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Catalytic conversions in water
Applied Catalysis A: General 194 –195 (2000) 435–442
Catalytic conversions in water Part 13. Aerobic oxidation of olefins to methyl ketones catalysed by a water-soluble palladium complex – mechanistic investigations Gerd-Jan ten Brink, Isabel W.C.E. Arends, Georgios Papadogianakis1 , Roger A. Sheldon ∗ Laboratory for Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Accepted 6 July 1999
Abstract Water-soluble palladium(II) bathophenanthroline is a stable, recyclable catalyst for the selective aerobic oxidation of terminal olefins to the corresponding 2-alkanones in a biphasic liquid–liquid system. Kinetic measurements indicate that the active catalyst is a homogeneous mononuclear species. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Water-soluble palladium complex; Wacker-type oxidation; Olefins; Catalysis; Mechanistic investigations
1. Introduction In a constant search for cleaner technologies, oxidation reactions are nowadays preferably carried out with oxidants such as dioxygen or hydrogen peroxide. These oxidants can be used with high-atom efficiency [1], producing only water as a by-product. There is a definite need for good catalysts that enable the use of these oxidants. Moreover, the use of water as a reaction medium makes these reactions even more attractive from an economic and environmental point of view. Most organic products are water-insoluble and their separation from a water-soluble catalyst is relatively straightforward. ∗ Corresponding author. Tel.: +31-15-2782683; fax: +31-15-2781415. E-mail address: [email protected] (R.A. Sheldon). 1 University of Athens, Department of Chemistry, Laboratory of Industrial Chemistry, Panepistimiopolis - Zografou, 15771, Athens, Greece.
The well-known Wacker process [2] is an example of homogeneous catalysis on a bulk scale that seems to meet these requirements. It uses dioxygen as the primary oxidant to oxidise ethylene to acetaldehyde and propylene to acetone. The process is carried out in water with 100% atom efficiency. Although at first glance this system seems perfectly environmentally friendly, a more detailed look reveals that it is ’less than green’ chemistry since the process requires large amounts of copper salts, chlorides and acid to maintain a catalytic cycle [3]. To avoid the use of these corrosive additives, co-catalysts such as the heteropolyacid H3 PMo6 V6 O40 [3,4] or a combination of benzoquinone with either iron(II) phthalocyanine [5,6] or heteropolyacids [7] have been developed. Recently [8], we reported on the aerobic oxidation of terminal and cyclic olefins catalysed by water-soluble palladium complexes of bidentate diamine ligands. A combination of palladium acetate and a water-soluble rigid bidentate diamine, notably bathophenanthroline disulfonate (PhenS*),
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Fig. 1. Wacker-type oxidations catalysed with water-soluble PhenS*Pd(OAc)2 .
gave the best results in terms of rate, stability and selectivity. This provided a system for performing Wacker-type reactions in water under neutral, copperand chloride-free conditions (Fig. 1). Here, we report the mechanistic investigations of this system aimed at establishing the nature of the catalytically active species. A particular focus of attention was the possible role of (giant) palladium clusters in the observed catalysis [9–11].
2. Experimental 2.1. Materials Bathophenanthroline disulfonic acid disodium salt (98%), Pd(OAc)2 , sodium sulfite (97%) (Acros), 1-hexene (97%) (Aldrich), Na(OAc).3H2 O (>99.5%) (Merck) and reagent grade NaOH (Baker) were used without further purification. Water was distilled before use. 2.2. Synthesis 2.2.1. Sodium 4-pentenylsulfonate 1 A mixture of 1-bromo-4-pentene [12] (300 mmol, 44.5 g) and sodium sulfite (345 mmol, 48.5 g) in de-aerated water (225 ml) was refluxed overnight under N2 to give a clear homogeneous aqueous phase. After cooling, water was removed under reduced pressure and the residue was extracted with petroleum ether 40–60. The residue was then boiled with 100% ethanol and filtered hot. After cooling the filtrate, a white solid was filtered off. The filtrate was
concentrated and a second crop of white solid was collected. The combined yields were dried in air at room temperature, giving 23 g (134 mmol, 45%) of 1. This procedure is the somewhat modified method of Houlton and Tartar [13]. 1 H-NMR, 300.17 MHz (D O, ref. tert-BuOH = 2 1.20 ppm): δ 5.91–5.76 (m, 1H, C4 –H), δ 5.10–4.98 (m, 2H, C5 –H), δ 2.86 (t, 2H, CH2 SO3 Na, J = 7.9 Hz), δ 2.14 (dt, 2H, C3 –H, J = 6.7 Hz), δ 1.78 (dt, 2H, C2 –H, J = 7.7 Hz). 13 C-NMR, 75.48 MHz (D O, ref. tert-BuOH = 2 31.2 ppm): δ 139.68 (C4 ), δ 116.99 (C5 ), δ 51.38 (C1 ), δ 33.27 (C3 ), δ 24.81 (C2 ). 2.2.2. Sodium 4-oxopentylsulfonate 2 Oxidation of 1 (10 mmol, 1.72 g) was carried out as described for other olefins, see below. In this particular case 8% oxygen in nitrogen was used as the oxidant. After reaction, water was removed under reduced pressure, giving 1.84 g (9.8 mmol, 98%) of 2 contaminated with catalyst. 1 H-NMR, 300.17 MHz (D O, ref. tert-BuOH = 2 1.20 ppm): δ 2.86 (t, 2H, CH2 SO3 Na, J = 7.6 Hz), δ 2.70 (t, 2H, C3 –H, J = 7.3 Hz), δ 2.17 (s, CH3 ), δ 1.91 (dt, 2H, C2 –H, J = 7.6 Hz). 13 C-NMR, 75.48 MHz (D O, ref. tert-BuOH = 2 31.2 ppm): δ 216.57 (C4 ), δ 51.44 (C1 –SO3 Na), δ 42.88 (C3 ), δ 31.89 (C5 ), δ 19.95 (C2 ). 2.3. Catalytic experiments The catalyst solutions were prepared by stirring Pd(OAc)2 (0.0224 g, 0.1 mmol) and PhenS* (0.0546 g, 0.1 mmol) overnight in water (42.5 ml) to give a clear yellow–orange solution. Standard catalytic experiments were carried out in a closed Hastelloy C autoclave (175 ml). The autoclave was cooled to ca. 0◦ C, charged with the catalyst solution (0.1 mmol catalyst in 42.5 ml water), olefin (10–20 mmol) and internal standard (n-heptane). The autoclave was pressurised with air and heated to 100◦ C (30 bar) for 10 h. After reaction the autoclave was cooled to 0◦ C and depressurised. Any volatile material was collected in a liquid nitrogen trap. The product mixture was extracted with Et2 O and the organic layer was dried over MgSO4 and analysed by GC. Recoveries were always 100 ± 2% with this procedure.
