NaY Zeolite: A Useful Catalyst for Nitrile Hydrolysis

Abstract

The NaY zeolite catalysed hydrolysis of nitriles to primary amides is reported. It is found that aryl nitriles with strong electron-withdrawing substituents and cyanopyridines are readily hydrolysed in the water suspension, while aliphatic nitriles do not react.

Introduction

Zeolites are effective catalysts in organic chemistry and their specificity in gas phase transforma- tions is greatly utilised in industry [1]. Alkylation reaction, polymerisation, cyclization [2], pho- toreduction [3], or preparation of nitroalkenes [4], occur in gas phase or with reactants sorbed within zeolite in inert solvent. Recently, several reports on the use of acidic zeolites (HY) in macrolactoniza- tion [5], acetalization [6], acetylation [7] and gem-diacetalization [8], as well as the synthesis and ap- plication of the first organic-functionalized zeolite-beta [9], prompted us to investigate the new cata- lytic possibilities of NaY zeolite.

Here we wish to report the application of NaY zeolite as reusable catalyst in the hydrolysis of nitriles to primary amides.

Results and Discussion

In our experiments the suspension of NaY zeolite and a nitrile in water (or methanol) was heated to reflux for a given period (

Table 1. Hydrolysis of nitriles into amides^a.

Entry	Nitrile	Time (h)	Yield (%)b	Amide
1		24 14 6	87 76 67
2		24	86 (84, 80, 82)c
3		24	92
4		48	16
5		48	NRd
6		48	NR
7		24	87
8		48	35
9		48	NR
10		48	21
11		24	35
12	NCCH2CH2CO2Et (20)	2	quant.	NCCH2CH2CO2H (21)

^a NaY / nitrile = 4 : 1 (w / w). ^b Yield of isolated compounds. ^c Yield of nicotinamide (4) with reused NaY zeolite. ^d No reaction.

), zeolite was filtered off, and products were separated (or directly crystallised). Most important observation is that nitriles are hydrolysed only to the amide stage. Cya- nopyridines and benzonitriles with electron-withdrawing substituents are readily hydrolysed in good yield (Table 1, entries 1-3, 7).

Benzonitrile (10, entry 6) was totally resistant to hydrolysis, while benzonitriles substituted with week inductive electron-withdrawing groups (with strong +R, 4-aminobenzonitrile (13) and 4-(5- cyanopentoxy)benzonitrile (16) [10], Table 1, entries 8 and 10) were hydrolysed, although in low yield (35% and 21%, respectively). Hydroxy substituent completely prevented the hydrolysis (15, Table 1, entry 9) probably as a consequence of phenoxy ion formation. The case of di-nitrile 16 is very interesting: it shows that hydrolysis proceeds by blocking the formation of phenoxy ion, while at the same time pointing to the resistance of aliphatic nitriles to hydrolysis. The resistance of aliphatic nitriles was confirmed by attempted hydrolysis of CH₃CN (not shown) and by hydrolysis of ethyl 3- cyanopropanoate 20 only to cyanoacid 21 (Table 1, entry 12). Benzylic cyano group was also found to be resistant to applied conditions (9, Table 1, entry 5), or was very slightly hydrolysed when CH₂CN was attached to the electron-withdrawing pyridine ring (7 →8, 16%, Table 1, entry 4).

Hydrolysis of succinodinitrile (18, Table 1, entry 11) afforded only 35% of 2-cyanopropanamide (19) and 65% of educt. It is interesting to note that much greater site-differentiation was achieved in enzymatic hydrolysis of α,ω-dinitriles into corresponding ω-cyanoacids [11].

In some cases, prolonged reaction time resulted in higher yields, as is given for 4-cyanopyridine (1) (Table 1, entry 1, 6 h (67%) → 24 h (87%)). The reusability of NaY catalyst was tested using 3- cyanopyridine (3, Table 1, entry 2). Four runs were performed with the same batch of the catalyst without significant loss of its activity.

In

Table 2. Solvolysis of 4-cyanopyridine (1).

Entry	Reactant [time (h)]	Product	Yield (%)a
1	H2O (24)	isonicotinamide (2)	87
2	CH3OH (14)		67 12
3	ethanol (10) propanol (10) 1-butanol (10)	NR
4	NH2NH2 × H2O (2)		67 18

^a Yield of isolated compounds.

the influence of reactant (solvent) is shown. Using methanol instead of water the imino ester 22 was obtained in good yield (67%; entry 2), while ethanol and higher homologues (propanol and 1-butanol) were ineffective. However, hydrazine hydrate afforded isoniazide (23, 67%) along with 18% of isonicotinamide (2).

