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Oxidative cleavage of vic-diols catalyzed by manganese (III) complexes in ionic liquids
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Catalysis Communications 9 (2008) 1282–1285 www.elsevier.com/locate/catcom
Oxidative cleavage of vic-diols catalyzed by manganese (III) complexes in ionic liquids S. Rian˜o, D. Ferna´ndez, L. Fadini * Departamento de Quı´mica, Facultad de Ciencias, Universidad Nacional de Colombia, Cra 30 No. 45-03, Bogota´ D.C., Colombia Received 17 October 2007; received in revised form 13 November 2007; accepted 14 November 2007 Available online 4 December 2007
Abstract The oxidative cleavage of vicinal diols allows the selective oxidation of diols in order to obtain aldehydes or ketones. In this work we present a novel catalytic system based on MnIII(salen) complexes dissolved in ionic liquids that allows high efficiency with low environmental impact in the oxidative cleavage of ethyleneglycol, pinacol, benzopinacol and hydrobenzoin. The use of ionic liquids in this reaction shows a positive effect of the solvent in the efficiency of the MnIII catalysts (increase of the yield in ionic liquids: 10– 60%). Ó 2007 Elsevier B.V. All rights reserved. Keywords: Oxidative cleavage; Ionic liquids; Salen; Mn(III)-catalyst; Vic-diol
1. Introduction The obtaining of aldehydes through the oxidative cleavage of vicinal diols is an important field in synthetic organic chemistry [1]. This type of reaction takes place in several important biochemical processes, such as the hydroxylated carbohydrates metabolism [2]. Nevertheless, although a wide range of systems from stoichiometric [1,3–7] to catalytic [8–12] have been tried, these are far away from the complete achievement in terms of activity, selectivity, atomic economy and environmental impact. Therefore, one of the aims of the organic synthesis is the development of new catalytic systems that allow the selective formation of aldehydes with non-polluting and non-toxic catalysts, using oxygen as an oxidant [9,13]. Our approach is based on the use of well known MnIII(salen) complexes [14,15] in ionic liquids (organic salts with low melting-point (<100 °C)) [16,17] as a catalytic system for the oxidative C–C bond cleavage of vic-diols to aldehydes with O2 as oxidant and a sacrificial aldehyde as a
*
Corresponding author. Fax: +57 1 316 5220. E-mail address: [email protected] (L. Fadini).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.028
co-reductant [18]. The highly polar ionic liquids (IL) can stabilize the cationic catalyst or the ionic intermediates, having as a consequence an increased activity of the catalyst [19–21]. Furthermore, the option to recycle the catalyst/IL system as a strategy to improve the deficiency of the homogeneous catalysts allows also an improved total turnover number (TON) [22]. The selected catalysts are achiral manganeseIII(salen) complexes ((N,N0 -bis(salicylidene)-1,2-ethylenediamine)manganese (III)), which have been intensely investigated owing to their excellent performance in asymmetric alkene epoxidation [15]. The properties relating to activity, oxygen affinity and ligand modifications of these Mn(salen) complexes are well known, but so far they have not been reported in the catalytic oxidative C–C bond cleavage of vicinal diols. On the other hand, due to the environmental and economic necessity to recover and reuse homogenous catalysts, they have been tried in the recycling of the catalyst/ionic liquid system for the Katsuki–Jacobsen epoxidation, showing that this type of catalyst can be reused without considerable loss of the activity [20,23]. The MnIII catalysts in ionic liquids therefore promise an alternative method for production of aldehydes and ketones from vicinal diols (Scheme 1).
S. Rian˜o et al. / Catalysis Communications 9 (2008) 1282–1285 catalyst: O O Mn N N N HO
OH
R R' R R' R = H, Me, Ph
30-60 °C, 2-6 h, ionic liquid, R''CHO, O2
O 2 R R' R = H, Me, Ph
Scheme 1. Oxidative cleavage of vicinal diols catalyzed by [Mn(salen)(Py)](OAc).