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Fig. 2. Reoxidation of Pd(0) with dioxygen.
The oxidation of 1 was followed by monitoring the oxygen uptake as a function of time. Samples were analysed by 1 H-NMR. 2.4. Apparatus 1 H-
and 13 C NMR spectra were recorded on a Bruker AC 300 spectrometer using tert-BuOH as an external reference. GC measurements were carried out with a Varian Star 3400 instrument equipped with a CP Sil 5-CB column (50 m × 0.53 mm). 3. Results and discussion The conversions of olefins to carbonyl compounds with palladium catalysts can be seen as the reduction of Pd2+ salts to zerovalent Pd(0) by these substrates. This probably is a critical stage in any catalytic process that involves a Pd2+ /Pd(0) redox couple, since clustering of atomic Pd(0) to give palladium black is a highly favourable and largely irreversible process [14]. Therefore, zerovalent Pd(0) has to be stabilised with regard to palladium black formation through interaction with solvents, acids or ligands. Direct re-oxidation of Pd(0) by dioxygen takes place when the zerovalent metal is stabilised in clusters in solvents such as ethylene carbonate [15] or DMSO [16–18]. In highly acidic, dry media, insertion of dioxygen into a palladium hydride [19] or protonation of palladium peroxide [20] to yield a palladium hydroperoxide will also re-oxidise zerovalent pal-
ladium (Fig. 2). However, formation of palladium hydroperoxide seems unlikely with the aqueous, neutral-to-basic reaction conditions used in our experiments. It is possible that the zerovalent palladium formed in our system is held by phenanthroline in a coagulated state that is small enough — possibly even monoatomic [21,22] — to allow for rapid re-oxidation to successfully compete with irreversible palladium black formation. This is analogous to the direct dioxygen oxidation of Pd(0) anchored on polyanilines [23] or polypyrroles [24], which have also been used as catalysts in Wacker-type reactions. Coordination of 2,20 -bipyridine or phenanthroline stabilises Pd2+ [25–27], resulting in a decrease of the redox potential of the Pd2+ /Pd(0) couple. This makes re-oxidation of Pd(0) more thermodynamically favourable and leaves Pd(0) less susceptible to clustering. Addition of chloride ions has a similar thermodynamic effect in the classical Wacker process, where Pd(0) is re-oxidised to PdCl4 2− instead of to PdCl2 [3]. Furthermore, it was proposed that chlorides kinetically accelerate Pd(0) re-oxidation either by keeping the surface area of Pd(0)n particles large for a facile re-oxidation and/or by participating in the transition state for Pd(0) re-oxidation [3]. Again, a comparison with bipyridine and phenanthroline ligands can be made, since complexation of these ligands to a metal may sterically hinder extensive clustering and accelerate electron transfer between a metal and its redox partner. This might also allow PhenS*Pd(0) less time for clustering.
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Fig. 3. Oxidation of 1-hexene as a function of time. Conditions: 0.1 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 10 or 20 mmol 1-hexene, 10 mmol NaOAc, p = 30 bar, air, t = 5–15 h, T = 100◦ C.
Fig. 4. The order in [olefin]. Calculated from: -d [4-pentenylsulfonate]/dt = -2d [O2 ]/dt. Conditions: 0.05 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 10 mmol 1, 10 mmol NaOAc, p = 10 bar, 8% O2 in N2 , T = 100◦ C.
3.1. Kinetics of olefin oxidation The water-soluble complex of PhenS*Pd(OAc)2 catalyses the aerobic oxidation of 1-hexene to 2-hexanone [8]. When the reaction is followed as a function of time using 10 mmol of substrate the rate is constant for 10 h until about 50% conversion is reached and then the rate decreases (Fig. 3). The decrease in rate after 10 h is not caused by the deterioration of the catalyst since when more 1-hexene is added the rate remains constant for a longer period of time. Up to 10 h the aqueous phase is saturated with olefin, which means that the concentration of the latter is constant and a pseudo-zero-order rate in [olefin] is observed. When about 5 mmol of 1-hexene is left the reaction rate becomes first order in [1-hexene]. Due to the biphasic nature of the reaction, it is difficult to take representative samples to follow the course of the reaction as a function of time. Hence, to determine the order in [olefin] we used a (completely) water-soluble olefin, sodium 4-pentenylsulfonate 1, as substrate and monitored oxygen uptake as a function of time. Calculation of the olefin conversion from these data revealed that the reaction is first order in [olefin] from t ∼ = 0, as shown in Fig. 4. The order in [palladium] was determined by reacting an excess of olefin (1-hexene) at various palladium concentrations. Fig. 5 shows that the order in [palladium] is 1/2. This is in agreement with the formation of a dimeric palladium species that is in equilib-
rium with a catalytically active palladium monomer (Fig. 6). The most likely candidate is a dimeric palladium–phenanthroline species with two bridging hydroxide anions [28,29]. This result contrasts with the first-order dependence on [palladium] found in the classical Wacker and related processes [30–32]. As previously reported [8], sodium acetate has a beneficial effect in stabilising the catalyst. This effect only became noticeable after the catalyst was recycled. Data with different amounts of NaOAc and NaOH are shown in Table 1. The results show that addition of sodium acetate or sodium hydroxide has no influence
Fig. 5. The order in [palladium] in 1-hexene oxidation. [Pd2 ] denotes the palladium dimer concentration. Experiments carried out with 0.025, 0.05, 0.066, 0.1 and 0.2 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 20 mmol 1-hexene, 10 mmol NaOAc, p = 30 bar, air, t = 5–15 h, T = 100◦ C.
