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DOI: 10.1039/C2RA21632A (Paper) RSC Adv., 2012, 2, 8789-8792

Sustainable H2O/ethyl lactate system for ligand-free Suzuki–Miyaura reaction†

Jie-Ping Wan ab, Chunping Wang a, Rihui Zhou a and Yunyun Liu *ab
aKey Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University, Nanchang, 330022, P. R. China
bCollege of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, P. R. China. E-mail: chemliuyunyun@gmail.com; Fax: +86 791 88120380; Tel: +86 791 88120380

Received 1st August 2012 , Accepted 1st August 2012

First published on 3rd August 2012

Abstract

A sustainable catalyst system consisted of H2O/ethyl lactate (EL), Pd(OAc)2 and K2CO3 has been developed as a generally applicable protocol for Suzuki–Miyaura reactions using various aryl bromides and iodides to incorporate arylboronic acids under ligand-free conditions. This protocol is advantageous owing to the employment of water and non-toxic, biomass available ethyl lactate as green media. In addition, as a co-solvent with water, ethyl lactate displayed evident superiority over conventional polar organic solvents such as DMF, DMSO and dioxane etc., which implied the additional potential function of EL as a ligand in these reactions.

Introduction

The discovery of the Suzuki–Miyaura reaction has brought revolutionary progress to synthetic organic chemistry by providing a universally applicable method of constructing sp2 C–C bonds via the coupling of aryl/vinyl halides and boranes.1 The past several decades have witnessed spectacular advances on the research of the Suzuki–Miyaura reaction, and the application of this classical transformation has nowadays permeated to the synthesis of natural products, agrochemicals, clinical medicines, intermediates of the chemical industry as well as functionalized materials.2 Among the current extensive research interests into the field of the Suzuki–Miyaura reaction, developing green and sustainable catalyst protocols represents one of the most widely concerned directions. In order to attain ideal results with eco-friendly processes, numerous efforts have been made to establishing green methodologies for the Suzuki–Miyaura reaction.

Currently, routes of designing eco-friendly protocols for the Suzuki–Miyaura reaction include alternating traditional phosphine ligands with environmentally benign ligands or ligand-free catalysis,3 reactions under mild conditions,4 reactions at low catalyst loading, transition metal-free conditions or employing reusable supported catalysts.5 However, most efforts on green Suzuki–Miyaura methodology have focused on the search of green and sustainable solvent mediated catalyst systems. A lot of different green media have been employed for developing the Suzuki–Miyaura reaction in eco-friendly conditions. Typical examples in this area include water-poly(ethylene glycol) (PEG),6 water–acetone,7 water–ionic liquid,8 water–MeCN,9 water–DMF,3d water–alcohol (EtOH, i-PrOH, BuOH) etc.,10 to name but a few. On the other hand, using only water as solvent for the Suzuki–Miyaura reactions had also been achieved with elegant results, the limit for these reactions was the need for prior decorated palladium catalysts with functional organic auxiliaries, harsh heating, special ligands or other external assistance in most cases.11 For those reactions performed under conventional heating with simple palladium catalysts such as PdCl2, Pd(OAc)2etc., it seems that the presence of organic solvent is still currently inevitable to guarantee satisfactory results.12 In this regard, the design of a catalyst system employing authentically nontoxic and biodegradable organic medium is a promising direction for establishing greener Suzuki–Miyaura reactions.

In recent years, nontoxic biomass available solvents have been regarded as highly potential sustainable media for organic synthesis.13 Solvents of this type such as glycerol,14 glycerol ethers15etc. have already shown broad applicability as media in organic synthesis. Ethyl lactate is another biomass-derivable solvent that possesses unique advantages of low production cost, nontoxicity (used as additive in food industry), low environmental impact (completely biodegradable), good compatibility with water, high boiling point (154 °C) and excellent solubility to organic compounds.16 Therefore, ethyl lactate is theoretically an ideal solvent as medium for sustainable organic synthesis. However, the research on EL mediated organic synthesis is presently in its infancy,17 to our best knowledge, the Suzuki–Miyaura reaction using EL or similar biomass derived solvents is not yet available. Herein, we report the first water–EL mixed green solvent mediated Suzuki–Miyaura reaction using a simple commercially available palladium catalyst under ligand free conditions.

