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Sullivan et al.
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
Oxidation of Anthracene by Thallium(111) Trifluoroacetate. Electron Spin Resonance and Structure of the Product Cation Radicals' Paul D. Sullivan," Egbert M. Menger, Department of chemistry, Ohio University, Athens, Ohio 4570 1
Allan H. Reddoch," and Donald H. Paskovich' Division of Chemistry, National Research Council of Canada, Ottawa, Canada, K I A OR6 (Received September 2lC 1977) Publication costs assisted by the National Research Council of: Canada
The oxidation of anthracene by thallium(II1) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) has been reinvestigated. We have interpreted the observed EPR spectra in terms of the 9-trifluoroacetoxyanthracene and 9,10-bis(trifluoroacetoxy)anthracenecation radicals and not the ion pair or hydrogen bonded species as previously suggested by other workers. The oxidations of 9-methyl- and 9,lO-dimethylanthracene by TTFA and of anthracene by thallium(II1) perfluoropropionate in perfluoropropionic acid are consistent with this interpretation.
Introduction Radical ions and their ion pairs have been the object of extensive research3 by many techniques in recent years. While there have been many EPR studies of ion pairs containing anion radical^,^ there are very few report^^,^ of cation radicals in ion pairs, even though cation radicals are well known. One of these reports5 is based on an EPR study of anthracene oxidized by thallium(II1) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA). There a spectrum was obtained which was analyzed in terms of three sets of four equivalent nuclei of spin 1/2 with coupling constants of 3.08, 1.54, and 0.25 G, and two sets of two nuclei of spin 1/2 with coupling constants of 4.62 and 1.03 G. It was suggested that this analysis could be explained by an anthracene cation radical in which the 9 and 10 protons are hydrogen bonded to fluorines on trifluoroacetate ions. Several details led us to reexamine this system. It seemed possible that some other analysis could give a better simulation of the EPR spectrum. Moreover, the structure, the perturbations of the coupling constants, and the stability of the proposed ion group all seemed surprising, particularly in comparison with other ion pair^.^,^ TTFA is a fairly strong, albeit extremely toxic, oxidant7 for organic compounds. It has been shownE to act as a one-electron oxidant yielding cation radicals of some benzene derivatives. However, it can also lead to oxidative couplingEand to t h a l l a t i ~ n . ~ In this paper we show that TTFA in TFA leads to trifluoroacetoxylation of the anthracene and that the reported5 EPR spectra arise from cation radicals of these products. Experimental Section Oxidations were carried out by adding a few drops of 0.8 M TTFA in TFA to a solution of the appropriate hydrocarbon in degassed TFA. Samples were sealed under vacuum. For some experiments an Eastman Chemical Co. solution of TTFA was used. For others the TTFA solution was prepared by dissolving T1203 in heated TFA in a manner similar to that of McKillop.lO Thallium(II1) perfluoropropionate (TPFP)was prepared similarly from T1203and perfluoropropionic acid (PFP). The spectrometers used were a Varian E-15 with a field-frequency lock and a Varian E-Line with a 12-in. Varian V4012 0022-365417812082-1158$01 .OO/O
magnet. The coupling constants and g factors were based on the perylene anion radical or on the phenalenyl radical as standards in a dual sample cavity. Sample temperature was measured with a microprobe thermocouple in the cavity. The optimum temperature for TTFA/TFA samples was between -10 and -20 "C.
