2-Arylnaphthoxazole-derived palladium(II) complexes as phosphine-free catalysts for Heck reactions

Polyhedron 26 (2007) 4389–4396 www.elsevier.com/locate/poly

2-Arylnaphthoxazole-derived palladium(II) complexes as phosphine-free catalysts for Heck reactions Hong Li, Yang-Jie Wu *, Chen Xu, Rong-Qiang Tian Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities, Zhengzhou University, Zhengzhou 450052, PR China Received 27 February 2007; accepted 22 May 2007 Available online 5 June 2007

Abstract Four air- and moisture-stable new palladium(II) complexes 2a–2c and 4 have been synthesized from easily available 2-arylnaphthoxazole derivatives. The detailed structures of 2c and 4 have been determined by single-crystal X-ray analysis. The Pd–N, Pd–C bonds in palladacycle complexes 2a–2c and the Pd–N, Pd–O bonds in complex 4 form the basis for five- and six-membered chelate rings, respectively. These complexes were applied as efficient phosphine-free catalysts for Heck reactions with aryl bromides and ethyl acrylate. Typically, in the presence of two equivalent n-Bu4NBr, using 0.01% of palladacycle complex 2c as catalyst and two equivalent of K2CO3 as base in DMF at 140 C provided coupled products in moderate to high yields.  2007 Published by Elsevier Ltd. Keywords: 2-Arylnaphthoxazole derivatives; Palladium(II) complexes; Crystal structures; Heck reactions

1. Introduction Palladium-catalyzed coupling reactions have been recognized as extremely powerful tools in organic synthesis for the formation of carbon–carbon or carbon–heteroatom bonds [1]. The Heck reaction is among the most important ones due to its high tolerance of functional groups and general applicability [2]. The reaction generally proceeds in the presence of palladium catalysts associated with phosphine ligands which could stabilize the active palladium intermediates [3]. However, most of the phosphine ligands are airand moisture-sensitive. It is therefore not surprising that the interest in the development of new catalytic systems for this process continues unabated [4]. Among them, palladacycles including CP, CN, CS, PCP, SCS, NCN types [5] have been shown to be extremely active in the promotion of Heck reactions because of their facile synthesis,

*

Corresponding author. Tel./fax: +86 371 67979408. E-mail address: [email protected] (Y.-J. Wu).

0277-5387/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.poly.2007.05.031

thermal stability and structural versatility [6]. Although cyclopalladated aromatic compounds are the choice systems for these catalysts [7], the use of oxazoles as ligands in Heck reactions is rare [8]. Accordingly, we considered it worthwhile to investigate whether 2-arylnaphthoxazole derivatives could be cyclopalladated to yield efficient catalysts. Herein, we prepared four new palladium complexes 2a–2c and 4 and examined their catalytic activity in Heck reactions. The results are presented below. 2. Experimental 2.1. General Reactions were monitored by thin-layer chromatography, which was carried out on silica gel (60F254) coated glass plates. Melting points were measured with the use of a WC-1 microscopic apparatus. Elemental analyses were conducted with a Carlo Erba 1160 elemental analyzer. IR spectra were collected on a Bruker VECTOR22 spectrophotometer in KBr pellets. 1H and 13C

