exploration of amino-functionalized ionic liquids as ligand and base for heck reaction

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Full PaperReceived: 7 September 2009 Revised: 29 December 2009 Accepted: 2 January 2010 Published online in Wiley Interscience: 2 March 2010

(www.interscience.com) DOI 10.1002/aoc.1624

Exploration of amino-functionalized ionicliquids as ligand and base for Heck reactionJie Liua, Hongqiang Liua and Lei Wanga,b∗

A kind of amino-functionalized ionic liquid has been prepared and investigated as ligand and base for the Heck reactionsbetween aryl iodides and bromides with olefins in the presence of a catalytic amount of Pd submicron powder in [Bmim]PF6.The reactions generated the corresponding products in excellent yields under mild reaction conditions. The generality of thiscatalytic system to the different substrates also gave satisfactory results. The key feature of the reaction is that Pd species andionic liquids were easily recovered and reused six times with constant activity. Copyright c© 2010 John Wiley & Sons, Ltd.

Keywords: Heck reaction; functionalized ionic liquids; Pd submicron powder

Introduction

Palladium-catalyzed coupling of olefins with aryl and vinylhalides, known as the Heck reaction,[1,2] pioneered by Heckand Mizoroki,[3] is one of the most investigated transition-metalcatalyzed carbon–carbon bond formation reactions in organicsynthesis.[4] Actually, the Heck reactions involving aryl iodidesand bromides are catalyzed by almost any Pd(II) or Pd(0) catalystprecursor, usually at elevated temperatures.[5] In addition, Heckreactions are generally carried out in homogeneous systems in thepresence of P-ligands, which are moisture- and air-sensitive andunrecoverable.[6] Because the Heck reaction products were foundto be important intermediates in the preparation of materials,[7]

natural products[8] and bioactive compounds,[9] this chemistryhas been focused on discovering a new generation of catalystsystems, such as palladacycles and Pd–carbene complexes.[10,11]

Nevertheless, several factors, such as the use of toxic, easilyoxidable phosphines, and the utilization of harmful solvents suchas DMF and volatile organic solvents, have hampered broadindustrial application.[12] In the last decade, palladium–phosphinecatalyzed Heck reactions in room temperature ionic liquids(RTILs) have gained increased attention in order to resolve oneor more of these problems.[13 – 15] In view of the increasingdemand for environmentally benign reaction processes, increasedefforts have been put towards investigating the Heck reaction,including searching for phosphine-free methods, using a ligand-free palladium catalyst system and carrying out the Heck reactionin non-conventional reaction media such as water, ionic liquidsor supercritical CO2.[16 – 18] As an extension to our research inrecyclable catalytic systems,[19] we were interested in the use ofnon-volatile RTILs as reaction media.[13,20] RTILs are liquids at oraround room temperature, they are salts that do not normallyneed to be melted by means of an external heat source, andhave a negligibly low vapor pressure (∼10−8 bar)[21] due to strongCoulombic interactions.[22] They are thus termed green solvents, incontrast to traditional volatile organic solvents, which makes themsuitable for industrial applications.[23,24] Over the past decade, ionicliquids have gained increasing attention as promising reactionmedia or catalysts for synthetic chemistry.[13,25] It is known that anionic liquid can act as a ligand or other function, except reactionmedia. Although RTILs are superior to conventional solvents in

many cases, only a very limited number of structures have beenutilized. Most of the recent investigations have employed theuse of 1,3-dialkylimidazolium salts.[26] It is desirable to develop asimpler and more concise synthetic procedure for the synthesis offunctionalized ionic liquids (FILs), which act as reaction mediaand other functions in the carbon–carbon bond formationreactions.[13]

Herein, we wish to report the design and synthesis of the amino-functionalized ionic liquids, which can act as reaction medium,ligand as well as base to the Pd-catalyzed Heck reaction on arecyclable basis. The general procedure for the preparation ofamino-functionalized ionic liquids is shown in Scheme 1.

Experimental

Materials and Methods

Melting points were recorded on a WRS-2B melting pointapparatus and are uncorrected. IR spectra were obtained on aNicolet Nexus 470 spectrophotometer. All 1H NMR and 13C NMRspectra were recorded on a 400 MHz Bruker FT-NMR spectrometer.TMS was used as an internal standard. Products were purified byflash chromatography on 230–400 mesh silica gel. The chemicalsand solvents were purchased from commercial suppliers (Aldrich,USA and Shanghai Chemical Company, China) and were usedwithout purification prior to use.

