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DOI: 10.1055/s-0032-1317963
The [RPPh3]2[Pd2X6] as a Catalyst Precursor for the Heck Cross-Coupling Reaction by in situ Formation of Stabilized Pd(0) Nanoparticles
Publication History
Received: 10 November 2012
Accepted after revision: 10 December 2012
Publication Date:
08 January 2013 (online)
Abstract
Pd(II) anionic, square planar complexes of the type [RPPh3]2[Pd2X2Cl4], where X = Cl, Br, have been applied for the first time as a catalyst precursor for the Heck reaction carried out in DMF at 140 °C. The highest yield was obtained for the most reducible ones, [MePPh3]2[Pd2Br2Cl4], in DMF in the presence of NaHCO3 as a base. It was found that during the reaction, phosphonium halide stabilized Pd(0) nanoparticles of about 10 nm, which have been formed in situ from the palladium(II) precursor and Pd(0) colloidal nanoparticles acts as the reservoir for Pd(II) species via activation of the metal surface through the oxidative addition of aryl halides.
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Since its discovery in the early 1970s, the Heck reaction has been applied to a diverse array of fields, ranging from natural products synthesis to materials science as well to bioorganic chemistry. This powerful carbon–carbon bond-forming process has been practiced on an industrial production of fine chemicals, such as octyl methoxycinnamate.[ 1 ] This kind of reaction possesses several advantages in comparison to the most classical organic methods, including their excellent functional group tolerance, mild reaction conditions, less waste, and sometimes fewer steps than the original stoichiometric routes.[ 2 ] In the past decade, new generations of homogeneous catalysts such as palladacycles, Pincer ligands, and more recently bulky electron-rich ligands have extended the scope of the reaction to all aromatic halides, even the poorly reactive aryl chlorides. In general, homogeneous catalysis suffers from several shortcomings, such as: the ligands are required to form the active catalyst, the ligands may be expensive or not commercially be available in bulk; furthermore, removal of residual ligand or palladium provides a challenging task for chemists in the pharmaceutical industry to reduce the palladium to a level that satisfies specifications required by regulators.[ 3 ] Herein, we report the use of a palladium complex of the type [RPPh3]2[Pd2X2Cl4] with the quaternary phosphonium salts for the first time as an efficient catalyst for Heck reaction of aryl halides with alkenes with quantitative yields.
Dimeric palladium(II) ammonium, imidazolium and pyridinium complexes have been previously used as catalysts for chemical reactions in literature.[4] [5] [6] [7] However, to the best of our knowledge, dimeric palladium(II) phosphonium complexes have never been used before as catalyst precursors for the chemical reactions.
Phosphonium salts display higher stability towards thermal and chemical degradation in comparison to their ammonium counterparts. A few reports are available for the preparation of dipalladium(II) complexes containing phosphonium counterion[8] [9] and these dipalladium complexes have never been used before as catalyst precursors. Kaufmann reported the application of hexadecyltributylphosphonium bromide as reaction medium for Heck reaction of aryl halides at 100 °C in 16–43 hours.[ 10 ] Capretta explored the effect of counter anion matched to the phosphonium cation on the yields of the Heck reaction. He found that the best counter anion were Cl and decanoate.[ 11 ] In this work, we prepared and examined the catalysts 1–5 with different halogen bridge and R group attached to the phosphonium salt in the Heck reaction (Figure [1]).
Our first goal was to synthesize the palladium(II) catalysts with the general formula [RPPh3]2[Pd2X2Cl4] with different substituent R on phosphonium cations and Cl or Br ions coordinated to Pd(II) as shown in Figure [1]. Phosphonium salts were prepared from the reaction of triphenylphosphine and alkyl or benzyl bromides/chlorides and the resulting phosphonium salts then were reacted with palladium chloride in toluene at reflux temperature to prepare the catalysts 1–5 and then the applications of the resulting catalysts were investigated in the Heck coupling of 4-bromoacetophenone to determine the effect of phosphonium salt counter cations and halogen bridges in the catalysts on the yield of the reaction. Figure [2] shows the results for the reaction of 4-bromoacetophenone and methyl acrylate using [RPPh3]2[Pd2X2Cl4] as the catalysts and NaHCO3 as the base. Results showed that in contrast to the Capretta, the counter anion on the phosphonium cation and bulkiness of the phosphonium cation plays no significant role in the outcome of the reaction, which may be due to the decreasing selectivity at a higher temperatures. Although the results for these catalysts were comparable, we chose [MePPh3]2[Pd2Cl6] (4) as the catalyst.
