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The Oxidation of Adamantane in Trifluoroacetic Acid
H.E. Curry-Hyde and R.F. Howe (Editors), Natural Gas Conversion I1 0 1994 Elsevier Science B.V. All rights reserved.
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The Oxidation of Adamantane in Trifluoroacetic Acid J.K. Beattie, S. Kacanic, S.J.MacLeman and A.F. Masters Department of Inorganic Chemistry, University of Sydney, NSW, 2006
1. INTRODUCTION Adamantane has been used as a model for saturated alkanes in C-H bond activation.1-6 Adamantane has four tertiary and twelve secondary hydrogen atoms. If all the hydrogens had the same reactivity in the oxidation of adamantane, this would lead to secondary and tertiary substitution products in a 3: 1 ratio. In radical reactions involving adamantane about 5 - 15% of the products are secondary substituted, the rest being tertiary substituted products.5 Ionic processes lead to an even higher percent of tertiary product^.^ In adamantane, the tertiary substituted products are 1-adamantyl derivatives and the secondary substituted products are 2adamantyl derivatives. An organoiron cluster reported by Barton et a13-6typically converts adamantane to 1-adamantanol(O.8%), 2-adamantanol(l.25%) and 2-adamantanone (17.2%). The iron cluster compound is formed in situ by the reaction of iron powder, pyridine-water, acetic acid, hydrogen sulfide, zinc dust and oxygen. In contrast, Barton et a13 report the use of iron acetate in acetic acid to afford 27.5% conversion of adamantane primarily to I-adamantanol. A variety of metals has been reported to oxidise adamantane in trifluoroacetic acid (TFA). Jones and Mellor8 reported the oxidation of adamantane by lead tetraacetate in TFAdichloromethane. Dichloromethane is used as a cosolvent to keep the adamantane in solution. Lead tetrakistrifluoroacetateis formed in situ and acts as the oxidant in these reactions. I-Adamantyl trifluoroacetate is the only product (94% yield) reported in these reactions. Hence the regioselectivity is 100% with respect to the tertiary position. Sen et a11v2 reported the oxidation of arenes, methane and adamantane in TFA by palladium(I1) acetate. Heating equimolar amounts of palladium(I1) acetate and adamantane in TFA at 80 "C for one hour results in the exclusive formation of 1-adamantyl trifluoroacetate in greater than 50% yield. By using palladium(I1) trifluoroacetate in acetonitrile in the presence of a few equivalents of TFA and irradiating with uv light, adamantane is converted to 1-adamantyl acetarnide (ca. 70%) and 2-adamantyl acetamide (ca. 30%) quantitatively.6 If the amount of palladium(I1) trifluoroacetate is decreased and copper trifluoroacetate is introduced as a cooxidant, small amounts of 1- and 2-adamantyl trifluoroacetates are also detected.
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2. OXIDATION WITH PALLADIUM COMPLEXES In the present work, adamantane has been reacted with palladium(I1) acetate in TFA at 80 "C. After six hours only about 10-15%of the adamantane has reacted to form 1-adamantyl trifluoroacetate. These results conflict with those of Sen who reported over 50% conversion in one hour. In a recent paper, Moiseev et a19 also described their failure to reproduce Sen's results. Sen reported that the fresher the palladium(I1) acetate sample, the higher the yields. We have observed variation in the properties of different preparations (IR, XRD, colour), but have not yet observed any significant effect on the reaction rate or yield. To overcome the uncertainty of the results from the palladium(I1) acetate samples with different histories, we have used bipyridylbisacetato palladium(I1). This compound has been characterised by Wilkinson. I 1 It is easier to recrystallise cleanly than is palladium(I1) acetate. Adamantane is also oxidised exclusively to I-adamantyl trifluoroacetate by bipyridylbisacetato palladium(I1) in refluxing TFA. The rate, product selectivity and yields are not appreciably different to those in the palladium(I1) acetate system. However, the bipyridylbisacetato palladium(I1) reaction is a more reproducible system than that of palladium(I1) acetate.
3. OXIDATION WITH COBALT(II1) TRIFLUOROACETATE The partial oxidation of methane by cobalt(II1) trifluoroacetate has been reported by Moiseev and coworkers.9 Cobalt(II1) trifluoroacetate has been used in the present work to oxidise adamantane to the trifluoroacetate ester. In this case, there is also a competing reaction in which the cobalt species is consumed without the concomitant oxidation of the substrate. This is accompanied by a change in the colour of the reaction mixture from dark green to red. This competing reaction can be minimised by lowering the temperature, but at the expense of longer reaction times.
