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Oxidation of cycloalkanes with molecular oxygen in the presence of salen metallocomplexes in thermomorphic conditions
Catalysis Communications 39 (2013) 102–105
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Oxidation of cycloalkanes with molecular oxygen in the presence of salen metallocomplexes in thermomorphic conditions Katarzyna Pamin a, Gianluca Pozzi b, Edyta Tabor c, Wiktor Bukowski d, Jan Połtowicz a,⁎ a
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR, via Camillo Golgi 19, 20133 Milano, Italy J.Heyrovsky Institute of Physical Chemistry, Structure and Dynamics in Catalysis, Dolejškova 2155/3, 18223 Prague 8, Czech Republic d Faculty of Chemistry, Rzeszów University of Technology, al. Powstańców Warszawy 6, 35-959 Rzeszów, Poland b c
a r t i c l e
i n f o
Article history: Received 26 February 2013 Received in revised form 30 April 2013 Accepted 30 April 2013 Available online 22 May 2013 Keywords: Cycloalkanes oxidation Schiff base Fluorinated ligand Thermomorphic effect
a b s t r a c t The oxidation of cycloalkanes with molecular oxygen, catalyzed by two groups of metallosalen complexes, was studied. The first group consisted of salen complexes with different metals such as Mn, Fe, Co, Ni, Cu, Zn, while the second group was composed of manganese salen complexes with different substituents (t-butyl electron-donating substituents and/or electron-withdrawing perfluoroalkyl substituents). Mn, Fe and Co salen complexes are the most active catalysts, while Ni, Cu and Zn salen complexes are far less efficient. The introduction of t-butyl electron-donating substituents into Mnsalen complex increases the catalytic activity and catalysts solubility in the reaction mixture. The introduction of perfluoroalkyl electronwithdrawing substituents enhances the catalytic activity and renders the solubility of the catalyst temperature dependent (thermomorphic behaviour), thus allowing one to recover them easily after the reaction by simply cooling the system to room temperature. The synthesis of two new manganese salen complexes with perfluoroalkyl substituents was elaborated. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxidation of cycloalkanes with molecular oxygen is one of the most important large-scale processes in chemical industry and one of the most attractive transformations in organic synthesis [1–5]. Transition metal complexes of macrocyclic ligands such as porphyrins, phthalocyanines and to a much lesser extent related complexes of chelating [N, N-bis(salicylidene)ethylenediamine] (salen) have been previously investigated as homogeneous catalysts in these processes [6–9]. Although viable conversion and selectivity levels have been achieved by ligand structure optimization, the practical application of these relatively expensive complexes in cycloalkanes oxidation reactions remains challenging mostly because they cannot be efficiently recovered and reused. Various immobilization techniques on solid support have been developed to overcome this problem [10–12], however there is still an obvious need for more convenient, easy to apply methods for the separation of these homogeneous catalysts from reaction mixtures. Two literature reports had previously demonstrated the potential of fluorous catalysis in the aerobic oxidation of cycloalkanes [13,14]. In those examples, metal complexes of macrocyclic nitrogen ligands bearing medium-sized perfluoroalkyl substituents, which ensured preferential solubility in perfluorocarbons (PFC), were used as catalysts. ⁎ Corresponding author. Tel.: +48 126395153; fax: +48 124252943. E-mail address: [email protected] (J. Połtowicz). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.04.026
Reactions were thus performed under PFC-organic biphase conditions, according to the concept introduced by Horváth and Rábai [15,16]. The organic and PFC layers demixed upon cooling at the end of the reaction, and the bottom PFC layer containing the fluorous catalyst was easily separated and efficiently recycled. More recently, the cost of PFC as well as their persistence in the environment has pushed research towards the development of fluorous separation strategies which minimize, or even eliminate the need for these fluorous solvents [17]. The one introduced independently by the groups of Gladysz [18,19] and Yamamoto [20,21] is particularly appealing. It is based on appropriately designed fluorous catalysts that exhibit highly temperature-dependent solubilities in organic solvents, being essentially insoluble at low temperatures while completely soluble at elevated temperatures. This thermomorphic character can be exploited in various ways [22], for instance by adding the solid fluorous catalyst to an organic solution of the reactants that is then warmed to dissolve the catalyst and reach one-phase reaction conditions. Upon completion of the reaction the catalyst is deposited by just cooling the system, and eventually recovered by solid–liquid phase separation. Recently, we described the application of metalloporphyrins with different substituents as catalysts for cycloalkanes oxidation with molecular oxygen. The influence of the different kinds of substituents on the catalytic activity was described in details [5]. In the aim of developing cleaner processes based on handy catalysts we have now investigated the use of salen complexes with different metals like Mn, Fe, Co, Ni, Cu, Zn and of manganese salen
K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
103
30
complexes bearing electron-withdrawing perfluoroalkyl substituents and/or t-butyl electron-donating substituents in the oxidation of cycloalkanes with molecular oxygen (Scheme 1).
25
2. Experimental
ketone alcohol
2.1. Synthesis Manganese, iron, cobalt, nickel, copper and zinc complexes of salen ligand 1 are well-known compounds and they were readily obtained following standard literature methods [23]. Functionalized salen derivatives 2–6 and the corresponding manganese complexes were also synthesized according to procedures described elsewhere [24–28]. Their spectroscopic data acquired from FT-IR, UV–vis and NMR techniques were in agreement with the reported literature data.
