RSC Advances REVIEW Suzuki–Miyaura reaction by heterogeneously supported Pd in water: recent studies Cite this: RSC Adv., 2015, 5, 42193
Susmita Paul,*a Md. Mominul Islamc and Sk. Manirul Islam*b Received 31st December 2014 Accepted 7th April 2015 DOI: 10.1039/c4ra17308b www.rsc.org/advances
This review summarizes the progress made essentially in the last fifteen years in the Suzuki–Miyaura coupling reaction by heterogeneous palladium catalysis in water as the sole solvent. The discussion focuses on the heterogenization of the palladium catalyst, efficiency and reusability of the heterogeneous catalysts as well as on the reaction conditions from a sustainable chemistry point of view.
Introduction For the last few decades, palladium remains the most useful transition metal catalyst in the array of transformations in organic synthesis, in particular for carbon–carbon bond formations.1 The unique nature of the palladium catalyst for selective reactions, easy tuning of the catalyst reactivity and selectivity by ligands or additives, and the high turnover numbers (TONs) and turnover frequencies (TOFs) using extremely small amounts of palladium (ppm or ppb levels) under milder conditions, are the main reasons for attracting researchers, and as a result of these facts a number of palladium catalysts are commercially available2 and employed in many areas, including natural product syntheses.3 Among the different types of palladium-catalyzed reactions, the Suzuki– Miyaura reaction, which is the reaction between aryl halides and arylboronic acids, represents possibly the most important and widely used one.4
Suzuki–Miyaura reaction
unactivated alkyl halides, enabling C(sp2)–C(sp3) and even C(sp3)–C(sp3) bond-forming processes.1h,i,7 The non-toxicity and simplicity related to the preparation of organoboron compounds (e.g. aryl, vinyl, alkyl),5b,c their relative stability to air and water, combined with relatively mild reaction conditions as well as the formation of nontoxic by-products, makes the Suzuki–Miyaura reaction an important method for enlarging the carbon skeleton. The general and widely accepted mechanism of the Suzuki– Miyaura reaction is depicted in Fig. 1. The rst step is the oxidative addition of palladium 1 to halide 2 to form the organopalladium species 3. Reaction of the organopalladium species with a base gives intermediate 4, which via transmetalation with boronate complex 6 forms the organopalladium species 8. Reductive elimination of the desired product 9 restores the original palladium catalyst 1.
Water – a green reaction medium From the academic as well as industrial viewpoints, alternative reaction media are of considerable concern at present for
The Suzuki–Miyaura reaction is characterised by the crosscoupling of two aryl subunits, one from an aryl boronic acid or its derivative and the other from an organohalide or -triate, to give a biaryl motif.1,5 The relative reactivity order is as follows: R–I > R-OTf > R–Br [ R–Cl. This reaction has become one of the most adaptable methods for the expansion of the carbon framework in organic molecules since its discovery in 1979.6 Amongst its wide applicability, the Suzuki–Miyaura reaction is particularly useful as a way of assembling conjugated diene and higher polyene systems of high stereoisomeric purity, as well as biaryl and related systems. Incredible progress has been made in the development of Suzuki–Miyaura coupling reactions of a
Department of Chemistry, University of Kalyani, Nadia, West Bengal, India. E-mail:
[email protected]; Fax: +91 33 2582 8282; Tel: +91 94 7419 7728
b
Department of Chemistry, University of Kalyani, Nadia, West Bengal, India. E-mail: manir65@rediffmail.com; Fax: +91 33 2582 8282; Tel: +91 33 2582 8750
c
Fig. 1 Schematic representation of the general mechanism of the Suzuki–Miyaura coupling reaction.
Department of Chemistry, University of Kalyani, Nadia, West Bengal, India
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making this palladium catalyzed cross-coupling process “greener” by minimizing the use of organic solvents.8 Water is the obvious foremost choice in conjunction with cost, environmental benets, and safety. The use of water in Pd-catalyzed cross coupling reactions dates back to the early development of the Suzuki–Miyaura coupling9 with the rst example being reported by Calabrese and co-workers in 1990.10 Since then, a large number of water-soluble Pd catalysts bearing hydrophilic ligands have been reported, and several reviews have been devoted to this subject.11 Attempts have been made for synthesizing water-soluble catalysts or water-soluble ligands,12 adding surfactants or phase-transfer agents,13 using organic co-solvents or inorganic salts as a promoter,14 and utilizing microwave heating or ultrasonic irradiation.15 The use of water as reaction medium would be practically green and welcoming only when any traces of organics or metals can be fully removed from the water used for the reaction.16 For metal catalyzing reactions in which the catalysts are to some extent water soluble, heterogeneous catalysis could solve this issue using a water insoluble support or catalyst which can easily be removed from the medium by ltration. Moreover, for large scale processes, organic products can be separated by simple decantation.
