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DOI: 10.1055/s-2007-1000863
1-Acetyl-2,3-Dimethylimidazolidine: A Novel Organic Reductant for Transfer Hydrogenation
Publikationsverlauf
Publikationsdatum:
21. Dezember 2007 (online)
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Abstract
1-Acetyl-2,3-dimethylimidazolidine was synthesized and was shown to be able to directly reduce a series of aromatic, aliphatic and α,β-unsaturated aldehydes as well as imines in high yields.
Key words
1-acetyl-2,3-dimethylimidazolidine - aldehydes - hydrogen transfer - imines - organic reductant
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References and Notes
Synthesis of 1-Acetyl-2,3-dimethylimidazolidine (1c) (Scheme 1):
2-Methylimidazoline (4.3 g, 51 mmol) was dissolved in CH2Cl2 (15 mL) at 0 °C, followed by the addition of Et3N (5.2 g, 51 mmol). This solution was then added dropwise to a solution of Ac2O (5.2 g, 51 mmol) in CH2Cl2 (5 mL). After the addition, the reaction mixture was stirred at r.t. for an additional 3 h. The resulting solution was treated with 10% NaHCO3 (40 mL). After stirring for 0.5 h, the mixture was extracted with CH2Cl2, and the organic layers were washed with brine, and dried over anhyd Na2SO4. Evaporation of CH2Cl2 in vacuo gave 1-acetyl-2-methylimidazoline (5.46 g) as an oil (yield: 85%). This product was sufficiently pure for the next step.
1-Acetyl-2-methylimidazoline (5.8 g, 46 mmol) and MeI (8.6 mL, 138 mmol) were refluxed in Et2O (30 mL) for 10 h. After cooling to r.t., acetone (15 mL) was added to the mixture. The mixture was stirred for several hours, and the solid obtained was collected by vacuum filtration to give 1-acetyl-2,3-dimethylimidazolinium iodide (8.5 g) as a white solid (yield: 69%). 1H NMR (300 MHz, CDCl3): δ = 2.39 (s, 3 H), 2.79 (s, 3 H), 3.41 (s, 3 H), 4.29 (m, 2 H), 4.46 (m, 2 H). 13C NMR (75 MHz, CDCl3): δ = 172.1, 171.2, 55.5, 49.3, 38.5, 28.1, 19.2. IR (KBr): 2941, 1728, 1654, 1446, 1388, 1298, 1174, 1037 cm-1.
1-Acetyl-2,3-dimethylimidazolinium iodide (0.80 g, 3 mmol) was dissolved in MeCN (10 mL) at 0 °C, followed by the addition of NaBH4 (0.125 g, 3.3 mmol). After the addition, the reaction mixture was stirred at r.t. for an additional 5 h. The reaction was quenched with H2O (50 mL). The mixture was extracted with CH2Cl2, and the organic layers were washed with brine, and dried over anhyd Na2SO4. Evaporation of CH2Cl2 in vacuo gave 1-acetyl-2,3-dimethylimidazolidine (1c; 0.42 g) as a clear oil (yield: 97%). 1H NMR (300 MHz, D2O): δ = 1.33-1.50 (3 × d, J = 6.3 Hz, 3 H), 2.05-2.11 (3 × s, 3 H), 2.63-2.69 (3 × s, 3 H), 3.27-3.88 (3 × m, 4 H), 4.76-4.90 (m, 1 H). IR (film): 2956, 2381, 1718, 1652, 1419, 1265, 1182 cm-1. MS: m/z = 142 [M+], 127 [M -15], 99 [M - 43]. Anal. Calcd for C7H14N2O: C, 59.12; H, 9.92; N, 19.70. Found: C, 59.24; H, 9.89; N, 19.75.
The deuterated 1c (C-2 position) was synthesized by the same procedures except that NaBD4 was used. 1H NMR (300 MHz, D2O): δ = 1.22-1.36 (3 × s, 3 H), 1.93-1.99 (3 × s, 3 H), 2.51-2.56 (3 × s, 3 H), 3.15-3.73 (3 × m, 4 H).
Scheme 1
General Procedure for the Transfer Hydrogenation Reaction with Imidazolidine 1c as ReductantProcedure Using Condition A: To a solution of substrate (1 mmol) in MeOH (2 mL) was added a solution of imidazolidine 1c (0.142 g, 1 mmol) in anhyd MeCN (2 mL). The reaction mixture was stirred under reflux and monitored by TLC. Upon completion, the solvent was removed in vacuo and the residue was chromatographed on silica gel to give the desired products.
Procedure Using Condition B: To a solution of substrate (1 mmol) and Mg(ClO4)2 (22.3 mg, 0.1 mmol) in MeOH (2 mL) was added a solution of imidazolidine 1c (0.142 g, 1 mmol) in anhyd MeCN (2 mL). The reaction mixture was stirred at r.t. and monitored by TLC. Upon completion, the solvent was removed in vacuo and the residue was chromatographed on silica gel to give the desired products.