Energy & Fuels 1999, 13, 927-933
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“FeS”-Assisted Scission of Strong Bonds in Phenoxydiphenylmethanes. Competition between Hydrogen Atom Transfer and Free Radical Rearrangement Pathways Tom Autrey,* John C. Linehan, Lauren Kaune, Tess R. Powers, Eric F. McMillan, Carrie Stearns, and James A. Franz Chemical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352 Received January 19, 1999. Revised Manuscript Received March 29, 1999
Model compound studies comparing rates of decomposition and product distributions from orthoand para-phenoxydiphenylmethanes [(PhO)PhCH2Ph] suggest that hydrogen atom abstraction from the model compounds, to yield a benzylic radical intermediate, competes with hydrogen atom transfer to the aryl rings from the reduced “FeS” catalyst. A free-radical rearrangement pathway involving o-phenoxydiphenylmethane, facilitated by the presence of the “FeS” catalyst, generated in situ from ferric oxyhydroxysulfate (OHS) and sulfur, leads to apparent Ar-OAr bond scission at temperatures significantly lower than expected for homolytic scission pathways. Thermolysis of the ortho isomer proceeds predominately through a pathway involving an intramolecular addition of the benzylic radical to the 1-position of the appended diphenyl ether, Ar1-5 participation, forming a spirodienyl radical intermediate. Scission of the C-O bond, followed by hydrogen atom abstraction, yields thermally labile o-(hydroxyphenyl)phenylmethane (oHPPM). Under the reaction conditions, at 390 °C, tautomerism of oHPPM to the keto isomer followed by homolysis of the weak C-C bond in the keto intermediate yields diphenylmethane and phenol. To unambiguously demonstrate the importance of the free-radical rearrangement pathway, products from the thermolysis of o-(4-methylphenoxy)diphenylmethane were quantitatively determined. Decomposition of this labeled diaryl ether at 390 °C in 9,10-dihydrophenanthrene containing OHS/sulfur yields 4-methyldiphenylmethane and phenol as the major products. Catalytic decomposition of the corresponding para isomer, p-(4-methylphenoxy)diphenylmethane, where the intramolecular free-radical rearrangement pathway is hindered, shows that the rate of decomposition is significantly slower than observed for the corresponding ortho isomer, and 4-methyldiphenyl ether and toluene are the major products. The selectivity observed for the product distribution in the catalytic thermolysis of the para isomer is consistent with a reversible hydrogen atom transfer pathway from the “FeS” catalyst.
Introduction Model compound studies have provided a fundamental understanding of the catalytic and “solvent-induced” thermal scission pathways of thermodynamically stable linkages in coal structures. Significant effort has been extended to understand the hydrogen atom transfer pathways responsible for the scission of the strong alkyl-aryl bonds encountered in diarylmethanes (ArCH2Ar) under both thermal1-14 and catalytic condi* Author to whom correspondence should be addressed. (1) Autrey, T.; Alborn, E. A.; Franz, J. A.; Camaioni, D. M. Energy Fuels 1995, 9, 420-428. (2) Autrey, T.; Powers, T.; Alborn, E. A.; Camaioni, D. M.; Franz, J. A. Coal Sci. Technol. 1995, 24, 1431-1434. (3) Autrey, T.; Alborn-Cleveland, E.; Camaioni, D. M.; Franz, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 627-631. (4) Autrey, S. T.; Camaioni, D. M.; Ferris, K. F.; Franz, J. A. Conf. Proc.sInt. Conf. Coal Sci., 7th 1993, 1, 336-339. (5) Autrey, T.; Gleicher, G. J.; Camaioni, D. M.; Ferris, K. F.; Franz, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 521528. (6) Autrey, T.; Franz, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 381-386.
