Cyclometalated Palladium NHC Complexes Bearing PEG Chains for

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Cyclometalated Palladium NHC Complexes Bearing PEG Chains for Suzuki−Miyaura Cross-Coupling in Water Felix Schroeter, Johannes Soellner, and Thomas Strassner* Physikalische Organische Chemie, TU Dresden, Bergstraße 66, 01069 Dresden, Germany

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S Supporting Information *

ABSTRACT: We present the synthesis and characterization of four new polyethylene glycol (PEG) substituted palladium complexes bearing a cyclometalated 2-phenylimidazole ligand and an N-heterocyclic carbene (NHC) ligand. A solid-state structure reveals the chelating binding mode and the coiling of the PEG chain in the auxiliary ligand. The PEG substitution significantly increased the solubility of the complexes in several solvents, enabling the efficient Suzuki−Miyaura crosscoupling reaction of aryl chlorides in an aqueous medium. Under optimized reaction conditions, sterically demanding biphenyl compounds with up to three ortho substituents were accessible in good to excellent yields.

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INTRODUCTION Suzuki−Miyaura cross-coupling is one of the most important C−C bond-forming processes,1,2 which allows access to biphenyls on a laboratory3−5 and industrial6,7 scale (Scheme 1a). After its initial discovery,8,9 a great deal of research has been conducted on improving this Nobel-prize-winning reaction:10−12 e.g., by optimization of ligand systems13−20 such as phosphines21−23 and NHCs24−29 or by the use of ligand-free systems, 30,31 nanoparticles, 32−36 or ionic liquids.37−39 In light of the need for greater sustainability in chemical processes,40,41 the concept of green chemistry42 has not left the Suzuki−Miyaura reaction untouched.43−45 In attempts to make the Suzuki−Miyaura reaction more environmentally friendly,46,47 the use of alternative solvents has become increasingly important.48−51 One of the most abundant and least toxic reaction media is water, and thus many catalyst systems have been developed to enable efficient Suzuki− Miyaura cross-couplings in this medium.52−58 Typically, to not sacrifice the ability to efficiently couple the less expensive but also less reactive aryl chlorides, organometallic palladium complexes have to be employed. One approach to ensure the solubility of these catalysts in water, among using intrinsically water soluble ligands (Scheme 1d),61,65−83 is the modification of common ligands by attaching polar groups.84−87 Especially sulfonates (Scheme 1c,e),60,62,88−104 carboxylic acids (Scheme 1f),105,106 carbohydrates,63,107,108 and PEG-ether chains (Scheme 1b,g)59,109−120 have been extensively used. In view of the encouraging results previously obtained by our group with PEGylated bis-NHC palladium complexes (Scheme 1g),64 which surpassed the reactivity of their non-PEGylated analogues,121,122 we turned our attention toward the modification of the highly efficient © XXXX American Chemical Society

cyclometalated 2-phenylimidazole NHC palladium complexes reported earlier.123 Similarly to analogous complexes,124−129 these complexes additionally bear an assisting ligand, which ensures their stability during storage, enables quick activation under catalytically relevant conditions, and then stabilizes catalytic intermediates.130 Therefore, we present the synthesis and characterization of four new cyclometalating 2-phenylimidazole NHC complexes with PEG side chains. These complexes have been successfully employed in Suzuki−Miyaura cross-coupling reactions in an aqueous medium. The scope of the reaction has been explored, showing that up to tris-ortho-substituted biphenyls can be synthesized in good to excellent yields.

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RESULTS AND DISCUSSION Synthesis and Characterization. The PEG-substituted ligand precursors 2 and 3 were accessible in excellent yields by standard nucleophilic substitution reactions from the parent imidazole and the PEGylated bromide 1131 following literature procedures (Scheme 2).64,132 Cyclometalation of 2 with palladium acetate afforded complex 4, similar to the reported synthesis of 5.123 Compound 4 is dimeric and exists as a mixture of cis and trans isomers in a ratio of about 1:6, as evident from NMR measurements. NOESY cross-peaks indicate the slow isomerization of both isomers in solution. In the following reaction, the dimeric structure is opened by addition of the NHC ligand, leading to compound 6 in 80% yield. Addition of excess potassium chloride to the reaction mixture ensured complete displacement of the acetato ligands. Compounds 7 and 8 were synthesized analogously, starting Received: August 23, 2018