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NMR spectra were recorded on a Bruker DPX-400 spectrometer in CDCl3 with TMS as an internal standard. High-resolution mass spectra were measured on a Waters Q-Tof Micro spectrometer. Preparative TLC was performed on dry silica gel plates and developed with selected appropriate solvents as eluent. Solvents were dried and freshly distilled prior to usage. 2-Arylnaphthoxazole derivatives 1a–1c, 3 [9,10] and Pd(OAc)2 [11] were prepared according to previously reported procedures. All aryl halides were purchased and used without further treatment. 2.2. Synthesis of palladium complexes 2.2.1. General method for the synthesis of palladacycles 2a–2c A mixture of 2-arylnaphthoxazoles 1a–1c (0.2 mmol) and palladium(II) acetate (0.2 mmol) in 2 mL of acetic acid was heated at 100 C under nitrogen for 6 h. After cooling to room temperature, water was added and the mixture was extracted with dichloromethane. Then the combined organic phases were dried over MgSO4, filtered and the solvent was removed under reduced pressure. The residue was purified by preparative TLC giving yellow product. Compound 2a: Yield 82%. Yellow solid, m.p. 190 C (dec.); Anal. Calc. for C38H26N2O6Pd2: C, 55.70; H, 3.20; N, 3.42. Found: C, 55.58; H, 3.11; N, 3.35%. IR (KBr): tmax 1598, 1572, 1522, 1452, 1414, 1382, 1345, 1297, 1279, 1246, 1128, 1092, 1041, 1001, 963, 884, 802, 767, 743, 724, 695 cm 1; 1H NMR (400 MHz, CDCl3): d 2.33 (s, 6H, CH3), 5.62–5.66 (m, 2H, Ar–H), 6.25–6.28 (m, 4H, Ar–H), 6.92 (d, J = 7.0 Hz, 2H, Ar–H), 7.43 (d, J = 8.9 Hz, 2H, Ar–H), 7.50–7.58 (m, 4H, Ar–H), 7.69 (d, J = 8.9 Hz, 2H, Ar–H), 7.86 (d, J = 8.4 Hz, 2H, Ar– H), 8.85 (d, J = 8.0 Hz, 2H, Ar–H) ppm; 13C NMR (100 MHz, CDCl3): d 25.0(CH3), 110.4(CH), 123.1(CH), 123.7(CH), 124.1(C), 124.3(CH), 125.8(CH), 126.6(CH), 126.7(CH), 128.1(CH), 128.3(CH), 128.9(C), 129.6(CH), 131.1(C), 133.8(C), 142.8(C), 146.0(C), 168.6(C@N), 181.5(C@O) ppm. Compound 2b: Yield 78%. Yellow solid, m.p. 199 C (dec.); Anal. Calc. for C40H30N2O6Pd2: C, 56.69; H, 3.57; N, 3.31. Found: C, 56.59; H, 3.46; N, 3.24%. IR (KBr): tmax 2921, 1599, 1572, 1518, 1463, 1406, 1385, 1346, 1278, 1248, 1140, 1103, 1039, 1001, 960, 884, 803, 739, 694 cm 1; 1H NMR (400 MHz, CDCl3): d 1.09 (s, 6H, CH3), 2.33 (s, 6H, CH3), 6.06 (d, J = 7.7 Hz, 2H, Ar–H), 6.13 (s, 2H, Ar–H), 6.82 (d, J = 7.5 Hz, 2H, Ar–H), 7.44 (d, J = 8.9 Hz, 2H, Ar–H), 7.49–7.52 (m, 2H, Ar–H), 7.55– 7.59 (m, 2H, Ar–H), 7.68 (d, J = 8.9 Hz, 2H, Ar–H), 7.86 (d, J = 8.0 Hz, 2H, Ar–H), 8.89 (d, J = 8.3 Hz, 2H, Ar–H) ppm; 13C NMR (100 MHz, CDCl3): d 21.1(CH3), 25.1(CH3), 110.5(CH), 123.0(CH), 124.0(C), 124.3(CH), 124.5(CH), 125.8(CH), 126.1(C), 126.2(CH), 126.6(CH), 128.2(CH), 131.6(C), 131.7(CH), 133.9(C), 140.3(C), 142.7(C), 145.9(C), 168.9(C@N), 181.4(C@O) ppm.