Results and discussion

We started to explore the method on the model reaction of iodobenzene 1a (0.5 mol) and phenylboronic acid 2a (0.75 mol) with palladium catalysis. In the presence of K2CO3, 1 mol% Pd(OAc), reactions performed at 60 °C in 3 mL of EL solvent, pure water and EL[thin space (1/6-em)]:[thin space (1/6-em)]water (2[thin space (1/6-em)]:[thin space (1/6-em)]1) and (1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixture suggested that 1[thin space (1/6-em)]:[thin space (1/6-em)]1 mixed EL and water was better than other media systems (entries 1–4, Table 1), further modification of the solvent proved that 2 mL of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 mixed EL[thin space (1/6-em)]:[thin space (1/6-em)]water gave even better result (entry 5, Table 1). With this established mixed solvent system, extensive investigation was conducted to evaluate the effect of temperature (entry 6, Table 1), base species (entries 7–9, Table 1) and catalyst loading (entries 10–11, Table 1). Through comparing the results from these experiments, optimal conditions turned out to be 1 mol% Pd(OAc)2, 2 eq of K2CO3 in 2 mL mixed EL[thin space (1/6-em)]:[thin space (1/6-em)]water (1[thin space (1/6-em)]:[thin space (1/6-em)]1) and 60 °C (entry 5, Table 1). Further experiments performed with PdCl2 as catalyst or with DMF–water, DMSO–water, ethyl acetate–water, dioxane–water as mixed media all gave inferior yield of the product 3a (entries 12–16, Table 1).
Table 1 Optimization of reaction conditionsa
ugraphic, filename = c2ra21632a-u1.gif
Entry Catalyst (mol %) Base Solvent (mL) Yield (%)b
a Unless specified, the general conditions: 1a (0.5 mmol), 2a (0.75 mmol) and base (2 eq) at 60 °C for 2 h. EL = ethyl lactate, EA = ethyl acetate. b Isolated yield based on 1a. c This reaction was performed at 45 °C.
1 Pd(OAc)2 (1) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (2[thin space (1/6-em)]:[thin space (1/6-em)]1) 26
2 Pd(OAc)2 (1) K2CO3 EL (3) 62
3 Pd(OAc)2 (1) K2CO3 H2O (3) 60
4 Pd(OAc)2 (1) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1.5[thin space (1/6-em)]:[thin space (1/6-em)]1.5) 56
5 Pd(OAc)2 (1) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 93
6c Pd(OAc)2 (1) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 47
7 Pd(OAc)2 (1) Na2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 62
8 Pd(OAc)2 (1) Et3N EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 53
9 Pd(OAc)2 (1) NaHCO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 44
10 Pd(OAc)2 (0.5) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 52
11 Pd(OAc)2 (0.25) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 44
12 PdCl2 (1) K2CO3 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 23
13 Pd(OAc)2 (1) K2CO3 DMF[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 70
14 Pd(OAc)2 (1) K2CO3 DMSO[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 57
15 Pd(OAc)2 (1) K2CO3 Dioxane[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 58
16 Pd(OAc)2 (1) K2CO3 EA[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 15

Based on the optimized results, we first examined the applicable scope by using different iodobenzenes and arylboronic acid derivatives. The results from this section were shown in Table 2. It was found that most aryl iodides reacted with phenylboronic acids to give corresponding biaryls with excellent yields since aryl iodides are known as active electrophiles in various cross coupling reactions. The heteroaryl substrate thiophen-2-boronic acid provided relatively lower yield of the corresponding product when reacted with 1a (entry 11, Table 2). Based on these typical examples, it was undoubted that the palladium-catalyzed green protocol consisted of EL–water was well suited for the Suzuki–Miyaura cross coupling of aryl iodide substrates with boronic acids. With these acquired results, we believed that this method should be also applicable for the reactions of aryl bromides. Therefore, we turned to examine the Suzuki–Miyaura reactions using aryl bromides as these reactions represented more practical application of a catalytic system for the Suzuki–Miyaura reaction. The reaction results acquired from a broad array of aryl/heteroaryl bromides and aryl boronic acids were outlined in Table 3. As expected, the reactions of aryl bromides have also shown excellent tolerance to this protocol albeit the reaction temperature was enhanced because of lower reactivity of these bromide substrates. Generally, aryl bromides with electron withdrawing as well donating groups in the ortho-, meta- and para-positions, and heteroaryl bromides were able to smoothly incorporate a class of different aryl boronic acids to give corresponding biaryls 3 in moderate to excellent yields. Based on the results from these present entries, the properties of the functional groups in both reactants didn't show any regular effects on the reaction efficiency. On the other hand, an attempt on employing functional aryl chlorides such as p-tolyl chloride as a reaction partner showed only trace transformation under present conditions.

Table 2 Suzuki–Miyaura cross coupling reactions of aryl iodides with boronic acids in ethyl lactate[thin space (1/6-em)]:[thin space (1/6-em)]H2O systema
ugraphic, filename = c2ra21632a-u2.gif
Entry R1 Ar Product Yield (%)b
a General reaction conditions: 1 (0.5 mmol), 2 (0.75 mmol) mixed in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O, stirred at 60 °C for 2 h in the presence of 0.005 mmol Pd(OAc)2 and 1 mmol K2CO3. b Isolated yield based on 1.
1 H Ph 3a 93
2 4-MeO Ph 3b 99
3 3-NO2 Ph 3c 91
4 2-MeO Ph 3d 81
5 2-OH Ph 3e 80
6 H 4-FC6H4 3f 98
7 H 4-ClC6H4 3g 97
8 H 4-MeOC6H4 3b 82
9 H 4-MeC6H4 3h 70
10 H 2-MeC6H4 3i 97
11 H Thiophene-2-yl 3j 60
12 H Naphth-1-yl 3k 92

