Palladium Precatalysts for Decarbonylative Dehydration of Fatty Acids

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Palladium Precatalysts for Decarbonylative Dehydration of Fatty Acids to Linear Alpha Olefins Anamitra Chatterjee, Sondre H. Hopen Eliasson, Karl Wilhelm Tornroos, and Vidar R. Jensen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02460 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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Palladium Precatalysts for Decarbonylative Dehydration of Fatty Acids to Linear Alpha Olefins Anamitra Chatterjee, Sondre H. Hopen Eliasson, Karl W. Törnroos, and Vidar R. Jensen* University of Bergen, Department of Chemistry, Allégaten 41, N-5007, Bergen, Norway ABSTRACT: Transition-metal-catalyzed decarbonylative dehydration of even-numbered fatty acids to give valuable oddnumbered alpha olefins has so far been performed using approaches in which the catalyst is formed in situ. We show that welldefined Pd(bis-phosphine) precatalysts eliminate the need for excess phosphine and selectively produce linear alpha olefins under mild conditions and with substantially increased turnover frequency (TOF) and substrate scope. In particular, the new precatalyst Pd(cinnamyl)Cl(DPEphos) (5) selectively converts substrates containing unprotected aliphatic-alcohol and amine functional groups to their corresponding linear alpha olefins.

KEYWORDS: decarbonylative dehydration, fatty acids, linear alpha olefins, palladium, renewable feedstocks To reach a more sustainable society, commodity chemicals should increasingly be produced from biomass feedstocks.1-3 One of the most important classes of such chemicals, which are building blocks in the production of polyethylene, synthetic oils, plasticizers, detergents and oilfield fluids,1-2, 4-5 is that of linear alpha olefins (LAOs), which are currently produced from fossil carbon sources in oligomerization processes leading to even-numbered LAOs.6-7 Thus, direct catalytic synthesis of LAOs from renewable feedstocks, either via decarboxylative or decarbonylative transformations, is an attractive goal that will give a more sustainable access to these important industrial intermediates.2 In addition, decarboxylation and decarbonylation of one of the most attractive starting materials for these processes, even-numbered fatty acids, available from a range of natural sources, lead to uncommon, odd-numbered LAOs. As shown in a recent, critical review,2 only bio-catalysts have so far given LAOs from fatty acids with TOFs higher than 1 min-1 at ambient temperatures, but the bio-catalytic processes are limited by low catalyst stability and low volumetric productivity.2, 8-9 The stability and productivity of the heterogeneous catalysts are higher, but they generally need harsh reaction conditions and are less selective for LAOs.2 Superior selectivity and scopes are typically offered by the homogeneous catalysts,2, 10-11 but achieving acceptable TOFs at low temperatures has proven to be a serious challenge.2 Particular attention in this respect has been paid to the decarbonylative dehydration reaction in recent years, and it has proven to be a promising reaction not only to produce LAOs but also in multistep organic synthesis.12 Methods based on in situ-generated transition-metal-catalysts based on palladium10, 13-19 , nickel20-22, rhodium17-18, iridium23-24 and iron25 have so far been developed for this reaction. The methods based on palladium have so far been superior, but still typically require high temperatures and often also distillation of the olefin product to achieve acceptable selectivity.10, 14, 16-19 In contrast, Gooßen13 and Scott15 reported high selectivity using a relatively moderate reaction temperature (110 oC) without in situ distillation, an important achievement given the difficulty in avoiding olefin double-bond-migration to internal positions.1-2 Howev-

er, these low-temperature approaches still required high catalyst loadings (~3 mol%) and resulted in relatively low (≤ 33) turnover numbers (TONs). The above palladium-catalyzed processes are approaches in which the palladium catalyst is generated in situ using excess phosphine compared to palladium. Whereas avoiding this excess would make for a greener and more atom-economic process,26 a recent molecular-level computational study27 also suggests that phosphine dissociates from palladium prior to the rate-determining transition state, implying that avoiding excess phosphine should, in principle, increase catalytic activity. This hypothesis can be tested by using well-defined precatalysts in which the phosphine ligands are already bonded to palladium, a strategy that has been met with success in palladium-catalyzed cross-coupling reaction.28-29 We here report the first investigation of well-defined Pd(bis-phosphine) precatalysts for decarbonylative dehydration of fatty acids to LAOs. These precatalysts may offer inspiration and starting points for rational catalyst design for a reaction in sore need of improved efficiency. To obtain data against which to compare the precatalysts and to obtain ideas as to promising ligands, we initially screened several palladium complexes and phosphine ligands (See the Supporting Information) using a standard in situ strategy, with stearic acid 1a as model substrate; see entries 19 in Table 1. First, a drop in conversion from 100% to 10% when reducing the PdCl2 loading from 3 mol% to 0.5 mol% (Table 1, entries 1-3) confirmed the previously observed,13, 15 detrimental effect of decreasing catalyst loading and phosphine/palladium ratio on the reaction output. We hypothesized that the reduced yield might stem from inefficient in situ catalyst formation at lower catalyst loading and phosphine/palladium ratio and that in situ approaches might also lead to off-cycle species reducing catalyst activity.28