Scope and limitations of heterogeneous catalysis For the synthesis of symmetrical and nonsymmetrical biaryls the palladium-catalyzed carbon–carbon coupling reaction remains an important method, and a broad variety of homogeneous catalytic systems have been developed to achieve this transformation,17 mainly because homogeneous catalysts display high activity and are better dened and understood. Although homogeneous catalysts have many advantages, the complications regarding the separation and recovery of the catalyst, and product contamination with traces of heavy metals, could not be ignored, which limit their applications in chemical and pharmaceutical industries,18 and become an issue of great economic and environmental concern especially for expensive and/or toxic heavy metal complexes.19 These limitations of homogeneous catalysis have resulted in the progress of new strategies for transition-metal catalysis which facilitate catalyst recovery and recycling.20 Recently, many recoverable, supported palladium catalysts have been reported to catalyze Suzuki–Miyaura coupling reactions such as polymers, biomaterials, porous silica, carbon nanotubes, polyurea, natural phosphates etc.21 However, some supported catalysts which are known as heterogeneous catalysts, oen resulted in a signicant loss of catalytic activity when reused and leaching of transition metal during the reaction,22 and the nature of the true catalyst is still unclear. The problem of distinguishing homogeneous from heterogeneous catalysis is an important question that arises more and more oen, in particular when heterogeneous systems are developed. Heterogeneous catalytic systems may partly dissolve to yield a homogeneous component which might be much more reactive than the parent metal surface. For this reason, in some
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cross-coupling reactions especially with facile substrates catalyzed by trace amounts of metal of various origins, checking is necessary for the genuine source of catalytic metal. Careful kinetic studies, ltration tests, selective poisons for catalysts in solid or soluble systems, and Rebek–Collman 3-phase tests are very helpful and informative to solve the question of heterogeneity.
Aim of the review The aim of this review is to provide an overview of heterogeneous palladium chemistry for the Suzuki–Miyaura crosscoupling reaction in water as the sole reaction medium or solvent. Since water is genuinely useful for green chemistry as a solvent itself, only protocols carried out in water are covered. One review article23 in this regard by Felpin and co-authors is worth mentioning. The reactions which are not mentioned in that article and the newer reports (up to September, 2014), including all the heterogeneous systems already mentioned by Felpin, are comprised in this review article. The reports consisting of semi-heterogeneous or quasi heterogeneous catalysts, reactions carried out by soluble supports and examples from the patent literature are discarded in this review. Supports are mainly divided into three categories, viz. inorganic, organic and hybrid of inorganic–organic materials, and the discussion is again subdivided according to necessity and for lucidness. Inorganic supports Palladium supported on carbon. Bumagin and Bykov have reported the cross-coupling of water-soluble 3-bromobenzoic acid with tetraphenylborate in neat water using Pd(0)/C (Scheme 1).24 This report is the rst example of a Pd/Ccatalyzed Suzuki–Miyaura reaction in neat water. The coupling between iodophenols and boronic acids at room temperature (Scheme 2) could be performed using K2CO3 as the base with a lower loading of Pd/C (0.3 mol%).25 The obtained yields were excellent and fairly independent of the nature of the boronic acid. The reactivity order decreased from iodophenol to bromophenol, and the successful reaction required higher temperatures. Aer completion of the reaction, the Pd/C catalyst was recovered by simple ltration and reused ve times with only a slight decrease in activity. For non-water-soluble aryl halides, a number of reports use surfactants as additives to increase the solubility. Arcadi and coworkers used cetyltrimethylammonium bromide (CTAB) for this purpose, which was found to be quite effective when combined with K2CO3 with a catalyst loading (Pd/C) of 5 mol%
Scheme 1
Use of NaB(Ph)4 as the phenylboronic acid substitute.
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Scheme 2
Cross-coupling of iodophenols with boronic acids.
Scheme 3
Suzuki–Miyaura reactions using CTAB as a surfactant.
(Scheme 3).26 Recycling of the ltered and used catalyst suffered from gradually diminished activity which raises the question of true heterogeneity. Xu and co-workers27 (Scheme 4) established that watersoluble bromoarenes react efficiently with sodium tetraphenylborate in reuxing water even in the presence of 0.0025 mol% of Pd/C when the reaction time was prolonged from 1 to 7 h. Comparison of the inorganic bases utilized showed the suitability of sodium bases over potassium ones. The reusability of the catalyst showed capability over ve cycles but with gradual loss of reactivity (Fig. 2). Coupling of aryl chlorides with aryl boronic acids using ligandless Pd/C in water has been described by Kohler and Lysen (Scheme 5).28 All reactions were performed under an ambient atmosphere to reduce homocoupling. Activated aryl chlorides reacted at lower palladium concentrations (0.2–0.5 mol%) while deactivated chloroarenes required higher catalyst concentrations (2.0 mol%) and longer reaction times (six hours). Addition of TBAB was found to be essential, and NaOH was found to be the superior base among the several bases tested. Aryl iodides and bromides could also be completely converted to the corresponding biaryls by a small variation of the reaction conditions. Recovery of the catalyst was performed by simple ltration through celite or by centrifugation. Not only boronic acids but also boronate esters and potassium triuoroborate salts were effective under the reaction conditions developed by these authors. The reactivation using iodine as the oxidizing agent [Pd(0) to Pd(II)] was necessary to improve the recycling ability of the catalyst, showing consistent activity over three cycles.
Scheme 4
Suzuki–Miyaura reactions by Xu and co-workers.
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Fig. 2 Proposed mechanism of the Pd/C-catalysed cross-coupling of aryl bromide with sodium tetraarylborate by Xu and co-workers.
Scheme 5
Approach by Kohler and co-workers.
