Thiopseudourea ligated palladium complexes: synthesis, characterization and application as catalysts for Suzuki–Miyaura, Sonogashira, Heck and Hiyama reactions

Abstract

A series of new non-phosphine 1-(2-picolyl)-3-benzoylsubstituted-2-benzyl-2-thiopseudourea ligands 2a–2f have been synthesized in two steps from commercially available benzoyl chlorides. Treatment of these ligands with Pd(OAc)₂ in a 1 : 1 molar ratio in dichloromethane (DCM) at room temperature provided convenient access to the corresponding N,N,O-tridentate palladium(II) complexes [Pd(OAc){ArCONHC(=N(CH₂Py)}SCH₂C₆H₅] (Ar = C₆H₅ (3a); 4-F-C₆H₄ (3b); 4-Br-C₆H₄ (3c); 4-I-C₆H₄ (3d); 4-Me-C₆H₄ (3e); 3,4,5-(OMe)₃-C₆H₂ (3f)) in almost quantitative yields. The new ligands and their palladium complexes were characterized by NMR, IR, ESIMS, and HRMS analysis. The molecular structure of complex 3c has been determined by X-ray single-crystal diffraction. These Pd(II) complexes have been used as catalysts for the Suzuki–Miyaura, Sonogashira, Heck and Hiyama cross-coupling reactions.

---

1. Introduction

The development of new non-phosphorus ligands and their palladium catalysts has become an important topic of research in organic synthesis especially for the C–C bond formation reactions. In contrast to phosphine ligands, many of the N-donor ligands are inexpensive, nontoxic and stable in air. The interesting properties of non-phosphine ligands and their Pd complexes, which can explain their popularity and growing applications, are mainly dependent on their facile synthesis, stability and easy handling. Moreover, the possibility of modulating their electronic and steric properties renders them an interesting and varied family of organometallic compounds. These interesting classes of non-phosphine ligands and their Pd catalysts represent a challenge to chemists not only in terms of their synthesis but also in terms of their structure, design, and the mode of coordination of the ligand to the metal. It is well known that the nature of different donor atoms in Pd catalysts could have a substantial effect on the catalytic behaviour. Therefore, a number of non-phosphine ligands containing different donor functionalities such as N-heterocyclic and carbocyclic carbenes,^1–15 oxazolines,^16–18 aryloximes,¹⁹ Schiff bases,^20,21 arylimines,^22,23 selenides,²⁴ guanidine,²⁵ simple amines,^26–29 thioureas,^30,31 pyridines,^32–34 imidazoles,^35–38 pyrazoles^39–41 and hydrazones^42–44 have been synthesized and their Pd catalysts have been studied for the C–C bond forming reactions. The stability, nontoxicity, easy handling of the boron precursors and easy workup allow the Suzuki reaction to occupy a special place among the different palladium-catalyzed C–C cross-coupling reactions.^45–49 Among the palladium complexes, anionic N-donor ligands were found to be efficient in enhancing the catalytic activity.^50–54 Recently, Kantam et al. have synthesized anionic uridate–palladium complex catalysts for Heck reactions.⁵⁵ We have developed a series of new thiopseudourea^56,57 ligands and their palladium(II) complexes and applied to Suzuki–Miyaura, Sonogashira, Heck and Hiyama cross-coupling reactions.

2. Results and discussion

2.1 Synthesis and characterization of thiopseudourea ligands

The new thiopseudourea ligands 2a–2f were prepared from commercially available benzoyl chlorides in two steps as shown in Scheme 1. The first step involves the reaction between in situ generated benzoyl derivatives such as aroyl isothiocyanates and picolyl amine in acetone at 0 °C to afford thiourea compounds 1a–1f in almost quantitative yields under a nitrogen atmosphere. In the second step, the compounds 1a–1f reacted with benzyl bromide and NaH in tetrahydrofuran at 0 °C to give the corresponding thiopseudourea ligands (2a–2f), which are white solids after purification by column chromatography. Compounds were characterized by NMR, IR and Mass spectroscopic techniques. The ¹H NMR spectra of all 2a–2f compounds showed a singlet in the region of 11.89–11.93 ppm corresponding to the –CONH group.

Scheme 1 Synthesis of thiopseudourea ligands.

2.2 Synthesis and characterization of palladium(II)–thiopseudourea complexes

Pd(II)–thiopseudourea complexes were synthesized by the treatment of a methylene chloride suspension of thiopseudourea ligands (2a–2f) with Pd(OAc)₂ in a 1 : 1 molar ratio for 6 h at room temperature, followed by filtration and removal of dichloromethane and acetic acid leading to the isolation of yellow solid Pd(II) complexes (3a–3f) in 94–96% yields (Scheme 2). It is important to note that the acetate counterion acts as a base in the deprotonation of the thiopseudourea ligand to give AcOH as the product. This process becomes the driving force of the complexation reaction and explains the high reaction yields. These complexes are insoluble in nonpolar solvents but highly soluble in chloroform, dichloromethane, acetonitrile and N,N-dimethylformamide. Complexes 3a–3f were unambiguously characterized by NMR, IR, ESI-MS, HRMS spectroscopy and X-ray crystallography in the case of complex 3c. From ¹H NMR spectra, the disappearance of the singlet peak in the range of 11.89–11.93 ppm corresponding to the amide proton of the thiopseudourea group could be associated to the formation of the Pd(II) complexes (see the ESI†). Further, the formation of Pd(II) complexes was confirmed by Electrospray Ionization (ESI) and High Resolution (HR) Mass Spectroscopies in their positive-mode spectra.

Scheme 2 Synthesis of thiopseudourea-based palladium(II) complexes.

2.3 X-Ray crystal structure of 3c

Slow evaporation of chloroform solution of complex 3c at 30 °C gave light yellow colored crystals of 3c suitable for single crystal X-ray analysis. The N,N,O-coordination of the 1-(2-picolyl)-3-(-(4-bromobenzoyl)-2-benzyl-2-thiopseudourea ligand in complex 3c was observed instead of expected N3-coordination.^21,58 This phenomenon might be due to the pronounced tendency of palladium(II) to form more stable six membered cyclic complexes, rather than a four-membered cyclic complex (Scheme 3). The crystal structure of 3c established an η3-bonded monomeric neutral palladium complex with the 1-(2-picolyl)-3-(4-bromobenzoyl)-2-benzyl-2-thiopseudourea ligand. The ligand is coordinated to Pd in a tridentate way via three different functionalities, namely, thiopseudouridate N(1), pyridine N(2) and benzoyl oxygen. The isolated OAc is bonded to Pd as a fourth coordinate. The bond angles around the Pd metal are not orthogonal at 82.81(9)°, 92.74(8)°, 96.29(9)° and 88.19(8)° for each of N1–Pd–N2, N1–Pd–O1, N2–Pd–O2 and O1–Pd–O2 respectively (Table 1), and the sum of the angles around palladium(II) is very close to 360°. Therefore, complex 3c adopts a distorted square planar geometry around the Pd(II) atom (Fig. 1). The complete crystallographic data of 3c are given in Table 8.