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Table 1. Comparison of in situ catalytic system with precatalysts.a

Entry

Pd precursor (mol%)

Ligand (mol%)

Yield [%]b

2a [%]c

1

PdCl2 (3)

DPEPhos (9)

100

97

2

PdCl2 (1)

DPEPhos (1)

35

97

3

PdCl2 (0.5)

DPEPhos (0.5)

10

97

4

Pd(OAc)2 (1)

DPEPhos (1)

-

-

5

Pd(dba)2 (1)

DPEPhos (1)

3

ND

6

Pd(PPh3)4 (1)

DPEPhos (1)

5

ND

7

PdCl2(PPh3)2 (1)

DPEPhos (1)

42

97

8

[Pd(cinnamyl)Cl]2 (0.25)

DPEPhos (0.5)

65

96

9

[Pd(cinnamyl)Cl]2 (0.25)

XantPhos (0.5)

22

96

10

4 (0.5)

-

30

97

11

5 (0.5)

-

88

99

12

6 (0.5)

-

40

96

13

7 (0.5)

-

5

ND

14

8 (0.5)

-

14

64

15

5 (0.5)

-

70

96d

a

Conditions: 1 mmol 1a, 2 mmol Ac2O, 2 mL DMPU, 15h. bDetermined as the average of two experiments by 1H NMR using methyl benzoate as an internal standard, with a maximum ∆yield = ±5%. cDetermined as the average of two experiments by 1H NMR, with a maximum ∆selectivity = ±1%. dIn absence of base.

The use of well-defined precatalysts should reduce the problems of inefficient catalyst formation and off-cycle species at the same time as avoiding excess phosphine, which is postulated to dissociate prior to the rate-determining transition state.27 Entry 8 makes it clear that tests of precatalysts should include the combination of cinnamyl and DPEPhos. The in situ combination of these two ligands, which has not previously been tested in decarbonylative dehydration, was found to be particularly promising at lower catalyst loadings (0.25 mol%, TON = 130). This ligand combination was included, as complex 5, among the five different Pd(bis-phosphine) precatalysts (complexes 4-8, Figure 1) selected for investigation.

Figure 1. Precatalysts screened for decarbonylative dehydration.

Whereas complexes 4 and 8 are commercially available, precatalysts 5-7 can be synthesized by a high-yield one-pot procedure (See the Supporting Information). All the precatalysts 4-8 can be stored under air without any decomposition. The new precatalyst 5 was characterized by X-ray crystallography (Figure 2). The geometry is as expected for Pd(η3allyl)(bis-phosphine) complexes30 and can be described as a distorted square pyramid in which the cinnamyl and phosphine ligands occupy the equatorial plane with chloride in the axial site. Looking at the details of the cinnamyl and phosphine ligands, the bis-phosphine bite angle (102.2°) is typical for DPEphos complexes,31 while the cinnamyl Pd−C bond distances are dissimilar, the Pd1-C3 bond (2.373(4) Å) being longer than the Pd1-C1 (2.133(4) Å) bond. This difference is similar to previously observed bonding asymmetry for the η3cinnamyl ligand,32 and is believed to facilitate the activation into an active Pd(0) species.32 The Pd-Cl bond (2.7216(10) Å) of 5 is 30-40 pm longer than Pd-Cl bonds of PdCl2(bisphosphine) complexes closely related to 4,31 but still within the sum of the van der Waals radii of palladium and chloride,33 prompting us to investigate the nature of this bond by quantum chemical calculations. Whereas natural bond orbital (NBO) analysis34 shows that the Pd-Cl bonds of 4 are polar bonds, with natural ionicity (NI, close to -0.5) far from the ionic limit of NI > |0.95| and covalent bond orders35 of 0.88, the Pd-Cl bond of 5 is almost purely ionic (NI = -0.92)36 and has a covalent bond order35 of only 0.37.