Compound 2c: Yield 76%. Yellow solid, m.p. 201 C (dec.); Anal. Calc. for C40H30N2O8Pd2: C, 54.62; H, 3.44; N, 3.19. Found: C, 54.55; H, 3.38; N, 3.10%. IR (KBr): tmax 2927, 1599, 1567, 1520, 1467, 1412, 1388, 1346, 1315, 1264, 1230, 1178, 1130, 1101, 1036, 1000, 960, 884, 800, 742, 696 cm 1; 1H NMR (400 MHz, CDCl3): d 2.31 (s, 6H, CH3), 2.79 (s, 6H, CH3), 5.79–5.84 (m, 4H, Ar– H), 6.86 (d, J = 8.3 Hz, 2H, Ar–H), 7.40 (d, J = 8.9 Hz, 2H, Ar–H), 7.48–7.50 (m, 2H, Ar–H), 7.52–7.54 (m, 2H, Ar–H), 7.66 (d, J = 8.9 Hz, 2H, Ar–H), 7.86 (d, J = 8.0 Hz, 2H, Ar–H), 8.85 (d, J = 8.2 Hz, 2H, Ar–H) ppm; 13C NMR (100 MHz, CDCl3): d 25.0(CH3), 53.8(CH3), 110.5(CH), 111.4(CH), 114.0(CH), 121.2(C), 123.7(C), 124.1(CH), 124.4(CH), 125.5(CH), 125.8(CH), 126.4(CH), 128.2(CH), 131.5(C), 133.7(C), 145.5(C), 145.7(C), 158.7(C), 169.0(C@N), 181.3(C@O) ppm. 2.2.2. General method for the synthesis of palladium complex 4 To a solution of Li2PdCl4 (0.2 mmol) in methanol (2 mL), a methanolic solution (2 mL) of corresponding 2-arylnaphthoxazole 3 (0.2 mmol) and NaOAc (0.2 mmol) was added at room temperature. Then, the solution was stirred for about 10 h and a precipitate was formed. The precipitate was filtered and washed with methanol. Compound 4: Yield 94%. Red solid, m.p. >270 C; IR (KBr): tmax 1601, 1555, 1528, 1465, 1429, 1383, 1322, 1248, 1151, 1133, 1069, 1004, 976, 885, 857, 797, 747, 704 cm 1; 1H NMR (400 MHz, CDCl3): d 6.21 (d, J = 8.5 Hz, 2H, Ar–H), 6.58–6.62 (m, 2H, Ar–H), 6.96– 6.70 (m, 2H, Ar–H), 7.57–7.61 (m, 2H, Ar–H), 7.64–7.68 (m, 2H, Ar–H), 7.71 (d, J = 8.9 Hz, 2H, Ar–H), 7.87 (d, J = 8.0 Hz, 2H, Ar–H), 7.90 (d, J = 8.9 Hz, 2H, Ar–H), 7.99 (d, J = 8.0 Hz, 2H, Ar–H), 9.46 (d, J = 8.4 Hz, 2H, Ar–H) ppm; HRMS (positive ESI) Calc. for C34H20N2O4Pd: 627.0536 [M+H]+, Found: 627.0533 [M+H]+. 2.3. General procedure for Heck reactions A 5 mL round-bottom flask was charged with appropriate aryl halides (0.5 mmol), ethyl acrylate (0.75 mmol), nBu4NBr (1.0 mmol) and base (1 mmol). The catalyst was introduced as a solvent solution (0.4 mg/mL) via syringe, and additional solvent was added to give a total volume of 2 mL. The reaction mixture was then placed in an oil bath and heated at 140 C until the starting aryl halides had been completely consumed, as monitored by thin-layer chromatography. After cooling to room temperature, water was added and the mixture was extracted with dichloromethane. Then the combined organic phases were dried over MgSO4, filtered and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel (the purified products were identified by comparison of melting points with the literature data or by 1H NMR).