Scheme 3 Stable and unstable bonding modes of thiopseudouridate/pyridyl ligands.

Table 1 Selected bond lengths [Å] and angles [°] of complex 3c

Bond lengths
Pd1–N1	1.945(2)	Pdl–N2	1.988(2)
Pdl–O1	1.977(19)	Pd1–O2	2.038(19)
Bond angles
N1–Pd1–N2	82.81(9)	N1–Pd1–O1	92.74(8)
N1–Pd1–O2	176.83(8)	N2–Pd1–O1	175.50(7)
N2–Pd1–O2	96.29(9)	O1–Pd1–O2	88.19(8)
N1–C8–N3	127.4(2)	N1–C8–S1	116.65(19)
N1–C16–C17	109.65(2)	N2–C17–C16	116.38(2)
N3–C7–O1	129.20(2)	C8–N1–C16	121.3(2)

---

Fig. 1 X-Ray structure of Pd(II) complex 3c. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radius.

3. Catalytic activity of Pd complexes

3.1 Suzuki reactions

The Suzuki–Miyaura reaction was selected in order to evaluate the catalytic activity of thiopseudouridate Pd(II) complexes. We have chosen the cross-coupling of 4-bromoacetophenone with phenylboronic acid as a model to optimize the reaction conditions. The results are summarized in Table 2. The reaction was performed in the presence of 0.01 mol% of 3a in the mixture of water and organic solvent (9.5 : 0.5 ratio) for about 6 h at 80 °C. We optimized the reaction conditions by screening with several bases (Table 2, entries 1–8) and solvents (entries 9–11). The best results were observed with the K₂CO₃−H₂O/DMF system (entry 3).

Table 2 Suzuki reaction between 4-bromoacetophenone and phenyl boronic acid^a

Entry	Solvent (9.5 : 0.5)	Base	Time (h)	Yield^b (%)
a Reaction conditions: 0.5 mmol of 4-bromoacetophenone, 0.75 mmol of phenylboronic acid, 1.0 mmol of K2CO3, 0.01 mol% 3a, solvent (2 mL), reaction temperature 80 °C. b Isolated yield.
1	H2O/DMF	Li2CO3	6	66
2	H2O/DMF	Na2CO3	6	74
3	H2O/DMF	K2CO3	3	92
4	H2O/DMF	K3PO4·3H2O	4	86
5	H2O/DMF	Na3PO4	4	62
6	H2O/DMF	KF·2H2O	6	56
7	H2O/DMF	NaHCO3	6	78
8	H2O/DMF	NEt3	6	56
9	H2O/MeOH	K2CO3	6	78
10	H2O/THF	K2CO3	6	61
11	H2O/EtOH	K2CO3	6	66

---

With the appropriate solvent and base in hand, we studied the relative activity of several thiopseudouridate Pd(II) complexes for the same model reaction and the results are shown in Table 3. The complex 3e exhibited higher activity than the other complexes (entry 5). The maximum turnover number (about 10⁵) for the Suzuki–Miyaura coupling reaction was obtained using 0.001 mol% of catalyst 3e at 120 °C for 24 h (entry 7), but no yield was observed at room temperature (entry 8). The reaction with 0.01 mol% Pd(OAc)₂ alone as a catalyst resulted in moderate yields (entry 10). Without catalyst no reaction occurred (entry 11). This shows the usefulness of the thiopseudourea ligands for the Suzuki–Miyaura cross-coupling reaction. The scope and limitations of the reaction were studied using aryl bromides and aryl chlorides with different arylboronic acids (Tables 4 and 5). The reaction afforded good to excellent yields of coupling products with both the electron-rich and electron-deficient aryl bromides irrespective of the position of the substituent (Table 4, entries 1–14). In particular deactivated electron rich 4-bromo N,N-dimethylbenzenamine requires more reaction time to give good yields compared to other substituted aryl bromides (entry 15).

Table 3 Relative catalytic activities of thiopseudourea Pd(II) complexes 3a–f^a

Entry	Catalyst (mol%)	T (°C)	Yield^b (%)
a Reaction conditions: 0.5 mmol of 4-bromoacetophenone, 0.75 mmol of phenylboronic acid, 1.0 mmol of K2CO3, 2 mL H2O/DMF (9.5 : 0.5), reaction time 3 h. b Isolated yield. c Reaction time: 24 h.
1	3a/0.01	80	92
2	3b/0.01	80	94
3	3c/0.01	80	92
4	3d/0.01	80	90
5	3e/0.01	80	96
6	3f/0.01	80	93
7	3e/0.001	120	96^c
8	3e/0.01	25	—
9	3e/0.01	50	61
10	Pd(OAc)2/0.2	80	72
11	—	80	—

---

Table 4 Suzuki reactions with different aryl bromides and arylboronic acids using complex 3e in water^a

Entry	Arylbromide	R	Time (h)	Yield^b (%)
a Reaction conditions: 0.5 mmol of aryl halide, 0.75 mmol of arylboronic acid, 1.0 mmol of K2CO3, 0.01 mol% 3e, 2 mL H2O/DMF (9.5 : 0.5), reaction temperature 80 °C. b Isolated yield.
1	4-Bromotoluene	H	3	99
2	4-Bromoanisole	H	3	94
3	4-Fluorobromobenzene	H	3	93
4	4-Bromobenzaldehyde	H	3	99
5	1,2-Dibromobenzene	H	3	76
6	Ethyl-4-bromobenzoate	H	4	95
7	4-Bromoacetophenone	H	3	99
8	2-Bromonaphthalene	H	4	97
9	2-(4-Bromophenyl)acetonitrile	H	3	97
10	3-Bromobenzonitrile	H	4	94
11	3-Bromotoluene	H	3	96
12	3-Bromoanisole	H	4	97
13	3-Bromobenzaldehyde	H	4	92
14	3-Bromoacetophenone	H	8	82
15	4-Bromo N,N′-dimethylbenzenamine	H	12	76
16	4-Bromotoluene	4-OMe	3	94
17	4-Bromoacetophenone	4-OMe	6	92
18	4-Bromoanisole	4-OMe	6	90
19	4-Bromotoluene	3-Me	3	90
20	4-Bromotoluene	4-Me	3	92
21	4-Bromoacetophenone	3-Me	3	95
22	3-Bromobenzaldehyde	4-CHO	3	90