The benecial effect of microwave heating was explored by Freundlich and Landis (Scheme 6)29 for the coupling of boronic acids with bromophenols. A variety of boronic acids were coupled at 120 C in aqueous potassium hydroxide for short reaction times (15 min). The reactions remained unsuccessful with the chloro substituent. The coupling of substituted bromophenol with potassium phenyl triuoroborate salt was less efficient than with phenyl boronic acid under the optimized reaction conditions. Arvela and Leadbeater reported a combined TBAB and microwave activation procedure for the cross-coupling of aryl chlorides with boronic acids (Scheme 7).30 For substrates bearing electron-withdrawing groups, the effects of
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Suzuki–Miyaura reactions using microwave irradiation by Freundlich and Landis.
Scheme 6
Scheme 7
Catalytic system developed by Leadbeater et al.
simultaneous cooling on the product yield were not signicant, attributed to the fact that the coupling reaction is faster than the decomposition of the chloride substrate, but with substrates bearing electron-neutral or electron-donating substituents, simultaneous cooling signicantly increased the product yield. Very fast reaction rates were observed in only ten minutes and the method was found to be efficient for aryl chlorides containing electron-withdrawing groups. The coupling of bromoarenes with tetraphenylborate (Scheme 8) has been described by Bai using a similar catalytic system.31 Excellent yields were obtained in less than twenty minutes at 120 C in the presence of K2CO3 as the base with a comparatively higher palladium loading (5 mol% Pd). The catalyst showed excellent recycling ability over more than ve cycles. Modied multi-walled carbon nanotubes have been proposed as a support for palladium nanoparticles for the crosscoupling in neat water.32 4-Dimethylaminopyridine (DMAP) stabilized palladium nanoparticles were prepared by mixing solutions of Na2PdCl4 and DMAP followed by the reduction with NaBH4. Thiol-modied multi-walled carbon nanotubes (MWCNTs), prepared by a carbon arc discharge method, were functionalized via an amide coupling reaction followed by
Scheme 8
Coupling of bromoarenes with tetraphenylborate by Bai.
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sequential treatment with HNO3, KMnO4, HClO4, citric acid, DMAP and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDAC) and 2-mercaptoethylamine hydrochloride (Fig. 3). Multi-walled carbon nanotube/DMAP-stabilized Pd nanoparticle composites (MWCNT/Pd–DMAP NP composites, catalyst 14) were prepared by the addition of a known amount of a DMAP-stabilized palladium nanoparticle dispersion to thiolmodied multi-walled carbon nanotubes under sonication. The 4-iodo-substituent gave 87% conversion within 10 min with 0.004 mol% of catalyst 14, while the bromo-substituent gave 53% conversion and the chloro-analogue gave 25% conversion with 0.024 mol% of catalyst 14 when reuxing for 6 h. The catalyst was recovered by ltration through a polycarbonate lter, and experimental studies showed good recyclability over six runs with leaching of the Pd species below the detection limit of AAS (Scheme 9). Graphene modied with palladium nanoparticles by reducing palladium acetate [Pd(OAc)2] in the presence of sodium dodecyl sulfate (SDS) was reported by Zhang and coworkers (Scheme 10)33 (SDS is used as both surfactant and the reducing agent). The palladium nanoparticle–graphene hybrids (Pd–graphene hybrids, catalyst 15) were characterized by spectrometric methods, and HRTEM showed that the mean size of the Pd nanoparticles dispersed on the graphene sheets is about 4 nm. Catalyst 15 acted as an efficient catalyst for the Suzuki– Miyaura reaction under aqueous and aerobic conditions, with the reaction reaching completion within 5 min. Bromobenzene and allyl iodides were also employed in this coupling reaction
Fig. 3 Schematic representation for the preparation of MWCNT/Pd– DMAP NP composites.
Scheme 9 Suzuki–Miyaura reaction of interest, using the MWCNT/ Pd–DMAP NP composites, between phenylboronic acid and 4-halobenzoic acids.
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Preparation of the nanocrystalline MgO-stabilised palladium catalyst 17.
Fig. 4
Scheme 10 Work by Zhang and co-workers.