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Fig. 6. The catalytic cycle proposed for olefin oxidation with aqueous soluble Pd(II) catalysts.
on the reaction rate. In summary, in our system the reaction rate for olefin oxidation follows the equation: rate ∼ = k [Pd2 ]1/2 [olefin] and the catalyst activity does not depend on pH or [acetate]. 3.2. Oxidation mechanism We propose the reaction mechanism shown in Fig. 6 to rationalise the observed results. In the first step a palladium dimer with two bridging hydroxy ligands is dissociated via coordination of the olefin, consistent Table 1 1-Hexene oxidation at various pH values and base concentrationsa NaOAc (mmol)
Cycle
pH (NaOH added)
TOFinitial (h−1 )
Selectivity (%)
0 0 0 2 5 10 10 10 0 0 10
1st 2nd 3rd 1st 1st 1st 2nd 3rd 1st 1st 1st
6.5 6.5 6.5 6.5 6.6 6.6 6.6 6.6 9 10 10
4.7 2.8 1.5 4.7 4.9 4.8 4.4 4.0 5.0 5.3 4.8
99 97 96 99+ 99+ 99+ 96 95 97 97 99+
a Conditions: 0.1 mmol PhenS*Pd(OAc) 2 in 42.5 ml H2 O, 20 mmol 1-hexene, p = 30 bar, air, t = 10 h, T = 100◦ C. (TOF in mmol olefin/mmol Pd/h; selectivity in mmol 2-hexanone/6 mmol of products).
with the observed 1/2 order in [palladium] and first order in [olefin]. This is followed by intra-molecular attack of hydroxide on the coordinated olefin, i.e. the reaction proceeds via cis-hydroxypalladation [30–32] (see later). The resulting -hydroxyalkyl palladium complex decomposes into the 2-alkanone and a zerovalent palladium species. The latter is re-oxidised with dioxygen, giving a palladium peroxide [33]. Coupling of this peroxide with one equivalent of zerovalent palladium–PhenS* regenerates the starting palladium dimer. 3.3. Comparison with the Wacker process: Hydroxypalladation mechanism The catalyst system described here shows a number of similarities to the Wacker process in terms of type of metal, solvent and type of product that is formed. Marked differences are observed, however, in the respective rate laws. In the Wacker process the rate ∼ = k [Pd2+ ] [olefin]/ [Cl− ]2 [H+ ] at low [Cl− ], while in our system the rate ∼ = k [Pd2 ]1/2 [olefin]. A second-order inhibition by chloride is observed in the Wacker process, since the active catalyst — PdCl4 2− — dissociates two equivalents of chloride, replacing them with one equivalent of olefin and one equivalent of water prior to the rate-limiting step. In contrast, such a displacement does not occur in our system.
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Fig. 7. Olefin oxidation by PdCl4 2− in the Wacker process according to Henry and Bäckvall.
The key step in the Wacker reaction is hydroxypalladation, for which two different mechanisms have been proposed (Fig. 7). Both provide an explanation for the observed kinetics of the Wacker process. Henry [30–32] proposed a mechanism involving rate limiting cis-hydroxypalladation. On the other hand, Bäckvall [34] favoured fast trans-hydroxypalladation followed by rate-limiting decomposition of a -hydroxyalkyl-palladium species. In the Henry mechanism the aqua-ligand dissociates a proton. This is followed by an intra-molecular attack of the resulting hydroxide ligand on the coordinated olefin. We propose that our system involves a similar mode of cis-hydroxypalladation. Outer sphere attack of a water molecule on the coordinated olefin, resulting in the release of a proton (Fig. 8), can be ruled out since the addition of base (NaOH or NaOAc) has no effect on the rate [34].
Henry also showed that coordination of chloride [35], (at high [Cl− ]), pyridine [36,37] or bidentate phosphine [38] instead of H2 O or OH− to palladium seriously hampers the reaction and gives mainly chloroethanol instead of acetaldehyde in the Wacker process. This emphasises the need for an aqua/hydroxy ligand coordinated to palladium, as in our system. Therefore, our findings indirectly support an intra-molecular cis-hydroxypalladation mechanism for the classical Wacker process. 3.4. Possible involvement of (giant) clusters Alternatively, our system could involve the formation of (giant) palladium clusters [9–11]. It is known that palladium–phenanthroline clusters are easily formed from Phen–Pd(OAc)2 in the presence of a reducing agent such as CO [39] or H2 [40] and
Fig. 8. Possible modes of attack on the coordinated olefin.