---

Table 5 Suzuki reactions with different aryl chlorides and arylboronic acids using complex 3e^a

Entry	Aryl chloride	R	Yield^b (%)
a Reaction conditions: 1 mmol of aryl halide, 1.3 mmol of arylboronic acid, 2.0 mmol of K2CO3, 1 mol% 3e, 0.5 mmol of TBAB, 2 mL of ethylene glycol, at 110 °C for 10 h. b Isolated yield.
1	4-CF3C6H4	H	94
2	4-CF3C6H4	OMe	82
3	4-NO2C6H4	H	96
4	4-NO2C6H4	OMe	64
5	4-COCH3C6H4	H	56
6	4-CH3OC6H4	H	—
7	4-CH3C6H4	H	—
8	C6H5	H	—

---

Further, this catalytic system also tolerates the para and meta substituted arylboronic acids such as 4-methoxy, 4-methyl, 3-methyl, and 4-formylphenylboronic acids which were coupled efficiently with electron-donating and electron-withdrawing aryl bromides with excellent yields of 90–95% within 6 h (entries 16–22).

On the other hand, we also examined the Suzuki–Miyaura cross-coupling reaction of aryl chlorides with phenylboronic acid using the same reaction conditions but found to be ineffective for the Suzuki reaction of aryl chlorides. In order to improve the yields, we have screened the reaction using various solvents and tetrabutylammoniumbromide (TBAB) as an additive. Thus the optimized reaction conditions for Suzuki reaction of aryl chlorides are 1 mol% 3e, 2 mmol K₂CO₃, 0.5 mmol TBAB, and 2 mL of ethylene glycol and heated at 110 °C for 10 h. The reactions of electron-deficient aryl chlorides such as 1-chloro-4-trifluoromethylbenzene, 1-chloro-4-nitrobenzene and 1-chloro-4-acetylbenzene proceeded smoothly with arylboronic acids at 110 °C and the products were isolated in good yields (Table 5, entries 1–5), whereas chlorobenzene, 4-chlorotoluene and 4-chloroanisole turned out to be inferior substrates for this reaction (entries 6–8).

3.2 Sonogashira reactions

We have also investigated the efficiency of the thiopseudourea–Pd complex for the Sonogashira cross-coupling reaction (Table 6).^59–61 Thus using 3e as catalyst, the reaction conditions were optimized for the coupling of phenylacetylene with iodobenzene. Among the bases (K₂CO₃, K₃PO₄ and LiOH·H₂O) tested LiOH·H₂O is the best system. All the reactions were performed with 0.01 mol% of catalyst 3e at 100 °C for 12 h. The Sonogashira reaction worked well with both electron-rich and electron-poor aryl iodides. Phenylacetylene reacted efficiently with aryl iodides providing good to excellent yields of the desired products (entries 1–7). However, the catalyst showed moderate activity toward bromobenzene (entry 8).

Table 6 Sonogashira reactions between phenylacetylene and different aryl iodides using complex 3e^a

Entry	Ar	X	Yield^b (%)
a Reaction conditions: 1 mmol of aryl halide, 1.2 mmol of phenylaetylene, 2.0 mmol of LiOH·H2O, 0.01 mol% 3e, 2 mL of DMF at 100 °C for 12 h. b Isolated yield.
1	C6H5	I	94
2	4-COCH3C6H4	I	96
3	4-NO2C6H4	I	92
4	2-CH3C6H4	I	74
5	3-CH3C6	I	88
6	4-ClC6H4	I	92
7	4-CH3OC6H4	I	72
8	Q6H5	Br	64

---

3.3 Heck reactions

The Heck reaction is one of the most important Pd catalyzed C–C coupling reactions.^62–64 It involves vinylation of aryl halides. Thiopseudourea–Pd complex 3e displayed high catalytic activity with LiOH·H₂O as base in DMF solvent at 135 °C for 12 h. The results showed (Table 7) that the aryl iodides and aryl bromides react with styrene to generate the coupled products in excellent yields (entries 1–7). No catalytic activity was observed in the case of aryl chlorides (entry 8).

Table 7 Heck reactions between styrene and different aryl halides using complex 3e^a

Entry	Ar	X	Yield^b (%)
a Reaction conditions: 1 mmol of aryl halide, 2 mmol of styrene, 2 mmol of LiOH·H2O, 0.001 mol% 3e, 2 mL of DMF at 135 °C for 12 h. b Isolated yield.
1	C6H5	I	94
2	4-COCH3C6H4	I	97
3	4-CH3C6H4	I	97
4	4-CH3OC6H4	I	96
5	4-CH3C6H4	Br	92
6	C6H5	Br	93
7	4-CH3OC6H4	Br	91
8	C6H5	C1	—

---

3.4 Hiyama reactions

In addition, a palladium-catalyzed Hiyama reaction was also investigated with our complex 3e.^65,66 The reactions of 4-bromotoluene with trimethoxy(phenyl)silane proceeded smoothly to afford the desired biphenyl product in high yields (Scheme 4).

Scheme 4 Hiyama reaction of 4-bromotoluene with trimethoxy(phenyl)silane using complex 3e.

4. Conclusions

New thiopseudourea ligands 2a–2f have been easily prepared in two steps and in high yields. On the basis of X-ray and spectroscopic results, 2a–2f compounds act as N,N,O-tridentate ligands by reacting with Pd(OAc)₂ in a 1 : 1 ratio to form Pd complexes 3a–3f in good yields. The obtained Pd complexes were found to be efficient catalysts for the Suzuki reaction with aryl bromides in aqueous solution and also for coupling of activated aryl chlorides in ethane-1,2-diol. Further, the same catalyst is applicable for Sonogashira, Heck and Hiyama cross-coupling reactions.