Yield, %
20
15
10
5
0
2.2. Catalyst characterization 2.2.1. UV–vis spectroscopy UV–vis measurements were performed on a Perkin Elmer Lambda 35 double beam spectrophotometer, using quartz cells of 1 cm optical path. Electronic spectra of the metallocomplexes and heterogenized metallocomplexes were measured in water solution with the concentration of 2 ⋅ 10−5 mol/L. 2.2.2. Infrared spectroscopy FT-IR spectra were recorded on a Nicolet 800 spectrometer in KBr pellets over the range of 400–4000 cm−1 under the atmospheric conditions. 2.2.3. NMR spectroscopy 1 H NMR (300 MHz), 13C NMR (75.4 MHz) and 19F NMR (282 MHz) spectra were recorded on a Brucker AC 300 spectrometer with tetramethylsilane (δ = 0), CDCl3 (δ = 77) and CFCl3 (δ = 0) as internal standards. 2.3. General procedure for the oxidation of cycloalkanes The oxidation of cyclooctane (cyclopentane or cyclohexane) was performed in a stainless steel batch reactor system at 393 K and under the air pressure of 10 atm, with the substrate to oxygen molar ratio set at 6.5. In a typical experiment, 60 mL of substrate containing the amount of catalyst providing a concentration of 3.3 × 10−4 M was introduced in the deaerated autoclave and the whole system was heated until a temperature of 393 K was reached. Air was then introduced and after 6 h the oxidation products were analyzed by GC Agilent 6890 N equipped with an Innowax (30 m) column. The yield values were verified by addition of an internal standard, chlorobenzene, at the end of the reaction. 3. Results and discussion Three members of homologous series, viz. cyclopentane, cyclohexane or cyclooctane were applied as substrates and studied in oxidation catalyzed by manganese salen complex Mn-1 (Fig. 1). The main
O
metallosalen catalyst O 2(air),120°C,10 atm,6 h 1
OH
+ 2
3
Scheme 1. Oxidation of cyclooctane with molecular oxygen in the presence of metallosalen complexes.
Fig. 1. Oxidation of different cycloalkanes by simple manganese salen complex Mn-1.
products of cycloalkanes oxidation with molecular oxygen were cycloketone and cycloalcohol. No oxidation occurred at 393 K in the absence of the catalyst. For cyclopentane, only 1.1% yield to ketone and 0.5% yield to alcohol were obtained in the presence of Mn-1 after 6 h. Under analogous conditions, cyclohexane was converted to ketone and alcohol with yield of 3.2% and 1.2%, respectively. Finally, the oxidation of cyclooctane catalyzed by Mn-1 resulted in 23.9% yield to ketone and 2.3% yield to alcohol. In this case, the amount of cyclooctane hydroperoxide in the reaction mixture evaluated by iodometric titration [29] after 6 h was only 0.08%, too low to have an impact on the results of the subsequent GC analysis. It is well-known that the selective catalytic oxidation of C\H bonds results in the formation of hydroperoxides [30–35]. These compounds in the course of the reaction upon the action of the catalyst convert to alcohol and ketone. In our reaction conditions cycloalkyl hydroperoxide in the presence of metallosalen catalysts is almost completely decomposed. Once the following order of catalytic activity was established: cyclooctane > cyclohexane > cyclopentane, the most reactive among investigated cycloalkanes was selected for further investigations. In a first series of experiments, the activity of various transition metal complexes of plain salen ligand 1 was evaluated. All these catalysts were active in the oxidation of cyclooctane (1) with molecular oxygen resulting in cyclooctanone (2) and cyclooctanol (3) as the main products. However, the results summarized in Table 1 show a clear influence of the complexed metal cation on catalytic activity. As one can see metallosalen complexes can be divided into two groups. Manganese, iron and cobalt salen complexes are the most active catalysts, while nickel, copper and zinc salen complexes are far less useful. This is in line with the results obtained in oxidation processes catalyzed by tetraarylporphyrin metallocomplexes [36,37]. Among plain salen metallocomplexes, cobalt complex Co-1 afforded the highest products yield while manganese catalyst Mn-1 showed the highest ratio of cyclooctanone to cyclooctanol and therefore the latter complex was selected for further studies. Following these preliminary assays, manganese complexes of modified salen ligands, 2–6 (Fig. 2) bearing electron-withdrawing n-C8F17 fluorous substituents and/or t-butyl electron-donating substituents on the 3,3′ and/or 5,5′ positions of the aryl ring, were also studied in the oxidation of cyclooctane with molecular oxygen. The data in Table 2 highlight the influence of the substitution pattern on the catalytic activity of these complexes. Regardless of the substituents nature, Mn-2–Mn-6 showed a higher catalytic activity than
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K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
Table 1 Oxidation of cyclooctane by salen complexes with different metals. Catalyst
MnSalen (Mn-1) CoSalen (Co-1) FeSalen (Fe-1) NiSalen (Ni-1) CuSalen (Cu-1) ZnSalen (Zn-1) [a] [b]
Cyclooctanone yield
Cyclooctanol yield
Mol c-on/100 mol of substrate
%[a]
TON[b]
Mol c-ol/100 mol of substrate
%[a]
TON[b]
4.2 6.2 3.4 1.1 0.8 0.6
23.9 35.2 19.3 6.3 4.6 3.4
9240 13,640 7480 2420 1760 1320
0.4 0.8 0.4 – – –
2.3 4.6 2.3 Traces Traces Traces
880 1760 880 – – –
C-one + C-ol yield%
C-one/C-ol ratio
26.1 39.8 19.7 6.3 4.6 3.4
10.5 7.8 8.5 – – –
Calculated on the base of oxygen quantity in batch reactor; Mol product × mol catalyst−1 × h−1.