2. Results and discussion Considering the results reported by Pedro et al., using a tetradentate o-phenylenedioxamate manganeseIII catalyst [12] in which the substrate that presents the smaller reactivity is benzopinacol, we decided to select the best reaction conditions using manganeseIII(salen) complexes in ionic liquids with the sterically hindered tetraphenyl-1,2-ethanediol as a substrate. The tested diols were 1,2-ethanediol (ethyleneglycol, R = R0 = H), 2,3-dimethyl-2,3-butanediol (pinacol, R = R0 = Me), 1,2-diphenyl-1,2-ethanediol (hydrobenzoin, R = Ph, R0 = H) and 1,1,2,2-tetraphenyl1,2-ethanediol (benzopinacol, R = R0 = Ph), so we could establish differences of reactivity as regards electronic and steric effects. Table 1 shows the results of the catalytic experiments (reaction conditions: 2–3 h, 30–60 °C, with constant bubbling of oxygen, 5 mol% of catalyst (in situ or isolated) and benzaldehyde or salicylaldehyde as sacrificial aldehyde) [24]. The catalytic activity of the Mn complexes toward the oxidative cleavage of vicinal diols shows better results in
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ionic liquids than in organic solvents. The best oxidation condition is the use of oxygen as an oxidant in the presence of excess aldehyde as a sacrificial reagent, the so-called Mukaiyama catalytic system [18,25]. Also in the in situ system (the catalyst is formed from 5% mol of Mn(OAc)2 with 1 equiv. of N,N0 -bis(salicylidene)1,2-ethylenediamine (salen), 1 equiv. of pyridine dissolved in dichloromethane/ionic liquid) there is improved activity if the reaction is carried out in ionic liquids as a solvent (entries 1–3, increasing of the yield from 18.4% in dichloromethane to 60.1% in [OPic][PF6]). The use of the isolated catalyst [Mn(salen)(Py)](OAc) allows us to obtain better yields (77.8% in dichloromethane and 82.9% in [OPic][PF6], entries 4–5). This indicates perhaps that the catalysis has a long induction time, and the formation of the catalytic species is better and faster if the isolated catalyst is used. Moreover, the Mn-catalyst in the ionic N-octyl-3-picolinium hexafluorophosphate is one of the more efficient systems for the C–C bond oxidative cleavage of 1,1,2,2tetraphenyl-1,2-ethanediol, allowing a quantitative yield (>99%) after 2 h at 60 °C (entry 7). This result is definitively better than that reported by Pedro and co-workers using the o-phenylendioxamate MnIII catalyst (in acetonitrile, at 40 °C, after 6 h: 10% yield) [12]. Although the role of the sacrificial aldehyde has not been clearly established, the Mukaiyama group proposed that for aerobic epoxidation reactions in the presence of this type of complex the reaction proceeds simultaneously with the co-oxidation of the aldehyde by a radicalic mechanism that generates a peroxyacid which reacts with the metallic complex to form an oxo intermediate [18,25]. We therefore evaluated the effect of two sacrificial aldehydes in the oxidative cleavage reaction of benzopinacol: benzaldehyde and salicylaldehyde.