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Fig. 9. The formation of palladium clusters.
consecutive oxidation with dioxygen. In our case, the olefin might play a similar role as a reductant. However, a number of observations militate against this. Firstly, cluster formation would most likely liberate free phenanthroline (Fig. 9), which is not found after reaction. Secondly, no induction period that would be required to form a cluster of some sort from homogeneous PhenS*Pd(OAc)2 is detected (Fig. 3). Thirdly, the kinetic observations are not consistent with the formation of clusters. In cluster catalysis, the reaction might be expected to show Michalis–Menten kinetics [40]. It would be first order in [cluster] and consequently approximately first order in [palladium], supposing that all palladium atoms form clusters of comparable size, structure and reactivity [41]. Catalysis with giant palladium clusters would also require the presence of strong acids such as HClO4 , or H2 SO4 , and would not proceed in neutral to basic aqueous solutions [42]. Fourthly, in giant clusters ca.10% of the surface area is uncovered, leaving enough room only for small molecules, such as ethylene, propylene, CO or acetic acid to react [43,44]. This is not consistent with the observed facile oxidation of cyclooctene [8]. Fifthly, (giant) palladium clusters are expected to give rise to double bond isomerisation in ␣-olefins and allylic alcohols giving internal olefins and (saturated) aldehydes, respectively. Although this would be suppressed to a large degree in cluster catalysis through the addition of water, this phenomenon is completely absent in our system. Furthermore, allylic oxidation
is highly favoured over the formation of 2-alkanones in reactions catalysed by giant palladium clusters [45]. The oxidation of propylene in water, catalysed by a giant Pd561 cluster, for example, yielded ca.3% of acetone, while allylic oxidation products — allyl alcohol (38%), acrolein (14%) and acrylic acid (20%) — were predominant [42]. Finally, it could be argued that 1—hexene would reduce Pd2+ to give a (giant) cluster in the first cycle, which would then continue as the actual catalyst. However, no significant change in the catalyst solution was observed with UV-VIS or light scattering techniques. Moreover, we observed that our recycled aqueous catalyst solution still gave stoichiometric oxidation under anaerobic conditions [46]. It is well known that a giant cluster cannot carry out (stoichiometric) oxidations in the absence of dioxygen Fig. 10 [40,45]. By the same token the observation of stoichiometric oxidation of 1-hexene with PhenS*Pd(OAc)2 in an inert atmosphere rules out the possibility that the reaction proceeds via a Pd(IV)/Pd(II) cycle in which Pd(II) is first oxidised by dioxygen.
4. Conclusion The results of kinetic investigations of the aerobic oxidation of terminal olefins catalysed by water-soluble palladium–phenanthroline catalysts are consistent with the active catalyst being a mononu-
Fig. 10. Oxidation with homogeneous Pd2+ vs. cluster Pdn+ in stoichiometric reactions with olefins.
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clear complex in equilibrium with a hydroxy-bridged dimer rather than a giant palladium cluster. The results are consistent with a mechanism that proceeds via cis-hydroxypalladation of the olefin. The catalyst is perfectly stable in water under the reaction conditions and the reactions are generally fast, selective and highly reproducible. Hence, the method should have broad applicability. References [1] R.A. Sheldon, J. Dakka, Catal. Today 19 (1994) 215. [2] R. Jira, in: B. Cornils, W.A. Herrmann (Eds.), Homogeneous Catalysis with Organometallic Compounds, VCH, Weinheim, 1997, p.374. [3] J.H. Grate, D.R. Hamm, S. Mahajan, in: J. Kosak, T. Johnson (Eds.), Catalysis of Organic Reactions, Marcel Dekker, Dordrecht, The Netherlands, 1994, p.213. [4] K.I. Matveev, Kinet. Catal. 18 (1977) 716. (Engl. transl.). [5] J.-E. Bäckvall, R.B. Hopkins, Tetrahedron Lett. 29 (1988) 2855. [6] J.-E. Bäckvall, R.B. Hopkins, H. Grennberg, M.A. Mader, A.K. Awasthi, J. Amer. Chem. Soc. 112 (1990) 5160. [7] T. Yokata, S. Fujibayashi, Y. Nishiyama, S. Sakaguchi, Y. Ishii, J. Mol. Catal. A: Chemical 114 (1996) 113. [8] Part 10: G.J. ten Brink, I.W.C.E. Arends, G. Papadogianakis, R.A. Sheldon, J. Chem. Soc. Chem. Commun. 1998, 2359. [9] I.I. Moiseev, in: M.E. Volpin (Ed.), Chemistry Reviews (Soviet Scientific Reviews), vol. 4, Harwood Academic, New York, p.139. [10] J.S. Bradley, in: G. Schmid (Ed.), Clusters and Colloids, VCH, Weinheim, 1994, p.459. [11] I.I. Moiseev, M.N. Vargaftik, in: R.D. Adams, F.A. Cotton (Eds.), Catalysis by Di- and Polynuclear Metal Cluster Complexes, VCH-Wiley, New York, 1998, p.395. [12] F.B. LaForge, N. Green, W.A. Gersdorff, J. Amer. Chem. Soc. 70 (1948) 3707. [13] H.G. Houlton, H.V. Tartar, J. Amer. Chem. Soc. 60 (1938) 544. [14] 1H ≈ 378.2 kJ/mol. Data obtained from J. Phys. Chem. Ref. Data, 11, suppl. 2 (1982). [15] T.F. Blackburn, J. Schwartz, J. Chem. Soc. Chem. Commun., 1977, 157. [16] K.P. Peterson, R.C. Larock, J. Org. Chem. 63 (1998) 3185. [17] R.C. Larock, T.R. Hightower, J. Org. Chem. 58 (1993) 5298.