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2.4. X-ray structure determination

3. Results and discussion

Intensity data of the complexes 2c and 4 were measured on a Rigaku-Raxis-IV X-ray diffractometer using graphite˚ ) at monochromated Mo Ka radiation (k = 0.71073 A 291(2) K. All data were collected using x 2h scan technique and corrected for Lorebtz-polarization effects. A correction for secondary extinction was applied. The two structures were solved by direct methods [12] and expanded using Fourier techniques. The full-matrix least-squares calculations on F2 were applied on the final refinement. Their raw data were corrected and the structures were solved using the SHELXTL-97 program [13]. Details of crystal structure determination of the complexes 2c and 4 are summarized in Table 1.

3.1. Synthesis and characterization of palladium complexes

Table 1 Crystal and structure refinement data for 2c and 4 Compound

2c

4

Empirical formula Formula weight Crystal system Crystal size (mm) Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z DCalc (g cm 3) l(Mo Ka) (mm 1) h Range () Data/restraints/parameters Goodness-of-fit on F2 R(F) [I > 2r(I)]

C40H30N2O8Pd2 879.46 monoclinic 0.20 · 0.18 · 0.17 P2(1)/c 8.3599(17) 15.124(3) 27.690(6) 90 98.38(3) 90 3463.6(12) 4 1.687 1.097 1.49–25.00 5725/0/470 1.082 R1 = 0.0499, wR2 = 0.1076 R1 = 0.0652, wR2 = 0.1152

C34H20N2O4Pd 626.92 monoclinic 0.20 · 0.18 · 0.18 C2/c 20.202(4) 8.4717(17) 14.692(3) 90 92.61(3) 90 2511.8(9) 4 1.658 0.786 2.02–27.51 2768/0/187 1.047 R1 = 0.0404, wR2 = 0.1010 R1 = 0.0475, wR2 = 0.1069

R indices (all data)

Cyclopalladation of 2-arylnaphthoxazole derivatives 1a–1c readily occurred using Pd(OAc)2 in acetic acid at 100 C under nitrogen to give yellow cyclopalladated complexes 2a–2c in moderate yields (Scheme 1). Unfortunately, the l-Cl bridged cyclopalladated complexes were not obtained by the reaction of 2-arylnaphthoxazole derivatives 1a–1c with Li2PdCl4 in MeOH in the presence of NaOAc as a proton scavenger at room temperature or 65 C, respectively. Complexes 2a–2c were fully characterized by elemental analysis, IR, 1H and 13C NMR. The IR spectra of 1a–1c show a sharp band at about 1637 cm 1 [9,10], while for complexes 2a–2c, this band shifted to 1599 cm 1, indicating that the nitrogen atom was coordinated to palladium through its lone pair. The appearance of the signal at about d 2.3 ppm in 1H NMR spectra and the signal at d 181 ppm in 13C NMR spectra suggested the presence of acetate group. Finally, the structure of complex 2c has been confirmed by X-ray diffraction study (Fig. 1).

O Pd(OAc)2

R

AcOH 100oC Pd

N OAc

O

2 2a-2c

R N 1a-1c R= H (a), CH3 (b), OCH3 (c)

O Li2PdCl4 / NaOAc R

×

MeOH r.t. or 65 oC

Fig. 1. Molecular structure of acetate-bridged dimer 2c.

Pd

N Cl 2

Scheme 1.

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Li2PdCl4 / NaOAc MeOH r.t. O O

N O Pd O N

N OH 3

O 4 Pd(OAc)2

CH2Cl2 r.t. Scheme 2.

Table 2 ˚ ) and angles () for 2c and 4 Selected bond lengths (A 2c Pd(1)–C(13) Pd(1)–O(7) Pd(1)–N(1) Pd(1)–O(5) Pd(2)–C(31) Pd(2)–O(6) Pd(2)–N(2) Pd(2)–O(8) C(11)–N(1) C(29)–N(2) C(13)–Pd(1)–O(7) 4 Pd(1)–O(2) Pd(1)–O(2)#1 Pd(1)–N(1) Pd(1)–N(1)#1 N(1)–C(11) N(1)–C(10) O(2)–C(13) C(11)–C(12) C(12)–C(13)

1.976(6) 2.034(4) 2.071(4) 2.141(4) 1.996(7) 2.030(4) 2.091(5) 2.126(4) 1.314(7) 1.318(8) 91.7(2) 1.999(3) 1.999(3) 2.029(3) 2.029(3) 1.319(4) 1.398(4) 1.320(4) 1.437(5) 1.418(5)

Fig. 3. Molecular structure of complex 4.