tions.15-24 The “FeS” catalyst, generated in situ under reducing conditions from 9,10-dihydrophenanthrene, elemental sulfur, and ferric oxyhydroxysulfate (OHS), has been used to induce the scission of strong aryl-CH2(7) Camaioni, D. M.; Autrey, S. T.; Franz, J. A. J. Phys. Chem. 1993, 97, 5791-5792. (8) Camaioni, D. M.; Autrey, S. T.; Franz, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 303-309. (9) Camaioni, D. M.; Autrey, S. T.; Salinas, T. B.; Franz, J. A. J. Am. Chem. Soc. 1996, 118, 2013-2022. (10) Savage, P. E. Energy Fuels 1995, 9, 590-598. (11) McMillen, D. F.; Malhotra, R.; Tse, D. S. Energy Fuels 1991, 5, 179. (12) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Fleming, R. H.; Ogier, W. C.; Nigenda, S. E. Fuel 1987, 66, 1611. (13) Malhotra, R.; McMillen, D. F. Energy Fuels 1993, 7, 227. (14) Stein, S. E. Acc. Chem. Res. 1991, 24, 350. (15) Autrey, T.; Linehan, J. C.; Steams, C. J.; Camaioni, D. M.; Kaune, L.; Franz, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 66-69. (16) Autrey, T.; Linehan, J. C.; Camaioni, D. M.; Kaune, L. E.; Watrob, H. M.; Franz, J. A. Catal. Today 1996, 31, 105-111. (17) Autrey, T.; Linehan, J.; Camaioni, D.; Powers, T.; Wartob, H.; Franz, J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 973. (18) Schmidt, E.; Song, C.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 580-583.
10.1021/ef990009q CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999
928 Energy & Fuels, Vol. 13, No. 4, 1999
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aryl bonds selectively in the presence of alkyl substituents at moderate liquefaction temperatures.16 A reversible hydrogen atom transfer from the catalyst surface to the aryl rings was proposed as an important mechanistic step responsible for the observed selectivity. On the other hand, there is little quantitative information on the mechanistic pathways leading to scission of the structurally similar diaryl ether linkage arylOaryl.25-27 Diaryl ethers, like their corresponding diarylmethane analogues, are thermodynamically stable linkages found in coal structures. For example, the unimolecular scission of the Ar-OAr bond in diphenyl ether, to generate a phenoxyl and phenyl radical as shown in eq 1, corresponds to a calculated half-life of hundreds of years at 400 °C.
Scheme 1
Scheme 2
∆Hrxn (kcal/mol) •
•
Ph-OPh f Ph + PhO ∆Hf 11.6 78.6 11.4
78.1
(1)
Diaryl ethers,28 eq 3, may be formed in competition with diphenylmethanes,29 eq 4, by retrogressive reaction pathways (radical cage recombination) during hydroliquefaction from thermally labile benzylphenyl ethers as shown in eqs 2-4.
ArCH2-OAr T [ArCH2•/•OAr] f ArCH3 + HOAr (2) [ArCH2•/•COAr] f ArO-ArCH3
(3)
[ArCH2•/•OAr] f ArCH2-ArOH
(4)
The intent of this present study is two-fold: (1) to examine the effectiveness of the “FeS” catalyst in promoting the scission of the strong Ar-OAr bond in diaryl ethers, and (2) to further test the hypothesis of free-radical pathways. The structure-reactivity relationship observed in alkyl-substituted diarylmethane decomposition studies provided strong evidence for the reversible hydrogen atom transfer pathway between the catalyst and the aromatic rings of the model compound as shown in Scheme 1. However, ionic intermediates formed by a two-step electron transferhydrogen atom transfer pathway as shown in Scheme 2 could not be completely ruled out. For example, the mechanism of catalytic-induced scission of a related diarylmethane, 4-(1-naphthylmethyl)bibenzyl (NBBM), (19) Linehan, J. C.; Matson, D. W.; Darab, J. G.; Autrey, S. T.; Franz, J. A.; Camaioni, D. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 720-722. (20) Farcasiu, M.; Smith, C. Energy Fuels 1991, 5, 87. (21) Wei, X.-Y.; Ogata, E.; Niki, E. Chem. Lett. 1991, 2199. (22) Wei, X.-Y.; Ogata, E.; Niki, E. Bull. Chem. Soc. Jpn. 1992, 65, 1114. (23) Wei, X.-Y.; Ogata, E.; Niki, E.; Zong, Z.-M. Energy Fuels 1992, 6, 868. (24) Wei, X.-Y.; Zong, Z.-M. Energy Fuels 1992, 6, 236. (25) McMillen, D. F.; Ogier, W. C.; Ross, D. S. J. Org. Chem. 1981, 46, 3322. (26) Afifi, A. I.; Chornet, E.; Thring, R. W.; Overend, R. P. Fuel 1996, 75, 509-516. (27) van Scheppingen, W.; Dorrestijn, E.; Arends, I.; Mulder, P.; Korth, H.-G. J. Phys. Chem. A 1997, 101, 5404-5411. (28) Winans, R. E.; Tomczyk, N. A. Fuel Preprints 1997, 42, 181. (29) Buchanan, A. C.; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895-9903.