[18] R.A.T.M. van Benthem, H. Hiemstra, J.J. Michels, W.N. Speckamp, J. Chem. Soc. Chem. Commun. 1994, 357. [19] T. Hosokawa, S.-I. Murahashi, Acc. Chem. Res. 23 (1990) 49. [20] F. Igersheim, H. Mimoun, Nouv. J. Chim. 4 (1980) 711. [21] A. Bontempi, E. Alessio, G. Chanos, G. Mestroni, J. Mol. Catal. 42 (1987) 67. [22] E. Alessio, G. Mestroni, J. Organometal. Chem. 291 (1985) 117. [23] T. Hirao, M. Higuchi, B. Hatano, I. Ikeda, Tetrahedron Lett. 36 (1995) 5925. [24] M. Higuchi, S. Yamaguchi, T. Hirao, Synlett. 1996, 1213. [25] R.P. Patel, S.K. Kulshreshtha, H. Mohan, J. Chem. Soc. Dalton Trans. 1993, 1245. [26] R.P. Patel, H. Mohan, S.K. Kulshreshtha, Radiat. Phys. Chem. 47 (1996) 571. [27] R.P. Patel, M.A. Vaidya, H. Mohan, S.K. Kulshreshtha, Radiat. Phys. Chem. 48 (1996) 41. [28] S.W. Wimmer, P. Castan, F.L. Wimmer, N.P. Johnson, Inorg. Chim. Acta 142 (1988) 13. [29] S.W. Wimmer, P. Castan, F.L. Wimmer, N.P. Johnson, J. Chem. Soc. Dalton Trans. 1989, 403. [30] P.M. Henry, J. Amer. Chem. Soc. 86 (1964) 3246. [31] P.M. Henry, J. Amer. Chem. Soc. 88 (1966) 1595. [32] P.M. Henry, Palladium Catalyzed Oxidation of Hydrocarbons, D. Reidel, Dordrecht, The Netherlands, 1980, p.41. [33] T. Jintoku, K. Takai, Y. Fujiwara, Y. Fuchita, K. Hiraki, Bull. Chem. Soc. Jpn. 63 (1990) 438. [34] J.E. Bäckvall, B. Åkermark, S.O. Ljunggren, J. Amer. Chem. Soc. 101 (1979) 2411. [35] J.W. Francis, P.M. Henry, Organometallics 10 (1991) 3498. [36] J.W. Franis, P.M. Henry, J. Mol Catal. 99 (1995) 77. [37] J.W. Franis, P.M. Henry, J. Mol Catal. 112 (1996) 317. [38] O. Hamed, P.M. Henry, Organometallics 17 (1998) 5184. [39] M.K. Starchevsky, M.N. Vargaftik, I.I. Moiseev, Kinet. Catal. 20, 1979, 955, (Engl. transl.). [40] M.N. Vargaftik, V.P. Zagorodnikov, I.P. Stolyarov, I.I. Moiseev, D.I. Kochubey, V.A. Likholobov, A.L. Chuvilin, K.I. Zamaraev, J. Mol. Catal. 53 (1989) 315. [41] I.I. Moiseev, T.A. Stromnova, M.N. Vargaftik, J. Mol. Catal. 86 (1994) 71. [42] P.I. Pasichnyk, M.K. Starchevsky, Yu. A. Pazdersky, V.P. Zagorodnikov, M.N. Vargaftik, I.I. Moiseev, Mendeleev Commun. 1994, 1. [43] I.I. Moiseev, M.N. Vargaftik, Russ. Chem. Revs. 59 (1990) 1133. [44] I.P. Stolyarov, M.N. Vargaftik, I.I. Moiseev, Kinet. Catal. 28, 1987, 1172. (Engl. transl.). [45] I.I. Moiseev, -Complexes in Liquid Phase Olefin Oxidation, Nauka, Moscow, 1970. [46] F.C. Phillips, Amer. Chem. J. 16 (1894) 255.
Catalytic conversions in water Part 13. Aerobic oxidation of olefins to methyl ketones catalysed by a water-soluble palladium complex – mechanistic investigations Gerd-Jan ten Brink, Isabel W.C.E. Arends, Georgios Papadogianakis1 , Roger A. Sheldon ∗ Laboratory for Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Accepted 6 July 1999
Abstract Water-soluble palladium(II) bathophenanthroline is a stable, recyclable catalyst for the selective aerobic oxidation of terminal olefins to the corresponding 2-alkanones in a biphasic liquid–liquid system. Kinetic measurements indicate that the active catalyst is a homogeneous mononuclear species. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Water-soluble palladium complex; Wacker-type oxidation; Olefins; Catalysis; Mechanistic investigations
1. Introduction In a constant search for cleaner technologies, oxidation reactions are nowadays preferably carried out with oxidants such as dioxygen or hydrogen peroxide. These oxidants can be used with high-atom efficiency [1], producing only water as a by-product. There is a definite need for good catalysts that enable the use of these oxidants. Moreover, the use of water as a reaction medium makes these reactions even more attractive from an economic and environmental point of view. Most organic products are water-insoluble and their separation from a water-soluble catalyst is relatively straightforward. ∗ Corresponding author. Tel.: +31-15-2782683; fax: +31-15-2781415. E-mail address: [email protected] (R.A. Sheldon). 1 University of Athens, Department of Chemistry, Laboratory of Industrial Chemistry, Panepistimiopolis - Zografou, 15771, Athens, Greece.
The well-known Wacker process [2] is an example of homogeneous catalysis on a bulk scale that seems to meet these requirements. It uses dioxygen as the primary oxidant to oxidise ethylene to acetaldehyde and propylene to acetone. The process is carried out in water with 100% atom efficiency. Although at first glance this system seems perfectly environmentally friendly, a more detailed look reveals that it is ’less than green’ chemistry since the process requires large amounts of copper salts, chlorides and acid to maintain a catalytic cycle [3]. To avoid the use of these corrosive additives, co-catalysts such as the heteropolyacid H3 PMo6 V6 O40 [3,4] or a combination of benzoquinone with either iron(II) phthalocyanine [5,6] or heteropolyacids [7] have been developed. Recently [8], we reported on the aerobic oxidation of terminal and cyclic olefins catalysed by water-soluble palladium complexes of bidentate diamine ligands. A combination of palladium acetate and a water-soluble rigid bidentate diamine, notably bathophenanthroline disulfonate (PhenS*),
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Fig. 1. Wacker-type oxidations catalysed with water-soluble PhenS*Pd(OAc)2 .
gave the best results in terms of rate, stability and selectivity. This provided a system for performing Wacker-type reactions in water under neutral, copperand chloride-free conditions (Fig. 1). Here, we report the mechanistic investigations of this system aimed at establishing the nature of the catalytically active species. A particular focus of attention was the possible role of (giant) palladium clusters in the observed catalysis [9–11].
