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DOI: 10.1055/s-2008-1078548
Palladium N-Heterocyclic Carbene Catalysts for Synthesis of Diaryl Ethers
Publication History
Publication Date:
02 July 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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References and Notes
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.