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ACS Catalysis used (see the Supporting Information). The base only influences the catalyst, probably by facilitating the activation, and is not a substrate of the reaction.15 Table 2. Substrate scope.a

Figure 2. X-ray crystal structure of 5 (hydrogen atoms and solvent omitted for clarity). Selected bond lengths: Pd1-C1 2.133(4) Å; Pd1-C2 2.200(4) Å; Pd1-C3 2.373(4) Å; Pd1-P1 2.3370(10) Å; Pd1-P2 2.2978(10) Å; Pd1-Cl1 2.7216(10) Å. Bite angle P2–Pd1– P1 = 102.19(4)°.

We compared the catalytic performance of the above precatalysts with that of our in situ systems (Table 1, entries 10-15 vs. 1-9). Complex 4 immediately confirms the benefit of a precatalyst by being three times as active as the corresponding in situ system (entry 10 vs. 3). Similarly, complex 6 is two times more active (entry 12 vs. 9). Although high selectivity and activity are observed for the in situ approach involving the cinnamyl ligand (entry 8), the advantages of using a precatalyst 5 are evident also in this case (entry 11). The new precatalyst 5 stands out as the best catalyst in our screening, with a TON of 176, a TOF (11.7 h-1, entry 11) more than eight times that of the corresponding PdCl2/DPEPhos mixture (1.3 h-1, entry 3), and nearly complete selectivity. This remarkable improvement is caused by the beneficial combination, in a precatalyst, of the cinnamyl and DPEPhos ligands. DPEPhos in combination with the smaller allyl ligand of 7 gives much lower catalytic activity (entry 13), while replacing the DPEPhos ligand of 5 by XantPhos to give 6 results in both lower activity and selectivity (entry 12). Having identified the best catalyst (5), we explored the influence of reaction conditions and found that the conditions used in the first series of experiments (Table 1) were essentially already optimal; see the Supporting Information. The reaction was found to benefit from highly polar solvents such as DMPU or NMP, which supports the idea that charge separation is important during the catalytic alkene formation step.27 Nonpolar solvents like toluene and xylene gave poor yield (16-23%, see the Supporting Information) and, for xylene, also poor selectivity (65%). For toluene, the yield could be improved by the use of high catalyst loadings, but only to the detriment of selectivity. Reactions using acetic anhydride as solvent gave no yield and catalyst loadings below 0.5 mol% significantly compromised the yield. Although the reaction works very well without external base (TON = 140, Table 1, entry 15), addition of catalytic amounts of organic bases such as NEt3 or pyridine improved the yield (TON = 170, entry 11). In contrast, no product was obtained when inorganic bases (K2CO3, NaOH, KOtBu, Na3PO4) were

a

Conditions: 1 mmol 1, 2 mmol Ac2O, 2 mL DMPU, 15h. bDetermined as the average of two experiments by 1H NMR using methyl benzoate as an internal standard, with a maximum ∆yield = ±5%. cDetermined as the average of two experiments by 1H NMR, with a maximum ∆selectivity = ±1%.

Having identified both the best catalyst and the optimal reaction conditions, we turned to investigate the scope of the transformation (Table 2). Naturally occurring long chain fatty acids provided the corresponding olefin in good yield and high LAO selectivity (entries 1-4). Remarkably, the selectivity is also high (> 90%) for allylbenzene derivatives 2g (entry 7) and 2i (entry 9), for which catalytic isomerization of the LAO products to the thermodynamically preferred internal olefins can be expected to be a significant side reaction. Equally remarkable is the fact that unprotected aliphatic alcohol and amine functionalities, which are prone to coordinate irreversibly to the metal center and deactivate the catalyst37-38 and for

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which catalytic decarbonylative dehydrogenation, despite attempts39, has not been reported earlier,2 are compatible with our catalyst and reaction conditions (entries 12-13). The strong hydrogen bonding of unprotected alcohols and amines in polar solvent (DMPU) is probably inhibiting the formation of esters and amides, respectively, upon treatment with excess acetic anhydride. Also the ester-functionalized olefin 2j was, as with an earlier in situ approach,13 obtained in satisfactory yield (entry 10). And even though the yield of the 2k is not impressive, entry 11 is still the first example of a brominefunctionalized olefin formed via decarbonylative dehydration. The lower yield is probably due to the volatility of the compound. On a preparative scale, 50 mmol of stearic acid was converted to the corresponding alpha olefin with a yield and selectivity similar to those of the small-scale reaction (see the Supporting Information), suggesting that our approach can be scaled up to prepare LAOs on a multi-gram scale.