but produced poor yields. The catalysts were recovered by simple centrifugation and reused successfully for ten consecutive runs. Palladium supported on metal oxides. A sepiolite-supported palladium(II) catalyst (catalyst 16) has been prepared by mixing sepiolite with an aqueous solution of [Pd(NH3)4]Cl2 at 298 K for 48 h, followed by centrifuging and washing with deionized water, and subsequent drying in vacuo at the same temperature. The prepared catalyst was successfully used for the crosscoupling of 4-bromophenol with phenylboronic acid or tetraphenylborate at room temperature (Scheme 11)34 with a palladium loading of 0.1 mol%. The Pd(II)/sepiolite catalyst could be reused three times without any apparent deactivation. Successful reaction could be achieved by decreasing the catalyst loading to 0.0001 mol% but with higher temperatures. Highly basic nanocrystalline magnesium oxide (NAP-MgO) as a palladium nanoparticle support has been exploited for the Suzuki–Miyaura coupling reaction (Scheme 12).35 Fig. 4 shows the schematic presentation for the preparation of Pd– NAP-MgO (catalyst 17). The cross-couplings of iodo- and bromoarenes with arylboronic acids were efficiently carried out in water, but the reactions involving chloroarenes were performed in DMA with catalyst 17. The cross-couplings were completed in
only 5 to 6 h at room temperature at a quite low loading (0.5 mol%); an even lower loading as low as 0.01 mol% is also effective with longer reaction times (40 h). It is supposed that the high activity of the catalyst is due to the nanostructured MgO material that possesses a high surface area (600 m2 g1) and a strong basicity. The catalyst was recyclable for all reactions up to ve cycles with almost consistent activity. Artok and co-workers prepared a highly active catalyst (catalyst 18) by loading NaY zeolite with Pd(NH3)4Cl2 (Scheme 13)36 (NaY zeolite (SiO2/Al2O3 molar ratio: 5.1)) by ion exchange, which gave good yields for the corresponding biphenyl compounds by cross-coupling soluble and insoluble aryl bromides with benzeneboronic acid with catalyst loadings of (0.01–0.001) mol% in water. Electron-rich bromoarenes were found to be much less reactive and required the use of surfactants such as TBAB or CTAB for better results. A possible instability of the catalytic system under localized heating was established by performing the reaction under microwave heating. The Pd/ZrO2 nanocatalyst formed by electrochemical impregnation of nanostructured tetragonal ZrO2 with palladium nanoparticles (PdNPs/ZrO2, catalyst 19) was demonstrated to be a very efficient catalyst in Suzuki–Miyaura reactions of aryl halides in water, by Nicola Cioffi and coworkers (Scheme 14).37 The catalyst efficiency was attributed to the stabilization of Pd nanophases provided by tetra alkyl ammonium hydroxide, which behaved both as a base as well as a PTC (phase transfer catalyst) agent. The Suzuki–Miyaura cross-coupling reactions were carried out in water at 90 C using
Scheme 11 Example of a cross-coupling with an ultra low catalyst
loading.
Scheme 13
Scheme 12 Examples of cross couplings with NAP-MgO–Pd(0) as the
Scheme 14
catalyst.
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Work by Artok and co-workers.
Suzuki–Miyaura reactions catalyzed by Pd-NPs/ZrO2 (catalyst 19) in water.
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aryl bromides and iodides as substrates and phenyl boronic acids. The supported catalyst could be recycled up to ten times without any appreciable loss of activity which was supported by an average yield of 83% for a number of aryl bromides, except for the less reactive electron-rich 4-bromoanisole. Palladium supported on hydroxyapatite. Hydroxyapatitesupported palladium(0) (Pd/HAP, catalyst 20) was prepared by stirring a mixture of hydroxyapatite and Pd(OAc)2 in ethanol followed by the dropwise addition of hydrazine hydrate (80%) under continuous stirring, and conditioning of the catalyst by reuxing for 6 h in each ethanol, toluene and acetonitrile. The conditioned catalyst was quite stable and could be used for several days. The TEM micrograph showed an average palladium particle diameter of about 20 nm on hydroxyapatite. Suzuki–Miyaura cross-couplings of bromoarenes with arylboronic acids were carried out with this catalyst in the presence of TBAB as a surfactant and K2CO3 as the base (Scheme 15).38 Paul and co-workers obtained excellent yields for biphenyl compounds from facile substrates using the prepared catalyst 20 (0.33 mol% Pd/HAP). The stability of the hydroxyapatite supported palladium catalyst was demonstrated by the recycling ability studied for the coupling of 4-bromoacetophenone with benzeneboronic acid over ve cycles with no apparent deactivation of the reused catalyst. Two types of these supported palladium catalyst, one by immobilization of [Pd(COD)Cl2] (COD ¼ 1,5-cyclooctadiene) on hydroxyapatite (catalyst 21) and another catalyst by subsequent reduction of the previous catalyst with sodium borohydride (catalyst 22), were prepared (Fig. 5) for the Suzuki–Miyaura coupling reaction in water.39 The catalyst with Pd2+, was found to be almost ve times more active than the reduced catalyst under similar reaction conditions. The best catalytic activities were observed in the presence of potassium carbonate as the base and tetrabutylammonium bromide as a promoter using the non-reduced catalyst and water as the solvent under aerobic conditions (Scheme 16). This catalyst system has been tested for different electronically neutral, electron-rich, electron-poor and
Review
Scheme 16 Suzuki–Miyaura reactions using the Pd–HAP catalyst 21.
sterically hindered aryl boronic acids, and several different aryl halides including aryl chlorides. More than one thousand turnovers and high selectivities toward the hetero-coupled products have been observed in most cases. A negligible drop in activity was observed over ten cycles. Palladium supported on mesoporous silica. Ordered mesoporous MCM-41 material has been used as a suitable support for uniformly sized palladium nanoparticles (Fig. 6, catalyst 22) by Sayari and Das, and has been explored in Suzuki–Miyaura
Scheme 15 Suzuki–Miyaura reactions using Pd/HAP as the catalyst.
Fig. 5
Synthetic outline for the synthesis of catalysts 21 and 21a.
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Fig. 6 Schematic outline of the synthesis of catalyst 22 from poreexpanded MCM-41 and supported monodispersed Pd nanoparticles.
