J. Org. Chem. 1984,49, 3142-3150
3142
Stoichiometry and Kinetics of p -Methoxytoluene Oxidation by Electron Transfer. Mechanistic Dichotomy between Side Chain and Nuclear Substitution C. J. Schlesener and J. K. Kochi* Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Received December 29, 1983
The oxidation of p-methoxytoluene by tris(phenanthroline)iron(III) or Fe(phen),,+ in acetonitrile containing pyridine bases affords both side chain and nuclear substitution producta in the form of the isomeric Nbenzylpyridinium ion I and N-anisylpyridinium ion 11, respectively. The relative rates of formation of I and I1 show an unusual and pronounced dependence on the structure of the base-pyridine yielding mainly 11, and 2,&lutidine producing exclusively I. The mechanistic dichotomy between these side chain and nuclear substitution producta is resolved by the complete analysis of the complex oxidation kinetics. Thus the mechanism in Scheme I1 involves initial electron transfer from p-methoxytoluene to Fe(phen)?+ to afford the p-methoxytoluene cation radical [PMT.] as a reactive intermediate which can be directly observed by transient ESR spectroscopy. Products I and I1 arise from [PMV.] and the pyridine base in a follow-up step with a second-order rate constant kz. The base dependence of kz together with the deuterium kinetic isotope effect for deprotonation help to delineate the separate transition states leading to side chain and nuclear substitution. The kinetics also allow the evaluation of the rate constant k l for electron transfer from p-methoxytoluene to Fe(phen)?+. The correlation of log kl with analogous values for electron transfer from a series of polymethylated benzenes according to the linear free energy relationship from the Marcus rate theory shows the essential outer sphere character of the transition state for electron transfer.
Introduction The oxidation of aromatic compounds either by transition metal complexes or electrochemical methods has attracted increasing attention as a route to the introduction of various functional groups.' Among these studies, the oxidative transformation of an alkyl substituent, particularly a methyl group, has been investigated at length since it bears directly on the industrially important conversion of p-xylene to terephthalic acid.2 Two major pathways have been identified in the oxidation of aralkanes-namely side chain and nuclear substitution as given below with ~ dichotomy has toluene as an illustrative e ~ a m p l e .This
-(
G C H , 0 4 c
O
C
H
3
+
2H+
side c h a i n substitution
A c O G C H I
+ 2H*
nuclear substitution
been identified in most of the previous studies of aromatic oxidations by extensive product studies using various types of alkyl aromatic hydrocarbon^.'^^ Electron transfer as a primary act in the oxidation of aromatic compounds was first proposed by Dewar and co-workers5 in their pioneering investigation of p-methoxytoluene (PMT) with manganese(II1) acetate as the oxidant. Their kinetic studies which established the inverse dependence on manganese(II) led to the postulation of the cation radical of p-methoxytoluene as the reactive intermediate. According to the mechanism in Scheme I, Scheme I ArCH3 + Mnnl ArCH3+.
Y ArCH3+. + Mn"
k
ArCH2. + MnIn ArCH2+ + HOAc
ArCH2. + H+
fast
(a) (b)
ArCH2++ Mn"
(4
ArCH20Ac + H+
(d)
fast
side chain substitution proceeds by an initial reversible electron transfer (step a), followed by a slow rate-limiting proton transfer (step b). Indeed, a complete kinetic analysis of this mechanism has been recently carried out by Eberson for PMT by using the heteropoly cobaltotungstate KSCoWI2Omas the oxidant.6 Our interest in PMT arises from the desire to exploit tris(phenanthroline)iron(III)or Fe(phen),3+ as an oxidant,' since it has a number of desirable properties for mechanistic studies of organic oxidation. Thus it is a reasonably potent oxidant (EoFe= 1-09V vs. SCE in CH3CN), but more importantly it is substitution inert and prone to undergo electron transfer by an outer sphere mechanism.8 In this study we wish to describe how F e ( ~ h e n ) can ~~+ be utilized in the oxidation of this prototypical methylareneQ
*-
Present address: Department of Chemistry, University of Houston, University Park, Houston, Texas 77004.