Preparation of Amino-functionalized IL 1[27]

Under nitrogen atmosphere, 1-methylimidazole (8.21 g,100 mmol) and 3-chloropropan-1-amine (9.36 g, 100 mmol) were

∗ Correspondence to: Lei Wang, Huaibei Coal Teachers College, Chemistry,Huaibei, Anhui 235000, People’s Republic of China.E-mail: [email protected]

a Department of Chemistry, Huaibei Coal Teachers College, Huaibei, Anhui235000, People’s Republic of China

b State Key Laboratory of Organometallic Chemistry, Shanghai Institute ofOrganic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’sRepublic of China

Appl. Organometal. Chem. 2010, 24, 386–391 Copyright c© 2010 John Wiley & Sons, Ltd.

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Amino-functionalized ionic liquids as ligand and base for Heck reaction

N N Cl NH2Reflux, 24h

EtOH, N2N N NH2

KOHNaBF4, r.t. 48 h

KOHKPF6, r.t. 48 h

Cl-

N N NH2

BF4-

N N NH2PF6

-

IL 2

IL 3

IL 1

+

IL 1

IL 1

R1R2

Pd Powder

[Bmim]PF6+ R1

R2IL 1, 2 or 3

N N

PF6-

[Bmim]PF6 =

Scheme 1. Synthesis of amino-functionalized ionic liquids and theirapplications in Heck reaction.

dissolved in 50 ml of dry ethanol under stirring. The resulting mix-ture was refluxed for 24 h under nitrogen protection. After removalof ethanol in vacuum, the solid residue was dissolved in water. Thenthe pH value of the solution was adjusted to 10 by the additionof potassium hydroxide. The obtained solution was concentratedunder vacuum and then extracted with ethanol–tetrahydrofuran(v/v, 1 : 1, 75.0 ml × 2). The combined extracts were concentratedto give the product IL 1 as a pale yellow viscous liquid (13.08 g,yield 73%).[27] 1H NMR (400 MHz, D2O): δ = 8.80 (s, 1 H), 7.54 (s,1 H), 7.48 (s, 1 H), 4.34 (t, J = 7.4 Hz, 2 H), 3.90 (s, 3 H), 3.08 (m, 2H), 2.28 (m, 2 H); 13C NMR (100 MHz, D2O): δ = 27.41, 35.89, 36.43,46.44, 122.15, 123.97, 136.19; IR (KBr): 3157, 2963, 2753, 1634,1579 cm−1. The 1H and 13C spectra were found to be in agreementwith the Fu and Liu.[27]


IL 1 (13.08 g, 73.0 mmol) subsequently through ion exchange withsodium tetrafluoroborate (8.72 g, 80.0 mmol) in ethanol–water(v/v, 1 : 1, 15.0 ml) was performed for 48 h at room temperature.The suspension was filtered to remove the precipitated bromidesalt and the organic phase was concentrated. The residue wasthen re-dissolved in small amount of chloroform (5.0 ml), andfiltered to remove the inorganic salt. The solvent was removedin vacuo to afford yellow viscous IL 2 (15.11 g, yield 91%).[28] 1HNMR (400 MHz, D2O): δ = 8.75 (s, 1 H), 7.58 (s, 1 H), 7.50 (s, 1 H),4.38 (t, J = 8.2 Hz, 2 H), 3.92 (s, 3 H), 3.14 (m, 2 H), 2.35 (m, 3 H);13C NMR (100 MHz, D2O): δ = 27.40, 35.87, 35.90, 46.39, 122.09,123.88, 136.26; IR (KBr): 3426, 3143, 2955, 2739, 2643, 2504, 2015,1574, 1506, 1457, 1339, 1285, 1232, 1169, 1085, 1021, 831, 756,621 cm−1. The 1H and 13C spectra were found to be in agreementwith Tan et al.[28]


IL 1 (13.08 g, 73.0 mmol) subsequently through ion exchangewith potassium hexafluorophosphate (14.72 g, 80.0 mmol) inethanol–water (v/v, 1 : 1, 15.0 ml) was performed for 48 h atroom temperature. The suspension was filtered to remove theprecipitated bromide salt and the organic phase was concentrated.The residue was then re-dissolved in small amount of chloroform