Table 1 Oxidative C–C bond cleavage of vic-diols catalyzed by manganeseIII(salen) complexes Entry
R and R0 substrate
Conditions
Solvent
%Yielda Ketone
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a
0
R = R = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Me R = R0 = Me R = R0 = Me R = H, R0 = H R = H, R0 = H R = H, R0 = H R = Ph, R0 = H R = Ph, R0 = H R = Ph, R0 = H
In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 60 °C, 2 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde
Dichloromethane 18.3 [BMIM][PF6] 48.6 [OPic][PF6] 60.1 Dichloromethane 77.8 [OPic][PF6] 82.9 [OPic][PF6] >99b Dichloromethane 40.7 [OPic][PF6] 56.2 Dichloromethane 43.2 [BMIM][PF6] 58.5 [OPic][PF6] 77.9 Dichloromethane [BMIM][PF6] [OPic][PF6] Dichloromethane [BMIM][PF6] [OPic][PF6]
Aldehyde
0 9.3c 30.2c 17.1 43.2 41.0
Yields (average of 2 runs) are for the isolated product, after purification by distillation or flash chromatography. No traces of the diol were detected after 2 h. c Product derived with 2,4-dinitrophenylhydrazine. [OPic][PF6]: N-octyl-3-picolinium hexafluorophosphate; [BMIM][PF6]: butylmethylimidazolium hexafluorophosphate. b
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S. Rian˜o et al. / Catalysis Communications 9 (2008) 1282–1285
The yields using benzaldehyde as co-reductant always showed better results (entries 4 vs. 7 and 5 vs. 8), indicating the important role of this aldehyde in the formation of the active species. The oxidative cleavage of 2,3-dimethyl-2,3-butanediol allowed the formation of acetone in moderate to good yields when the catalysis was carried out in dichloromethane (43.2%), in [BMIM] [PF6] (58.5%) or in [OPic][PF6] (77,9%; entries 9–11). Since the yields for the acetone or benzophenone formation are similar, it is possible to assert that the steric hindrance does not seem to be determinant for the substrate–catalyst interaction. On the other hand, the results with ethyleneglycol to form formaldehyde [26] show only low yields (up to 30.2%), and no appreciative amounts of the corresponding acid have been detected. In the oxidative cleavage of 1,2-diphenyl-1,2-ethanediol, the formation of benzaldehyde is slightly superior: 17.1% in dichloromethane and close to 40% in the ionic liquids (entries 15–17), comparable with the yields reported by Pedro et al. [12] with only traces of the corresponding acid. According to these results, it appears that for this oxidative cleavage of vic-diols, it is necessary to use substrates with substituents that can weaken the C–C bond, which is consistent with the results reported by Okamoto et al. [13]. Although the oxidation cleavage of diols catalyzed by transition metal complexes has been reported by different groups, a really satisfactory mechanism that explains the interaction between the substrates and the catalyst has still to be detailed. Several groups [12,18] proposed that the reaction is catalyzed by an oxomanganese active species, which is formed by a sacrificial aldehyde that must be present in stoichiometrical quantities (or in excess). Using DFT and UHF calculations, we tried to understand how the oxo manganese complex catalyzes the oxidation cleavage of the diols. To model this reaction, the use of a
B3LYP/LANL2DZ level of theory has been established [27,28] (as implemented in the PCGAMESS package), since this method has been successfully applied in the modelling of Katsuki–Jacobsen epoxidation reactions (using the same active species) [29–31]. In order to simplify the computational model, ethyleneglycol was chosen as the substrate, the oxomanganese(V) chloride as the catalyst (the neutral species to simplify the underlying calculations) and a simplified salen ligand (without the lateral aromatic ring of the salicylaldehyde) [30,31]. Geometrical optimizations were done with all the reagents and products prior to any further calculations. As it is known that transition metal systems often exhibit spin crossing in their reaction paths, every single reaction step was calculated using multiplicities 1, 3 and 5, while in some cases the calculations could not be done owing to SCF convergence problems. The calculations were therefore done with UHF, which can improve the problems caused in DFT calculations when the system is not well represented by a single slater determinant (several contributing multiplicities). The first step to get the reaction approximation’s coordinates of the substrate was the mapping of the electrostatic potential of the diol and the manganese complex. The substrate and the complex showed a marked negative potential of almost equal magnitude around their active oxygen atoms. Thus, the oxygen–oxygen approach to form a peroxide intermediate as in an iron porphyrine compound is almost impossible owing to electrostatic repulsion. The diol approaching the oxomanganese complex formed an intermediate with multiplicity 5, in which a proton from the diol was transferred to the manganese complex (Fig. 1a). A local minimum was found when both protons of the diol had been donated. This geometry involved the formation of two molecules of formaldehyde
Fig. 1. Intermediates in the interaction between ethylenglycol and the oxomanganese compound: a) first deprotonation of the diol; b) formation of the products and the aqua manganese complex.