0022-3263/84/1949-3142$01.50/0
(1) (a) Beletskaya, I. P.; Makhon'kov, D. I. Russ. Chem. Reu. 1981,50, 534. (b) Tomilov, A. P. Ibid. 1961, 30, 639. (2) For a review, see: Sheldon, R. A.; Kochi, J. K. 'Metal-Catalyzed Oxidation of Organic Compounds"; Academic Press: New York, 1982; Chapters 5 and 10. (3) (a) Eberson, L.; Nyberg, K. J.Am. Chem. SOC. 1966,88,1686. (b) Eberson, L.; Nyberg, K. Acc. Chem. Res. 1973,6,106. (c) For the toluene cation radical, see: Komatsu, T.; Lund, A.; Kinell, P.-0. J. Phys. Chem. 1972, 76,1721. (4) (a) Heiba, E. 1.; Deasau, R. M. Koehl, W. J., Jr. J. Am. Chem. SOC. 1969,41,6830. (b) Hanotier, J.; Hanotier-Bridoux, M.; de Radzitzky, P. J. Chem. SOC.. Perkin Trans. 2 1973 381. (c) Ross. S.D.: Finkelstein. M.: Petersen, R. C. J. Org. Chem. 1970,35, 781. (d) Blum,'Z.; Cedheim, L.; Nyberg, K. Acta Chem. Scand., Ser. B 1975, B29,715. (e) Shaw, M. J.; Weil, J. A.; Hyman, H. H.; Filler, R. J. Am. Chem. SOC.1970, 92, 5096. (f) Shaw, M. J.; Hyman, H. H.; Filler, R. J. Org. Chem. 1971,36, 2918. (9) Eberson, L.; Oberrauch,E. Acta Chem. Scand., Ser. B 1981,B35,193. (5) Andrulis, P. J.; Dewar, M. J. S.;Dietz, R.; Hunt, R. L. J. Am. Chem. SOC.1966,88, 5473. (6) Eberson, L. J. Am. Chem. SOC.1983, 105, 3192. (7) Schilt, A. A. "Analytical Applications of 1,lO-Phenanthroline and Related Compounds"; Pergamon: Oxford, 1969. (8) See, e.g.: (a) Dulz, G.; Sutin, N. Inorg. Chem. 1963,2, 917. (b) Diebler, H.; Sutin, N. J.Phys. Chem. 1964, 68, 174. ( c ) Wilkins, R. G.; Yelin, R. E. Inorg. Chem. 1968, 7,2667. (d) Wong, C. L.; Kochi, J. K. J. Am. Chem. SOC.1979,101,5593. (e) Fukuzumi, S.;Wong, C. L.; Kochi, J. K. J.Am. Chem. SOC. 1980,102, 2928. (9) For some examples of oxidation studies of PMT and related compounds using chemical oxidants and electrochemical methods, see: (a) Eberson, L. J. Am. Chem. SOC.1967,89,4669. (b) Eberson, L.; J6nsson, L.; Wistrand, L.-G. Acta Chem. Scand., Ser. B 1978, B32,520. (c) Nyberg, K.; Wistrand, L.-G. J.Org. Chem. 1978,43,2613. (d) Marrocco, M.; Brilmyer, G. J. Org. Chem. 1983,48,1487. (e) Nilsson, A.; Palmguist, U.; Petterson, T.; Ronlan, A. J. Chem. SOC.,Perkin Trans. 1 1978,708. (0 Uemura, S.; Ikeda, T.; Tanaka, S.; Okano, M. J. Chem. SOC.,Perkin Trans. 1 1979, 2574.