(5.0 ml), and filtered to remove the inorganic salt. The solventwas removed in vacuo to afford yellow viscous IL 3 (18.13 g, yield87%).[29] 1H NMR (400 MHz, D2O): δ = 8.78 (s, 1 H), 7.52 (s, 1 H),7.44 (s, 1 H), 4.29 (t, J = 7.5 Hz, 2 H), 3.84 (s, 3 H), 3.02 (m, 2 H), 2.22(m, 2 H); 13C NMR (100 MHz, D2O): δ = 27.42, 36.10, 36.49, 46.45,122.13, 122.89, 136.18; IR (KBr): 3432, 3147, 2980, 2973, 2634, 1594,1576, 1521, 1507, 1463, 1172, 848, 757, 622 cm−1. The 1H and 13Cspectra were found to be in agreement with Wu et al.[29]

General Procedure for Heck Reaction Catalyzed by Amino-functionalized IL 3 and Pd Submicron Powder

Under nitrogen atmosphere, an oil bath with a round-bottomedflask containing 4-iodoanisole (234 mg, 1.0 mmol), n-butyl acrylate(128 mg, 1.0 mmol), Pd submicron powder (1.0 mg, 0.01 mmol)and amino-functionalized IL 3 (285 mg, 1.0 mmol) in [Bmim]PF6

(2.0 ml) was heated to 120 ◦C during a period of 24 h. Aftercooling to room temperature, the product was extracted fromthe mixture with EtOAc (5.0 ml × 3). The combined organic layerswere washed with H2O and brine, dried over MgSO4, and thesolvent evaporated under reduced pressure. Typically, purificationby chromatography of the crude mixture was performed to givethe desired pure product in 93% yield (234 mg).

(E)-Stilbene

White solid; m.p. 122–123 ◦C (lit.[30] 120–122 ◦C). IR (KBr): 3022,1586, 1494, 1457, 1366, 967, 760, 693 cm−1. 1H NMR (400 MHz,CDCl3): δ = 7.35–7.50 (m, 4 H), 7.31–7.33 (m, 4 H), 7.23 (t,J = 7.5 Hz, 2 H), 7.09 (s, 2 H). 13C NMR (100 MHz, CDCl3): δ = 137.3,128.6, 128.4, 127.6, 126,5. The 1H and 13C spectra were found tobe in agreement with Cui et al.[31]

(E)-4-Methoxystilbene

Light yellow solid; m.p. 136–138 ◦C (lit.[32] 135–137 ◦C). IR (KBr):2961, 1604, 1512, 1446, 1293, 1025, 965, 862, 753, 686 cm−1. 1HNMR (400 MHz, CDCl3): δ = 3.73 (s, 3H), 6.90 (d, J = 8.7 Hz, 2 H),6.90–6.96 (m, 2 H), 7.00 (d, J = 15.9 Hz, 1 H), 7.23 (t, J = 7.5 Hz,2 H), 7.35–7.41 (m, 4 H). 13C NMR (100 MHz, CDCl3): δ = 159.3,137.6, 130.1, 128.6, 128.2, 127.7, 127.2, 126.6, 126.2, 114.1, 55.3.The 1H and 13C spectra were found to be in agreement with Cuiet al.[31]

(E)-4-Methylstilbene

Light yellow solid; m.p. 121–122 ◦C (lit.[30] 119–120 ◦C). IR (KBr):3022, 2914, 2848, 1606, 1444, 967, 810, 743, 698 cm−1. 1H NMR(400 MHz, CDCl3): δ = 2.45 (s, 3 H), 7.15–7.29 (m, 4 H), 7.35–7.42(m, 3 H), 7.52 (d, J = 7.9 Hz, 2 H). 13C NMR (100 MHz, CDCl3):δ = 137.5, 134.5, 129.4, 128.6, 127.6, 127.3, 126.4, 126.3, 21.2. The1H and 13C spectra were found to be in agreement with Cui et al.[31]