Results Anthracene. In our hands oxidation of anthracene with TTFA in TFA led to the appearance of several different EPR spectra over a period of time. The first spectrum which was observed was due to the anthracene cation radical. This spectrum was not always observed and, even when it was obtained, its lifetime was quite short. Another spectrum appeared within a matter of minutes which was identical with that reported by Eloranta and S i p p ~ l a . ~ This latter spectrum, when obtained with the Eastman reagents, changed slowly over a period of hours at -10 " C to give a third and final spectrum which is different from any previously reported. This third spectrum appeared only occasionally with the TTFA which we synthesized. However it was readily obtained when either trifluoroacetic anhydride (TFAn) or A1203was added to the solution of anthracene in TTFA/TFA. The analysis of this last spectrum was relatively easy and unambiguous yielding three groups of spin 1/2 nuclei containing four, four, and six nuclei, respectively, with splittings of 3.09, 1.20, and 0.27 G and a g factor of 2.00275. The spectrum and a simulation are shown in Figure 1. The second spectrum which was previously interpreted5 in terms of the splittings given in the Introduction has been reanalyzed, and we propose a different interpretation. Our analysis is made in terms of four groups of two equivalent nuclei of spin 1 / 2 with splittings of 3.41, 2.94, 1.60, and 1.08 G, a single nucleus of spin 1 / 2 of splitting 6.58 G, and three nuclei of spin 112 of splitting 0.27 G. The g factor is 2.00267. A simulation using those parameters is compared with the experimental spectrum in Figure 1. The agreement is quite good considering the experimental difficulty in obtaining a spectrum which is not contaminated with the final spectrum and seems to be better than that shown previo~sly.~ When anthracene was allowed to react with TPFP in PFP, a spectrum was obtained which closely resembled the second spectrum above except that the group of three 0 1978 American Chemical Society
Oxidation of Anthracene by Thallium(II1) Trifluoroacetate
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
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TABLE I Splitting constants at positionsa Compound Anthracene 3.08 1.39 1.39 3.08 6.60 6.60 2.00259 2.00267 2.94b 0.27d 6.58 3.41b 1.60' 9-Trifluoroacetoxyanthracene ' 1.08' 0.27d 2.00275 3.09 0.27d 1.20 1.20 9,1O-Bis(trifluoroacetoxy )anthracene+ 3.09 2.5 2b 7.60 0. 30d 1.34' 0.83' 9-Methyl-10-trifluoroacetoxyanthracene' 3.07 7.95 7.95 2.54 1.19 1.19 2.54 9,10-Dimethylanthracenet 1.63 1.01 2.86 0.27e 6.50 9-Perfluoropropionoxyanthracene+ 3.39 a All splitting constants in Gauss, t O . O 1 G ; g factors, rtO.00001. The assignments for the 1,8 and 4,5 positions may be reversed. ' The assignments for the 2,7 and 3,6 positions may be reversed. d CF, splittings. e CF, splittings. +
Figure 2. (a) The experimental spectrum observed on oxidation of anthracene with lTFA in TFA at -10 O C . The spectrum was observed 15 min after the sample was prepared. (b) A simulation of the 9trifluoroacetoxyanthracenecation radical using the parameters shown in Table I.
Figure 1. (a) The experimental spectrum observed for anthracene in TTFA/TFA with TFAn. The spectrum was taken at -10 OC. (b) A simulation of the 9, IO-bis(trif1uoroacetoxy)anthracenecation radical using the parameters given in Table I.