0 1984 American Chemical Society
J. Org. Chem., Vol. 49, No. 17, 1984
p-Methoxytoluene Oxidation by Electron Transfer
3143
Table I. Oxidation Products of p-Methoxytolueneby Tris(phenanthroline)iron(III) in the Presence of Pyridine and 2,B-Lutidine as Added Bases' p-methoxy-
toluene Fe(phen)39+ base [B] initial final initial final (mmol) (mmol) (mmol) (mmol) (mmol) pyridine 0.50 0.11 1.00 0.05 (1.01) 2,6-lutidine 0.50 0.25 1.00 0.33 (1.00)
Q 0
c o
ft 0 In
n
Q
1
400
I
I
I
500
600
700
Wavelength
,
800
nm
Figure 1. Spectral changes durin the oxidation of 1.9 X 10" M of pmethoxytoluene by 2.0 x d M Fe(phe&% in acetonitrile containing 3.1 X lom2M pyridine and 0.1 M LiCIOI. Initial iron(II1) indicated as 0. Numbers refer to time, t = 4, 14,28, and 360 s after mixing. The final spectrum of iron(I1)indicated by dashed line, was recorded after the reaction mixture was diluted -12-fold.
to afford second-order rate constants for the formation of the cation radical, as well as the second-order rate constants for its deprotonation by pyridine bases. Moreover the latter allows the mechanism of the side chain and the nuclear substitution of PMT to be delineated in a quantitative manner. Results The oxidation of p-methoxytoluene (PMT) by tris(phenantholine)iron(III)in acetonitrile solution containing pyridine as an added base is accompanied by the rapid change in color from blue to red. The distinctive color change is indicative of the reduction of the iron(II1) complex Fe(phen),3+to the iron(II) complex Fe(phen)t+.' The course of oxidation is illustrated by the temporal changes in the absorption spectrum during PMT oxidation in Figure 1. Note the absorption spectrum (daahed curve) at the end of the oxidation corresponds to that of the reduced ferrous complex Fe(phen)32+. Stoichiometry and Products of PMT Oxidation by Iron(II1). The stoichiometry for the iron(II1) oxidation of p-methoxytoluene was determined by two independent methods involving (a) the reduction of the iron(I1I) complex and (b) the disappearance of the methylarene. The reduction of the iron(II1) complex was determined quantitatively by spectral titration using the method of continuous variatiodOof Fe(phen)3a+,as monitored at 560 nm (see the Experimental Section). The consumption of PMT was measured directly by quantitative gas chromatography using the internal standard method. The results from the spectral titration and the gas chromatographic analysis indicated a ratio of 2.1 and 1.8, respectively, for the relative amount of iron(I1) and PMT consumed. Accordingly, the stoichiometry corresponds to an overall 2-electron oxidation, viz.,
[Bl
p-CH3C6H40CH3+ 2 F e ( ~ h e n ) ~ ~ + [P-CH~C~H~OCH +~2Fe(~hen),~+ ]~. (1) where the quantity in brackets represents the oxidation products of p-methoxytoluene to be described below. (IO) Roseotti, F. J. C.; Rwotti, H. 'Determination of Stability Constants";McGraw-Hill, Inc.: New York, 1981; p 47.