remains a controversy. In the presence of Black Pearls, the observation of a charge buildup has been sugested to imply the formation of a radical cation formed by an electron transfer pathway. However, it has been argued30 that formation of a NBBM radical cation would not lead directly to the observed products formed in the presence of either Black Pearls or “FeS”. An alternative mechanism, electron transfer followed by hydrogen atom transfer, was proposed to explain both the buildup of charge and the observed product selectivity.31 It has been suggested that a comparison of diarylmethanes with the diaryl ether analogues would provide a test of the radical versus radical ion pathways under the reaction conditions where strong bond scission is facilitated by the presence of the “FeS” catalyst.32 Specifically, scission of ArO(+) from [Ar-OAr]+ bond scission, is far less favorable than scission of ArCH2(+) from [Ar-CH2Ar]+ bond scission. However, involvement of the radical intermediates were suggested to favor ArOAr bond scission compared to Ar-CH2Ar. One approach would be to examine a series of model compounds that contain both structural units within the same “probe molecule.” Utilizing a single probe molecule ensures that the rate of electron transfer (ET) to either diphenyl ether (DPE) or diphenylmethane (DPM) does not control the product selectivity. Given this constraint, a series of phenoxydiphenylmethanes (PDM) were prepared to examine the direct competition of Ar-OAr and Ar-CH2Ar bond scission pathways from a common intermediate. If cationic intermediates are formed under the reaction conditions (Scheme 2), scission of benzyl cation, from intermediate (CH+), is so favorable, relative to scission of phenoxyl cation, from intermediate (DH+), that no C-O bond scission would be observed. On the other hand, if radical intermediates are formed under the reaction conditions (30) Penn, J. H.; Wang, J. Energy Fuels 1994, 8, 421. (31) Franz, J. A.; Camaioni, D. M.; Alnajjar, M. S.; Autrey, T.; Linehan, J. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 203-207. (32) McMillen, D. F.; Malhotra, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 221-227.
Scission of Strong Bonds in Phenoxydiphenylmethanes
the ratio of C-O to C-C bond scission should be proportional to the stability of the radical intermediates leading to bond scission. Therefore, examination of the ratio of Ar-OAr to Ar-CH2Ar bond scission, i.e., the ratio of PhOPh/PhCH2Ph could give some insight into the involvement of either ionic or radical intermediates.
Energy & Fuels, Vol. 13, No. 4, 1999 929 Scheme 3
Experimental Section Methylene chloride (GC capillary grade), methanol, phenol, toluene, biphenyl, benzene, 2-hydroxydiphenylmethane, 4-hydroxydiphenylmethane, 4-bromotoluene, and bromobenzene were obtained from Aldrich and used as received. The 9,10-dihydrophenanthrene (Aldrich, 94%) was distilled and recrystallized from methanol. Ferric oxyhydroxysulfate prepared by the RDTS method was available from previous work.33 NMR spectra of the model compounds, in CDCl3, were collected on a Varian VXR-300 NMR spectrometer operating at 299.9 and 75.4 MHZ for 1H and 13C, respectively. TMS was used as an internal standard for the 1H, and chloroform was used as the internal standard in the 13C spectra. Mass spectral data was obtained from an HP 5971 series Mass Selective Detector connected to a HP 5890 series II GC. The model compounds o-phenoxydiphenylmethane, p-phenoxydiphenylmethane, p-(4-methylphenoxy)diphenylmethane, and o-(4-methylphenoxy)diphenylmethane were synthesized from the appropriate (ortho or para) hydroxydiphenylmethane and arylbromide (4-bromotoluene or bromobenzene) according to literature procedures.34 The model compounds were distilled under reduced atmosphere prior to use and their purity was determined to be greater than 98% by 1H and 13C NMR and by GC/MS. p-Phenoxydiphenylmethane, white solid with mp 4546° C. MS (EI) m/z 260. 