Table 3 Suzuki–Miyaura cross coupling reactions of aryl bromides with boronic acids in ethyl lactate[thin space (1/6-em)]:[thin space (1/6-em)]H2O systema
ugraphic, filename = c2ra21632a-u3.gif
Entry Ar1 Ar2 Product Yield (%)b
a General reaction conditions: 4 (0.5 mmol), 2 (0.75 mmol) mixed in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 EL[thin space (1/6-em)]:[thin space (1/6-em)]H2O, stirred at 80 °C for 2 h in the presence of 0.005 mmol Pd(OAc)2 and 1 mmol K2CO3. b Isolated yield based on 4.
1 Ph Ph 3a 81
2 4-OHC6H4 Ph 3l 93
3 3-MeOC6H4 Ph 3m 76
4 4-AcC6H4 Ph 3n 65
5 2-OHCC6H4 Ph 3o 60
6 Ph 4-MeOC6H4 3b 43
7 Ph 4-FC6H4 3f 56
8 Ph Naphtha-1-yl 3k 62
9 4-AcC6H4 4-MeC6H4 3p 81
10 4-AcC6H4 4-FC6H4 3q 88
11 2-OHCC6H4 4-ClC6H4 3r 96
12 2-OHCC6H4 4-FC6H4 3s 62
13 4-MeC6H4 4-ClC6H4 3t 43
14 3-MeOC6H4 Napth-1-yl 3u 50
15 Naphtha-2-yl Ph 3v 48
16 Pyridine-2-yl Ph 3w 45

The practical reaction results of aryl boronic acids to both aryl iodides and bromides encouraged us to further explore the reactivity of different bromo- and iodoaryl dihalides to investigate the chemoselectivity of these bihaloaryls in the Suzuki–Miyaura coupling. Interestingly, when subjected to 80 °C heating in the presence of 2 equiv of phenylboronic acid, the mono-coupled biaryl products 3x–3z were obtained as single or major products and the doubly coupled triaryl product was not observed or isolated as a minor product (Scheme 1). The chemoselectivity was different from most previously reported catalyst protocols wherein triaryls were usually provided as the main or sole products.5k,7,18 The excellent chemoselectivity of our approach on the production of bromo-substituted biaryls was advantageous for the synthesis of unsymmetrical triaryls via stepwise cross coupling using different aryl boronic acids.


Chemoselective Suzuki–Miyaura reactions of bromoaryl iodides.
Scheme 1 Chemoselective Suzuki–Miyaura reactions of bromoaryl iodides.

Conclusions

In conclusion, we developed a sustainable system consisting of EL–water media in the presence of 1 mol% Pd(OAc)2. This protocol tolerated the reactions of a broad array of aryl halides and aryl boronic acids under mild reaction conditions. Unique advantages of this methodology were the nontoxicity, no environmental impact, low cost and easy availability of the reaction media as well as ligand-free conditions. More importantly, since the biomass available EL was rarely employed in organic synthesis as an eco-friendly solvent, successful employment of EL as solvent for Suzuki–Miyaura reactions marks a notable extension on biomass organic solvent-based green synthesis. The present methodology also provides a useful complement to known catalyst systems for Suzuki–Miyaura reactions.

Experimental

General procedure for Suzuki–Miyaura reactions using aryl iodides

To a 25 mL round bottom flask was charged aryl iodide (0.5 mmol), aryl boronic acid (0.75 mmol), Pd(OAc)2 (0.005 mmol) and K2CO3 (1 mmol). EL (1 mL) and water (1 mL) were added and the mixture was heated at 60 °C for 2 h (TLC). After cooling down to room temperature, the mixture was extracted with ethyl acetate (3 × 10 mL). The combined organic phases were dried over anhydrous Na2SO4. The solvent was then removed under reduced pressure and the residue was subjected to silica gel chromatography to give pure product.

General procedure for Suzuki–Miyaura reactions using aryl bromides and bromophenyl iodides.

To a 25 mL round bottom flask was charged aryl bromide/bromophenyl iodide (0.5 mmol), aryl boronic acid (0.75 mmol), Pd(OAc)2 (0.005 mmol) and K2CO3 (1 mmol), EL (1 mL) and water (1 mL) were added and the mixture was heated at 80 °C for 2 h (TLC). After cooling down to room temperature, the mixture was extracted with ethyl acetate (3 × 10 mL). The combined organic phases were dried over anhydrous Na2SO4. The solvent was then removed under reduced pressure and the residue was subjected to silica gel chromatography to give pure product.

Acknowledgements

Financial support from NSFC of China (No. 21102059), NSF of Jiangxi Province (No. 20114BAB213005), a Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University and a grant from the Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201220) are gratefully acknowledged.

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Footnote

† Electronic Supplementary Information (ESI) available: General experimental information, characterization data as well as copies of 1H and 13C NMR spectra of all products. See DOI: 10.1039/c2ra21632a/
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