C(13)–Pd(1)–N(1) O(7)–Pd(1)–N(1) C(13)–Pd(1)–O(5) O(7)–Pd(1)–O(5) N(1)–Pd(1)–O(5) C(31)–Pd(2)–O(6) C(31)–Pd(2)–N(2) O(6)–Pd(2)–N(2) C(31)–Pd(2)–O(8) O(6)–Pd(2)–O(8) N(2)–Pd(2)–O(8)

81.8(2) 173.4(2) 176.0(2) 87.0(2) 99.5(2) 90.5(2) 81.3(3) 171.4(2) 176.5(2) 86.2(2) 102.1(2)

O(2)–Pd(1)–O(2)#1 O(2)–Pd(1)–N(1) O(2)#1–Pd(1)–N(1) O(2)–Pd(1)–N(1)#1 O(2)#1–Pd(1)–N(1)#1 N(1)–Pd(1)–N(1)#1 C(11)–N(1)–Pd(1) C(10)–N(1)–Pd(1) C(13)–O(2)–Pd(1)

169.4(2) 89.4(1) 90.9(1) 90.9(1) 89.4(1) 177.8(2) 118.9(2) 133.5(2) 119.1(2)

The reaction of 2-arylnaphthoxazole derivative 3 with Li2PdCl4 in MeOH in the presence of NaOAc as a proton scavenger at room temperature afforded quantitatively red coordination compound 4, but no cyclopalladated complex formed. On the other hand, when Pd(OAc)2 was stirred in CH2Cl2 with 3 at room temperature, the same product – complex 4 was obtained (Scheme 2). Complex 4 was fully characterized by elemental analysis, IR and 1H NMR. The tC@N at 1601 cm 1 in the IR spectrum of 4 is lower than that of 3 (1641 cm 1) [9], due to the intramolecular coordination of nitrogen to palladium. Nevertheless, because of its low solubility in CDCl3 and other solvents, we were unable to obtain its 13C NMR spectra with high enough resolution. Fortunately, the structure of complex 4 was unequivocally confirmed by X-ray diffraction analysis (Fig. 3).

Fig. 2. One-dimensional chain structure of complex 2c.

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Fig. 4. The p–p interactions in complex 4.

3.2. X-ray diffraction studies: crystal structures of palladium complexes 2c and 4 The crystal structures of complexes 2c and 4 were determined, and the crystal data and refinement details for both ˚) structures are listed in Table 1. Selected bond length (A and angles () are listed in Table 2. Perspective view of the molecular structure for complex 2c, together with the corresponding atom-labeling scheme, is given in Fig. 1. The Pd atom in complex 2c is in a squareplanar environment bonded to nitrogen, carbon atoms of the naphthoxazole ligand and two oxygen atoms (one from each of the acetate ligands). The Pd–N and Pd–C bonds form the basis for five-membered chelate rings. The ˚ , which is regarded as Pd1


Pd2 distance is 2.8735(8) A non-bonding, the covalent radius of square-planar Pd(II) ˚ [14]. The Pd1 has been estimated as approximately 1.31 A and Pd2 are bridged by two acetate ligands and the coordination planes of the Pd atoms are not coplanar (dihedral angle is 24.0). This leads to interligand repulsions on the ‘‘open’’ side of the molecule. Fig. 2 shows that in the crystal of 2c there exist intermolecular Pd


