NzO-scavenger experiments continue the investigation into the decomposition mechanism,
Experimental Section by John T. SearsIb Nuclear Engineering Department, Brookhaven National Laboratory, Upton. New York 11973 (Received August 28. 1 9 6 8 )
Primary processes in the irradiation-induced decomposition of nitrous oxide, which is a potential dosimetry s y ~ t e m , ~are J still incompletely understood. The present investigation presents results of the irradiation of solid, liquid, and high density (to 0.7 g/cc) gas which emphasize the relative constancy of G ( Nz). Additional
All gases were simply purified by freeze-thaw vacuum techniques. Low-pressure y-ray experiments were conducted in all-glass break-seal vessels (500 to 550 cc) . I n other experiments zzzRn CY rays (and daughter particles) were deposited internally in a 90-cc sealed vessel until the radon was exhausted. zlOPoa rays (1) (a) This work was performed under the auspices of the U. S. Atomic Energy Commission. (b) Esso Research and Engineering Co., Linden, N. J. 07036, (2) P. Harteck and 9. Dondes, NucZeonlcs, 14, 66 (1956). (3) R . W. Hummel and J. A. Hearne. Nature, 188, 734 (1960). Volume YS,Number 4 April 1869