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DOI: 10.1021/acs.organomet.8b00607 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Compounds 3 and 7 showed a pronounced hygroscopicity. The previously reported compound 9 was used for a comparison of the catalytic activity.123 Single crystals suitable for an X-ray solid-state structure determination of complex 6 were obtained by slow evaporation of a solution of the compound in a mixture of benzene and hexanes. Compound 6 formed monoclinic crystals, in the space group P21/n (Figure 1). The unit cell contains four molecules

Scheme 1. Suzuki−Miyaura Cross-Coupling Reaction and Representative Ligand Systems for Catalysis in Water

Figure 1. ORTEP representation of the solid-state structure of 6. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): C16−Pd1 2.026(2), Pd1−N2 2.058(2), Pd1− Cl1 2.405(2), Pd1−C17 2.011(2), N2−Pd1−C16 80.20(6), C16− Pd1−C17 94.42(7), C16−Pd1−C17-N4 100.5(1).

a

b

of 6. The palladium atom is coordinated in a distorted-squareplanar fashion, with the cyclometalating ligand in the palladium coordination plane. The Pd−C bond lengths of 2.026(2) Å (C16−Pd1) and 2.011(2) Å (Pd1−C17) meet the expectations for Pd−C single bonds. The 5-ring chelate leads to an N2−Pd1−C16 angle of 80.20(6)°, significantly lower than for the ideal square-planar geometry. This opens up space for the bulky monodentate NHC ligand, which is located trans to the nitrogen atom of the chelating ligand, leading to angles of 94.42(7)° (C16−Pd1−C17) and 94.31(5)° (C-17−Pd1-Cl1). To minimize steric interactions, the NHC ligand is oriented almost perpendicularly to the palladium coordination plane.

c

Cross-coupling reaction. Reference 59. Reference 60. dipp = (2,6diisopropyl)phenyl. dReference 61. eReference 62. fReference 63. g Reference 64.

from compound 3 as the NHC ligand precursor. In solution, these compounds exist in different rotamers, as reported for similar compounds,133 which complicates the corresponding NMR spectra. The formation of the complexes has been confirmed by elemental analyses, ESI-MS measurements, and, for compound 6, by a solid-state structure determination.

Scheme 2. Overview of the Synthesized Compounds and Investigated Complexesa

a

Reaction conditions: (i) 2-phenylimidazole (1 equiv), KOH (1 equiv), MeCN, room temperature, 24 h, quantitative; (ii) Pd(OAc)2 (0.93 equiv), AcOH, 110 °C, 1.5 h, 72%; (iii) (2,6-diisopropyl)phenyl)imidazole (1 equiv), THF, 80 °C, 5 days, 85%; (iv) 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (2 equiv), K2CO3 (10 equiv), KCl (12 equiv), THF, 50 °C, 16 h, 80%; (v) 3 (2 equiv), K2CO3 (10 equiv), KBr (12 equiv), THF, 50 °C, 16 h, 43%; (vi) 5 (0.5 equiv), K2CO3 (5 equiv), KBr (6 equiv), THF, 50 °C, 2 days, 56%. Abbreviations: PEG3 = 2-(2methoxyethoxy)ethoxy, dipp = (2,6-diisopropyl)phenyl. B


Article

Organometallics The PEG chain attached to the cyclometalating ligand also extends out of the palladium coordination plane in a coiled arrangement. This fact, along with the bulky IPr (1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) ligand, prevents the complex from π stacking. In the packing of 6, one PEG chain is embedded between the imidazolium ring of the chelating ligand of one adjacent molecule and the NHC backbone of another adjacent molecule. Thus, there is no intermolecular interaction of two PEG chains, although they appear to be close (Figure S1 in the Supporting Information). Catalysis. For the assessment of the catalytic activity of the complexes in the Suzuki−Miyaura cross-coupling, we chose the reaction between 4-chlorotoluene and phenylboronic acid (Table 1). The initial reaction conditions were inspired by our

Table 2. Screening of the Concentrations and the Catalyst in the Suzuki−Miyaura Cross-Coupling Reactiona

entry

Table 1. Screening of Solvent and Base in the Suzuki− Miyaura Cross-Coupling Reactiona

entry

mol % 6

solvent

1 2 3 4

0.1 0.1 0.1 0.1

H2O H2O/dioxane H2O/toluene H2O/EtOH

base (equiv) K3PO4 K3PO4 K3PO4 K3PO4

5 6 7 8

0.2 0.2 0.2 0.2

H2O/EtOH H2O/EtOH H2O/EtOH H2O/EtOH

K3PO4 (2) K2CO3 (2) Na2CO3 (2) NaOH (2)

(2) (2) (2) (2)

yield (%)

TON

1 8 13 13

10 80 130 130

63 58 55 65

315 290 275 325

cat. (mol %)

PhB(OH)2 (equiv)

base (equiv)

yield (%)

TON

1 2 3 4 5

6 6 6 6 6

(0.2) (0.2) (0.2) (0.2) (0.2)

1.1 1.1 1.1 1.1 1.1

NaOH NaOH NaOH NaOH NaOH

(1.0) (1.5) (2.0) (2.5) (3.0)

19 51 65 74 64

95 255 325 370 320

6 7

6 (0.2) 6 (0.2)

1.3 1.5

NaOH (2.5) NaOH (2.5)

82 78

410 390

8 9 10 11 12

4 5 7 8 9

(0.2) (0.2) (0.2) (0.2) (0.2)

1.5 1.5 1.5 1.5 1.5

NaOH NaOH NaOH NaOH NaOH

(2.5) (2.5) (2.5) (2.5) (2.5)

2 0 27 15 59

10 0 135 75 295

13 14 15 16

6 6 6 6

(0.1) (0.3) (0.4) (0.5)

1.1 1.1 1.1 1.1

NaOH NaOH NaOH NaOH

(2.0) (2.0) (2.0) (2.0)

53 68 88 100

530 227 220 200

a

Reaction conditions: 4-chlorotoluene (1 mmol), H2O (1 mL), EtOH (0.5 mL).

chose to compare the activity of 6 to that of the other precatalysts in this reaction (Table 2, entries 8−12). The nonNHC complexes 4 and 5 were completely inefficient. However, the replacement of one dipp substituent by a PEG chain in the NHC ligand of 7 and 8 led to significantly lower catalytic activity, which is caused by the decreased steric demand of the ligand.137,138 Complex 9 was also less efficient than 6, which might be caused by the solubility differences of both compounds. Since optimizing the amount of base and boronic acid did not lead to quantitative conversion, we chose to screen the catalyst load instead (Table 2, entries 13−16). While the highest TON is achieved at 0.1 mol % catalyst load, only increasing the amount of 6 to 0.5 mol % led to a quantitative yield. Under these conditions, a slight excess of boronic acid and 2 equiv of sodium hydroxide are sufficient to achieve quantitative conversion in this reaction. Under the optimized reaction conditions, we explored the scope of the reaction (Scheme 3). Upon varying the aryl halide, we found that electron-withdrawing substituents were tolerated well, leading to quantitative yields of 10b,c. However, the reaction is sensitive to electron-donating groups. The reaction leading to 10e was the only reaction without full conversion. In other reactions such as in the synthesis of 10d, the lower yields are caused by hydrodechlorination139,140 and hydrodeboronation135,136 side reactions, which typically occur under protic conditions. The catalyst is not sensitive to coordinating groups, such as in 10f, and tolerates various arylboronic acids (10h−j). In addition, the yield of orthosubstituted biphenyls, such as 10g,i, was as high as for the other substrates, while typically lower yields would be expected.123,134 Thus, we synthesized different biphenyls with

a

Reaction conditions: 4-chlorotoluene (1 mmol), phenylboronic acid (1.1 mmol), H2O (1 mL), cosolvent (0.5 mL).

previous results obtained in organic solvents (Table 1).122,123,134 However, the pronounced differences between water and organic solvents made it necessary to reoptimize the reaction conditions. When we used potassium phosphate as a base in water without additives, we only obtained trace amounts of product 10a (Table 1, entry 1). The yield could be improved by using an organic cosolvent (Table 1, entries 2− 4). Ethanol, as a nontoxic cosolvent, proved to be optimal for this reaction (Table 1, entry 4), but the yield was still as low as 13%. When the catalyst load was doubled, the yield significantly improved to 63%, while the turnover number (TON) more than doubled (Table 1, entry 5). Next, we looked at the influence of the base. While carbonate bases are slightly less efficient than potassium phosphate (Table 1, entries 6 and 7), sodium hydroxide was slightly more efficient (Table 1, entry 8). We thus continued the optimization with this inexpensive, atom-economical base. Next, we looked at the influence of the amount of base in the reaction (Table 2, entries 1−5). The highest yield of 74% was achieved when 2.5 equiv of sodium hydroxide was used. Next, we increased the amount of boronic acid in the reaction (Table 2, entries 6 and 7). Under the reaction conditions, hydrodeboronation might be an important side reaction.135,136 However, the yield was only slightly increased, which indicates that decomposition of the boronic acid is not the yield-limiting factor in our case. Instead, quick formation of palladium black suggested that catalyst decomposition is critical in this reaction. To assess the influence of the catalyst itself, we C


Organometallics

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Scheme 3. Reaction Scopea

Article

EXPERIMENTAL SECTION

General Information. The following compounds were synthesized according to literature procedures: 1,131 5,123 9,123 1-(2,6diisopropylphenyl)imidazole,141 and 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride.142 All reactions were performed under an argon atmosphere, unless stated otherwise. Solvents of at least 99.0% purity were used in all reactions in this study. Toluene was distilled from calcium hydride and stored over molecular sieves (4 Å). Dioxane was stored over molecular sieves (4 Å). Dry acetonitrile, DCM, and THF were obtained from an MBraun Solvent Purification System. Ethanol was used as received. All other chemicals were obtained from common suppliers and used without further purification. 1H and 13C NMR spectra were acquired on Bruker NMR Avance 300 and Bruker DRX 500 spectrometers. 1H and 13C NMR spectra were referenced internally using the solvent resonances (1H 7.26 ppm and 13C 77.0 ppm for CDCl3). Chemical shifts are given in ppm and coupling constants J in Hz. Elemental analyses were performed by the microanalytical laboratory of our institute on a Hekatech EA 3000 Euro Vector elemental analyzer. GC-MS measurements were carried out on an Agilent 7890A gas chromatograph and an Agilent 5975 Series MSD. ESI-MS measurements were conducted on a Bruker Esquire MS equipped with an ion trap detector and an ESI source. 1-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)-2-phenyl-1H-imidazole (2). In air, 2-phenylimidazole (450 mg, 3.12 mmol), 1 (716 mg, 3.12 mmol), and KOH (350 mg, 6.24 mmol, 2 equiv) were dissolved in acetonitrile (5 mL) and stirred at room temperature for 24 h. Water (15 mL) and DCM (15 mL) were added, the aqueous phase was extracted with DCM (3 × 30 mL), and the combined organic layers were dried over sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (ethyl acetate/methanol 20/1). The product was dried under high vacuum. It was obtained as a yellow solid (904 mg, 3.11 mmol, quantitative). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.58−7.64 (m, 2 H, Har), 7.38−7.49 (m, 3 H, Har), 7.20 (d, 3J = 1.3 Hz, 1 H, NCH), 7.14 (d, 3J = 1.3 Hz, 1 H, NCH), 4.19 (t, 3J = 5.4 Hz, CH2), 3.76 (t, 3J = 5.4 Hz, CH2), 3.56−3.65 (m, CH2), 3.49−3.55 (m, 2 H, CH2), 3.37 (s, 3 H, OCH3). 13C NMR (126 MHz, CDCl3): δ (ppm) 147.7 (s, C), 130.1 (s, C), 129.2 (s, CH), 128.9 (s, CH), 128.5 (s, CH), 127.8 (s, CH), 121.1 (s, CH), 71.8 (s, CH2), 70.7 (s, CH2), 70.5 (s, CH2), 70.4 (s, CH2), 59.0 (s, CH3), 46.6 (s, CH2). Anal. Calcd for C16H22N2O3 (290.36): C, 66.18; H, 7.64; N, 9.65. Found: C, 66.22; H, 7.96; N, 9.52. Mp: 85 °C. 1-(2,6-Diisopropylphenyl)-3-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)imidazolium Bromide (3). In a pressure tube, 2,6diisopropylphenylimidazole (704 mg, 3.08 mmol) and 1 (700 mg, 3.08 mmol) were dissolved in THF (10 mL) and stirred in air at 80 °C for 5 days. The solvent was evaporated under reduced pressure, and the residue was washed with a mixture of diethyl ether and ethyl acetate (1/1, 2 × 20 mL). The product was dried under high vacuum. It was obtained as a brown, hygroscopic oil (1.2 g, 2.63 mmol, 85%). 1 H NMR (300 MHz, CDCl3): δ (ppm) 9.92 (s, 1 H, NCHN), 8.43 (s, 1 H, NCH), 7.52 (t, 3J = 7.7 Hz, 1 H, p-Har), 7.30 (d, 3J = 7.7 Hz, 2 H, m-Har), 7.10 (t, 3J = 1.5 Hz, 1 H, NCH), 4.98−5.11 (m, CH2), 3.94−4.06 (m, CH2), 3.65−3.79 (m, CH2), 3.58−3.65 (m, CH2), 3.51−3.58 (m, CH2), 3.36−3.43 (m, 2 H, CH2), 3.21 (s, 3 H, OCH3), 2.28 (spt, 3J = 6.8 Hz, 2 H, CHiPr), 1.21 (d, 3J = 6.8 Hz, 6 H, CH3,iPr), 1.14 (d, 3J = 6.8 Hz, 6 H, CH3,iPr). 13C NMR (75 MHz, CDCl3): δ (ppm) 145.5 (s, m-Car), 138.3 (s, NCHN), 131.9 (s, p-CHar), 130.3 (s, i-Car), 124.9 (s, NCH), 124.7 (s, m-CHar), 123.5 (s, NCH), 71.7 (s, CH2), 70.2 (s, 2 CH2), 70.1 (s, CH2), 69.3 (s, CH2), 58.8 (s, OCH3), 50.4 (s, CH2), 28.7 (s, CH,iPr), 24.3 (s, CH3,iPr), 24.2 (s, CH3,iPr). Anal. Calcd for C22H35BrN2O3·0.5H2O (455.43): C, 56.89, H, 7.81; N, 6.03. Found: C, 56.71; H, 7.98; N, 6.19. Complex 4. Compound 2 (419 mg, 1.44 mmol, 1.08 equiv) and palladium acetate (300 mg, 1.34 mmol) were dissolved in acetic acid (40 mL). Argon was bubbled through the mixture for 30 min. Then, the mixture was stirred at 110 °C for 1.5 h. The solvent was removed under reduced pressure, and the residue was dissolved in DCM (20 mL) and washed with water (2 × 10 mL). The aqueous phase was

a

Reaction conditions: aryl chloride (1 mmol), aryl boronic acid (1.1 mmol), NaOH (2 mmol), 6 (0.5 mol %), H2O (1 mL), EtOH (0.5 mL), 24 h.

more than one ortho substituent and still achieved excellent yields for compounds 10k−m. However, bis-ortho-substituted boronic acids were less reactive, producing compound 10m in just 48% yield. As a major side reaction, hydrodeboronation occurred, which diminished the available amount of boronic acid. Switching to more electron rich boronic acids helps in this case, which allowed for the synthesis of 10n in 80% yield. However, our attempts to synthesize tetra-ortho-substituted biphenyls (e.g., 2,2′,4,4′,6,6′-hexamethylbiphenyl) only led to trace amounts of the desired product, while nearly quantitative amounts of the hydrodechlorination and hydrodeboronation side products were formed.

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CONCLUSION We have synthesized and characterized four new cyclometalated palladium NHC complexes bearing PEG side chains. These greatly increase the solubility of the complexes in different solvents. The binding mode of the ligands and thus the structure of the complexes was confirmed by solid-state structure determination of compound 6. Complex 6 was also shown to be an active precatalyst in the Suzuki−Miyaura crosscoupling of the less reactive, cheap aryl chlorides in a water− ethanol mixture. Sodium hydroxide was used as an inexpensive base. Under the optimized reaction conditions, sterically demanding biphenyl compounds with up to three ortho substituents were accessible in very good yields. D


Article

Organometallics extracted with DCM (20 mL), and the combined organic layers were dried over sodium sulfate. After removal of the solvent under reduced pressure, column chromatography on silica gel (ethyl acetate/ methanol 19/1 → 9/1 → DCM/methanol 9/1), and drying under high vacuum, the product was obtained as a yellowish brown oil (438 mg, 0.48 mmol, 72%). 1H NMR (500 MHz, CDCl3): δ (ppm) 6.91 (dd, J = 7.6 Hz, J = 0.9 Hz, 2 H, Har), 6.73−6.81 (m, 4 H, Har), 6.68 (td, J = 7.3 Hz, J = 1.9 Hz, 2 H, Har), 6.43 (d, 3J = 1.6 Hz, 2 H, NCH), 6.37 (d, 3J = 1.6 Hz, 2 H, NCH), 3.98−4.15 (m, 2 H, NCH2a), 3.81− 3.94 (m, 2 H, NCH2b), 3.62−3.78 (m, 4 H, CH2), 3.52−3.62 (m, 12 H, CH2), 3.48 (q, 3J = 4.5 Hz, 4 H, MeOCH2), 3.35 (s, 6 H, OCH3), 2.21 (s, 6 H, CH3). Mixture of trans/cis isomers (approximately 6:1). For clarity, only the peaks of the trans isomer are given. 13C NMR (126 MHz, CDCl3): δ (ppm) 181.2 (s, C(O)), 151.6 (s, Car), 147.5 (s, Car), 134.4 (s, Car), 132.7 (s, CHar), 126.3 (s, CHar), 125.7 (s, NCH), 122.7 (s, CHar), 119.7 (s, CHar), 119.6 (s, NCH), 71.8 (s, CH2), 70.8 (s, CH2), 70.57 (s, CH2), 70.59 (s, CH2), 69.2 (s, CH2), 59.0 (s, OCH3), 47.0 (s, NCH2), 24.6 (s, CH3). Mixture of trans/cis isomers (approximately 6:1). For clarity, only the peaks of the trans isomer are given. Anal. Calcd for C36H48N4O10Pd2 (909.64): C, 47.54; H, 5.32; N, 6.16. Found: C, 47.56; H, 5.05; N, 6.41. ESI-MS: 395.2 ([L − Pd]+), 851.3 ([L2Pd2(OAc)]+). Complex 6. 1,3-Bis(2,6-diisopropylphenyl)imidazolium chloride (283 mg, 0.67 mmol, 2 equiv), compound 4 (303 mg, 0.33 mmol), KCl (298 mg, 4.00 mmol, 12 equiv), and potassium carbonate (465 mg, 3.33 mmol, 10 equiv) were suspended in THF (15 mL) and were stirred at 50 °C for 16 h. The mixture was filtered, the solvent was evaporated under reduced pressure, and the residue was purified by column chromatography on silica gel (DCM/ethyl acetate 9/1). The product was dissolved in diethyl ether (10 mL) and precipitated by addition of hexanes (40 mL). After drying under high vacuum at 50 °C, the product was obtained as a light yellow solid (436 mg, 0.53 mmol, 80%). 1H NMR (600 MHz, CDCl3): δ (ppm) 7.36 (t, 3J = 7.9 Hz, 2 H, Har), 7.27−7.31 (m, 3 H, Har, NCH), 7.21 (s, 2 H, Har), 7.16 (d, 3J = 1.5 Hz, 1 H, NCH), 7.15 (d, 3J = 1.5 Hz, 1 H, NCH), 7.08 (dd, 3J = 7.9 Hz, 4J = 1.1 Hz, 1 H, Har), 6.88 (td, 3J = 7.6 Hz, 4J = 1.3 Hz, 1 H, Har), 6.79 (td, 3J = 7.4, 4J = 1.3 Hz, 1 H, Har), 6.72 (dd, 3J = 7.5 Hz, 4J = 0.8 Hz, 1 H, Har), 6.65 (d, 3J = 1.9 Hz, 1 H, NCH), 4.26 (t, 3J = 5.6 Hz, 2 H, NCH2), 3.76 (t, 3J = 5.6 Hz, 2 H, NCH2CH2), 3.51−3.54 (m, 3 H, CH2), 3.47−3.51 (m, 3 H, CH2), 3.40−3.44 (m, 2 H, CH2), 3.31 (s, 3 H, OCH3), 3.30 (spt, 3J = 6.0 Hz, 4 H, CHiPr), 1.48 (d, 3J = 6.4 Hz, 6 H, CH3), 1.16 (d, 3J = 6.8 Hz, 6 H, CH3), 1.01 (d, 3J = 6.8 Hz, 6 H, CH3), 0.78 (d, 3J = 6.4 Hz, 6 H, CH3). 13C NMR (151 MHz, CDCl3): δ (ppm) 180.1 (s, Car) 152.6 (s, Car), 152.2 (s, Car), 147.8 (s, Car), 145.0 (s, Car), 138.0 (s, CHar), 137.4 (s, Car), 136.1 (s, Car), 129.8 (s, CHar), 127.4 (s, CHar), 125.9 (s, CHar), 124.8 (s, CHar), 124.2 (s, CHar), 123.9 (s, CHar), 122.6 (s, CHar), 120.9 (s, CHar), 120.1 (s, CHar), 71.8 (s, CH2), 70.8 (s, CH2), 70.52 (s, CH2), 70.49 (s, CH2), 69.4 (s, CH2), 68.5 (s, CH2), 59.0 (s, OCH3), 47.4 (s, CH2), 29.0 (s, CHiPr) 28.4 (s, CHiPr), 26.5 (s, CH3,iPr), 26.1 (s, CH3,iPr), 23.2 (s, CH3,iPr), 23.0 (s, CH3,iPr). Anal. Calcd for C43H57ClN4O3Pd (819.82): C, 63.00; H, 7.01; N, 6.83. Found: C, 63.03; H, 7.39; N, 6.70. ESI-MS: 783.6 ([LL′Pd]+). Mp: 78 °C. Complex 7. Compound 3 (120 mg, 0.26 mmol, 2 equiv), compound 4 (120 mg, 0.13 mmol), KBr (188 mg, 1.58 mmol, 12 equiv), and potassium carbonate (184 mg, 1.32 mmol, 10 equiv) were suspended in THF (10 mL) and stirred at 50 °C for 16 h. The mixture was filtered off, the solvent was evaporated under reduced pressure, and the residue was purified by column chromatography on silica gel (DCM/ethyl acetate 9/1). After drying under high vacuum, the product was obtained as a yellow, hygroscopic oil (96 mg, 0.11 mmol, 43%). NMR: complex spectrum. Please see the Supporting Information. Anal. Calcd for C38H55BrN4O6Pd·0.7H2O (850.2): C, 53.29; H, 6.99; N, 6.55. Found: C, 52.90; H, 6.59; N, 6.49. ESI-MS: 769.6 ([LL′Pd]+). Complex 8. Compound 3 (169 mg, 0.37 mmol, 2 equiv), compound 5 (120 mg, 0.19 mmol), KBr (266 mg, 2.2 mmol, 12 equiv), and potassium carbonate (260 mg, 1.9 mmol, 10 equiv) were suspended in THF (15 mL) and stirred at 50 °C for 2 days. The mixture was filtered, the solvent was evaporated under reduced

pressure, and the residue was purified by column chromatography on silica gel (hexanes/ethyl acetate/methanol 50/50/1 → 25/25/1 → DCM/methanol 10/1). After drying under high vacuum, the product was obtained as a yellow solid (150 mg, 0.21 mmol, 56%). NMR: complex spectrum. Please see the Supporting Information. Anal. Calcd for C32H43BrN4O3Pd (718.04): C, 53.53; H, 6.04; N, 7.80. Found: C, 53.22; H, 6.28; N, 7.51. ESI-MS: 637.4 ([LL′Pd]+). Mp: 67 °C. General Catalytic Procedure. All solid reagents and a magnetic stirring bar were placed in a 10 mL crimp vial, and the vial was capped by a butyl rubber septum. The vial was evacuated and subsequently filled with argon three times. Then, the solvent and all liquid reagents were added via syringe. The vial was placed in a preheated aluminum block and stirred for the indicated time. For determination of the GC yield, DCM (2 mL) and dodecane (175 mg), as an internal standard, were added, and aliquots of 50 μL of the organic layer were placed onto a plug of silica and rinsed with DCM (3 mL). The obtained solution was analyzed by GC-MS. The yield was determined using a calibration curve. All yields given are an average of two separate runs. For the determination of the isolated yields, water (5 mL) was added and the mixture was extracted with DCM (3 × 10 mL). The solvent was evaporated under reduced pressure, the residue was dissolved in a suitable eluent (5 mL, see the Supporting Information), and the solution was filtered over silica. For reactions without full conversion, the raw product was purified by column chromatography. The product was dried under reduced pressure at 50 °C. Product characterization data are given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00607. Characterization data for the synthesized compounds, details for the solid-state structure determination of compound 6, and NMR spectra (PDF) Accession Codes

CCDC 1863544 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.S.: [email protected]. ORCID

Thomas Strassner: 0000-0002-7648-457X Notes

The authors declare no competing financial interest.

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REFERENCES

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Cyclometalated Palladium NHC Complexes Bearing PEG Chains for

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