Microwave-assisted conversion of oximes into nitriles in thepresence of a zeolite

Adrienn Hegedüs,a Agnieszka Cwik,a Zoltán Hell,*a Zoltán Horváth,b Ágota Esekc andMária Uzsokic

a Department of Organic Chemical Technology, Budapest University of Technology andEconomics, H-1521 Budapest, Hungary. E-mail: zhell@mail.bme.hu

b Erdokémia-ker Ltd, H-1108 Budapest, Gyömroi út 132-136, Hungaryc Chinoin Co. Ltd, Member of Sanofi-Synthélabo Group, H-1325 Budapest, P.O.Box 110,

Hungary

Received 24th July 2002First published as an Advance Article on the web 8th November 2002

Aromatic aldoximes were converted to the corresponding nitriles in a microwave oven in the presence of amolecular sieve-type modified zeolite, Ersorb, under solvent-free conditions. The yields depended on thesubstituent of the oxime.

Introduction

Nitriles are important synthons in organic chemical syntheses.1There are two essential general methods for their preparation;(i) reaction of an alkyl or aryl halide with alkali or other metalcyanide. In this case the extremely poisonous cyanides maycause problems so the technologies obviously involve thetedious and expensive treatment of the waste water. (ii)Conversion of an aldehyde into oximes and dehydration of thelatter into nitrile. Unfortunately the common dehydrating agentsare insufficient for this purpose and more active reagents arerequired. A number of compounds have been described for thistransformation, these include dicyclohexyl carbodiimide in thepresence of CuCl2,2 selenium dioxide in chloroform,3 chlor-osulfonyl isocyanate,4 triphenyl phosphine and carbon tetra-chloride in acetonitrile5 or the Burgess reagent.6 There are alsomethods published for the direct conversion of aldehydes tonitriles using reagents such as hydroxylamine in toluene in thepresence of pyridine,7 or in the presence of selenium dioxide,8or phosphoric acid,9 hydroxylamine hydrochloride in thepresence of formic acid10 or MgSO4/p-toluenesulfonic acid,11

N-tosylimines,12 or trimethylsilyl azide.13 Most of thesemethods, however, require corrosive, toxic, expensive orcommercially unavailable reagents. Thus the development of asimpler but efficient method would be an interesting synthetictarget, as well as being beneficial from an environmental pointof view.

Ersorb-4 (E4) is a commercially available clinoptylolite-typezeolite material with high silicium content (Si+Al ratio = 5+1).The original mineral is modified with ionic exchange and withother water-phase technologies followed by a thermal treatmentyielding a Ca–K mixed cation-based adsorbent with 4 Å poresize. E4 has a specific surface of 50 m2 g21 (determined by theBET-method with nitrogen at the temperature of liquidnitrogen) and has a slightly surfacial acidic character. The highsilicium content leads to high stability up to 500–600 °C. E4 canadsorb small molecules such as water, hydrochloric acid,ammonia, methanol or hydrogen sulfide. The small pore sizedoes not allow the adsorption of molecules larger than themethanol in the pores. Based upon this adsorption ability, E4has been used as spectral drying agent in both the gas and liquidphases, suitable for the dehydration of gaseous hydrochloricacid or even liquid chlorine and sulfur dioxide. Recently we

described that E4 showed good activity in different condensa-tion reactions such as in the acylation of amino acids witharomatic acid chlorides14 and in the cyclization of b-aminoalco-hols with carboxylic acids to oxazolidine derivatives.15

Results and discussion

As part of the investigation of the applicability of differentErsorb types in organic syntheses we examined the reaction ofaldehydes with hydroxylamine hydrochloride in the presence ofE4. The aim of this examination was the development of a one-pot transformation of aldehydes into nitriles. Recently amodified montmorillonite (K10/FeCl3) has already been suc-cessfully used for this transformation.16

Aldehydes react spontaneously with hydroxylamine hydro-chloride without any catalyst. In this step E4 would serve onlyto bind the liberated hydrochloric acid and water. TLCexamination of the reaction mixtures of different substitutedaromatic aldehydes with hydroxylamine hydrochloride in thepresence of E4 in boiling toluene showed that the formation ofthe oximes occurred but no formation of the appropriate nitrile

Green ContextTightening and more universal legislation over the use ofhazardous chemicals may indirectly hinder or prevent themanufacture of many important chemical products. Thusthe manufacture of organic nitrates via the traditionalnucleophite substitution route using highly toxic ioniccyanides may prove to be too difficult to operate. Sincenitriles have so many useful applications we should seek toestablish alternative, non-cyanide routes which themselvesare not significantly environmentally threatening. Here wesee a significant development in the cyanide-free route tonitriles starting from oximes and using a benign andreuseable zeolitic catalyst. JHC

This journal is © The Royal Society of Chemistry 2002

618 Green Chemistry, 2002, 4, 618–620 DOI: 10.1039/b207273b

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was observed except in the case of 4-(dimethylamino)benzalde-hyde, when 69% of 4-(dimethylamino)benzonitrile hydro-chloride was isolated, and for benzaldehyde, when GCexamination of the crude reaction mixture showed the formationof benzonitrile with 42% conversion.

