Indian Journal of Chemistry Vol. 408, December 2001, pp. 1239- 1 24 1

Note

Microwave-induced solvent-free, rapid and efficient synthesis of conjugated nitroalkenes

using sulfated zirconia

B P Bandgar* & S P Kasture School of Chemical Sciences, S.R.T.Marathwada University,

Nanded-43 1 606, India Fax : 0091-2462-2619.

Received 11 December 2000; accepted 11 June 2001

Sulfated Zirconia catalyzed synthesis of conjugated nitroalke­nes has been carrried out under solvent-free condition using sul­fated zirconia. Recyclability of catalyst, isolation of pure products by simple filtration and evaporation are important features of this method.

Nitroalkenes are pharmacologically valuable sub­stances l -

4 and are useful as insecticidal agents.5-6 They

have proved to be valuable precursors to a wide vari­ety of target molecules. The utility of nitroalkenes in organic synthesis is largely due to their ease of con­version into a variety of functionalities.7-s In addition, they are powerful dienophiles in Diels-Alder reactions and readily undergo addition reactions with many dif­ferent nucleophiles. S A few nitroalkenes also occur in nature.9

The classical preparation of nitroalkenes involves several reagents 1

0- 13 to effect the Henry condensation

followed by dehydration of the resultant b-nitro alcohols. 14 The utility of nitroalkenes prompted us to investigate environmentally benign synthetic protocols under solvent free 'dry' conditions lO for the Henry reaction which involves the use of excess amount of nitroalkanes. We now report herein a solventless synthesis of conjugated nitroalkenes using microwaves in the presence of sulfated zirconia as a heterogenous catalyst.

The method reduces longer reaction times and requirement of large quantities of nitroalkanes that normally play the dual role of a solvent as well as a reactant in the conventional solution phase chemistry.

:> MW R, /NO, =O+RCH NO > =C + H 0 , ' Sulfated Zirconia � 'R '

Microwave heating has been used for a wide variety of applications including the rapid synthesis of organic compounds. 15 The condensation aldehydes or ketones with equimolar amount of nitroalkanes using a catalytic amount of sulfated zirconia coupled with the pulsed microwave irradiation is found to be an ideal system that affords high yields of the conjugated nitroalkenes directly without the isolation of the intermediate b-nitroalcohols. That the effect is not purely thermal 1 5 is supported by the fact that using an alternate heating mode (oil bath) at temperature of 100 °C, the reaction of cyclohexanone (Entry-30) was incomplete in less than 15 hr. The results are summarized in Table I The reaction of nitromethane, nitroethane and phenynitromethane with variety of aldehydes and ketones was carried out. The isolation of the pure products by simple filtration and evaporation is an additional important feature of this method. The products were obtained in good yields with exclusively (E) geometryl2- 1 3 and no contamination with the corresponding (Z) isomers.

In conclusion, the use of sulfated zirconia as a het­erogenous catalyst for microwave induced synthesis of conjugated nitroalkenes from aldehydes or ketones and nitroalkanes under solvent free condition is a vi­able alternative to the existing procedures. Further­more, this method is advantageous because of rapid reaction rates, easy isolation of pure products in good yields by simple filtration and evaporation. The recy­clability of the catalyst is worth mentioning .

Experimental Section A zirconium salt, such as zrOCh or ZrO(N03h, with aqueous ammonia was converted into zirconium hydroxide and then to the sulfated zirconium hydroxide with a H2S04 or (N�hS04 solution. The resultant sulfated zirconium hydroxide must be calcined in air at 550-650°C to generate sulfated zirconia (SOlI Zr02 ) which is solid super acid 104

times stronger than 100% sulfuric acid. IS Sulfated zirconia was supplied by MEL Chemicals, England and was used without activation/calcination. A Kelvinator T-37 model microwave oven, was used for microwave irradiation at its 100% power and 760 watt. IR spectra were recorded on Bomem MB 104 FTIR spectrometer whereas I H NMR and I 3C NMR

1240 INDIAN JCHEM. SEC B, DECEMBER 2001

Table I-Synthesis of conjugated nitroalkenes catalyzed by sulfated zirconia

Entry RI R2 R Time Yielda•b m.p Lit.m.p (sec) (%) °C °C

3a 2-N02C6H4 H H 1 5 80 102 1 0417

3b 3-N02C6H4 H H 1 5 82 124 1 24-2617

3c 4-CIC6H4 H H 40 92 1 1 1 1 1 1 17

3d 2.4-CI2C6H3 H H 20 74 1 18 1 2017

3e C6H5 H H 30 78 66 58-59 16

3f 4-0HC6H4 H H 30 68 1 68 1 68-69 16

3g 3-Me.4-0HC6H3 H H 30 54 167 167-68 16

3h 4-0Me C6H4 H CH3 40 78 46 44-45 16

3i 4-0HC6H4 H CH3 45 72 124 1 24-2616

3j C6HS H CH3 40 76 64 64-65 16

3k C6H5 CH3 H 120 83 Oil

31 4-N02C6H4 H C6H5 35 75 140 14 1 17 3m 4-CIC6H� H C6H5 40 80 127 1 2i7

3n (CH2)4 H 90 88 Oil

30 (CH2)5 H 90 89 Oil

(a) Yields are of pure isolated products. (b) Products were characterized by their physical constants, IR, IH NMR, J3C NMR and com-

earison with authentic sameles.(c) These comeound were eurified further b;t chromatograeh;t.

were recorded on a Perkin-Elmer 90 MHz NMR instrument. General Procedure

A mixture of aldehyde or ketone (5 mmoles) and nitroalkane (5 mmoles) was placed in a 25 ml beaker containing sulfated zirconia ( 1 00 mg) and exposed to pulsed microwave irradiation using an unmodified microwave oven operating at its 1 00% power. The reaction mixture was cooled to room temperature ( I min) and irradiated again for 10 to 1 5 sec. After completion of the reaction (TLC), the reaction mix­ture was filtered and the catalyst was washed with ether (4 x 10 mL). The solvent was removed under reduced pressure to afford the products in good yields and in nearly pure form; if necessary products were further purified by column chromatography using pe­troleum ether:ethyl acetate (9 : 1 ) as an eluent.

Acknowledgement We thank MEL Chemicals, England for the

generous gift of sulfated zirconia.

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NOTES 1 24 1

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