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An Evaluation of the Potential of Radical Cation Chemistry for Coal Conversion Processes John W. Larsen* and Thomas P. Eskay Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received October 11, 1995X
The cleavage of the radical cations of benzyl phenyl ether and 4,4′-dimethoxybibenzyl at low temperature (82 °C) has been studied. The cation radicals are formed by one-electron oxidation using FeIII(1,10-phenanthroline)3(ClO4)3. Cleavage of benzyl phenyl ether has been achieved using a catalytic amount of Fe(III) and air to reoxidize Fe(II) to Fe(III). While the dominant chemical process is cleavage of the bond β to the aromatic rings, substantial amounts of less reactive rearranged and higher molecular weight materials are formed. These competing reactions are expected to become more important in diffusionally limited coals. We conclude that radical cation mechanisms are not good candidates for low-temperature coal conversion processes because of the propensity of the systems to undergo aromatic substitution reactions and rearrangements to less reactive structures, processes which do not lead to depolymerization.
Introduction It has been pointed out that bond cleavage reactions proceeding through radical cations occur at low temperatures and may provide an attractive route for coal depolymerization.1-5 The attractiveness of radical cation chemistry for coal depolymerization is shown most clearly by the data in Table 1, taken from ref 1. Cleavage of bonds β to an aromatic ring in radical cations requires much less energy than the homolysis of the same bond in the neutral precursor.1-10 A good example is benzyl phenyl ether where the bond strength in the neutral molecule is 53 kcal/mol and half that (or less!) in the radical cation. Consideration of coal structures such as Shinn’s reveals the presence of numerous bonds which should cleave very readily if the appropriate radical cations could be formed in the coal.11 One-electron oxidation of structures which occur in coals can be achieved quite readily by numerous reagents including such low cost and easily accessible species as Fe(III). This combination of factors makes radical cation chemistry an attractive possibility for low-temperature coal depolymerization. Unfortunately, there exist several side reactions that might prevent radical cation decomposition from being useful for coal depolymerization. Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Camaioni, D. M. J. Am. Chem. Soc. 1990, 112, 9475-9483. (2) Penn, J. H. Electron Transfer Reactions of Coal Model Compounds. Final Report to the Gas Research Institute, Chicago, IL; GRI88/0335, 1989. (3) Farcasiu, M.; Smith, C. Energy Fuels 1991, 5, 83-87. (4) Penn, J. H.; Smith, R. S. Tetrahedron Lett. 1986, 27, 3475-3478. (5) Penn, J. H.; Liu, Y.-Q.; Yassini, P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36(2), 605-608. (6) Camaioni, D. M.; Franz, J. A. J. Org. Chem. 1984, 49, 16071613. (7) Penn, J. H.; Deng, D.-L.; Aleshire, S. K. J. Org. Chem. 1988, 53, 3572-3582. Penn, J. H.; Deng, D.-L. Tetrahedron 1992, 48, 48234830. (8) Walling, C.; El-Taliawa, M.; Amarnath, K. J. Am. Chem. Soc. 1984, 106, 7573-7578. (9) Popielarz, R.; Arnold, D. R. J. Am. Chem. Soc. 1990, 112, 30683082. (10) Okamota, A.; Arnold, D. R. Can. J. Chem. 1985, 2340-2342. (11) Shinn, J. H. Fuel 1984, 63, 1187-1195. X
S0887-0624(95)00209-X CCC: $12.00
Scheme 1. Reactions of Radical Cations
The network of reactions that a radical cation might undergo is shown in Scheme 1. Radical cations have only one electron in their highest occupied molecular orbital (HOMO) and thus form quite stable complexes with other aromatics. These complexes can easily further react to form substitution products.12,13 The radical cation can also lose a proton to form the radical which can hydrogen abstract, undergo substitution reactions, or be oxidized to give the cation that in turn has its own familiar chemistry.14-16 Camaioni and Franz have demonstrated that proton loss, not C-C bond cleavage, is the predominant pathway for diarylethanes (ArCH2CH2Ar1) unless they are substituted on the ethane linkage (ArCHR-CH2Ar, R ) alkyl, OH).6 (12) Norman, R. O. C.; Thomas, C. B.; Wilson, J. S. J. Chem. Soc., Perkin Trans. 1 1973, 325-332. (13) Ledwith, A. Acc. Chem. Res. 1972, 5, 133-139. (14) Baciocchi, E.; Bartoli, D.; Rol, C.; Ruzziconi, R.; Sebastiani, G. V. J. Org. Chem. 1986, 51, 3587-3592. (15) Baciocchi, E.; Rol, C.; Mandolini, L. J. Org. Chem. 1977, 42, 3682-3686. (16) Schlesener, C. J.; Kochi, J. K. J. Org. Chem. 1984, 49, 31423150.