Table 8 Crystallographic data for 3c

a The asymmetric unit also contains one acetic acid solvate which was grossly disordered and was excluded using SQUEEZE subroutine in PLATON [A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, D65, 148–155].
CCDC number	823203
Empirical formula	C23H20BrN3O3PdS, C2H4O2^a
Formula weight	664.84
Temperature	294(2) K
Wavelength	0.71069 Å
Crystal system	Triclinic
Space group	P1̄
Unit cell dimensions	a = 7.724(5) Å	α = 68.935(5)°
	b = 13.385(5) Å	β = 83.939(5)°
	c = 14.000(5) Å	γ = 89.447(5)°
Volume	1342.5(11) A^3,
Z	2
Density (calculated)	1.645 Mg m^−3
Absorption coefficient	2.297 mm^−1
F(000)	664
Crystal size	0.14 × 0.11 × 0.06 mm^3
θ range for data collection	1.63 to 25.00°
Index ranges	−9 ≤ h ≤ 9, −15 ≤ k ≤ 15, −16 ≤ l ≤ 16
Reflections collected	12 886
Independent reflections	4716 [R(int) = 0.0207]
Completeness to θ = 25.00°	99.7%
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameters	4716/0/290
Goodness-of-fit on F^2	1.077
Final R indices [I < 2σ(I)]	R 1 = 0.0278, wR2 = 0.0767
R indices (all data)	R 1 = 0.0298, wR2 = 0.0784
Largest diff. peak and hole	0.462 and −0.676 e Å^−3

---

5. Experimental section

5.1 General procedure for the preparation of ligands 1a–1f

To a solution of NH₄SCN (0.912 g, 12 mmol) in 20 mL of acetone aroyl chloride (10 mmol) was added dropwise at 0 °C and stirred for 30 min. To this, picolylamine (1.08 g, 10 mmol) was added at the same temperature and allowed to stir for an additional 3 h at room temperature. The mixture was concentrated to a solid; water (50 mL) was added, and extracted with ethyl acetate (2 × 50 mL). The organic layer was washed with brine (20 mL), dried over MgSO₄, and filtered, after which the solvent was removed in vacuo. The crude product was purified by column chromatography on silica gel using ethyl acetate/hexane as the eluent to give the desired products (1a–f) in 80–90% yield.

1-(2-Picolyl)-3-benzoyl-thiourea (1a). White solid, mp: 163 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ (ppm) 11.60 (s, 1H), 9.12 (s, 1H), 8.64 (d, J = 4.53 Hz, 1H), 7.84–7.19 (m, 2H), 7.70–7.47 (m, 4H), 7.34 (d, J = 7.55 Hz, 1H), 7.22 (t, J = 6.79 Hz, 1H), 5.00 (d, J = 4.53 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 179.4, 166.5, 154.4, 149.0, 139.6, 133.1, 131.6, 128.7, 128.4, 127.9, 127.3, 122.3, 121.6, 50.4. IR (KBr, cm⁻¹): 3188, 1666, 1547, 1498, 1251, 1169, 704. ESI-MS (m/z) (M + H)⁺ = 272. HRMS: calcd for C₁₄H₁₃N₃NaOS (M + Na)⁺ = 294.067, found: 294.069.

1-(2-Picolyl)-3-(4-fluorobenzoyl)-thiourea (1b). White solid, mp: 176 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.57 (s, 1H), 9.15 (s, 1H), 8.63 (d, J = 4.53 Hz, 1H), 7.96–7.92 (m, 2H), 7.68 (t, J = 7.55 Hz, 1H), 7.33 (d, J = 8.30 Hz, 1H), 7.24–7.14 (m, 3H), 4.98 (d, J = 4.53 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 179.4, 167.5, 165.6, 164.1, 154.5, 149.4, 136.8, 130.3, 130.2, 128.1, 122.6, 121.8, 116.6, 116.4, 50.7. IR (KBr, cm⁻¹): 3165, 1687, 1544, 1496, 1250, 1171, 759. ESI-MS (m/z) (M + H)⁺ = 290. HRMS: calcd for C₁₄H₁₂N₃NaOFS (M + Na)⁺ = 312.0582, found: 312.0582.

1-(2-Picolyl)-3-(4-bromobenzoyl)-thiourea (1c). White solid, mp: 162 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ 11.59 (s, 1H), 9.02 (s, 1H), 8.63 (d, J = 5.12 Hz, 1H), 7.77 (d, J = 8.79 Hz, 2H), 7.70–7.62 (m, 3H), 7.33 (d, J = 7.32 Hz, 1H), 7.25–7.21 (m, 1H), 4.98 (d, J = 5.12 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 179.4, 166.3, 154.5, 149.1, 144.1, 136.5, 129.4, 128.7, 127.3, 122.3, 121.5, 50.5. IR (KBr, cm⁻¹): 3173, 1678, 1545, 1503, 1255, 1170, 754. ESI-MS (m/z) (M + H)⁺ = 350. HRMS: calcd for C₁₄H₁₃N₃OSBr (M + H)⁺ = 349.9962, found: 349.9949.

1-(2-Picolyl)-3-(4-iodobenzoyl)-thiourea (1d). White solid, mp: 178 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.55 (s, 1H), 9.02 (s, 1H), 8.65 (d, J = 4.39 Hz, 1H), 7.87 (d, J = 8.05 Hz, 2H), 7.70–7.62 (m, 3H), 7.33 (d, J = 7.32 Hz, 1H), 7.25–7.21 (m, 1H), 4.98 (d, J = 4.39 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 179.2, 165.8, 154.3, 149.3, 138.3, 136.8, 131.2, 128.8, 122.6, 121.8, 101.2, 50.7. IR (KBr, cm⁻¹): 3350, 3208, 1678, 1541, 1478, 1246, 1169, 753. ESI-MS (m/z) (M + Na)⁺ = 420. HRMS: calcd for C₁₄H₁₃N₃OSI (M + H)⁺ = 397.9824, found: 397.9814.

1-(2-Picolyl)-3-(4-methylbenzoyl)-thiourea (1e). White solid, mp: 135 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.60 (s, 1H), 9.04 (s, 1H), 8.63 (d, J = 4.53 Hz, 1H), 7.79 (d, J = 8.30 Hz, 2H), 7.67 (t, J = 7.55 Hz, 1H), 7.33–7.26 (m, 3H), 7.21 (t, J = 7.55 Hz, 1H), 4.98 (d, J = 4.53 Hz, 2H), 2.43 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 175.4, 165.8, 149.2, 137.0, 132.0, 132.3, 131.6, 131.5, 130.7, 129.1, 128.6, 122.7, 122.0, 50.6, 29.7. IR (KBr, cm⁻¹): 3177, 1670, 1546, 1483, 1247, 1160, 746. ESI-MS (m/z) (M + H)⁺ = 286. HRMS: calcd for C₁₅H₁₅N₃NaOS (M + Na)⁺ = 308.0833, found: 308.0842.