1H: δ 3.956 (CH2). 13C{1H}: 157.5 (1C), 155.4 (1C), 141.1 (1C), 136.1 (1C), 130.1 (2C), 129.6 (2C), 128.9 (2C), 128.5 (2C), 126.1 (1C), 123.0 (1C), 119.0 (2C), 118.6 (2C), 41.19 (1C, CH2). o-Phenoxydiphenylmethane, viscous colorless oil. MS (EI) m/z 260. 1H: δ 3.983 (CH2). 13C{1H}: 157.7 (1C), 154.5 (1C), 140.5 (1C), 132.9 (1C), 131.0 (1C), 129.6 (2C), 129.0 (2C), 128.3 (2C), 127.6 (1C), 125.9 (1C), 123.9 (1C), 122.6 (1C), 119.4 (1C), 117.9 (1C), 35.98 (1C, CH2). p-(4-Methylphenoxy)diphenylmethane, viscous colorless oil. MS (EI) m/z 274. 1H: δ 3.942 (CH2), 2.314 (CH3). 13C{1H}: 156.0 (1C), 155.0 (1C), 141.2 (1C), 135.6 (1C), 135.0 (1C), 130.2 (2C), 130.0 (2C), 128.9 (2C), 128.5 (2C), 126.1 (1C), 118.9 (2C), 118.5 (2C), 41.17 (1C, CH2), 20.68 (1C, CH3). o-(4-Methylphenoxy)diphenylmethane, viscous colorless oil. MS (EI) m/z 274. 1H: δ 3.990 (CH2), 2.306 (CH3). 13C{1H}: 155.0 (1C), 140.6 (1C), 132.5 (1C), 132.2 (1C), 131.4 (1C), 130.9 (1C), 130.1 (2C), 129.0 (2C), 128.3 (2C), 127.5 (1C), 125.9 (1C), 123.4 (1C), 118.8 (1C), 118.1 (2C), 35.97 (1C, CH2), 20.62 (1C, CH3). Kinetics were performed by weighing 5 mg of the catalyst “premix” (consisting of a 1:1 by weight mixture of ferric oxyhydroxysulfate and elemental sulfur) and 10 µL of a mixture of the model compound and biphenyl (33) Linehan, J. C.; Matson, D. W.; Darab, J. G. Energy Fuels 1994, 8, 56. (34) Ungnade, H. E.; Orwell, E. F. Organic Synthesis, Collective Vol. III, 2-methoxydiphenyl ether; Vol. III, pp 566-568.
(internal GC standard) into a 5 mm o.d. (3.5 mm i.d.) Pyrex tube. 9,10-Dihydrophenanthrene (100 mg) was added and the tubes were sealed under vacuum. The reaction vessels were immersed in a temperaturecontrolled fluidized sand bath. Heat-up time to 390 °C was determined to be less than 5 min. The stability of the temperature was (3 °C over the time of the experiment, 90 min. At various intervals, samples were removed and cooled rapidly in liquid nitrogen before preparation for product analysis. The reaction mixture was dissolved in methylene chloride solution containing tert-butylbenzene (external GC standard). The products were quantified by GC (HP-5890 equipped with a FID, on-column injection, 15 m by 0.25 mm i.d. HP-5 capillary column with a 1 µm film thickness). A temperature program held at 40 °C for 1 min followed by a 10°/min ramp to 250 °C at which temperature the column was held until all compounds eluted (approximately 40 min). Ab intitio quantum chemical calculations were performed using the GAUSSIAN 9435 program. Optimized geometries of the radicals were determined using density functional theory. Results Four model compounds, shown in Scheme 3, p-phenoxydiphenylmethane (pPDM), p-(4-methylphenoxy)diphenylmethane (pMPDM), o-phenoxydiphenylmethane (oPDM), and o-(4-methylphenoxy)diphenylmethane (oMPDM) containing both a diarylmethane (ArCH2Ar) and diaryl ether (ArOAr) linkage were investigated in this work to elucidate the mechanism of “FeS”-induced scission of strong Ar-XAr bonds. The “FeS” catalyst is formed in situ from a 1:1 by weight mixture of ferric oxyhydroxysulfate (OHS) and elemental sulfur under reducing conditions. In general the rate of disappearance of the four model compounds examined showed first-order behavior as observed in the previous studies.16 A plot of ln(Ct/Co) versus time is displayed in Figure 1, where Ct is the concentration of the model compound at time t and Co is the initial concentration of the model compound. While the rates of decomposition of the phenoxydiphenylmethanes showed first-order (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrezewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision E-2; Gaussian, Inc.: Pittsburgh, 1995.