products: mol (I) Ia 0.M6(9) IIa 0.35 (69) IIb 0 (lO-fold excess PMT (8-60 kinetics were obtained throughout each run. The validity of the mM) and varying amounts of base (pyridine or 2,6-lutidine at procedure was indicated by the constancy of k, up to high con4-125 mM) in a 1.0-cm quartz cuvette, preflushed with argon. versions. Furthermore the same reliability of the method was [The same results were obtained when Schlenk techniques were shown by the invariance of k, evaiuated in kinetic runs at different used to exclude air.] The absorbance of either Fe(phen)$+ at 510 at 650 nm (c 540 M-l nm ( t 1.1 x lo4 M-' cm-') or Fe(~hen),~+ cm-') was monitored at intervals ranging from 1to 10 s allowed (38) Equation 22 should be comparedwith the general rate expressions derived ear1ier:'when p = k-lCo/k,[B] approaches infinity, Le., G = 1. by the use of a Hewlett-Packard 8450A UV-vis spectrometer keeping the total volume constant at 2 mL. The mole fraction
XF,of Fe(phen):+ was varied from 0.15 to 0.85 by using eight
+
3150 J. Org. Chem., Vol. 49, No. 17, 1984
Schlesener and Kochi
I
X
I
c3
acetonitrile owing to the rapid reaction of [PMT+.] with this solvent. However a reversible cyclic voltammogram could be obtained at relatively high scan rates in trifluoroacetic acid containing 5 %vol trifluoroacetic anhydride. Using a gold microelectrode by a technique described earlier,40a peak separation of 80-90 mV could be obtained from which EoA,= 1.256 & 0.029 for PMT. For comparison, the values of EoA, for 2-methoxytoluene, 3-methoxytoluene, and 2,6-dimethoxytoluene are 1.32 0.02, 1.44 0.01, and 1.30 f 0.01 V vs. Ag/AgC1O4 in trifluoroacetic acid containing 5 %vol trifluoroacetic anhydride. ~ 0.870 ~ + i 0.002 Under the same conditions EOF, for F e ( ~ h e n ) is V vs. Ag/AgClO,. The corresponding values for the tris 5-chloroand 5nitrophenanthroline derivatives are 0.963 f 0.001 and 1.073 0.002 v. Evaluation of the Free Energy Terms. For the application to the Marcus equation, the free energy change AG was determined from the standard oxidation potentials, EoA,and E°Fe,using the expression AG = ?[EoA,- EoFe] wp. The work term of the product wp was taken to be 1.7 kcal mol-', as previously described." unfortunately there is ambiguity as to the proper valuea of E'A, and EoFeto use, since the standard oxidation potential of methylbenzenes cannot be measured by cyclic voltammetry in acetonitrile. Two approaches have been employed. In the previous study,'4 we uniformly applied a solvent correction of 0.12 V to the values of EoA,measured in trifluoroacetic acid. Alternatively, EOF, (0.87 V vs. Ag/AgClO,) as measured in trifluoroacetic acid can be used directly with EoA,in the same solvent (listed in Table IV) on the (tenuous) basis that the solvent effect in the two redox couples would cancel out. The values of the intrinsic barrier AGO*calculated from eq 17 using this assumption are listed in column 5 of Table IV. Note that these values for hexamethylbenzene, pentamethylbenzene,durene, and prehnitene differ somewhat from those presented earlier" owing to the ambiguity in the solvent correction mentioned above; but otherwise the conclusions are the same. Electron Spin Resonance Study of [PMT+.] as a n Intermediate. Since arene cation radicals are known to be much more persistent in trifluoroacetic acid than in acetonitri1e;l the observation of [PMT+.] during the thermal reaction of PMT and F e ( ~ h e n ) was ~ ~ +carried out in the following manner. A trifluoroacetic acid solution of 1.3 X M PMT contained in an ESR tube was degassed and then frozen. A solution of 2.3 X 10" M F e ( ~ h e n ) ~ ( Pmade F ~ ) ~up in trifluoroacetic acid containing approximately 5 %vol water was added to the tube, quickly degassed and frozen. The sealed tube was warmed rapidly just until it thawed. The contents were rapidly mixed and the ESR spectrum in Figure 4a was recorded at a fast scan rate since the oxidation under these conditions was complete within a few minutes. The relatively broad lines in the spectrum could thus be attributed to the less than optimum conditions for these spectral measurements. The isotropic value of 2.0033 is based on an external DPPH standard. The computer simulated spectrum in Figure 4b is based on the proton hyperfiie splittings listed in ref 12, and a peak-to-peak line width of 1.7 gauss. A broad underlying ESR signal with 25% of the amplitude was imposed as a background for the simulation.