1-(2-Picolyl)-3-(3,4,5-trimethoxybenzoyl)-thiourea (1f). White solid, mp: 184 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.59 (s, 1H), 8.97 (s, 1H), 8.63 (d, J = 4.94 Hz, 1H), 7.67 (t, J = 7.91 Hz, 1H), 7.32 (d, J = 7.91 Hz, 1H), 7.21 (t, J = 5.93 Hz, 1H), 7.06 (s, 2H), 4.99 (d, J = 4.94 Hz, 2H), 3.92 (s, 6H), 3.87 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 179.3, 166.2, 154.4, 153.2, 149.2, 142.4, 136.6, 126.7, 122.5, 121.7, 104.8, 60.8, 56.3, 50.6. IR (KBr, cm⁻¹): 3209, 1664, 1539, 1498, 1219, 1170, 756. ESI-MS (m/z) (M + H)⁺ = 362. HRMS: calcd for C₁₇H₁₉N₃NaO₄S (M + Na)⁺ = 384.0993, found: 384.0990.

5.2 General procedure for the preparation of ligands 2a–2f

To a solution of 1a–1f (5 mmol), NaH (60% in mineral oil, 0.35 g, 9.2 mmol) in THF (20 mL), benzyl bromide (0.6 mL, 5 mmol) was added dropwise at 0 °C and stirred for 3 h. At the end the reaction mixture was neutralised with aqueous NH₄Cl and the organic layer was extracted with ethyl acetate (2 × 50 mL), dried over MgSO₄, filtered, and the solvent was removed in vacuo. The crude product was purified by column chromatography on silica gel using ethyl acetate/hexane as the eluent to give 2a–2f as white solids with 80–85% yield.

1-(2-Picolyl)-3-benzoyl-2-benzyl-2-thiopseudourea (2a). White solid, mp: 147 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.91 (s, 1H), 8.62 (d, J = 4.87 Hz, 1H), 8.25 (d, J = 6.83 Hz, 2H), 7.66 (t, J = 9.75 Hz, 1H), 7.44 (t, J = 7.80 Hz, 1H), 7.40–7.36 (m, 4H), 7.29–7.17 (m, 5H), 4.71 (d, J = 4.87 Hz, 2H), 4.61 (s, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 176.0, 172.9, 154.7, 149.5, 137.6, 136.8, 131.6, 129.5, 129.0, 128.6, 127.9, 127.5, 122.6, 121.2, 48.7, 35.5; IR (KBr, cm⁻¹): 3167, 1682, 1600, 1554, 1350, 1339, 1290, 753. ESI-MS (m/z) (M + H)⁺ = 362. HRMS: calcd for C₂₁H₂₀N₃OS (M + H)⁺ = 362.1327, found: 362.1337.

1-(2-Picolyl)-3-(4-fluorobenzoyl)-2-benzyl-2-thiopseudourea (2b). White solid, mp: 117 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.89 (s, 1H), 8.64 (d, J = 4.34 Hz, 1H), 8.26 (t, J = 8.30 Hz, 2H), 7.69 (t, J = 7.55 Hz, 1H), 7.40 (d, J = 6.98 Hz, 2H), 7.32–7.25 (m, 5H), 7.06 (t, J = 8.68 Hz, 2H), 4.71 (s, 2H), 4.59 (s, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 174.9, 172.8, 163.4, 163.4, 154.6, 149.5, 136.9, 132.0, 131.9, 131.8, 128.9, 128.6, 127.5, 122.6, 121.2, 115.0, 114.7, 48.7, 35.5. IR (KBr, cm⁻¹): 3164, 1612, 1557, 1371, 1352, 1303, 782. ESI-MS (m/z) (M + H)⁺ = 380. HRMS: calcd for C₂₁H₁₉N₃OFS (M + H)⁺ = 380.1232, found: 380.1249.

1-(2-Picolyl)-3-(4-bromobenzoyl)-2-benzyl-2-thiopseudourea (2c). White solid, mp: 132 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.90 (s, 1H), 8.63 (d, J = 4.53 Hz, 1H), 8.11 (d, J = 8.30 Hz, 2H), 7.68 (t, J = 7.17 Hz, 1H), 7.52 (d, J = 8.49 Hz, 2H), 7.39 (d, J = 6.98 Hz, 2H), 7.32–7.25 (m, 5H), 4.71 (s, 2H), 4.58 (s, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 175.1, 173.7, 154.5, 149.6, 136.8, 131.1, 128.9, 128.6, 127.5, 126.5, 122.6, 121.3, 48.7, 35.5. IR (KBr, cm⁻¹): 3181, 1600, 1553, 1369, 1349, 1297, 770. ESI-MS (m/z) (M + H)⁺ = 440. HRMS: calcd for C₂₁H₁₉N₃OSBr (M + H)⁺ = 440.0432, found: 440.0431.

1-(2-Picolyl)-3-(4-iodobenzoyl)-2-benzyl-2-thiopseudourea (2d). White solid, mp: 129 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.81 (s, 1H), 8.57 (d, J = 4.15 Hz, 1H), 7.91 (d, J = 8.30 Hz, 2H), 7.72–7.59 (m, 3H), 7.35 (d, J = 6.61 Hz, 2H), 7.27–7.15 (m, 5H), 4.64 (s, 2H), 4.52 (s, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 175.2, 173.1, 154.4, 149.4, 137.4, 137.1, 136.9, 131.3, 131.1, 128.8, 128.6, 127.5, 122.6, 121.3, 99.2, 48.6, 35.4. IR (KBr, cm⁻¹): 3175, 1598, 1553, 1389, 1352, 1292, 765. ESI-MS (m/z) (M + H)⁺ = 488. HRMS: calcd for C₂₁H₁₉N₃OSI (M + H)⁺ = 488.0293, found: 488.0276.

1-(2-Picolyl)-3-(4-methylbenzoyl)-2-benzyl-2-thiopseudourea (2e). White solid, mp: 111 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.91 (s, 1H), 8.61 (d, J = 4.34 Hz, 1H), 8.16 (d, J = 7.93 Hz, 2H), 7.68 (t, J = 7.17 Hz, 1H), 7.41 (d, J = 6.98 Hz, 2H), 7.32-7.18 (m, 7H), 4.72 (s, 2H), 4.61 (s, 2H), 2.40 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 176.0, 172.5, 154.8, 149.5, 142.0, 136.8, 135.0, 129.6, 128.9, 128.6, 127.4, 122.5, 121.2, 48.7, 35.4, 21.5. IR (KBr, cm⁻¹): 3164, 1599, 1553, 1349, 1291, 766. ESI-MS (m/z) (M + H)⁺ = 376. HRMS: calcd for C₂₂H₂₂N₃OS (M + H)⁺ = 376.1483, found: 376.1492.

