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Synlett 2008(12): 1781-1784  
DOI: 10.1055/s-2008-1078548
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Palladium N-Heterocyclic Carbene Catalysts for Synthesis of Diaryl Ethers

Mitat Akkoça, Nevin Gürbüza, Engin Çetinkayab, Ismail Özdemir*a
a Chemistry Department, Faculty of Science and Arts, Inönü University, 44280 Malatya, Turkey
b Department of Chemistry, Ege University, 35100 Bornova-zmir, Turkey
Fax: +90(422)3410212; e-Mail: iozdemir@inonu.edu.tr;
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Publikationsverlauf

Received 3 March 2008
Publikationsdatum:
02. Juli 2008 (online)

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Abstract

Novel functionalized 1,3-dialkylimidazolinium (LH) salts as precursors for N-heterocyclic carbenes (NHCs) have been prepared and successfully applied in the palladium-catalyzed synthesis of diaryl ethers and arylation of benzaldehydes. An efficient catalyst system was prepared in situ from Pd(OAc)2, 1,3-dialkylimidazolinium bromides (LHBr), and NaH.

Key words

C-O bond formation - etherification - palladium ­coupling - N-heterocyclic carbene.

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20

General Procedure for the Synthesis of 1,3-Dialkyl-imidazolinium Bromides 2: To a solution of 1-(methoxy-ethyl)imidazoline (1 mmol) in DMF (3 mL), alkyl halide (1.1 mmol) was added and the resulting solution was stirred for 1 h at r.t. and heated for 12 h at 80 ˚C. Et2O (10 mL) was added to the reaction mixture. A white solid was precipitated in this period. The precipitate was then crystallized from EtOH-Et2O (1:2).
1-(2-Methoxyethyl)-3-(cyclohexylmethyl)imidazolinium Bromide (2a): yield: 2.65 g (87%); mp 29 ˚C. IR: 1465 (C=N) cm- ¹. ¹H NMR (300.13 MHz, CDCl3): δ = 3.11 (s, 3 H, OMe), 3.38 (t, J = 4.8 Hz, 2 H, NCH2CH 2O), 3.59 (t, J = 4.8 Hz, 2 H, NCH 2CH2O), 3.79, 3.89 (m, 4 H, NCH2CH2N), 0.72-1.41 (m, 11 H, CH2C6 H 11), 3.15 (d, 2 H, CH 2C6H11), 9.20 (s, 1 H, 2-CH). ¹³C NMR (75.46 MHz, DMSO): δ = 47.3, 48.8 (NCH2CH2N), 68.6 (OMe), 58.5 (NCH2 CH2O), 53.8 (NCH2CH2O), 49.2 (CH2C6H11), 25.0, 25.6, 34.9, 39.7 (CH2 C 6H11), 158.2 (2-CH). Anal. Calcd for C13H25N2OBr: C, 51.15; H, 8.25; N, 9.18. Found: C, 51.23; H, 8.18; N, 9.24.
1-(2-Methoxyethyl)-3-(2,3,5,6-tetramethylbenzyl)im-idazolinium Bromide (2b): yield: 3.22 g (91%); mp 156-157 ˚C. IR: 1649 (C=N) cm- ¹. ¹H NMR (300.13 MHz, CDCl3): δ = 2.20 [s, 6 H, CH2C6H(CH 3)4-2,6], 2.22 [s, 6 H, CH2C6H(CH 3)4-3,5], 3.32 (s, 3 H, OMe), 3.60 (t, J = 8.0 Hz, 2 H, NCH2CH 2O), 4.04 (t, J = 8.0 Hz, 2 H, NCH 2CH2O), 3.76-3.81 (m, 4 H, NCH2CH2N), 4.89 [s, 2 H, CH 2C6H(CH3)4-2,3,5,6], 6.97 [s, 1 H, CH2C6 H(CH3)4-2,3,5,6], 9.22 (s, 1 H, 2-CH). ¹³C NMR (75.46 MHz, CDCl3): δ = 14.9 [CH2C6H(CH3)4-2,6], 19.5 [CH2C6H(CH3)4-3,5], 48.4 [CH2C6H(CH3)4-2,3,5,6], 47.0 (NCH2 CH2N), 68.3 (OMe), 57.9 (NCH2 CH2O), 45.9 (NCH2CH2O), 127.3, 131.8, 132.9, 133.7 [CH2 C 6H(CH3)4], 157.5 (2-CH). Anal. Calcd for C17H27N2OBr: C, 57.47; H, 7.66; N, 7.88. Found: C, 57.39; H, 7.61; N, 7.93.
1-(2-Methoxyethyl)-3-(2,3,4,5,6-pentamethylbenzyl)im-idazolinium Bromide (2c): yield: 3.28 g (89%); mp 182-183 ˚C. IR: 1648 (C=N) cm- ¹. ¹H NMR (CDCl3): δ = 2.22 [s, 6 H, CH2C6 (CH 3)5-2,6], 2.25 [s, 3 H, CH2C6 (CH 3)5-4], 2.30 [s, 6 H, CH2C6 (CH 3)5-3,5], 3.35 (s, 3 H, OMe), 3.63 (t, J = 8.0 Hz, 2 H, NCH2CH 2O), 4.09 (t, J = 8.0 Hz, 2 H, NCH 2CH2O), 3.83-3.88 (m, 4 H, NCH2CH2N), 4.91 [s, 2 H, CH 2C6(CH3)5-2,3,4,5,6], 9.15 (s, 1 H, 2-CH). ¹³C NMR (75.46 MHz, CDCl3): δ = 17.1 [CH2C6(CH3)5-2,3,5,6], 17.4 [CH2C6(CH3)5-4], 49.6 [CH2C6(CH3)5], 69.4 (OMe), 59.0 (NCH2 CH2O), 48.3 (NCH2CH2O), 47.5, 48.2 (NCH2 CH2N), 125.9, 133.6, 136.7 [CH2 C 6(CH3)5], 157.5 (2-CH). Anal. Calcd for C18H29N2OBr: C, 58.53; H, 7.91; N, 7.58. Found: C, 58.47; H, 7.81; N, 7.62.
General Procedure for the Diaryl Ether Formation: A dried Schlenk tube was charged with NaH (1.4 mmol) and purged with argon. The phenol (1.1 mmol) and toluene (3 mL) were added, and the mixture was stirred at 100 ˚C for 30 min under argon. The reaction mixture was cooled to r.t. and the tube was charged with Pd(OAc)2 (2.0 mmol%), 1,3-dialkyimidazolinium bromides 2 (4.0 mmol%) and aryl chloride (1.0 mmol). The mixture was heated at 100 ˚C for 20 h. At the conclusion of the reaction, the mixture was cooled, extracted with Et2O, filtered through a pad of silica gel with copious washings, concentrated and purified by flash chromatography on silica gel. The purity of the compounds was checked by GC and yields are based on the aryl chloride.