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no coordination site available for the metal to bind these groups and deactivate the catalyst. In summary, we report here for the first time well-defined Pd(bis-phosphine) precatalysts for decarbonylative dehydration of carboxylic acids. The precatalysts are more catalytically active than traditional in situ systems, and the new, discrete palladium complex 5 has a TOF more than eight times higher than standard in situ protocols at 110 oC. Precatalyst 5 has a broad substrate scope and gives unprotected alcohol and amine functionalized olefins with good yields and achieves high selectivity even for allylbenzene derivatives. These capabilities together with the fact that high selectivity is obtained without in situ distillation make our precatalysts, and in particular complex 5, an attractive starting point for further developments of catalysts for decarbonylative dehydration of fatty acids.

EXPERIMENTAL SECTION

Scheme 1. Plausible reaction mechanism.

The reaction mechanism (Scheme 1) presumably involves the initial transformation of the carboxylic acid to the mixed anhydride17, 27 and activation of the precatalyst to Pd(0),40 followed by oxidative addition, decarbonylation, alkene formation, and catalyst regeneration.27 The performance of the well-defined precatalyst systems suggests that a 1:1 ratio between palladium and bis-phosphine ligand is beneficial and is consistent with the mechanistic suggestion that excess phosphine should be avoided27 at the same time as a Pd(bisphosphine) complex is present from the start and does not have to be formed (with the help of excess phosphine) in situ. The bis-phosphine ligand is probably bound to the metal throughout the catalytic cycle, as the compatibility with unprotected alcohol and amine functionalities suggests that there is

Procedure for Pd-catalyzed decarbonylative dehydration. Pd catalyst (0.005 mmol, 0.50 mol%) and stearic acid 1a (1 mmol, 1 equiv) was added to a 20 × 150 mm Kimble glass tube equipped with a magnetic stir bar. The tube was sealed with a rubber septum, evacuated and refilled with argon (3 times) using Schlenk technique. Anhydrous DMPU (2 mL) was then first added, followed by acetic anhydride (2 mmol, 2 equiv), and next NEt3 (9 mol% or excess). The reaction tube was placed in a preheated 110 °C oil bath and stirred for 15 h. The oil bath was removed to cool the reaction tube, followed by the addition of methyl benzoate (internal standard, 1 mmol, 1 equiv) and stirring of the resulting reaction mixture for 1 min. Et2O (ca. 10 mL) was added and the organic layer was washed with saturated NH4Cl solution followed by H2O and brine. Evaporation of the solvent afforded a crude mixture which was analyzed by 1H NMR using CDCl3 as solvent. Since NMR integrals are directly related to molarity, the molar ratio of product to the internal standard was obtained from the corresponding integral ratios calculated for the same number of protons in the internal standard and the olefin products. Synthesis of complex 5. A dry Schlenk flask equipped with a magnetic stir bar was charged with [Pd(Cinnamyl)Cl]2 (259 mg, 0.50 mmol, 0.50 equiv.) followed by DPEPhos (538 mg, 1.00 mmol, 1.00 equiv.). The flask was fitted with a rubber septum, and it was evacuated and backfilled with argon. This evacuation/argon backfill cycle was repeated two additional times. Anhydrous dichloromethane (2 mL) was added via syringe, and the reaction mixture was stirred at room temperature for 60 minutes. Diethyl ether was then added to fully precipitate the product. The solid materials were then collected by suction filtration, washed with additional diethyl ether, and dried in vacuo. The title compound was obtained as a yellow solid (636 mg, 85%). Single crystals suitable for X-ray diffraction were obtained by allowing Et2O to diffuse into a saturated solution of 5 in dichloromethane at -20 oC. The crystal structure was confirmed by X-ray crystallography (Figure 2), the CIF file for complex 5 and tabular material are in the Supporting Information. Details of the quantum chemical calculations. The geometries of complexes 4 and 5 were optimized in gas-phase using the hybrid range-separated functional ωB97X-D41-43 as implemented in Gaussian 0944. This functional includes empirical