1-(2-Picolyl)-3-(3,4,5-trimethoxybenzoyl)-2-benzyl-2-thiopseudourea (2f). White solid, mp: 122 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 11.87 (s, 1H), 8.65 (d, J = 4.53 Hz, 1H), 7.70 (t, J = 7.17 Hz, 1H), 7.56 (s, 2H), 7.44 (d, J = 6.61 Hz, 2H), 7.33–7.22 (m, 5H), 4.74 (s, 2H), 4.62 (s, 2H), 3.88 (s, 3H), 3.81 (s, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 175.1, 169.9, 154.6, 152.6, 149.6, 141.2, 136.9, 128.8(d), 127.5, 122.7, 121.3, 106.7, 60.8, 56.0, 48.7, 35.3. IR (KBr, cm⁻¹): 3186, 1688, 1572, 1333, 1282, 765. ESI-MS (m/z) (M + H)⁺ = 452. HRMS: calcd for C₂₄H₂₆N₃O₄S (M + H)⁺ = 452.1644, found: 452.1665.

5.3 General procedure for the preparation of palladium complexes (3a–3f)

To a flask containing ligand 2a–2f (0.5 mmol) and Pd(OAc)₂ (0.5 mmol), 15 mL of CH₂Cl₂ was added at room temperature and stirred for 12 h. The reaction mixture washed with water to remove AcOH generated in the reaction mixture. The solvent was removed under reduced pressure and the resulting yellow solid was dried under high vacuum to obtain the pure complexes (3a–3f) in 94–96% yield.

Complex 3a. Light yellow solid, mp: 239 °C. ¹H NMR (500 MHz, CDCl₃, TMS): δ 8.21–8.14 (m, 3H), 7.78 (t, J = 7.80 Hz, 1H), 7.40 (t, J = 7.80 Hz, 1H), 7.35–7.20 (m, 9H), 4.95 (s, 2H), 4.64 (s, 2H), 2.16 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.4, 163.3, 162.5, 150.6, 148.6, 138.6, 136.6, 135.2, 131.5, 130.0, 129.9, 128.9, 128.7, 127.8, 127.5, 123.3, 120.5, 61.8, 36.9, 23.8. IR (KBr, cm⁻¹): 2922, 1712, 1615, 1528, 1463, 1386, 1354, 1248, 718. ESI-MS (m/z) (M–OAc)⁺ = 466. HRMS: calcd for C₂₁H₁₈N₃OSPd (M–OAc)⁺ = 466.0205, found: 466.0210.

Complex 3b. Light yellow solid, mp: 189 °C. ¹H NMR (500 MHz, CDCl₃, TMS): δ 8.18–8.12 (m, 3H), 7.80 (t, J = 8.79 Hz, 1H), 7.33–7.19 (m, 7H), 6.94 (t, J = 8.79 Hz, 2H), 4.93 (s, 2H), 4.60 (s, 2H), 2.15 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.6, 167.4, 165.2, 163.4, 162.4, 148.6, 138.7, 136.3, 132.5, 132.3, 132.2, 131.4 (d), 128.8, 128.7, 127.6, 123.4, 120.5, 115.0, 114.6, 61.8, 37.0, 23.7. IR (KBr, cm⁻¹): 2920, 1721, 1573, 1531, 1451, 1390, 1358, 1249, 758. ESI-MS (m/z) (M–OAc)⁺ = 484. HRMS: calcd for C₂₁H₁₇N₃OFS Pd (M–OAc)⁺ = 484.0111, found: 484.0093.

Complex 3c. Light yellow solid, mp: 219 °C. ¹H NMR (500 MHz, CDCl₃, TMS): δ 8.21 (d, J = 5.65 Hz, 1H), 8.04 (d, J = 9.04 Hz, 2H), 7.84 (t, J = 7.91 Hz, 1H), 7.45 (d, J = 7.91 Hz, 2H), 7.37–7.24 (m, 7H), 4.98 (s, 2H), 4.63 (s, 2H), 2.19 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.7, 165.3, 163.5, 162.4, 148.6, 138.7, 136.3, 134.2, 131.5, 131.1, 128.8, 128.7, 127.6, 126.5, 123.4, 120.5, 61.8, 37.0, 23.7. IR (KBr, cm⁻¹): 2912, 1719, 1619, 1578, 1529, 1459, 1390, 1356, 1247, 752. ESI-MS (m/z) (M–OAc)⁺ = 544. HRMS: calcd for C₂₁H₁₇N₃OSBrPd (M–OAc)⁺ = 543.9310, found: 543.9328.

Complex 3d. Light yellow solid, mp: 197 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 8.15 (d, J = 6.04 Hz, 1H), 7.84–7.76 (m, 3H), 7.62 (d, J = 9.05 Hz, 2H), 7.32–7.19 (m, 7H), 4.89 (s, 2H), 4.56 (s, 2H), 2.15 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.6, 165.7, 163.6, 162.4, 148.8, 138.8, 137.2, 136.3, 134.8, 131.5, 128.8, 128.7, 127.6, 123.5, 120.4, 99.2, 61.8, 37.0, 23.5. IR (KBr, cm⁻¹): 2922, 1726, 1610, 1574, 1521, 1448, 1386, 1347, 750. ESI-MS (m/z) (M–Ac)⁺ = 608. HRMS: calcd for C₂₁H₁₇N₃OSPdI (M–OAc)⁺ = 591.9171, found: 591.9162.

Complex 3e. Light yellow solid, mp: 196 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 8.20 (d, J = 5.28 Hz, 1H), 8.09 (d, J = 8.30 Hz, 2H), 7.79 (t, J = 7.55 Hz, 1H), 7.39–7.22 (m, 7H), 7.14 (d, J = 8.30 Hz, 2H), 5.02 (s, 2H), 4.66 (s, 2H), 2.38 (s, 3H), 2.21 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.9, 166.2, 163.2, 162.5, 148.6, 142.0, 138.6, 136.7, 132.5, 130.0, 128.9, 128.6, 127.5, 123.3, 120.5, 61.8, 36.9, 23.7, 21.5. IR (KBr, cm⁻¹): 2918, 1720, 1605, 1525, 1434, 1389, 1351, 1255, 751. ESI-MS (m/z) (M–OAc)⁺ = 480. HRMS: calcd for C₂₂H₂₀N₃OSPd (M–OAc)⁺ = 480.0361, found: 480.0353.