As the one-pot reaction proved to be unsuccessful, weprepared the pure benzaldoximes by a known method,17 andsubjected these oximes to the dehydration reaction using E4 inboiling toluene or dioxane. Unfortunately TLC examination ofthese reactions mixtures showed only a small amount ofnitrile.

There has been increasing interest in the application ofmicrowave irradiation to chemical reactions over recent years.The publications about these experiments usually describe shortreaction times (often a few minutes only), an increase in thepurity of the resulting products and enhancement of chemicalyield. Organic syntheses on solid or solid supported reagentscoupled with microwaves, often in dry media, have particularimportance from many points of view. The workup of thereaction mixture is easier, the solid reagents are often recyclableand the solvent-free technique reduces the costs of thetechnology and protects the environment at the same time.There are some examples for the preparation of nitriles inmicrowave-assisted reactions starting from aldehydes andhydroxylamine hydrochloride in the presence of alumina,18

dibutyltin oxide supported on alumina,19 from aldoximes in thepresence of p-toluenesulfonyl chloride20 or from N,N-dime-thylhydrazones with oxone supported on wet alumina.21

In our experiments we used a Prolabo Synthewave 402focused microwave reactor. The aldoxime and E4 wereirradiated without solvent for 3–10 min with constant micro-wave power. The appropriate nitriles were obtained with goodyield. The results are summarized in Table 1. The irradiationtime and power required for good conversion depended stronglyon the substituent of the aldehyde. Thus, in the reaction of4-(dimethylamino)benzaldoxime higher microwave power re-sulted in the formation of resinous byproducts. The best resultwas obtained at 210 W (entries 2–4). 3-Nitrobenzaldoximeproduced a violent reaction at higher microwave power; at 300W in 4 min the temperature of the reaction mixture reached themaximal allowed temperature of the instrument (250 °C), so theirradiation was automatically stopped by the security system ofthe instrument. On the other hand, good results were obtained at

150 W irradiation power for 10 min (entries 9–11). Reaction ofthe 2-bromo and the 4-methoxy derivatives gave excellentyields (entries 7 and 8, respectively) but 2- and 4-chlor-obenzaldoxime gave poorer results irrespective of the irradia-tion power (entries 5 and 6). In this case longer reaction timeshad no positive effect on the yield instead resulting in theformation of resinous byproducts.

Both E4 and the microwave irradiation were necessary forgood conversion. The irradiation of the oximes alone ormixtures of the aldehyde and hydroxylamine hydrochloride inthe absence of E4 failed to give any reaction. No solvent wasrequired, the solid oximes melted in a few minutes whichresulted in a significant acceleration of the reaction as indicatedby the fast increase of the temperature of the reaction mixture.The catalyst filtered out from the reaction mixture was reusableafter a simple treatment (washing with acetone and drying at120 °C for 2 h) without significant loss of activity.

In conclusion, we have found that the simple zeolite Ersorb isa good catalyst in a new, solvent-free, microwave-assistedmethod for the conversion of aromatic aldoximes into nitriles.Compared with other methods a fast and environmentally-friendly synthesis of aromatic nitriles has been developed.

Experimental

Typical procedure for the reaction in microvawe oven

A mixture of the aldehyde (6 mmol) and E4 (1 g) was irradiatedfor 3–10 min with 150–300 W power (see Table 1). Then themixture was diluted with acetone, the catalyst was filtered offand the filtrate was evaporated. The structures of the nitrileswere confirmed by their melting point and IR spectra.

Acknowledgement

This work was supported by the Hungarian Research FundPrograms (OTKA Grant No. T-037757). The authors thank DrMária Balogh (Chinoin) for helpful discussion.

Table 1 Microwave-assisted transformation of aldoximes into nitriles

Entry RA RB RÚReactiontime/min

Microwavepower/W Yielda (%) Mp/°C lit. mp (bp)/°C

1 H H H 10 300 65 (82)b —c (40–41/1 Torr)22

2 H H (CH3)2N 10 300 32 75 75–7723

3 H H (CH3)2N 10 240 57 76–77 75–7723

4 H H (CH3)2N 10 210 95 75 75–7723

5 H H Cl 10 240 30 97–101 95–9624

6 H H Cl 10 300 30 91 95–9624

7 Br H H 5 300 90 —c 5625

8 H H OCH3 10 300 83 63 62–6326

9 H NO2 H 4 300 54 (61)b 103 11527

10 H NO2 H 5.5 240 50 (57)b 101 11527

11 H NO2 H 10 150 75 (89)b 111 11527

12 Cl H H 10 300 37 (41)b 43 46–4728

13 OH H H 10 300 59 (65)b 95 95–9629

14 H OCH3 H 10 300 65 (71)b —c 2330

a Preparative yield. b Yield estimated by GC. c Liquid.

Green Chemistry, 2002, 4, 618–620 619

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