H hydrogen bonds ˚ , Pd1F


H7P = (Pd1F


H5P = Pd1O


H5F = 3.162 A

˚ ), which are attributed to construct Pd1O


H7F = 3.020 A the 1D chain structure of complex 2c. Fig. 3 shows that the Pd atom in complex 4 is in a slightly distorted square-planar environment (deviation ˚ ), ligand 3 coordinates to Pd(II) as from plane 0.1016 A a ON bidentate ligand. In contrast to complex 2c, the Pd–N and Pd–O bonds form the basis for six-membered ˚ ) bond chelate rings in complex 4. The Pd–N (2.029(3) A length is similar to that of the related complex ˚ and 2.014(6) A ˚ ) [15], while it is shorter than (2.026(7) A ˚ and 2.091(5) A ˚ ). It can that of complex 2c (2.071(4) A be seen from Fig. 4 that the two naphthoxazole rings of neighboring molecules are almost parallel with each ˚ (diheother with mean interplanar distance of 3.3305 A dral angle is 0.0), indicating strong intermolecular p–p stacking interaction between the two neighboring molecules. 3.3. Catalytic activity of palladium complexes in Heck reactions Palladacycles have been thoroughly investigated as precatalysts in C–C bond formation reactions. One of the

Table 3 Influence of bases, solvents and catalysts on Heck reactions of 4-bromotoluene with ethyl acrylate

O Br +

O

Cat. / Base OC2H5

OC2H5

Additive / Solvent

Entry

Cat. (mol%)

Base

Additive

Solvents

T (C)

Yielda (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2a (0.01) 2b (0.01) 2c (0.01) 4 (0.02)

K2CO3 K2CO3 Et3N CsF K3PO4 Cs2CO3 KF Æ H2O Na2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

none n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr n-Bu4NBr

DMF DMF DMF DMF DMF DMF DMF DMF dioxane toluene xylene DMA DMF DMF DMF

140 140 140 140 140 140 140 140 100 110 140 140 140 140 140

38 93 52 68 90 65 88 71 44 60 90 92 93 95 90

Reaction conditions: 4-bromotoluene (0.5 mmol), ethyl acrylate (0.75 mmol), base (1.0 mmol), additive (1.0 mmol), solvent (1 mL), 24 h reaction time. a Isolated yields (average of two runs) based on 4-bromotoluene.

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Table 4 Heck reactions of aryl halide with ethyl acrylate catalyzed by 2c

O OC2H5 Entry

ArX

1

O

Cat. 2c / K2CO3

ArX +

n-Bu4NBr / DMF / 140 oC

Cat. 2c (mol%)

Time (h)

2c (0.01)

16

Ar

OC2H5 Yielda (%)

Product

O

Br

96

OC2H5 2c (0.01)

2

16

93

O

Br

OC2H5

3

2c (0.01)

H3CO

24

Br

4

H3CO

2c (0.01)

N

Cl

OC2H5

O

12

Br

F3 C

2c (0.01)

7

95

O Cl

2c (0.01)

F3C

OC2H5

12

Br

6

88

O N

2c (0.01)

5

OC2H5

24

Br

93

O

96

OC2H5

12

96

O

Br OC2H5

OHC

OHC 2c (0.01)

8

24

Br

89

O OC2H5

2c (0.01)

9

24

Br

10

OC2H5

2c (0.01)

16

O

Br

Br

95

OC2H5

2c (0.01)

11

72

O

24

90

O OC2H5

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Table 4 (continued) Entry

ArX

12

Cat. 2c (mol%)

Time (h)

2c (0.01)

16

Yielda (%)

Product

O

Br N

2c (0.01)

16

N

OC2H5

N

14

2c (0.01)

Br

24

OC2H5

S 2c (0.01)

24

OC2H5

S 2c (0.01)

70

O

Br

16

76

O

S

15

88

O

Br

S

OC2H5

N

13

79

24

trace

O

Cl

OC2H5 2c (0.1)