The influence of the amount of catalyst on product distribution is exemplified with methanolysis of 4-cyanopyridine (1) (

Table 3. Methanolysis of 4-cyanopyridine (1): dependence on catalyst-to-substrate ratio (reflux 15 h).

Entry	1	2	3	4	5	6	7
NaY / 1 (w/w)	4	2	1	0.5	0.2	0.1	0.06
22 / 1 (GC ratio)	73 : 27	65 : 35	63 : 35	60 : 38	61 : 32	66 : 32	51 : 47

). Product formation started with 6% (w/w) of catalyst, and the increase of zeolite part did not significantly affect the product distribution, what, beside its already shown reus- ability, confirms the true catalytic nature of NaY zeolite.

Conclusion

We have shown that NaY zeolite can be used in the simple procedure as a reusable catalyst for hy- drolysis of aromatic nitriles, primarily of cyanopyridines and benzonitriles possessing electron- withdrawing groups. Contrary to hydrolysis under acidic conditions, benzyl- and alkanenitriles are sta- ble under conditions applied, so enabling their further selective transformations. In addition, NaY zeo- lite can also be used for imino ester preparation as an alternative to Pinner synthesis.

Experimental

General

Melting points were determined on a Mikro-Heiztisch Boetius PHMK apparatus and were not corrected. IR spectra were recorded on Perkin-Elmer spectrophotometer FT-IR 1725X. ¹H and ¹³C NMR spectra were recorded on a Varian Gemini-200 or Bruker AM-250 spectrometers. Chemical shifts were expressed as ppm (δ) values and coupling constants (J) in Hz. Mass spectra were taken on a Finnigan- MAT 8230 spectrometer, as indicated below.

In our experiments, NaY zeolite with following characteristics was used [12]: crystallinity 100%; SiO₂ [%] 63.80; Al₂O₃ [%] 22.90; NaO [%] 13.30; molar ratio SiO₂ / Al₂O₃: 4.73; specific area (B.E.T) 850 m² / g; pore volume 0.32 cm³ / g; diameter of crystallite 3.5 μm; diameter of granulate 150 μm; pH of water suspension 10.05. Zeolite was pre-dried only for methanolysis reaction, in order to suppress the formation of an amide, and for acetylation reactions.

Hydrolysis of Nitriles - General

The suspension of a nitrile (200 mg) and zeolite (800 mg) in water (5 ml) was heated to reflux (for details see Table 1). The hot reaction mixture was filtered and zeolite was washed with water (and/or methanol). When catalyst was reused, it was dried on air overnight. Pure amides were crystallised di- rectly from the crude product mixture or were purified by column chromatography (SiO₂ or RP-18). All isolated compounds were fully characterised by spectroscopic and analytical methods.

The data of known compounds were compared with literature data given in [13], and that refers to:

• 2: mp 152-154°C, [14] mp 155-157°C
• 4: mp 128-129°C, [13, p. C-474] mp. 129-131°C
• 6: mp 106-108°C, [13, p. C-474] mp 107-108°C
• 12: mp 201-202.5°C, [13, p. C-197] mp 201.4°C
• 14: mp 184-186°C, [13, p. C-183] mp 183°C
• 21: mp 48-50°C, [11] mp 49.5-51°C.

2-Pyridylacetamide (8)

Mp 120-121°C. (colourless needles, water). IR (KBr) cm⁻¹: 3377, 3188, 3112, 3017, 1678, 1646, 1597, 1570, 1439, 1402. ¹H NMR (CD3OD, 200 MHz): 8.51-8.43 (m, 1H, H-C(6')), 7.78 (td, J = 7.8, 1.8, H-C(4')), 7.40 (d, J = 8.0, H-C(3')), 7.30 (ddd, J = 8, 5, 1, H-C(5')), 3.75 (s, 2H-C(2)). ¹³C NMR (CD3OD, 50 MHz): 174.97 (C1), 156.82 (C2'), 149.86 (C6'), 138.76 (C4'), 125.69 (C3'), 123.59 (C5'), 45.12 (C2). Anal. calc. for C₇H₈N₂O (136.06): C 61.75, H 5.92, N 20.57, found: C 62.08, H 5.57, N 20.83.

