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Aerobic oxidation of naturally occurring α-bisabolol catalyzed by palladium(II) salts as sole catalysts
Applied Catalysis A: General 524 (2016) 126–133
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Aerobic oxidation of naturally occurring ␣-bisabolol catalyzed by palladium(II) salts as sole catalysts Luciana A. Parreira a,b , Débora C. Gonc¸alves c , Luciano Menini c , Elena V. Gusevskaya a,∗ a b c
Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil Departamento de Química e Física, Campus de Alegre—CCA, Universidade Federal do Espírito Santo, 29500-000 Caixa Postal 16 Alegre, ES, Brazil Campus de Alegre Instituto Federal de Educac¸ão, Ciência e Tecnologia do Espírito Santo, 29500-000 Caixa Postal 47 Alegre, ES, Brazil
a r t i c l e
i n f o
Article history: Received 11 May 2016 Received in revised form 16 June 2016 Accepted 18 June 2016 Available online 23 June 2016 Keywords: Bio-renewables ␣-Bisabolol Oxidation Oxygen Palladium Terpenoids
a b s t r a c t ␣-Bisabolol, a sesquiterpene found in various essential oils, has numerous direct commercial uses in cosmetic and therapeutic formulations. In the present work, we report two novel liquid-phase processes for the catalytic oxidation of ␣-bisabolol with dioxygen, in which palladium salts (PdCl2 or Pd(OAc)2 ) are employed as sole catalysts. The addition of co-catalysts conventionally used for the re-oxidation of reduced palladium species or special ligands for their stabilization is not required. The reactions occur in non-acidic solvents, i.e., aqueous methanol or dimethylacetamide, and give exclusively the products derived from the interaction of the acyclic olefinic bond with palladium, whereas the other olefinic bond remains intact. The method allows to perform the green and simple synthesis of poly-functionalized compounds potentially useful for fine chemical industry as fragrance ingredients and synthetic intermediates, starting from a bio-renewable substrate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Terpenes are natural compounds present in a variety of essential oils, often as main components. They are widely used as ingredients and raw materials in flavor&fragrance and pharmaceutical industries due to specific olfactory characteristics and interesting therapeutic properties [1,2]. In particular, a sesquiterpenic alcohol ␣-bisabolol occurs in high concentrations in chamomile, candeia and sage essential oils (up to 50, 85 and 90%, respectively) [3]. As natural sources do not currently satisfy a high commercial demand for ␣-bisabolol, it is also produced synthetically from other sesquiterpenic alcohols farnesol and nerolidol [4–7]. ␣-Bisabolol is a component in over 1000 cosmetic, fragrance and therapeutic formulations [3,8]. Despite of such important and numerous direct uses, the catalytic oxidation of ␣-bisabolol, which contains two olefinic bonds, can open new perspectives for the applications of this compound. It has been reported that ␣-bisabolol derivatives, both natural and synthetic, also show an important therapeutic potential presenting, for example, anti-inflammatory and anti-tumoral activities [5,9,10].
∗ Corresponding author. E-mail address: [email protected] (E.V. Gusevskaya). http://dx.doi.org/10.1016/j.apcata.2016.06.027 0926-860X/© 2016 Elsevier B.V. All rights reserved.
The palladium catalyzed oxidation of olefins is a convenient and widely used in organic synthesis route to various oxygenated compounds, in particular, for the industrial transformation of ethylene into acetaldehyde (the Wacker process) [11–13]. The attractive feature of these reactions from economic and environmental viewpoints is the use of molecular oxygen as the terminal oxidant. On the other hand, the disadvantage of most of the conventional palladium-based catalytic systems for aerobic oxidations is a requirement for auxiliary co-catalysts. The role of these co-catalysts is to act as reversible co-oxidants capable to re-oxidize reduced palladium species before their irreversible agglomeration in a bulk metal and to be re-oxidized back by molecular oxygen. CuCl2 is the most commonly used co-catalyst in these systems (Wacker catalyst); however, corrosion and selectivity issues often become a problem due to the high concentrations of chloride ions and acidity of reaction solutions. Heteropoly compounds, nitrates and p-benzoquinone (BQ) have been studied among other compounds in palladium-based systems to replace CuCl2 and create more benign catalytic systems which do not contain chloride ions [14–21]. Furthermore, in the past decade several examples of the aerobic oxidation of olefins catalyzed by palladium(II) complexes in the absence of auxiliary co-oxidants have been published [22–33]. In these systems, the regeneration of palladium was performed directly by dioxygen through the sta-
L.A. Parreira et al. / Applied Catalysis A: General 524 (2016) 126–133
1.90-2.10, 2H, m 30.87 30.81
1.64, s 23.23
6
1
7
1.60-1.70, 1H, m 1.15-1.30, 1H, m 23.96 23.23
5
134.06 133.73
9
4 2
5.36, br.s 120.35 120.52
3
OH
14
13
12
74.05 74.62
1.55-1.65, m 43.08 44.15
1.75-1.85, 1H, m 1.90-2.10, 1H, m 26.82 26.40
1.31, s 29.71
1.08, s 22.87 22.88
8
127
OH 70.51 70.59
5.66-5.72, m 142.19 140.10
11
10 2.20, br.d 3 J = 7.2 Hz 42.83 46.58
5.66-5.72, m 121.84 123.50
15 1.31, s 29.71
Fig. 1. The attribution of the NMR signals for product 3 (two isomers).
1.90-2.00, m 31.09; 31.13 1.63 s; 1.64 s 23.41; 23.46
7
1.25-1.35, 1H, m 1.65-1.85, 1H, m 23.31; 24.03
6
9
1.24 s; 27.98 1.12 s; 22.75
4
134.15 133.48
2 5.36, br.s 120.85 121.31
O
5
1 3
1.27 s 25.49; 29.75
14
8
10
1.80-1.90, 1.90-2.00, 35.94 1.70-1.80, 2.05-2.15, 31.70
15
70.03 74.38
1.21; 1.24 s 30.99; 31.07
12
75.49 74.38
1.40-1.50, m; 45.58 1.65-1.75, m; 42.96
1.70-1.90, 1H, m 1.95-2.15, 1H, m 26.12; 26.78
13
5.59 d, 3J=10.4, 131.52 5.67, br.s; 134.38
11 1H, m 1H, m
5.80-5.85. m; 122.78 5.67, br.s; 120.46
1H, m 1H, m
Fig. 2. The attribution of the NMR signals for product 5 (two isomers).
bilization of palladium(0) species in solutions by special ligands or by coordinating solvents. Within our current project aiming to add value to natural essential oils by catalytic reactions, we have studied the Pd(II) catalyzed oxidation of various naturally occurring alkenes and alkenyl alcohols, i.e., nerolidol ␣-terpineol and linalool [17–21,27,28]. Our particular interest is currently directed to the oxidation of ␣-bisabolol as a route to interesting poly-functionalized sesquiterpenic compounds, which would by hardly accessible by other synthetic methods [34,35]. Before starting the project, we could find very limited information on the catalytic oxidation of ␣-bisabolol [36,37]. Both these publications reported the processes in which hydrogen peroxide or other peroxo derivatives were used as stoichiometric oxidants. As far as we know, two our recently published works [34,35] are the only available reports on the application of catalytic chemistry of palladium to the oxidation of ␣-bisabolol. As the attempts to apply the conventional Wacker catalyst (PdCl2 /CuCl2 ) to ␣-bisabolol were not encouraging in terms of reaction selectivity, we first tried to find suitable co-catalysts which could be an alternative to cupper chloride. The catalytic combinations of palladium(II) acetate with either BQ [34] or cupper(II) acetate [35] allowed to perform the aerobic oxidation of ␣-bisabolol in acetic acid or aqueous methanol media, respectively,
under chloride free conditions to obtain several novel oxygenated sesquiterpenoid compounds. Further, we have started a search for the systems which do not require auxiliary co-catalysts at all. Herein, we present novel processes for the palladium catalyzed direct-dioxygen-coupled oxidation of ␣-bisabolol. The reactions occur under mild nonacidic conditions and involve molecular oxygen as the stoichiometric oxidant and palladium salts (PdCl2 or Pd(OAc)2 ) as the sole catalysts. Oxygenated poly-functionalized compounds derived from bio-renewable ␣-bisabolol are potentially useful in fine chemical industry as fragrance ingredients and synthetic intermediates. 2. Experimental Natural ␣-(−)-bisabolol originated from the candeia (Eremanthus erythropappus) extract was received from CITRÓLEO Ind. Com. Óleos Essenciais as a kind donation. Palladium (II) acetate (>99.9%) was purchased from Aldrich and palladium (II) chloride (99.9%) from Strem. The copper content in both palladium salts determined by atomic absorption spectroscopy was less than 0.005%. Lithium chloride and copper (II) acetate were heated for dehydration. The catalytic tests were performed in a stainless steel autoclave (total volume of 100 mL) with magnetic stirring. The reaction progress was monitored by gas chromatography (GC) be
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Fig. 3. The attribution of the NMR signals for product 6.