As demonstrated in Figure [2] the cis/trans selectivities for catalysts 1–5 are similar. Therefore, it can be concluded that: (1) if the homogenous Pd(II) acts as active catalyst, phosphonium cation may be far from the palladium(II) center, or (2) the active Pd(0) species with Pd(0) colloidal nanoparticles or highly active forms of low coordinated Pd(0) species are the active sites of reaction, so, the type of bridge or aryl/alkyl groups of phosphonium catalyst does not significantly affect the yield and selectivity of the reaction. To determine that the Pd(II) or Pd(0) is involved in this reaction, we performed the mercury poisoning test with Hg(0). This test is based on the deactivation of colloidal metal via amalgam formation. The addition of excess of Hg(0) to the Heck reaction showed an extremely strong inhibition of the catalytic activity, which implies the presence of active Pd(0) species.
In all of the Heck reactions under study, with different substrates, a thick colloid of Pd(0) was formed after about two minutes. Its formation was first signalized by the change of the red color of Pd(II) solution to the almost black Pd(0) after about two minutes due to the change in the UV–VIS spectra of the catalyst. As shown in Figure [3], palladium(II) complex exclusively reduced to Pd(0) and the band at λ = 267 nm, which has been assigned to [MePPh3]2[Pd2Br2Cl4], disappeared. Decomposition of this complex leads to the formation of phosphonium chloride that may act as a stabilizer for the palladium nanoparticles.[ 12 ]
Figure [4] shows the TEM image of the reaction solution after about two minutes of reaction of 4-bromoacetophenone and methyl acrylate. This image clearly indicated the formation of Pd nanoparticles during the Heck reaction having an average diameter of 10 nm. The cyclic voltammetry (CV) diagram of the catalysts 1–5 (Figure [5]) showed that the order of reducibility is R = Me > R = Bu > R = Bn > R = anthrylmethyl and X = Br > X = Cl. From the cyclic voltammetry diagram, it was determined that [MePPh3]2[Pd2Br2Cl4] has the highest susceptibility for reduction to palladium nanoparticles. As can be seen from Figure [2], this catalyst has the highest yield in the Heck reaction.
These results indicate that the Pd(0) nanoparticles may act as a reservoir for Pd(II) species via activation of the metal surface with haloarenes.[ 13 ] Therefore, we suggest the possible catalytic cycle for this reaction as described in Scheme [1].
Initially, for finding the best conditions for coupling of 4-bromoacetophenone and methyl acrylate in the presence of catalyst 4, several conditions including the effects of solvent, base, temperature, reaction time and the amount of catalysts on the yield of the coupling reaction were investigated.
a GC yields. Reaction conditions: 4-bromoacetophenone (0.25 mmol), methyl acrylate (1.5 mmol), catalyst (0.01 mmol), base (0.25 mmol), DMF (1.5 mL), 140 °C (oil bath) and 45 min.
b The amount of base used was 2 equiv.
As is demonstrated in Figure [6], this reaction leads to the formation of trans isomer exclusively in 99% conversion based on GC. Several bases were tested for the reaction and the results showed that the effect of bases on the reaction was significant (Table [1]). When no base was used in this reaction, only trace amount of the desired product was detected. A comparative study of various bases, such as KOH, Et3N, Na2CO3, NaHCO3 and K2CO3, revealed that NaHCO3 gave the best result. Using, two equivalents of NaHCO3 gave the coupling reaction in highest yields.
The amount of catalysts was also evaluated by using various concentration of [MePPh3]2[Pd2Br2Cl4]. The highest yield was obtained for 0.04 mmol, whereas at lower and higher concentrations of the catalyst precursor, the obtained yields were lower. It is probably due to the formation of larger palladium nanoparticles at higher concentrations.