4. OXIDATION WITH NON-METALLIC SPECIES Hydrogen peroxide has been used with palladium(I1) acetate as a catalytic co-oxidant by Sen2 in the oxidation of methane in TFA. In the present work hydrogen peroxide has been used to oxidise adamantane directly. When equimolar amounts of adamantane and hydrogen peroxide were refluxed in trifluoroacetic anhydride, over 90% conversion to 1-adamantan01 occurred. Nitric acid has also been used to oxidise adamantane. In these reactions equimolar quantities of nitric acid and adamantane were refluxed in TFA until evolution of brown fumes ceased. By gc analysis, all the adamantane had been consumed and had reacted to give exclusively 1adamantanol.
5. EXPERIMENTAL Palladium(I1) acetate and bipyridylbisacetato palladium(I1) was prepared by the method of Wilkinson!O Cobalt(II1) trifluoroacetate was prepared by the method of Tang.' 1
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Adamantane (Aldrich), trifluoroacetic acid (Aldrich) and decalin (Merck) were used as received. In a typical oxidation reaction, the oxidant (1.10 mmol)(palladium acetate, palladium bipyridylbisacetate, cobalt(II1) trifluoroacetate, hydrogen peroxide or nitric acid) and adamantane (1.10 mmol) were refluxed in trifluoroacetic acid (20 mL) for a specified period of time. The reaction mixture was quenched with water (20 mL), adjusted to ca. pH 8 with sodium carbonate solution (ca. 100 mL, 20% w/v), extracted with ether and the extracts dried (anh. MgS04). The organic material was then weighed and extracted into a heptane/decalin solution for gc analysis. Decalin was used as an internal standard for gc.
5. CONCLUSIONS A high selectivity for oxidation at the tertiary carbon has been demonstrated in most, if not all, of the above reactions with TFA as the solvent. In the oxidation of adamantane in 100% TFA, oxidation to the I-adamantyl derivative exclusively is observed. When the TFA is diluted with for example dichloromethane or acetonitrile, there is some oxidation to give a 2adamantyl derivative, but the 1-adamantyl derivative is still the major product. It is likely that the trifluoroacetate substituent substantially deactivates the hydrocarbon, so only one product is obtained in reactions where the solvent is 100% TFA. Whether or not this selectivity is due only to solvent effects needs to be investigated.
In the above conversions of adamantane, it is clear that the purity of the oxidant contributes to the extent of the reaction. In the case of palladium(I1) acetate, different batches (whether bought or synthesised) may have different reactivities. On the other hand, bipyridylbisacetato palladium(I1) can be prepared as clean yellow crystals with different batches having the same properties. Hence the problem of oxidant purity effecting the extent of the reaction can be overcome. 6. REFERENCES 1. E. Gretz, T.F. Oliver and A. Sen, J. Am. Chem. Soc., 109, (1987), 8109-1 1. 2. L.-C. Kao, A.C. Hutson and A. Sen, J. Am. Chem. SOC.,113, (1991), 700-1. 3. D.H.R. Barton, M.J. Gastiger and W.B. Motherwell, J. Chem. SOC., Chem. Commun., (1983), 731-3. 4. G. Balavoine, D.H.R. Barton, J. Boivin, P. Lecoupanec and P. Lelandais, New J. Chem., 13, (1989), 691-700. 5. D.H.R. Barton and D. Doller, Acc. Chem. Res., 25, (1992), 504-12. 6. J. Muzart and F. Henin, C.R. Acad. Sci., 307, (2), (1988), 479-82. 7. R.C. Bingham and P. von R. Schleyer, Fortschr. Chem. Forsch., 18, (1971), 1. 8. S.R. Jones and J.M. Mellor, J.C.S. Perkin I, (1976), 2576-81. 9. I.P. Stolarov, M.N. Vargaftik, D.I. Shishkin and 1.1. Moiseev, J. Chem. SOC., Chem. Commun., (1991), 938-9. 10. T.A. Stephenson, S.M. Morehouse. A.R. Powell, J.P. Heffer and G. Wilkinson, J. Chem. SOC.,(1965), 3632-40. 11. R. Tang and J.K. Kochi, J. Inorg. Nucl. Chem, 35, (1973), 3845.