simple unsubstituted Mn-1 complex. As can be seen from Table 2 the following order of catalytic activity was obtained: Mn−6 > Mn−5 > Mn−2≥Mn−4≥Mn−3 > Mn−1: Complex Mn-6 bearing four n-C8F17 substituents was thus the most active among the studied catalysts. It is known that the introduction of electron-withdrawing substituents into the metallocomplex increases its catalytic activity due to the decrease of the negative charge on the metal center [6,7,38,39]. Therefore, this result did not come unexpected. It is interesting to note, however, that manganese complex Mn-5 with four t-butyl electron-donating substituents shows a similar catalytic activity as complex Mn-6 with four electron-withdrawing fluorous substituents. This finding is in agreement with the positive role exerted by electron-donating substituents in hydroxylation reactions catalyzed by metalloporphyrins [40]. As in that case the introduction of electrondonating substituents into salen ligand rises the electron density of the metal center and weakens the bond between the metal center and oxygen. Complex Mn-3 with four substituents (two electron-donating and two electron-withdrawing) displayed almost the same activity as Mn-4 and Mn-2 bearing only two alkyl and two fluorous substituents, respectively. It appears that opposite effects brought about by t-butyl and n-C8F17 compensate each other in this manganese complex. Again this situation has parallels in metalloporphyrins. For instance, the half-wave potential of ZnTPP is almost equal to the half-wave potential of substituted ZnTPFPPMe8 where the aryl rings of the TPP ligand are perfluorinated and its β-pyrrole positions are occupied by methyl groups [41]. In this case the strong electron-withdrawing character induced by the presence of 20 fluorine substituents is compensated by the electron-donating properties of 8 methyl groups. Functionalization of the salen ligand with fluorous and/or alkyl substituents affects not only the catalytic behaviour of the corresponding manganese complexes but also their physico-chemical properties (Table 3). In particular, complexes Mn-4 and Mn-5 bearing lipophilic t-butyl groups exhibited enhanced affinity for apolar solvents with respect to the
N Z
N
OH HO X
Z X
plain salen complex Mn-1. Thus, contrary to the latter, Mn-4 and Mn-5 complexes were readily soluble in cyclooctane at room temperature. The highly temperature-dependent solubility of fluorous catalysts Mn-2 and Mn-6 in cyclooctane was even more interesting. These complexes were in fact completely insoluble in cyclooctane at room temperature but became soluble upon heating at the reaction temperature (393 K). Such a thermomorphic behaviour allowed one to perform the aerobic oxidation of cyclooctane under homogeneous conditions where the fluorous catalysts worked the best. They were easily recovered from the reaction mixture upon cooling to room temperature and precipitation, and they were reused three times with only a slight decrease in catalytic activity as exemplified in Table 2 in the case of Mn-6. 4. Conclusions In conclusion, it was shown that cyclooctane was the most reactive among the studied substrates (cyclopentane, cyclohexane or cyclooctane) in oxidation catalyzed by the simple manganese salen complex. The introduction of fluorous substituents and/or t-butyl electron-donating substituents into the salen ligand structure has an influence not only on the catalytic properties of the corresponding metal complexes but also on their solubility behaviour in the organic phase. It is also important to state that salen metallocomplexes are suitable catalysts in the aerobic oxidation of cyclooctane to cyclooctanone and cyclooctanol and their recovery and recycling can be facilitated by choosing fluorous electron-withdrawing substituents as ligand modifiers. In the case of cyclooctane oxidation the thermomorphic behaviour of fluorous salen metallocomplexes permits one to obtain separation of these catalysts as solid by simply cooling the reaction mixture, without resorting to any additional perfluorocarbon phase. This approach meets the growing need for more environmentally acceptable processes (green processes) suitable for industry, which enhance the efficiency of organic transformations and reduce the amount of waste materials at the same time. The
Ligand
X
Z
1
H
H
2
H
n-C8F17
3
H
t-butyl
4
t-butyl
n-C8F17
5
t-butyl
t-butyl
6
n-C8F17
n-C8F17
Fig. 2. Structures of investigated salen ligands.
K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
105
Table 2 Oxidation of cyclooctane by manganese salen complexes with different substituents. Catalyst
Mnsalen (Mn-1) Mnsalen(n-C8F17)2 (Mn-2) Mnsalen(n-C8F17)2 (t-butyl)2 (Mn-3) Mnsalen(t-butyl)2 (Mn-4) Mnsalen(t-butyl)4 (Mn-5) Mnsalen(n-C8F17)4 (Mn-6) Mnsalen(n-C8F17)4 (Mn-6)[c] Mnsalen(n-C8F17)4 (Mn-6)[d] [a] [b] [c] [d]
Cyclooctanone yield
Cyclooctanol yield
Mol c-on/100 mol of substrate
%[a]
TON[b]
Mol c-on/100 mol of substrate
%[a]
TON[b]
4.2 5.6 5.3 5.4 7.9 9.2 9.0 8.7
23.9 31.8 30.1 30.7 44.9 52.3 51.1 49.4
9240 12,320 11,660 11,880 17,380 20,240 19,800 19,140
0.4 0.6 0.5 0.7 1.1 1.2 1.2 0.9
2.3 3.4 2.8 4.0 6.3 6.8 6.8 5.1
880 1320 1100 1540 2420 2640 2640 1980
C-one + C-ol yield, %
C-one/C-ol Ratio
26.1 35.2 33.0 34.6 51.2 59.1 57.4 54.5
10.5 9.3 10.6 7.7 7.7 7.2 8.1 9.7
Calculated on quantity of oxygen in batch reactor; Mol product × mol catalyst−1 × h−1; Second run; Third run.
