Natural Kaolin Supported Sulfuric Acid as an Efficient Catalyst for Selective Hydrolysis of Nitriles to Amides

나이트릴을 아마이드로 선택적 가수분해하는데 효과적인 촉매로서의 자연 고령토에 흡착된 황산

RESULTS AND DISCUSSION

Natural kaolin is very cheap and shows good potential as support material. Currently, strong interests in such natural supports are due to ecofriendly demands in many modern industrial applications.20

In a typical procedure, nitrile (4 mmol) were dissolved in water (10 mL) in the presence of kaolin and refluxed for 24 h. The best result was obtained by acidified kaolin with sulfuric acid (2% w/w). After completion of the reaction (monitored by TLC), the crude product was extracted with ethyl acetate in 55-80% yields. The results are shown in Table 1. In general, aliphatic and aromatic nitriles were successfully converted into amides. Two industrially important amides, acrylic amide and methacrylic amid were obtained in 78% and 74% yields, respectively, from corresponding nitriles.

Table 1.aIsolated yields. bThe products were characterized by IR and 1H NMR spectroscopy and also their melting points are compared with authentic samples.

Careful neutralization of all reaction mixtures were carried out to pH = 7 for exact monitoring of reaction for possible formation of carboxylic acid. In all reactions we only obtained amide without any contamination with carboxylic acid which was monitored with TLC, IR and NMR spectra. Only in the case of (Table 1, entry 4) about 5% carboxylic acid was formed. Then the product was purified for obtaining the pure corresponding amide for further characterization. Acidic work up destroyed the product complexes with metal cations of kaolin and increased the isolated yields but the catalyst was deformed and did not regenerate.

The products were characterized by IR and 1H NMR spectroscopy and also their melting points are compared with authentic samples. The disappearance of one strong and sharp absorption band (CN stretching band), and the appearance of two NH2 stretching bands in 3370 and 3320 cm-1 and carboxamide stretching in 1650 cm-1 in the IR spectra, were evidence for the formation of primary amides. In 1H NMR spectrum of acrylic amide, three doublet of doublets were observed for three vinylic protons at δ 5.57, 6.07 and 6.17 with expectedly vicinal and geminal splitting. NH2 group shows two singlets at δ 7.1 and 7.5 due to amidic resonance.

The mechanism of this selective transformation is not clear. We assume that kaolin / sulfuric acid (2% w/w) acts as a Lewis or Bronsted acid making the nitrile more susceptible to nucleophilic addition. Perhaps, an efficient coordination of amide-intermediate with kaolin stopped the hydrolysis in amide stage until work up.

In conclusion, we have developed a novel and highly efficient method for the synthesis of primary amides from nitriles in the presence of the acidified kaolin (2% w/w) under reflux conditions in water. The significant advantages of this methodology are good yields without carboxylic acid contamination, a simple work-up procedure, and easy preparation and handling of the catalyst. Also, the catalyst and solvent are both environmentally safe that means green chemistry.

 

EXPERIMENTAL

All materials and solvents were purchased from Merck and Fluka. Melting points were determined in open capillary tubes in an Electrothermal IA 9700 melting point apparatus. 1H NMR spectra were recorded on a Bruker-100 MHz and 500 MHz instruments using tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a Shimadzu-IR 470 spectrophotometer. All of the products are known products and all of the isolated products gave satisfactory IR and NMR spectra.

Preparation of the Acidified Kaolin with Sulfuric Acid (2% w/w)

Kaolin (7.5 g) was treated with concentrated sulfuric acid (0.15 g, 0.08 mL) and stirred for 1 h. The prepared acidified kaolin (2% w/w), was stored for further applications.

Preparation of Primary Amides From Nitriles: Typical Procedure: The acidified kaolin (2% w/w) (150 mg) was added to a solution of nitrile (4 mmol) in water (10 mL) and refluxed for 24 h. After completion of the reaction (as indicated by TLC), the reaction mixture was cooled to room temperature and neutralized with sodium hydroxide solution (4 N) to pH=7 carefully. The reaction mixture was filtered and extracted with ethyl acetate (2×20 mL). The organic layer dried over sodium sulfate and evaporated. The crystalline amide was obtained after recrystallization from H2O-EtOH. The products were obtained in 55-80% yields. All the products are known compounds and the spectral data and melting points were identical to those reported in the literature.

Acrylamide: Yield 78%; mp 84-85 ℃; IR (KBr) 3370, 3320, 1650 cm-1; 1H NMR (DMSO-d6, 500 MHz) δ 7.5 (b, 1H, NH), 7.1 (b, 1H, NH), 6.18 (dd, 1H, J1 = 16Hz, J2 = 10 Hz), 6.07 (dd, 1H, J1 = 16Hz, J2 = 2.2Hz), 5.58 (dd, 1H, J1 = 10Hz, J2 = 2.2Hz).

4-Methoxybenzamide: Yield 69%; mp 164-165 ℃; IR (KBr) 3330, 3320, 1640 cm-1; 1H NMR (DMSO-d6, 500 MHz) δ 7.84 (d, 2H, J = 7Hz), 7.17 (b, 2H, NH2), 6.95 (d, 2H, J = 7Hz), 3.78 (s, 3H).

4-Bromobenzamide: Yield 65%; mp 190 ℃; IR (KBr) 3350, 3200, 1660 cm-1; 1H NMR (Acetone-d6, 100 MHz) δ 7.75 (AB q, 4H), 6.8 (b, 2H, NH2).

3-Methylbenzamide: Yield 66%; mp 93-94 ℃; IR (KBr) 3350, 3200, 1660 cm-1; 1H NMR (Acetone-d6, 100 MHz) δ 7.8 (m, 2H), 7.4 (m, 4H), 2.3 (s, 3H).

n-Hexamide: Yield 61%; mp 100-101 ℃; IR (KBr) 3350, 3200, 1660 cm-1; 1H NMR (Acetone-d6, 100 MHz) δ 6.5 (b, 2H, NH2), 2.55 (t, 2H, J = 8Hz), 2.1 (m, 2H), 1.1-1.7 (m, 4H), 0.9 (t, 3H, J = 8Hz).