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reactions (Scheme 17).40 Although the reactions were carried out in water, the recycling experiments and mechanistic considerations were evaluated in EtOH solvent. A truly heterogeneous mechanism was proven in EtOH. Palladium supported on hydrotalcite. Ruiz and co-workers opted for the use of a Mg/Al hydrotalcite-supported palladium(II) (catalyst 23) for a single example of the Suzuki–Miyaura cross-coupling reaction involving bromobenzene and phenylboronic acid at room temperature (Scheme 18).41 The supported catalyst was prepared by mixing appropriate amounts of palladium acetate, pyridine and hydrotalcite at 80 C for 1 h, aer which the solid was ltered off and washed with toluene. The catalyst thus obtained was named HT-Pd(AcO2)Py2 (catalyst 23). Only 52% conversion was obtained. Optimization studies showed that the use of sodium dodecyl sulphate as a surfactant was crucial for an acceptable conversion among those tested for this purpose, which included anionic, cationic and neutral surfactants. Palladium supported on porous glass. The catalytic activity of Pd supported on porous glass (catalyst 24) in the Suzuki– Miyaura reaction was studied by Ondruschka and co-workers under aerobic conditions (Scheme 19).42 The catalyst was prepared by dissolving Pd(OAc)2 (20 mg, 0.09 mmol) in dichloromethane containing the porous glass support (1 g; TRISOPOR) followed by removal of the solvent in vacuo and calcination of the catalyst precursor for 2 h at 300 C in a muffle furnace to obtain the catalyst with a Pd loading of 1 wt%. For varying catalyst loadings, different amounts of Pd were dissolved in dichloromethane (e.g. 10 mg for 0.5 wt% and 5 mg for 0.25 wt%). The reactions were carried out in water under
microwave irradiation. The effects of the catalyst preparation process (calcination time and temperature), as well as the base, substrate, and boron compound used in the coupling reaction were investigated in relation to the reusability of the catalyst. Among the bases used to recalcinate the catalyst, HNEt2 and NEt3 were successful. Substitutions in the ortho, meta, or para positions of the aryl halide showed a negligible inuence on the yield of the desired coupling product. Except for the phenol-type substrates, all other substrates required the addition of the phase-transfer catalyst tetra-n-butylammonium bromide (TBAB) to enhance their solubility in the solvent (deionized water). The classical order of reactivity of aryl iodides > bromides > chlorides was conrmed. Palladium supported on natural phosphate. F. Aziz and coworkers have reported a convenient method for the preparation of a recyclable and heterogeneous natural phosphatesupported palladium catalyst (catalyst 25) by treatment of natural phosphate (NP) with PdCl2(PhCN)2 in acetone and its application for the synthesis of biaryls via Suzuki–Miyaura couplings using water as solvent (Scheme 20).43 Aryl bromides and heteroaryl bromides efficiently reacted with arylboronic acids providing a useful way for the synthesis of aryl-substituted nitrogen heterocycles. However, the coupling reaction of 2bromothiophene and phenylboronic acid did not occur even with increased catalyst loading or reaction time. A considerable steric effect was observed when the reaction was carried out with sterically hindered 2-bromo-m-xylene and phenylboronic acid which led to the desired product in poor yields. Under identical conditions, the aryl chlorides bearing electronwithdrawing groups reacted in good yields, but no conversion was obtained in the coupling of aryl chlorides bearing electrondonating groups. Catalyst 25 was recovered by simple ltration and the product yields for the 2nd and 3rd cycle were nearly the same (93%) but reduced during the 4th cycle (88%). No leaching of the catalyst to the organic layer was reported. Organic support
Scheme 17
Examples of Pd/MCM-41-catalyzed Suzuki–Miyaura
reaction.
Scheme 18
Suzuki–Miyaura cross-coupling by Ruiz and co-workers.
Scheme 19 Suzuki–Miyaura reactions by Ondruschka and co-
workers.
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Palladium supported on polystyrene. Incorporation of nanosized Pd particles into a hyper-crosslinked polystyrene matrix by reduction was developed, and used for the Suzuki– Miyaura coupling reaction in water.44 Catalyst 26 was prepared by mixing an acidic solution of PdCl2 (PdCl2 (83 mg), 2 ml of H2O and 0.2 ml of concentrated HCl)) with a pre-washed and dried Macronet MN100 resin (950 mg of the MN100 resin in 10 ml ask). The resin was allowed to swell for 10 min in the
Scheme 20 Heterogeneous Suzuki–Miyaura couplings of aryl bromides and aryl chlorides with phenylboronic acid using PdNP.
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mixture followed by the addition of sodium formate and sodium hydroxide. The resulting mixture was then heated for 10 min at 80 C. The obtained grey beads were washed with water and MeOH, and dried in vacuo (1 mm Hg) under heating (90 C). The average palladium content in the resin was found to be 3.75%. Electron microscopy analysis of the milled Pd catalyst showed an average size of the Pd nanoparticles almost equal to 12.5 nm. The catalyst was employed in the coupling of aryl bromides and chlorides (with greater amounts of the catalyst, viz. double of the amount used for the bromo substrates) with phenylboronic acid, with water being the preferred solvent (Scheme 21). The reuse of the polymer-supported Pd resin was performed analogously to the Suzuki–Miyaura procedure. Palladium nanoparticles stabilized onto linear polystyrene (catalyst 27) by thermal decomposition of Pd(OAc)2 was examined in the Suzuki–Miyaura reaction in 1.5 M aqueous KOH solution (Scheme 22).45 A fairly uniform particle size of 2.3 0.3 nm was obtained and ICP-AES revealed that the catalyst contained an average of 2.5 mmol g1 of Pd. The immobilization degree of palladium was dependent on the molecular weight of polystyrene, while the size of the nanoparticles was not. The cross-coupling reaction of bromobenzene with p-methylphenylboronic acid proceeded efficiently to give 4-methylbiphenyl in 99% yield. Both electron-rich and electron-decient aryl bromides were reactive under these reaction conditions, affording the desired coupling products in high yields. The catalyst could be recovered by simple ltration. The average yield of 4-methylbiphenyl from the 1st through to the 10th recovered catalysts was 99%. No leaching of palladium into the solution during the reaction was observed by ICP-AES. Worth mentioning is that the reaction proceeded well with aryl chloride and the catalyst could even be recycled.