Table 3 Solubility of manganese salen complexes in the reaction media at room temperature. Catalyst
Solubility in reaction medium at room temperature
Mnsalen (Mn-1) Mnsalen(n-C8F17)2 (Mn-2) Mnsalen(t-butyl)2 (Mn-3) Mnsalen(n-C8F17)2(t-butyl)2 (Mn-4) Mnsalen(t-butyl)4 (Mn-5) Mnsalen(n-C8F17)4 (Mn-6)
Slightly soluble Insoluble Soluble Insoluble Soluble Insoluble
synthesis of two new manganese salen complexes with perfluoroalkyl substituents was elaborated. References [1] In: D.H.R. Barton, A.E. Martell, E.D.T. Sawyer (Eds.), The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum Press, New York, 1993. [2] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chemistry–A European Journal 17 (2011) 6256–6262. [3] H. Yu, F. Peng, J. Tan, X. Hu, H. Wang, J. Zheng, Angewandte Chemie International Edition in English 50 (2011) 3978–3982. [4] P.E. Ellis Jr., J.E. Lyons, Coordination Chemistry Reviews 105 (1990) 181–193. [5] J.E. Lyons, P.E. Ellis Jr., Applied Catalysis A: General 84 (1992) L1–L6. [6] In: R.A. Sheldon (Ed.), Metalloporphyrins in Catalytic Oxidation, Marcel Dekker, Basel, Hongkong, 1994. [7] In: F. Montanari, L. Cassela (Eds.), Metalloporphyrins Catalyzed Oxidation, Kluwer Academic Publisher, Dordrecht, 1994. [8] J. Połtowicz, J. Haber, Journal of Molecular Catalysis A: Chemical 220 (2004) 43–51. [9] C.-C. Guo, M.-F. Chu, Q. Liu, Y. Liu, D.-C. Guo, X.-Q. Liu, C.-C. Guo, M.-F. Chu, Q. Liu, Y. Liu, D.-C. Guo, X.-Q. Liu, Applied Catalysis A: General 246 (2003) 303–309. [10] K.J. Balkus Jr., J.P. Ferraris, Journal of Physical Chemistry 94 (1990) 8019–8120. [11] D.E. De Vos, P.A. Jacobs, Catalysis Today 57 (2000) 105–114. [12] D. Woehrle, O. Suvorova, R. Gerdes, O. Bartels, L. Lapok, N. Baziakina, S. Makarov, A. Slodek, Journal of Porphyrins and Phthalocyanines 8 (2004) 1020–1041. [13] J.-M. Vincent, A. Rabion, V.K. Yachandra, R.H. Fish, Angewandte Chemie International Edition in English 36 (1997) 2346–2349. [14] G. Pozzi, M. Cavazzini, S. Quici, S. Fontana, Tetrahedron Letters 43 (1997) 7605–7608. [15] I.T. Horváth, J. Rabái, Science 266 (1994) 72–75.
[16] I.T. Horváth, Accounts of Chemical Research 31 (1998) 641–650. [17] In: J.A. Gladysz, D.P. Curran, I.T. Horváth (Eds.), Handbook of Fluorous Catalysis, Viley- VCH, Weinheim, 2004. [18] M. Wende, R. Meier, J.A. Gladysz, Journal of the American Chemical Society 123 (2001) 11490–11491. [19] M. Wende, J.A. Gladysz, Journal of the American Chemical Society 125 (2003) 5861–5872. [20] K. Ishihara, S. Kondo, H. Yamamoto, Synlett 2001 (09) (2001) 1371–1374. [21] K. Ishihara, A. Hasegawa, H. Yamamoto, Synlett 2002 (08) (2002) 1299–1301. [22] J.A. Gladysz, V. Tesevic, Regulated systems for multiphase catalysis, in: W. Leitner, M. Holscher (Eds.), Topics in Organometallic Chemistry, vol. 23, Springer Verlag, Heidelberg, 2008, pp. 67–91. [23] P. Pfeiffer, E. Breith, E. Lubbe, T. Tsumaki, Justus Liebigs Annalen der Chemie 503 (1933) 84–130. [24] A. Bukowska, W. Bukowski, J. Noworól, Journal of Molecular Catalysis A 203 (2003) 95–99. [25] A. Bukowska, W. Bukowski, J. Noworól, Journal of Molecular Catalysis A 225 (2005) 7–10. [26] G. Pozzi, F. Cinato, F. Montanari, S. Quici, Chemical Communications (1998) 877–878. [27] G. Pozzi, M. Cavazzini, F. Cinato, F. Montanari, S. Quici, European Journal of Organic Chemistry (1999) 1947–1955. [28] M. Cavazzini, G. Pozzi, S. Quici, I. Shepperson, Journal of Molecular Catalysis A: Chemistry 204–205 (2003) 433–441. [29] R.D. Mair, A.J. Graupner, Analytical Chemistry 36 (1964) 194–204. [30] G.B. Shulpin, Y.N. Kozlov, L.S. Shulpina, A.R. Kudinov, D. Mandelli, Inorganic Chemistry 48 (2009) 10480–10482. [31] G.B. Shulpin, Y.N. Kozlov, L.S. Shulpina, P.V. Petrovskiy, Applied Organometallic Chemistry 24 (2010) 464–472. [32] E. Roduner, W. Kaim, B. Sarkar, V.B. Urlacher, J. Pleiss, R. Glaser, W.-D. Einicke, G.A. Sprenger, U. Beifuß, E. Klemm, C. Liebner, H. Hieronymus, S.-F. Hsu, B. Plietker, S. Laschat, ChemCatChem 5 (2013) 82–112. [33] J.E. Lyons, P.E. Ellis Jr., H.K. Myers Jr., Journal of Catalysis 155 (1995) 59–73. [34] M.W. Grinstaff, M.G. Hill, Y.A. Labinger, H.B. Gray, Science 264 (1994) 1311–1313. [35] G.B. Shulpin, Journal of Molecular Catalysis A: Chemical 189 (2002) 39–66. [36] B. Meunier, Chemical Reviews 92 (1992) 1411–1456. [37] D. Mansuy, Coordination Chemistry Reviews 125 (1993) 129–141. [38] J. Połtowicz, E. Tabor, K. Pamin, J. Haber, Inorganic Chemistry Communications 8 (2005) 1125–1127. [39] L. Cavallo, H. Jacobsen, Journal of Organic Chemistry 68 (2003) 6202–6207. [40] E. Baciocchi, T. Boschi, C. Galli, A. Lapi, P. Tagliatesta, Tetrahedron 53 (1997) 4497–4502. [41] J.A. Hodge, M.G. Hill, H.B. Gray, Inorganic Chemistry 34 (1995) 809–812.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Oxidation of cycloalkanes with molecular oxygen in the presence of salen metallocomplexes in thermomorphic conditions Katarzyna Pamin a, Gianluca Pozzi b, Edyta Tabor c, Wiktor Bukowski d, Jan Połtowicz a,⁎ a
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR, via Camillo Golgi 19, 20133 Milano, Italy J.Heyrovsky Institute of Physical Chemistry, Structure and Dynamics in Catalysis, Dolejškova 2155/3, 18223 Prague 8, Czech Republic d Faculty of Chemistry, Rzeszów University of Technology, al. Powstańców Warszawy 6, 35-959 Rzeszów, Poland b c
a r t i c l e
i n f o