(E)-4-Cyanostilbene

White solid; m.p. 117–119 ◦C (lit.[33] 117.4–117.7 ◦C). IR (KBr): 3024,2918, 2225, 1601, 973, 826, 759 cm−1. 1H NMR (400 MHz, CDCl3):δ = 7.05 (d, J = 15.8 Hz, 1 H), 7.31–7.39 (m, 3 H), 7.51–7.60 (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 141.8, 136.2, 132.3, 132.2, 128.7,128.5, 127.0, 126.8, 126.6, 118.9, 110.4. The 1H and 13C spectra werefound to be in agreement Cui et al.[31]

Appl. Organometal. Chem. 2010, 24, 386–391 Copyright c© 2010 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/aoc

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J. Liu, H. Liu and L. Wang

(E)-4-(Trifluoromethyl)stilbene

Light yellow solid; m.p. 132–134 ◦C (lit.[34] 132–134 ◦C). IR (KBr):3025, 2846, 1626, 1554, 1418, 1326, 1167, 1111, 1070, 973,827 cm−1. 1H NMR (400 MHz, CDCl3): δ = 7.58 (m, 4 H), 7.52(d, J = 7.2 Hz, 2 H), 7.39 (m, 2 H), 7.29 (m, 1 H), 7.18 (d, J = 16.2 Hz,1 H), 7.09 (d, J = 16.5 Hz, 1 H).13C NMR (100 MHz, CDCl3):δ = 140.6,136.2, 132.1, 132.0, 131.3, 128.6, 128.1, 127.2, 126.7, 126.6, 125.6,125.45. The 1H and 13C spectra were found to be in agreementwith Warner and Sutherland.[35]


Light yellow solid; m.p. 65–67 ◦C (lit.[36] 66–67 ◦C). IR (KBr): 3039,2854, 1619, 1558, 1497, 1452, 1342, 1169, 1116, 962, 805, 696 cm−1.1H NMR (400 MHz, CDCl3): δ = 7.75 (s, 1 H), 7.68 (m, 1 H), 7.55–7.47(m, 4 H), 7.42–7.34 (m, 2 H), 7.27 (m, 1 H), 7.16 (d, J = 16.3 Hz, 1 H),7.12 (d, J = 16.3 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 138.4,136.9, 131.2, 130.8, 129.6, 129.1, 128.9, 128.4, 127.2, 126.8, 124.3,124.2, 123.1. The 1H and 13C spectra were found to be in agreementwith Cui et al.[31]


Light yellow liquid.[37] IR (film): 3030, 2850, 1621, 1550, 1428,1341, 1162, 1113, 965, 815, 695 cm−1. 1H NMR (400 MHz, CDCl3):δ = 7.70–7.09 (m, 11 H). 13C NMR (100 MHz, CDCl3): δ = 137.2,136.5, 132.7, 131.8, 131.7, 129.5, 128.7, 128.4, 127.5, 127.3, 127.1,126.2, 124.5. The 1H and 13C spectra were found to be in agreementwith Wang and Wnuk.[37]

(E)-2-Methylstilbene

Light yellow solid; m.p. 31–32 ◦C (lit.[38] 30–32 ◦C). IR (KBr): 2923,1604, 1492, 1454, 967, 763 cm−1. 1H NMR (400 MHz, CDCl3):δ = 7.56–7.49 (m, 3 H), 7.36–7.13 (m, 6 H), 7.07 (d, J = 16.2 Hz, 2H), 2.38 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 137.6, 136.2, 135.8,130.4, 130.0, 128.7, 127.6, 127.6, 126.5, 126.2, 125.4, 20.2. The 1Hand 13C spectra were found to be in agreement with Lindhardtet al.[39]

n-Butyl (E)-3-(4-methoxyphenyl)prop-2-enoate

Light yellow liquid.[31] IR (film): 2956, 2930, 1603, 1462, 982,826 cm−1. 1H NMR (400 MHz, CDCl3): δ = 0.94 (t, J = 7.8 Hz, 3H), 1.40–1.46 (m, 2 H), 1.64–1.71 (m, 2 H), 3.80 (s, 3 H), 4.19 (t,J = 6.9 Hz, 2 H), 6.31 (d, J = 15.4 Hz, 1 H), 6.88 (d, J = 8.2 Hz, 2H), 7.45 (d, J = 8.2 Hz, 2 H), 7.63 (d, J = 15.8 Hz, 1 H).13C NMR(100 MHz, CDCl3): δ = 167.2, 161.9, 144.0, 129.5, 127.2, 115.6,114.2, 64.1, 55.2, 30.7, 19.1, 13.6. The 1H and 13C spectra werefound to be in agreement with Cui et al.[31]