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secondary product formation. However, the experiments were heated for at least 30 min to ensure that the chemistry was occurring at the final temperature and not during the heat-up time. Thermolysis of p-Phenoxydiphenylmethane (pPDM). Thermolysis of the pPDM in 9,10-DHP containing the catalyst precursor for 90 min at 390 °C, 11% conversion, leads to the formation of diphenyl ether and toluene as the major products and diphenylmethane as a minor product. An observed selectivity of twelve C-C (eq 7) to one C-O (eq 8) bond scission events was determined by comparing the ratio of diphenyl ether (PhOPh) to diphenylmethane (PhCH2Ph) after 90 min at 390 °C. major
p-(PhO)-PhCH2-Ph 98 PhOPh + PhCH3 (7) Figure 1. Observed first-order rates of model compound bond scission in 9,10-dihydrophenanthrene using OHS/sulfur at 390 °C. (×) p-phenoxydiphenylmethane (pPDM), (2) o-phenoxydiphenylmethane (oPDM), (+) p-(4-methylphenoxy)diphenylmethane (pMPDM), (9) o-(4-methylphenoxy)diphenylmethane (oMPDM). Table 1. Product Distributions and Measured Rates of Thermolysis of Phenoxydiphenylmethanes at 390 °C in 9,10-Dihydrophenanthrene in the Presence and Absence of OHS/S model compound conditionsa pPDM
catalytic
oPDM
thermal catalytic
pMPDM
thermal catalytic
oMPDM
thermal catalytic thermal
k (s-1) 4(2×
10-5
productsb,c
C-C/C-Od
DPE (12) DPM (1)
9 ( 2 × 10-6 12 ( 1 × 10-5 DPE (6) DPM (24) 5 ( 1 × 10-5 3 ( 1 × 10-5 MDPE (10) DPM (0.5) 4 ( 2 × 10-6 37 ( 3 × 10-5 MDPE (2.5) MDPM (35) 13 ( 2 × 10-5
12/1
1/4
20/1
1/14
Catalytic 9,10-dihydrophenanthrene (DHP) + OHS/sulfur; thermal DHP. b DPE, diphenyl ether; DPM, diphenylmethane; MDPE, 4-methyldiphenyl ether; MDPM, 4-methyldiphenylmethane. c Products reported as (%) observed based on starting concentration of model compounds. d Ratio of carbon-carbon to carbon-oxygen bond scission determined from products. a
behavior, both the rate of decomposition and the product distribution varied significantly for the different isomers. To monitor the selectivity the ratio of diphenylmethane (DPM), i.e., apparent C-O bond scission (eq 5), to diphenyl ether (DPE), i.e., apparent C-C (eq 6) were quantitatively analyzed. The results are summarized in Table 1.
PhO-PhCH2Ph f PhCH2Ph + PhOH (DPM)
(5)
PhOPhCH2-Ph f PhOPh + PhCH3 (DPE)
(6)
The mass balance of the observed products in the catalysis experiments ranged from 80% to 95%. Typically the yields of phenol and toluene were less than expected. The deficit of products in part is likely due to alkylation of the solvent.36 Reactions were run to low conversion when possible to avoid complications due to
minor
p-(PhO)-PhCH2Ph 98 PhCH2Ph + PhOH
(8)
The rate of disappearance of pPDM in the presence of “FeS” is roughly four times the thermal background and shows first-order behavior. Analysis of the catalytic decomposition data shown in Figure 1 yields a total rate of pPDM thermolysis of (4 ( 2) × 10-5 s-1. Thermolysis of o-Phenoxydiphenylmethane (oPDM). Thermolysis of the oPDM in the presence of 9,10-DHP and the catalyst precursor for 60 min at 390 °C, 50% conversion, leads to the formation of diphenylmethane, phenol, diphenyl ether, and a trace of xanthene. A plot of ln(Ct/Co) vs time is shown in Figure 1. Analysis of the slope provides a combined first-order rate of decay (1.2 ( 0.1) × 10-4 s-1 for oPDM that is greater than twice the “FeS” induced decay of the pPDM and 2.5 times faster than the thermal background. The products are the same as observed in the thermolysis of pPDM; however, the product distribution is notably different. The observed ratio of PhCH2Ph to PhOPh is 4 to 1, suggesting a significantly different pathway leading to bond scission, eq 9.
o-(PhO)-Ph-CH2Ph f PhCH2Ph/PhOPh 4/1
(9)
Thermolysis of p-(4-Methylphenoxy)diphenylmethane (pMPDM). Thermolysis of the pMPDM in 9,10-DHP containing the catalyst precursor for 90 min at 390 °C, 10% conversion, leads to the formation of 4-methyldiphenyl ether and toluene as the major observable products and a minor yield of diphenylmethane. The ratio of 4-methyldiphenyl ether, C-C bond scisssion, to DPM, C-O bond scission is 20 to 1 after 90 min.
p-(4-CH3PhO)-Ph-CH2Ph f 4-CH3PhOPh/PhCH2Ph (10) A plot of ln(Ct/Co) vs time is shown in Figure 1. Analysis of the slope provides a combined first-order rate of decay (3 ( 1) × 10-5 s-1 for pMPDM comparable (36) This appears to be a general observation, see ref 25. Two small peaks in the GC/MS trace appeared after the solvent phenanthrene was eluted with the parent ion expected for a benzylated phenanthrene (m/z ) 268).