S. Rian˜o et al. / Catalysis Communications 9 (2008) 1282–1285
and a molecule of an aqua manganese complex (Fig. 1b). Finally, all transition states and minima were verified by hessian calculations, showing the correct amount of imaginary frequencies. The changes in energy along the whole reaction were also calculated, showing a favourable change of 93.65 kJ/mol for the reagents towards the first minimum and 56.62 kJ/mol from the reagents towards the second minimum. Even if these energies are not exact, owing to the simplification of the catalyst, these values indicate that the reaction involves an energetic stabilization. Further thermodynamic and kinetic studies are in progress in order to elucidate the whole catalytic cycle. Finally, both intermediates and transition states showed high areas of charge separation in the electrostatic potential maps. This suggests that the use of solvents with high dielectric constants as well as the use of cationic manganese complexes would cause further stability of the intermediates and the transition states. This explains that the use of high polar solvents almost certainly decreases the activation energies, elucidating the positive effect of ionic liquids in the catalytic oxidative cleavage of vicinal diols. 3. Conclusions The complex [Mn(salen)(Py)]OAc presents catalytic activity for the oxidative C–C bond cleavage of vic-diols with oxygen as oxidant and an aldehyde as co-reductant. Ketones from steric hindered substrates like 1,1,2,2-tetraphenyl-1,2-ethanediol (benzopinacol) have been formed (with yields up to 99%, 2 h at 60 °C)), showing better results than the previously reported systems based on manganese (III) catalysts. The use of ionic liquids as solvents in this reaction allows us to improve the results considerably, increasing the yield up to 60%. These results corroborate the positive effect of the use of ionic liquids with cationic catalysts. DFT and UHF calculations let us clarify the possible interaction between the substrates and the catalyst, related to an ionic mechanism. Further mechanistic studies to determine the catalytic cycle and the possibility of recycle the catalytic system in ionic liquids are in progress. Acknowledgements The authors thank Prof. Edgar Daza and Prof. Andre´s Reyes (Universidad Nacional de Colombia) for assistance in the calculation studies.
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References [1] T.K.M. Shing, in: B.M. Trost, I. Fleming (Eds.), Comprehensive Organic Synthesis, Pergamon Press, Oxford, 1991, p. 703. [2] B. Meunier, Biomimetic Oxidations Catalyzed by Transition Metal Complexes, Imperial College Press, London, 2000. [3] F.P. Greenspan, H.M. Woodburn, J. Am. Chem. Soc. 76 (1954) 6345. [4] W.S. Trahanovsky, L.H. Young, M.H. Bierman, J. Org. Chem. 34 (1969) 869. [5] D.H.R. Barton, J.-P. Finet, Pure. Appl. Chem. 59 (1987) 937. [6] J.I.C. Lena, E. Altinel, N. Birlirakis, S. Arseniyadis, Tetrahedron Lett. 43 (2002) 2505. [7] O. Sesenoglu, J.I.C. Lena, E. Altinel, N. Birlirakis, S. Arseniyadis, Tetrahedron-Asymmetr. 16 (2005) 995. [8] D.H.R. Barton, W.B. Motherwell, A. Stobie, J. Chem. Soc., Chem. Commun. (1981) 1232. [9] T.R. Felthouse, J. Am. Chem. Soc. 109 (1987) 7566. [10] E. Takezawa, S. Sakaguchi, Y. Ishii, Org. Lett. 1 (1999) 713. [11] Y. Ishii, T. Nakano, in: United States patent; US 6,506,943 B1, 2003. [12] S. Barroso, G. Blay, I. Fernandez, J.R. Pedro, R. Ruiz-Garcia, E. Pardo, F. Lloret, M.C. Munoz, J. Mol. Catal. A, Chem. 243 (2006) 214. [13] T. Okamoto, K. Sasaki, S. Oka, J. Am. Chem. Soc. 110 (1988) 1187. [14] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, J. Am. Chem. Soc. 112 (1990) 2801. [15] C. Baleizao, H. Garcia, Chem. Rev. 106 (2006) 3987. [16] J.D. Holbrey, K.R. Seddon, Clean Prod. Process. 1 (1999) 223. [17] J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002) 3667. [18] T. Mukaiyama, T. Yamada, Bull. Chem. Soc. Jpn. 68 (1995) 17. [19] P. Dyson, Transit. Metal Chem. 27 (2002) 353. [20] T. Welton, Coordin. Chem. Rev. 248 (2004) 2459. [21] S. Chowdhury, R.S. Mohan, J.L. Scott, Tetrahedron 63 (2007) 2363. [22] V.I. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615. [23] C.E. Song, E. Roh, J. Chem. Commun (2000) 837. [24] The standard catalysis procedure: 0.27 mmol of the diol is dissolved in 2 mL of dichloromethane and added to a round-bottomed flask that contains 5.0 mL of the ionic liquid and 0.014 mmol (5 mol%) of [Mn(salen)(Py)](OAc). The organic solvent is evaporated and the system is bubbled constantly with oxygen at 30 °C. One equivalent of the sacrificial aldehyde (0.27 mmol, benzaldehyde or salicylaldehyde) is added and the addition is repeated after 1 and 2 hours. After three hours the oxygen bubbling is suspended and 5 mL of heptane are added to the system. The products are extracted (5 5 mL) and purified (flash chromatography) with 2,2,4-trimethyl pentane/ethylacetate 9:1 or distilled with reduced pressure. The yields are for isolated products. [25] T. Mukaiyama, Aldrichim. Acta 29 (1996) 59. [26] The products have been distilled (via cannula) during the reaction and put directly in a solution of 2,4-dinitrophenylhydrazine. The obtained hidrazone was filtered, washed (heptane), dried, characterized (IR, 1H NMR) and quantified. [27] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [28] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [29] L. Cavallo, H. Jacobsen, J. Phys. Chem. A 107 (2003) 5466. [30] L. Cavallo, H. Jacobsen, Eur. J. Inorg. Chem. (2003) 892. [31] H. Jacobsen, L. Cavallo, Phys. Chem. Chem. Phys. 6 (2004) 3747.
Catalysis Communications 9 (2008) 1282–1285 www.elsevier.com/locate/catcom
Oxidative cleavage of vic-diols catalyzed by manganese (III) complexes in ionic liquids S. Rian˜o, D. Ferna´ndez, L. Fadini * Departamento de Quı´mica, Facultad de Ciencias, Universidad Nacional de Colombia, Cra 30 No. 45-03, Bogota´ D.C., Colombia Received 17 October 2007; received in revised form 13 November 2007; accepted 14 November 2007 Available online 4 December 2007
Abstract The oxidative cleavage of vicinal diols allows the selective oxidation of diols in order to obtain aldehydes or ketones. In this work we present a novel catalytic system based on MnIII(salen) complexes dissolved in ionic liquids that allows high efficiency with low environmental impact in the oxidative cleavage of ethyleneglycol, pinacol, benzopinacol and hydrobenzoin. The use of ionic liquids in this reaction shows a positive effect of the solvent in the efficiency of the MnIII catalysts (increase of the yield in ionic liquids: 10– 60%). Ó 2007 Elsevier B.V. All rights reserved. Keywords: Oxidative cleavage; Ionic liquids; Salen; Mn(III)-catalyst; Vic-diol
1. Introduction The obtaining of aldehydes through the oxidative cleavage of vicinal diols is an important field in synthetic organic chemistry [1]. This type of reaction takes place in several important biochemical processes, such as the hydroxylated carbohydrates metabolism [2]. Nevertheless, although a wide range of systems from stoichiometric [1,3–7] to catalytic [8–12] have been tried, these are far away from the complete achievement in terms of activity, selectivity, atomic economy and environmental impact. Therefore, one of the aims of the organic synthesis is the development of new catalytic systems that allow the selective formation of aldehydes with non-polluting and non-toxic catalysts, using oxygen as an oxidant [9,13]. Our approach is based on the use of well known MnIII(salen) complexes [14,15] in ionic liquids (organic salts with low melting-point (<100 °C)) [16,17] as a catalytic system for the oxidative C–C bond cleavage of vic-diols to aldehydes with O2 as oxidant and a sacrificial aldehyde as a
*
Corresponding author. Fax: +57 1 316 5220. E-mail address: [email protected] (L. Fadini).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.028