Furthermore, the effect of the solvent was investigated (Table [2]) and polar aprotic solvents were found to be better solvents than water and alcohols and the best result (99.5%) was obtained for the reaction in DMF. For DMAc and NMP, the conversion was 79.2% and 97.7% respectively, whereas for water, MeCN and ethanol, no products were obtained. In addition, temperature as an important factor for this reaction was examined by choosing the range of 100–140 °C and the results are shown in Figure [7].
a GC yields. Reaction conditions: 4-bromoacetophenone (0.25 mmol), methyl acrylate (1.5 mmol), catalyst (0.01 mmol), NaHCO3 (0.5 mmol), solvent (1.5 mL), 140 °C (oil bath) and 45 min.
By employing the mentioned optimized conditions, other substrates including, aryl iodides, bromides, chlorides and olefins were coupled with methyl acrylate, methyl methacrylate, acrylic acid and styrene in the presence of catalyst 4.
a Reaction conditions: aryl halide (1 mmol), alkene (1.2 mmol), [MePPh3]2[Pd2Br2Cl4] (0.04 mmol), NaHCO3 (2 mmol), DMF (1.5 mL) at 140 °C without protection of inert atmosphere.
As shown in Table [3], all aryl iodides were rapidly converted into the corresponding Heck products with excellent yields (Table [3], entries 8, 11, 17 and 18). Although the coupling of bromoarenes with an electron-donating substituent (entry 4) was considerably decreased, the coupling reaction of both electron-deficient and electron-rich aryl bromides with the olefins proceeded smoothly with good to excellent yields. The coupling reactions with methyl acrylate, methyl methacrylate and acrylic acid were faster than that with styrene.
The catalyst was also confirmed to be very active for 1,1-disubstituted alkene substrates. For example, methyl methacrylate was transformed into α-methyl cinnamate derivatives in good yields and only small amounts of diarylated products were detected (Table [3], entries 15–18).
Aryl chlorides needed longer times and provided lower yields. This lower reactivity may be attributed to the reluctance of the aryl–chloride bond at oxidative addition step.[ 1 ]
In this contribution we have demonstrated the applicability of dimeric palladium(II) phosphonium complexes in Heck coupling reactions and investigated the effect of several reaction parameters such as temperature, amount of catalyst, base and starting compounds on catalytic activity.[ 14 ] By using the Hg(0) test, UV–VIS spectroscopy, transmission electron microscopy and cyclic voltammetry, we concluded that Pd(0) colloidal nanoparticles act as reservoir for Pd(II) species via activation of the metal surface with haloarenes.
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Acknowledgment
We gratefully acknowledge the funding support received for this project from the Isfahan University of Technology (IUT), IR Iran (A.R.H.) and Grant GM 33138 (A.E.R.) from the National Institutes of Health, USA. Further financial support from the Center of Excellency in Chemistry Research (IUT) is gratefully acknowledged.
Supporting Information
- for this article is available online at http://www.thieme-connect.com.accesdistant.sorbonne-universite.fr/ejournals/toc/synlett.
- Supporting Information
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References and Notes
- 1 Littke AF, Fu GC. J. Am. Chem. Soc. 2001; 123: 6989
- 2 Consorti CS, Flores FR, Dupont J. J. Am. Chem. Soc. 2005; 127: 12054
- 3 Schils D, Stappers F, Solberghe G, van Heck R, Coppens M, Van den Heuvel D, Van der Donck P, Callewaert T, Meeussen F, Bie ED, Eersels K, Schouteden E. Org. Process Res. Dev. 2008; 12: 530
- 4 Bouquillon S, du Moulinet d’Hardemare A, Averbuch-Pouchot M.-T, Hénin F, Muzart J. Polyhedron 1999; 18: 3511