Scission of Strong Bonds in Phenoxydiphenylmethanes
Energy & Fuels, Vol. 13, No. 4, 1999 931
to the observed rate of decay of the pPDM and 7.5 times faster than the thermal background. Thermolysis of o-(4-Methylphenoxy)diphenylmethane (oMPDM). Thermolysis of the oMPDM in 9,10-DHP containing the catalyst precursor for 30 min at 390 °C, 64% conversion, leads to the formation of phenol and 4-methyldiphenylmethane as the major products and 4-methyldiphenyl ether as a minor product. The ratio of 4-methyldiphenylmethane to 4-methyldiphenyl ether (14 to 1), eq 11, provides a measure of rearrangement to direct “FeS” induced bond scission, vide infra.
∆Hrxn (AH•) )
o-(4-CH3PhO)-PhCH2Ph f 4-CH3PhCH2Ph/MePhOPh (11) 14/1
∆∆Hrxn (HA) ∼ ∆∆Hfo (HA) )
A plot of ln(Ct/Co) verses time is shown in Figure 1. Analysis of the slope provides a combined first-order rate of decay (3.7 ( 0.3) × 10-4 s-1 for oMPDM that is ca. 12 times faster than the decay of pMPDM and 2.5 times greater than the thermal background. At longer reaction times, the yield 4-methyldiphenylmethane begins to decrease concomitant with increase in the yield of toluene. Discussion The goals of the present work were two-fold: (1) to examine the effectiveness of the catalyst in promoting the scission of the strong Ar-OAr bond in diaryl ethers, and (2) to further test the hypothesis of free-radical pathways. Comparison of products formed and the rates of decomposition of the four different isomers lend additional support to the hydrogen transfer hypothesis and demonstrate the importance of a free-radical rearrangement pathway. Para-Substituted Isomers. The “FeS” induced scission of p-phenoxydiphenylmethane (pPDM) and p-(4methylphenoxy)diphenylmethane (pMPDM) lead to the formation of diphenyl ether and 4-methyldiphenyl ether in favor of diphenylmethane by a ratio of between 12 and 20 to 1, respectively. Scission of the phenoxyl cation, from intermediate CH+ shown in Scheme 2, is predicted to be thermodynamically insignificant compared to scission of benzyl cation, from the intermediate DH+. Two results are notable. First, the observation of diphenylmethane rules out any significant contribution from a cationic intermediate. And second, the observed selectivity is consistent with a hydrogen atom transfer mechanism leading to two intermediates, AH• and BH• shown in Scheme 1. The difference in rates of bond scission are directly proportional to the difference in heats of formation of the two intermediates, consistent with hydrogen atom transfer by a mechanism involving a late transition state.16 Therefore, the difference in heat of reaction for addition of a hydrogen atom, ∆∆Hrxn (HA), eq 12, to form intermediate AH•, eqs 13 and 14, and intermediate BH•, eqs 15 and 16, should be directly proportional to the experimentally measured selectivity. Substitution of the terms in eqs 14 and 16 into eq 12 yields eq 17 and provides a means to calculate the selectivity, provided the ∆Hfo of the radical intermediates AH• and BH• are known or can be calculated. PDM refers generically to any of the four phenoxydiphenyl-
methanes examined in this work.