Article history: Received 26 February 2013 Received in revised form 30 April 2013 Accepted 30 April 2013 Available online 22 May 2013 Keywords: Cycloalkanes oxidation Schiff base Fluorinated ligand Thermomorphic effect
a b s t r a c t The oxidation of cycloalkanes with molecular oxygen, catalyzed by two groups of metallosalen complexes, was studied. The first group consisted of salen complexes with different metals such as Mn, Fe, Co, Ni, Cu, Zn, while the second group was composed of manganese salen complexes with different substituents (t-butyl electron-donating substituents and/or electron-withdrawing perfluoroalkyl substituents). Mn, Fe and Co salen complexes are the most active catalysts, while Ni, Cu and Zn salen complexes are far less efficient. The introduction of t-butyl electron-donating substituents into Mnsalen complex increases the catalytic activity and catalysts solubility in the reaction mixture. The introduction of perfluoroalkyl electronwithdrawing substituents enhances the catalytic activity and renders the solubility of the catalyst temperature dependent (thermomorphic behaviour), thus allowing one to recover them easily after the reaction by simply cooling the system to room temperature. The synthesis of two new manganese salen complexes with perfluoroalkyl substituents was elaborated. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxidation of cycloalkanes with molecular oxygen is one of the most important large-scale processes in chemical industry and one of the most attractive transformations in organic synthesis [1–5]. Transition metal complexes of macrocyclic ligands such as porphyrins, phthalocyanines and to a much lesser extent related complexes of chelating [N, N-bis(salicylidene)ethylenediamine] (salen) have been previously investigated as homogeneous catalysts in these processes [6–9]. Although viable conversion and selectivity levels have been achieved by ligand structure optimization, the practical application of these relatively expensive complexes in cycloalkanes oxidation reactions remains challenging mostly because they cannot be efficiently recovered and reused. Various immobilization techniques on solid support have been developed to overcome this problem [10–12], however there is still an obvious need for more convenient, easy to apply methods for the separation of these homogeneous catalysts from reaction mixtures. Two literature reports had previously demonstrated the potential of fluorous catalysis in the aerobic oxidation of cycloalkanes [13,14]. In those examples, metal complexes of macrocyclic nitrogen ligands bearing medium-sized perfluoroalkyl substituents, which ensured preferential solubility in perfluorocarbons (PFC), were used as catalysts. ⁎ Corresponding author. Tel.: +48 126395153; fax: +48 124252943. E-mail address: [email protected] (J. Połtowicz). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.04.026
Reactions were thus performed under PFC-organic biphase conditions, according to the concept introduced by Horváth and Rábai [15,16]. The organic and PFC layers demixed upon cooling at the end of the reaction, and the bottom PFC layer containing the fluorous catalyst was easily separated and efficiently recycled. More recently, the cost of PFC as well as their persistence in the environment has pushed research towards the development of fluorous separation strategies which minimize, or even eliminate the need for these fluorous solvents [17]. The one introduced independently by the groups of Gladysz [18,19] and Yamamoto [20,21] is particularly appealing. It is based on appropriately designed fluorous catalysts that exhibit highly temperature-dependent solubilities in organic solvents, being essentially insoluble at low temperatures while completely soluble at elevated temperatures. This thermomorphic character can be exploited in various ways [22], for instance by adding the solid fluorous catalyst to an organic solution of the reactants that is then warmed to dissolve the catalyst and reach one-phase reaction conditions. Upon completion of the reaction the catalyst is deposited by just cooling the system, and eventually recovered by solid–liquid phase separation. Recently, we described the application of metalloporphyrins with different substituents as catalysts for cycloalkanes oxidation with molecular oxygen. The influence of the different kinds of substituents on the catalytic activity was described in details [5]. In the aim of developing cleaner processes based on handy catalysts we have now investigated the use of salen complexes with different metals like Mn, Fe, Co, Ni, Cu, Zn and of manganese salen
K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
103
30
complexes bearing electron-withdrawing perfluoroalkyl substituents and/or t-butyl electron-donating substituents in the oxidation of cycloalkanes with molecular oxygen (Scheme 1).
25
2. Experimental
ketone alcohol
2.1. Synthesis Manganese, iron, cobalt, nickel, copper and zinc complexes of salen ligand 1 are well-known compounds and they were readily obtained following standard literature methods [23]. Functionalized salen derivatives 2–6 and the corresponding manganese complexes were also synthesized according to procedures described elsewhere [24–28]. Their spectroscopic data acquired from FT-IR, UV–vis and NMR techniques were in agreement with the reported literature data.