Table 1 Oxidation of ␣-bisabolol with dioxygen in methanol.a Run
Extra water (vol%)
T (◦ C)
P (atm)
Time (h)
Conversion (%)
c
8 8 8 15 4 8 8 8 8
80 80 60 60 60 60 60 60 60
10 10 5 10 10 10 10 10 10
5 47 37 7 10 7 10 9 10 2 9
87 55 57 27 32 47 51 48 50 92 83 100 50
2 3 4 5 6 7c 8d 9e
TOFb (h−1 )
Selectivity (%) 2
3
4
63 45 48 43 45 32 33 46 45 48 81 – tr.
8 67 38 37 52 50 27 26 37 7 – tr.
10 99 9 10 10 12 9 12 5 10 – tr.
4.8 4.4 1.8 2.2 2.6 2.3 2.0 18.0 2.0
a Conditions: ␣-bisabolol (0.20 M), [Pd(OAc)2 ] (0.01 M); gas phase – O2 , total volume 12 mL. Conversion and selectivity were obtained from GC data and referred to the amounts of the reacted substrate; tr. – trace amounts. b Average rate (turnover frequency) of the substrate conversion per mol of palladium for the first reaction hour. c Cu(OAc)2 added (0.05 M). d Pd(OAc)2 was replaced by PdCl2 . e Pd(OAc)2 was replaced by Pd(OCOCF3 )2 .
OH
OH
Pd(OAc)2 (cat)
2 (R=CH 3) 3 (R=H)
OR
(up to 85%)
O2, CH3OH 1
O
4
(~10%)
Scheme 1. Oxidation of ␣-bisabolol in methanol.
periodical sampling. Typically, ␣-(−)-bisabolol, palladium catalyst, water and bornyl acetate (used as a GC standard, 0.10 M) were dissolved in indicated concentrations in a specified solvent (12 mL total volume). The solution was put in the autoclave and the indicated pressure was attained with dioxygen as the gas phase. An oil bath was used to heat the reactor and to maintain the reaction temperature. The samples of the reaction solutions for the GC analysis were taken using a special device maintaining the pressure inside the reactor. GC analyses were performed on a Shimadzu GC-2010 Plus chromatograph using a Rtx-Wax 30 m or Rtx-5MS 30 m capillary column and a flame ionization detector. Conversions and selectivities were calculated using as a reference the amount
of the substrate reacted. Average turnover frequencies, TOF, were measured for the first reaction hour (in most cases up to 20–40% conversions). The identification of the products was performed by 1 H and 13 C NMR and GC–MS spectroscopy after their isolation from the reaction solutions by a column chromatography (silica gel 60); hexane and dichloromethane and their solutions were used as eluents. NMR experiments were conducted on a Bruker 400 MHz instrument (CDCl3 as the solvent, TMS as the internal standard). Mass spectra were acquired on a Shimadzu QP2010-PLUS equipment (70 eV).
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Table 2 Oxidation of ␣-bisabolol with dioxygen in dimethylacetamide.a Run
1 2 3c 4 5 6d 7d 8 9 10 11 12e
Added water (vol%)
15 15 15 15 15 15 15 15 15 8 0 15
[PdCl2 ] (mM)
10 none none 20 5 10 10 10 10 10 10 10
T (◦ C)
60 60 60 60 60 60 80 40 30 60 60 60
Time (h)
2 4 4 1 6 4 1 6 11 10 4 13
Conversion (%)
95 0 0 90 57 93 94 86 70 95 45 70
TOFb (h−1 )
Selectivity (%) 3
4
5
6
13 – – 16 13 10 10 10 10 11 tr. –
2 – – 7 8 2 tr. 11 6 3 3 20
35 – – 30 38 35 27 25 19 25 14 tr.
36 – – 37 22 34 21 40 51 30 24 tr.
11.0 – – 9.0 9.6 6.0 18.8 8.0 4.8 9.0 4.0 1.0
a Conditions: ␣-bisabolol (0.20 M), [Pd(OAc)2 ] (0.01 M); 10 atm of O2 , total volume 12 mL. Conversion and selectivity were obtained from GC data and referred to the amounts of the reacted substrate; tr. – trace amounts. b Average rate (turnover frequency) of the substrate conversion per mol of palladium for the first reaction hour. c LiCl was added (0.02 M). d 5 atm. e PdCl2 was replaced by Pd(OAc)2 .
Product 2 (isomers 2a and 2b) (first description was published in [35]): 2a: MS (70 eV, EI): m/z (%): 202 (1) [M+ −H2 O−CH3 OH)], 139 (32), 125 (9), 121 (18), 95 (46), 82 (100), 71 (18%), 67 (32); 2b (longer GC retention time): MS (70 eV, EI): m/z (%): 202 (1) [M+ −H2 O−CH3 OH)], 125 (4), 95 (4), 73 (100), 67 (3). For NMR data see [35]. Product 3 (novel compound as far as we know, isomers 3a and 3b): 3a: MS (70 eV, EI): m/z (%): 220 (1) [M+ −H2 O], 202 (1) [M+ −H2 O−H2 O)], 147 (15), 139 (30), 125 (31), 121 (30), 99 (38), 95 (69), 85 (61), 82 (100), 67 (46), 59 (61). 3b: MS (70 eV, EI): m/z (%): 220 (1) [M+ −H2 O], 202 (1) [M+ −H2 O−H2 O)], 147 (22), 125 (31), 95 (36), 94 (51), 85 (100), 82 (25), 79 (37), 67 (26), 59 (90), 43 (94). The attribution of NMR signals is shown in Fig. 1. Product 4 (first description was published in [29]): MS (70 eV, EI): m/z (%): 220 (1) [M+ ], 187 (1) [M+ −H2 O−CH3 ], 132 (18), 125 (100), 107 (61), 95 (14), 93 (13), 67 (18). For NMR data see [34]. Product 5 (novel compound as far as we know): MS (70 eV, EI): m/z (%): 220 (2) [M+ ], 205 (3) [M+ −CH3 ], 187 (2) [M+ −CH3 −H2 O], 125 (100), 107 (34), 83 (12), 43 (46). Product 5 showed only one peak on chromatograms; however, the NMA analysis revealed the existence of two isomers with very similar spectra. The attribution of NMR signals is shown in Fig. 2. Product 6 (known as norbisabolide): MS (70 eV, EI): m/z (%): 194 (28) [M+ ], 179 (4) [M+ −CH3 ], 161 (8) [M+ −CH3 −H2 O], 134 (22), 121 (64), 119 (22), 99 (100), 93 (78), 81 (19), 79 (19), 71 (28), 67 (22), 42 (49). The attribution of NMR signals is shown in Fig. 3. 3. Results and discussion 3.1. Oxidation of ˛-bisabolol in methanol solutions The data on the oxidation of ␣-bisabolol (1) by dioxygen in aqueous methanol solutions containing Pd(OAc)2 in catalytic amounts are collected in Table 1. In our previous work [35], Cu(OAc)2 was used as the co-catalyst together with Pd(OAc)2 for the oxidation of ␣-bisabolol. The main reaction products were compounds 2 (up to 85% selectivity) and 4 (ca. 10% selectivity) (Scheme 1). Both products were novel bisabolane derivatives formed due to the oxidation of the acyclic olefinic bond of the substrate. The example of the reaction in the system containing Cu(OAc)2 is presented in Table 1 (run 1). The oxidation of ␣-bisabolol in the bimetallic system was accelerated by the increase in the palladium concentration; however, it was (quite unexpectedly) only slightly dependent on the concen-
tration of copper. Encouraged by this observation, we performed the process in the absence of copper acetate. Despite the decrease in selectivity for product 2 and reaction stagnation at ca. 50% conversion, the reaction was still catalytic with respect to palladium and showed the dioxygen-coupled turnover number (TON) of nearly 10 (Table 1, run 2). No formation of the palladium mirror on the walls of the reactor was detected at the end of the reaction. Varying reaction parameters (Table 1, runs 3–6), we found that the reaction could be nearly completed in 9 h at 60 ◦ C and 10 atm to give methyl ether 2 and corresponding diol 3 in 85% combined selectivity and comparable amounts (Table 1, run 6). Heterocyclic compound 4 was detected in these runs in small amounts (5–10%). All three products were formed due to the oxidation of the acyclic C C bond with the participation as nucleophiles of external methanol or water molecules to give products 2 and 3, respectively, or the internal tethered hydroxyl group to give cyclization product 4. In addition to the products specified in Table 1, bisabolol oxide B (identified by MS) was formed in most of the runs in small amounts. Diol 3, a novel compound to the best of our knowledge, was separated after the reaction as a mixture of two isomers 3a and 3b. The NMR spectra of these isomers are very similar (Fig. 1, Experimental Section). Both 3a and 3b seem to have a trans configuration at the acyclic double bond as the hydrogens at C-10 and C-12 correlated in the NOE spectrum and the chemical shift values for allylic carbons C-13 and C-10 in both isomers were close. We suppose that the structural difference between the two isomers of diol 3 is related to the configuration at carbon C-8, similarly to what we suggested for the isomers of ether 2 [35]. Compound 4 was reported for the first time in our previous work as a major oxidation product formed from ␣-bisabolol in the acetic acid solutions containing palladium acetate and BQ in catalytic amounts [34]. It is surprising that the reactions with and without Cu(OAc)2 occurred at similar rates and with similar selectivities for diol products 2 and 3 and for cyclization product 4 (Table 1, run 7 vs. run 6). However, differently from “palladium solo” reactions, in the bimetallic system methyl ether 2 was the main diol product, whereas diol 3 itself was detected only in small amounts (Table 1, runs 1 and 7). It can be suggested that Lewis acidity of copper ions favored the etherification of diol 3 with methanol to give ether 2. However, the second hydroxyl group in diol 3 did not undergo etherification in these systems. The more plausible explanation looks a suggestion that the presence of copper ions increases the relative contribution of methanol as the nucleohile at the oxidation
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O
5 (up to 40%) PdCl2 (cat) OH O
O 2, DMA 1
O
6 (up to 50%) OH
OH
3 (10-15%) Scheme 2. Oxidation of ␣-bisabolol in dimethylformamide.