n-Butyl (E)-cinnamate

Light yellow liquid.[31] IR (film): 3065, 2958, 1708, 1631, 1462, 1326,766, 688 cm−1. 1H NMR (400 MHz, CDCl3): δ = 0.98 (t, J = 7.3 Hz,3 H), 1.42–1.56 (m, 2 H), 1.70–1.72 (m, 2 H), 4.21 (t, J = 6.7 Hz, 2 H),6.47 (d, J = 16.0 Hz, 1 H), 7.35–7.38 (m, 3 H), 7.51–7.53 (m, 2 H),7.65 (d, J = 16.0 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 167.2,144.4, 134.6, 130.1, 128.7, 128.1, 118.2, 64.3, 30.7, 19.1, 13.6. The 1Hand 13C spectra were found to be in agreement with Cui et al.[31]

n-Butyl (E)-3-(4-methylphenyl)prop-2-enoate

Light yellow liquid.[31] IR (film): 3012, 2956, 1719, 1604, 1517, 984,810 cm−1. 1H NMR (400 MHz, CDCl3): δ = 0.86 (t, J = 7.4 Hz, 3H), 1.29–1.38 (m, 2 H), 1.55–1.62 (m, 2 H), 2.25 (s, 3 H), 4.10 (t,J = 6.7 Hz, 2 H), 6.31 (d, J = 16.0 Hz, 1 H), 7.06 (d, J = 8.0 Hz, 2H), 7.30 (d, J = 8.0 Hz, 2 H), 7.58 (d, J = 16.0 Hz, 1 H). 13C NMR(100 MHz, CDCl3): δ = 167.1, 144.4, 140.4, 131.63, 129.5, 127.9,117.0, 64.2, 30.7, 21.3, 19.1, 13.6. The 1H and 13C spectra werefound to be in agreement with Cui et al.[31]

Ethyl (E)-3-(3-nitrophenyl)prop-2-enoate

Light yellow solid; m.p. 74–75 ◦C (lit.[40] 74.3–74.6 ◦C). IR (KBr):3073, 2983, 1717, 1645, 1525, 1483, 1353, 1328, 1187, 997, 871,747, 666 cm−1. 1H NMR (400 MHz, CDCl3): δ = 8.53 (t, J = 1.7 Hz,1 H), 8.21(dd, J = 8.4, 1.6 Hz, 1 H), 8.17 (d, J = 8.0 Hz, 1 H), 7.76 (d,J = 16.4 Hz, 1 H), 7.71 (t, J = 8.0 Hz, 1 H), 6.84 (d, J = 16.0 Hz, 1 H),4.21 (q, J = 7.2 Hz, 2 H), 1.26 (t, J = 7.0 Hz, 3 H). 13C NMR (100 MHz,CDCl3): δ = 166.8, 149.2, 142.6, 136.4, 134.0, 130.2, 124.4, 122.8,121.4, 59.6, 14.6. The 1H and 13C spectra were found to be inagreement with Bouziane et al.[41]

n-Butyl (E)-3-(4-nitrophenyl)prop-2-enoate

Light yellow solid; m.p. 63–65 ◦C (lit.[32] 64–65 ◦C). IR (KBr): 3058,2958, 1709, 1601, 1493, 1308, 982, 759 cm−1. 1H NMR (400 MHz,CDCl3): δ = 0.97 (t, J = 7.4 Hz, 3 H), 1.40–1.50 (m, 2 H), 1.67–1.74(m, 2 H), 4.24 (t, J = 6.5 Hz, 2 H), 6.56 (d, J = 15.8 Hz, 1 H), 7.69–7.73(m, 3 H); 8.23–8.25 (m, 2 H). 13C NMR (100 MHz, CDCl3): δ = 165.7,148.2, 141.2, 140.3, 128.4, 123.8, 122.3, 64.5, 30.4, 18.9, 13.4. The 1Hand 13C spectra were found to be in agreement with Cui et al.[31]