3-Cyanopropanamide (19)

Mp 86-88°C (colourless amorphous solid). IR (KBr) cm⁻¹: 3414, 3225, 2293, 2248, 1681, 1619, 1421. ¹H NMR (D2O, 200 MHz) δ: 2.80-2.65 (m, 4H, 2H-C(2), 2H-C(3)). ¹³C NMR (D₂O, 50 MHz) δ: 177.90 (C1), 123.04 (C4), 32.38 (C2), 15.12 (C3). MS (CI, isobutane): 197 (2 M⁺+1). Anal. calc. for C₄H₆N₂O (98.11): C 48.97, H 6.16, found: C 49.34, H 6.08.

4-(5-Cyanopentoxy)benzamide (17)

Colourless solid mp 97-101°C. IR (KBr) cm⁻¹: 3466, 2140, 1614, 1566, 1401. ¹H NMR (CD₃OD, 200 MHz): 7.84 (AA'BB', J = 6.8, 2, H-C(2), H-C(6)), 6.98 (AA'BB', J = 6.8, 2, H-C(3), H-C(5)), 4.06 (t, J = 6.2, 2H-C(1')), 2.49 (t, J = 6.8, 2H-C(5')), 1.90-1.57 (m, 6H). MS (EI, 70 eV): 232 (M⁺, 25), 137 (33), 121 (100), 96 (14), 55 (8), 41(5). Anal. calc. for C₁₃H₁₆N₂O₂ (232.12): C 67.22, H 6.94, N 12.06, found: C 67.54, H 6.90, N 12.54.

Iminoester 22

The suspension of 4-cyanopyridine (1, 200 mg) and zeolite (800 mg) in methanol (5 ml) was heated to reflux for 14 h. Hot reaction mixture was filtered and zeolite was washed with methanol. Crude product was chromatographed on Lobar RP-18 column (eluent: CH₃OH / H₂O = 40 : 60) affording the analytical sample of iminoester 22 (175 mg, 67%), isonicotinamide (2, 28 mg, 12%) and educt 1 (36 mg, 18%). 22: mp = 45°C (colourless needles, diisopropyl ether). IR (KBr) cm⁻¹: 3290, 3220, 3034, 1651, 1603, 1556, 1445, 1348, 1313, 1108, 1087. ¹H NMR (200 MHz, DMSO-d₆): δ 9.50 (bs, HN=C), 8.76 (AB, J = 4, H-C(2), H-C(6)), 7.80 (AB, J = 4, H-C(3), H-C(5)), 3.82 (s, CH₃O_-). ¹³C NMR (50 MHz, DMSO-d₆, DEPT): δ 164.14 (C=N), 150.60 (C2 and C6), 138.82 (C4) 121.08 (C3 and C5), 53.46 CH₃. MS CI (isobutane): 137 (MH⁺). Anal. calc. for C₇H₈N₂O (136.15): C 61.75, H 5.92, N 20.57, found: C 62.04, H 5.89, N 19.96.

Isoniazid (23)

The suspension of 4-cyanopyridine (1, 1.00 g) and zeolite (1.00 g) in hydrazine hydrate (1.00 ml) and water (8 ml) was heated at 90°C for 2 h. Hot reaction mixture was filtered and zeolite was washed with hot ethanol. Crude product crystallised affording 478 mg of 23, and the rest was chromatographed on SiO₂ column (eluent: methanol / EtOAc (1:9)). Isoniazid (23) was obtained in combined yield of 67% (883 mg), together with 211 mg (18%) of isonicotinamide (2). 23: mp 169°C (colourless needles, ethanol). [13, p. C-475] mp 171-173°C. IR (KBr) cm ⁻¹: 3111, 3050, 1667, 1635, 1557. ¹H NMR (DMSO-d₆, 250 MHz): 10.13 (s, 1H), 8.76-8.67 (m, 2H, H-C(2), H-C(6)), 7.80-7.68 (m, 2H, H-C(3), H-C(5)), 4.77-4.60 (m, 2H). ¹³C NMR (DMSO-d₆, 62.5 MHz): 164.02 (C=O), 150.26 (C2 and C6), 140.30 (C4), 121.06 (C3 and C5). Anal. calc. for C₆H₇N₃O (137.17): C 52.54, H 5.15, N 30.64; found: C 52.31, H 5.27, N 30.83.