© 1996 American Chemical Society
Potential of Radical Cation Chemistry for Coal Conversion
Energy & Fuels, Vol. 10, No. 4, 1996 965
Table 1. A Comparison of Radical Cation and Homolytic Bond Dissociation Energies (from Ref 1) bond dissociation energies (kcal/mol)
a
R1-R2
R1•
R2+
radical cationa
neutral
bibenzyl 1,2-di(p-tolyl)ethane benzyl phenyl ether benzyl phenyl sulfide ethylbenzene n-propylbenzene isobutylbenzene neopentylbenzene neopentylbenzene
PhCH2 MePhCH2 PhO PhS Me Et i-Pr t-Bu PhCH2
PhCH2 MePhCH2 PhCH2 PhCH2 PhCH2 PhCH2 PhCH2 PhCH2 t-Bu
26 (27) 27 (24) 12 (27) 30 (38) 43 (40) 35 (38) 26 (37) 21 (33) 17 (22)
62 61 53 52 76 73 71 68 68
AM1 calculated values, experimental values from thermodynamic cycles in parentheses. R1 is a radical and R2 is a cation.
When substituted on the ethane linkage, the radical cations cleave.6,17 Nucleophilic attack on the radical cation is possible but will probably not be very important in reactions of coals since the supply of anionic nucleophiles is limited.16 Finally, the radical and cation formed in radical cation cleavage have their own wellestablished chemistries which can result in undesirable products. The questions which we are addressing in this paper are: for functional groups present in coals, what is the balance between these possible radical cation reaction pathways and is that balance such that radical cations are attractive intermediates on which to base a coal depolymerization process. Three other groups have considered radical cation chemistry as a possible route for low-temperature coal depolymerization. The fundamental studies of Camaioni have already been referred to.1,6 Penn studied the decomposition of a pair of gem diols in acetonitrile using FeIII(1,10-phenanthroline)3(PF6)3 as the oxidant and observed quantitative bond cleavage.2,18 Benzyl phenyl ether and benzyl phenyl sulfide are also readily cleaved using organic single electron oxidants such as TCNQ and DDQ.4,7 Farcasiu has suggested that radical cation intermediates might be involved in the carboncatalyzed pyrolysis of several coal model compounds.3 It is necessary to choose an oxidant to remove one electron from the compounds to be studied. Of greatest interest to us is FeIII(1,10-phenanthroline)3(ClO4)3 (FeIIIPhen) because it is a well-studied well-characterized, outer-sphere single-electron oxidant19-21 and might lead to the development of a low-cost catalytic oxidizing system in which Fe(III) oxidizes the organic and the Fe(II) thus formed is reoxidized to Fe(III) by air. It is a reasonably strong oxidizing agent having an oxidation potential of 1.09 V vs SCE in acetonitrile and it has been shown to oxidize alkylbenzenes.22 Because its normal mechanism is outer-sphere electron transfer, complexation of the iron with the aromatic to be oxidized is not necessary. In addition to FeIIIPhen, we briefly explored the use of another pair of one-electron oxidizing agents. Triarylamminium salts are known to form radical cations (17) Maslak, P.; Chapman, W. H., Jr.; Vallombroso, T. M., Jr.; Watson, B. A. J. Am. Chem. Soc. 1995, 117, 12380-12389. (18) Penn, J. H.; Deng, D.-L.; Chai, K.-J. Tetrahedron Lett. 1988, 29, 3635-3638. (19) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley Interscience: New York, 1988. (20) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (21) Bunton, C. A. In Oxidation in Organic Chemistry; Wiberg, K. B., Ed.; Academic Press: New York, 1985. (22) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 3567-3577.