n-Butyl (E)-3-(4-cyanophenyl) prop-2-enoate

Light yellow liquid.[32] IR (film): 2959, 2886, 2221, 1715, 1641,1606, 1411, 987, 836 cm−1. 1H NMR (400 MHz, CDCl3): δ = 0.97 (t,J = 7.6 Hz, 3 H), 1.34–1.48 (m, 2 H), 1.59–1.64 (m, 2 H), 4.21 (t,J = 6.8 Hz, 2 H), 6.50 (d, J = 15.7 Hz, 1 H), 7.61–7.73 (m, 5 H). 13CNMR (100 MHz, CDCl3): δ = 166.2, 142.1, 138.6, 132.3, 128.0, 121.0,117.9, 113.3, 64.5, 30.5, 19.3, 13.8. The 1H and 13C spectra werefound to be in agreement with Cui et al.[31]

Ethyl (E)-3-(2-trifluoromethylphenyl)prop-2-enoate

Light yellow liquid.[42] IR (film): 3083, 2985, 1723, 1635, 1489, 1389,1317, 1165, 1125,1037, 980, 766, 652 cm−1. 1H NMR (400 MHz,CDCl3): δ = 8.08 (d, J = 15.8 Hz, 1 H), 7.75–7.46 (m, 4 H), 6.42(d, J = 15.8 Hz, 1 H), 4.28 (q, J = 7.1 Hz, 2 H), 1.34 (t, J = 6.0 Hz,3 H).13C NMR (100 MHz, CDCl3): δ = 166.8, 142.3, 132.5, 131.6,128.2, 126.6, 125.3, 122.4, 61.6, 14.2. The 1H were found to be inagreement with Chatterjee et al.[42]

Ethyl (E)-3-(4-trifluoromethylphenyl)prop-2-enoate

Light yellow solid; m.p. 31–33 ◦C (lit.[43] 31–32 ◦C). IR (KBr): 3076,2984, 1712, 1643, 1417, 1325, 1284, 1170, 1069, 985, 833 cm−1.1H NMR (400 MHz, CDCl3): δ = 7.68 (d, J = 15.9 Hz, 1 H), 7.61(m,4 H), 6.50 (d, J = 15.9 Hz, 1 H), 4.29 (q, J = 7.2 Hz, 2 H), 1.35 (t,J = 7.0 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 166.2, 142.7,137.6, 132.1, 129.5, 128.2, 125.5, 120.7, 60.6, 14.4. The 1H and 13Cspectra were found to be in agreement with Chen et al.[44]

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Results and Discussion

The synthesis of amino-functionalized IL 1, 2 and 3 is illustrated inScheme 1. They were readily prepared through a straightforwardtwo-step procedure from commercially available starting materialsand reagents in good yields. The N-methyl imidazole wasreacted with 3-chloropropan-1-amine with 1 : 1 molar ratioin ethanol under reflux temperature for 24 h to afford IL1 in 73% yield. This chloride salt (IL 1) then reacted withsodium tetrafluoroborate or potassium hexafluorophosphateat room temperature in ethanol–water for 48 h to obtainthe corresponding amino-functionalized ionic liquids containingtetrafluoroborate or hexafluorophosphate anions, IL 2 and 3,respectively. The ionic liquids were further purified by drying ina vacuum to remove the residual starting materials, reagents ororganic solvents.

In order to evaluate the catalytic activity of Pd-catalyzed Heckreaction in the presence of IL 1, 2 and 3, initially, we focused ourattention on the reaction of 4-iodoanisole with n-butyl acrylateunder the reaction conditions involving 4-iodoanisole (1.0 mmol),n-butyl acrylate (1.0 mmol), Pd submicron powder (0.01 mmol), IL1, 2 or 3 (1.0 mmol), at 120 ◦C in [Bmim]Cl, [Bmim]BF4 or [Bmim]PF6

(2.0 ml), respectively. It was found that IL 3 in [Bmim]PF6 exhibitsa high activity to palladium-catalyzed Heck reaction, while thecorresponding BF4

− and Br− ionic liquids, IL 2 with [Bmim]BF4

and IL 1 with [Bmim]Cl, demonstrated the lower catalytic activity(Table 1, entries 1, 3, 5, 7 and 8). The influence of anions infunctionalized ionic liquids on the catalytic activity of palladium-catalyzed Heck reaction is Br− < BF4

