Useful Mimics for Phenol Oxidants in Catalytic Delignification?

Chapter 17

Metal-Schiff Base Complexes: Useful Mimics for Phenol Oxidants in Catalytic Delignification? Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch017

Jussi Sipilä, Anssi Haikarainen, Pekka Pietikäinen, Gösta Brunow, Timo Repo, Juha Anturaniemi, and Markku Leskelä University of Helsinki, Department of Chemistry, Ρ.Ο. Box 55, 00014 Helsinki, Finland

The catalytic effect of metal-Schiff base complexes in phenol oxidation reactions has been investigated. In the first experimental set the effects of the metal and the structure of the ligand on the catalytic oxidation reactions were studied using salen type complexes and following the oxidation of coniferyl and sinapyl alcohols. The most promising complexes were then examined with monomeric and dimeric lignin models in systems relevant to practical bleaching conditions. The results indicate that Schiff-bases catalyze the oxidation of lignin model compounds either by one- or two-electron oxidation mechanism and that the mechanism is dependent on the bulk of the substrate. Sterically bulky β-ether models reacted by formation of biphenyls whereas monomeric models with benzyl alcohol structures gave products with benzylic carbonyl groups. Only small amounts of quinoid structures were found among the reaction products.

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© 2001 American Chemical Society In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch017

Introduction In recent years increasing environmental concerns have forced industry to develop "green" technologies in their production processes. In the pulp and paper industry most concerns have been focused on bleaching where alternative bleaching agents are required to replace chlorine containing reagents (/). Today several techniques in this field are already a reality. Oxygen, ozone and hydrogen peroxide used in sequences, provide commercially available bleaching technologies (2) and xylanases have been used successfully to enhance lignin removal from pulps (3). The use of oxidation catalysts such as laccase with mediators (4), polyoxometallates (5) and binuclear Mn-complexes (6) have shown to be potentially available technologies for chlorinefree-bleaching. Laccase, polyoxometallate and binuclear Mn-complex approaches in bleaching are based on the idea of using "biomimetic" procedures for residual lignin degradation. However, although the published material relating to these procedures indisputably show their positive effects on bleaching parameters, little is known so far on their detailed action in lignin removal from pulp. In the present study we have focused particularly on the chemical aspects involved in "biomimetic" approaches in bleaching. For this purpose we have mimicked the oxidation capacity of different enzymes known to be relevant in lignin biodégradation (7) by synthetically obtained oxidation catalysts, so called metalsalen compounds of type 1 and 2. The salen compounds are structurally simple and easy to handle allowing the use of conventional practical organic chemistry methods for the studies.

1

2

a) R ! = R = H b) R i = H , R =S0 Na e ) R i = i - B u , R = CH2PPh CI 2

2

3

2

3

M = Mn(lll), Fe (III), Co(ll), Cu(ll)

In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

288 •OH

•OH

OH

OH


3

4

Results and Discussion Dehydrogenase polymerization of coniferyl and sinapyl alcohols catalyzed by metal-salen complexes The salen complexes 1 and 2 were prepared according to literature procedures (8) with slight modifications and the metal center was introduced using standard methods (9). In a typical oxidation experiment 3 (coniferyl alcohol) or 4 (sinapyl alcohol) was dissolved in 2:1 water-dioxane solution buffered to pH 3. Low pH was used to mimic the activity maximum of lignin peroxidase (10). The salen complex (Co, Mn or Fe, 0.05 mol. equiv.) and H 0 (1 mol. equiv., in three portions) were then added to this solution and the reaction followed by TLC until the disappearance of the substrate. In experiments where the oxidant was oxygen (Co and Cu), the reaction solution was stirred either in an open flask or with slight overpressure of dioxygen using a balloon. In some instances imidazole was used to mimic the axial ligand of enzymes (10). The DHP (synthetic lignin) formed in the oxidation was separated from the reaction solution by extraction with ethyl acetate, acetylated and analyzed by NMR. The DHPs were compared to those produced by HRP (horseradish peroxidase)/H 0 oxidation system. The results obtained in experiments with 3 are presented in Table 1. The results indicate that, in general, metal salen compounds show substantial phenol oxidative capacity under conditions assumed to prevail during lignin bioconversions. The order of reactivity seems to be roughly Fe ~ Co > Mn » Cu. With copper, coniferyl alcohol was oxidized only in the case of bulky 1c. The long reaction time with Culc is in accordance with the findings that copper containing enzymes, like laccase, has been reported to take several days and to stop at the dimeric stage (11). The effect of imidazole, a mimic for internal ligands in enzymes, is also pronounced: when added to the reaction mixture it significantly accelerates the 2