∆∆Hrxn (HA) ) ∆Hrxn (AH•) - ∆Hrxn (BH•) (12) PDM + H• f AH•
(13)
∆Hfo (AH•) - [∆Hf (PDM) + ∆Hfo (H•)] (14) PDM + H• f BH•
(15)
•
∆Hrxn (BH ) ) ∆Hfo (BH•) - [∆Hfo (PDM) +∆Hfo (H•)] (16) ∆Hfo (AH•) - ∆Hfo (BH•) (17) The observed selectivity, at 390 °C, as measured by the yield of DPE (and benzyl radical), products arising from radical intermediate AH•, compared to the yield of DPM (and phenoxyl radical), products arising from radical intermediate BH• suggests a ∆∆Hrxn (HA) of ca. 3-4 kcal/mol. Unfortunately, no thermochemical data is available specifically for the ∆Hfo of the radical adducts shown in Scheme 1. However, both an empirical and a theoretical approach can provide insight into the ∆∆Hrxn (HA) as shown in eq 17. A comparison of the bond dissociation energies of hydrocarbons with and without R-hydroxy groups can provide an empirical estimate of the magnitude of stabilization provided by an R-hydroxy group. For example, the additional stabilization provided by an R-hydroxy group for a comparable cross conjugate stabilized radical, R-hydroxydiphenylmethane versus diphenylmethane, is expected to be between 3 and 6 kcal/mol.37-41 Theoretical methods can be used to calculate the ∆Hfo for a pair of structurally similar “parent” isomers. The energy differences between 1-methyl-4-hydroxy-cyclohexadienyl radical (E) and 1-hydroxy-4-methyl-cyclohexadienyl radical (F) shown in Scheme 4 were estimated by ab initio electronic structure calculations (Gaussian 94). Geometries were optimized using density functional theory (B3LYP/6-31G*) and at the HF/6-31G* and MP2)FU/6-31G* levels of theory. Energies were corrected to 298 K using HF/6-31G* harmonic frequencies. Radical E is predicted to be more stable than radical F by 4.4 kcal/mol at the B3LYP/6-31G* level, 3.72 kcal/mol at the MP2)FU/6-31G**//B3LYP/6-31G* projected MP2 level of theory and by 3.78 kcal/mol at the spin projected (PMP2) MP2)FU/6-31G* level of theory. The energy differences at the MP2 level of theory are presumed to be more accurate for comparison of the two radicals. Assuming the ∆∆Hfo between the radical intermediates E and radical intermediate F are comparable to the ∆∆Hfo between the radical intermediates (37) The measured BDE of diphenylmethane is between 80.6 (ref 38) and 81.8 kcal/mol (ref 39) compared to the measured BDE of diphenylmethanol between 75.4 (ref 40) and 78 kcal/mol (ref 41). (38) Stein, S. E.; Brown, R. L. J. Am. Chem. Soc. 1991, 113, 787793. (39) Bordwell, F. G.; Cheng, J.-P.; Harrelson, J. A., Jr. J. Am. Chem. Soc. 1988, 110, 1229-1231. (40) Arnaut, L. G.; Caldwell, R. A. J. Photochem. Photobiol. A: Chem. 1992, 65, 15-20. (41) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493.
932 Energy & Fuels, Vol. 13, No. 4, 1999 Scheme 4
Scheme 5
AH• and BH•, a direct comparison is possible between the ∆∆Hrxn (HA) determined by experimental selectivity and the ∆∆Hrxn (HA)* predicted by MP2 theory. The observation of Ar-OAr bond scission from both pPDM and pMPDM suggests the ionic pathway proposed in Scheme 2 is not important in the “FeS”-induced scission of Ar-XAr bonds. On the other hand, the clear agreement between the experimentally calculated ∆∆Hrxn (HA), estimated from the observed product selectivity, and the ∆∆Hrxn (HA) calculated from the theoretical results at the MP2 level of theory provide strong support for the hydrogen atom transfer pathway shown in Scheme 1. Ortho-Substituted Isomers. The “FeS” induced scission of o-phenoxydiphenylmethane (oPDM) is both quantitatively and qualitatively different than observed for the corresponding para isomer, pPDM. Two results are notable; (1) the rate of disappearance of the ortho isomer is faster than the rate of disappearance of the para isomer, and (2) the ratio of apparent C-O to C-C bond scission observed in the ortho isomers (1 to 4) is nearly the inverse of that observed in the para isomers (12 to 1)! This surprising finding led us to suspect a possible free-radical rearrangement pathway as shown in Scheme 5. There is significant precedence for 1,5 radical shifts, e.g., hexenyl radical clocks,42 and the Ar1-5 radical rearrangement of alkyl-substituted arenes.43 Directly relevant to the present work, Cadogan and co-workers have shown that generation of the 2-phenoxybenzyl radical generated in the gas phase at 750 °C leads to (42) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317-323. (43) Winstein, S.; Hech, R.; Lapporte, L.; Baird, R. Experientia 1955, 12, 138-141.