*
-
X
c
-J I
0
IO
20
30
40
50
Time, s Figure 7. Kinetic treatment according to eq 22 of the rate data for the oxidation of 1.8 X M p-methoxytoluene, 1.25 X lo-' M pyridine, and 1.3 X lo-" M Fe(phen)as+in acetonitrile containing 0.1 M LiCIOI. Curve I represents G = (1- Ao/Aa)/(l - cnr/tn) = 0.72. Curve I1 represents G = 1.0. Curve I11 represents G = 0. Table V. Electron Transfer lutidine, Fe(phen)$+, mM mM G 0.124 0.83 21.6 43.2 0.131 0.78 0.80 43.2 0.134 21.6 0.134 0.85
Kinetics for PMT" y
P
0.76 0.66
10.8 5.7
0.69
7.4 9.5
0.76
kl M-'d 6.7 6.9 7.7 5.6
"In acetonitrile solutions containing 18.5 mM PMT and 0.1 M LiCIOl at 25 OC.
*
+
concentrations of either PMT or pyridine. In practice we generally found eq 29 to be the experimentally feasible form of the rate expression in eq 2 (in which G = 1))since most of the oxidations were too fast to set accurately t,, to the zero point of mixing at t = 0. The practical effect of the correction using G is illustrated by the true line I in Figure 7 with the slope k,. By comparison, curve I1 represents the same rate data plotted as eq 2 (i.e., G = 1) and shows an initial concave behavior owing to the error in determining t = 0. For comparison, curve I11 is the rate data plotted as -In x vs. time &e., G = 0). Evaluation of t h e Electron-Transfer Rate Constant for PMT. The kinetics runs for the determination of the deprotonation rate constant kH in eq 7 were typically carried out at base concentrations which were sufficiently low so that the rate expression in eq 2 (or equivalently eq 29) applied. In terms of the mechanism in Scheme II,this condition represented electron transfer in full equilibrium followed by rate-limiting proton transfer, i.e., k2[B] > k-JFe(II)]. Consequentlywe were san for help with the ESR experiments, C. Amatore for forced to treat the intermediate kinetics situation in which the the electrochemical measurements and many helpful disgeneral rate expression in eq 15applied, viz., base concentrations cussions, J. Goncalves for the computer interface of t h e sufficient to allow k,[B] = k-l[Fe(II)], as listed in Table V. In spectral data, and t h e National Science Foundation for eq 15, the experimental rate constant ke'= 2k-l[PMT]o/(1 + p ) , financial support. where p = k-l[Fe(III)]o/k2[B]. The coefficient y in eq 15 was obtained from the linear regression, and it equals Gp/(l + p ) . Registry No. Ia.PF6, 91084-08-1; Ib.PF6, 91084-10-5; IIa.PF6, Reliable values of kl were obtained in this manner as listed in 91084-12-7; p-methoxytoluene, 104-93-8; tris(phenanthro1ine)column 6. The full details of the kinetics treatment for the rate iron(III), 13479-49-7; pyridine, 110-86-1;2,6-lutidine, 108-48-5; expression in eq 15 are described elsewhere.% deuterium, 7782-39-0. Electrochemical Measurements of P W and F e ( ~ h e n ) ~ ~ . Supplementary Material Available: Table of [Fe(III)]as ~~+ The standard oxidation potentials of PMT and F e ( ~ h e n )were a function of time for Figure 2a (1page). Ordering information measured by cyclic voltammetric methode in which Eo was taken is given on any current masthead page. as (E; + E 3/2 where E "and E; and E; are the peak potentials of the a n d c and cathdic waves, respectively.sg Unfortunately the cyclic voltammogram of p-methoxytoluene is irreversible in (40) Howell, J. 0.; Goncalves, J.; Amatore, C.; Klasinc, L.; Wightman, R. M.; Kochi, J. K.J. Am. Chem. Soc., in press. (41) Compare Hammerich, 0.; Moe, N. S.;Parker, V. D. J. Chem. SOC., Chem. Commun. 1972, 156. (39) Klingler, R. J.; Kochi, J. K. J.Phys. Chem. 1981,86, 1731.