step (to give ether 2) as compared to water (to give diol 3), possibly, due to the formation of copper methoxy complexes. In blank reactions without Pd(OAc)2 , none of oxidation products 2, 3 and 4 was expectedly detected in reaction solutions. The decrease in the oxygen pressure did not affect significantly the product distribution and initial turnover frequency (TOF) (Table 1, run 3 vs run 6). However, the reaction at 5 atm turned stagnated at ca. 30% conversion with the formation of inactive palladium metal. Therefore, to ensure the effective re-oxidation of palladium the reaction should be performed under higher oxygen pressure. It is remarkable that the reaction with ␣-bisabolol proceeds in aqueous methanol solutions with dioxygen-coupled turnovers without the use of auxiliary co-oxidants or special auxiliary ligands for the stabilization of reduced palladium species. In this context, it is worthwhile to cite the example of the related “palladium solo” reaction which we have found in the literature. As early as in 1978, Hosokawa et al. described the oxidative cyclization of allylphenol substrates with molecular oxygen catalyzed by palladium(II) acetate alone in the solutions of aqueous methanol [38]. We suppose that palladium(0) species in these systems are able to form relatively stable complexes with the solvent and/or with the substrate which decelerates their aggregation. In other words, the coordination enables the reoxidation of palladium(0) species with dioxygen to occur faster than palladium deactivation due to clustering into inactive bulk metal. The nature of the olefinic substrate is very important for the possibility to run the process without Cu(OAc)2 , i.e., to perform a “palladium solo” reaction. Although the related substrate 3,7dimethyl-1,6-octadien-3-ol (linalool) did react with dioxygen in the presence of the bimetallic Pd(OAc)2 + Cu(OAc)2 catalytic combination [19], the attempts to perform the reaction with Pd(OAc)2 alone resulted in the appearance of the palladium mirror on the walls of the reactor (not shown in the table). Palladium black was also formed at the attempt to oxidize another monoterpenic acyclic alcohol, 3,7-dimethyl-2,6-octadien-1-ol (nerol), in the absence of Cu(OAc)2 . All three substrates: linalool, nerol, and ␣-bisabolol, contain in their molecules the same fragment with a trisubstituted acyclic double bond and namely this bond is oxidized in our reactions. It seems that the presence of a six-membered unsaturated ring in the ␣-bisabolol molecule plays a key role in its capacity to stabilize the reduced palladium species in the course of the catalytic process thus allowing to maintain the catalytic cycle without the need for auxiliary co-catalysts.
The nature of anionic ligands on palladium is also remarkably important for the successful operation of palladium as the sole catalyst in the oxidation of ␣-bisabolol. The reactions with the catalysts generated from PdCl2 and Pd(OCOCF3 )2 showed very poor selectivities for products 2, 3 and 4, with numerous unidentified peaks being detected by GC (Table 1, runs 8 and 9). 3.2. Oxidation of ˛-bisabolol in dimethylacetamide solutions In 2006, Kaneda and co-workers disclosed that PdCl2 catalyzed the oxidation of some terminal [39] and internal [26] alkenes with molecular oxygen in dimethylacetamide (DMA) solutions without the addition of co-oxidants or special ligands. Latter, we have extended the method to the oxidation of styrene [27] and naturally occurring allyl benzenes [28], the substrates containing terminal olefinic bonds. It has been suggested that DMA as a coordinating solvent stabilizes palladium(0) species thus allowing to perform direct-dioxygen-coupled catalytic cycles. Based on these results, we decided to apply the approach to the oxidation of ␣-bisabolol using dioxygen as the sole oxidant (Table 2). The reactions were performed in aqueous DMA solutions containing catalytic amounts of PdCl2 and, in most of the runs, 10 atm of oxygen to ensure the efficient capture of the reduced palladium species. Compounds 5 and 6 were detected as major products in ca. 70% combined selectivity along with smaller amounts of diol 3 (10–15%) and tetrahydrofuran product 4 (5–10%) (Scheme 2). Compounds 5 and 6 were isolated from the reaction solutions and identified by GC–MS and NMR spectroscopy. Product 5, a novel compound to the best of our knowledge, was observed by NMR as two not GC separable isomers with very similar spectra (Fig. 2, Experimental Section). As natural ␣-(−)-bisabolol has R configurations at carbons 4 and 8, we suppose that the partial inversion of the configuration occurs at C-8, which is linked to the involved in the catalytic transformation OH group, so that the two isomers of 5 differ from each other by geometry at C-8. Product 6 has been identified as a ␥-butyrolactone compound shown in Scheme 2, which is known as norbisabolide, a natural compound extracted from Atalantia Monophylla citric plant [35] (NMR attributions are shown in Fig. 3), Experimental Section. Compound 6 was described by Solabannavar et al. [40]. All identified products detected at the oxidation of ␣-bisabolol in DMA solutions were originated by the interaction of the acyclic olefinic bond with palladium. Products 3, 4 and 5 keep the carbon skeleton of the original molecule, whereas the formation of
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131
Graphical abstract.doc revised manuscript.doc Scheme 3. Suggested mechanism for the formation of products 2, 3, 4 and 5.
ketone 6 involves its oxidative cleavage. The endocyclic double bond remained intact in all these products. ␣-Bisabolol readily reacted with PdCl2 in aqueous DMA solutions (15 vol% of water). A nearly complete conversion occurred in a two-hour reaction at 60 ◦ C to give compounds 5 and 6 in ca. 35% selectivity each (Table 2, run 1). The reaction was catalytic in palladium showing TON of ca. 20. Thus, DMA efficiently stabilizes the palladium(0) species so that their reaction with dioxygen occurs faster than the precipitation of palladium black. In blank reactions, with no PdCl2 or with LiCl instead of PdCl2 , the conversion of ␣-bisabolol was not observed (Table 2, runs 2 and 3). It is important, that no formation of even trace amounts of compound 6, which might be the product of the non-catalytic oxidative cleavage of the ␣-bisabolol molecule, was detected in these runs. The reaction rate was found to be roughly proportional to the PdCl2 amounts as initial TOF values in the experiments with different PdCl2 concentrations were similar (Table 2, runs 1, 4 and 5). With 10 mol% of PdCl2, the reaction was nearly completed within 1 h to give products 3, 4, 5 and 6 with 90% combined selectivity. Bisabolol oxide B was mainly responsible for the rest of the mass balance in this system also. The reaction could be accelerated by the increase in the oxygen pressure (Table 2, cf. runs 1 and 6). Under 5 atm, a nearly complete conversion occurred in 4 h with no formation of the palladium mirror. The changes in the oxygen pressure and catalyst amounts did not affect significantly a product distribution. The positive effects of palladium and oxygen on reaction kinetics suggest that the rate determining step of the whole process can be the regeneration of palladium by dioxygen. It should be mentioned that in the reaction under atmospheric pressure, the precipitation of palladium metal was observed. The reaction was much faster at 80 ◦ C being nearly completed for 1 h vs. 4 h at 60 ◦ C; however, selectivity was lower, especially for product 6 (Table 2, run 7 vs. run 6). The tendency was confirmed in another set of the run (Table 2, runs 1, 8 and 9): at lower temperature the contribution of the carbon–carbon bond cleavage to give
product 6 became higher at the expense of product 5. However, the combined selectivity for products 5 and 6 remained nearly constant (ca. 70%). The process can be performed at nearly ambient temperature. Although the reaction rate became lower, the selectivity of 51% obtained for norbisabolide (product 6) was the best value we could achieve so far (Table 2, run 9). At lower water concentrations, the reaction was slower and less selective (Table 2, cf. runs 1, 10 and 11). The accelerating effect of water was also reported for the Pd/(−)-sparteine [41] and PdCl2 [27,28] catalyzed oxidation of terminal olefins with molecular oxygen in DMA solutions. However, water concentrations higher than 15 vol% can not be recommended due to miscibility problems.