nuclei had been reduced to two nuclei. The coupling constants are given in Table I. 9-Methylanthracene. Oxidation with TTFA in TFA gave an EPR spectrum which is different from the reported spectrum of the 9-methylanthracene cation radicalll and which we have interpreted in terms of four groups of two equivalent nuclei of spin 1 / 2 and two groups of three equivalent nuclei of spin 1 / 2 (see Table I). 9,10-Dimethylanthracene, Perylene, Durene, and Hexamethylbenzene. Upon oxidation with TTFA in TFA, 9,lO-dimethylanthracene and perylene gave EPR spectra (Figure 2) which are identical with the reported spectra of their respective cation radicals.11J2 Durene and hexamethylbenzene also yielded their cation radicals as Elson and Kochi have previously reported.6 In this connection, Svanholm and Parker have questioned13 the assertion that the HMB cation radical is the source of the spectrum of Elson and Kochi, arguing
that the species is too unstable and suggesting another substituted benzene of lower symmetry as the source of the spectrum. However, our spectra show 13 well-resolved lines, 0.3-G peak-to-peak width, with intensities which agree within about 1%with the expected intensities for 18 equivalent protons. It would be difficult to find a species other than HMB'. which would give such a spectrum. Naphthalene. Naphthalene with TTFA in TFA gave a moderately resolved spectrum which may be analyzed in terms of two groups of eight nuclei of spin 1 / 2 with coupling constants of 1.04 and 2.76 G. This is consistent with the formation of the naphthalene dimer cation, (C1(&)2+, which has previously been obtained14 by the oxidation of naphthalene with SbC15. Benzo(a)pyrene. Oxidation of benzo(a)pyrene with TTFA in TFA gave a well-resolved EPR spectrum which has not been analyzed in detail but which is not consistent with a benzo(a)pyrene cation radi~a1.l~
Discussion TTFA in TFA can without a doubt act as a one-electron oxidant of suitable aromatic substrates. This is seen above for 9,lO-dimethylanthracene and perylene and has been previously observed by Elson and Koch? for several tetra-, penta-, and hexaalkylated benzenes. In several other cases
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The Journal of Physical Chemistry, Vol. 82, No. 70, 1978
Scheme I
AH,
__ -e
AH, = anthracene
AH,+.
AH,+. t CF,CO,-
--t
CF,CO,AH,.
--PI+ -y+
-e
CF,CO,AH
-e-
CF,CO,AH + CF,CO,AH+, CF,CO,AH+. t CF,CO,'
(CF,CO,),A
+
(CF,CO,),AHs
=L(CF,CO,),A+.
oxidative coupling can occur prior to the observation of an EPR spectrum of the coupled product. Toluene, for example, gives the EPR spectrum of 4,4'-dimethylbiphenyl cation radical,8 and EPR spectra have also been detected from the coupled products of anisole, l-methoxynaphthalene, 1-methylnaphthalene, and 2,6-dimethylphenol among others.16 Two other reactions which might occur with TTFA in TFA include thallation reactionsg and trifluoroacetoxylations.17J8Indeed the anthracene cation radical is known to react in TFA to give 9-trifluoroacetoxyanthracene as a reaction product.17 We believe that the EPR results are most consistent with this latter reaction. Thus, anthracene reacts to produce g-trifluoroacetoxyanthracene, which in turn reacts further to produce 9,10-bis(trifluoroacetoxy)anthracene. It is the cation radicals of these two products which give rise to the observed EPR spectra. Thus the 9,10-bis(trifluoroacetoxylanthracene cation radical yields the final spectrum observed for anthracene. The six equivalent nuclei of 0.27 G are assigned to the fluorine nuclei of the trifluoroacetoxy groups; the other two groups of four nuclei are assigned to the protons at positions 1 and 2. The other results may then be similarly interpreted, the second spectrum from anthracene being due to the 9-trifluoroacetoxyanthracene cation radical with the splitting constants assigned as indicated in Table I. This interpretation is supported by the results obtained with anthracene and TPFP in PFP. Here the only significant difference between the spectrum of this system and that of 9-trifluoroacetoxyanthracene' is the loss of one of the three nuclei with a 0.27-G splitting. This indicates that this splitting is indeed from equivalent fluorines and that the coupling with the electron spin is by way of the carboxy group rather than directly with the terminal fluorines. The g factors for the mono- and disubstituted anthracenes, 2.00267 and 2.00275, are also consistent with the above interpretation, since they are slightly higher than that of the anthracene cation in TFA, 2.00259 by our measurements, These increases reflect the interaction of the electron spin with the oxygens of one and two carboxy groups, respectively. Similarly, 9-methylanthracene is believed to give rise to cation radical, the 9-methyl-10-trifluoroacetoxyanthracene and the splitting constants can be assigned by analogy with the other compounds. Benzo(a)pyrene probably gives rise t o the 6-trifluoroacetoxybenzo(a)pyrene,although the spectrum is still too complex to analyze. The trifluoroacetoxylation of anthracene is similar to the oxidative substitution of arenes by cobalt(II1) trifluoro-
Sullivan et al.