− < PF6−. For comparison,

the experimental results also revealed that, in the absence ofamino-functionalized ionic liquids, 1, 2 and 3, Pd submicronpowder clearly showed no catalytic activity to the Heck reaction(Table 1, entries 2, 4 and 6). When the reaction was carried outin [Bmim]BF4 with additional of K2CO3 (2.0 mmol) added to thereaction system, only a trace amount of the desired Heck couplingproduct was isolated (Table 1, entry 9). This is presumably due tothe effective N-ligand of IL 1, 2 and 3, in the palladium powdercatalyzed reaction.[13]

Encouraged by this result, we continued our research to furtheroptimization of the reaction conditions. We then turned ourattention to investigating the effects of palladium source on theHeck reaction. Pd submicron powder was screened as the optimalone in the presence of 1.0 mol% amount at 120 ◦C, whereas otherpalladium sources such as PdCl2, Pd(Cl)2(PPh3)2 and Pd(PPh3)4

were substantially less effective (Table 2). Although Pd(OAc)2 alsoachieved satisfactory yield (92%), palladium submicron powderwas superior to Pd(OAc)2 in the recovery and reuse procedure forthe further consideration.

After exploring a wide array of reaction conditions at the outsetof our studies, we were pleased to find that the treatment of4-iodoanisole and n-butyl acrylate in the presence of 1.0 mol% Pdsubmicron powder and using amino-functionalized IL 3 (1.0 equiv.)in [Bmim]PF6 at 120 ◦C for 24 h generated the expected productin excellent yield (93%, Table 1, entry 4 and Table 2, entry 1).

To investigate the scope of the present method, the Heckalkenation of n-butyl acrylate and ethyl acrylate with a variety ofiodoarenes and bromoarenes, containing electron-withdrawingor electron-donating substituents, was investigated. The results inTable 3 indicated that the conversions, regioselectivities (>99%trans products) and yields were satisfactory under optimizedreaction conditions (Table 3, entries 1–12). In addition, todetermine the scope of this catalytic system, other olefins

Table 1. Effect of the amino-functionalized ionic liquids on the Heckreactiona

H3CO

I

CO2Bu-nH3CO

CO2Bu-nPd powder(Submicron)

+

Entry Amino-functionalized IL Common IL Yieldb (%)

1 IL 1 [Bmim]Cl 31

2 – [Bmim]Cl 0

3 IL 2 [Bmim]BF4 76

4 – [Bmim]BF4 trace

5 IL 3 [Bmim]PF6 93

6 – [Bmim]PF6 trace

7 IL 1 [Bmim]PF6 58

8 IL 2 [Bmim]PF6 85

9 – [Bmim]BF4 tracec

a 4-Iodoanisole (234 mg, 1.0 mmol), n-butyl acrylate (128 mg,1.0 mmol), Pd submicron powder (1.0 mg, 0.01 mmol), amino-functionalized IL (1.0 mmol) and in common IL (2.0 ml) at 120 ◦Cfor 24 h.b Isolated yield.c In the presence of K2CO3 (2.0 mmol).

Table 2. Effect of palladium source on the Heck reactiona

H3CO

I

CO2Bu-nH3CO

CO2Bu-nPd Source

+IL 3

Entry Palladium source Yieldb (%)

1 Pd submicron powder 93

2 Pd(OAc)2 92

3 PdCl2 51

4 Pd(Cl)2(PPh3)2 60

5 Pd(PPh3)4 34

a 4-Iodoanisole (234 mg, 1.0 mmol), n-butyl acrylate (128 mg,1.0 mmol), Pd source (0.01 mmol) and IL 3 (285 mg, 1.0 mmol) in[Bmim]PF6 (2.0 ml) at 120 ◦C for 24 h.b Isolated yields.

substrates, such as styrene, were also examined, and goodresults were also obtained under the identical reaction conditions(Table 3, entries 13–24). It should be noted that the alkenationof haloarenes was tolerant of ortho- and meta-substitution ofthe aryl iodide and afforded the desired products in excellentyields (Table 3, entries 4, 7 and 18–20). However, when arylchlorides served as organic halide substrates, poor yields ofthe corresponding products were obtained under the samereaction conditions, owing to the much lower reactivity oftheir carbon–chlorine bond.[13,45,46] In addition, the result of theolefination reaction of iodobenzene with internal olefins, such asethyl cinnamate, was found to be negative (Table 3, entry 25).