2. Experimental 2.1. Materials Bathophenanthroline disulfonic acid disodium salt (98%), Pd(OAc)2 , sodium sulfite (97%) (Acros), 1-hexene (97%) (Aldrich), Na(OAc).3H2 O (>99.5%) (Merck) and reagent grade NaOH (Baker) were used without further purification. Water was distilled before use. 2.2. Synthesis 2.2.1. Sodium 4-pentenylsulfonate 1 A mixture of 1-bromo-4-pentene [12] (300 mmol, 44.5 g) and sodium sulfite (345 mmol, 48.5 g) in de-aerated water (225 ml) was refluxed overnight under N2 to give a clear homogeneous aqueous phase. After cooling, water was removed under reduced pressure and the residue was extracted with petroleum ether 40–60. The residue was then boiled with 100% ethanol and filtered hot. After cooling the filtrate, a white solid was filtered off. The filtrate was
concentrated and a second crop of white solid was collected. The combined yields were dried in air at room temperature, giving 23 g (134 mmol, 45%) of 1. This procedure is the somewhat modified method of Houlton and Tartar [13]. 1 H-NMR, 300.17 MHz (D O, ref. tert-BuOH = 2 1.20 ppm): δ 5.91–5.76 (m, 1H, C4 –H), δ 5.10–4.98 (m, 2H, C5 –H), δ 2.86 (t, 2H, CH2 SO3 Na, J = 7.9 Hz), δ 2.14 (dt, 2H, C3 –H, J = 6.7 Hz), δ 1.78 (dt, 2H, C2 –H, J = 7.7 Hz). 13 C-NMR, 75.48 MHz (D O, ref. tert-BuOH = 2 31.2 ppm): δ 139.68 (C4 ), δ 116.99 (C5 ), δ 51.38 (C1 ), δ 33.27 (C3 ), δ 24.81 (C2 ). 2.2.2. Sodium 4-oxopentylsulfonate 2 Oxidation of 1 (10 mmol, 1.72 g) was carried out as described for other olefins, see below. In this particular case 8% oxygen in nitrogen was used as the oxidant. After reaction, water was removed under reduced pressure, giving 1.84 g (9.8 mmol, 98%) of 2 contaminated with catalyst. 1 H-NMR, 300.17 MHz (D O, ref. tert-BuOH = 2 1.20 ppm): δ 2.86 (t, 2H, CH2 SO3 Na, J = 7.6 Hz), δ 2.70 (t, 2H, C3 –H, J = 7.3 Hz), δ 2.17 (s, CH3 ), δ 1.91 (dt, 2H, C2 –H, J = 7.6 Hz). 13 C-NMR, 75.48 MHz (D O, ref. tert-BuOH = 2 31.2 ppm): δ 216.57 (C4 ), δ 51.44 (C1 –SO3 Na), δ 42.88 (C3 ), δ 31.89 (C5 ), δ 19.95 (C2 ). 2.3. Catalytic experiments The catalyst solutions were prepared by stirring Pd(OAc)2 (0.0224 g, 0.1 mmol) and PhenS* (0.0546 g, 0.1 mmol) overnight in water (42.5 ml) to give a clear yellow–orange solution. Standard catalytic experiments were carried out in a closed Hastelloy C autoclave (175 ml). The autoclave was cooled to ca. 0◦ C, charged with the catalyst solution (0.1 mmol catalyst in 42.5 ml water), olefin (10–20 mmol) and internal standard (n-heptane). The autoclave was pressurised with air and heated to 100◦ C (30 bar) for 10 h. After reaction the autoclave was cooled to 0◦ C and depressurised. Any volatile material was collected in a liquid nitrogen trap. The product mixture was extracted with Et2 O and the organic layer was dried over MgSO4 and analysed by GC. Recoveries were always 100 ± 2% with this procedure.
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437
Fig. 2. Reoxidation of Pd(0) with dioxygen.