Complex 3f. Light yellow solid, mp: 180 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 8.23 (d, J = 5.28 Hz, 1H), 7.84 (t, J = 7.55 Hz, 1H), 7.45 (s, 2H), 7.40–7.25 (m, 7H), 5.01 (s, 2H), 4.67 (s, 2H), 3.85 (s, 3H), 3.72 (s, 6H), 2.17 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 178.5, 165.7, 163.1, 162.5, 152.3, 148.2, 138.7, 136.7, 130.4, 128.7, 127.5, 123.4, 120.5, 107.2, 107.1, 61.7, 60.7, 55.8, 55.7, 36.6, 23.7. IR (KBr, cm⁻¹): 2937, 1715, 1567, 1528, 1462, 1383, 1356, 1257, 758. ESI-MS (m/z) (M–OAc)⁺ = 556. Anal. calcd. for C₂₄H₂₄N₃O₄SPd (M–OAc)⁺ = 556.0522, found: 556.0529.

5.4 General procedure for the Suzuki–Miyaura cross-coupling reaction of aryl bromides

A mixture of aryl bromide (0.5 mmol), phenylboronic acid (0.75 mmol), potassium carbonate (1.0 mmol), H₂O (1.9 mL), and Pd complex 3e in freshly prepared DMF solution (0.00005 mmol in 0.1 mL DMF) was stirred at 80 °C for a desired reaction time. Further, the reaction mixture was extracted with ethyl acetate and dried over MgSO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel using ethyl acetate/hexane as the eluent to give the corresponding coupling products.

5.5 General procedure for the Suzuki–Miyaura cross-coupling reaction of aryl chlorides

A mixture of aryl chloride (1 mmol), phenylboronic acid (1.3 mmol), potassium carbonate (2.0 mmol), TBAB (0.5 mmol), and Pd complex 3e (1 mol% 5.3 mg) in ethylene glycol (2 mL) was stirred at 110 °C for 10 h. To the cooled solution water was added, extracted with ethyl acetate and dried over MgSO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel using ethyl acetate/hexane as the eluent to give the desired products.

5.6 General procedure for Sonogashira reaction of aryl iodides

In a typical experiment, the 25 mL RB-flask was charged with aryl iodides (1 mmol), phenylacetylene (1.2 mmol), LiOH·H₂O (2 mmol) and the catalyst (3e) (0.01 mol%) in N,N-dimethylformamide (2 mL). The reaction mixture was stirred at 100 °C for 12 h. Then the reaction mixture was cooled to room temperature and diluted with ethyl acetate (20 mL) and washed with brine water. The combined organic phase was dried over anhydrous Na₂SO₄. After the removal of solvent, the residue was subjected to column chromatography on silica gel using ethyl acetate and hexane to afford the Sonogashira product in high purity.

5.7 General procedure for Heck reaction of aryl halides

In a typical experiment, the 25 mL RB-flask was charged with aryl bromides (1 mmol), styrene (2 mmol), LiOH·H₂O (2 mmol) and the catalyst (3e) (0.001 mol%) in N,N-dimethylformamide (2 mL). The reaction mixture was stirred at 135 °C for 12 h. Then the reaction mixture was cooled to room temperature and diluted with ethyl acetate (20 mL) and washed with brine water. The combined organic phase was dried over anhydrous Na₂SO₄. After the removal of solvent, the residue was subjected to column chromatography on silica gel using ethyl acetate and hexane to afford the Heck product in high purity.

5.8 General procedure for Hiyama reaction of aryl bromides

In a typical experiment, the 25 mL RB-flask was charged with aryl bromide (1 mmol), trimethoxy(phenyl)silane (1.2 mmol), LiOH·H₂O (2 mmol) and the catalyst (3e) (0.05 mol%) in ethylene glycol (2 mL). The reaction mixture was stirred at 100 °C for 12 h. Then the reaction mixture was cooled to room temperature and diluted with ethyl acetate (20 mL) and washed with brine water. The combined organic phase was dried over anhydrous Na₂SO₄. After the removal of solvent, the residue was subjected to column chromatography on silica gel using ethyl acetate and hexane to afford the biphenyl product in high purity.

Acknowledgements

K.S., P.S.P. and K.B. thank Council of Scientific and Industrial Research (CSIR), New Delhi, for senior research fellowships.