co-reductant [18]. The highly polar ionic liquids (IL) can stabilize the cationic catalyst or the ionic intermediates, having as a consequence an increased activity of the catalyst [19–21]. Furthermore, the option to recycle the catalyst/IL system as a strategy to improve the deficiency of the homogeneous catalysts allows also an improved total turnover number (TON) [22]. The selected catalysts are achiral manganeseIII(salen) complexes ((N,N0 -bis(salicylidene)-1,2-ethylenediamine)manganese (III)), which have been intensely investigated owing to their excellent performance in asymmetric alkene epoxidation [15]. The properties relating to activity, oxygen affinity and ligand modifications of these Mn(salen) complexes are well known, but so far they have not been reported in the catalytic oxidative C–C bond cleavage of vicinal diols. On the other hand, due to the environmental and economic necessity to recover and reuse homogenous catalysts, they have been tried in the recycling of the catalyst/ionic liquid system for the Katsuki–Jacobsen epoxidation, showing that this type of catalyst can be reused without considerable loss of the activity [20,23]. The MnIII catalysts in ionic liquids therefore promise an alternative method for production of aldehydes and ketones from vicinal diols (Scheme 1).
S. Rian˜o et al. / Catalysis Communications 9 (2008) 1282–1285 catalyst: O O Mn N N N HO
OH
R R' R R' R = H, Me, Ph
30-60 °C, 2-6 h, ionic liquid, R''CHO, O2
O 2 R R' R = H, Me, Ph
Scheme 1. Oxidative cleavage of vicinal diols catalyzed by [Mn(salen)(Py)](OAc).
2. Results and discussion Considering the results reported by Pedro et al., using a tetradentate o-phenylenedioxamate manganeseIII catalyst [12] in which the substrate that presents the smaller reactivity is benzopinacol, we decided to select the best reaction conditions using manganeseIII(salen) complexes in ionic liquids with the sterically hindered tetraphenyl-1,2-ethanediol as a substrate. The tested diols were 1,2-ethanediol (ethyleneglycol, R = R0 = H), 2,3-dimethyl-2,3-butanediol (pinacol, R = R0 = Me), 1,2-diphenyl-1,2-ethanediol (hydrobenzoin, R = Ph, R0 = H) and 1,1,2,2-tetraphenyl1,2-ethanediol (benzopinacol, R = R0 = Ph), so we could establish differences of reactivity as regards electronic and steric effects. Table 1 shows the results of the catalytic experiments (reaction conditions: 2–3 h, 30–60 °C, with constant bubbling of oxygen, 5 mol% of catalyst (in situ or isolated) and benzaldehyde or salicylaldehyde as sacrificial aldehyde) [24]. The catalytic activity of the Mn complexes toward the oxidative cleavage of vicinal diols shows better results in
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ionic liquids than in organic solvents. The best oxidation condition is the use of oxygen as an oxidant in the presence of excess aldehyde as a sacrificial reagent, the so-called Mukaiyama catalytic system [18,25]. Also in the in situ system (the catalyst is formed from 5% mol of Mn(OAc)2 with 1 equiv. of N,N0 -bis(salicylidene)1,2-ethylenediamine (salen), 1 equiv. of pyridine dissolved in dichloromethane/ionic liquid) there is improved activity if the reaction is carried out in ionic liquids as a solvent (entries 1–3, increasing of the yield from 18.4% in dichloromethane to 60.1% in [OPic][PF6]). The use of the isolated catalyst [Mn(salen)(Py)](OAc) allows us to obtain better yields (77.8% in dichloromethane and 82.9% in [OPic][PF6], entries 4–5). This indicates perhaps that the catalysis has a long induction time, and the formation of the catalytic species is better and faster if the isolated catalyst is used. Moreover, the Mn-catalyst in the ionic N-octyl-3-picolinium hexafluorophosphate is one of the more efficient systems for the C–C bond oxidative cleavage of 1,1,2,2tetraphenyl-1,2-ethanediol, allowing a quantitative yield (>99%) after 2 h at 60 °C (entry 7). This result is definitively better than that reported by Pedro and co-workers using the o-phenylendioxamate MnIII catalyst (in acetonitrile, at 40 °C, after 6 h: 10% yield) [12]. Although the role of the sacrificial aldehyde has not been clearly established, the Mukaiyama group proposed that for aerobic epoxidation reactions in the presence of this type of complex the reaction proceeds simultaneously with the co-oxidation of the aldehyde by a radicalic mechanism that generates a peroxyacid which reacts with the metallic complex to form an oxo intermediate [18,25]. We therefore evaluated the effect of two sacrificial aldehydes in the oxidative cleavage reaction of benzopinacol: benzaldehyde and salicylaldehyde.