2

2


2

Àr

β-β


β-5

β-Ο-4

Table 1. Dehydrogenative polymerization of coniferyl alcohol (CA) in aqueous dioxane at pH 3 (oxidant H 0 ) 2

Catalysts Mnla Mnlb Mnlc Mn2 Felc Cola Culc Mnla + imid. Fela + imid. Cola + imid. Cula + imid. HRP

β-5:β-β:β-0-4 2:3:2 2:3:1 3:2:1 2:2:1 2:2:1 3:3:1 1:1:1 3:3:1 3:3:1

2:2:3

2

Time for CA disappearance 18 h 10 h 35 min lh 2h 35 min 3 days 1.5 h 15 min no reaction no reaction lh

oxidation. In general, oxidations with salen catalysts seem to take place faster than with HRP. The results further indicate that increasing the bulk in the ligand results in faster oxidations. One explanation might be that bulky substituents increase the catalytic effect by preventing the ligand from settling into a planar conformation. In the case of Culc we have been able to show the twisted conformation by X-ray analysis (Figure 1) (12). An increase in the bulk of the ligand in the case of Mn has only a slight effect on the product distribution. This was also the case with other metals. In the case of Cola, changing the oxidant from hydrogen peroxide to oxygen did not change the product distribution.



290

Figure 1. The β-5 dimer formed upon oxidation of 3 with the chiral complex 2 was investigated by chiral HPLC and found to be racemic. The lack of regiospecifity and enantioselectivity in the coupling reactions indicate a random radical coupling mechanism in salen catalyzed dehydrogenation reactions in aqueous solutions. In experiments with sinapyl alcohol we were particularly interested in finding an explanation why sinapyl alcohol produces β-Ο-4 -rich DHP when oxidized by FeCl but gives predominantly β-β dimers in hydrogen peroxide/HRP systems (7 J). One explanation for this phenomenon might be that in FeCl oxidation the HC1 liberated catalyzes the addition of water to β-Ο-4type quinone methides and forces the system to produce β-Ο-4 type polymer (13). Indeed, at lower pH the polymerization of 4 using Mnl a/hydrogen peroxide oxidation system gave larger amounts of β-Ο-4 type structures than the experiments with HRP/hydrogen peroxide system at pH 3. 3

3

Oxidation of monomeric lignin model compounds catalyzed by Mn- and Cocomplexes

The experiments described above indicate that Fe, Mn and Co all form salen-type complexes capable of catalysing the oxidation of phenolic materials. We decided to concentrate on Mn and Co as some results have suggested that hydrogen peroxide oxidations with Fe cause unwanted side reactions, such as production of hydroxyl radicals. To examine our catalysts under bleaching conditions, we first studied the activity of Co-catalysts in the oxidation of benzyl alcohols 5 (vanillyl alcohol) and 7 (veratryl alcohol) with dioxygen in aqueous solution at pH 10 at two temperatures, 80 and 100 °C. In addition to Co-salen Cola and Cosulphosalen Colb we also examined the reactivities of Co-acacen 9, Co-amethylsalen 10, Co-4-hydroxysalen 11. Conversions to corresponding



291

5R = H , R = O H , R = = H L

2

3

6R = M e , R = O H , R = H 1

2

3

7R = H , R = O M e , R = H 1

2

3

8R = Me, R = OMe, R = H 1

2

3


292


aldehydes (the only products found apart from a small amount of polymeric material) were followed by *H-NMR (Table 2). The results were then compared to ambient temperature oxidations using Mnlb with hydrogen peroxide and Cola with dioxygen. These were performed in 1:1 methanol-water solvent systems, at pH 10 (Table 3).