Yield, %
20
15
10
5
0
2.2. Catalyst characterization 2.2.1. UV–vis spectroscopy UV–vis measurements were performed on a Perkin Elmer Lambda 35 double beam spectrophotometer, using quartz cells of 1 cm optical path. Electronic spectra of the metallocomplexes and heterogenized metallocomplexes were measured in water solution with the concentration of 2 ⋅ 10−5 mol/L. 2.2.2. Infrared spectroscopy FT-IR spectra were recorded on a Nicolet 800 spectrometer in KBr pellets over the range of 400–4000 cm−1 under the atmospheric conditions. 2.2.3. NMR spectroscopy 1 H NMR (300 MHz), 13C NMR (75.4 MHz) and 19F NMR (282 MHz) spectra were recorded on a Brucker AC 300 spectrometer with tetramethylsilane (δ = 0), CDCl3 (δ = 77) and CFCl3 (δ = 0) as internal standards. 2.3. General procedure for the oxidation of cycloalkanes The oxidation of cyclooctane (cyclopentane or cyclohexane) was performed in a stainless steel batch reactor system at 393 K and under the air pressure of 10 atm, with the substrate to oxygen molar ratio set at 6.5. In a typical experiment, 60 mL of substrate containing the amount of catalyst providing a concentration of 3.3 × 10−4 M was introduced in the deaerated autoclave and the whole system was heated until a temperature of 393 K was reached. Air was then introduced and after 6 h the oxidation products were analyzed by GC Agilent 6890 N equipped with an Innowax (30 m) column. The yield values were verified by addition of an internal standard, chlorobenzene, at the end of the reaction. 3. Results and discussion Three members of homologous series, viz. cyclopentane, cyclohexane or cyclooctane were applied as substrates and studied in oxidation catalyzed by manganese salen complex Mn-1 (Fig. 1). The main
O
metallosalen catalyst O 2(air),120°C,10 atm,6 h 1
OH
+ 2
3
Scheme 1. Oxidation of cyclooctane with molecular oxygen in the presence of metallosalen complexes.
Fig. 1. Oxidation of different cycloalkanes by simple manganese salen complex Mn-1.
products of cycloalkanes oxidation with molecular oxygen were cycloketone and cycloalcohol. No oxidation occurred at 393 K in the absence of the catalyst. For cyclopentane, only 1.1% yield to ketone and 0.5% yield to alcohol were obtained in the presence of Mn-1 after 6 h. Under analogous conditions, cyclohexane was converted to ketone and alcohol with yield of 3.2% and 1.2%, respectively. Finally, the oxidation of cyclooctane catalyzed by Mn-1 resulted in 23.9% yield to ketone and 2.3% yield to alcohol. In this case, the amount of cyclooctane hydroperoxide in the reaction mixture evaluated by iodometric titration [29] after 6 h was only 0.08%, too low to have an impact on the results of the subsequent GC analysis. It is well-known that the selective catalytic oxidation of C\H bonds results in the formation of hydroperoxides [30–35]. These compounds in the course of the reaction upon the action of the catalyst convert to alcohol and ketone. In our reaction conditions cycloalkyl hydroperoxide in the presence of metallosalen catalysts is almost completely decomposed. Once the following order of catalytic activity was established: cyclooctane > cyclohexane > cyclopentane, the most reactive among investigated cycloalkanes was selected for further investigations. In a first series of experiments, the activity of various transition metal complexes of plain salen ligand 1 was evaluated. All these catalysts were active in the oxidation of cyclooctane (1) with molecular oxygen resulting in cyclooctanone (2) and cyclooctanol (3) as the main products. However, the results summarized in Table 1 show a clear influence of the complexed metal cation on catalytic activity. As one can see metallosalen complexes can be divided into two groups. Manganese, iron and cobalt salen complexes are the most active catalysts, while nickel, copper and zinc salen complexes are far less useful. This is in line with the results obtained in oxidation processes catalyzed by tetraarylporphyrin metallocomplexes [36,37]. Among plain salen metallocomplexes, cobalt complex Co-1 afforded the highest products yield while manganese catalyst Mn-1 showed the highest ratio of cyclooctanone to cyclooctanol and therefore the latter complex was selected for further studies. Following these preliminary assays, manganese complexes of modified salen ligands, 2–6 (Fig. 2) bearing electron-withdrawing n-C8F17 fluorous substituents and/or t-butyl electron-donating substituents on the 3,3′ and/or 5,5′ positions of the aryl ring, were also studied in the oxidation of cyclooctane with molecular oxygen. The data in Table 2 highlight the influence of the substitution pattern on the catalytic activity of these complexes. Regardless of the substituents nature, Mn-2–Mn-6 showed a higher catalytic activity than
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Table 1 Oxidation of cyclooctane by salen complexes with different metals. Catalyst
MnSalen (Mn-1) CoSalen (Co-1) FeSalen (Fe-1) NiSalen (Ni-1) CuSalen (Cu-1) ZnSalen (Zn-1) [a] [b]
Cyclooctanone yield
Cyclooctanol yield
Mol c-on/100 mol of substrate
%[a]
TON[b]
Mol c-ol/100 mol of substrate
%[a]
TON[b]
4.2 6.2 3.4 1.1 0.8 0.6
23.9 35.2 19.3 6.3 4.6 3.4
9240 13,640 7480 2420 1760 1320
0.4 0.8 0.4 – – –
2.3 4.6 2.3 Traces Traces Traces
880 1760 880 – – –
C-one + C-ol yield%
C-one/C-ol ratio
26.1 39.8 19.7 6.3 4.6 3.4
10.5 7.8 8.5 – – –
Calculated on the base of oxygen quantity in batch reactor; Mol product × mol catalyst−1 × h−1.