Autrey et al.
the formation of 2-benzylphenol.44 In this present work rearrangement of the initially formed benzylic radical of oPDM by an analogous radical cyclization pathway would yield o-(hydroxyphenyl)diphenylmethane (oHPDM). The inability to observe oHPDM under the reaction conditions is attributed to the thermal lability of hydroxydiphenylmethanes. For example, thermolysis of (2hydroxyphenyl)phenylmethane (oPPM) in tetralin yields phenol and toluene at 400 °C with an observed rate of 3 × 10-6 s-1, significantly faster than predicted by a simple homolysis pathway.25 The thermolysis was proposed to occur by a pathway involving homolysis of the weak carbon-carbon bond controlled by an enol/keto equilibrium (Scheme 5). Thus, the greater stability of the leaving group, diphenylmethyl radical from the keto form of oHPDM, compared to the benzyl radical from the keto form of oPPM, results in a lower barrier for C-C bond scission from the keto intermediate consistent with the observed thermal decomposition rate of o-phenoxydiphenylmethane (oPDM), (5 ( 1) × 10-5 s-1 at 390 °C in 9,10-DHP. The rate enhancement (12 ( 1) × 10-5 s-1 observed in the presence of the “FeS” catalyst is likely due to the facilitated formation of the initial radical intermediate, an acceleration of the enol/keto tautomerism or a combination of both. To validate the Ar1-5 radical rearrangement hypothesis p-(4-methyl)phenoxydiphenylmethane (pMPDM) and o-(4-methyl)phenoxydiphenylmethane (pMPDM) were synthesized. These model compounds provide a label on the aromatic ring containing the phenoxyl group and permit a quantitative comparison of radical rearrangement versus direct scission pathways. The control experiment, thermolysis of pMPDM, occurs at a rate comparable to pPDM, and the selectivity shows a preference for C-C to C-O bond scission, ca. 20 to 1. Assuming between a 10-20 to 1 selectivity for C-C versus C-O bond scission by hydrogen transfer from the catalyst to the model compound, thermolysis of oMPDM provides a measure of radical rearrangement versus direct C-O bond scission. Direct C-O bond scission by a hydrogen atom transfer to the ipso position of the central ring bearing the phenoxyl group will yield p-cresol (CH3PhOH) and diphenylmethane (DPM) as the major products. On the other hand, if rearrangement of oMPDM occurs as shown in Scheme 5, both phenol (PhOH) and 4-methyldiphenylmethane (MDPM) will be observed. Quantitative analysis of the reaction mixture shows the products expected for the radical rearrangement pathway 4-methyldiphenylmethane (MeDPM) and only a minor yield of 4-methyldiphenyl ether (MeDPE). The ratio of MeDPM to MeDPE (14 to 1) observed suggests the rearrangement pathway is significantly faster than the hydrogen atom transfer from the catalyst to the ipso position of oMPDM. Conclusions Two distinct reaction pathways operate in both the thermal and “FeS” induced scission of ortho- and parasubstituted diaryl ethers. Scission of the Ar-CH2Ar bond in both p-phenoxydiphenylmethane and 4-methyl(44) Cadogan, J. I. G.; Huthchingson, H. S.; McNab, H. J. Chem. Soc., Perkin Trans. 1991, 1, 385-393.
Scission of Strong Bonds in Phenoxydiphenylmethanes
p-phenoxydiphenylmethane is favored over scission of the Ar-OAr bond. These results are consistent with previous observations that the rate-controlling step involves the reversible transfer of a hydrogen atom from the catalyst surface to the ipso position of the substituted arene. The branching ratio is controlled by the stability of the radical intermediates. On the other hand, scission of either o-phenoxy(diphenylmethane) or 4-methyl-o-phenoxy(diphenylmethane) occurs predominately through a radical rearrangement pathway involving the formation of a thermally labile (o-hydroxyphenyl)diphenylmethane. In the presence of the “FeS” catalyst either enhanced formation of initial radical intermediate or acceleration of the enol/keto tautomerism or a combination of both leads to rate enhancement compared to the thermal decomposition pathways. This work illustrates the importance of a general pathway for ortho-assisted scission of strong Ar-OAr bonds in coal structures. In general, ortho substitution of diaryl ethers may facilitate Ar-OAr bond scission through radical rearrangement
Energy & Fuels, Vol. 13, No. 4, 1999 933
pathways both in the presence and absence of hydroliquefaction catalysts. Work is in progress to measure the thermal activation barrier for the radical rearrangement pathway in 2-phenoxylbenzyl radical.45 Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Research, Chemical Sciences Division, Process and Techniques Branch. The work was conducted at Pacific Northwest Laboratory, which is operated for the U.S. Department of Energy under Contract DE-ACO6-76RL0 1830. T.A. thanks Dr. Buchanan (ORNL) for bringing our attention to the Cadagon work. Support for L.K., C.S., T.R.P., and E.F.M. was provided through AWUNW under grant DE-FG06-89ER-75522 with the U.S. Department of Energy. EF990009Q (45) Franz, J. A.; Linehan, J. C.; Crump, A. E.; Kaune, L.; Alnajjar, M. S.; Camaioni, D. M.; Autrey, T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 12-14.