3.3. Reaction mechanism The acyclic olefinic bond of ␣-bisabolol was the only one oxidized in “palladium solo” reactions in both methanol and DMA solutions. No products derived from the oxidation of the endocyclic olefinic bond were registered in detectable amounts. In this context, it seems remarkable the difference between these systems and the Pd(OAc)2 /BQ system described in our previous work [34], which promoted the oxidation of both olefinic bonds in ␣-bisabolol. The mechanism proposed for the formation of products 2, 3, 4 and 5 is shown in Scheme 3. As it is generally accepted that the palladium promoted oxidations of alkenyl alcohols involve an oxypalladation step [4–45], we suggest such interaction as a key reaction feature in our process. The Markovnikov oxypalladation of the acyclic double bond leads to products 2, 3 and 5. In this case, the nucleophile attacks a trisubstituted vinylic carbon atom. The antiMarkovnikov oxypalladation leads to product 4. The Markovnikov type interaction is a more favorable alternative because palladium in corresponding -alkyl palladium intermediates is linked to a sterically more accessible carbon atom. Probably for this reason, compound 4 was detected only as the minor product in all the runs. -Alkyl palladium intermediates A (the precursors of diol
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OH
1
[Pd]
OH
[Pd]
OH OH
O
O
hydroxyaldehyde
hydroxycarboxylic acid - H2O
O hemiacetal
[Pd]
O
OH - 2[H]
O
6
Scheme 4. Suggested mechanism for the formation of ␥-butyrolactone 6.
3 and ether 2) are formed through the nucleophilic attack by the external molecule of water or methanol. The participation of the internal hydroxyl group at the oxypalladation step leads to the molecule cyclization. The hydroxyl group can attack both vinylic carbons coordinated on palladium to form either a tetrahydrofuran or tetrahydropyran ring (intermediates B and C, respectively). Finally, the abstraction of -hydrogens in -alkyl intermediates A, B and C results in the formation of products 2 (or 3), 4 and 5. It is interesting that the direction of the ␣-bisabolol cyclization (furan vs. pyran products) is affected by the nature of anionic groups on palladium. The reactions with Pd(OAc)2 (see Table 1 and our previous works [34,35]) give preferably five-membered product 4, which is the result of the anti-Markovnikov oxypalladation. On the other hand, in the system with PdCl2 the major cyclization product is six-membered compound 5, which arises from the Markovnikov oxypalladation. Moreover, when PdCl2 was substituted by Pd(OAc)2 , not only the reaction became slower and less selective, but also the cyclization selectivity was switched almost completely from six-membered compound 5 to five-membered product 4 (Table 2, run 12 vs. run 1). ␥-Butyrolactone 6 is the product of the oxidative cleavage of the acyclic double bond, the reaction much less common in the catalytic chemistry of palladium than transformations through the oxypalladation step shown in Scheme 3. H. Jiang et al. have recently reported that palladium acetate catalyzes the oxidative cleavage of multiple C C bonds in alkenes [46] and alkynes [47] by molecular oxygen to give carbonyl and carboxyl products (aldehydes, ketones or carboxylic acid derivatives). Based on these literature data we are suggesting two possible routes to product 6 from ␣bisabolol (Scheme 4). The oxidative cleavage of the olefinic bond could give a hydroxyaldehyde, which then undergoes a spontaneous intramolecular cyclization resulting in a five-membered hemiacetal. The subsequent palladium catalyzed dehydrogenation of the hemiacetal would give ␥-lactone 6. Alternatively, the product of the oxidative cleavage of ␣-bisabolol could be a hydroxycarboxylic acid, whose spontaneous cyclization would give directly product 6. 4. Conclusion In summary, we have developed two novel liquid-phase processes for the oxidation of ␣-bisabolol in aqueous methanol or dimethylacetamide solutions using molecular oxygen as the stoichiometric oxidant. The reactions are catalyzed solely by the commercially available palladium salts, Pd(OAc)2 or PdCl2 . ␣-
Bisabolol, a renewable sesquiterpene found in essential oils of various plants, contains two olefinic bonds, both trisubstituted. All reaction products arise exclusively from the oxidation of the acyclic olefinic bond. Although the endocyclic double bond is usually involved in palladium catalyzed reactions with more facility, it remains intact. In general, internal acyclic olefinic bonds in non-functionalized alkenes are very reluctant to Wacker-type oxidations, with only few examples being reported so far [26]. The success in the oxidation of ␣-bisabolol was probably enabled by the existence of the hydroxyl group in a ␥-position to the acyclic double bond, which could favour the interaction of th double bond with palladium. These simple catalytic reactions represent environmentally and commercially interesting routes to poly-functionalized products potentially useful in fine chemical industry as fragrance ingredients and synthetic intermediates. The use of the renewable biomass-based substrate, low-cost nonacidic non-corrosive solvents, palladium salts as the sole catalysts and dioxygen as the stoichiometric oxidant is an advantage particularly important for the green chemistry concept. Acknowledgments The authors thank the Citróleo Ind. Com. Óleos Essenciais for the donating of ␣-(−)-bisabolol, Dr. Patrícia Fontes Pinheiro (Universidade Federal do Espírito Santo) for the help in contacting the Citróleo industry and the CNPq, FAPEMIG, FAPES and INCT Catálise (Brazil) for funding this work. References [1] C. Sell (Ed.), The Chemistry of Fragrances: from Perfumer to Consumer, vol. 2, second ed., RSC Publishing, Dorset, UK, 2006, pp. 52–88. [2] H. Mimoun, Chimia 50 (1996) 620–625. [3] G.P.P. Kamatou, A.M. Viljoen, J. Am. Oil Chem. Soc. 87 (2010) 1–7. [4] O. Taglialatela-Scafati, F. Pollastro, L. Cicione, G. Chianese, M.L. Bellido, E. ¨ Z. Toker, G. Appendino, J. Nat. Prod. 75 (2012) 453–458. Munoz, H.C. Ozen, [5] K. Massonne, K.P. Pfaff, J. Schubert, G. Gottwald (BASF), DE 102005053329 (2007). [6] K. Massonne, K.P. Pfaff, J. Schubert, G. Gottwald (BASF), DE 102005053338 (2007). [7] A.L.P. de Meireles, M. dos Santos Costa, K.A. da Silva Rocha, E.V. Gusevskaya, Appl. Catal. A 502 (2015) 271–275. [8] K. Russell, S.E. Jacob, Dermatitis 21 (2010) 57–58. [9] M. Piochon, J. Legault, C. Gauthier, A. Pichette, Phytochemistry 70 (2009) 228–236. [10] A.P. da Silva, M.V. Martini, C.M.A. de Oliveira, S. Cunha, J.E. de Carvalho, A.L.T.G. Ruiz, C.C. da Silva, Eur. J. Med. Chem. 45 (2010) 2987–2993. [11] W. Wu, H. Jiang, Acc. Chem. Res. 45 (2012) 1736–1748. [12] B.V. Popp, S.S. Stahl, Top. Organomet. Chem. 22 (2007) 149–189. [13] J.A. Keith, P.M. Henry, Angew. Chem. Int. Ed. 48 (2009) 9038–9049. [14] A. Heumann, K.J. Jens, M. Réglier, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 42, Wiley, New York, 1994, pp. 542–576. [15] T. Yokota, A. Sakakura, M. Tani, S. Sakaguchi, Y. Ishii, Tetrahedron Lett. 43 (2002) 8887–8891. [16] J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed. 47 (2008) 3506–3523. [17] J.A. Gonc¸alves, E.V. Gusevskaya, Appl. Catal. A 258 (2004) 93–98. [18] J.A. Gonc¸alves, O.W. Howarth, E.V. Gusevskaya, J. Mol. Catal. A 185 (2002) 95–104. [19] M.G. Speziali, P.A. Robles-Dutenhefner, E.V. Gusevskaya, Organometallics 26 (2007) 4003–4009. [20] M.G. Speziali, V.V. Costa, P.A. Robles-Dutenhefner, E.V. Gusevskaya, Organometallics 28 (2009) 3186–3192. [21] A.C. Bueno, Á.O. de Souza, E.V. Gusevskaya, ChemCatChem 4 (2012) 1382–1388. [22] C.N. Cornell, M.S. Sigman, Inorg. Chem. 46 (2007) 1903–1909. [23] S.S. Stahl, Science 309 (2005) 1824–1826. [24] J. Muzart, Chem. Asian J. 1 (2006) 508–515. [25] K.M. Gligorich, M.S. Sigman, Angew. Chem. Int. Ed. 45 (2006) 6612–6615. [26] T. Mitsudome, K. Mizumoto, T. Mizugaki, K. Jitsukawa, K. Kaneda, Angew. Chem. Int. Ed. 49 (2010) 1238–1240. [27] A.C. Bueno, Á.O. de Souza, E.V. Gusevskaya, Adv. Synth. Catal. 351 (2009) 2491–2495. [28] L.A. Parreira, L. Menini, J.C. da Cruz Santos, E.V. Gusevskaya, Adv. Synth. Catal. 352 (2010) 1533–1538. [29] P.B. White, J.N. Jaworski, G.H. Zhu, S.S. Stahl, ACS Catal. 6 (2016) 3340–3348. [30] M.S. Sigman, E.W. Werner, Acc. Chem. Res. 45 (2012) 874–884. [31] A.B. Weinstein, S.S. Stahl, Angew. Chem. Int. Ed. 51 (2012) 11505–11509.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Aerobic oxidation of naturally occurring ␣-bisabolol catalyzed by palladium(II) salts as sole catalysts Luciana A. Parreira a,b , Débora C. Gonc¸alves c , Luciano Menini c , Elena V. Gusevskaya a,∗ a b c
Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil Departamento de Química e Física, Campus de Alegre—CCA, Universidade Federal do Espírito Santo, 29500-000 Caixa Postal 16 Alegre, ES, Brazil Campus de Alegre Instituto Federal de Educac¸ão, Ciência e Tecnologia do Espírito Santo, 29500-000 Caixa Postal 47 Alegre, ES, Brazil
a r t i c l e
i n f o
Article history: Received 11 May 2016 Received in revised form 16 June 2016 Accepted 18 June 2016 Available online 23 June 2016 Keywords: Bio-renewables ␣-Bisabolol Oxidation Oxygen Palladium Terpenoids
a b s t r a c t ␣-Bisabolol, a sesquiterpene found in various essential oils, has numerous direct commercial uses in cosmetic and therapeutic formulations. In the present work, we report two novel liquid-phase processes for the catalytic oxidation of ␣-bisabolol with dioxygen, in which palladium salts (PdCl2 or Pd(OAc)2 ) are employed as sole catalysts. The addition of co-catalysts conventionally used for the re-oxidation of reduced palladium species or special ligands for their stabilization is not required. The reactions occur in non-acidic solvents, i.e., aqueous methanol or dimethylacetamide, and give exclusively the products derived from the interaction of the acyclic olefinic bond with palladium, whereas the other olefinic bond remains intact. The method allows to perform the green and simple synthesis of poly-functionalized compounds potentially useful for fine chemical industry as fragrance ingredients and synthetic intermediates, starting from a bio-renewable substrate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Terpenes are natural compounds present in a variety of essential oils, often as main components. They are widely used as ingredients and raw materials in flavor&fragrance and pharmaceutical industries due to specific olfactory characteristics and interesting therapeutic properties [1,2]. In particular, a sesquiterpenic alcohol ␣-bisabolol occurs in high concentrations in chamomile, candeia and sage essential oils (up to 50, 85 and 90%, respectively) [3]. As natural sources do not currently satisfy a high commercial demand for ␣-bisabolol, it is also produced synthetically from other sesquiterpenic alcohols farnesol and nerolidol [4–7]. ␣-Bisabolol is a component in over 1000 cosmetic, fragrance and therapeutic formulations [3,8]. Despite of such important and numerous direct uses, the catalytic oxidation of ␣-bisabolol, which contains two olefinic bonds, can open new perspectives for the applications of this compound. It has been reported that ␣-bisabolol derivatives, both natural and synthetic, also show an important therapeutic potential presenting, for example, anti-inflammatory and anti-tumoral activities [5,9,10].
∗ Corresponding author. E-mail address: [email protected] (E.V. Gusevskaya). http://dx.doi.org/10.1016/j.apcata.2016.06.027 0926-860X/© 2016 Elsevier B.V. All rights reserved.
The palladium catalyzed oxidation of olefins is a convenient and widely used in organic synthesis route to various oxygenated compounds, in particular, for the industrial transformation of ethylene into acetaldehyde (the Wacker process) [11–13]. The attractive feature of these reactions from economic and environmental viewpoints is the use of molecular oxygen as the terminal oxidant. On the other hand, the disadvantage of most of the conventional palladium-based catalytic systems for aerobic oxidations is a requirement for auxiliary co-catalysts. The role of these co-catalysts is to act as reversible co-oxidants capable to re-oxidize reduced palladium species before their irreversible agglomeration in a bulk metal and to be re-oxidized back by molecular oxygen. CuCl2 is the most commonly used co-catalyst in these systems (Wacker catalyst); however, corrosion and selectivity issues often become a problem due to the high concentrations of chloride ions and acidity of reaction solutions. Heteropoly compounds, nitrates and p-benzoquinone (BQ) have been studied among other compounds in palladium-based systems to replace CuCl2 and create more benign catalytic systems which do not contain chloride ions [14–21]. Furthermore, in the past decade several examples of the aerobic oxidation of olefins catalyzed by palladium(II) complexes in the absence of auxiliary co-oxidants have been published [22–33]. In these systems, the regeneration of palladium was performed directly by dioxygen through the sta-
L.A. Parreira et al. / Applied Catalysis A: General 524 (2016) 126–133
1.90-2.10, 2H, m 30.87 30.81
1.64, s 23.23
6
1
7
1.60-1.70, 1H, m 1.15-1.30, 1H, m 23.96 23.23
5
134.06 133.73
9
4 2
5.36, br.s 120.35 120.52
3
OH
14
13
12
74.05 74.62
1.55-1.65, m 43.08 44.15
1.75-1.85, 1H, m 1.90-2.10, 1H, m 26.82 26.40
1.31, s 29.71
1.08, s 22.87 22.88
8
127
OH 70.51 70.59
5.66-5.72, m 142.19 140.10
11
10 2.20, br.d 3 J = 7.2 Hz 42.83 46.58
5.66-5.72, m 121.84 123.50
15 1.31, s 29.71
Fig. 1. The attribution of the NMR signals for product 3 (two isomers).
1.90-2.00, m 31.09; 31.13 1.63 s; 1.64 s 23.41; 23.46
7
1.25-1.35, 1H, m 1.65-1.85, 1H, m 23.31; 24.03
6
9
1.24 s; 27.98 1.12 s; 22.75
4
134.15 133.48
2 5.36, br.s 120.85 121.31
O
5
1 3
1.27 s 25.49; 29.75
14
8
10
1.80-1.90, 1.90-2.00, 35.94 1.70-1.80, 2.05-2.15, 31.70
15
70.03 74.38
1.21; 1.24 s 30.99; 31.07
12
75.49 74.38
1.40-1.50, m; 45.58 1.65-1.75, m; 42.96
1.70-1.90, 1H, m 1.95-2.15, 1H, m 26.12; 26.78
13
5.59 d, 3J=10.4, 131.52 5.67, br.s; 134.38
11 1H, m 1H, m
5.80-5.85. m; 122.78 5.67, br.s; 120.46
1H, m 1H, m
Fig. 2. The attribution of the NMR signals for product 5 (two isomers).
bilization of palladium(0) species in solutions by special ligands or by coordinating solvents. Within our current project aiming to add value to natural essential oils by catalytic reactions, we have studied the Pd(II) catalyzed oxidation of various naturally occurring alkenes and alkenyl alcohols, i.e., nerolidol ␣-terpineol and linalool [17–21,27,28]. Our particular interest is currently directed to the oxidation of ␣-bisabolol as a route to interesting poly-functionalized sesquiterpenic compounds, which would by hardly accessible by other synthetic methods [34,35]. Before starting the project, we could find very limited information on the catalytic oxidation of ␣-bisabolol [36,37]. Both these publications reported the processes in which hydrogen peroxide or other peroxo derivatives were used as stoichiometric oxidants. As far as we know, two our recently published works [34,35] are the only available reports on the application of catalytic chemistry of palladium to the oxidation of ␣-bisabolol. As the attempts to apply the conventional Wacker catalyst (PdCl2 /CuCl2 ) to ␣-bisabolol were not encouraging in terms of reaction selectivity, we first tried to find suitable co-catalysts which could be an alternative to cupper chloride. The catalytic combinations of palladium(II) acetate with either BQ [34] or cupper(II) acetate [35] allowed to perform the aerobic oxidation of ␣-bisabolol in acetic acid or aqueous methanol media, respectively,
under chloride free conditions to obtain several novel oxygenated sesquiterpenoid compounds. Further, we have started a search for the systems which do not require auxiliary co-catalysts at all. Herein, we present novel processes for the palladium catalyzed direct-dioxygen-coupled oxidation of ␣-bisabolol. The reactions occur under mild nonacidic conditions and involve molecular oxygen as the stoichiometric oxidant and palladium salts (PdCl2 or Pd(OAc)2 ) as the sole catalysts. Oxygenated poly-functionalized compounds derived from bio-renewable ␣-bisabolol are potentially useful in fine chemical industry as fragrance ingredients and synthetic intermediates. 2. Experimental Natural ␣-(−)-bisabolol originated from the candeia (Eremanthus erythropappus) extract was received from CITRÓLEO Ind. Com. Óleos Essenciais as a kind donation. Palladium (II) acetate (>99.9%) was purchased from Aldrich and palladium (II) chloride (99.9%) from Strem. The copper content in both palladium salts determined by atomic absorption spectroscopy was less than 0.005%. Lithium chloride and copper (II) acetate were heated for dehydration. The catalytic tests were performed in a stainless steel autoclave (total volume of 100 mL) with magnetic stirring. The reaction progress was monitored by gas chromatography (GC) be
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Fig. 3. The attribution of the NMR signals for product 6.