Review
In a similar study, linear polystyrene-stabilized PdO nanoparticles (PS–PdONPs, catalyst 28) were prepared in water by thermal decomposition of Pd(OAc)2 in the presence of polystyrene, and the Pd nanoparticles (PS–PdNPs) were also prepared using NaBH4 and phenylboronic acid as reductants.46 The catalytic activity of PS–PdONPs was found slightly higher than that of PS–PdNPs for the Suzuki–Miyaura coupling reaction in water probably due to the presence of oxygen and the size effect. The TEM image of catalyst 28 showed a uniform particle size of 2.3 0.3 nm. Under optimized conditions, the Suzuki–Miyaura coupling reaction of bromobenzene with 4methylphenylboronic acid in 1.5 M KOH aqueous solution at 80 C for 1 h proceeded efficiently to give 4-methylbiphenyl in 99% yield (Scheme 23). Both electron-rich and electrondecient aryl bromides were reactive, affording the desired coupling products in high yields. However, the reaction of chlorobenzene gave a lower yield. The catalyst was recovered by ltration and was recycled for 10 times without any loss of activity. No leaching of palladium into the reaction occurred during the reaction, as conrmed by ICP-AES. Polypyrrole–palladium nanocomposite-coated cross-linked polystyrene latex particles (PS/PPy–Pd, catalyst 29) have been applied with an excellent catalytic activity to the Suzuki– Miyaura coupling reaction in water (Scheme 24).47 The catalyst was prepared by adding an aqueous solution of PdCl2 and NaCl to a premixed aqueous dispersion of pyrrole and polystyrene. The polymerization was allowed to proceed for 7 days at 200 rpm. The PS/PPy–Pd particles were subsequently puried by repeated centrifugation–redispersion cycles followed by freezedrying overnight. The potency of the PS/PPy–Pd particles as a catalyst with low metal loading (0.03 mol% of Pd) was examined in the Suzuki–Miyaura coupling reaction of various aryl halides with arylboronic acids in 1.5 mol L1 aqueous potassium carbonate solution as test reactions. Steric hindrance did not matter as was observed from the high yield of 2,4-o-
Scheme 21 Suzuki–Miyaura reactions of phenylboronic acid with
arylbromides and an arylchloride.
Scheme 22
Suzuki–Miyaura reactions using polystyrene stabilized Pd.
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Scheme 23
Suzuki–Miyaura reactions using PS–PdONPS.
Scheme 24
Suzuki–Miyaura reactions using PS/PPy–Pd.
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dimethylbiphenyl from the coupling reaction of 2-bromotoluene with 4-methylphenylboronic acid. ICP-AES analyses conrmed that both the aqueous phase and the organic phase contained barely detectable levels of palladium and that the Pd loading in the particles did not change even aer the h run, which indicated that no/little Pd nanoparticles detached from the PS/PPy–Pd particles. Palladium immobilized by polymer-supported phosphine ligands. Wang and co-workers reported an exceedingly straightforward and competent catalytic system for the coupling of aryl bromides with sodium tetraphenylborate in water under focused microwave conditions.48 The coupling reaction was completed within 15 to 20 min under the applied reaction conditions involving 1 mol% of the catalyst. The heterogeneous palladium catalyst, consisting of a complex of PdCl2 bonded to a polystyrene–diphenylphosphine ligand, is fairly stable for years at room temperature under aerobic conditions. Potassium carbonate was the choice as the base, and TBAB as the phasetransfer catalyst. Various aryl and heteroaryl bromides were successfully coupled to NaBPh4 under microwave heating (Scheme 25). The heterogeneous palladium catalyst could be easily recovered by ltration and recycled at least ten times with unfailing activity. An amphiphilic resin-supported triarylphosphine–palladium complex bound to a polyethylene glycol–polystyrene gra copolymer (PEG–PS resin) has been described for the crosscoupling of aryl iodides with boronic acids using KOH as the base.49 The PEG–PS resin-supported palladium–monophosphine complex Pd–PEP (32) was readily prepared by treatment of the resin-supported phosphine (31) with an excess amount of di(m-chloro)bis(h-allyl)dipalladium(II) ([PdCl(h3C3H5)]2) (Pd/P > 1/1) followed by the removal of not immobilized [PdCl-(h3-C3H5)]2 by washing with chloroform (Scheme 26). This catalytic system was found to be more active than the comparable usual homogeneous palladium–phosphine complexes under the same reaction conditions (Scheme 27). No examples with chloroarenes were reported but good yields were obtained with aryl iodides and bromides under mild conditions (25 C). In another report, another resin-supported palladium catalyst (PS–PEG-adppp) for the Suzuki–Miyaura coupling reaction in water has been described by Uozumi and co-workers by merely changing the ligand.50 The catalyst 35 was prepared by treatment of PSPEG–NH2 with diphenylphosphinomethanol (Scheme 28) in toluene–MeOH at 25 C for 3 h to give PS–PEGadppp with a quantitative loading value of 0.32 mmol g1. The
Scheme 25
Suzuki–Miyaura reactions by Wang et al.