acetate in TFA reported by Kochi et a1.18 The reaction in this case was believed to occur via the production of an intermediate cation radical, Co(II1) acting as a one-electron oxidant. Similarly, on reaction with TTFA, the first step is believed to be a one-electron transfer leading to a cation radical. The cation radical then reacts with a trifluoroacetate ion to give a neutral radical intermediate, which in turn eliminates a proton to yield the trifluoroacetoxylated product. The trifluoroacetoxy product can then be oxidized to give the cation radical. A similar series of reactions can lead to the 9,10-bis(trifluoroacetoxy)anthracene cation radical as outlined in Scheme I. The fact that the bis(trifluoroacetoxy)anthracene cation radical was obtained with the Eastman TTFA, but was only rarely seen with the TTFA which we synthesized, was present, suggests that the former unless TFAn or A1203 system was drier and that water interferes with the rate and extent of the second trifluoroacetoxylation. These observations may be related to the results of Hammerich and Parker,19J0who showed that water interferes with the anodic oxidation of anthracene because of competing hydroxylation reactions.
Conclusion We have obtained an improved analysis and simulation of the EPR spectrum from the oxidation of anthracene with TTFA in TFA. This analysis and other experiments show that the 9 proton of the anthracene is replaced by a trifluoroacetoxy group. There is thus no evidence for ion pairs or hydrogen-bonded species. The results are best interpreted in terms of trifluoroacetoxylated derivatives. It is obvious that oxidations with TTFA to produce cation radical intermediates should be approached with caution since the parent cation radical may react further to give a substituted or coupled product. Acknowledgment. This study was supported in part (P.D.S. and E.M.M.) by Grant No. CHE 76-041666 from the National Science Foundation. We are grateful to D. J. Northcott for synthesizing TTFA and TPFP and for preparing some of the samples and obtaining their spectra.
References and Notes Issued as N.R.C.C. No. 16547. N.R.C.C. Guest Worker 1976-1977. M. Szwarc, Ed., "Ions and Ion Pairs in Organic Reactions", Wiley-Interscience, New York, N.Y., 1972. G. Goez-Morales and P. D. Sullivan, J. Am. Chem. Soc., 96, 7232 (1974). J. Eloranta and A. Sippula, Finn. Cbem. Lett., 170 (1975). A. H. Reddoch, J . Cbem. Pbys., 43, 225 (1965). R. J. Ouellette in "Oxidation In Organic Chemistry", Part 8 , W. S. Trahanovsky, Ed., Academic Press, New York, N.Y., 1973, Chapter 3. I. H. Elson and J. K. Kochi, J . Am. Cbem. SOC.,95, 5060 (1973). A. McKilloD et al., J . Am. Chem. Soc., 93, 4841 (1971). A. McKillop et al., Tetrahedron Left., 2423 (1969). J. R. Bolton, A. Carrington, and A. D. Mclachlan, Mol. Pbys., 5, 31 (1962). A. Carrington, F. Dravnieks, and M. C. R. Symons, J . Cbem. SOC., 947 (1959). U. Svanholm and V. D. Parker, Tetrahedron Leff., 471 (1972). 0. W. Howarth and G. K. Fraenkel, J. Cbem. fbys., 52,6258 (1971). E. M. Menger, R. B. Spokane, and P. D. Sullivan, Biocbem. Biopbys. Res. Commun., 71, 610 (1976). P. D. Sullivan, J. Y. Fong, and E. M. Menger, unpublished results. U. Svanholm and V. D. Parker, J. Am. Cbem. Soc., 98,2942 (1976). J. K. Kochi, R. T. Tang, and T. Bernath, J . Am. Cbem. SOC.,95, 7114 (1973). 0. Hammerich and V. D. Parker, J. Cbem. SOC.,Cbem. Commun., 245 (1974). 0 . Hammerich and V. D. Parker, J. Am. Cbem. SOC.. 96, 4289 (1974).