readily and are readily available.13,23,24 We have used one of these. Ammonium cerium(IV) nitrate is another well-characterized single-electron oxidant known to form radical cations of simple alkyl benzenes and we have made use of this oxidant.14,15,25 The organic molecules selected for study are benzyl phenyl ether (1) and 4,4′-dimethoxybibenzyl (7). The ether linkage is known to occur in coals, especially lowrank coals, and the products of the reaction were expected to be relatively easy to isolate and characterize. Benzyl phenyl ether was chosen in the hope that it would provide information in a system where bond cleavage should be competitive with deprotonation. This system will provide information on the fate of the radical and cation formed in the cleavage. Dimethoxybibenzyl should be easy to oxidize to its radical cation and provide the opportunity of comparing carboncarbon bond cleavage with the carbon-oxygen bond cleavage in the benzyl-phenyl ether. Once again, the products should be easy to isolate and characterize. As pointed out by a reviewer, other groups might be better model coal structures, but the radicals and cations formed from benzyl phenyl ether will be typical of those formed by radical cation cleavages in coals and their chemical behavior will be representative. Results and Discussion Benzyl phenyl ether (1) was treated with a slight molar excess of FeIIIPhen in refluxing acetonitrile for 2 h (sufficient time for complete reaction) at 82 °C. The reaction mixture was analyzed by GC and GC mass spectrometry, and product identifications are based on comparison with authentic samples on three different GC columns. Control experiments demonstrated that the workup procedure did not alter the product distribution. The products are shown in eq 1 and the product distributions from two separate runs are presented in Table 2. The mass balance is not 100% and the “missing” material is presumably higher molecular weight products which would not elute from the GC column. No gaseous products are expected. We will return to this point later. These products are those expected from the decomposition of the benzyl phenyl ether radical cation as shown in Scheme 1. The phenoxy radical resulting from the decomposition will abstract a hydrogen, probably (23) Kricks, L. J.; Ledwith, A. J. Chem. Soc., Perkin Trans. 1 1973, 294-297. (24) Bellville, D. J.; Wirth, D. D.; Bauld, N. L. J. Am. Chem. Soc. 1981, 103, 718-720. (25) Trahanovsky, W. S.; Brixius, D. W. J. Am. Chem. Soc. 1973, 95, 6778-6780.
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Table 2. Products and Yields from the Oxidation of Benzyl Phenyl Ether by FeIIIPhen in Refluxing Acetonitrile for 2 h (mol % ( sd)
a
run
2 (PhOH)
3 (PhCH2NHOAc)
4 (PhCHO)
5 (PhCH2PhOH)
6 (PhCH2PhOH)
mass balance
1a 2b
52 ( 2 57 ( 1
64 ( 4 63.4 ( 0.2
17 ( 1 16.7 ( 0.1
2(1 2.7 ( 0.1
3(1 3(1
78 ( 2 82 ( 1
70 mol % excess FeIIIPhen, [1] ) 0.051 M. b 17 mol % excess FeIIIPhen, [1] ) 0.055 M.
Table 3. Products and Yields from the Perchloric Acid and Neutralized FeIIIPhen Cleavage Reaction of Benzyl Phenyl Ether (1) in Refluxing Acetonitrile for 2 h (mol % ( sd)a reaction
2
3
5
6
4
mass balance
HClO4 FeIIIPhen pretreated w/K2CO3c FeIIIPhen pretreated w/Al2O3d
67 ( 4 40.4 ( 0.4 58 ( 4
65 ( 4 38 ( 4 47 ( 1
24 ( 1 8(1 11 ( 1
5(1 1e 1.3 ( 0.2
2(1 4(2 7(4
95 ( 5 78 ( 3 79 ( 6
b
a Mole % given as moles of product/total starting moles of 1. b 20 mol % excess HClO , [1] ) 0.55 M, reflux. c 70% conversion of 1 4 observed, 11 mol % excess FeIIIPhen, [1] ) 0.055 M. d 90% conversion of 1 observed, 11 mol % excess FeIIIPhen, [1] ) 0.053 M. e Determined from only one injection.