Table 1 Oxidative C–C bond cleavage of vic-diols catalyzed by manganeseIII(salen) complexes Entry
R and R0 substrate
Conditions
Solvent
%Yielda Ketone
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a
0
R = R = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Ph R = R0 = Me R = R0 = Me R = R0 = Me R = H, R0 = H R = H, R0 = H R = H, R0 = H R = Ph, R0 = H R = Ph, R0 = H R = Ph, R0 = H
In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde In situ (5% mol Mn(OAc)2, salen, pyridine), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 60 °C, 2 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. benzaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde 5 mol% [Mn(salen)(Py)](OAc), 30 °C, 3 h, 3 equiv. salicylaldehyde
Dichloromethane 18.3 [BMIM][PF6] 48.6 [OPic][PF6] 60.1 Dichloromethane 77.8 [OPic][PF6] 82.9 [OPic][PF6] >99b Dichloromethane 40.7 [OPic][PF6] 56.2 Dichloromethane 43.2 [BMIM][PF6] 58.5 [OPic][PF6] 77.9 Dichloromethane [BMIM][PF6] [OPic][PF6] Dichloromethane [BMIM][PF6] [OPic][PF6]
Aldehyde
0 9.3c 30.2c 17.1 43.2 41.0
Yields (average of 2 runs) are for the isolated product, after purification by distillation or flash chromatography. No traces of the diol were detected after 2 h. c Product derived with 2,4-dinitrophenylhydrazine. [OPic][PF6]: N-octyl-3-picolinium hexafluorophosphate; [BMIM][PF6]: butylmethylimidazolium hexafluorophosphate. b
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The yields using benzaldehyde as co-reductant always showed better results (entries 4 vs. 7 and 5 vs. 8), indicating the important role of this aldehyde in the formation of the active species. The oxidative cleavage of 2,3-dimethyl-2,3-butanediol allowed the formation of acetone in moderate to good yields when the catalysis was carried out in dichloromethane (43.2%), in [BMIM] [PF6] (58.5%) or in [OPic][PF6] (77,9%; entries 9–11). Since the yields for the acetone or benzophenone formation are similar, it is possible to assert that the steric hindrance does not seem to be determinant for the substrate–catalyst interaction. On the other hand, the results with ethyleneglycol to form formaldehyde [26] show only low yields (up to 30.2%), and no appreciative amounts of the corresponding acid have been detected. In the oxidative cleavage of 1,2-diphenyl-1,2-ethanediol, the formation of benzaldehyde is slightly superior: 17.1% in dichloromethane and close to 40% in the ionic liquids (entries 15–17), comparable with the yields reported by Pedro et al. [12] with only traces of the corresponding acid. According to these results, it appears that for this oxidative cleavage of vic-diols, it is necessary to use substrates with substituents that can weaken the C–C bond, which is consistent with the results reported by Okamoto et al. [13]. Although the oxidation cleavage of diols catalyzed by transition metal complexes has been reported by different groups, a really satisfactory mechanism that explains the interaction between the substrates and the catalyst has still to be detailed. Several groups [12,18] proposed that the reaction is catalyzed by an oxomanganese active species, which is formed by a sacrificial aldehyde that must be present in stoichiometrical quantities (or in excess). Using DFT and UHF calculations, we tried to understand how the oxo manganese complex catalyzes the oxidation cleavage of the diols. To model this reaction, the use of a