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Scheme 26 Schematic diagram for the synthesis of the resin-supported triarylphosphine–palladium complex.
Scheme 27
Suzuki–Miyaura reactions by Uozumi et al.
palladium complex of the bisphosphine ligand PS–PEG-adppp was prepared by mixing [PdCl(h3-C3H5)]2 in toluene at 25 C for 15 min to give [PS–PEG-adppp-Pd-(h3-C3H5)]Cl (catalyst 35) in a quantitative yield. Altogether, ninety six combinations of eight aromatic halides and twelve different boronic acids were reported from which a clear idea about the extremely efficient and stable heterogeneous catalyst 35 could be obtained. The reactions were carried out in aqueous K2CO3 at 85 C (Scheme 29). Catalyst 35 can be recovered by simple ltration and reused without any loss of activity.
Scheme 28
Synthetic approach for catalyst 35.
Scheme 29
Suzuki–Miyaura reactions by Uozumi et al.
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A similar strategy was followed by the same author to introduce the asymmetric Suzuki–Miyaura cross-coupling reaction,51 and achieved by anchoring chiral imidazoindolephosphine to an amphiphilic polystyrene–polyethylene glycol copolymer (PS–PEG) resin (Scheme 30). Excellent yields with very good enantioselectivities (88–99% ee) were obtained at room temperature in water with the aid of a large excess of TBAF (10 equiv.) and boronic acid (5 equiv.). A large amount (10 mol%) of the palladium catalyst was required, but it could be reused aer simple ltration with consistent results. Ikegami and co-workers designed a self-assembled complex of palladium and a non-crosslinked amphiphilic polymer supported through phosphines (Scheme 31).52 The catalyst support was prepared by random polymerization of 4-diphenylstyrylphosphine (37) with 12 equiv. of N-isopropylacrylamide (38) in the presence of 4 mol% AIBN, which gave 39 in 89% yield. Catalyst 40 was prepared by self-assembly of 39 and (NH4)2PdCl4. The catalytic activity of this palladium-network catalyst 40 has been investigated for Suzuki–Miyaura reaction in reuxing water where Na2CO3 was the ultimate choice as a base (Scheme 32). The protocol allowed the reaction of aryl bromides and aryl iodides as substrates, but aryl triates remained unaffected. Only trace amounts of the highly active palladium-network complex (50–500 ppm) were required for a successful reaction. The catalyst was found to achieve the coupling of unusual alkenyl halides and alkenylboronic acids at a low catalyst concentration (500 ppm). The recyclability of catalyst 40 was
Review
Suzuki–Miyaura reactions with the network catalyst 40 by Ikegami et al.
Scheme 32
examined for the preparation of biphenyl for up to ten consecutive cycles with a consistent activity. Uozumi and co-workers further developed the idea of a novel palladium complex embedded in a three-dimensional network complex (Scheme 33).53 A novel 3D palladium-network complex catalyst 45 was obtained by self-assembly of PdCl2 and C3trisphosphine 44, which was prepared from the commercially available 2,4,6-tris-(bromomethyl)mesitylene (41) in four steps.
Selected examples of the asymmetric Suzuki–Miyaura reaction by Uozumi et al.
Scheme 30
Scheme 33 Scheme 31 Preparation of the self-assembled catalyst 40.
42202 | RSC Adv., 2015, 5, 42193–42221
Preparation of the palladium network complex catalyst
45.
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Review
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Catalyst 45 showed a high catalytic efficiency at a low loading of 0.05 mol% palladium, and twenty seven examples of the Suzuki–Miyaura reaction in reuxing water with a variety of bromo- and iodoarenes were reported (Scheme 34). Finally, the catalytic complex displayed reusability properties over four successive cycles. Polymer-supported oxime-based ligands. The group of Kirschning reported in 2004 an insoluble pyridine–aldoxime palladium catalyst active under microwave heating in Suzuki– Miyaura reactions in water (Scheme 35).54 Although the exact structure of the catalyst was not elucidated, the absence of palladium–carbon bonds excluded any palladacycle-type structure. Experiments showed that under microwave activation water as the solvent turned out to be superior to toluene under the catalytic conditions. In the subsequent studies, in order to improve its lifetime the authors covered catalyst 46 (1 mol%) with an Irori Kan™. Under optimized conditions, catalyst 46 with K2CO3 as a base and TBAB as a phase-transfer agent showed good activity for the coupling of various substituted boronic acids with p-chloro-, p-bromo-, p-iodo- and p-triuoromethylsulfonylacetophenone (Scheme 35). Catalyst 46 was reused for the coupling of 4-bromoacetophenone with benzeneboronic acid, and 93% conversion was observed aer the 14th run. Following their earlier studies, Kirschning et al. considered the aqueous Suzuki–Miyaura reactions of another closely related catalytic system prepared from a 2-pyridine aldoximebased Pd(II) complex covalently anchored onto a glass–polymer composite material (catalyst 47).55 Aryl and heteroaryl bromides were efficiently coupled with boronic acids at fairly low palladium loadings (0.7 mol%) with the aid of TBAB as the surfactant and KOH as the base, under both thermal (100 C) or microwave heating (160 C) conditions (Scheme 36). For chloroarenes, only the coupling of 4-chloroacetophenone has been reported. The catalyst could be reused at least seven times with consistent activity regardless of the source of heating.