simple unsubstituted Mn-1 complex. As can be seen from Table 2 the following order of catalytic activity was obtained: Mn−6 > Mn−5 > Mn−2≥Mn−4≥Mn−3 > Mn−1: Complex Mn-6 bearing four n-C8F17 substituents was thus the most active among the studied catalysts. It is known that the introduction of electron-withdrawing substituents into the metallocomplex increases its catalytic activity due to the decrease of the negative charge on the metal center [6,7,38,39]. Therefore, this result did not come unexpected. It is interesting to note, however, that manganese complex Mn-5 with four t-butyl electron-donating substituents shows a similar catalytic activity as complex Mn-6 with four electron-withdrawing fluorous substituents. This finding is in agreement with the positive role exerted by electron-donating substituents in hydroxylation reactions catalyzed by metalloporphyrins [40]. As in that case the introduction of electrondonating substituents into salen ligand rises the electron density of the metal center and weakens the bond between the metal center and oxygen. Complex Mn-3 with four substituents (two electron-donating and two electron-withdrawing) displayed almost the same activity as Mn-4 and Mn-2 bearing only two alkyl and two fluorous substituents, respectively. It appears that opposite effects brought about by t-butyl and n-C8F17 compensate each other in this manganese complex. Again this situation has parallels in metalloporphyrins. For instance, the half-wave potential of ZnTPP is almost equal to the half-wave potential of substituted ZnTPFPPMe8 where the aryl rings of the TPP ligand are perfluorinated and its β-pyrrole positions are occupied by methyl groups [41]. In this case the strong electron-withdrawing character induced by the presence of 20 fluorine substituents is compensated by the electron-donating properties of 8 methyl groups. Functionalization of the salen ligand with fluorous and/or alkyl substituents affects not only the catalytic behaviour of the corresponding manganese complexes but also their physico-chemical properties (Table 3). In particular, complexes Mn-4 and Mn-5 bearing lipophilic t-butyl groups exhibited enhanced affinity for apolar solvents with respect to the
N Z
N
OH HO X
Z X
plain salen complex Mn-1. Thus, contrary to the latter, Mn-4 and Mn-5 complexes were readily soluble in cyclooctane at room temperature. The highly temperature-dependent solubility of fluorous catalysts Mn-2 and Mn-6 in cyclooctane was even more interesting. These complexes were in fact completely insoluble in cyclooctane at room temperature but became soluble upon heating at the reaction temperature (393 K). Such a thermomorphic behaviour allowed one to perform the aerobic oxidation of cyclooctane under homogeneous conditions where the fluorous catalysts worked the best. They were easily recovered from the reaction mixture upon cooling to room temperature and precipitation, and they were reused three times with only a slight decrease in catalytic activity as exemplified in Table 2 in the case of Mn-6. 4. Conclusions In conclusion, it was shown that cyclooctane was the most reactive among the studied substrates (cyclopentane, cyclohexane or cyclooctane) in oxidation catalyzed by the simple manganese salen complex. The introduction of fluorous substituents and/or t-butyl electron-donating substituents into the salen ligand structure has an influence not only on the catalytic properties of the corresponding metal complexes but also on their solubility behaviour in the organic phase. It is also important to state that salen metallocomplexes are suitable catalysts in the aerobic oxidation of cyclooctane to cyclooctanone and cyclooctanol and their recovery and recycling can be facilitated by choosing fluorous electron-withdrawing substituents as ligand modifiers. In the case of cyclooctane oxidation the thermomorphic behaviour of fluorous salen metallocomplexes permits one to obtain separation of these catalysts as solid by simply cooling the reaction mixture, without resorting to any additional perfluorocarbon phase. This approach meets the growing need for more environmentally acceptable processes (green processes) suitable for industry, which enhance the efficiency of organic transformations and reduce the amount of waste materials at the same time. The
Ligand
X
Z
1
H
H
2
H
n-C8F17
3
H
t-butyl
4
t-butyl
n-C8F17
5
t-butyl
t-butyl
6
n-C8F17
n-C8F17
Fig. 2. Structures of investigated salen ligands.
K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
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Table 2 Oxidation of cyclooctane by manganese salen complexes with different substituents. Catalyst
Mnsalen (Mn-1) Mnsalen(n-C8F17)2 (Mn-2) Mnsalen(n-C8F17)2 (t-butyl)2 (Mn-3) Mnsalen(t-butyl)2 (Mn-4) Mnsalen(t-butyl)4 (Mn-5) Mnsalen(n-C8F17)4 (Mn-6) Mnsalen(n-C8F17)4 (Mn-6)[c] Mnsalen(n-C8F17)4 (Mn-6)[d] [a] [b] [c] [d]
Cyclooctanone yield
Cyclooctanol yield
Mol c-on/100 mol of substrate
%[a]
TON[b]
Mol c-on/100 mol of substrate
%[a]
TON[b]
4.2 5.6 5.3 5.4 7.9 9.2 9.0 8.7
23.9 31.8 30.1 30.7 44.9 52.3 51.1 49.4
9240 12,320 11,660 11,880 17,380 20,240 19,800 19,140
0.4 0.6 0.5 0.7 1.1 1.2 1.2 0.9
2.3 3.4 2.8 4.0 6.3 6.8 6.8 5.1
880 1320 1100 1540 2420 2640 2640 1980
C-one + C-ol yield, %
C-one/C-ol Ratio
26.1 35.2 33.0 34.6 51.2 59.1 57.4 54.5
10.5 9.3 10.6 7.7 7.7 7.2 8.1 9.7
Calculated on quantity of oxygen in batch reactor; Mol product × mol catalyst−1 × h−1; Second run; Third run.