Notes and references

1. D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev., 2000, 100, 39–92.
2. Y. Han, H. V. Huynh and G. K. Tan, Organometallics, 2007, 26, 6581–6585.
3. J. Ye, W. Chen and D. Wang, Dalton Trans., 2008, 4015–4022.
4. N. Marion and S. P. Nolan, Acc. Chem. Res., 2008, 41, 1440–1449.
5. X. Zhang, Z. Xi, A. Liu and W. Chen, Organometallics, 2008, 27, 4401–4406.
6. D. Meyer, M. A. Taige, A. Zeller, K. Hohlfeld, S. Ahrens and T. Strassner, Organometallics, 2009, 28, 2142–2149.
7. T. Tu, J. Malineni, X. Bao and K. H. Dotz, Adv. Synth. Catal., 2009, 351, 1029–1043.
8. C. W. K. Gstottmayr, V. P. W. Bohm, E. Herdtweck, M. Grosche and W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1363–1365.
9. O. Navarro, R. A. Kelly and S. P. Nolan, J. Am. Chem. Soc., 2003, 125, 16194–16195.
10. C. W. Bielawski, A. G. Tennyson and V. M. Lynch, J. Am. Chem. Soc., 2010, 132, 9420–9429.
11. C. W. Bielawski, D. M. Khramov, E. L. Rosen, J. A. V. Er, P. D. Vu and V. M. Lynch, Tetrahedron, 2008, 64, 6853–6862.
12. S. P. Nolan, N. Marion, O. Navarro, J. Mei, E. D. Stevens and N. M. Scott, J. Am. Chem. Soc., 2006, 128, 4101–4111.
13. P. Nun, J. Martinez and F. Lamaty, Synlett, 2009, 1761–1764.
14. S. P. Nolan, O. Navarro, H. Kaur and P. Mahjoor, J. Org. Chem., 2004, 69, 3173–3180.
15. F. Glorius, G. Altenhoff, R. Goddard and C. W. Lehmann, Angew. Chem., Int. Ed., 2003, 42, 3690–3693.
16. B. Tao and D. W. Boykin, Tetrahedron Lett., 2002, 43, 4955–4957.
17. R. A. Gossage, H. A. Jenkins and P. N. Yadav, Tetrahedron Lett., 2004, 45, 7689–7691.
18. S. Lee, J. Organomet. Chem., 2006, 691, 1347–1355.
19. L. Botella and C. Najera, J. Organomet. Chem., 2002, 663, 46–57.
20. G. A. Grasa, A. C. Hillier and S. P. Nolan, Org. Lett., 2001, 3, 1077–1080.
21. K. M. Wu, C. A. Huang, K. F. Peng and C.-T. Chen, Tetrahedron, 2005, 61, 9679–9687.
22. J. Zhou, X. Guo, C. Tu, X. Li and H. Sun, J. Organomet. Chem., 2009, 694, 697–702.
23. D. F. Brayton, T. M. Larkin, D. A. Vicic and O. Navarro, J. Organomet. Chem., 2009, 694, 3008–3111.
24. Q. Yao, E. P. Kinney and C. Zheng, Org. Lett., 2004, 6, 2997–2999.
25. S. Li, Y. Lin, J. Cao and S. Zhang, J. Org. Chem., 2007, 72, 4067–4072.
26. B. Tao and D. W. Boykin, J. Org. Chem., 2004, 69, 4330–4335.
27. J.-H. Li and W.-J. Liu, Org. Lett., 2004, 6, 2809–2811.
28. Y.-X. Xie, J.-H. Li and D.-L. Yin, Chin. J. Org. Chem, 2006, 26, 1155–1163.
29. D. Mingji, B. Liang, C. Wang, Z. You, J. Xiang, G. Dong, J. Chen and Z. Yang, Adv. Synth. Catal., 2004, 346, 1669–1673.
30. D. Yang, Y. C. Chen and N. Y. Zhu, Org. Lett., 2004, 6, 1577–1580.
31. W. Chen, R. Li, B. Han, B. J. Li, Y. C. Chen, Y. Wu, L. S. Ding and D. Yang, Eur. J. Org. Chem., 2006, 1177–1184.
32. M. R. Buchmeiser and K. Wurst, J. Am. Chem. Soc., 1999, 121, 11101–11107.
33. T. Kawano, T. Shinomaru and I. Ueda, Org. Lett., 2002, 4, 2545–2547.
34. C. Najera, J. G. Molto, S. Karlstrom and L. R. Falvello, Org. Lett., 2003, 5, 1451–1454.
35. S. B. Park and H. Alper, Org. Lett., 2003, 5, 3209–3212.
36. J. C. Xiao, B. Twamley and J. M. Shreeve, Org. Lett., 2004, 6, 3845–3847.
37. S. Haneda, C. Ueba, K. Eda and M. Hayashi, Adv. Synth. Catal., 2007, 349, 833–835.
38. K. Kawamura, S. Haneda, Z. Gan, K. Eda and M. Hayashi, Organometallics, 2008, 27, 3748–3752.
39. A. Mukherjee and A. Sarkar, Tetrahedron Lett., 2005, 46, 15–18.
40. V. Montoya, J. Pons, V. Branchadell, J. G. Anton, X. Solans, M. F. Bardia and J. Ros, Organometallics, 2008, 27, 1084–1091.
41. F. Li and T. S. A. Hor, Adv. Synth. Catal., 2008, 350, 2391–2400.
42. T. Mino, Y. Shirae, M. Sakamoto and T. Fujita, Synlett, 2003, 882–884.
43. T. Mino, Y. Shirae, M. Sakamoto and T. Fujita, J. Org. Chem., 2005, 70, 2191–2194.
44. T. Mino, Y. Shirae, M. Sakamoto, Y. Sasai and T. Fujita, J. Org. Chem., 2006, 71, 6834–6839.
45. P. Srinivas, P. R. Likhar, H. Maheswaran, B. Sridhar, K. Ravikumar and M. L. Kantam, Chem. Eur. J., 2009, 15, 1578–1581.
46. M. L. Kantam, P. Srinivas, J. Yadav, P. R. Likhar and S. Bhargava, J. Org. Chem., 2009, 74, 4882–4885.
47. K. W. Jung, K. S. Yoo, J. O. Neill, S. Sakaguchi, R. Giles and J. H. Lee, J. Org. Chem., 2010, 75, 95–101.
48. K. W. Jung, J. Jarusiewicz, Y. Choe, K. S. Yoo and C. P. Park, J. Org. Chem., 2009, 74, 2873–2876.
49. H. M. Lee and Y.-C. Chang, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2011, E67, m467.
50. F. Li, L. Zhanga, J. Wu, L. Shi and C. Xia, Tetrahedron Lett., 2011, 52, 3897–3901.
51. E. U. Wurthwein, J. K. Eberhardt and R. Frohlich, J. Org. Chem., 2003, 68, 6690–6694.
52. K. I. J. Szabo and N. Selander, Chem. Rev., 2011, 111, 2048–2076.
53. M. P. Song, X. Q. Hao, Y. N. Wang, J. R. Liu, K. L. Wang and J. F. Gong, J. Organomet. Chem., 2010, 695, 82–89.
54. N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483.
55. M. L. Kantam, P. Srinivas, K. Srinivas, P. R. Likhar, B. Sridhar, K. V. Mohan and S. Bhargava, J. Organomet. Chem., 2011, 696, 795–801.
56. A. F. Ferris, O. L. Salerni and B. A. Schutz, J. Chem. Soc., 1965, 6650–6651.
57. I. D. Linney, I. M. Buck, E. A. Harper, S. B. Kalindjian, M. J. Pether, N. P. Shankley, G. F. Watt and P. T. Wright, J. Med. Chem., 2000, 43, 2362–2370.
58. C. G. Agoston, Z. Miskolczy, Z. Nagy and I. Sovago, Polyhedron, 2003, 22, 2607–2615.
59. Y. He and C. Cai, J. Organomet. Chem., 2011, 696, 2689–2692.
60. J. Zhang, M. Daković, Z. Popovic, H. Wu and Y. Liu, Catal. Commun., 2012, 17, 160–163.
61. B. S. Zhang, C. Wang, J. F. Gong and M. P. Song, J. Organomet. Chem., 2009, 694, 2555–2561.
62. R. F. Heck and J. P. Nolley, J. Org. Chem., 1972, 37, 2320–2322.
63. I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009–3066.
64. A. C. Hillier, G. A. Grasa, M. S. Viciu, H. M. Lee, C. Yang and S. P. Nolan, J. Organomet. Chem., 2002, 653, 69–82.
65. Z. S. Gu, L. X. Shao and J. M. Lu, J. Organomet. Chem., 2012, 700, 132–134.
66. I. Blaszczyk and A. M. Trzeciak, Tetrahedron, 2010, 66, 9502–9507.

---

Footnote

† Electronic supplementary information (ESI) available: Additional characterization studies supporting the results which include NMR and X-ray data. CCDC reference number 823203. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cy00542e

---