Table 1 Oxidation of ␣-bisabolol with dioxygen in methanol.a Run
Extra water (vol%)
T (◦ C)
P (atm)
Time (h)
Conversion (%)
c
8 8 8 15 4 8 8 8 8
80 80 60 60 60 60 60 60 60
10 10 5 10 10 10 10 10 10
5 47 37 7 10 7 10 9 10 2 9
87 55 57 27 32 47 51 48 50 92 83 100 50
2 3 4 5 6 7c 8d 9e
TOFb (h−1 )
Selectivity (%) 2
3
4
63 45 48 43 45 32 33 46 45 48 81 – tr.
8 67 38 37 52 50 27 26 37 7 – tr.
10 99 9 10 10 12 9 12 5 10 – tr.
4.8 4.4 1.8 2.2 2.6 2.3 2.0 18.0 2.0
a Conditions: ␣-bisabolol (0.20 M), [Pd(OAc)2 ] (0.01 M); gas phase – O2 , total volume 12 mL. Conversion and selectivity were obtained from GC data and referred to the amounts of the reacted substrate; tr. – trace amounts. b Average rate (turnover frequency) of the substrate conversion per mol of palladium for the first reaction hour. c Cu(OAc)2 added (0.05 M). d Pd(OAc)2 was replaced by PdCl2 . e Pd(OAc)2 was replaced by Pd(OCOCF3 )2 .
OH
OH
Pd(OAc)2 (cat)
2 (R=CH 3) 3 (R=H)
OR
(up to 85%)
O2, CH3OH 1
O
4
(~10%)
Scheme 1. Oxidation of ␣-bisabolol in methanol.
periodical sampling. Typically, ␣-(−)-bisabolol, palladium catalyst, water and bornyl acetate (used as a GC standard, 0.10 M) were dissolved in indicated concentrations in a specified solvent (12 mL total volume). The solution was put in the autoclave and the indicated pressure was attained with dioxygen as the gas phase. An oil bath was used to heat the reactor and to maintain the reaction temperature. The samples of the reaction solutions for the GC analysis were taken using a special device maintaining the pressure inside the reactor. GC analyses were performed on a Shimadzu GC-2010 Plus chromatograph using a Rtx-Wax 30 m or Rtx-5MS 30 m capillary column and a flame ionization detector. Conversions and selectivities were calculated using as a reference the amount
of the substrate reacted. Average turnover frequencies, TOF, were measured for the first reaction hour (in most cases up to 20–40% conversions). The identification of the products was performed by 1 H and 13 C NMR and GC–MS spectroscopy after their isolation from the reaction solutions by a column chromatography (silica gel 60); hexane and dichloromethane and their solutions were used as eluents. NMR experiments were conducted on a Bruker 400 MHz instrument (CDCl3 as the solvent, TMS as the internal standard). Mass spectra were acquired on a Shimadzu QP2010-PLUS equipment (70 eV).
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Table 2 Oxidation of ␣-bisabolol with dioxygen in dimethylacetamide.a Run
1 2 3c 4 5 6d 7d 8 9 10 11 12e
Added water (vol%)
15 15 15 15 15 15 15 15 15 8 0 15
[PdCl2 ] (mM)
10 none none 20 5 10 10 10 10 10 10 10
T (◦ C)
60 60 60 60 60 60 80 40 30 60 60 60
Time (h)
2 4 4 1 6 4 1 6 11 10 4 13
Conversion (%)
95 0 0 90 57 93 94 86 70 95 45 70
TOFb (h−1 )
Selectivity (%) 3
4
5
6
13 – – 16 13 10 10 10 10 11 tr. –
2 – – 7 8 2 tr. 11 6 3 3 20
35 – – 30 38 35 27 25 19 25 14 tr.
36 – – 37 22 34 21 40 51 30 24 tr.
11.0 – – 9.0 9.6 6.0 18.8 8.0 4.8 9.0 4.0 1.0
a Conditions: ␣-bisabolol (0.20 M), [Pd(OAc)2 ] (0.01 M); 10 atm of O2 , total volume 12 mL. Conversion and selectivity were obtained from GC data and referred to the amounts of the reacted substrate; tr. – trace amounts. b Average rate (turnover frequency) of the substrate conversion per mol of palladium for the first reaction hour. c LiCl was added (0.02 M). d 5 atm. e PdCl2 was replaced by Pd(OAc)2 .
Product 2 (isomers 2a and 2b) (first description was published in [35]): 2a: MS (70 eV, EI): m/z (%): 202 (1) [M+ −H2 O−CH3 OH)], 139 (32), 125 (9), 121 (18), 95 (46), 82 (100), 71 (18%), 67 (32); 2b (longer GC retention time): MS (70 eV, EI): m/z (%): 202 (1) [M+ −H2 O−CH3 OH)], 125 (4), 95 (4), 73 (100), 67 (3). For NMR data see [35]. Product 3 (novel compound as far as we know, isomers 3a and 3b): 3a: MS (70 eV, EI): m/z (%): 220 (1) [M+ −H2 O], 202 (1) [M+ −H2 O−H2 O)], 147 (15), 139 (30), 125 (31), 121 (30), 99 (38), 95 (69), 85 (61), 82 (100), 67 (46), 59 (61). 3b: MS (70 eV, EI): m/z (%): 220 (1) [M+ −H2 O], 202 (1) [M+ −H2 O−H2 O)], 147 (22), 125 (31), 95 (36), 94 (51), 85 (100), 82 (25), 79 (37), 67 (26), 59 (90), 43 (94). The attribution of NMR signals is shown in Fig. 1. Product 4 (first description was published in [29]): MS (70 eV, EI): m/z (%): 220 (1) [M+ ], 187 (1) [M+ −H2 O−CH3 ], 132 (18), 125 (100), 107 (61), 95 (14), 93 (13), 67 (18). For NMR data see [34]. Product 5 (novel compound as far as we know): MS (70 eV, EI): m/z (%): 220 (2) [M+ ], 205 (3) [M+ −CH3 ], 187 (2) [M+ −CH3 −H2 O], 125 (100), 107 (34), 83 (12), 43 (46). Product 5 showed only one peak on chromatograms; however, the NMA analysis revealed the existence of two isomers with very similar spectra. The attribution of NMR signals is shown in Fig. 2. Product 6 (known as norbisabolide): MS (70 eV, EI): m/z (%): 194 (28) [M+ ], 179 (4) [M+ −CH3 ], 161 (8) [M+ −CH3 −H2 O], 134 (22), 121 (64), 119 (22), 99 (100), 93 (78), 81 (19), 79 (19), 71 (28), 67 (22), 42 (49). The attribution of NMR signals is shown in Fig. 3. 3. Results and discussion 3.1. Oxidation of ˛-bisabolol in methanol solutions The data on the oxidation of ␣-bisabolol (1) by dioxygen in aqueous methanol solutions containing Pd(OAc)2 in catalytic amounts are collected in Table 1. In our previous work [35], Cu(OAc)2 was used as the co-catalyst together with Pd(OAc)2 for the oxidation of ␣-bisabolol. The main reaction products were compounds 2 (up to 85% selectivity) and 4 (ca. 10% selectivity) (Scheme 1). Both products were novel bisabolane derivatives formed due to the oxidation of the acyclic olefinic bond of the substrate. The example of the reaction in the system containing Cu(OAc)2 is presented in Table 1 (run 1). The oxidation of ␣-bisabolol in the bimetallic system was accelerated by the increase in the palladium concentration; however, it was (quite unexpectedly) only slightly dependent on the concen-
tration of copper. Encouraged by this observation, we performed the process in the absence of copper acetate. Despite the decrease in selectivity for product 2 and reaction stagnation at ca. 50% conversion, the reaction was still catalytic with respect to palladium and showed the dioxygen-coupled turnover number (TON) of nearly 10 (Table 1, run 2). No formation of the palladium mirror on the walls of the reactor was detected at the end of the reaction. Varying reaction parameters (Table 1, runs 3–6), we found that the reaction could be nearly completed in 9 h at 60 ◦ C and 10 atm to give methyl ether 2 and corresponding diol 3 in 85% combined selectivity and comparable amounts (Table 1, run 6). Heterocyclic compound 4 was detected in these runs in small amounts (5–10%). All three products were formed due to the oxidation of the acyclic C C bond with the participation as nucleophiles of external methanol or water molecules to give products 2 and 3, respectively, or the internal tethered hydroxyl group to give cyclization product 4. In addition to the products specified in Table 1, bisabolol oxide B (identified by MS) was formed in most of the runs in small amounts. Diol 3, a novel compound to the best of our knowledge, was separated after the reaction as a mixture of two isomers 3a and 3b. The NMR spectra of these isomers are very similar (Fig. 1, Experimental Section). Both 3a and 3b seem to have a trans configuration at the acyclic double bond as the hydrogens at C-10 and C-12 correlated in the NOE spectrum and the chemical shift values for allylic carbons C-13 and C-10 in both isomers were close. We suppose that the structural difference between the two isomers of diol 3 is related to the configuration at carbon C-8, similarly to what we suggested for the isomers of ether 2 [35]. Compound 4 was reported for the first time in our previous work as a major oxidation product formed from ␣-bisabolol in the acetic acid solutions containing palladium acetate and BQ in catalytic amounts [34]. It is surprising that the reactions with and without Cu(OAc)2 occurred at similar rates and with similar selectivities for diol products 2 and 3 and for cyclization product 4 (Table 1, run 7 vs. run 6). However, differently from “palladium solo” reactions, in the bimetallic system methyl ether 2 was the main diol product, whereas diol 3 itself was detected only in small amounts (Table 1, runs 1 and 7). It can be suggested that Lewis acidity of copper ions favored the etherification of diol 3 with methanol to give ether 2. However, the second hydroxyl group in diol 3 did not undergo etherification in these systems. The more plausible explanation looks a suggestion that the presence of copper ions increases the relative contribution of methanol as the nucleohile at the oxidation
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O
5 (up to 40%) PdCl2 (cat) OH O
O 2, DMA 1
O
6 (up to 50%) OH
OH
3 (10-15%) Scheme 2. Oxidation of ␣-bisabolol in dimethylformamide.