OH OCH2
FeIIIPhen CH3CN
1
CH2NHCOCH3
+
+ 2
CHO
3
4
OH
+ HO 5
CH2
(1)
6
from the solvent, to given phenol (2). The benzylic carbonium ion will also react with solvent to give an intermediate which will be rapidly hydrolyzed by water to give benzylacetamide (3). This pathway is known and is common when benzylic carbonium ions are electrochemically generated in acetonitrile.26 The rearranged products 5 and 6 can be formed by either an inter- or an intramolecular pathway. Note that these molecules will not readily cleave thermally and their formation in a coal conversion process would replace a labile bond by a strong bond. There are two routes to formation of benzaldehyde (4): (1) the benzylic cation may react with water followed by oxidation either by perchlorate or Fe(III) or (2) the benzylic cation may be trapped and oxidized by perchlorate. The observed products are completely consistent with the decomposition of the benzyl phenyl ether radical cation formed by oneelectron oxidation of benzyl phenyl ether. Note that the presence of a sacrificial nucleophile is necessary to trap the carbocation. Control experiments are necessary to demonstrate that the observed products did indeed arise from the radical cation. The problem is that the cleavage of the C-O bond in benzyl phenyl ether is known to occur in the presence of acid.5,27 The FeIIIPhen used in the reaction was crystallized from perchloric acid. It is possible that traces of acid could be present and could be partially responsible for the observed reaction. Benzyl phenyl ether was treated with a slight molar excess of perchloric acid in refluxing acetonitrile for 2 h. The products isolated were the same as those produced when benzyl phenyl ether was treated with FeIIIPhen. Product distributions (see Table 3) are generally similar to those obtained using FeIIIPhen although not identical. (26) Eberson, L.; Nyberg, K. Tetrahedron Lett. 1966, 22, 2389-2393. (27) Hart, L. S.; Waddington, C. R. J. Chem. Soc., Perkin Trans. 2 1985, 1607-1612.
It is possible that acid-induced bond cleavage is contributing to the products observed during oxidation with FeIIIPhen. To further investigate this point, the FeIIIPhen was treated to remove all traces of acid. This was done by treating the complex and reaction solvent with solid potassium carbonate or by passing it through a column of activated alumina. These two samples of FeIIIPhen were then used to oxidize benzyl phenyl ether, and the product distributions are shown in Table 3. In these reactions, 100% conversion was not observed, presumably because some of the Fe(III) was reduced to Fe(II) during the pretreatment step. Control experiments demonstrated that FeIIIPhen was slowly reduced to Fe(II) when the complex was left in contact with either K2CO3 or Al2O3 in acetonitrile. The product ratios obtained with the treated FeIIIPhen are significantly different than those observed with perchloric acid or with the untreated FeIIIPhen. This demonstrates that there is an acid component to the contribution to the products observed with untreated FeIIIPhen. There is a significant increase in the amount of rearranged product formed with the deacidified FeIIIPhen. A comparison of product ratios in Tables 2 and 3 suggests that roughly 15% of the cleavage reported in Table 2 was due to acid catalysis and 85% was due to FeIIIPhen. Another potential route for acid-induced cleavage of benzyl phenyl ether exists. Fe(III) is a strong Lewis acid. If the phenanthroline complex dissociates, then it may be possible for Fe(III) to coordinate with the ether oxygen and induce the cleavage processes. There is strong evidence that this does not occur. The stability constant for FeIIIPhen is high (log K > 15) so there is little uncomplexed Fe(III). Also, FeIIIPhen is not stable in excess phenanthroline. It very rapidly oxidizes the phenanthroline, being itself reduced to Fe(II). If the complex were to partially dissociate, it would be destroyed as undissociated FeIIIPhen oxidized the freed phenanthroline. FeIIIPhen is stable in refluxing acetonitrile for at least 72 h. If the complex were dissociating, one would expect that undissociated FeIIIPhen would oxidize the free phenanthroline and thus Fe(III) would slowly be converted to Fe(II). To further investigate the possibility of this reaction, benzyl phenyl ether was treated with iron(III) chloride in refluxing acetonitrile. The products observed were the same as those resulting from the perchloric acid catalyzed reaction. However, the reactions were much slower than