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The Oxidation of Adamantane in Trifluoroacetic Acid J.K. Beattie, S. Kacanic, S.J.MacLeman and A.F. Masters Department of Inorganic Chemistry, University of Sydney, NSW, 2006
1. INTRODUCTION Adamantane has been used as a model for saturated alkanes in C-H bond activation.1-6 Adamantane has four tertiary and twelve secondary hydrogen atoms. If all the hydrogens had the same reactivity in the oxidation of adamantane, this would lead to secondary and tertiary substitution products in a 3: 1 ratio. In radical reactions involving adamantane about 5 - 15% of the products are secondary substituted, the rest being tertiary substituted products.5 Ionic processes lead to an even higher percent of tertiary product^.^ In adamantane, the tertiary substituted products are 1-adamantyl derivatives and the secondary substituted products are 2adamantyl derivatives. An organoiron cluster reported by Barton et a13-6typically converts adamantane to 1-adamantanol(O.8%), 2-adamantanol(l.25%) and 2-adamantanone (17.2%). The iron cluster compound is formed in situ by the reaction of iron powder, pyridine-water, acetic acid, hydrogen sulfide, zinc dust and oxygen. In contrast, Barton et a13 report the use of iron acetate in acetic acid to afford 27.5% conversion of adamantane primarily to I-adamantanol. A variety of metals has been reported to oxidise adamantane in trifluoroacetic acid (TFA). Jones and Mellor8 reported the oxidation of adamantane by lead tetraacetate in TFAdichloromethane. Dichloromethane is used as a cosolvent to keep the adamantane in solution. Lead tetrakistrifluoroacetateis formed in situ and acts as the oxidant in these reactions. I-Adamantyl trifluoroacetate is the only product (94% yield) reported in these reactions. Hence the regioselectivity is 100% with respect to the tertiary position. Sen et a11v2 reported the oxidation of arenes, methane and adamantane in TFA by palladium(I1) acetate. Heating equimolar amounts of palladium(I1) acetate and adamantane in TFA at 80 "C for one hour results in the exclusive formation of 1-adamantyl trifluoroacetate in greater than 50% yield. By using palladium(I1) trifluoroacetate in acetonitrile in the presence of a few equivalents of TFA and irradiating with uv light, adamantane is converted to 1-adamantyl acetarnide (ca. 70%) and 2-adamantyl acetamide (ca. 30%) quantitatively.6 If the amount of palladium(I1) trifluoroacetate is decreased and copper trifluoroacetate is introduced as a cooxidant, small amounts of 1- and 2-adamantyl trifluoroacetates are also detected.
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2. OXIDATION WITH PALLADIUM COMPLEXES In the present work, adamantane has been reacted with palladium(I1) acetate in TFA at 80 "C. After six hours only about 10-15%of the adamantane has reacted to form 1-adamantyl trifluoroacetate. These results conflict with those of Sen who reported over 50% conversion in one hour. In a recent paper, Moiseev et a19 also described their failure to reproduce Sen's results. Sen reported that the fresher the palladium(I1) acetate sample, the higher the yields. We have observed variation in the properties of different preparations (IR, XRD, colour), but have not yet observed any significant effect on the reaction rate or yield. To overcome the uncertainty of the results from the palladium(I1) acetate samples with different histories, we have used bipyridylbisacetato palladium(I1). This compound has been characterised by Wilkinson. I 1 It is easier to recrystallise cleanly than is palladium(I1) acetate. Adamantane is also oxidised exclusively to I-adamantyl trifluoroacetate by bipyridylbisacetato palladium(I1) in refluxing TFA. The rate, product selectivity and yields are not appreciably different to those in the palladium(I1) acetate system. However, the bipyridylbisacetato palladium(I1) reaction is a more reproducible system than that of palladium(I1) acetate.
3. OXIDATION WITH COBALT(II1) TRIFLUOROACETATE The partial oxidation of methane by cobalt(II1) trifluoroacetate has been reported by Moiseev and coworkers.9 Cobalt(II1) trifluoroacetate has been used in the present work to oxidise adamantane to the trifluoroacetate ester. In this case, there is also a competing reaction in which the cobalt species is consumed without the concomitant oxidation of the substrate. This is accompanied by a change in the colour of the reaction mixture from dark green to red. This competing reaction can be minimised by lowering the temperature, but at the expense of longer reaction times.