step (to give ether 2) as compared to water (to give diol 3), possibly, due to the formation of copper methoxy complexes. In blank reactions without Pd(OAc)2 , none of oxidation products 2, 3 and 4 was expectedly detected in reaction solutions. The decrease in the oxygen pressure did not affect significantly the product distribution and initial turnover frequency (TOF) (Table 1, run 3 vs run 6). However, the reaction at 5 atm turned stagnated at ca. 30% conversion with the formation of inactive palladium metal. Therefore, to ensure the effective re-oxidation of palladium the reaction should be performed under higher oxygen pressure. It is remarkable that the reaction with ␣-bisabolol proceeds in aqueous methanol solutions with dioxygen-coupled turnovers without the use of auxiliary co-oxidants or special auxiliary ligands for the stabilization of reduced palladium species. In this context, it is worthwhile to cite the example of the related “palladium solo” reaction which we have found in the literature. As early as in 1978, Hosokawa et al. described the oxidative cyclization of allylphenol substrates with molecular oxygen catalyzed by palladium(II) acetate alone in the solutions of aqueous methanol [38]. We suppose that palladium(0) species in these systems are able to form relatively stable complexes with the solvent and/or with the substrate which decelerates their aggregation. In other words, the coordination enables the reoxidation of palladium(0) species with dioxygen to occur faster than palladium deactivation due to clustering into inactive bulk metal. The nature of the olefinic substrate is very important for the possibility to run the process without Cu(OAc)2 , i.e., to perform a “palladium solo” reaction. Although the related substrate 3,7dimethyl-1,6-octadien-3-ol (linalool) did react with dioxygen in the presence of the bimetallic Pd(OAc)2 + Cu(OAc)2 catalytic combination [19], the attempts to perform the reaction with Pd(OAc)2 alone resulted in the appearance of the palladium mirror on the walls of the reactor (not shown in the table). Palladium black was also formed at the attempt to oxidize another monoterpenic acyclic alcohol, 3,7-dimethyl-2,6-octadien-1-ol (nerol), in the absence of Cu(OAc)2 . All three substrates: linalool, nerol, and ␣-bisabolol, contain in their molecules the same fragment with a trisubstituted acyclic double bond and namely this bond is oxidized in our reactions. It seems that the presence of a six-membered unsaturated ring in the ␣-bisabolol molecule plays a key role in its capacity to stabilize the reduced palladium species in the course of the catalytic process thus allowing to maintain the catalytic cycle without the need for auxiliary co-catalysts.
The nature of anionic ligands on palladium is also remarkably important for the successful operation of palladium as the sole catalyst in the oxidation of ␣-bisabolol. The reactions with the catalysts generated from PdCl2 and Pd(OCOCF3 )2 showed very poor selectivities for products 2, 3 and 4, with numerous unidentified peaks being detected by GC (Table 1, runs 8 and 9). 3.2. Oxidation of ˛-bisabolol in dimethylacetamide solutions In 2006, Kaneda and co-workers disclosed that PdCl2 catalyzed the oxidation of some terminal [39] and internal [26] alkenes with molecular oxygen in dimethylacetamide (DMA) solutions without the addition of co-oxidants or special ligands. Latter, we have extended the method to the oxidation of styrene [27] and naturally occurring allyl benzenes [28], the substrates containing terminal olefinic bonds. It has been suggested that DMA as a coordinating solvent stabilizes palladium(0) species thus allowing to perform direct-dioxygen-coupled catalytic cycles. Based on these results, we decided to apply the approach to the oxidation of ␣-bisabolol using dioxygen as the sole oxidant (Table 2). The reactions were performed in aqueous DMA solutions containing catalytic amounts of PdCl2 and, in most of the runs, 10 atm of oxygen to ensure the efficient capture of the reduced palladium species. Compounds 5 and 6 were detected as major products in ca. 70% combined selectivity along with smaller amounts of diol 3 (10–15%) and tetrahydrofuran product 4 (5–10%) (Scheme 2). Compounds 5 and 6 were isolated from the reaction solutions and identified by GC–MS and NMR spectroscopy. Product 5, a novel compound to the best of our knowledge, was observed by NMR as two not GC separable isomers with very similar spectra (Fig. 2, Experimental Section). As natural ␣-(−)-bisabolol has R configurations at carbons 4 and 8, we suppose that the partial inversion of the configuration occurs at C-8, which is linked to the involved in the catalytic transformation OH group, so that the two isomers of 5 differ from each other by geometry at C-8. Product 6 has been identified as a ␥-butyrolactone compound shown in Scheme 2, which is known as norbisabolide, a natural compound extracted from Atalantia Monophylla citric plant [35] (NMR attributions are shown in Fig. 3), Experimental Section. Compound 6 was described by Solabannavar et al. [40]. All identified products detected at the oxidation of ␣-bisabolol in DMA solutions were originated by the interaction of the acyclic olefinic bond with palladium. Products 3, 4 and 5 keep the carbon skeleton of the original molecule, whereas the formation of
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Graphical abstract.doc revised manuscript.doc Scheme 3. Suggested mechanism for the formation of products 2, 3, 4 and 5.
ketone 6 involves its oxidative cleavage. The endocyclic double bond remained intact in all these products. ␣-Bisabolol readily reacted with PdCl2 in aqueous DMA solutions (15 vol% of water). A nearly complete conversion occurred in a two-hour reaction at 60 ◦ C to give compounds 5 and 6 in ca. 35% selectivity each (Table 2, run 1). The reaction was catalytic in palladium showing TON of ca. 20. Thus, DMA efficiently stabilizes the palladium(0) species so that their reaction with dioxygen occurs faster than the precipitation of palladium black. In blank reactions, with no PdCl2 or with LiCl instead of PdCl2 , the conversion of ␣-bisabolol was not observed (Table 2, runs 2 and 3). It is important, that no formation of even trace amounts of compound 6, which might be the product of the non-catalytic oxidative cleavage of the ␣-bisabolol molecule, was detected in these runs. The reaction rate was found to be roughly proportional to the PdCl2 amounts as initial TOF values in the experiments with different PdCl2 concentrations were similar (Table 2, runs 1, 4 and 5). With 10 mol% of PdCl2, the reaction was nearly completed within 1 h to give products 3, 4, 5 and 6 with 90% combined selectivity. Bisabolol oxide B was mainly responsible for the rest of the mass balance in this system also. The reaction could be accelerated by the increase in the oxygen pressure (Table 2, cf. runs 1 and 6). Under 5 atm, a nearly complete conversion occurred in 4 h with no formation of the palladium mirror. The changes in the oxygen pressure and catalyst amounts did not affect significantly a product distribution. The positive effects of palladium and oxygen on reaction kinetics suggest that the rate determining step of the whole process can be the regeneration of palladium by dioxygen. It should be mentioned that in the reaction under atmospheric pressure, the precipitation of palladium metal was observed. The reaction was much faster at 80 ◦ C being nearly completed for 1 h vs. 4 h at 60 ◦ C; however, selectivity was lower, especially for product 6 (Table 2, run 7 vs. run 6). The tendency was confirmed in another set of the run (Table 2, runs 1, 8 and 9): at lower temperature the contribution of the carbon–carbon bond cleavage to give
product 6 became higher at the expense of product 5. However, the combined selectivity for products 5 and 6 remained nearly constant (ca. 70%). The process can be performed at nearly ambient temperature. Although the reaction rate became lower, the selectivity of 51% obtained for norbisabolide (product 6) was the best value we could achieve so far (Table 2, run 9). At lower water concentrations, the reaction was slower and less selective (Table 2, cf. runs 1, 10 and 11). The accelerating effect of water was also reported for the Pd/(−)-sparteine [41] and PdCl2 [27,28] catalyzed oxidation of terminal olefins with molecular oxygen in DMA solutions. However, water concentrations higher than 15 vol% can not be recommended due to miscibility problems.