B3LYP/LANL2DZ level of theory has been established [27,28] (as implemented in the PCGAMESS package), since this method has been successfully applied in the modelling of Katsuki–Jacobsen epoxidation reactions (using the same active species) [29–31]. In order to simplify the computational model, ethyleneglycol was chosen as the substrate, the oxomanganese(V) chloride as the catalyst (the neutral species to simplify the underlying calculations) and a simplified salen ligand (without the lateral aromatic ring of the salicylaldehyde) [30,31]. Geometrical optimizations were done with all the reagents and products prior to any further calculations. As it is known that transition metal systems often exhibit spin crossing in their reaction paths, every single reaction step was calculated using multiplicities 1, 3 and 5, while in some cases the calculations could not be done owing to SCF convergence problems. The calculations were therefore done with UHF, which can improve the problems caused in DFT calculations when the system is not well represented by a single slater determinant (several contributing multiplicities). The first step to get the reaction approximation’s coordinates of the substrate was the mapping of the electrostatic potential of the diol and the manganese complex. The substrate and the complex showed a marked negative potential of almost equal magnitude around their active oxygen atoms. Thus, the oxygen–oxygen approach to form a peroxide intermediate as in an iron porphyrine compound is almost impossible owing to electrostatic repulsion. The diol approaching the oxomanganese complex formed an intermediate with multiplicity 5, in which a proton from the diol was transferred to the manganese complex (Fig. 1a). A local minimum was found when both protons of the diol had been donated. This geometry involved the formation of two molecules of formaldehyde
Fig. 1. Intermediates in the interaction between ethylenglycol and the oxomanganese compound: a) first deprotonation of the diol; b) formation of the products and the aqua manganese complex.
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and a molecule of an aqua manganese complex (Fig. 1b). Finally, all transition states and minima were verified by hessian calculations, showing the correct amount of imaginary frequencies. The changes in energy along the whole reaction were also calculated, showing a favourable change of 93.65 kJ/mol for the reagents towards the first minimum and 56.62 kJ/mol from the reagents towards the second minimum. Even if these energies are not exact, owing to the simplification of the catalyst, these values indicate that the reaction involves an energetic stabilization. Further thermodynamic and kinetic studies are in progress in order to elucidate the whole catalytic cycle. Finally, both intermediates and transition states showed high areas of charge separation in the electrostatic potential maps. This suggests that the use of solvents with high dielectric constants as well as the use of cationic manganese complexes would cause further stability of the intermediates and the transition states. This explains that the use of high polar solvents almost certainly decreases the activation energies, elucidating the positive effect of ionic liquids in the catalytic oxidative cleavage of vicinal diols. 3. Conclusions The complex [Mn(salen)(Py)]OAc presents catalytic activity for the oxidative C–C bond cleavage of vic-diols with oxygen as oxidant and an aldehyde as co-reductant. Ketones from steric hindered substrates like 1,1,2,2-tetraphenyl-1,2-ethanediol (benzopinacol) have been formed (with yields up to 99%, 2 h at 60 °C)), showing better results than the previously reported systems based on manganese (III) catalysts. The use of ionic liquids as solvents in this reaction allows us to improve the results considerably, increasing the yield up to 60%. These results corroborate the positive effect of the use of ionic liquids with cationic catalysts. DFT and UHF calculations let us clarify the possible interaction between the substrates and the catalyst, related to an ionic mechanism. Further mechanistic studies to determine the catalytic cycle and the possibility of recycle the catalytic system in ionic liquids are in progress. Acknowledgements The authors thank Prof. Edgar Daza and Prof. Andre´s Reyes (Universidad Nacional de Colombia) for assistance in the calculation studies.
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