Table 2. The oxidation of (5) and (7) oxidised by dioxygen and catalysed by Co-complexes at pH 10 Catalyst Substrate 100°C (% Conv.) 80°C (% Conv.) la 5 91 83 lb 86 5 38 95 9 5 27 10 88 5 6 11 60 5 53 la 7 71 25 43 lb 7 22 9 28 7 21 10 25 7 28 11 71 7 4

The unsubstituted Co-salen l a seems to work best, giving the highest conversions and practically showing no temperature effect between 80 °C and 100 °C with the substrate 5 (Table 2). The result is somewhat surprising as one might expect catalyst l b , with electron attracting sulphonyl groups, to exhibit enhanced catalytic properties compared to l a (and also to 11) (14). The results also show that the change in temperature has a significant effect only in the case of vanillyl alcohol 5; the conversion of non-phenolic veratryl alcohol 7 to aldehyde remained practically at the same level at both temperatures (80 °C and 100 °C). Most probably this phenomenon is due to the presence/absence of a free phenolic group in the substrate. In the experiments with Mn-hydrogen peroxide system (Table 3), the conversions of structurally most simple benzyl alcohols 5 and 7 are about the same level as in oxidations with Co-catalysts at 80 °C (Table 2). For some reason, compound 6 was completely unreactive under the reaction conditions. Also, the Co-catalyst Cola seems to lose most of its activity at the lower temperature.


293


Table 3. Oxidation of "monomelic" models 5 - 8 by Mn-sulfosalen lb and Co-salen la at ambient temperature at pH 10. Catalyst Substrate Conversion (%) Mnlb 5 70 Mnlb 0 6 Mnlb 65 7 Mnlb 50 8 Cola 10 5

Oxidation of dimeric lignin model compounds catalyzed by Mn- and Cocomplexes

The experiments with dimeric lignin models were performed in 1:1 water-methanol solution (pH 10). The molar ratio of substrate to catalysts was 1:0.05. In studies with Mnlb the oxidant was hydrogen peroxide and the reactions were performed at room temperature. For Cola and Colb, the oxidant was dioxygen and the reactions were performed under refluxing conditions, with a slight overpressure of dioxygen. The products were identified by NMR.

Oxidation of dimer 12 with Mnla yielded biphenyl 16, a radical coupling product, as the sole product. The oxidation of 12 with Colb gave back starting material. With Cola, H NMR showed weak signals around 11 ppm and 9 ppm suggesting minor degradation of the substrate. Dimer 13 turned out to be totally l


294


unreactive in the reaction systems investigated. Even the addition of pyridine as an axial ligand (75) in experiments with Cola and l b had no effect suggesting the total inertness of non-phenolic dimers in the oxidation system.

To elucidate the reactivity of chromophoric structures in residual lignins we then studied the reactivity of (£)-4,4'-dihydroxy-3,3'dimethoxystilbene (14). The manganese complex l a quantitatively oxidised 14 to diol 15. When the oxidation was performed with Cola or Colb, a considerable amount of vanillin was formed as indicated by a singlet at 9.78 ppm in the H NMR spectrum of the oxidation mixture. !

Conclusions

The results of this study demonstrate that both Mn- and Co-salen catalysts have clear capacity to catalyse phenol oxidation under conditions applicable to bleaching processes. The results indicate that in aqueous solvent Mn catalysts with hydrogen peroxide are able to oxidize lignin models either to form carbonyl compounds or radical coupling products. With structurally simple lignin models Mn-salens catalyze the oxidation of the benzylic position to a carbonyl (a 2-electron oxidation) whereas in the case of bulky substrates, oneelectron oxidation predominates and biphenyl structures are formed. Cobalt catalysts, on the other hand, seem to oxidize model compounds to give carbonyl structures. The Co-catalyzed oxidations, however, seem to be restricted to monomeric substrates. The recent findings in the field of bleaching chemistry has established that residual lignins contain substantial amounts of β-ether


295 structures, and that catalysts such as the salen compounds clearly assist removal of residual lignin from the pulps. Our results, however, demonstrate that these catalysts will preferentially polymerize bulky substrates like β-ethers in alkaline solutions. Further studies are therefore required to reveal the relevant mechanisms of catalytic bleaching.