3.3. Reaction mechanism The acyclic olefinic bond of ␣-bisabolol was the only one oxidized in “palladium solo” reactions in both methanol and DMA solutions. No products derived from the oxidation of the endocyclic olefinic bond were registered in detectable amounts. In this context, it seems remarkable the difference between these systems and the Pd(OAc)2 /BQ system described in our previous work [34], which promoted the oxidation of both olefinic bonds in ␣-bisabolol. The mechanism proposed for the formation of products 2, 3, 4 and 5 is shown in Scheme 3. As it is generally accepted that the palladium promoted oxidations of alkenyl alcohols involve an oxypalladation step [4–45], we suggest such interaction as a key reaction feature in our process. The Markovnikov oxypalladation of the acyclic double bond leads to products 2, 3 and 5. In this case, the nucleophile attacks a trisubstituted vinylic carbon atom. The antiMarkovnikov oxypalladation leads to product 4. The Markovnikov type interaction is a more favorable alternative because palladium in corresponding -alkyl palladium intermediates is linked to a sterically more accessible carbon atom. Probably for this reason, compound 4 was detected only as the minor product in all the runs. -Alkyl palladium intermediates A (the precursors of diol
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OH
1
[Pd]
OH
[Pd]
OH OH
O
O
hydroxyaldehyde
hydroxycarboxylic acid - H2O
O hemiacetal
[Pd]
O
OH - 2[H]
O
6
Scheme 4. Suggested mechanism for the formation of ␥-butyrolactone 6.
3 and ether 2) are formed through the nucleophilic attack by the external molecule of water or methanol. The participation of the internal hydroxyl group at the oxypalladation step leads to the molecule cyclization. The hydroxyl group can attack both vinylic carbons coordinated on palladium to form either a tetrahydrofuran or tetrahydropyran ring (intermediates B and C, respectively). Finally, the abstraction of -hydrogens in -alkyl intermediates A, B and C results in the formation of products 2 (or 3), 4 and 5. It is interesting that the direction of the ␣-bisabolol cyclization (furan vs. pyran products) is affected by the nature of anionic groups on palladium. The reactions with Pd(OAc)2 (see Table 1 and our previous works [34,35]) give preferably five-membered product 4, which is the result of the anti-Markovnikov oxypalladation. On the other hand, in the system with PdCl2 the major cyclization product is six-membered compound 5, which arises from the Markovnikov oxypalladation. Moreover, when PdCl2 was substituted by Pd(OAc)2 , not only the reaction became slower and less selective, but also the cyclization selectivity was switched almost completely from six-membered compound 5 to five-membered product 4 (Table 2, run 12 vs. run 1). ␥-Butyrolactone 6 is the product of the oxidative cleavage of the acyclic double bond, the reaction much less common in the catalytic chemistry of palladium than transformations through the oxypalladation step shown in Scheme 3. H. Jiang et al. have recently reported that palladium acetate catalyzes the oxidative cleavage of multiple C C bonds in alkenes [46] and alkynes [47] by molecular oxygen to give carbonyl and carboxyl products (aldehydes, ketones or carboxylic acid derivatives). Based on these literature data we are suggesting two possible routes to product 6 from ␣bisabolol (Scheme 4). The oxidative cleavage of the olefinic bond could give a hydroxyaldehyde, which then undergoes a spontaneous intramolecular cyclization resulting in a five-membered hemiacetal. The subsequent palladium catalyzed dehydrogenation of the hemiacetal would give ␥-lactone 6. Alternatively, the product of the oxidative cleavage of ␣-bisabolol could be a hydroxycarboxylic acid, whose spontaneous cyclization would give directly product 6. 4. Conclusion In summary, we have developed two novel liquid-phase processes for the oxidation of ␣-bisabolol in aqueous methanol or dimethylacetamide solutions using molecular oxygen as the stoichiometric oxidant. The reactions are catalyzed solely by the commercially available palladium salts, Pd(OAc)2 or PdCl2 . ␣-
Bisabolol, a renewable sesquiterpene found in essential oils of various plants, contains two olefinic bonds, both trisubstituted. All reaction products arise exclusively from the oxidation of the acyclic olefinic bond. Although the endocyclic double bond is usually involved in palladium catalyzed reactions with more facility, it remains intact. In general, internal acyclic olefinic bonds in non-functionalized alkenes are very reluctant to Wacker-type oxidations, with only few examples being reported so far [26]. The success in the oxidation of ␣-bisabolol was probably enabled by the existence of the hydroxyl group in a ␥-position to the acyclic double bond, which could favour the interaction of th double bond with palladium. These simple catalytic reactions represent environmentally and commercially interesting routes to poly-functionalized products potentially useful in fine chemical industry as fragrance ingredients and synthetic intermediates. The use of the renewable biomass-based substrate, low-cost nonacidic non-corrosive solvents, palladium salts as the sole catalysts and dioxygen as the stoichiometric oxidant is an advantage particularly important for the green chemistry concept. Acknowledgments The authors thank the Citróleo Ind. Com. Óleos Essenciais for the donating of ␣-(−)-bisabolol, Dr. Patrícia Fontes Pinheiro (Universidade Federal do Espírito Santo) for the help in contacting the Citróleo industry and the CNPq, FAPEMIG, FAPES and INCT Catálise (Brazil) for funding this work. References [1] C. Sell (Ed.), The Chemistry of Fragrances: from Perfumer to Consumer, vol. 2, second ed., RSC Publishing, Dorset, UK, 2006, pp. 52–88. [2] H. Mimoun, Chimia 50 (1996) 620–625. [3] G.P.P. Kamatou, A.M. Viljoen, J. Am. Oil Chem. Soc. 87 (2010) 1–7. [4] O. Taglialatela-Scafati, F. Pollastro, L. Cicione, G. Chianese, M.L. Bellido, E. ¨ Z. Toker, G. Appendino, J. Nat. Prod. 75 (2012) 453–458. Munoz, H.C. Ozen, [5] K. Massonne, K.P. Pfaff, J. Schubert, G. Gottwald (BASF), DE 102005053329 (2007). [6] K. Massonne, K.P. Pfaff, J. Schubert, G. Gottwald (BASF), DE 102005053338 (2007). [7] A.L.P. de Meireles, M. dos Santos Costa, K.A. da Silva Rocha, E.V. Gusevskaya, Appl. Catal. A 502 (2015) 271–275. [8] K. Russell, S.E. Jacob, Dermatitis 21 (2010) 57–58. [9] M. Piochon, J. Legault, C. Gauthier, A. Pichette, Phytochemistry 70 (2009) 228–236. [10] A.P. da Silva, M.V. Martini, C.M.A. de Oliveira, S. Cunha, J.E. de Carvalho, A.L.T.G. Ruiz, C.C. da Silva, Eur. J. Med. Chem. 45 (2010) 2987–2993. [11] W. Wu, H. Jiang, Acc. Chem. Res. 45 (2012) 1736–1748. [12] B.V. Popp, S.S. Stahl, Top. Organomet. Chem. 22 (2007) 149–189. [13] J.A. Keith, P.M. Henry, Angew. Chem. Int. Ed. 48 (2009) 9038–9049. [14] A. Heumann, K.J. Jens, M. Réglier, in: K.D. Karlin (Ed.), Progress in Inorganic Chemistry, vol. 42, Wiley, New York, 1994, pp. 542–576. [15] T. Yokota, A. Sakakura, M. Tani, S. Sakaguchi, Y. Ishii, Tetrahedron Lett. 43 (2002) 8887–8891. [16] J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed. 47 (2008) 3506–3523. [17] J.A. Gonc¸alves, E.V. Gusevskaya, Appl. Catal. A 258 (2004) 93–98. [18] J.A. Gonc¸alves, O.W. Howarth, E.V. Gusevskaya, J. Mol. Catal. A 185 (2002) 95–104. [19] M.G. Speziali, P.A. Robles-Dutenhefner, E.V. Gusevskaya, Organometallics 26 (2007) 4003–4009. [20] M.G. Speziali, V.V. Costa, P.A. Robles-Dutenhefner, E.V. Gusevskaya, Organometallics 28 (2009) 3186–3192. [21] A.C. Bueno, Á.O. de Souza, E.V. Gusevskaya, ChemCatChem 4 (2012) 1382–1388. [22] C.N. Cornell, M.S. Sigman, Inorg. Chem. 46 (2007) 1903–1909. [23] S.S. Stahl, Science 309 (2005) 1824–1826. [24] J. Muzart, Chem. Asian J. 1 (2006) 508–515. [25] K.M. Gligorich, M.S. Sigman, Angew. Chem. Int. Ed. 45 (2006) 6612–6615. [26] T. Mitsudome, K. Mizumoto, T. Mizugaki, K. Jitsukawa, K. Kaneda, Angew. Chem. Int. Ed. 49 (2010) 1238–1240. [27] A.C. Bueno, Á.O. de Souza, E.V. Gusevskaya, Adv. Synth. Catal. 351 (2009) 2491–2495. [28] L.A. Parreira, L. Menini, J.C. da Cruz Santos, E.V. Gusevskaya, Adv. Synth. Catal. 352 (2010) 1533–1538. [29] P.B. White, J.N. Jaworski, G.H. Zhu, S.S. Stahl, ACS Catal. 6 (2016) 3340–3348. [30] M.S. Sigman, E.W. Werner, Acc. Chem. Res. 45 (2012) 874–884. [31] A.B. Weinstein, S.S. Stahl, Angew. Chem. Int. Ed. 51 (2012) 11505–11509.
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