The oxidation of 1 was followed by monitoring the oxygen uptake as a function of time. Samples were analysed by 1 H-NMR. 2.4. Apparatus 1 H-
and 13 C NMR spectra were recorded on a Bruker AC 300 spectrometer using tert-BuOH as an external reference. GC measurements were carried out with a Varian Star 3400 instrument equipped with a CP Sil 5-CB column (50 m × 0.53 mm). 3. Results and discussion The conversions of olefins to carbonyl compounds with palladium catalysts can be seen as the reduction of Pd2+ salts to zerovalent Pd(0) by these substrates. This probably is a critical stage in any catalytic process that involves a Pd2+ /Pd(0) redox couple, since clustering of atomic Pd(0) to give palladium black is a highly favourable and largely irreversible process [14]. Therefore, zerovalent Pd(0) has to be stabilised with regard to palladium black formation through interaction with solvents, acids or ligands. Direct re-oxidation of Pd(0) by dioxygen takes place when the zerovalent metal is stabilised in clusters in solvents such as ethylene carbonate [15] or DMSO [16–18]. In highly acidic, dry media, insertion of dioxygen into a palladium hydride [19] or protonation of palladium peroxide [20] to yield a palladium hydroperoxide will also re-oxidise zerovalent pal-
ladium (Fig. 2). However, formation of palladium hydroperoxide seems unlikely with the aqueous, neutral-to-basic reaction conditions used in our experiments. It is possible that the zerovalent palladium formed in our system is held by phenanthroline in a coagulated state that is small enough — possibly even monoatomic [21,22] — to allow for rapid re-oxidation to successfully compete with irreversible palladium black formation. This is analogous to the direct dioxygen oxidation of Pd(0) anchored on polyanilines [23] or polypyrroles [24], which have also been used as catalysts in Wacker-type reactions. Coordination of 2,20 -bipyridine or phenanthroline stabilises Pd2+ [25–27], resulting in a decrease of the redox potential of the Pd2+ /Pd(0) couple. This makes re-oxidation of Pd(0) more thermodynamically favourable and leaves Pd(0) less susceptible to clustering. Addition of chloride ions has a similar thermodynamic effect in the classical Wacker process, where Pd(0) is re-oxidised to PdCl4 2− instead of to PdCl2 [3]. Furthermore, it was proposed that chlorides kinetically accelerate Pd(0) re-oxidation either by keeping the surface area of Pd(0)n particles large for a facile re-oxidation and/or by participating in the transition state for Pd(0) re-oxidation [3]. Again, a comparison with bipyridine and phenanthroline ligands can be made, since complexation of these ligands to a metal may sterically hinder extensive clustering and accelerate electron transfer between a metal and its redox partner. This might also allow PhenS*Pd(0) less time for clustering.
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Fig. 3. Oxidation of 1-hexene as a function of time. Conditions: 0.1 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 10 or 20 mmol 1-hexene, 10 mmol NaOAc, p = 30 bar, air, t = 5–15 h, T = 100◦ C.
Fig. 4. The order in [olefin]. Calculated from: -d [4-pentenylsulfonate]/dt = -2d [O2 ]/dt. Conditions: 0.05 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 10 mmol 1, 10 mmol NaOAc, p = 10 bar, 8% O2 in N2 , T = 100◦ C.
3.1. Kinetics of olefin oxidation The water-soluble complex of PhenS*Pd(OAc)2 catalyses the aerobic oxidation of 1-hexene to 2-hexanone [8]. When the reaction is followed as a function of time using 10 mmol of substrate the rate is constant for 10 h until about 50% conversion is reached and then the rate decreases (Fig. 3). The decrease in rate after 10 h is not caused by the deterioration of the catalyst since when more 1-hexene is added the rate remains constant for a longer period of time. Up to 10 h the aqueous phase is saturated with olefin, which means that the concentration of the latter is constant and a pseudo-zero-order rate in [olefin] is observed. When about 5 mmol of 1-hexene is left the reaction rate becomes first order in [1-hexene]. Due to the biphasic nature of the reaction, it is difficult to take representative samples to follow the course of the reaction as a function of time. Hence, to determine the order in [olefin] we used a (completely) water-soluble olefin, sodium 4-pentenylsulfonate 1, as substrate and monitored oxygen uptake as a function of time. Calculation of the olefin conversion from these data revealed that the reaction is first order in [olefin] from t ∼ = 0, as shown in Fig. 4. The order in [palladium] was determined by reacting an excess of olefin (1-hexene) at various palladium concentrations. Fig. 5 shows that the order in [palladium] is 1/2. This is in agreement with the formation of a dimeric palladium species that is in equilib-
rium with a catalytically active palladium monomer (Fig. 6). The most likely candidate is a dimeric palladium–phenanthroline species with two bridging hydroxide anions [28,29]. This result contrasts with the first-order dependence on [palladium] found in the classical Wacker and related processes [30–32]. As previously reported [8], sodium acetate has a beneficial effect in stabilising the catalyst. This effect only became noticeable after the catalyst was recycled. Data with different amounts of NaOAc and NaOH are shown in Table 1. The results show that addition of sodium acetate or sodium hydroxide has no influence
Fig. 5. The order in [palladium] in 1-hexene oxidation. [Pd2 ] denotes the palladium dimer concentration. Experiments carried out with 0.025, 0.05, 0.066, 0.1 and 0.2 mmol PhenS*Pd(OAc)2 in 42.5 ml H2 O, 20 mmol 1-hexene, 10 mmol NaOAc, p = 30 bar, air, t = 5–15 h, T = 100◦ C.
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439
Fig. 6. The catalytic cycle proposed for olefin oxidation with aqueous soluble Pd(II) catalysts.
on the reaction rate. In summary, in our system the reaction rate for olefin oxidation follows the equation: rate ∼ = k [Pd2 ]1/2 [olefin] and the catalyst activity does not depend on pH or [acetate]. 3.2. Oxidation mechanism We propose the reaction mechanism shown in Fig. 6 to rationalise the observed results. In the first step a palladium dimer with two bridging hydroxy ligands is dissociated via coordination of the olefin, consistent Table 1 1-Hexene oxidation at various pH values and base concentrationsa NaOAc (mmol)
Cycle
pH (NaOH added)
TOFinitial (h−1 )
Selectivity (%)
0 0 0 2 5 10 10 10 0 0 10
1st 2nd 3rd 1st 1st 1st 2nd 3rd 1st 1st 1st
6.5 6.5 6.5 6.5 6.6 6.6 6.6 6.6 9 10 10
4.7 2.8 1.5 4.7 4.9 4.8 4.4 4.0 5.0 5.3 4.8
99 97 96 99+ 99+ 99+ 96 95 97 97 99+
a Conditions: 0.1 mmol PhenS*Pd(OAc) 2 in 42.5 ml H2 O, 20 mmol 1-hexene, p = 30 bar, air, t = 10 h, T = 100◦ C. (TOF in mmol olefin/mmol Pd/h; selectivity in mmol 2-hexanone/6 mmol of products).