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atom-atom dispersion, and has proved to reproduce X-ray geometries of transition metal complexes with high accuracy.45 Whereas the input geometry of 4 was constructed by modifying a similar molecular structure from X-ray diffraction46 using Spartan,47 the initial structure of 5 was that reported in this work (Figure 2). The geometries were optimized to a maximum force of 1.5 · 10−5 au using the opt=Tight option. Numerical integrations were performed with the ultrafine grid of Gaussian (Int=Ultrafine option, pruned, 99 radial shells and 590 angular points per shell) and the SCF density-based convergence criterion was tightened to 10-10 (option SCF=(Conver=10)). The located stationary points were characterized by the eigenvalues of the analytically calculated Hessian matrix. For palladium, the Stuttgart 28-electron relativistic effective core potential (ECP28MDF) was used in conjunction with the accompanying correlation consistent valence double-ζ plus polarization (cc-pVDZ-PP) basis set.48 All other atoms were described by standard correlation consistent valenc double-ζ plus polarization (cc-pVDZ)49-50 basis sets obtained from the EMSL basis set exchange web site.51 NBO6.0 was used to analyze the Pd-Cl bonds of complexes 4 and 5.34

AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Detailed experimental procedure and spectral data. X-ray data for Pd-complex 5.

ACKNOWLEDGMENT The authors gratefully acknowledge the Research Council of Norway for financial support via the Idélab initiative and the BIOTEK2021 program (grant number 238851) and for CPU and storage resources granted through the NOTUR (NN2506K) and NORSTORE (NS2506K) supercomputing programs. S.H.H.E. acknowledges the University of Bergen for a doctoral fellowship. Dr. Bjarte Holmelid and Mr. Wietse Smit are thanked for assistance with the HRMS analyses and 31P NMR spectroscopy, respectively.

ABBREVIATIONS DPEPhos, (Oxydi-2,1 phenylene)bis(diphenylphosphine); XantPhos, 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene; DMPU, 1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; ND, not determined.

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to illustrate the lack of covalent contribution to the Pd-Cl interaction. This bond thus had to be specified explicitly via the $CHOOSE keyword in order to be included in the NBO analysis. (37) Obora, Y.; Ishii, Y., Catalysts 2013, 3, 794. (38) Ammann, S. E.; Rice, G. T.; White, M. C., J. Am. Chem. Soc. 2014, 136, 10834-10837. (39) Le Notre, J.; Scott, E. L.; Franssen, M. C. R.; Sanders, J. P. M., Green Chem. 2011, 13, 807-809. (40) Melvin, P. R.; Balcells, D.; Hazari, N.; Nova, A., ACS Catalysis 2015, 5, 5596-5606. (41) Chai, J. D.; Head-Gordon, M., Phys. Chem. Chem. Phys. 2008, 10, 6615-20. (42) Wu, Q.; Yang, W., J. Chem. Phys. 2002, 116, 515-524. (43) Becke, A. D., J. Chem. Phys. 1997, 107, 8554-8560. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Gaussian, Inc.: Wallingford, CT, USA, 2009. (45) Minenkov, Y.; Singstad, A.; Occhipinti, G.; Jensen, V. R., Dalton Trans. 2012, 41, 5526-5541. (46) Allen, F. H., Acta cryst. B 2002, 58, 380-8. (47) Spartan '08, Wavefunction, Inc.: Irvine, CA, 2008. (48) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H., J. Chem. Phys. 2007, 126, 124101. (49) Woon, D. E.; Dunning, T. H., J. Chem. Phys. 1993, 98, 1358-1371. (50) Dunning, T. H., J. Chem. Phys. 1989, 90, 1007-1023. (51) Feller, D., J. Comput. Chem. 1996, 17, 1571-1586.

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Scheme 1. Plausible reaction mechanism. 96x111mm (300 x 300 DPI)

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Figure 1. Precatalysts screened for decarbonylative dehydration. 116x96mm (300 x 300 DPI)

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Figure 2. X-ray crystal structure of 5 (hydrogen atoms and sol-vent omitted for clarity). Selected bond lengths: Pd1-C1 2.133(4) Å; Pd1-C2 2.200(4) Å; Pd1-C3 2.373(4) Å; Pd1-P1 2.3370(10) Å; Pd1-P2 2.2978(10) Å; Pd1-Cl1 2.7216(10) Å. Bite angle P2–Pd1–P1 = 102.19(4)°. 254x190mm (300 x 300 DPI)

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