17

24

19

O

Cl

OC2H5 2c (0.01)

18

O2N

19

Cl

23

O O2 N

2c (0.1)

O2N

24

Cl

24

OC2H5 45

O O2 N

OC2H5

Reaction conditions: ArX (0.5 mmol), ethyl acrylate (0.75 mmol), K2CO3 (1.0 mmol), n-Bu4NBr (1.0 mmol), DMF (1 mL), 140 C, 24 h reaction time. a Isolated yields (average of two runs) based on ArX.

most popular cross-couplings is Heck reactions. A wide variety of substrates has been used to design catalyst precursors endowed with good activity in Heck reactions. So we thought it interesting to study whether the air-stable palladium(II) complexes 2a–2c and 4 would be efficient catalysts for Heck reactions. Our initial exploration of reaction conditions focused on the coupling of 4-bromotoluene with ethyl acrylate. The results are shown in Table 3. It showed that the reaction occurred smoothly in air and afforded the product in excellent yield in the presence of additive n-Bu4NBr which could react as reducing agent to generate active Pd(0) species and stabilize the intermediates by coordination or formation of ion-pairs in the catalytic cycle [16] (entries 1 and 2). After screening a variety of bases, K2CO3 was found to give the best result (entries 2–8). Then, a quick survey of solvents indicated that DMF was superior to others (entries 2, 9–12). With the appropriate base (K2CO3) and solvent (DMF) in hand, the relative activities of palladium com-

plexes 2a–2c and 4 for the same model reaction were studied (entries 2, 13–15). All the complexes exhibited high activity with catalyst loadings as low as 0.01%. Among them, complex 2c was slightly more active than others probably because of its electron-donating methoxy group (entry 14). Under the optimized conditions (K2CO3, DMF, 140 C), the substrate scope of Heck reactions was examined. As shown in Table 4, a variety of electronically and structurally diverse aryl bromides including heteroaryl bromides could be coupled efficiently with ethyl acrylate catalyzed by 0.01 mol% of complex 2c. The coupled products were isolated in moderate to high yields for non-activated and deactivated aryl bromide substrates in this catalytic system after 16 or 24 h (entries 1–4). For activated aryl bromides, such as 4-bromochlorobenzene, and 4-bromobenzotrifluoride, it was not surprising that excellent yields were obtained (entries 5–7). When ortho-substituents were used, Heck reactions also could occur efficiently (entries 8–11).

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Even with the very sterically hindered 1-bromo-2-methylnapthalene, the reaction gave the coupled product in 90% isolated yields (entry 11). Coupling of heteroaryl bromides with ethyl acrylate proceeded smoothly and moderate yields were obtained (entries 12–15). In contrast to corresponding aryl bromides, this catalyst system showed decreased or even no activity for the coupling of aryl chlorides (entries 1 and 16). In the case of chlorobenzene, 2c was almost inactive under the optimized reaction conditions (entry 16). Increasing catalyst loading to 0.1% gave 19% yield (entry 17). For activated aryl chlorides, such as 4-chloronitrobenzene, the yield of the coupled product could be reached 45% by using 0.1% of 2c (entry 19). 4. Conclusions

[3]

[4]

[5]

In summary, we have found that air- and moisturestable new palladium complexes 2a–2c and 4, especially palladacycle 2c, behave as efficient phosphine-free catalysts for Heck reactions of aryl bromides with ethyl acrylate. Further investigations on the catalytic activity of this kind of palladium complexes are currently underway in our laboratory. Acknowledgements We are grateful to the National Natural Science Foundation of China (Project 20472074) and the Innovation Fund for Outstanding Scholar of Henan Province (Project 0621001100) for the financial support given to this research.

[6]

[7]

Appendix A. Supplementary material CCDC 620288 and 620290 contain the supplementary crystallographic data for 2c and 4. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/ conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, fax: (+44) 1223-336-033, or e-mail: deposit @ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2007.05.031. References

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