- 5 Dullius JE. L, Suarez PA. Z, Einloft S, de Souza RF, Dupont J, Fischer J, De Cian A. Organometallics 1998; 17: 815
- 6 Zawartka W, Gniewek A, Trzeciaka AM, Ziłkowski JJ, Pernak J. J. Mol. Catal. A: Chem. 2009; 304: 8
- 7 Zawartka W, Trzeciak AM, Ziłkowski JJ, Lis T, Ciunik Z, Pernak J. Adv. Synth. Catal. 2006; 348: 1689
- 8 Baig S, Richard B, Serp P, Mijoule l, Hussein K, Guihéry N, Barthelat J, Kalck P. Inorg. Chem. 2006; 45: 1935
- 9 Makitova DD, Krasochka ON, Atovmyan LO, Letuchii YA, Roshchupkina OS. Coord. Chem. (Engl. Transl.) 1987; 13: 928
- 10 Kaufmann DE, Nouroozian M, Henze H. Synlett 1996; 1091
- 11 Gerritsma DA, Robertson A, McNulty J, Capretta A. Tetrahedron Lett. 2004; 45: 7629
- 12 Bhattacharya S, Srivastava A, Sengupta S. Tetrahedron Lett. 2005; 46: 3557
- 13 Precht MH. G, Scholten JD, Dupont J. Molecules 2010; 15: 3441
- 14 Typical Procedure for the Heck Arylation of Aryl Halides with Methyl Acrylate: Palladium complex ([MePPh3]2[Pd2Br2Cl4]) (0.01 mmol), methyl acrylate (0.5 mmol), aryl halides (0.25 mmol), NaHCO3 (0.5 mmol) and DMF (1.5 mL) were added to a 5-mL round-bottomed flask equipped with an efficient condenser and stirred at 140 °C in an oil bath for the time specified in Table 3. The course of the reaction was monitored by periodically taking samples and analyzing them by means of GC analysis. After cooling of the reaction mixture to r.t., the aqueous solution was extracted with hexane, the organic phase was dried over MgSO4, and the solvent was then removed under vacuum. Column chromatography on silica gel afforded the desired product. The product was characterized by 1H NMR and IR.
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References and Notes
- 1 Littke AF, Fu GC. J. Am. Chem. Soc. 2001; 123: 6989
- 2 Consorti CS, Flores FR, Dupont J. J. Am. Chem. Soc. 2005; 127: 12054
- 3 Schils D, Stappers F, Solberghe G, van Heck R, Coppens M, Van den Heuvel D, Van der Donck P, Callewaert T, Meeussen F, Bie ED, Eersels K, Schouteden E. Org. Process Res. Dev. 2008; 12: 530
- 4 Bouquillon S, du Moulinet d’Hardemare A, Averbuch-Pouchot M.-T, Hénin F, Muzart J. Polyhedron 1999; 18: 3511
- 5 Dullius JE. L, Suarez PA. Z, Einloft S, de Souza RF, Dupont J, Fischer J, De Cian A. Organometallics 1998; 17: 815
- 6 Zawartka W, Gniewek A, Trzeciaka AM, Ziłkowski JJ, Pernak J. J. Mol. Catal. A: Chem. 2009; 304: 8
- 7 Zawartka W, Trzeciak AM, Ziłkowski JJ, Lis T, Ciunik Z, Pernak J. Adv. Synth. Catal. 2006; 348: 1689
- 8 Baig S, Richard B, Serp P, Mijoule l, Hussein K, Guihéry N, Barthelat J, Kalck P. Inorg. Chem. 2006; 45: 1935
- 9 Makitova DD, Krasochka ON, Atovmyan LO, Letuchii YA, Roshchupkina OS. Coord. Chem. (Engl. Transl.) 1987; 13: 928
- 10 Kaufmann DE, Nouroozian M, Henze H. Synlett 1996; 1091
- 11 Gerritsma DA, Robertson A, McNulty J, Capretta A. Tetrahedron Lett. 2004; 45: 7629
- 12 Bhattacharya S, Srivastava A, Sengupta S. Tetrahedron Lett. 2005; 46: 3557
- 13 Precht MH. G, Scholten JD, Dupont J. Molecules 2010; 15: 3441
- 14 Typical Procedure for the Heck Arylation of Aryl Halides with Methyl Acrylate: Palladium complex ([MePPh3]2[Pd2Br2Cl4]) (0.01 mmol), methyl acrylate (0.5 mmol), aryl halides (0.25 mmol), NaHCO3 (0.5 mmol) and DMF (1.5 mL) were added to a 5-mL round-bottomed flask equipped with an efficient condenser and stirred at 140 °C in an oil bath for the time specified in Table 3. The course of the reaction was monitored by periodically taking samples and analyzing them by means of GC analysis. After cooling of the reaction mixture to r.t., the aqueous solution was extracted with hexane, the organic phase was dried over MgSO4, and the solvent was then removed under vacuum. Column chromatography on silica gel afforded the desired product. The product was characterized by 1H NMR and IR.