Potential of Radical Cation Chemistry for Coal Conversion
Energy & Fuels, Vol. 10, No. 4, 1996 967
Table 4. Products and Yields (mol % ( sd) from the Reactions of 4,4-Dimethoxybibenzyl (7) in Refluxing Acetonitrile for 3h conversion (%) reaction
7
8
mass balance (%)
(7) 0.045 M 5 mol % excess FeIIIPhen (7) 0.054 M 65 mol % excess FeIIIPhen
29 ( 1 27 ( 1
6.2 ( 2 7.4 ( 3
73 ( 1 78 ( 2
Table 5. Reaction of 4,4-Dimethoxybibenzyl with Single-Electron Oxidants in Refluxing Acetonitrile conversion (%) reaction
7
8
mass balance (%)
(7) 0.045 M 31 mol % excess Ar3N•+3a 18 h (7) 0.048 M 31 mol % excess Ceb 19 h
36 ( 1 97 ( 1
44 ( 2
64 ( 1 25 ( 2c
a Tris(p-bromophenyl)amminium SbCl . b Ammonium cerium(IV) nitrate. c Small amounts of two additional products were detected by 6 GC analysis. Conservative estimates based upon peak area suggest these products could account for at most 10-20 additional mol %. With this estimate the mass balances still less than 50%.
those which occur with FeIIIPhen. Only 40% conversion of benzyl phenyl ether in 3 h was observed with FeCl3 compared to 100% conversion in 2 h with FeIIIPhen. We conclude that Fe(III)-catalyzed decomposition of benzyl phenyl ether is not a significant competing process when FeIIIPhen is used. In all of the reactions so far reported, the mass balance is not 100%. For most of the FeIIIPhen oxidations, only about 80% of the products can be detected by gas chromatography. The “missing” products could well be higher molecular weight species. We used 252Cf plasma desorption mass spectrometry (PDMS) to search for higher molecular weight products. PDMS is a mass spectrometric technique that is capable of volatilizing, ionizing, and detecting species having molecular weights as high as 25 000.28,29 Cleavage is minimized and the principal ion detected is the molecular ion. To apply this technique, the FeIIIPhen complex was removed by passing the reaction mixture through silica gel. The acetonitrile solution of the products was then deposited on an aluminized Mylar disk and subjected to mass spectrometry. The PDMS mass spectra contained several peaks at m/e > 200, specifically m/e 364, 454, and 545. These peaks are not present in the background spectra, are not observed when mixtures of 1-5 are analyzed, and can be fit relatively easily by benzyl phenyl ether dimers and their mono- and dibenzylated derivatives. Polybenzylphenols will also fit this structure but are significantly less probable. We could not fit the remaining high mass peaks (m/e 206, 221, 280, and 390) to reasonable structures. Formation of the dimer is quite consistent with the known cation radical chemistry expressed in Scheme 1. We wished to extend this chemistry to the cleavage of stronger bonds, so bibenzyl was treated with FeIIIPhen in refluxing acetonitrile and in refluxing N,NDMF. No cleavage products were detected and starting material was recovered. This could be due to (1) failure to oxidize or (2) failure of the C-C bond to cleave. To distinguish between these two possibilities, neopentylbenzene was reacted with FeIIIPhen in acetonitrile. The bond strength of the neopentylbenzene radical cation is 5 kcal/mol less than that of bibenzyl, and it should cleave 900 times faster.1 After 24 h refluxing in acetonitrile, 97% of the neopentylbenzene was recovered unreacted. About 1% benzylacetamide was formed as (28) Larsen, J. W.; Lapucha, A. R.; Wernett, P. C.; Anderson, W. R. Energy Fuels 1994, 8, 258-265. (29) Cotter, R. J. Anal. Chem. 1988, 60, 781A.
well as a small amount of an unidentified product. It is likely that formation of the radical cation is not occurring with bibenzyl and neopentylbenzene. We next studied 4,4-dimethoxybibenzyl (7) chosen because the oxidation of p-methoxytoluene by FeIIIPhen is known.16 Almost 30% conversion was obtained in 3 h with the only isolated product being p-anisaldehyde (8) (see Table 4). Its formation can be explained in a straightforward way using the mechanisms presented in Scheme 1. Once again, the mass balance was less than 100%, so the reaction mixture was subjected to PDMS and a number of high molecular weight peaks were obtained. The peaks at m/e 482 and 602 are consistent with the formation of the dimer and pmethoxybenzylated dimer of dimethoxybibenzyl. Loss of methyl or p-methoxybenzyl would then give rise to the peaks at m/e 467 and 361. Alternatively, the peaks at m/e 361, 482, and 602 could be mono-, di-, and trimethoxybenzylated starting material. Reasonable structures for the peaks m/e 294, 415, and 655 have escaped us. Once again, it is clear that dimerization is an important reaction. So far, only one oxidant has been used. One expects that the chemistry of the radical cations will be significantly independent of the nature of the oxidant used to form them, but scenarios in which this is not so are conceivable. Ammonium cerium(IV) nitrate was used in acetonitrile and 98% conversion of dimethoxybibenzyl was observed in 19 h. The principal isolated product was p-anisaldehyde (8) and the mass balance was less than 50% as shown by the data in Table 5. Another single-electron oxidant, tris(p-bromophenyl)amminium hexachloroantimonate, was used in refluxing acetonitrile. After 18 h, 64% of the dimethoxybibenzyl was recovered and no cleavage products were isolated by GC analysis. With both of these oxidants, PDMS mass spectra showed the formation of high molecular weight products. The ultimate inexpensive oxidant is air, and radical cation based processes for coal depolymerization will only be possible if it is possible to use an oxidant as cheap as air. We therefore carried out reactions using a catalytic amount of FeIIIPhen while passing a stream of air through the system. This approach worked. Using 27 mol % of FeIIIPhen with acetonitrile, 99% conversion of benzyl phenyl ether was obtained in 30 h. In the absence of the air flow, only 20% conversion was achieved. Product distributions and mass balances are shown in Table 6. When the experiment was carried
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Table 6. Reaction of Benzyl Phenyl Ether with Less Than Stoichiometric Amounts of FeIIIPhen with and without Air
a
yield (%)
reaction
conversion of 1 (1%)
2
3
5
6
4
aira no airb
99 ( 1 20 ( 1
31 ( 1 12 ( 1
37 ( 2 12 ( 1
17 ( 1 2.4 ( 0.5
7.1 ( 0.2
3(1
mass balance (%) 60 ( 1 94 ( 1
28 mol % FeIIIPhen, [1] ) 0.051 M, reaction time, 30 h. b 17 mol % FeIIIPhen, [1] ) 0.048 M, reaction time, 22 h.
out with air flow but without the addition of FeIIIPhen, benzyl phenyl ether was recovered unreacted.30 The product distributions found in this oxidation are somewhat different than those previously observed. There is significantly more rearranged product and less bond cleavage. This result is not surprising. Several of the secondary reactions depend on the FeIIIPhen concentration, and this will be quite different in the air reactions. Oxidation of intermediates by O2 may also be occurring. We did not follow up on this observation because of the many unattractive features of radical cation chemistry for coal depolymerization. The most reasonable explanation for these results is the reoxidation of Fe(II) by oxygen in the air to form a catalytic cycle. Radical cations are formed by oxidation with Fe(III) and the Fe(II) thereby produced is reoxidized to Fe(III) by air. Radical cation chemistry is not attractive for use in coal depolymerization processes. It has an attractive feature: low-temperature bond cleavage reactions. In addition to bond cleavage, there are condensation reactions to give higher molecular weight species and rearrangement reactions which result in bonds which are more stable than those in the original benzyl phenyl ether. As this chemistry is moved from dilute solution in acetonitrile to the rigid environment and high concentrations found in native coals, both dimerization and rearrangement reactions should be enhanced making the chemistry even less attractive. Experimental Section All compounds were commercially available, except for FeIIIPhen, 4,4′-dimethoxybibenzyl, and benzyl phenyl ether (synthesis below). Phenanthrene (Baker) was recrystallized from ethanol, 4-benzylphenol (Aldrich) was recrystallized from n-hexane, and bibenzyl (Baker) was recrystallized from ethanol. 4,4′-Dimethoxybibenzyl, bibenzyl, benzyl phenyl ether, and FeIIIPhen were dried to constant weight in vacuum (0.4 Torr) over P2O5 prior to use. Ultraviolet spectra were obtained on a Perkin-Elmer Lambda 5 UV/vis spectrophotometer between 200 and 900 nm with a slit width of 2 nm using 1 cm quartz cells. The solvent for both reference and sample was anhydrous acetonitrile. NMR spectra were obtained on a JEOL FX90Q FT NMR spectrometer. Plasma desorption mass spectra were obtained on a BIO ION 20. Benzyl Phenyl Ether (1). Benzyl phenyl ether was synthesized by a modification of a procedure by Pirkle.31 In an oven-dried, nitrogen-purged, three-neck 50 mL flask fitted with a condenser and equipped with a magnetic stir bar was placed 5.29 g (38.3 mmol) of anhydrous K2CO3, 2.7 mL (30 mmol) of phenol, and 35 mL of dried acetone (dried by shaking over CaSO4 and distilling under nitrogen31). The resulting mixture was stirred for 2 h prior to the addition of 3.8 mL (34 mmol) of benzyl chloride. The mixture was then heated to reflux for 3 days. The reaction solution was then filtered (30) Larsen, J. W.; Kushal, P., unpublished results. (31) Pirckle, W. H.; Schreiner, J. L. J. Org. Chem. 1981, 46, 49884991. (32) Perrin, D. D.; Armarego, W. L. F. Purification of Organic Compounds, 3rd ed.; Pergamon Press: New York, 1988.
Table 7. Effect of Workup on Measured Product Distributions (mol %) compd
samplea 1 (%)
samplea 2 (%)
control (%)
2 1 3 4 5
100 99 99 99 106
99 101 98 100 108
98 99 100 100 108
a
Subjected to workup.
repeatedly washing the solid with acetone. The filtrate was then rotary evaporated to produce a yellow oil product Rsv ) 0.58 on Baxter MK6F/F254 TLC plates 1:4 CH2Cl2:petroleum ether bp 20-40 °C. Flash chromatography of the oil with grade 60 (230-400 mesh) silica gel using the same solvent system given for TLC produced a white crystalline powder which was recrystallized from ethanol twice. 1H NMR 6.87.5 (m, 10 H), 5.0 (s, 2 H). Mp ) 39 °C, lit. 39 °C. Iron(III) (1,10-Phenanthroline)3(perchlorate)3, FeIIIPhen. Iron(III) (1,10-phenanthroline)3(perchlorate)3 was prepared by a literature method.33 4,4′-Dimethoxybibenzyl (7). 7 was synthesized using a procedure by Trahanovsky.5 Reactions of Benzyl Phenyl Ether with FeIII(1,10phenanthroline)3(ClO4)3. The following is a representative procedure. An oven-dried two-neck 25 mL reaction flask containing a magnetic stir bar was purged with dry N2 while hot, capped, and allowed to cool in a desiccator. After cooling, the flask was weighed and moved into a glovebox (N2 atmosphere). Dry 1 was placed into the flask and the reaction vessel was removed from the glovebox and reweighed. It contained 0.0534 g (2.90 × 10-4 mol) of 1. It was then equipped with an oven-dried condenser and placed under a N2 purge. An acetonitrile solution of FeIIIPhen was prepared in an oven-dried crimp top vial with 0.776 g of dry FeIIIPhen and 10.558 of anhydrous CH3CN (Aldrich). A 4.28 g sample of this solution was then injected into the reaction vessel with an oven-dried syringe and the reaction vessel was immediately lowered into an oil bath at 100 °C. The amount of FeIIIPhen added was 0.293 g (3.28 × 10-4 mol), a 13% molar excess. After 2 h, the reaction flask was removed from the oil bath and 15 drops of saturated aqueous NaHCO3 were added. The resulting solution was allowed to stir for 15 min prior to the addition of MgSO4. Prior to gas chromatography analysis, 0.0619 g of phenanthrene, the internal standard, was added to the reaction mixture and the solution was stirred. Authentic samples of the reaction products were weighed into acetonitrile, carried through the workup procedure, and analyzed by gas chromatography in duplicate. These results are compared in Table 7 with the analysis of the same solution before workup. The procedures used give accurate product distributions. Chromatography: Benzyl Phenyl Ether Reactions. Gas chromatography was carried out using a (15 m × 0.25 mm i.d.) J&W DB-FFAP column in a Hewlett-Packard 5880A gas chromatograph equipped with a FID detector. The carrier gas was He at a flow rate of 0.5 cm3/min and the split ratio was 115:1. The column flow rate was determined by a methane injection at 180 °C. The analysis was carried out using the following program and an injection size of 0.3 µL: 180 °C for 4.5 min, a 10 °C/min temperature increase to 240 (33) Ford-Smith, M. H.; Sutin, N. J. Am. Chem. Soc. 1961, 83, 18301834.