Suzuki–Miyaura reactions with the network complex 45 by Uozumi et al.
Scheme 34
Scheme 35
Selected examples of cross-couplings using the catalyst
46.
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Scheme 36 Selected examples using the catalytic system developed by Kirchning et al.
In another report following the previous one, Kirschning and co-workers explored an alternative approach for immobilization of an oxime carbapalladacycle (48) onto polyvinylpyridine as the support (Scheme 37).56 The polymeric phase was prepared from a heated solution (70 C) of the monomers vinylpyridine and divinylbenzene with AIBN in a nonpolar solvent. While aryl chlorides were more efficiently coupled in water, aryl bromides were preferentially reacted with boronic acids in toluene under these catalytic conditions (Scheme 38). The protocol is associated with the use of TBAB (0.5 equiv.) and microwave activation. The protocol was equally applicable as a thermal, microwave or continuous ow method. Taking advantage of a rich experience in oxime carbapalladacycle catalysts for cross-coupling reactions in organic and aqueous media,57 Najera and co-workers prepared the palladated Kaiser oxime resin catalyst 50 as an active precatalyst for different types of the Suzuki–Miyaura reaction (Scheme 39).58
Immobilization of the oxime carbapalladacycle onto polyvinylpyridine.
Scheme 37
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Scheme 38
Review
Cross-coupling reactions developed by Kirschning et al.
Scheme 40
Synthesis of dipyridylmethylamine-based palladium
complex 51.
Scheme 39 Selected examples from the work of Na´jera and co-
workers.
Several examples were investigated involving the cross-coupling of aryl bromides, and allyl and benzyl chlorides with aryl-, alkyland alkenylboronic acids in neat water. Aryl bromides were efficiently cross-coupled with benzeneboronic acid, but aryl chlorides were poorly reactive. Alkylboronic acid, trivinylboroxine and trimethylboroxine reacted with aryl bromides in the presence of TBAB as an additive. Analysis of the solution showed moderate metal leaching which allowed the reuse of the recovered catalyst 50 with a gradually decreased catalytic activity. Polymer-supported pyridine ligands. Based on earlier studies59 on dipyridyl based ligands for palladium complexation, showing good catalytic activity for C–C bond-forming reactions, Najera and co-workers explored a dipyridyl–palladium complex anchored to a styrene–maleic anhydride copolymer (Scheme 40).60 Only three haloarenes were examined for the coupling with benzeneboronic acid in the presence of K2CO3 as a base and the supported palladium catalyst 51 (Scheme 41). Although the activated 4-chloroacetophenone is
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Scheme 41 Cross-coupling reactions according to the work of Najera and co-worker.
reactive, compared to the bromides it requires a much higher palladium loading (4.5 mol% vs. 0.1 mol%) and TBAB as a phase-transfer agent. Microwave irradiation was found to be disadvantageous for the reaction. Recycling studies showed good yields for up to three or four cycles.
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Palladium nanoparticles xed in the layer of core–shell poly(styrene-co-4-vinylpyridine) microspheres (catalyst 52) were found to be catalytically active for Suzuki–Miyaura crosscouplings in water (Scheme 42).61 The supported catalyst was prepared by adding an aqueous solution of PdCl2 into the colloidal dispersion of the core–shell PS-co-P4VP microspheres at room temperature followed by the dropwise addition of excess NaBH4 aqueous solution. The resultant colloidal dispersion was puried by dialyzing against water at room temperature for 4 days. Transmission electron microscopy (TEM) analyses evidenced that the palladium nanoparticles were uniformly distributed with an average size of 4.4 nm on the polyvinylpyridine shell. Optimization studies revealed that hydrophobic reagents were best coupled with Et3N as a base, while hydrophilic substrates preferably required K2CO3. The catalytic system was examined for the coupling of benzeneboronic acid with a range of unchallenging bromo- and iodoarenes. Chloroarenes, however, were almost unreactive under the same reaction conditions. Recycling studies carried out for the coupling of 4-bromoacetophenone with benzeneboronic acid showed 99% yield of the targeted biaryl compound throughout ve consecutive runs. The average size of the palladium nanoparticles remained the same during recycling. A novel heterogeneous transition-metal catalyst comprising a polymer-supported terpyridine palladium(II) complex (catalyst 53) was prepared (Scheme 43) and found to promote the Suzuki–Miyaura reaction in water under aerobic conditions
with high to excellent yields.62,63 The Suzuki–Miyaura crosscoupling reaction of iodobenzene with phenylboronic acid was carried out with K2CO3 (2 equiv.) in the presence of polymeric catalyst 53 (5 mol% Pd) in water to give biphenyl in 93% yield (Scheme 44). A variety of boronic acids and halo arenes with different types of substitution at different positions showed almost excellent yields, thereby proving that the substrate or reactant structures do not affect the reaction yield. The catalyst was recovered by simple ltration and directly reused several times without loss of catalytic activity (ICP-AES analysis (detection limit of Pd:
