Synthesis of novel NHC-pyrrole-NHC C-N-C Pincer proligands
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Hong Jiang
Abstract
Two novel pyrrole-functionalized bis(benzoimidazolium) salts, which are novel NHC-pyrrole-NHC C-N-C Pincer proligands were synthesized in high yield. Both of these compounds are fully characterized by 1H and 13C NMR, IR and elemental analysis.
Among polydentate N-heterocyclic carbenes (NHCs), an important class of tridentate chelating bis-NHCs ligands is Pincer ligands in which a donor-functional group is introduced between the two carbene edges (Pugh and Danopoulos, 2007; Poyatos et al., 2009). The Pincer ligands provide the ubiquitous coordination sphere and have attracted a lot of attention due to their good thermal stability and good rigidity (Choi et al., 2011; Selander and Szabo, 2011). The organometallic complexes with Pincer ligands have been widely studied in catalytic application (Albrecht and van Koten, 2001; Singleton, 2003; van der Boom and Milstein, 2003). A variety of atoms have been used in such structures, but, to the best of our knowledge, no example of N donor from pyrrole has been reported. Herein we report the preparation of a new type of NHC-pyrrole-NHC C-N-C Pincer proligands from pyrrole in multiple steps.
The C-N-C Pincer proligand 4 was prepared from pyrrole in an overall 34% yield in four steps (Scheme 1). Firstly, 2,5-bis(dimethylaminomethyl)pyrrole (1) (Heaney, 1997) was prepared by Mannich reaction of pyrrole in a basic medium with iminium ion generation from dimethylamine and formaldehyde. This reaction is very efficient and over 80% of 2,5-disubstituted pyrrole was reported to be isolated by chromatography. However, only 50% yield was achieved in our experiment due to the substantial loss of the product during distillation. The alkylation reaction of the tertiary amine arm of substituted pyrrole with methyl iodine gave quaternary ammonium salt 2 (Kim and Elsenbaumer, 1998) in high yield. The substitution reaction of quaternary ammonium iodide with imidazole gave compound 3 (Simons et al., 2002) in 85% yield. The substitution of imidazole-substituted pyrrole with methyl iodide generated NHC-pyrrole-NHC C-N-C Pincer proligand 4. The imidazolinium iodide 4 was converted quantitatively to the imidazolinium hexafluorophosphate 5 by treatment with NH4PF6 in water. Compounds 1–3 are known compounds, and their 1H NMR spectra were essentially identical with those of the authentic samples. Their IR data are reported herein. The final new products 4 and 5 are characterized by spectroscopy (1H NMR, 13C NMR and IR) and elemental analysis.
Experimental section
General
All reagents were commercially available and used without further purification. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker DPX spectrometer at room temperature. IR spectra were recorded in KBr pellets on a FTIR-Tensor 27 spectrometer. Melting points were detected by microscope melting point apparatus. Elemental analyses were performed on a EuroVektor Euro EA-300 elemental analyzer.
2,5-Bis(dimethylaminomethyl)pyrrole (1)
An aqueous solution of 37–40% formaldehyde (40 mmol, 3.16 g) was treated with dimethylamine hydrochloride (40 mmol, 3.4 g) followed by addition of pyrrole (20 mmol, 1.34 g). The mixture was stirred on an ice bath for 1 h, then allowed to warm to room temperature, and stirred for an additional 12 h before dropwise treatment with an aqueous 20% solution of NaOH to pH 7–8. The mixture was extracted three times with ether and the extract was concentrated on a rotary evaporator. The oily residue was distilled (75–80°C/1–2 mm Hg) to yield 1.74 g (48%) of product 1; IR: 3643, 3464, 2697, 1596, 1469, 1357, 1251, 1196, 1173, 1043, 1012, 990, 967, 847, 762, 712 cm-1; 1H NMR (CDCl3): δ 8.55 (br. s, 1H), 5.90 (d, J = 2.8 Hz, 2H), 3.37 (s, 4H), 2.20 (s, 12H) [1H NMR (Heaney, 1997; 60 MHz, CDCl3): 9.80 (br. s, 1H), 5.88 (d, 2H), 3.38 (s, 4H), 2.18 (s, 12H)].
2,5-Bis[(trimethylammonio)methyl]pyrrole diiodide (2)
A solution of compound 1 (50 mmol, 9.06 g) in THF (30 mL) was cooled on an ice bath and treated dropwise with methyl iodide (150 mmol, 7.6 mL). The mixture was stirred at room temperature for 5 h, then treated with acetone (20 mL), and stirred for an additional 12 h. The white precipitate of 2 was collected by filtration: yield 22.3 g (96%); IR: 3433, 3008, 1586, 1486, 1401, 1375, 1207, 1096, 1056, 1005, 982, 969, 945, 912, 872, 801, 751 cm-1; 1H NMR (D2O): δ 6.49 (s, 2H), 4.41 (s, 4H), 2.97 (s, 18H); 13C NMR (DMSO-d6): 121.5, 114.8, 61.2, 51.7. [13C NMR (Kim and Elsenbaumer, 1998): 121.4, 114.8, 61.1, 51.7].
2,5-Bis[(imidazol-1-yl)methyl]pyrrole (3)
A mixture of compound 2 (2.73 g, 6 mmol), KOH (1.34 g, 24 mmol) and imidazole (1.63 g, 24 mmol) in dichloromethane (40 mL) was stirred at room temperature for 16 h. The solution was filtered and the solvent was removed under reduced pressure to yield a light red solid of 3 that was crystallized from water: yield 1.16 g (85%); an off-white solid; mp 145–146°C; IR: 3010, 2880, 2758, 1668, 1628, 1544, 1507, 1432, 1393, 1339, 1271, 1222, 1204, 1168, 1112, 1073, 1028, 997, 918, 822, 794, 756, 723, 663, 615 cm-1; 1H NMR (DMSO-d6): δ 11.10 (s, 1H, pyrrole N-H), 7.60 (s, 2H, N-C(H)-N), 7.09 (s, 2H, CH), 6.84 (s, 2H, NCH), 5.96 (s, 2H, NCH), 5.03 (s, 4H, CH2). [1H NMR (Simons et al., 2002) (300 MHz DMSO-d6): 5.03 (s, 4H), 5.96 (s, 2H), 6.84 (s, 2H), 7.09 (s, 2H), 7.60 (s, 2H), 11.10 (s, 1H); 13C NMR (Simons et al., 2002) (100 MHz, DMSO-d6): 136.9, 128.4, 127.8, 119.2, 107.8, 42.7.
Compound 4
A mixture of compound 3 (3.1 g, 13.6 mmol) and methyl iodide (10 mL) in a heavy wall pressure tube was heated at 60°C for 10 h. After cooling the solid product 4 was filtered and washed with anhydrous ethanol: yield 5.98 g (86%); IR: 3221, 3172, 3136, 3093, 3035, 2951, 1631, 1569, 1502, 1381, 1339, 1318, 1235, 1187, 1160, 1032, 994, 871, 794, 769, 757, 649, 615. 1H NMR (DMSO-d6): δ 11.34 (s, 1H), 9.08 (s, 2H), 7.69 (m, 4H), 6.21 (m, 2H), 5.35 (s, 4H), 3.86 (s, 6H); 13C NMR (DMSO-d6): δ 136.2, 125.7, 123.8, 122.0, 109.9, 45.2, 36.0. Anal. Calcd for C14H19I2N5 (511.14): C, 32.90; H, 3.75; N, 13.70. Found: C, 32.76; H, 3.71; N, 13.87.
Compound 5
A solution of compound 4 (0.51 g, 1.0 mmol) in water (5 mL) was added dropwise to a saturated solution of NH4PF6 in water (20 mL). The resultant white solid of 5 was filtered and washed with water: yield 0.54 g (99%); IR: 3432, 3174, 1630, 1574, 1357, 1330, 1317, 1196, 1160, 1147, 1111, 1048, 1025, 884, 808, 773, 758, 731, 625 cm-1; 1H NMR (DMSO-d6): δ 11.35 (s, 1H), 9.01 (s, 2H), 7.65 (m, 4H), 6.20 (m, 2H), 5.34 (s, 4H), 3.84 (s, 6H); 13C NMR (DMSO-d6): δ 136.2, 125.8, 123.9, 122.1, 110.0, 45.4, 35.9. Anal. Calcd for C14H19F12N5P2 (546.08 g mol-1): C, 30.73; H, 3.50; N, 12.80. Found: C, 30.62; H, 3.42; N, 12.97.
We gratefully acknowledge the Project of Laboratory Development of Jiangsu Normal University (L2011Y32), Qing Lan Project (08QLT001 and 08QLD006), the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry at Jilin University (2013-03), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and National Natural Science Foundation of China (NSFC) (21071121) for financial support of this work.
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Articles in the same Issue
- Masthead
- Masthead
- Preliminary Communication
- Synthesis of novel NHC-pyrrole-NHC C-N-C Pincer proligands
- Research Articles
- Synthesis of new tetracyclic paullone derivatives as potential CDK inhibitors
- Keto furanylidene building blocks from silyl ethers of monoalkynylated β-keto carbonyls with iron(III) chloride hexahydrate-iodine
- Synthesis of perhydro-N-(2,2-disubstituted-3-aminopropyl) heterocycles as potentially bioactive compounds and fragments for combinatorial chemistry
- Pirfenidone structural isosteres: design, synthesis and spectral study
- Synthesis and characterization of new metallophthalocyanines bearing macrocyclic N3O2 groups on peripheral positions
- Synthesis and antimicrobial evaluation of substituted benzimidazolyl fluoroquinolones under conventional and microwave irradiation conditions
- Synthesis of novel 2,6-bis(5-t-butylbenzo[b]furan-2-ylcarbonyl)pyridines
- An efficient synthesis of 2-substituted benzoxazoles using cerium(III) chloride/sodium iodide as catalyst
- Efficient one pot synthesis of triazolotriazine, pyrazolotriazine, triazole, isoxazole and pyrazole derivatives
- Synthesis of novel trifluoromethyl-containing heterocycle-fused troponoid compounds