Potential of Radical Cation Chemistry for Coal Conversion °C, and 9 min at 240 °C with the injector temperature at 300 °C and the detector at 250 °C. The above program and conditions gave the following retention times in minutes: phenol 3.66, benzyl phenyl ether 6.89, benzylacetamide 9.52, phenanthrene 11.33, 2-benzylphenol 14.12, 4-benzylphenol 18.01. All results were response factor corrected, using phenanthrene as an internal standard, by response factors determined at the time of the analysis. At least two response factor determinations were made for each analysis performed. Reactions: Dimethoxybibenzyl (7) with FeIIIPhen. The flask, prepared as described for benzyl phenyl ether reaction, was then charged with 0.5 mL of sulfolane/5% 3-methylsulfolane. A solution containing 0.237 g (0.264 mmol) of FeIIIPhen in 4.5 mL of sulfolane/5% 3-methylsulfolane was then completely transferred to the reaction flask via a transfer needle and the flask was immediately lowered into an oil bath at 86 °C and stirring was started. After 2 h the reaction solution was removed from the oil bath. After cooling, 7 drops of saturated aqueous NaHCO3 was added and the solution was allowed to stir for 0.5 h; 0.0516 g of phenanthrene (the internal standard) was added to the solution and the solution was diluted with 15 mL of CH3CN. A 1 mL portion of this solution was removed, diluted with 5 mL of CH3CN, dried over MgSO4, and used in the GC analysis. Chromatography: 4,4′-Dimethoxybibenzyl (7) Reactions. Gas chromatography was carried out using a (10 m × 0.53 mm i.d.) Supelco Nukol column in a Hewlett-Packard 5890 Series II gas chromatograph equipped with an FID detector and on column injector port. Details are provided as supporting information (see paragraph at the end of the text).
Energy & Fuels, Vol. 10, No. 4, 1996 969 Reactions: Bibenzyl and Neopentylbenzene with FeIIIPhen. The procedure developed for benzyl phenyl ether was used. Prior to gas chromatography analysis, 0.0497 g of phenanthrene, the internal standard, was added to the reaction mixture and the solution was stirred. One milliliter of this solution was drawn off and diluted with 16 mL of acetonitrile for GC analysis. Reactions with neopentylbenzene were carried out in the same manner except the neopentylbenzene was transferred to the reaction vessel in an acetonitrile solution. Chromatography: Bibenzyl and Neopentylbenzene. Gas chromatography was carried out using a (10 m × 0.53 mm i.d.) Supelco Nukol column in a Hewlett-Packard 5890 Series II gas chromatograph equipped with an FID detector and on column injector port. Details are provided as supporting information.
Acknowledgment. We are grateful to the United States Department of Energy for partial support of this work. Parveen Kushal carried out preliminary experiments on the FeIIIPhen system. Supporting Information Available: Details of analyses for bibenzyl, neopentylbenzene, and 4,4′-dimethoxybibenzyl (3 pages). Ordering information is given on any current masthead page. EF950209B