with the observed 1/2 order in [palladium] and first order in [olefin]. This is followed by intra-molecular attack of hydroxide on the coordinated olefin, i.e. the reaction proceeds via cis-hydroxypalladation [30–32] (see later). The resulting -hydroxyalkyl palladium complex decomposes into the 2-alkanone and a zerovalent palladium species. The latter is re-oxidised with dioxygen, giving a palladium peroxide [33]. Coupling of this peroxide with one equivalent of zerovalent palladium–PhenS* regenerates the starting palladium dimer. 3.3. Comparison with the Wacker process: Hydroxypalladation mechanism The catalyst system described here shows a number of similarities to the Wacker process in terms of type of metal, solvent and type of product that is formed. Marked differences are observed, however, in the respective rate laws. In the Wacker process the rate ∼ = k [Pd2+ ] [olefin]/ [Cl− ]2 [H+ ] at low [Cl− ], while in our system the rate ∼ = k [Pd2 ]1/2 [olefin]. A second-order inhibition by chloride is observed in the Wacker process, since the active catalyst — PdCl4 2− — dissociates two equivalents of chloride, replacing them with one equivalent of olefin and one equivalent of water prior to the rate-limiting step. In contrast, such a displacement does not occur in our system.
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Fig. 7. Olefin oxidation by PdCl4 2− in the Wacker process according to Henry and Bäckvall.
The key step in the Wacker reaction is hydroxypalladation, for which two different mechanisms have been proposed (Fig. 7). Both provide an explanation for the observed kinetics of the Wacker process. Henry [30–32] proposed a mechanism involving rate limiting cis-hydroxypalladation. On the other hand, Bäckvall [34] favoured fast trans-hydroxypalladation followed by rate-limiting decomposition of a -hydroxyalkyl-palladium species. In the Henry mechanism the aqua-ligand dissociates a proton. This is followed by an intra-molecular attack of the resulting hydroxide ligand on the coordinated olefin. We propose that our system involves a similar mode of cis-hydroxypalladation. Outer sphere attack of a water molecule on the coordinated olefin, resulting in the release of a proton (Fig. 8), can be ruled out since the addition of base (NaOH or NaOAc) has no effect on the rate [34].
Henry also showed that coordination of chloride [35], (at high [Cl− ]), pyridine [36,37] or bidentate phosphine [38] instead of H2 O or OH− to palladium seriously hampers the reaction and gives mainly chloroethanol instead of acetaldehyde in the Wacker process. This emphasises the need for an aqua/hydroxy ligand coordinated to palladium, as in our system. Therefore, our findings indirectly support an intra-molecular cis-hydroxypalladation mechanism for the classical Wacker process. 3.4. Possible involvement of (giant) clusters Alternatively, our system could involve the formation of (giant) palladium clusters [9–11]. It is known that palladium–phenanthroline clusters are easily formed from Phen–Pd(OAc)2 in the presence of a reducing agent such as CO [39] or H2 [40] and
Fig. 8. Possible modes of attack on the coordinated olefin.
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Fig. 9. The formation of palladium clusters.
consecutive oxidation with dioxygen. In our case, the olefin might play a similar role as a reductant. However, a number of observations militate against this. Firstly, cluster formation would most likely liberate free phenanthroline (Fig. 9), which is not found after reaction. Secondly, no induction period that would be required to form a cluster of some sort from homogeneous PhenS*Pd(OAc)2 is detected (Fig. 3). Thirdly, the kinetic observations are not consistent with the formation of clusters. In cluster catalysis, the reaction might be expected to show Michalis–Menten kinetics [40]. It would be first order in [cluster] and consequently approximately first order in [palladium], supposing that all palladium atoms form clusters of comparable size, structure and reactivity [41]. Catalysis with giant palladium clusters would also require the presence of strong acids such as HClO4 , or H2 SO4 , and would not proceed in neutral to basic aqueous solutions [42]. Fourthly, in giant clusters ca.10% of the surface area is uncovered, leaving enough room only for small molecules, such as ethylene, propylene, CO or acetic acid to react [43,44]. This is not consistent with the observed facile oxidation of cyclooctene [8]. Fifthly, (giant) palladium clusters are expected to give rise to double bond isomerisation in ␣-olefins and allylic alcohols giving internal olefins and (saturated) aldehydes, respectively. Although this would be suppressed to a large degree in cluster catalysis through the addition of water, this phenomenon is completely absent in our system. Furthermore, allylic oxidation
is highly favoured over the formation of 2-alkanones in reactions catalysed by giant palladium clusters [45]. The oxidation of propylene in water, catalysed by a giant Pd561 cluster, for example, yielded ca.3% of acetone, while allylic oxidation products — allyl alcohol (38%), acrolein (14%) and acrylic acid (20%) — were predominant [42]. Finally, it could be argued that 1—hexene would reduce Pd2+ to give a (giant) cluster in the first cycle, which would then continue as the actual catalyst. However, no significant change in the catalyst solution was observed with UV-VIS or light scattering techniques. Moreover, we observed that our recycled aqueous catalyst solution still gave stoichiometric oxidation under anaerobic conditions [46]. It is well known that a giant cluster cannot carry out (stoichiometric) oxidations in the absence of dioxygen Fig. 10 [40,45]. By the same token the observation of stoichiometric oxidation of 1-hexene with PhenS*Pd(OAc)2 in an inert atmosphere rules out the possibility that the reaction proceeds via a Pd(IV)/Pd(II) cycle in which Pd(II) is first oxidised by dioxygen.
4. Conclusion The results of kinetic investigations of the aerobic oxidation of terminal olefins catalysed by water-soluble palladium–phenanthroline catalysts are consistent with the active catalyst being a mononu-
Fig. 10. Oxidation with homogeneous Pd2+ vs. cluster Pdn+ in stoichiometric reactions with olefins.
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