To screen the recyclability of this catalytic system, aftercarrying out a reaction, isolating the product from the reactionmixture, washing with solvents, drying and recovering amino-functionalized IL 3, Pd species and [Bmim][PF6], fresh startingmaterials were charged into the reaction system. The reactions


39

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J. Liu, H. Liu and L. Wang

Table 3. Heck reactions catalyzed by Pd submicron powder andamino-functionalized IL 3a

Entry Olefin Aryl halide Yieldb (%)

1 H2C CH–CO2C4H9-n p-CH3OC6H4I 93

2 H2C CH–CO2C4H9-n C6H5I 92

3 H2C CH–CO2C4H9-n p-CH3C6H4I 90

4 H2C CH–CO2C2H5 m-NO2C6H4I 78

5 H2C CH–CO2C4H9-n p-NO2C6H4I 92

6 H2C CH–CO2C4H9-n p-CNC6H4I 91

7 H2C CH–CO2C2H5 o-CF3C6H4I 89

8 H2C CH–CO2C2H5 p-CF3C6H4I 90

9 H2C CH–CO2C4H9-n p-CH3OC6H4Br 83

10 H2C CH–CO2C4H9-n p-CH3C6H4Br 82

11 H2C CH–CO2C4H9-n p-NO2C6H4Br 89

12 H2C CH–CO2C4H9-n p-CNC6H4Br 87

13 C6H5CH CH2 C6H5I 84

14 C6H5CH CH2 p-CH3OC6H4I 82

15 C6H5CH CH2 p-CH3C6H4I 85

16 C6H5CH CH2 p-CNC6H4I 92

17 C6H5CH CH2 p-CF3C6H4I 85

18 C6H5CH CH2 m-CF3C6H4I 89

19 C6H5CH CH2 o-CF3C6H4I 88

20 C6H5CH CH2 o-CH3C6H4I 87

21 C6H5CH CH2 p-CH3OC6H4Br 73

22 C6H5CH CH2 p-CH3C6H4Br 80

23 C6H5CH CH2 p-CNC6H4Br 89

24 C6H5CH CH2 C6H5Br 86

25 C6H5CH CH–CO2C2H5 C6H5I 0

a Alkene (1.0 mmol), aryl halide (1.0 mmol), Pd submicron powder(1.0 mg, 0.01 mmol), IL 3 (285 mg, 1.0 mmol) in [Bmim]PF6 (2.0 ml) at120 ◦C for 24 h.b Isolated yield.

Table 4. Successive trials by using recoverable Pd submicron powderand IL 3a

Recycle Pd

H3CO

I

CO2Bu-nH3CO

CO2Bu-n+

IL 3

Trial Yieldb (%) Trial Yieldb (%)

1 93 4 92

2 93 5 92

3 92 6 90

a 4-Iodoanisole (234 mg, 1.0 mmol), n-butyl acrylate (128 mg,1.0 mmol), Pd submicron powder (1.0 mg, 0.01 mmol), IL 3 (285 mg,1.0 mmol) in [Bmim]PF6 (2.0 ml) at 120 ◦C for 24 h.b Isolated yield.

still proceeded well. Pd, IL 3 and [Bmim]PF6 were recycled six timeswithout decreases in product yields.

Conclusion

In conclusion, we have developed a kind of amino-functionalizedionic liquids as ligand and base for the Heck reactions betweenaryl iodides and bromides with olefins in the presence of a catalytic

amount of Pd submicron powder in [Bmim]PF6 under base-freeand phosphine-free reaction conditions. The reactions generatedthe corresponding products in excellent yields under mild reactionconditions. It should be pointed out that Pd species and ionicliquids can be easily recycled and reused with the same efficaciesfor six cycles. Currently, further efforts to extend the applicationof the system in other palladium-catalyzed transformations areunderway in our laboratory.

Acknowledgments

We gratefully acknowledge financial support by the NationalNatural Science Foundation of China (no. 20772043), and the KeyProject of Science and Technology of the Department of Education,Anhui Province, China (no. ZD2007005-1).

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exploration of amino-functionalized ionic liquids as ligand and base for heck reaction

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