4. OXIDATION WITH NON-METALLIC SPECIES Hydrogen peroxide has been used with palladium(I1) acetate as a catalytic co-oxidant by Sen2 in the oxidation of methane in TFA. In the present work hydrogen peroxide has been used to oxidise adamantane directly. When equimolar amounts of adamantane and hydrogen peroxide were refluxed in trifluoroacetic anhydride, over 90% conversion to 1-adamantan01 occurred. Nitric acid has also been used to oxidise adamantane. In these reactions equimolar quantities of nitric acid and adamantane were refluxed in TFA until evolution of brown fumes ceased. By gc analysis, all the adamantane had been consumed and had reacted to give exclusively 1adamantanol.
5. EXPERIMENTAL Palladium(I1) acetate and bipyridylbisacetato palladium(I1) was prepared by the method of Wilkinson!O Cobalt(II1) trifluoroacetate was prepared by the method of Tang.' 1
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Adamantane (Aldrich), trifluoroacetic acid (Aldrich) and decalin (Merck) were used as received. In a typical oxidation reaction, the oxidant (1.10 mmol)(palladium acetate, palladium bipyridylbisacetate, cobalt(II1) trifluoroacetate, hydrogen peroxide or nitric acid) and adamantane (1.10 mmol) were refluxed in trifluoroacetic acid (20 mL) for a specified period of time. The reaction mixture was quenched with water (20 mL), adjusted to ca. pH 8 with sodium carbonate solution (ca. 100 mL, 20% w/v), extracted with ether and the extracts dried (anh. MgS04). The organic material was then weighed and extracted into a heptane/decalin solution for gc analysis. Decalin was used as an internal standard for gc.
5. CONCLUSIONS A high selectivity for oxidation at the tertiary carbon has been demonstrated in most, if not all, of the above reactions with TFA as the solvent. In the oxidation of adamantane in 100% TFA, oxidation to the I-adamantyl derivative exclusively is observed. When the TFA is diluted with for example dichloromethane or acetonitrile, there is some oxidation to give a 2adamantyl derivative, but the 1-adamantyl derivative is still the major product. It is likely that the trifluoroacetate substituent substantially deactivates the hydrocarbon, so only one product is obtained in reactions where the solvent is 100% TFA. Whether or not this selectivity is due only to solvent effects needs to be investigated.
In the above conversions of adamantane, it is clear that the purity of the oxidant contributes to the extent of the reaction. In the case of palladium(I1) acetate, different batches (whether bought or synthesised) may have different reactivities. On the other hand, bipyridylbisacetato palladium(I1) can be prepared as clean yellow crystals with different batches having the same properties. Hence the problem of oxidant purity effecting the extent of the reaction can be overcome. 6. REFERENCES 1. E. Gretz, T.F. Oliver and A. Sen, J. Am. Chem. Soc., 109, (1987), 8109-1 1. 2. L.-C. Kao, A.C. Hutson and A. Sen, J. Am. Chem. SOC.,113, (1991), 700-1. 3. D.H.R. Barton, M.J. Gastiger and W.B. Motherwell, J. Chem. SOC., Chem. Commun., (1983), 731-3. 4. G. Balavoine, D.H.R. Barton, J. Boivin, P. Lecoupanec and P. Lelandais, New J. Chem., 13, (1989), 691-700. 5. D.H.R. Barton and D. Doller, Acc. Chem. Res., 25, (1992), 504-12. 6. J. Muzart and F. Henin, C.R. Acad. Sci., 307, (2), (1988), 479-82. 7. R.C. Bingham and P. von R. Schleyer, Fortschr. Chem. Forsch., 18, (1971), 1. 8. S.R. Jones and J.M. Mellor, J.C.S. Perkin I, (1976), 2576-81. 9. I.P. Stolarov, M.N. Vargaftik, D.I. Shishkin and 1.1. Moiseev, J. Chem. SOC., Chem. Commun., (1991), 938-9. 10. T.A. Stephenson, S.M. Morehouse. A.R. Powell, J.P. Heffer and G. Wilkinson, J. Chem. SOC.,(1965), 3632-40. 11. R. Tang and J.K. Kochi, J. Inorg. Nucl. Chem, 35, (1973), 3845.