Table 3 Solubility of manganese salen complexes in the reaction media at room temperature. Catalyst
Solubility in reaction medium at room temperature
Mnsalen (Mn-1) Mnsalen(n-C8F17)2 (Mn-2) Mnsalen(t-butyl)2 (Mn-3) Mnsalen(n-C8F17)2(t-butyl)2 (Mn-4) Mnsalen(t-butyl)4 (Mn-5) Mnsalen(n-C8F17)4 (Mn-6)
Slightly soluble Insoluble Soluble Insoluble Soluble Insoluble
synthesis of two new manganese salen complexes with perfluoroalkyl substituents was elaborated. References [1] In: D.H.R. Barton, A.E. Martell, E.D.T. Sawyer (Eds.), The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum Press, New York, 1993. [2] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chemistry–A European Journal 17 (2011) 6256–6262. [3] H. Yu, F. Peng, J. Tan, X. Hu, H. Wang, J. Zheng, Angewandte Chemie International Edition in English 50 (2011) 3978–3982. [4] P.E. Ellis Jr., J.E. Lyons, Coordination Chemistry Reviews 105 (1990) 181–193. [5] J.E. Lyons, P.E. Ellis Jr., Applied Catalysis A: General 84 (1992) L1–L6. [6] In: R.A. Sheldon (Ed.), Metalloporphyrins in Catalytic Oxidation, Marcel Dekker, Basel, Hongkong, 1994. [7] In: F. Montanari, L. Cassela (Eds.), Metalloporphyrins Catalyzed Oxidation, Kluwer Academic Publisher, Dordrecht, 1994. [8] J. Połtowicz, J. Haber, Journal of Molecular Catalysis A: Chemical 220 (2004) 43–51. [9] C.-C. Guo, M.-F. Chu, Q. Liu, Y. Liu, D.-C. Guo, X.-Q. Liu, C.-C. Guo, M.-F. Chu, Q. Liu, Y. Liu, D.-C. Guo, X.-Q. Liu, Applied Catalysis A: General 246 (2003) 303–309. [10] K.J. Balkus Jr., J.P. Ferraris, Journal of Physical Chemistry 94 (1990) 8019–8120. [11] D.E. De Vos, P.A. Jacobs, Catalysis Today 57 (2000) 105–114. [12] D. Woehrle, O. Suvorova, R. Gerdes, O. Bartels, L. Lapok, N. Baziakina, S. Makarov, A. Slodek, Journal of Porphyrins and Phthalocyanines 8 (2004) 1020–1041. [13] J.-M. Vincent, A. Rabion, V.K. Yachandra, R.H. Fish, Angewandte Chemie International Edition in English 36 (1997) 2346–2349. [14] G. Pozzi, M. Cavazzini, S. Quici, S. Fontana, Tetrahedron Letters 43 (1997) 7605–7608. [15] I.T. Horváth, J. Rabái, Science 266 (1994) 72–75.
[16] I.T. Horváth, Accounts of Chemical Research 31 (1998) 641–650. [17] In: J.A. Gladysz, D.P. Curran, I.T. Horváth (Eds.), Handbook of Fluorous Catalysis, Viley- VCH, Weinheim, 2004. [18] M. Wende, R. Meier, J.A. Gladysz, Journal of the American Chemical Society 123 (2001) 11490–11491. [19] M. Wende, J.A. Gladysz, Journal of the American Chemical Society 125 (2003) 5861–5872. [20] K. Ishihara, S. Kondo, H. Yamamoto, Synlett 2001 (09) (2001) 1371–1374. [21] K. Ishihara, A. Hasegawa, H. Yamamoto, Synlett 2002 (08) (2002) 1299–1301. [22] J.A. Gladysz, V. Tesevic, Regulated systems for multiphase catalysis, in: W. Leitner, M. Holscher (Eds.), Topics in Organometallic Chemistry, vol. 23, Springer Verlag, Heidelberg, 2008, pp. 67–91. [23] P. Pfeiffer, E. Breith, E. Lubbe, T. Tsumaki, Justus Liebigs Annalen der Chemie 503 (1933) 84–130. [24] A. Bukowska, W. Bukowski, J. Noworól, Journal of Molecular Catalysis A 203 (2003) 95–99. [25] A. Bukowska, W. Bukowski, J. Noworól, Journal of Molecular Catalysis A 225 (2005) 7–10. [26] G. Pozzi, F. Cinato, F. Montanari, S. Quici, Chemical Communications (1998) 877–878. [27] G. Pozzi, M. Cavazzini, F. Cinato, F. Montanari, S. Quici, European Journal of Organic Chemistry (1999) 1947–1955. [28] M. Cavazzini, G. Pozzi, S. Quici, I. Shepperson, Journal of Molecular Catalysis A: Chemistry 204–205 (2003) 433–441. [29] R.D. Mair, A.J. Graupner, Analytical Chemistry 36 (1964) 194–204. [30] G.B. Shulpin, Y.N. Kozlov, L.S. Shulpina, A.R. Kudinov, D. Mandelli, Inorganic Chemistry 48 (2009) 10480–10482. [31] G.B. Shulpin, Y.N. Kozlov, L.S. Shulpina, P.V. Petrovskiy, Applied Organometallic Chemistry 24 (2010) 464–472. [32] E. Roduner, W. Kaim, B. Sarkar, V.B. Urlacher, J. Pleiss, R. Glaser, W.-D. Einicke, G.A. Sprenger, U. Beifuß, E. Klemm, C. Liebner, H. Hieronymus, S.-F. Hsu, B. Plietker, S. Laschat, ChemCatChem 5 (2013) 82–112. [33] J.E. Lyons, P.E. Ellis Jr., H.K. Myers Jr., Journal of Catalysis 155 (1995) 59–73. [34] M.W. Grinstaff, M.G. Hill, Y.A. Labinger, H.B. Gray, Science 264 (1994) 1311–1313. [35] G.B. Shulpin, Journal of Molecular Catalysis A: Chemical 189 (2002) 39–66. [36] B. Meunier, Chemical Reviews 92 (1992) 1411–1456. [37] D. Mansuy, Coordination Chemistry Reviews 125 (1993) 129–141. [38] J. Połtowicz, E. Tabor, K. Pamin, J. Haber, Inorganic Chemistry Communications 8 (2005) 1125–1127. [39] L. Cavallo, H. Jacobsen, Journal of Organic Chemistry 68 (2003) 6202–6207. [40] E. Baciocchi, T. Boschi, C. Galli, A. Lapi, P. Tagliatesta, Tetrahedron 53 (1997) 4497–4502. [41] J.A. Hodge, M.G. Hill, H.B. Gray, Inorganic Chemistry 34 (1995) 809–812.