Acknowledgement


This research has been funded partly by the Technical Development Center of Finland (TEKES) for the project "SEKAVA", Sellun katalyyttinen valkaisu.

References

1. Kringstad, K. P.; Lindström, Κ. Environ. Sci. Technol. 1984, 18, 236A.; Hise, R.; Wright, B. T.; Swanson, S. E. Chemosphere 1990, 20, 1723.; Rajan, P. S.; Chen, C. -L.; Gratzl, J. S.; Hise, R. G. Holzforschung 1994, 48, 117. Suppl.; Rajan, P. S.; Chen, C. -L.; Gratzl, J. S. Holzfoschung 1996, 50, 165. 2. Almberg, L.; Jamieson, Α.; Waldestam, S. Pulp & Paper 1980, 54, 92.; Hsu, C. L.; Hsieh, J. S. Tappi J. 1986, 69, 125.; Laxén, T.; Ryynänen, H.; Henricson, K. Paperi ja Puu 1990, 72, 504.; J. L. Colodette, J. L.; Singh, U . P.; Ghosh, A. K.; Singh, R. P. Pulp & Paper 1993, 67, 139. 3. Li, J.; Paice, M . G.; Macleod, J. M.; Jurasek, L. J. Pulp Paper Sci. 1996, 22, J207. 4. Bourbonnais, R.; Paice, M . G. Tappi J. 1996, 79, 199. 5. Weinstock, I. Α.; Atalla, R. H.; Reiner, R. S.; Moen, Μ. Α.; Hammel, Κ. E.; Houtman, C. J.; Hill, C. L. New J. Chem. 1996, 20, 269. 6. Patt, R.; Mielisch, H. -J.; Kordsachia, O.; Schubert, H. -L. International Pulp Bleaching Conference, Helsinki, Finland, June 1-5, Proceedings, Book 1, 1998, p. 111. 7. Hammel, Κ. E.; Jensen Jr., Κ. Α.; Mozuch, M. D.; Landucci, L. L.; Tien, M.; Rease, E. A. J. Biol. Chem. 1993, 268, 12274.; Kishi, K.; Wariishi, H.; Marquez, L.; Dunford, H. B.; Gold, M . H. Biochemistry 1994, 33, 8694.; Paice, M . G.; Bourbonnais, R.; Reid, I.; Archibald, F. S. 9 International Symposium on Wood and Pulping Chemistry (ISWPC), Montreal, 1997, p. PL1-1. 8. See for example Zhang, W.; Jacobsen, Ε. N . J. Org. Chem. 1991, 56, 2296. 9. Mukherjee, A. K.; Rây, P. J. Indian Chem. Soc. 1955, 32, 633. th


296 10. Poulos, T. L.; Edwadrs, S. L.; Wariishi, H; Gold, M . H. J. Biol. Chem. 1993, 268, 4429. 11. Okusa,K.; Miyakoshi, T.; Chen, C. -L. Holzforschung 1996, 50, 15. 12. Haikarainen, Α.; Sipilä, J.; Pietikäinen, P.; Pajunen, Α.; Mutikainen, I. manuscript, in preparation. 13. Sipilä, J.; Brunow, G.; Tunninen, P.; Niemi, T. 9 International Symposium on Wood and Pulping Chemistry (ISWPC), June 2-12, Montreal, Canada, 1997, B2-1. 14. For the electronic and other effects see Palucki, M . ; Finney, N. S.; Pospisil, P. J.; Gueler, M . L.; Ishida, T.; Jacobsen, Ε. N. J. Am. Chem. Soc. 1998, 120, 948. 15. For the effect of pyridine in related oxidations see for example Bozell, J. J.; Hames, B. R.; Dimmel, D. R. J. Org. Chem. 1995, 60, 2398.


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Useful Mimics for Phenol Oxidants in Catalytic Delignification?

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